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Gw mh-3-effect of sea water intrusionon gw quality in & around kelwa mahim village, distt. thane1
1.
2.
3. 1.1 Introduction:
Ground water is a vital resource for communities and ecosystems of the
coastal zones in India. The control of sea water ingression into the fresh water
coastal aquifers has become the matter of great concern in the coastal areas of India
due to the increase in groundwater withdrawl of fresh water supplies which overlay
saline water.
The Maharashtra State has a coastline of 720 Kilometers. The proximity of
coastal aquifers to seawater creates unique issues with respect to the sustainability
of fresh groundwater in coastal regions. Groundwater is a subject of rising social
concern, especially in coastal zones where it is increasingly mobilized to satisfy
water demands for agriculture and domestic uses. Ground water withdrawal for
drinking water supply, agriculture, industry and other uses in coastal area has
increased many folds during the last decade. Ground-water development obviously
depletes the ground water storage in the aquifer thereby reducing the hydraulic head
of fresh water inland aquifer. Due to decrease in hydraulic head in fresh water
aquifer the sea water migrates towards the inland side causing the deterioration in
groundwater quality.
Overpumping of groundwater in Kelwa-Mahim coastal area has been noticed
since the year 1984. Government of Maharashtra therefore imposed ban on
construction and energization of new wells. But the heavy groundwater extraction
activity continued thereafter, resulting in the depletion of groundwater level and
deterioration of groundwater quality,too. In addition to the problem of
overexploitation,the salt pan activities and increasing industrial waste in the adjoining
area also deteriorated the groundwater quality. The primary concern relating to
increasing salinity in groundwater is associated with potential limitations on
groundwater usage.Thus posing serious threats to the availability of the safe and
sustainable groundwater for drinking and irrigation purpose.
Sea water ingression study was undertaken in the Kelwa-Mahim coastal area
in Palghar taluka of Thane district with a view to study the extent and deleterious
impact of Sea water ingression in the coastal fresh water aquifer and accordingly,
frame the groundwater management strategy for reducing the sea water ingression
so as to provide pure and sustainable groundwater solution to the community. This
‘Purpose Driven Study’ was undertaken in Hydrology Project –phase II.
4. Geological,Hydrogeological and Geophysical investigations has been done by
Groundwater Surveys and Development Agency (GSDA),while the core logging and
its Petrological and Petrographical study and Isotope Radioactivity study was done
with technical support from Geological Survey of India(GSI) and Bhabha Atomic
Research Center (BARC),respectively.The project work commenced from the year
2008 and completed in year 2012 (four hydrological cycles).
1.2 Aim and Objectives:
Aim : “To study the extent of Sea water ingression in the coastal fresh water aquifer
and to frame the groundwater management strategy for reducing the sea water
ingression so as to provide pure and sustainable groundwater solution to the
community”.
Objectives:
§ To study the effect of tides on groundwater levels in the inland aquifer and to
develop the relation between these two.
§ To demarcate the extent of sea water ingression into the inland fresh
groundwater aquifer and to delineate the subsurface position of the zone of
mixing (fresh water-salt water interface).
§ To study the rate of change in chemical quality of groundwater and to
recommend the preventative measures.
§ To study the detailed petrology and petrography of the sub surface geological
formation and observe the depth wise effect of sea water ingression.
§ To study the hydraulic parameters of the prevailing groundwater system by
hydrogeological and radio isotope study.
§ To frame the groundwater management strategy for recovery of the area
affected by sea water ingression and also to reduce further sea water
ingression in the inland ground water system.
1.3 Methodology adopted:
In order to achieve the above objectives following methodologies have been
adopted as:
• Basic data collection: - Village maps, list of drinking and irrigation dug wells;
bore wells, cropping pattern, land use pattern, and all other socio economic
parameters.
5. • Mass communication: - Techniques involving Information, Education and
Communication (IEC) have been used for having interaction and participation
of the community. Workshops were conducted at village levels.
• Reconnaissance survey: - Preliminary geological and hydrogeological
survey of the area including mapping of surface geological subunits, major
drainages, surface water bodies, etc.
• Detailed hydrogeological surveys: - Includes well inventory of all the wells
(315 wells), fixation and monitoring of observation wells (47), aquifer
performance tests (11), construction piezometers for monitoring of deep
water levels and installation of an automatic DWLR.
• Core drilling: - Two boreholes of depth 30 meter each are drilled by
Geological Survey of India (GSI) using core drilling techniques at two pre
identified sites for detailed petrological and petrographical study.
• Geophysical investigation: - Includes 47 Vertical Electrical Soundings
(VES), synthesis and interpretation resistivity parameters.
• Hydro chemical Investigations: - Includes periodical collection of
groundwater samples from observation wells, synthesis and interpretation of
water quality parameters.
• Radio Isotope study: - Environmental and radioactive study is conducted in
collaboration with Bhabha Atomic Research Centre (BARC).
• Preparation of final report:- Includes collective analysis of all above
information, preparation sea water ingression model of the study area,
preparation of the final report, framing the groundwater management strategy
for reducing the seawater ingression in the study area, with community
participation.
1.4 Location and extent of the area:
The study area lies between the north latitudes 190
35’ & 190
41’ and east
longitudes 720
42’00” & 720
47’30” (Fig.1.4.1). It covers an area about 74 sq.kms of
6. villages Kelva ,Mahim, and their habitations in Palghar taluka of Thane district. The
area falls in quadrant C-2 of Survey of India Toposheet no.47A/14 and 47A/10 and
watershed WF-19.
The area is situated due north-west of Thane , the district headquarter at
about 103 kms and about 8-10 kms due south -west of Palghar, the taluka
headquarter.
Fig.1.4.1. Location map of the study area
1.5 Demographics:
The study area comprises the villages kelva ,Mahim, and their habitations in
Palghar taluka of Thane district. Total population of the area is about 13,500 as per
2001 census, now it may be around 15,800. Mostly tribal and konkani community is
observed in the area. Agriculture, fishery and salt pan activity are the main source of
7. income. Agricultural produce include rice, chikku, banana, coconut, beetle nuts,
vegetables, etc. Chikku grown in this area are sent allover India.
1.6 Physiography and drainage:
The area is almost flat with gentle slope towards the west. The elevation
decreases from 11-12 m.amsl in the east to 1-2 m.amsl at the extreme west. The
two creeks divide the area into two parts, the northern one is Mahim and the
southern is Kelwa Village area. The sea water ingress and retreads along these
creeks during tide times. The area borders the Arabian Sea on west side with a sea
shore of 15 kms. East west slope of the area is about 1 to 1.5 m/km. The varieties of
depositional and erosional landforms are commonly found in the area. Mud flats,
tidal marshes, and mangrove swamps are developed along the creek at some
places.
The area is drained by the local stream known as Paneri and by the creek in
the south. The area is divided by two creeks, one in the north and other in the south.
These creeks run from coastal inland up to considerable distance, carrying tidal
water. (Fig.1.4.1 and 1.6.1)
1.7 Climate:
The climate of Thane district is characterized by high humidity nearly all
round the year, oppressive summer season, and well distributed rainfall during the
south west monsoon season. The year can be divided into three seasons.
Rainy season- June to October
Winter season – November to February
Summer season - March to June
The area experiences a tropical, moderately humid and semi humid type
climate, throughout the year. It is rather cool during the rainy and winter season.
Relative humidity is high. The average temperature ranges between 17o
C to 32o
C.
9. 1.7.1 Temperature:
The nearest meteorological observatory at Dahanu is located near to the
study area. The temperatures are slightly lower during cold season and higher in the
hot season than at Dahanu in the eastern part of Thane district in the study area.
The temperatures progressively increase after February till May which is the hottest
month with mean daily temperature at 32.90
C. In summer season, the temperatures
may sometimes go above 370
C in the study area. With the onset of monsoon in the
second week of June the temperature decreases. From October when the south
west monsoon withdraws, the day temperatures increase and during October and
November the days are as hot as in summer while nights become progressively
cooler. After November, temperature decreases and in January which is the coldest
month, the mean daily maximum temperature is 27.70
C and the mean minimum is
16.80
C. In the cold season, cold waves sometimes affect and the night temperatures
go down to 100
C.
1.7.2 Winds:
In Thane area, winds are generally moderate except in the latter half of the
summer and during the south west monsoon season they are stronger. Winds during
May and the monsoon months blow mainly from directions between the northwest
and southwest. During rest of the year, winds blow from directions between north
and east in the mornings and between west and north in the afternoon.
Some of the cyclonic storms in the Arabian Sea in the latter part of summer
and post-monsoon season either move in northerly direction in the vicinity of the
coast. On such occasions the area experiences torrential rains with winds
sometimes reaching gale force. Thunderstorms occur in the latter part of the summer
season and in October.
10. Table: 1.7.1- Normal Temperature, Relative Humidity and Mean Wind speed at
Dahanu station
Month
Normal
Rainfall
(mm)
Mean
Maximum
Temperature
0
C
Mean
Minimum
Temperature
0
C
Relative
Humidity in
percentage
0830 1730
Mean
Wind
speed in
(Km/hr)
January 3.1 27.7 16.8 68 71 12.1
February 1.0 28.2 17.5 67 70 12.5
March 1.0 30.3 21.0 68 67 14.0
April 3.8 32.0 23.9 73 70 14.9
May 16.5 32.9 26.8 78 75 16.5
June 360.4 32.1 26.4 86 81 18.4
July 1006.6 29.7 25.1 91 86 25.3
August 632.2 29.1 24.8 89 84 24.3
September 320.8 29.6 24.3 87 79 16.3
October 99.8 31.7 23.0 77 74 10.8
November 18.5 31.9 20.0 68 71 9.9
December 3.6 29.7 17.9 67 71 10.7
Annual 2467.3 30.4 22.3 77 75 15.5
1.8 Rainfall:
Thane district receives rainfall from the southwest monsoon. Average annual
rainfall of the Thane district is 2441.7 mm. On an average, there are 83 rainy days
(i.e. rainfall more than 2.5 mm) in a year. Nearest rain gauge station to the study
area is Palghar, the taluka headquarter. It receives an average annual rainfall 2669
mm (Table-1.8.1, Fig: 1.8.1).
Annual rainfall and rainy days at Palghar rain gauge station
0
500
1000
1500
2000
2500
3000
3500
2000
2001
2002
2003
2004
2005
2006
2007
2008
2009
2010
2011
2012
Years
Rainfallinmm
0
20
40
60
80
100
120
Rainydays
Annual rainfall in mm Rainy days
Fig: 1.8.1: Rainfall and rainy days at Palghar rain gauge station
11. Table: 1.8.1: Rainfall and rainy days at Palghar rain gauge station
Year Annual rainfall in mm Rainy days
2000 2055.90 69
2001 2656.70 95
2002 2077.40 65
2003 2307.80 99
2004 2123.50 85
2005 3198.00 82
2006 2611.10 85
2007 2016.50 91
2008 2831.20 86
2009 2645.20 75
2010 3169.88 103
2011 2323.76 97
2012 2017.30 83
12.
13. 2.1 Introduction:
The nature and lateral and vertical extent of aquifers are controlled by litho
logy, stratigraphy and structure of rock formations. Litho logical characteristics of
rocks are reflected in their storage and yield properties. Stratigraphy gives the
chronological order in which the rock formations were laid down on the surface of the
earth, that is, the order of superposition of rocks from which the presence or
absence of aquifers below a rock formation can be predicted.
The present chapter deals with the regional as well as local geological set up
of the area and its Stratigraphic position. It also deals with the petrology and
petrography of cores of sub surface geological formations.
2.2 Geology and Structural features of Thane district :
The Thane district is covered almost entirely by Deccan basalt of Upper
Cretaceous to Paleogene age except for a few patches of alluvium occurring in the
river valleys. The Deccan Trap basalt is capped by laterite at a few places.
The Deccan basalt flows in the district are classified as ‘pahoehoe’ and ‘aa’
and are normally aphyric to feldspar phyric. At places, some of the feldspar flows are
quite extensive and serve as reliable regional markers for grouping the flows into
various formations. Three different megacryst horizons viz. (M1, M2, and M3) have
been identified and on the basis of these marker horizons the lava pile has been
divided into six formations. The lowermost Salher formation comprises of 11 aphyric
flows which is followed by lower Ratangarh formation comprising 7 fine grained
feldspar phyric flows. Upper Ratangarh formation comprising 6 aphyric to feldspar
phyric ‘aa’ flows appear next in the sequence. The uppermost Karla Formation
comprises of 3 compound pahoehoe flows of aphyric nature (GSI, 2001).
The basaltic pile of the area is profusely intruded by rocks of Borivali
formation which included doleritic and basaltic dykes, tuffs and agglomerates. The
frequency of dykes is much more in the northwestern part where the N-S trending
dyke is very conspicuous. Dykes trending in NW-SE, NE-SW and E-W are also
observed. Frequency of dykes becomes relatively less in the south eastern part. At
places triangular or rectilinear dyke patterns are noticed.
14. In the area to the west of Thane, an arcuate fault exposed a sequence of
various volcanic and plutonic rocks which are different from the rocks exposed east
of the fault. A sequence of fine grained ‘aa’ flows occupies about 60 % of the area.
The flows are aphyric in nature. These flows at places are interlayer with tuffs and
agglomerates and volcanic clastics occurring at different stratigraphic levels.
A gabbro pluton emplaced in Borivali formation having a width of about 2 km
and length of about 10 km is seen tapering due north and south. Its contact on west
and east are faulted. The fault on eastern side is a boundary fault separating Borivali
formation from normal pahoehoe formations.
Laterite of the Cenozoic age occurs as small isolated cappings on the top of
Tungar hill, Mahuli peak and east of Bena Malang (Fig. 2.2.1).
Fig.2.2.1: Geology of Thane district
2.3 Geology of the study area:
Eastern part of the study area is occupied by basalt lava flows. Basalt flows
are highly weathered and its thickness extends up to 8-10 m.bgl at some locations.
Where as, the western part of the villages p a r a l l e l to the sea coast (coastal
area) is characterized by the occurrence of alluvium deposit. These are recent
15. sedimentary (detrital) deposits. They are compact and consist of shell fragments,
sand, and gravel cemented together. Beach rock or calcareous grit is also found
along the coast. The thickness of the beach rock varies from 2 - 8 m. with low
angle seaward dip. These rocks are locally known as ‘Kakra’ and overlie basaltic
flows (Fig.2.3.1).
The Varieties of depositional and erosional landforms are commonly found in
the area. Mud flats, tidal marshes, and mangrove swamps are developed along
the creek at some places.
Medium to coarse-grained sand mixed with shell fragments forms the upper
layer of the soil. Mostly, the soil found in the area is black soil. Soil thickness
varies from 0.3 m to 2 m from east to west. Soil cover is followed by weathered
basalts in eastern portion and by recent alluvial deposits in western portion.
Fig.2.3.1: Geology of Study area (Kelwa –Mahim villages)
2.4 Core logging and Petrography of core samples:
To understand the subsurface geology, mineralogical characteristics and
depth wise impact of sea water ingression, two boreholes of 30 m length each
16. w e r e d r i l l e d i n t h e Kelwa-Mahim area. Drilling was followed by a run wise
core logging and collection of 90 nos. of samples based on the mineralogical
variation in the litho units and study of petrology and petrography of the core
samples.
Two boreholes were drilled at (1) Harnewadi and (2) Mangalali, (Fig.2.3.1).
Drilling were commenced on 28.05.2011 and completed on 08.06.2011 by the
Drilling Division, GSI, Central Region, Nagpur and core logging was carried out
at Regional Drill Core Repository, GSI, Central Region, Nagpur. The brief
outcome of core logging and petrography of core samples are given below.
2.4.1 Logging of core samples :
A] Borehole No.1 – Haranewadi :
Location: On Palghar road (Latitude 190
40’20” and Longitude 720
43’50”)
In this borehole two distinct types of basaltic flows were recorded (1)
massive and (2) amygdaloidal. Massive basalt belongs to Aa flow, whereas
amygdaloidal basalts exhibit typical compound Pahoehoe flow features. A total of
four flows were identified during the logging based on the top and bottom criteria.
Soil cover is present at the top of borehole. It extends up to 2.85 m followed by the
weathered massive basalt of flow-I. The soil is grey to buff colored gritty and
pebbly with basalt fragments. At places presence of clay is also recorded
(Fig.2.4.1A)
17. Fig.2.4.1A: Lithological succession in bore hole -1 (Haranewadi)
Flow-I extends up to 10.65 m depth below the soil cover and it is
characterized by its highly weathered and fragmented nature. Amygdales filled
with zeolites are present in the top of the flow-I. During the logging, clayey muddy
soil and fragmentary rock is recorded in between the flow-I especially at the bottom
of every run, which is indicative of a fracture zone.
Three distinct flows are recorded belonging to compound Pahoehoe below flow-I.
All these flows are characterized by the presence of amygdales in various
proportions and intense fracture fillings by zeolites, siliceous, calcareous and
chlorophaeitic material. These fractures show displacement at places along with
brecciation of the country rock.
The flow-II extends from 10.65 to 18.50 m and it is composed of fine
grained, grey, brownish to greenish black coloured amygdaloidal basalt with
amygdales filled by zeolites, quartz and chlorophaeitic material. Amygdales
recorded are of different sizes and shapes. In run 6/3, gradual decrease in
amygdule size is noticed from top to bottom. Number of sub vertical fractures is
18. recorded in this flow which is filled by siliceous, calcareous, chlorophaeitic and
zeolites rich vein lets. Pinching and swelling nature of the veins is also noticed. Due
to fracturing at places the basalt is highly fragmented. Bottom part of the run 5/3
and 5/4 consists of unconsolidated sandy material indicating presence of
fracture/joints.
Flow-III is intersected between 18.50 to 25.20 m. It is fine grained, grey,
brownish to greenish black coloured amygdular basalt with amygdales filled by
zeolites, quartz and chlorophaeitic material. High density of amygdales is observed
in the flow. Numbers of fractures are noticed traversing the core perpendicularly;
sub vertically and also vertically, filled by siliceous, calcareous, chlorophaeitic and
zeolites rich veins (Photo-1: Plate-I). These veins show braided nature and are of
up to 3 cm thick (at 23 m depth). Dismembering of the host rock fragments along
these veins is ubiquitous. Chloritization adjacent to the veins is also noticed. Step
like displacement (right lateral slips) of thin veins (up to 3 mm) are present in the run
7/4.
Flow-IV is recorded between 25.20 to 30 m and it is fine grained, grey to
brownish amygdular basalt. Number of calcite and siliceous veins cut at right angle
to the flow. Pinching and swelling nature of these veins is also noticed. A 0.5 cm
thick horizontally disposed calcite vein is recorded at the bottom of the run (8/3)
(Photo.2.Plate-I). Veins cutting, vertically, horizontally and at an angle to the core
are noticed in run 9/1. Angular to sub angular basaltic fragments are found in these
veins (run 9/1). Along the contact of the veins ferruginization is noticed, especially
more at the top of the run 8/3 with decrease in density towards lower part.
B] Borehole No.2 – Mangalali :
Location: on Mahim-Kelwe road (Latitude 190
36’30” and Longitude 720
44’00”)
In contrast to the Harnewadi borehole, here the presence of detrital
limestone layer is noticed at the top, followed by massive basalt. The amygdular
basalt which is found in Harnewadi area is missing here. Four distinct flows are
identified during the core logging. Near the surface dark buff to grey coloured silty,
clayey soil is recorded up to a depth of 6.17m.bgl. Below it, detrital limestone is
recorded from 6.17 to 9.50 m. Size of the individual clasts varies from 1 mm to 0.5
cm. These grains are well rounded and moderately sorted. There is a gradual
19. increase in the grain size with depth (Photo.3. Plate-I), which defines graded
bedding.
This detrital limestone exhibits typical graded bedding with repeated cycles
of sedimentation (> 10 cycles). Pebbles of cherty, siliceous, carbonate material
along with basaltic rock fragments are recorded and at places they are poorly
sorted in nature. A few gastropod fossils (megascopic) of the size 3 mm wide and 2
cm long are recorded. Unconsolidated pebbles are found at the top of the run which
are followed by poorly sorted pebble rich zone (size varying from 0.2 cm to ≥ 5 cm)
with calcareous cementing material; where as at bottom of the run is composed of
grey sandy and silty material as recorded in run 3/4.
Four distinct basaltic flows are recorded between 9.50 to 30 m from the
surface in this borehole i.e., Flow-I (9.50 to 18.10 m), Flow-II (18.10 to 21.00),
Flow-III (21.00 to 27.00 m) and Flow-IV (27.00 to 30.00)(Fig.2.4.1B).
The top flow (Flow-I) is a fine to medium grained highly weathered basalt at
the top of the flow which show gradual increase in grain size with depth. Intense
fracture fillings by quartz and calcite veins are noticed. Release of iron oxide due to
weathering is recorded. Sandy material is also observed within the flow-I indicating
the presence of intense fracture zones.
In Flow-II, fine to medium grained dark grey massive basalt with fracture
filling by quartz, calcite and chlorophaeitic veins is recorded. These veins are
cutting the core sub- vertically to 45° with respect to the flow (run 6/2) (Photo.4:
Plate-I). Veins have been displaced by minor right lateral shifts (step like).
Occurrence of loose sandy material in between the runs suggests presence of
fracture zone. The contact between the flow-I and II is also marked by the
presence of sandy material.
20. Fig.2.4.1B: Lithological succession in bore hole -2 (Mangal ali)
Flow-III is also medium grained and massive dark grey coloured basalt with
fracture filling by chlorophaeitic material and quartz-calcite veins. These veins cut
the core at 45° angle and displacement along these veins is also noticed.
The bottom most flow (Flow-IV) is characterized by the presence of
plagioclase phenocryst and abundance of glass. It is medium grained, porphyritic
and dark grey coloured basalt with fracture filling by chlorophaeitic material, quartz
and calcite veins. These veins cut the core at 45° angle and displacement along
these veins is also noticed viz., dragging of basaltic fragments, stretched chloritic
material at the contact of the veins. This flow is also characterized by intense
fractures with networking.
21. 2.4.2 Fracture Pattern:
Three sets of fractures are recorded during the borehole logging viz., (a)
vertical to sub-vertical to the flow or parallel to the axis of drill core, (b) at 45° to the
flow/axis of the drill core and (c) horizontal to the flow i.e. perpendicular to axis of
the drill core. All these fractures are filled by secondary quartz, calcite and
chlorophaeitic material. Vertical and sub- vertical fractures are dominant followed by
the fractures cutting 45° to axis of the drill core (Photo.5: Plate-I). They are
generally interconnected and at places show minor displacement (Photo.6: Plate-I).
Frequency of the fractures varies from flow to flow in the individual borehole.
Fracture density is more at Haranwadi core samples than that of at Mangalali.
2.4.3 Petrography of core samples:
A total of 90 core samples were collected from the two bore holes based on
the mineralogical variation for detailed petrography study. The petrography
characteristics of these rocks are detailed below.
A] Borehole No.1:
In the borehole drilled in the Haranwadi village four distinct flows were
identified during the borehole logging. Top flow is marked by massive basalt and
the remaining flows are of amygdular basalt. Flow wise petrography description of
the rock is as follows.
In the flow-I, massive basalt is recorded. It is a medium grained, sparsely
porphyritic rock, mainly composed of plagioclase feldspar, clinopyroxenes
(augite), glass (along with palagonite) and opaques (Magnetite+ilmenite).
Porphyritic, sub-ophitic, intergranular and interstitial textures are noticed. A few
phenocrysts of both plagioclase and augite are noticed in a finer matrix. Very few
enstatite grains are recorded. Plagioclase laths vary in size and show resorbed
margins. Zoning and Carlsbad twinning is very prominent in plagioclase at places
(Photo.5. Plate-II). Phenocryst of plagioclase shows segregation. Opaques
especially needle shaped ilmenites are abundant at the bottom of the flow.
Chlorophaeitic veins are observed in samples HGSDA-5. Top of the flow-I is glass
rich and contains amygdales filled with quartz and zeolites. Intense ferruginization is
also noticed.
22. Flow-II is inequigranular medium grained amygdular basalt. It is mainly
composed of calc-plagioclase (~35 %), pyroxenes (~25%) (with dominant augite
and minor enstatite), opaque minerals (~10%) and g l a s s (~30 % with palagonite).
Amygdales of quartz, calcite, zeolites and choloropheate are noticed filling the
vesicles. These vesicles are of varying sizes and shapes and at places they are
interconnected by secondary quartz and calcite veins. Replacement of zeolites by
calcite is ubiquitous in sample MGSDA-17. Density of the amygdales is very high at
the top of the flow-II. Glass is dominant in the flow. Alteration of glass has resulted
in development of brown to yellow palagonite. Porphyritic, glomeroporphyritic, sub-
ophitic, intergranular and interstitial textures are observed (Photo.6: Plate-II). A few
phenocrysts of both plagioclase feldspar and augite are noticed in a fine grained
glass rich matrix constituted by plagioclase, augite and glass. Plagioclase laths
vary in size and show oscillatory zoning at places. Phenocryst of plagioclase
feldspar and augite show segregation. This flow is intensely fractured and filled with
secondary veins of calcite, zeolite, chlorophaeite and ferruginous material. These
veins are anatomizing in nature with varying width (Photo.7: Plate-II). Some of the
quartz veins exhibit pinching and swelling nature.
Flow-III is an inequigranular, fine to medium grained amygdular basalt. It is
mainly composed of calc- plagioclase feldspar, pyroxenes (with dominant augite
and minor enstatite), olivine, opaque minerals (magnetite and ilmenite) and glass
(with palagonite). The vesicles are filled by quartz, calcite, zeolite and
choloropheaite. These vesicles are of varying sizes and shapes and at places they
are interconnected (Photo.8: Plate-II). Partial to complete replacement of zeolites by
calcite is noticed in amygdales (Photo.9 & 10: Plate-II). Density of the amygdales is
very high. Glass is dominant in the flow especially in sample no. HGSDA-28;
where it is made up to 80%. Alteration of glass has resulted in development of
brown to yellow palagonite. Porphyritic, glomeroporphyritic, ophitic to sub-ophitic,
intergranular and interstitial textures are noticed. A few phenocrysts of both
plagioclase and augite are noticed in a fine grained glass rich matrix constituted by
plagioclase, augite and glass. Very few enstatite and olivine grains are present.
Plagioclase laths vary in size and show oscillatory zoning at places. Phenocryst of
plagioclase and augite show segregation. This flow also shows intense fracturing
and filled with secondary veins of calcite, zeolite, chlorophaeite and ferruginous
23. material. These veins are braided in nature with varying width. Some of the
quartz veins exhibit pinching and swelling nature and contains caught up patches of
basalt. Earlier formed calcite and iron oxide rich veins are traversed by quartz
veins. Three generations of quartz veins are noticed.
Flow-IV is inequigranular, fine to medium grained amygdular basalt. It is
mainly composed of plagioclase feldspar, pyroxene (with dominant augite and minor
enstatite), opaque minerals (magnetite and ilmenite) and glass (with palagonite).
Olivine is subordinate to minor. Amygdales of quartz, calcite, zeolite and
choloropheate are noticed. The size and shapes of vesicles vary and they are
interconnected at places. Density of the amygdales is very high. Glass is dominant
in the flow especially in sample HGSDA-36 where it is made up to 70%. Alteration
of glass has resulted in development of brown to yellow palagonite. Porphyritic,
glomeroporphyritic, sub-ophitic, intergranular and interstitial textures are noticed. A
few phenocrysts of both plagioclase and augite are noticed in a fine grained glass
rich matrix constituted by plagioclase, augite and glass. Very few enstatite
grains are present. Euhedral olivine crystals are present as serpentinized
pseudomorphs (Photo.11 Plate-III).Plagioclase shows crude alignment probably
indicating flow direction. Plagioclase laths vary in size and show oscillatory zoning
at places. Phenocrysts of plagioclase and augite show segregation. This flow also
shows intense fracturing and filled by secondary veins of quartz, zeolite,
chlorophaeite and ferruginous material. These veins are parallel and braided in
nature with varying width.
B] Borehole no.02:
In the Mangalali borehole, calcareous sedimentary rock is noticed at the top
followed by four distinct flows of massive basalt towards depth. Petrographic
description of these rocks is detailed below.
Borehole encountered detrital limestone towards top. Detrital limestone is
composed of mainly lithic fragments made up of micritic limestone (at places rich
in heavy minerals and microfossils) and basalt fragments and very few s quartz,
chalcedony, enstatite , augite, plagioclase and opaques grains as clasts. Calcite
(sparite) is the cementing material (Photo.12 & 13, Plate-III). Presence of
24. microfossils like well developed foraminiferas is ubiquitous (Photo.14, Plate-III)
(confirmed from Paleontology Division). Heavy minerals (such as opaques,
diopside, rutile etc.) are also noticed within the lithic fragments along with
microfossils (Photo.15, & 16, Plate-III, Photo. 17, Plate-IV). The clasts are well
rounded with poor to moderate sphericity and poorly sorted. Basaltic fragments
show intense ferruginization and are glass rich. Secondary calcite veins are noticed
cross cutting the lithic fragments.
In the flow-I, massive basalt is recorded. It is a medium grained, sparsely
porphyritic and mainly composed of plagioclase feldspar, clinopyroxenes (augite),
glass (along with palagonite) and opaques (magnetite and ilmenite) (Photo.18,
Plate-IV). Porphyritic, ophitic to sub-ophitic, intergranular and interstitial textures are
noticed. A few phenocrysts of both plagioclase and augite are noticed in a finer
matrix. Rarely enstatite grains are recorded. Plagioclase laths vary in size and show
resorbed margins. Oscillatory zoning and Carlsbad twinning is very prominent in
plagioclase at places. Top of the flow is highly altered and weathered with very few
amygdales filled with cryptocrystalline quartz (chalcedony). Glass is dominant in
the flow i.e. up to 40%. Intense alteration of glass has resulted in
development of brown to yellow palagonite. This flow has been affected by
intense fracturing and vein filling with quartz, calcite and chloritic material (Photo.19,
Plate-IV). Calcite is present in the veins and also in the matrix adjacent to
veins (Photo.20, Plate-IV) Iron rich veins are also present. Three generations of
veins are recorded in the sample MGSDA-11, in which Fe rich veins are the
youngest one. Fine veins of chalcedony are observed. The chloritic veins show
indications of movement (slips) as noticed in sample MGSDA-16 (Photo.21, Plate-
IV).
Flow-II is constituted by fine to medium grained basalt. It is highly
fractured at places with emplacement of veins of quartz, calcite and
chlorophaeitic material along the fractures. It is sparsely porphyritic rock, mainly
composed of plagioclase feldspar, clinopyroxenes ( augite), o r t h o p y r o x e n e
( enstatite), g l a s s ( along w i t h p a l a g o n i t e ) a n d opaques (Magnetite &
ilmenite). Orthopyroxenes are present in minor amounts. Porphyritic, ophitic to sub-
ophitic, intergranular and interstitial textures are observed (Photo.22, Plate-IV and
Photo.23 & 24, Plate-V). A few phenocrysts of both plagioclase and augite are
25. noticed in a finer matrix. Plagioclase laths vary in size. Oscillatory zoning and
Carlsbad twinning is very prominent in plagioclase at places. Top of the flow is
highly altered and weathered with very few amygdales filled with quartz. Glass is
dominant in the flow. Intense alteration of glass has resulted in development of
brown to yellow palagonite. This flow has been affected by intense fracturing
and vein filling with quartz, calcite and chloritic material. Iron rich veins are also
present. Parallel veins of chlorophaeitic material and chalcedony is noticed on
sample MGSDA-21.
Flow-III is fine to medium grained basalt, sparsely porphyritic. It is highly
fractured at places and filled by veins of quartz, calcite and chlorophaeitic material.
It is mainly composed of plagioclase feldspar (45 %), clinopyroxenes (augite around
25 %), orthopyroxene (enstatite up to 2%), glass (along with palagonite nearly 25
%) and opaques (magnetite+ilmenite 3 %). Porphyritic, ophitic to sub-ophitic,
intergranular and interstitial textures are noticed. A few phenocrysts of both
plagioclase and augite are noticed in a finer matrix. Plagioclase laths vary in size
and show resorbed margins. Oscillatory zoning and Carlsbad twinning is very
prominent in plagioclase at places. Glass is dominant in the flow. Intense alteration
of glass has resulted in development of brown to yellow palagonite. Small veinlets of
quartz are also present. Parallel veins of iron and chlorophaeitic material are also
noticed.
Flow-IV is fine to medium grained porphyritic basalt. It is highly fractured at
places filled with veins of quartz, calcite and chlorophaeitic material. It is sparsely
porphyritic rock and composed mainly of plagioclase feldspar (~ 45%),
clinopyroxenes (augite ~ 25 %), glass (along with palagonite ~ 19%) and
opaques (magnetite+ilmenite ~ 10%). Olivine (1 %) is present in minor amount,
Porphyritic, glomeroporphyritic, ophitic to sub-ophitic, intergranular and interstitial
textures are observed. A few phenocrysts of plagioclase, olivine and augite are
noticed in a finer matrix. Plagioclase laths vary in size and show resorbed
margins. Oscillatory zoning and Carlsbad twinning is very prominent in
plagioclase at places. Olivine is showing serpentinization along the grain
boundaries and fractures (Photo.25, Plate-V). Some serpentinized olivine
pseudomorphs are also recorded. At places clustering of plagioclase phenocrysts is
also recorded. Altered glass is dominant in the flow. Glass has been altered to
26. brown to yellow palagonite. Ilmenite is the dominant opaque phase and occurs as
needles. Iron oxide veins occur as fracture fillings (Photo.26, Plate-V). Chloritic
veins exhibit pinch and swell nature. Late fractures cross cutting earlier veins are
recorded and are filled by quartz (chalcedony) and calcite (Photo. 27 & 28, Plate-V).
Thus the Kelwa-Mahim study area is covered by basalt lava flows
belonging to Sahyadri Group of Deccan volcanic province of upper Cretaceous to
lower Eocene age. Four flows including Aa and pahoehoe types are encountered at
Haranwadi lacation; whereas the upper amygdaloidal flow is missing at Mangalali
and is replaced by detrital limestone (lithified sediments).
All the flows are characterized by intense fracturing (three sets) with
vein fillings of zeolites, quartz, and carbonate and chlorophaeitic material. These
fracture fillings show displacement at places along with brecciation of the country
rock. Pinching and swelling nature of the veins is also noticed. Chloritization
adjacent to the veins is noticed. Iron rich veins along with calcite are ubiquitous.
Interconnecting vesicles in amygdular basalts is also noteworthy. Presence of
fractures in all the flows and vesicles in amygdular flows suggest that there is
ample space for groundwater to interact with the country rock.
Presence of detrital limestone at the top in Mangalali borehole with
gastropods and foraminifers suggest their deposition in brackish water condition.
Fracture filling and replacement by calcareous (calcite) and iron rich material in the
basaltic flows is conspicuous.
28. 3.1 Introduction:
Groundwater is the only source of fresh water for the coastal area. The
demand for groundwater is increasing every year due to growing population and
urbanization. On the other side the peculiar hydrologic, geologic and geomorphic
features restrict the availability of groundwater. In Kelwa –Mahim coastal area, the
coastal alluvium and/or detrital sedimentary beach rock followed by the weathered
basalt forms the shallow groundwater system; that holds and supply the fresh
ground water. Thickness of this fresh water aquifer is very limited, and hence it’s
potential. On the other side its demand for irrigation and other domestic purpose has
increased many folds, which is reflected in depleting ground water levels and also in
increasing the salinity of groundwater at depth. Thus a proper understanding of the
groundwater system is important in order to formulate future development and
management strategies.
Present chapter deals with the hydrogeologic setting of groundwater system,
and the prevailing hydrodynamic conditions in the Kelwa-Mahim coastal area.
3.2 Groundwater occurrence:
Alluvium and/or detrital sedimentary beach rock and weathered basalt are the
main water bearing formations and act as an aquifer in these villages. Alluvium is the
terrestrial sediments located parallel to the coast line and deposited by the sea
action. These include beach rock, intercalated sand and sediments of varying size.
The detrital limestone (lithified beach sediments) exhibits typical graded bedding
with repeated cycles of sedimentation (> 10 cycles). Pebbles of cherty, siliceous,
carbonate material along with basaltic rock fragments are recorded and at places
they are poorly sorted in nature.
The upper part of the basalt is highly weathered while the middle and lower part of
the flow is fractured and jointed. The weathering and joints attribute the secondary
porosity to the basalt flow; hence act as an aquifer in the area. The groundwater
inflows in wells are mainly concentrated along the vertical contact of alluvium and
basalt flow.
Thickness of this shallow groundwater system varies from 8 to 10 mbgl; and
yields fresh groundwater in general. There are 1068+ irrigation wells in the area,
29. 75% of which are mainly concentrated in the alluvial part parallel to the sea coast.
They mainly tap this shallow groundwater system with high potential. The depth of
dug well shows variation from 4 to 10 mbgl.
The dug-cum-bore wells and bore wells are also present in the area tapping
the groundwater from depth in peak season. These are deep up to 30+
mbgl. The
dug wells near the coastal region do not pierce the fresh compact basalt, as the
alluvium is thicker in this part. The irrigation wells in the basalt are less in number
and are sparsely distributed over the large area. Depth to the static water level
ranges from 1 to 4 mbgl in winter & 2 to 7.5 mbgl in summer, with a fluctuation of 1
to 4m.
Utilization of groundwater for agricultural purposes is on a large scale.
Coconut, chikku, banana, betel nut (Supari), betel leaves (Nagveli Pan), vegetables
and chilly are the main crops grown in the area. Traditional flood irrigation system is
adopted. Since last decade there is increased stress on the groundwater due to the
withdrawal for Industrial purpose also thus aquifer is further subjected to exploitation.
Groundwater level trend at Mahim observation station is depicted in the following
hydrograph (Fig.3.2.1).
FIG.3.2.1: A HYDROGRAPH SHOWING LONG TERM GROUNDWATER LEVEL TREND AT
MAHIM OBSERVATION STATION.
30. 3.3 Hydrodynamic condition in the area:
Overall 47 dug wells were selected on grid basis as an observation wells for
periodic monitoring of the groundwater levels. Pre-monsoon and post-monsoon
static water levels were recorded from all the observation wells. As most of the
irrigation wells were irrigation pumping wells, efforts have been made to record the
water level before onset of the pumping i.e. early in the morning, and in case of
delay, static water levels were recorded on the basis of reported information or by
observing the water mark in the well. The static water level data is analyzed and
interpreted in terms of depth to water level variation in the basin, seasonal water
level fluctuation i.e. rise or depletion. Water level contour maps or isobaths for every
season are prepared by using the SURFER (Ver.7) software package. Seasonal
hydrodynamic conditions of the area are discussed in the following sections.
3.3.1. Depth to the water table maps (DWL) or Isobaths:
The depth of the water table or isobaths of the water table depict the
inequalities in the position of the water table with respect to ground surface and are
useful in delineating recharge and discharge areas, locating sites for sinking wells
and dealing with drainage, artificial recharge or other problems in which the depth of
the water table is critical (Karanth, 1999). The depth of water table maps or isobaths
are prepared for winter and summer season of each observation years, by plotting
the depth of water tables as recoded from forty seven observation wells, on a base
map. The isobaths thus prepared show the variation in the depth to water table
conditions over the area.
A. Pre-monsoon (summer) isobaths (Summer 2008):
A depth to water level contour map or isobaths for pre-monsoon (summer) is
prepared (Fig.3.3.1.1). It is observed that the contours are highly scattered with the
general depth range from 2 to 10 mbgl. The western part of the area shows relatively
shallow depth to water levels, while it increases in middle and towards the eastern
part. Maximum depth is observed in the north-eastern part of the area.
31. B. Post monsoon (winter) isobaths (winter 2008):
A depth to water level contour map or isobaths for post-monsoon (winter) is
prepared (Fig.3.3.1.2). Depth of water level ranges between 0 to 5 mbgl. The
western part of the area shows relatively shallow depth to water levels, while it
increases in middle and towards the eastern part. Maximum depth is observed in the
north-eastern part of the area.
Thus from both the isobaths for two season it is observed that the
groundwater occurs at shallow depth along and near to the coast; whereas it
increases towards the east. The shallow groundwater level is attributed to the
alluvium dominant groundwater system, whereas in the eastern part where
weathered basalt aquifer geometry prevails relatively deep groundwater levels is
observed. And also the number of wells in this area are less and sparse.
Kelve
Hanumanpada
Palipada
Mahim
Haranvadi
Wagulsar
Makunsar
1 2 3 4
56
78
9
10
11
12
13
1415
16
1718
19
20
24
25
26
27
28
29 30
31
34
36
37
40
42
43
44
45
46
47
2
3
4
5
6
8
10
1935'1938'1941'
72 42' 72 45' 72 47'
DWL (pre monsoon)
N
32. FIG.3.3.1.1: SEASONAL VARIATION IN DWL -SUMMER, 2008
Kelve
Hanumanpada
Palipada
Mahim
Haranvadi
Wagulsar
Makunsar
1 2 3 4
56
78
9
10
11
12
13
1415
16
1718
19
20
24
25
26
27
28
29 30
31
34
36
37
40
42
43
44
45
46
47
0
1
2
3
4
5
N
1935'1938'1941'
72 42' 72 45' 72 47'
DWL post monsoon
FIG.3.3.1.2: SEASONAL VARIATION IN DWL - WINTER, 2008
3.3.2. Fluctuations of the water table (WTF):
The water table represents the ground water reservoir level and changes in
its level represent changes in the groundwater storage. A decline in the water table
represents groundwater abstraction in excess of increment, while a rise represents
groundwater increment in excess of abstraction. The magnitude of the water table
fluctuation depends on climatic factors, rainfall intensity and amount, drainage,
topography, and geological conditions. Primarily, the water table fluctuation is
governed by the specific yield of the material in the zone of water table fluctuation.
All factors remaining the same, water table fluctuation is inversely proportional to
specific yield (Karanth, 1999). Also, under a given set of hydrogeological conditions,
33. the water level rise during a rainfall season will be comparatively more in years of
higher rainfall.
Water table fluctuation maps showing the rise or fall of the water table in a
specific time interval are prepared from water level data of observation wells. The
spatial variation in fluctuation of water levels is observed and a water table
fluctuation map is prepared for the period of summer-2006 to winter-2006
(Fig.3.3.2.1). The figure clearly shows almost uniform fluctuation of water table over
the area, except few patches. Average water table fluctuation in the area is from 3.5
to 4.5 m..
Kelve
Hanumanpada
Palipada
Mahim
Haranvadi
Wagulsar
Makunsar
1 2 3 4
56
78
9
10
11
12
13
1415
16
1718
19
20
24
25
26
27
28
29 30
31
34
36
37
40
42
43
44
45
46
47
0
1
2
3
4
5
6
N
1935'1938'1941'
72 42' 72 45' 72 47'
Water level fluctuation
FIG.3.3.2.1: WATER TABLE FLUCTUATION IN THE AREA
34. 3.3.3 - Ground water movement:
Groundwater is in constant motion from a point of recharge to a point of
discharge, in accordance with laws governing flow of fluids in porous media. Ground
water moves in the direction of decreasing head or potential. The change in head
per unit distance is the hydraulic gradient. The maximum hydraulic gradient is in the
direction of flow line. Ground water flow (water level elevation) maps are prepared to
know the direction of groundwater movement and its gradient, to identify the
recharge and discharge areas, and to know the relative variation in permeability of
the ground water bearing horizon. The spot values of water level elevations recorded
from all the observation wells above mean sea level (amsl) in meters are plotted on
a base map, and equipotential lines are drawn using the software SURFER version
7.0, so as to prepare the ground water flow map (Fig.3.3.3.1 &3.3.3.2). The flow
lines or stream lines which are at right angles to the tangent of the equipotential lines
(Todd, 1980) are also drawn, to show the direction of ground water movement in the
area.
Groundwater elevation in the area varies from -1 m amsl to 12 m amsl, thus
having a groundwater head of 13 meters. The equipotential lines (groundwater
elevation contours) are relatively closely placed in the coastal north –south trending
part. A ground water ridge is observed parallel to the coast passing through the
Kelwe and Mahim village area. Groundwater moves away from this area towards
west and east direction. The gradient is steep in this part of the area and becomes
moderate towards east.
It also indicates and alerts that as long as groundwater table is within 10 msl
depth, the groundwater will have movement away from the village area and towards
the sea, but as it depletes below10 msl; the movement will be reversed. The creek
water may seeps into the aquifer when ground water level depletes below 7 msl as
the sea water retreats to a height of 5 msl.
Thus the possibility of sea water ingression is from both, sea side and also
from creek side.
36. 1 2 3 4
56
78
9
10
11
12
13
1415
16
1718
19
20
24
25
26
27
28
29 30
31
34
36
37
40
42
43
44
45
46
47
Kelve
Hanumanpada
Palipada
Mahim
Haranvadi
Wagulsar
Makunsar
-1
0
1
2
3
4
5
6
7
8
9
0.5
1.5
2.5
3.5
4.5
5.5
6.5
7.5
8.5
9.5
Reference Vectors
0.00978160558978256 10.3304374798476
1935'1938'1941'
72 42' 72 45' 72 47'
N
Pre monsoon wl amsl
FIG.3.3.3.2: GROUND WATER MOVEMENT DURING WINTER 2008
3.3.4 - Effect of tides on groundwater levels in the inland aquifer:
Effect of tides on groundwater levels in the inland aquifer is studied by
correlating the fluctuation of sea water level during tides in a day with the ground
water level changes in piezometer during the same period of the day. A sample plot
for the four observation day is presented in Fig.3.3.4.1, which shows marginal
depletion in groundwater level during 12 to 18 hrs and during the same time sea
water level also retreads. But this groundwater depletion during that period can also
37. be due to the groundwater pumping during day time for irrigation purpose. Thus a
clear cut relation ship between tidal water level changes in sea level and
groundwater level is not observed.
Variation of sea water level during tides and respective groundwater
level fluctuation during a day at Vadrai Piezometer (DWLR).
-6
-5
-4
-3
-2
-1
0
1
2
3
4
5
6
0.00 4.00 8.00 12.00 16.00 20.00 24.00
Time in Hours for a day
SwawaterlevelabovemslandGWlevelinPZ
bgl
Fig.3.3.4.1: Sea water level and ground water level fluctuation during a day
(4 sample days)
38.
39. 4.1- Introduction:
Geophysical investigations are the best tools for indirectly mapping the
subsurface rock formations and structures. There are large numbers of problems
connected with groundwater exploration and exploitation that could be investigated
with the help of geophysical methods. Of these, the most important are the location
of groundwater bearing formations and estimation of their thicknesses and depths.
Delineation of weathered zones, valley fills, fresh-salt water interfaces, and
groundwater flow directions are some other problems that can be solved with these
methods. Structural controls which help groundwater accumulation by preventing
sub-surface run-off can also be studied through these methods.
Essentially, geophysical investigations comprises measurement and
interpretation of signals from natural or induced physical phenomena generated as a
result of spatial changes in one or more physical properties of a sub-terranean
formation. These signals measured, repetitively at certain points of space and time,
are appropriately interpreted, in terms of the geological structures or the features
which have good groundwater potential or are indicative of good aquifers.
Among all the geophysical methods used for groundwater exploration, the
electrical resistivity method is the most widely applied method all over the globe.
This is because of its efficacy to detect the water bearing horizons, besides being
simple and inexpensive to carry out the field investigations (Gangadhara Rao, 1992).
Electrical resistivity method can be successfully employed for ground water
investigations, where a good electrical resistivity contact exists between the water
bearing formation and the underlying rock (Zohdy et al., 1974).In general, the matrix
minerals in the rocks are normally high resistive. However, rocks containing
interstitial fluids conduct the current electrolytically. As a result, weathered and
fractured water bearing formations show low resistivity value as compared to the
hard and massive rock formations.
In a vertical pile of horizontally disposed basalt flows in Deccan volcanic
province (DVP), the groundwater accumulation and movement is mainly
concentrated along the weathered and jointed portion of the basalt flow units, and
along the contact between these units (vesicular amygdaloidal basalt unit and
compact basalt unit). These water bearing horizons (layers) can be easily pin
pointed through the electrical resistivity investigations.
40. The present chapter deals with the application and results of the electrical
resistivity data interpretation for identifying the course of sea water ingression in the
Kelwe-Mahim coastal area.
4.2 - Electrical resistivity investigations and presentation of sounding results:
Forty seven vertical electrical soundings (VES) using Schlumberger electrode
configuration were conducted uniformly in the Kelwe-Mahim coastal area, with a
maximum current electrode spacing of 100 meters. The instrument ‘SAS 300
Terrameter’ (make - ABEM, Sweden) is used for this purpose. Most of the VES were
taken near the observation wells so that the correlation can be made between
observed well litholog and interpreted data in terms of thickness, depth, and water
bearing and transmitting capacity of the aquifers.
Parameters obtained after the interpretation of geosounding data can be
presented in different forms to compare with the hydrogeological findings, to arrive at
the conclusion regarding the lithological and structural control on the groundwater
accumulation and movement. Geosounding results of the Kelwe-Mahim area are
presented in the following forms, as:
A. Geoelectrical models
B. Apparent Isoresistivity maps
C. Iso-strip resistivity maps
4.2.1- Geoelectrical models (Interpretation of VES data):
The apparent resistivity data thus obtained for every VES station is processed
and plotted on log-log graph paper of the same modulus as that of the standard
master curves, with half current electrode spacing (AB/2) in meter on abscissa and
apparent resistivity (ρa) in Ohm.m on ordinate, and geoelectrical (VES) curves are
obtained. Geoelectrical sounding data may be interpreted in a qualitative or
quantitative manner. In the qualitative analysis of VES data, the most important
factor is the shape of the curve from which it is possible to decipher the number of
layers and their resistivity relationships. Quantitative interpretation of the sounding
curves can be done by analytical and empirical methods.
Curve matching technique – (Analytical method)
The geoelectrical curves thus prepared are interpreted in terms of layer parameters
i.e thickness and resistivity of individual layers. The layer parameters were initially
41. obtained using curve matching technique with the help of standard master curves as
presented by Orellana and Mooney (1966). These parameters were used as initial
model for computer assisted interpretation software IPI2 win version 3.0.1a
(Bobachev, 2003) distributed by Geoscan M. Ltd, Moscow, Russia. This software
helps in interactive semi-automated interpretation of the field data. Information of
observation dug wells was incorporated and layered earth models from VES
interpretation were kept as simple as possible by not allowing results with too many
thin layers. The VES data is interpreted up to five layers. Modeling of the resistivity
data for the study area displayed the following nine types of geoelectrical curves, in
combination (Fig.4.2.1.1).
The general range of resistivity for I, II, III, IV, and V layers varies from 5 to
500, Ohm.m except at few locations where it is exceptionally high. The thickness of
layers varies from 0.5 to 15-20 meters. The first layer is relatively thin with the
general thickness range from 0.5 to 5 m, while the thickness of II and III layers range
up to 15or 20 m. The fourth layer has thickness in the range of 10 to 20m.The II, and
III, layers are generally the water bearing horizons where they have the resistivity in
the range of 50-60 ohm.m.
Nine types of curves obtained in the area are correlated with the
hydrogeological setting. (Table 4.2.1.1 and 4.2.1.2)
Table- 4.2.1.1 - Types of geoelectrical earth models
Sr.No. Type of Geoelectrical earth model No. of models (VES)
01 A – type 8
02 H – type 9
03 K – type 9
04 Q – type 1
05 HA – type 3
06 HK – type 1
07 KH – type 14
09 KQH – type 1
10 QH – type 1
42. Fig.4.2.1.1- Geoelectrical sounding curves from the Kelwe-Mahim area
(IPI2 win layer earth models)
K1 VES NO 1 K2 VES NO 2
K3 VES NO 3 K4 VES NO 4
K5 VES NO 5 K6 VES NO 6
K7 VES NO 7 K 8 VES NO 8
43. K9 VES NO 9 K10 VES NO 10
K11 VES NO 11 K12 VES NO 12
K13 VES NO 13 K14 VES NO 14
44. K15 VES NO 15
M1 VES NO 16 M2 VES NO 17
M3 VES NO 18 M4 VES NO 19
45. M5 VES NO 20 M6 VES NO 21
M7 VES NO 22 M8 VES NO 23
M9 VES NO 24 M10 VES NO 25
M11 VES NO 26 M12 VES NO 27
46. M13 VES NO 28 M14 VES NO 29
M15 VES NO 30 M16 VES NO 31
M17 VES NO 32 M18 VES NO 33
47. M19 VES NO 34 M20 VES NO 35
M21 VES NO 36 M22 VES NO 37
M23 VES NO 38 M24 VES NO 39
48. M25 VES NO 40 M26 VES NO 41
M27 VES NO 42 M28 VES NO 43
M29 VES NO 44 M30 VES NO 45
M31 VES NO 46 M32 VES NO 47
49. 4.2.2- Apparent resistivity or Isoresistivity maps:
Apparent resistivity or Isoresistivity maps can be prepared by plotting the
apparent resistivity values for chosen electrode spacing at the corresponding
stations on the base map, and by contouring these values. As the electrode spacing
increases, the effective depth of investigation increases; therefore an apparent
resistivity values contoured for different depths can reveal the variations in the
resistivity at different depths. This will help in establishing the lithological or structural
control on the groundwater accumulation and movement. The apparent resistivity
map gives a rough idea about the subsurface variations only, and can not be used
for quantitative estimates.
To understand the variations in the apparent resistivity values over the area,
the apparent resistivity values obtained for the electrode spacing of 1.5,6,12,18, and
25 meters of depth are plotted on a base map and contoured using the software
SURFER ver.7.0 (Fig.4.2.2.1). Apparent resistivity map for the depth of 1.5 m shows
high resistivity along north-south trend passing through the Kelwe and Mahim
village, and the resistivity increases towards the coast. Resistivity highs at these
locations are aligned elliptically along the NNW-SSE directions. Similar trend of
resistivity lows and highs are observed for the depth of 3, 10, 20, and 30 meters,
with little variations in the values of apparent resistivity. Resistivity highs obtained for
near surface layers dilutes with the depth. This indicates high resistivity formation at
surface along this zone.
All the Isoresistivity maps are stacked so as to obtain the 3D-view of the
variations in apparent resistivity values over the basin (Fig.4.2.2.1)
52. 4.2.3- Iso-strip resistivity maps:
The ‘strip resistivity’ value is the average resistivity value corresponding to the
two electrode separation but is not a simple mean of two apparent resistivity values.
It is obtained as
ρst = (A2B2 – A1B1)/[( A2B2/ρa2)- (A1B1/ρa1)]
Where, A1B1 – is the first current electrode separation
A2B2 – is the second current electrode separation
ρa1 - is the apparent resistivity for A1B1 position
ρa2 - is the apparent resistivity for A2B2 position
Strip resistivity values for the strips of 1.5 to 6 m, 6 to 12 m, 12 to 18 m, of
depth are calculated from the VES data and are contoured for each strip so as to
obtain the Iso-strip resistivity maps for each strip of desired depth of investigation. All
these Iso-strip resistivity maps are then stacked one another so as to get the 3-D
view of lateral as well as vertical variation of strip resistivity in the basin (Fig.4.2.3.1).
From figure, it is observed that the strip resistivity contours are aligned elliptically
along the NNW-SSE direction having resistivity highs concentrated in the middle part
and lows in the eastern part of the area. The narrow weak zone passing E-W
thorough creek south of Kelwe village becomes more pronounced with the depth.
Thus the presentation of the Iso-resistivity plots in such forms reveal the 3-D
picture of the resistivity variations in the area. The relative resistivity lows and highs
can be easily traced out and interpreted in terms of groundwater accumulation and
movement.
High resistivity zone along NNW-SSE passing through Kelwe -Mahim village
area is attributed to the occurrence of detrital limestone (Gritty sediments) as
observed in the core sample of Mangal Ali. These sediments of high resistance to
the flow of current through it and hence the high resistivity alignment occurs parallel
to the coast line.
55. 4.2.4- Geoelectrical cross sections
An attempt is made to adopt an integrated approach to evaluate the pseudo
depth sections and geo-electric cross sections in consonance with horizontal and
vertical derivative transformations of the VES data. The derivative can be regarded
as a measure of change in slope of the fitted line or rate of change in the predicted
values. If the predicted value increases or decreases at a constant rate within an
interval, the derivative over that interval will be near zero. A large positive or
negative derivative indicates an abrupt change in slope, perhaps caused by a jump
in the average value or the presence of a sharp peak. The horizontal derivative is the
derivative by horizontal distance. The extrema of the horizontal derivative are likely
to mark the places where one horizontally layered model is changed radically to the
other. The vertical derivative is the derivative by spacing and is to highlight the
characteristic points of the sounding curves: extrema as zero points and bending as
extrema (Narendra and Rajendra Prasad, 2005). For this purpose the software
IPI2win (Bobachev, 2003) is used.
Three Geoelectrical cross sections along the chosen profiles passing through
different VES locations are also prepared (Fig.4.2.4.1 to 4.2.4.3), which represents
pseudo resistivity map, vertical resistivity variation, and horizontal and vertical
transformations. These sections can reveal the subsurface geometry of the
geological formation. The sub surface horizontal and vertical discontinuities and the
subsurface fracture zones can be better distinguished using Horizontal and vertical
transform functions of the VES data. A shorter spacing between the VES increases
the resolution of the technique.
The section no. 01 runs along SSE-NNW and encompasses the VES
nos.10,13,15,42,43,44,47,26,31,and29 (k10 +k13 +k15 +m27 +m28 +m29
+m32+m11+m16+m14) covering a length of 6.9 Kms. The section no. 02 runs along
W-E and encompasses the VES nos.28, 26, 24, 34, 36, and 40
(m13+m11+m9+m19+m21+m25) covering a length of 4.2 Kms. The section no.03
runs along N-S and It encompasses the VES nos. 21, 32, 23, 40, 38, 39, 8,
7(m6+m17+m8+m25+m23+m24+k8+k7) and covering a length of 6.8 Kms.
61. 5.1 Introduction:
Water quality analysis is one of the most important issues in groundwater
studies. The hydro chemical study reveals the zones and quality of water that are
suitable for drinking, agricultural and industrial purposes. Further, it is possible to
understand the change in quality due to rock water interaction or any type of
anthropogenic influence. Water quality gets modified in the course of movement of
water through the hydrological cycle and through the operation of the processes
such as; evaporation, transpiration, selective uptake by vegetation,
oxidation/reduction, cation exchange, dissociation of minerals, precipitation of
secondary minerals, mixing of waters, leaching of fertilizer sand manure, pollution
and lake/sea, biological process, etc.
Groundwater consists of seven major chemical elements as; Ca+2
, Mg+2
,
Na+1
, K+1
, Cl-1
, HCO3-1
and SO4-2
. Concentrations of these elements in water
determine its suitability for drinking, agricultural and industrial purposes.
The parameters studied for identifying the sea water ingression in any area include
Sodium and chloride concentration, chloride bicarbonate ratio, and concentration of
total dissolved solids (TDS).
The present chapter deals with the chemical analysis of these parameters
and discussion on the results of interpretation in terms of sea water ingression in the
Kelwe-Mahim coastal area.
5.2 Materials and method:
Groundwater quality of the Kelwe-Mahim area was monitored for a period of
five years i.e. from 2006 to 2011. Water samples were collected during pre-monsoon
and post monsoon period from dug wells and bore wells which were earmarked as
observation wells (47 locations). Samples were analysed at regional chemical
laboratory of Groundwater Survey and Development Agency, Konkan Bhavan, Navi
Mumbai.
The samples were analyzed for bicarbonate, hardness, chloride, sulfate,
sodium, potassium, calcium, and magnesium using standard methods for the
examination of water. The pH and electrical conductivity (EC) were measured in the
field, and anionic parameters of the water samples were measured within few hours
of the sampling. Total dissolved solids (TDS) were calculated using EC values,
62. considering the relative ion concentrations. The bicarbonate, alkalinity, hardness (as
CaCO3) and chloride were determined by standard titration methods, whereas flame
atomic absorption spectrometry (Systronics, India) was used for the determination of
cations. Sulfate was measured using the spectrophotometer (Systronics,India). All
instruments were calibrated appropriately according to the commercial grade
calibration standards prior to the measurements.
5.3 Results and discussion:
The geochemistry of groundwater is influenced by factors such as the rock
type, residing time in the rock, previous composition of the groundwater and other
characteristics of the flow path. Groundwater usually maintains a constant
composition with time and may vary only slightly from well to well due to slower
movement and longer residing time as compared to surface.
The results obtained from the analysis chemical parameters are discussed in
the following paragraphs as:
5.3.1 Characterization of ground water (hydro chemical facies):
Quality of most of the groundwater samples are of fresh water, and about
10% of the total samples are found to be saline (EC;5000 to 25000 µS/cm). Hyper
saline samples are also found at locations close to salt pans with EC up to 80,000
µS/cm. Groundwater in the area belong mainly to Na-Mg-HCO3, Na-Ca-HCO3, Na-
Mg-Cl, Na- Mg-Cl-HCO3, Na-SO4-Cl and Na-Cl type of hydro chemical facies . A
gradual migration of facies from Mg-HCO3 type to Na-Mg-Cl-HCO3 type to Na-Cl
type is noticed, which indicates migration of saline water into fresh aquifer.
The dominant anions are Cl- and HCO3- and cations are Na+ and Mg2+.
Piper trilinear plot of groundwater collected during May 2010 depicts three groups,
as shown in Fig. 5.3.1.1. Freshwater samples are mostly Mg-HCO3 type whereas
two saline clusters are Na-Mg-Cl and Na-Cl (Na-SO4-Cl) types. High magnesium in
fresh groundwater indicates contribution of weathered products derived from basaltic
rocks. In the case of saline waters, high Na+and Cl- are observed indicating
contribution of marine sources. Groundwater samples collected during Feb. 2012
show that freshwater samples have similar hydro chemical facies as observed in pre
monsoon (June 2010) period. However, a gradual migration of hydrochemical facies
63. from Mg-HCO3 type to Na-Mg-Cl-HCO3 type to Na-Mg-Cl (Na-Cl) type is noticed, as
shown in Fig. 5.3.1.2. It is to be noted that groundwater withdrawals are high during
this season in the year. This indicates migration of saline front into fresh aquifers due
to withdrawal of fresh groundwater.
Fig.5.3.1.1 Piper trilinear plot of major ion data – May 2010
Fig.5.3.1.2 Piper trilinear plot of major ion data – Feb. 2012
64. 5.3.2 Chloride-Bicarbonate ratio:
Cl/HCO3 ratio is an indicator of sea water ingression and if it is greater than
one then it indicates sea water ingression. Chloride –bicarbonate ratio is calculated
for all the samples collected during from the year 2006 to 2011(Table 5.3.2.1). Most
of the samples show chloride –bicarbonate ratio greater than one, and also TDS
greater than 2000 mg/l is observed in most of the samples, thus indicate the
possibilities of sea water ingression into the fresh water aquifer.
Cl/HNO3 ratio plotted against TDS on simple graph paper shows linear
relation ship (Fig.5.3.2.1). Thus high Cl/HNO3 indicates high TDS content and hence
sea water ingression.
C l/HNO3
ratio
Vs
TDS
(Oct-‐2006)
0
1000
2000
3000
4000
5000
6000
7000
0.0 2.0 4.0 6.0 8.0 10.0 12.0 14.0 16.0
Cl/HNO3
ratio
TDS
in
mg/l
Fig.5.3.2.1: A plot Cl/HNO3 Vs TDS content for season October, 2006
Table 5.3.2.1: No. of water samples showing Cl/HNO3 >1 and TDS>2000 mg/l.
Month&year of sampling No.of samples showing
Cl/HNO3>1
No.of samples showing
TDS>2000 mg/l
May 2006 18 12
June 2006 19 9
August 2006 8 3
65. October 2006 8 5
January 2008 6 3
February 2008 5 4
April 2008 13 5
May 2008 12 5
February 2009 9 2
May 2009 11 7
December 2010 6 2
January 2011 4 3
5.3.3 Variation of TDS concentration (ISO-TDS map):
The ISO-TDS maps (Fig.5.3.3.1A &B) prepared for pre monsoon and post
monsoon seasons exhibits the higher concentration of TDS in eastern part of the
area, whereas it is within limit in the NNW-SSE trending coastal part of the Kelwe-
Mahim village area.
Samples collected from or near to the salt pan area shows high TDS
concentration, indicating the mixing of salt pan water and fresh ground water in this
area. Thus contribution of salt water from salt pan activity is noticeable.
66. 1 2 3 4
56
78
9
10
11
12
13
1415
16
1718
19
20
21
23
24
25
26
27
28
29 30
31
32
34
35
36
37
38
39
40
4142
43
44
45
46
47
48
49
0
500
1000
1500
2000
5000
10000
15000
20000
26000
TDS in mg/l
0 12.7 25.4 38.1
kms
193519381941
72 42 72 45 72 47
Latitudeindegree
Longitude in degree
1 2 3 4
56
78
9
10
11
12
13
1415
16
1718
19
20
21
23
24
25
26
27
28
29 30
31
32
34
35
36
37
38
39
40
4142
43
44
45
46
47
48
49
0
500
1000
1500
2000
2500
3000
3500
4000
4500
TDS range in mg/l
0 12.7 25.4 38.1
kms
193519381941
Latitudeindegree
Longitude in degree
72 42 72 45 72 47
A] Pre monsoon season B] Post monsoon season
Fig. 5.3.3.1: ISO TDS maps of the area for Pre monsoon (A) and post monsoon (B)
season.
67. 5.3.4 Vertical variation of salinity (Vertical EC profile):
Depth wise variation of Electrical conductivity (EC) is measured using depth sampler at a
few bore well sites from the area (Fig. 5.3.4.1 to 5.3.4.3) to identify depth of
stratification. During pre monsoon period (May 2010), it is found that a gradual
increase in EC is noticed in all the monitored wells. The shallow depths show less EC
compared to deeper ones. Groundwater at Vadrai site is fresh to highly saline, EC
increases from 2000 µS/cm at shallow depth to 25, 000 µS/cm, in the deeper parts. In
the case of Lalbhat site, groundwater is highly saline in both the zones and EC increases
from 24, 000 to 50, 000 µS/cm while in the case of Ambedkar Ali site EC increases from
2000 to 3500 µS/cm gradually.
From the depth profile of EC in Vadrai and Mangal Ali sites during Feb.
2012(Fig.5.3.4.4), it can be observed that the change in EC is rather abrupt. At Vadrai
site, the EC changes are similar to premonsoon period. At Mangal Ali site, EC varies
from 3200 µS/cm at shallow depth to 7500 µS/cm in deeper depths.
From these EC distribution profiles it can be concluded that point of stratification
lies between 15 to 20 m bgl depending on the topographic relief and proximity to the
saline water bodies like sea and creek water.
68. Fig.5.3.4.1: EC profile at Ambedkar Ali piezometer
Fig.5.3.4.2: EC profile at Vadrai piezometer
Fig.5.3.4.3: EC profile at Lalbhat piezometer.
71. 6.1 Introduction:
A collaborative project was carried out by Isotope Applications Division of
Bhabha Atomic Research Centre, Mumbai and Groundwater Surveys and Development
Agency (GSDA), Maharashtra to identify the source of salinity and understand the flow
dynamics of the groundwater system in and around Kelwa and Mahim villages of
Palghar taluka, Thane district, Maharashtra. Samples were collected during May 2010 and
Feb. 2012 for analysis of hydrochemical parameters and environmental isotopes (2H,
18O & 3H), and three radiotracer (82Br radioisotope) experiments were conducted for
determining groundwater velocity.
The present chapter deals with the environmental isotopes and injected tracer
approaches employed in the study and results of interpretation in terms of sea water
ingression in the Kelwe-Mahim coastal area.
6.2 Techniques employed:
In order to identify the source of salinity and understand the flow dynamics of
the groundwater system in the study area following techniques were employed as:
6.2.1 Environmental isotope approach:
Environmental tracers have been established as potential tools in tracing
groundwater and also its contaminants. Environmental isotopes are used as a
modern, specific and reliable technique in understanding various hydrological
processes (IAEA, 1993; Clark and Fritz, 1997). The application of isotope techniques in
hydrology is based on the use of naturally occurring stable isotopes or/and unstable
(radioactive) isotopes. Generally environmental stable isotopes (2H, 18O, 13C etc.) are
used to determine the origin of water and its constituents and environmental
radioisotopes (3H and 14C) are used for dating groundwater.
Since the natural variations of environmental stable isotopes (18O and 2H) are
usually very small, the only technique that can routinely measure precise isotopic ratios is
mass spectrometry such as Isotope Ratio Mass Spectrometer (IRMS). The isotopic
variation is reported as δ value in permil (‰) deviations against a standard (Gonfiantini,
1981), where R represents isotopic ratio of heavier to lighter isotope.
72. δ2
H or δ18
O (‰) = [(R sample-R standard)/R standard] x 1000 --------(1)
where - R = 2
H/1
H or 18
O/16
O
The standard almost universally accepted for oxygen and hydrogen stable isotope
variations in natural waters is SMOW (Standard Mean Ocean Water). It corresponds to
a hypothetical water having both oxygen and hydrogen isotopic ratio equal to the mean
isotopic ratios of ocean water. For hydrogen and oxygen-18 isotope analyses, 25 mL of
water samples were collected in airtight polyethylene bottles and the measurements
were carried out using GEO 2020 (Europa) mass spectrometer. The precision of
measurement for δ2H is ± 0.1‰ and δ18O is ± 0.05‰.
For tritium measurement, 500mL water samples were collected in airtight
polyethylene bottle. 250 mL of distilled sample was electrolytically enriched at a low
temperature of about 1 to 4oC and sample - scintillator mixture (8:12 mL) taken in a
20 ml polythene vial was counted in an ultra low background (0.5 cpm) liquid
scintillation counter (Quantulus model 1220). The 3H values are expressed in tritium unit
(TU). One TU of sample has 3H/1H ratio equals to 1/1018, which corresponds to 0.12
Bq/kg of water. The minimum detection limit for this method is 0.5 TU (3σ) for 500
minutes counting. The counting efficiency and the calibration factor of the counter
were about 25% and 70 TU/cpm respectively(Nair,1983).
6.2.2 Injected tracer approach:
Injected radioactive tracers viz., 82Br 51Cr, 58Co and 60Co, 131I and 99Tc, which
can be measured in situ, have been proved to be suitable and handy for most of the
hydrological applications. High sensitivity of measurement, high specific activity, ease in
mixing with water and negligible density effects to the groundwater system, enable
radioactive tracers as preferable to other tracers. Selection of radioactive tracer is
based on the purpose and duration of the study while meeting safety requirements.
Injected radioactive tracers are used to determine groundwater recharge, rate of
groundwater movement and its direction, seepages from canals and reservoirs and
aquifer parameters (Tirumalesh et al., 2007, Rao, 1984, Sukhija et al., 1996). There are
two approaches commonly used for finding out the dynamics of groundwater; single
well method and multiple well method.
73. A] Single Well Technique:
The groundwater filtration velocity determination in a single well technique is
based on tracer dilution principle (Drost et al., 1968). This is also known as point dilution
method. At a desired depth in a borehole, groundwater column is labeled with a
radiotracer and well mixed. The decrease in tracer concentration (count rate) could be
due to natural decay of the tracer and/or physical transportation of the tracer by the
flowing groundwater. To know the tracer dilution with time, loggings were carried out
at different intervals of time and depths. Groundwater filtration velocity (Vf) is
computed using equation,
Vf = (V/αF.t) ln (Co/Ct)-------------------------------------------------(2)
Where, V is dilution volume, F is borehole cross section, C0 & Ct tracer
concentration initial & at time t and α is a constant which depends on hydraulic
conductivity of the well screen and the aquifer, and taken as 2. A schematic
representation is shown in Fig. 6.2.2.1
Fig.6.2.2.1: Schematic representation of bore hole logging
B] Multiple well techniques:
A radiotracer is introduced into an injection well and the activity is monitored in
the downstream boreholes. The direction of maximum activity corresponds to the
direction of flow. The linear velocity can be calculated using the equation,
Vt = X/T -------------------------------------------------(2)
74. Where X = distance between injected and monitoring well, T = time of tracer flow.
6.3 Sampling and measurement:
Two field sampling programs were carried out during May 10-11, 2010 and
January 31 to February 2,2012. About 50 samples were collected in the first sampling
(May 2010) and second sampling (Feb.2012) programs. Water samples were
collected from different sources, like groundwater tapping alluvium and weathered &
fractured basalt aquifers, creek water, sea water, salt pans and river for the analysis of
physico-chemical, chemical and isotope parameters.
For measuring filtration velocity of the groundwater, point dilution method was
applied. Radiotracer experiments were conducted at three locations viz., Lalbhat, Mangal
Ali and Vadrai. For the experiment 82Br in the form of NH4Br solution (200 µCi, t1/2 =
36 hrs) was injected in the bore hole and allowed to mix thoroughly in the water
column. 82Br activity loggings were carried out at different time intervals using NaI
scintillation detector coupled with rate meter. The groundwater velocity was computed
based on decay corrected radioactivity profiles.
6.4 Results and discussion:
Results obtained from the environmental and radio isotope study are
summarized in the following paragraphs.
6.4.1 Environmental isotopes:
During premonsoon, about 21 samples were analyzed for stable isotopes and
tritium content. Isotope variation is found to be very wide from -2.5 to +6.2‰ for
δ18O and -10 to +29‰ for δ2H. Very high enrichment in stable isotope data indicates
highly evaporated water. In order to verify evaporation effect and impact of seawater,
δ2H versus δ18O was plotted (Fig. 6.4.1.1). Samples fall on the best fit line with a slope of
4.6 indicating evaporation effect. Samples belonging to group (a) fall very close to the
GMWL and are relatively depleted. These samples are mainly recharged by precipitation.
75. Samples of group (b) represent enriched composition (δ18O: 1 to 6‰) showing high
degree of evaporation. Majority of these samples belong to locations close to salt pans
and creek. Since water deriving from these sources is highly evaporated it can be
expected that contribution of these sources leads to evaporated composition of
resulting groundwater. This indicates influence of salt pans on local groundwaters. There
are also instances of contribution of other surface waters like stagnant water bodies,
streams etc. in the case of samples from custom chowki and some piezometers from
Mahim.
Groundwater data of Feb. 2012 season indicate majority of the samples fall
around GMWL with δ18O ranging from - 2.7 to -1 ‰, indicating precipitation dominant
recharge (shown encircled, Fig. 6.4.1.2), which is an expected change due to contribution
from monsoonal rains during July to September months. The slope of the best fit line is
found to be 3.7 which indicate evaporation effect. A significant number of samples
show enriched signature (δ18O: 0.2 to 2.5 ‰), which could be attributed to
contribution of evaporated surface sources like salt pans or creek as noted in the case of
premonsoon season. Seasonal variation in isotope data is noted in all the locations
except in some wells belonging to Lalbhat, Vadrai and Dhavangepada villages. In
general, premonsoon samples are mostly enriched in stable isotope composition
compared to Feb. month samples. This indicates enhanced contribution of surface
sources during dry season.
Fig. 6.4.1.1: δ2H versus δ18O composition of groundwater collected from different
sources during premonsoon
76. Fig. 6.4.1.2: δ2H versus δ18O composition of groundwater collected from different
sources during Feb. 2012
In order to confirm the source of salinity, EC and δ18O correlations are
evaluated (Fig. 6.4.1.3 and 6.4.1.4). Samples affected by saltpans and creek water
fall in a single cluster (Fig. 6.4.1.3). These samples include, Mahim mithaya gram
panchayat, wells near Kelwa and Mahim salt pans. Sample from Temkepada also show
77. high EC values (40,000 – 50,000 µS/cm) with enriched δ18O content (2 – 4 ‰)
indicating contribution of salt pan water which is further evaporated. In Feb. 2012, most
of the samples fall along the X- axis indicating evaporation dominance in these
locations as compared to mixing with saline surface water (Fig. 6.4.1.4). In both pre
monsoon and Feb. month periods, except samples from Lalbhat (Fig.6.4.1.3) and Vadrai
(Fig. 6.4.1.4), no sample fall on the seawater-freshwater mixing line. Groundwaters at
Lalbhat and Vadrai sites are mixture of freshwater and seawater and are under
stagnant condition, which is further verified by radiotracer experiments.
Fig. 6.4.1.3: EC versus δ18O composition of groundwater collected from different
sources during premonsoon
Fig. 6.4.1.4: EC versus δ18O composition of groundwater collected from different
sources during Feb. 2012
78. Environmental tritium is an indicator for differentiating modern recharge (since
1960) from old recharge (prior to 1960). Tritium values above 2 T.U. indicate recharge
during modern period while lesser values signify old groundwater. In the study area, the
tritium values of groundwaters range between 2 and 7.5 T.U. indicating modern recharge.
There are also instances of high tritium (11 – 12.5 T.U.), which could be attributed to
anthropogenic contamination. Wastes from dye and watch industries can contribute to
high tritium in groundwater. These locations are in the vicinity of canal carrying wastes
from different industries of this region.
6.4.2 Groundwater Velocity by radio tracer technique:
Groundwater velocity was determined at three sites, one during premonsoon and
two during Feb. 2012 period. The wells for radiotracer experiment were chosen such
that groundwater from both alluvium and weathered & fractured basalt aquifers
contribute to the well. The radioactive 82Br activity profiles (decay corrected) at these
sites are shown in Fig. 6.4.2.1-6.4.2.3.
Fig. 6.4.2.1: 82Br activity loggings at Lalbhat site during premonsoon.
80. At lalbhat site, there is no clear decrease in activity with time throughout the water
column indicating that the groundwater is in stagnant condition (Fig.
6.4.2.1).The calculated groundwater velocity is in order of 0.5 cm/day. There is no
change in EC during Feb. 2012 sampling which further shows that groundwater is
under near stagnation. From the stable isotopic evidence, sea water contribution can be
concluded while environmental tritium values point to is modern recharge.
At Mangelali site, 82Br activity profiles indicate a clear distinction in groundwater
velocity in shallow and deep zones (Fig.6.4.2.2). At shallow depths, up to 6 m bgl,
groundwater is under dynamic condition with velocity varying from 0.2 to 6 cm/day. A
large decrease in 82Br activity is observed between T=60 and T=120 which is due to
withdrawal of tidal currents. Since this site is close to the coast, tidal influence on the
shallow zone groundwater levels can be expected. Below 9 m bgl, the 82Br activity doesn’t
vary much with time indicating stagnant condition.
At Vadrai site, 82Br activity depth profiles indicate a similar pattern as in the
case of Mangelali site. Shallow zone shows active groundwater flows with rates 3.5 to
21.5 cm/day while deeper parts the velocity in the order of 0.5 cm/day. Weathered and
fractured basalt aquifer at this site is found to be near-stagnant condition. High velocity
in shallow zones at this site could be attributed to induced flow conditions due to
withdrawal of groundwater for irrigation. This is also supported by deeper water levels (15
m bgl) compared to other sites where water levels are very shallow (2-5 m bgl).
81. Fig. 6.4.2.3: 82Br activity loggings at Vadrai site during Feb. 2012
Based on environmental isotopic inferences, groundwaters with precipitation
recharge are differentiated from those with dominant surface water contribution. High
degree of evaporation was found in groundwaters close to creek and salt pan areas. In
general, premonsoon samples are mostly enriched in stable isotope composition
compared to Feb. month samples, which indicates enhanced contribution of surface
sources during dry season. Seasonal variation in isotope data is noted in all the
locations except at Lalbhat, Vadrai and Dhavangepada piezometers.
Stable isotope (δ18O) correlations with s a l i n i t y (chloride content)
further confirm that groundwaters from Mahim mithaya gram panchayat, Kelwa
mithagram and Temkepada have significant contribution from salt pan activities.
Groundwaters of Lalbhat and Vadrai sites fall on seawater-freshwater mixing line indicating
seawater intrusion at these sites. Environmental tritium values range between 2 and 7.5
T.U indicating modern recharge while high values (> 10 T.U.) could be due to industrial
wastes.
82. Radiotracer experiments show a clear distinction in groundwater velocity in
shallow and deep zones. Shallow groundwaters which are mainly derived from alluvial
formations are found to be relatively dynamic as compared to deeper groundwaters which
are derived from the weathered and fracture basalt formations. The groundwater
velocity is in the range of 0.5-20 cm/day in the shallow zone while it is near stagnant
condition in deeper parts. High groundwater velocity in shallow zones during February
month could be attributed to induced flow conditions due to withdrawal of groundwater
for irrigation.
.
83.
84. 7.1- Summary:
The results obtained from Geology, Hydrogeology, hydrochemistry,
Geophysical investigations, and isotope studies undertaken in the Kelwe-Mahim
coastal area; are summarised and concluded in the following paragraphs.
Sea water ingression study was undertaken in the Kelwa-Mahim coastal area
in Palghar taluka of Thane district with a view to study the extent of Sea water
ingression in the coastal fresh water aquifer and to frame the groundwater
management strategy for reducing the sea water ingression so as to provide pure
and sustainable groundwater solution to the community.
The study area lies between the north lattitudes 190
35’53” & 190
41’08” and
east longitudes 720
42’00” & 720
47’00”. It covers an area about 74 sq.kms of kelva
and Mahim villages in Palghar taluka of Thane district. The area falls in quadrants C-
2 of Survey of India Toposheet nos.47 A/14 and 47 A/10 and watershed WF-19.
The area is situated due north-west of Thane District, the district headquarter
at about 103 kms and about 8-10 kms due south -west of Palghar, the taluka
headquarter. The area is almost flat with gentle slope towards the west. The
elevation decreases from 11-12 m.msl in the east to 1-2 m.msl at the extreme west.
The two creeks divide the area into two parts, the northern one is Mahim and the
southern is Kelwa village area. The sea water ingress and retreads along these
creeks during tide times. The area receives an annual rainfall of about 2500 mm.
The project work commenced from the year 2006 and completed in the year
2012..
Following results are obtained as against the set objectives for the study:
A] Hydrogeological study:-
The area is covered by coastal alluvium and basalt flows. The coastal
alluvium is followed by weathered /jointed basalt flows. These units act as the main
water bearing formations in the study area.
Alluvium is the terrestrial sediments located parallel to the coast line and
deposited by the sea action. These include beach rock, intercalated sand and
sediments of varying size. These constitute the shallow alluvial aquifer system within
the beach and littoral terrace, and margins of mud flats. The mud zone which is
under the constant influence of high tides supports only the growth of the mangroves
85. and is mainly responsible for providing the saline water to the jointed / fractured
formations that occurs below the mud areas.
The upper part of the basalt is highly weathered while the middle and lower
part of the flow is fractured and jointed. The weathering and joints attribute the
secondary porosity to the basalt flow; hence act as an aquifer in the area. The
groundwater inflows in wells are mainly concentrated along the vertical contact of
alluvium and basalt flow.
There are 1068+ irrigation wells in the area. The 75% of the total
irrigation wells are mainly concentrated in the alluvial part parallel to the
sea coast. The wells mainly tap the shallow alluvium having depth
ranging from 3.5 to 8.1 mbgl. The dug wells near the coastal region do
not pierce the fresh compact basaltic rock. However, bore wells and
dug-cum-bore wells which are of 30-40 m depth taps the basalt flows
below 10-12 m.bgl.
The static water level ranges from 1 to 4m.bgl, in winter and 2 to
7.5 m. bgl, in summer. The specific yield of the coastal alluvium ranges
from 0.07 to 0.09, while that of basalt is 0.019 to 0.04. The
Transmissivity of the coastal alluvium ranges from 3 to 193 m2
/day,
while that of basalt is 3 to 98m2
/day.
Long term pre and post monsoon ground water level from shallow
aquifer in the area shows marginal decline in the trend. Thus, indicating
stress of ground water exploitation on the shallow aquifer.
B] Effect of tides on groundwater levels in the inland aquifer:-
Effect of tides on groundwater levels in the inland aquifer is studied by
correlating the fluctuation of sea water level during tides in a day with the ground
water level changes in piezometer during the same period of the day. It shows
marginal depletion in groundwater level during 12 to 18 hrs and during the same
time sea water level also retreads. But this groundwater depletion during that period
86. can also be due to the groundwater pumping during day time for irrigation purpose.
Thus a clear cut relation ship between tidal water level changes in sea level and
groundwater level is not observed.
C] Geophysical study:- Analysis of 47 VES can be summarised as:
§ There exists an N-S trending, linear high resistivity zone parallel to the coast
line in the western most part of the study area.
§ The resistivity decreases towards the eastern part of the study area and
indicates the saline tract in the eastern part of the area.
§ High resistivity linear zone along the coast acts as a barrier for sea water
ingression from the western side.
§ The high concentration of saline water in the eastern part is due to the
presence of sediment deposition or fractures.
D] Groundwater chemistry:-
§ Most of the groundwater samples are fresh in quality, and about 10% of the
total samples are found to be saline (EC;5000 to 25000 µS/cm). Hyper saline
samples are also found at locations close to salt pans with EC up to 80,000
µS/cm, Hydro chemical facies are mainly dominated by Na-type and Mg-type.
A gradual migration of facies from Mg-HCO3 type to Na-Mg-Cl-HCO3 type to
Na-Cl type is noticed, which indicates migration of saline water into fresh
aquifer.
§ Cl/HCO3 ratio is greater than 1 in most of the samples which is indicative of
sea water ingression.
§ An Iso-TDS map shows higher concentration of TDS in eastern part of the
area.
§ Vertical distribution of groundwater salinity indicates that freshwater-sea
water interface is about 15-20 m.bgl
87. E] Petrology and petrography of core samples:-
§ The Kelwa-Mahim area is covered by Deccan basaltic flows belonging to
Sahyadri Group of Upper Cretaceous to Lower Palaeocene age.
§ Two boreholes of 30 m deep each were drilled in Harnewadi and Mangalali
villages. In the borehole drilled in Harnewadi two types of basaltic flows were
recorded viz.,(1) massive and (2) amygdular. Massive basalt belongs to Aa
Aa flow, whereas amygdaloidal basalts exhibit typical compound Pahoehoe
flow features. A total of four flows were identified during the logging.
§ The Mangalali borehole comprises detrital limestone at the top followed by
althrough the massive basalt comprising four Aa Aa type flows.All the flows
are mainly composed of plagioclase, clinopyroxenes (augite), glass along
with palagonite and magnetite & ilmenite. At places minor enstatite and
olivine are also noticed. Presence of glass is noticed up to 40 % and glass is
invariably altered to palagonite.
§ Presence of fractures in all the flows and vesicles in amygdular flows
suggest that there is ample space for groundwater to interact with the country
rock.
§ Presence of detrital limestone at the top in Mangalali borehole with
gastropods and foraminiferas suggest their deposition in brackish water
condition.
F] Radio Isotope study:-
Inferences drawn from radio isotope studies are as follows:
§ Environmental isotopic studies indicate high degree of evaporation in
groundwater close to creek and salt pan areas. Contribution of these surface
sources enhanced during the dry season.
§ Stable isotope (δ18
O) correlations with salinity (chloride content) further
confirm that groundwater from Mahim mithagar gram panchayat, Kelwa
mithagar and Tambakepada have significant contribution from salt pan
activities. Ground water from Lalbhat and Vadrai sites fall on seawater-
freshwater mixing line indicating seawater intrusion at theses sites.
