Presented by Timothy D. Oden at the Texas Water Conservation Association Conference in The Woodlands, Texas - March 2014

1. U.S. Department of the Interior
U.S. Geological Survey
Use of Environmental Tracers for
Estimating Rates of Groundwater
Recharge to the Gulf Coast Aquifer
System in Montgomery County,
Texas
by Timothy D. Oden
Prepared in Cooperation with Lone Star Groundwater
Conservation District
Texas Water Conservation Association
70TH Annual Convention
The Woodlands, Texas
March 6, 2014

2. Groundwater Recharge is Not Easily
Measured
 Recharge is not easily measured, often is
estimated from other methods
 Coupled recharge determinant methods
would be helpful
 age-dating (this study)
 water-table fluctuation
 chloride tracer method (saturated or unsaturated
zone)
 Streamflow-hydrograph separation

3. What is Groundwater Recharge...
 ...generally, the replenishment of water to a
groundwater flow system
 ...is an integral part of the hydrologic cycle
that may have started as precipitation
 ...precipitation that first infiltrates the
exposed sediments at land surface, moves
downward through the sediments until it
reaches the water table, at which point it is
part of the groundwater flow system of the
Gulf Coast Aquifer

4. Recharge and the Hydrologic Cycle

5. What is NOT Groundwater Recharge...
 ...equivalent to infiltration
most water that infiltrates at land surface is
returned to the atmosphere
 ...equated with percolation
percolation is the movement of water through the
unsaturated zone
 ...to be confused with aquifer yield
this is the amount an aquifer can yield to pumping
 ...the same as sustainable yield
recharge can be less than sustainable yield

6. Environmental Tracers in this Study
 Modern (1940s to present)
 Chlorofluorocarbons (CFCs)
 Sulfur hexafluoride (SF6)
 Tritium (3H)
 Helium/ Tritium (3He/3H)
 Intermediate (100-1000 years)
 Helium-4 (4He)?, maybe
 Paleowaters (1,000 to 40,000 years)
 Carbon-14 (14C)
 Not dating the water, but a substance IN the
water

7. The Model – Piston Flow
 Piston flow is the simplest transport
assumption in groundwater age dating
 Assumes no mixing or dispersion has altered
the concentration after entering the system
 Most likely, an oversimplification
 effects of mixing and dispersion beyond the scope
of this study

8. Variables needed for Recharge Rate
Estimation
 The known or to be to determined:
 H, aquifer thickness
 z, depth in the aquifer
 x, width of aquifer outcrop
 xo, distance from well to beginning of flow path
 e, porosity

9. Idealized flow in simple aquifer
type
Modified from Cook and Herczeg, 2000
Piston flow
– Lines of
equal age
Recharge
x
H
z
xo
e of the aquifer

10. Study Design
 2008
 37 wells, collected CFC, SF6, 3He/3H, 3H, 4He, and
dissolved gas data
 Chicot – 17 wells
 Evangeline – 13 wells
 Jasper – 7 wells
 2011
 24 wells, collected 14C, major ions, 4He and
dissolved gas data (select wells)
 Chicot – 7 wells
 Evangeline - 8
 Jasper – 9

11. Chicot Recharge Estimates
 Based on 14 wells –
 Not all wells sampled in the Chicot
Aquifer for this study were usable for
recharge determination
 Ranged from 0.2 to 7.2 inches per year
 About 0.4 to 14.6 percent of normal
annual precipitation*
*normal annual precipitation from 1971-2000,
COOP Weather Station 411956, Conroe, Texas

12. Chicot
Aquifer
1.9
2.4
ND
2.7
2.1
3.4
ND
7.2
2.3
3.8
ND1.0
0.4
0.9
0.2 3.7
4.8
Explanation
Recharge
Value (1.9),
inches per
year
ND, not
determined

13. Evangeline Recharge Estimates
 Based on 9 wells –
 Not all wells sampled in the Evangeline Aquifer for
this study were usable for recharge determination
 Ranged from less than 0.1 to 2.8 inches per year
 About 0.2 to 5.67 percent of normal annual
precipitation

14. Evangeline
Aquifer
0.2ND
0.1
2.8
2.4
ND
0.1
1.51.0
ND
ND
0.1
<.1
Explanation
Recharge
Value (0.1),
inches per
year
ND, not
determined

15. Jasper Recharge Rates
 Based on 10 wells –
 Not all wells sampled in the Jasper Aquifer for this
study were usable for recharge determination
 Ranged from less than 0.1 to 0.5 inches per year
 About 0.2 to 1.01 percent of normal annual
precipitation

16. Jasper
Aquifer
ND
0.1
0.1
0.1
0.1
0.5
<.1
<.1
0.1
<.1
0.1
Explanation
Recharge
Value (0.1),
inches per
year
ND, not
determined

17. Uncertainty –
Unconfined
System
modifying each
variable, the
rate changes –
blue and black circles, values from the report
apparent gw age
gw age, +10 percent
gw age, -10 percent
20 percent, porosity
25 percent, porosity
30 percent, porosity
aquifer thickness, +50 ft
aquifer thickness, -50 ft
Chicot Evangeline
Jasper

18. Uncertainty –
Confined
System
blue and black circles, values from the report
apparent gw age
gw age, +10 percent
gw age, -10 percent
20 percent, porosity
25 percent, porosity
30 percent, porosity
aquifer thickness, +50 ft
aquifer thickness, -50 ft
modifying each
variable, the
rate changes –
JasperEvangeline

19. Limitations of this study
1) Highly variable hydrogeology on a regional
scale
2) Piston flow is likely oversimplification
3) Porosity, used were previously determined
4) Possibility of mixing within system not
considered
5) The rates are point (site) specific
6) Interpretation of the environmental tracer
data can be complicated along the flow path

20. Publications
 Oden, T.D., 2011, Groundwater environmental tracer data collected
from the Chicot, Evangeline, and Jasper aquifers in Montgomery
County and adjacent counties, Texas, 2008: U.S. Geological
Survey Data Series 580, 37 p., http://pubs.usgs.gov/ds/580/
 Oden, T.D. and Truini, Margot, 2013, (revised May 31, 2013)
Estimated rates of groundwater recharge to the Chicot, Evangeline
and Jasper aquifers by using environmental tracers in
Montgomery and adjacent counties, Texas, 2008 and 2011: U.S.
Geological Survey Scientific Investigations Report 2013–5024, 50
p., http://pubs.usgs.gov/sir/2013/5024/
 Oden, T.D., and Delin, G.N., 2013, Groundwater recharge to the
Gulf Coast aquifer system in Montgomery and Adjacent Counties,
Texas: U.S. Geological Survey Fact Sheet 2013–3043, 6 p.,
http://pubs.usgs.gov/fs/2013/3043/.

21. Thanks!
Timothy Oden
toden@usgs.gov
303-236-1470