APPLICATION OF GROUND-WATER DATING TECHNIQUES FOR EVALUATING THE SUSCEPTIBILITY OF AQUIFERS AND PUBLIC-SUPPLY WELLS TO CONTAMINATION National Water Quality Monitoring Council Meeting May 9, 2006 Sandra M. Eberts John Karl Böhlke Leon Kauffman USGS National Water Quality Assessment Program (NAWQA) U.S. Department of the Interior U.S. Geological Survey

TALK OUTLINE Considerations For Interpretation Of Environmental-Age Tracers In Public-Supply Wells Tracer Concentrations & Age Distributions In Selected Aquifers And Public-Supply Wells USGS NAWQA Transport of Anthropogenic & Natural Contaminants (TANC) to Supply Wells Study

GROUND-WATER DATING STUDY AREAS Central Valley aquifer system High Plains aquifer Glacial deposits Floridan aquifer system Transport of Anthropogenic and Natural Contaminants (TANC) to Supply Wells Study Rio Grande aquifer system Edwards-Trinity aquifer system

1,1,1 trichloroethane cis-1,2 dichloroethene trichloroethene tetrachloroethene MTBE freon-11 arsenic uranium 7 yrs APPARENT AGES MAY NOT BE SUFFICIENT TO FORECAST SUPPLY WELL SUSCEPTIBILITY 25 yrs chloroform trichloroethene arsenic uranium 5 yrs High Plains bromoform chloroform tetrachloroethene nitrate atrazine DEA diuron arsenic uranium cis-1,2 dichloroethene trichloroethene tetrachloroethene nitrate arsenic uranium 20 yrs Central Valley High Plains Floridan Glacial deposits Preliminary – Subject to Revision

SELECTED ENVIRONMENTAL TRACERS USED TO INFER GROUND-WATER AGE Cook and Böhlke, 2000 Age: Time Elapsed Since Recharge To The Water Table  Tracer-Based Ages Are Apparent Ages  Tracer-Based Ages Are Model Dependent

a PFM = piston-flow model A water sample is a mixture of waters from all flow lines that reach the sampling point. If the flow lines are associated with a narrow range of ages, the apparent age of the sample may reasonably represent the travel time from the point of recharge. AGE INTERPRETATION -- Piston Flow Model Assumption -- a Zuber, 1986

a PFM = piston-flow model b, c, d EM = exponential model eEPM = exponential piston- flow model Additional models LM = linear LPM = linear-piston DM = dispersion Lumped-parameter models … simple mathematical functions that describe age distributions for various combinations of recharge area, aquifer geometry, and sample type (Zuber, 1986) AGE INTERPRETATION -- Lumped Parameter Models for Ground-Water Mixtures -- a b c d e

Modified from Plummer, written commun., 2006 AGE INTERPRETATION -- Tracer Plots --  Can Be Used To Recognize Cases Of Piston Flow And Binary Mixing With Tracer Free Fraction (Dilution)  Elimination Of Some Mixing Models Piston Flow Exponential Mixing Binary Mixing

AGE INTERPRETATION -- Flow Model Calibration and Simulation of Age Distributions -- SF 6 Flow-Weighted Particle Tracking: 4.14 pptv Supply-Well Sample: 4.53 pptv 3 H Flow-Weighted Particle Tracking: 1.98 TU Supply-Well Sample: 2.1 TU Floridan Aquifer System Public-Supply Well Preliminary – Subject to Revision Crandall, written commun., 2006

AGE DISTRIBUTIONS IN GROUND WATER MIXTURES -- Implications for Susceptibility Age, In Years Fraction of Sample TRACERMODEL3 (Böhlke, 2006) Water Samples With Identical Mean Ages

FLORIDAN AQUIFER SYSTEM -- SIMULATED AGE DISTRIBUTION IN SUPPLY WELL -- Recharge to Well, In Percent Age, In Yrs 0.35 Floridan Preliminary – Subject to Revision Not to Scale BINARY MIXTURE: Piston Flow (v. young) and Exponential Piston Flow (older) Crandall, written commun., 2006

FLORIDAN AQUIFER SYSTEM -- AGE TRACER DATA -- TRACERMODEL1b; H, TU Public Supply Well Surficial Aquifer Binary Mixing Model Exponential Mixing Model Piston Flow Model Exponential Piston Flow Model xP/xE = 15 Samples Preliminary – Subject to Revision 3 H/ 3 H(0), TU Confining Unit Shallow Upper Floridan Deep Upper Floridan

Recharge to Well, In Percent Age, In Yrs CENTRAL VALLEY AQUIFER SYSTEM -- SIMULATED AGE DISTRIBUTION IN SUPPLY WELL -- Preliminary – Subject to Revision Not to Scale Exponential Piston Flow Central Valley Burow, written commun., 2006

0.8 SF 6, pptv CENTRAL VALLEY AQUIFER SYSTEM -- AGE TRACER DATA -- Preliminary – Subject to Revision Public Supply Well 3 H/ 3 H(0), TU Binary Mixing Model Exponential Mixing Model Piston Flow Model Exponential Piston Flow Model xP/xE = 0.6 Samples TRACERMODEL1b;

GLACIAL DEPOSITS -- SIMULATED AGE DISTRIBUTION IN SUPPLY WELL -- Recharge to Well, In Percent Age, In Yrs Preliminary – Subject to Revision Not to Scale BINARY MIXTURE: Exponential ( v. young) and Piston Flow (older) Glacial deposits Starn, written commun., 2006

0.8 3 H/ 3 H(0), TU 3 H, TU Public Supply Well 0.6 GLACIAL DEPOSITS -- AGE TRACER DATA -- Preliminary – Subject to Revision TRACERMODEL1b; Binary Mixing Model Exponential Mixing Model Piston Flow Model Exponential Piston Flow Model xP/xE = 3.0 Samples 20.

Floridan Apparent Age: 3 H/ 3 He; 4 Yrs SF 6 ; 6 Yrs Preliminary – Subject to Revision COMPARISON OF SIMULATED AGE DISTRIBUTIONS IN SUPPLY WELLS Glacial deposits Age, In Yrs Recharge to Well, In Percent Apparent Age: 3 H/ 3 He; 7 Yrs Central Valley Apparent Age: 3 H/ 3 He; 38 yrs SF 6 25; Yrs 1,600 gal/min 660 gal/min 72 gal/min

Age Distributions Are More Useful Than Apparent Mean Ages For Understanding Susceptibility Ground-Water Flow Models Calibrated To Age Tracer Concentrations Can Be Used To Simulate Age Distributions It May Be Possible To Describe Age Distributions In Public-Supply Wells Using Lumped-Parameter Models (Simple Mathematical Functions) Tracer-Tracer Plots Can Sometimes Provide Insight Into Age Distributions In Public-Supply Wells CONCLUSIONS