use of environmental tracers for estimating rates of groundwater recharge to the gulf coast aquifer...
DESCRIPTION
Presented by Timothy D. Oden at the Texas Water Conservation Association Conference in The Woodlands, Texas - March 2014TRANSCRIPT
U.S. Department of the InteriorU.S. Geological Survey
Use of Environmental Tracers for Estimating Rates of Groundwater Recharge to the Gulf Coast Aquifer System in Montgomery County, Texasby Timothy D. Oden
Prepared in Cooperation with Lone Star Groundwater Conservation District
Texas Water Conservation Association70TH Annual ConventionThe Woodlands, TexasMarch 6, 2014
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
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
Recharge and the Hydrologic Cycle
What is NOT Groundwater Recharge...
...equivalent to infiltrationmost water that infiltrates at land surface is
returned to the atmosphere
...equated with percolationpercolation is the movement of water through the
unsaturated zone
...to be confused with aquifer yieldthis is the amount an aquifer can yield to pumping
...the same as sustainable yieldrecharge can be less than sustainable yield
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
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
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
Idealized flow in simple aquifer type
Modified from Cook and Herczeg, 2000
Piston flow – Lines of equal age
Rechargex
H
z
Flow path
xo
e of the aquifer
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
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
ChicotAquifer
1.9
2.4
ND2.7
2.1
3.4
ND
7.2
2.33.8
ND1.0
0.4
0.90.2 3.7
4.8
ExplanationRecharge Value (1.9), inches per year
ND, not determined
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
EvangelineAquifer
0.2ND0.1
2.82.4
ND
0.11.51.0
ND
ND
0.1<.1
ExplanationRecharge Value (0.1), inches per year
ND, not determined
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
JasperAquifer
ND
0.10.1
0.1
0.1
0.5
<.1
<.1
0.1
<.1
0.1
ExplanationRecharge Value (0.1), inches per year
ND, not determined
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
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
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
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/.
