1. Age-dating of Groundwater Lecture at Washington University, St. Louis April 11, 2007 Publication # UCRL-PRES-229859 By M. Lee Davisson Lawrence Livermore National Laboratory

2. Helps answer: How much is there? How long will it last? What is the source of contamination? What is the risk of a contaminant? What is the value of groundwater ages?

3. Darcy Equation Q is Darcy velocity K is intrinsic aquifer property is hydraulic head Can we measure the necessary parameters? v is actual microscopic velocity  is porosity

4. DistanceCan be measured between two groundwater wells. But what is the distance between a recharge point and a well? Groundwater elevation in wells measured with great accuracy At larger scales topography will suffice KCannot be measured in the field Difficult to measure in the laboratory Sensitive to geographic and depth scale Source of most uncertainty in hydrogeologic analysis MaterialHydraulic conductivity (m/s) Clay10 -11 to 10 -8 Silt, sandy silts, clayey sands, till 10 -8 to 10 -6 Silty sands, fine sands10 -7 to 10 -5 Well-sorted sands, glacial outwash 10 -5 to 10 -3 Well-sorted gravel10 -4 to 10 -3 What about fractured rock? Water about karst? 10 -2 to 10 -11 !

5. time Can be measured by tracers or other markers of time Can be measured with variable accuracy Can be measured over a wide age range Age-Dating Methods Natural radioactivity Climate change Inadvertent Tracers  Tritium  Chlorofluorocarbons  Krypton-85  Stable isotopes  Dissolved contaminants Intentional Tracers  Sulfur-hexafluoride  Noble gases  Dyes

6. SUPPLY DEMAND Agriculture Urban Recreation Environmental How much is there? Demand = Supply Natural recharge rates difficult to measure directly Age-dates of groundwater older than human occupation provide natural recharge rate Age-dates of youngest groundwater provide modern recharge rates

7. What is the distance traveled by the groundwater? In basins with little elevation gain, distance approximately equals depth to groundwater well extraction level In basins with large elevation differences, recharge sources need to be determined Tropical Arid Large elevation change Small elevation change Distance Groundwater Travels Increases

8. Many choices of naturally-occurring isotopes for age-dating Which ones behave most like water? Natural radioactivity

9. Isotopic age-dating methods Unstable isotopes with relatively high decay constants Either natural abundances or concentration spikes created by nuclear fallout N = measured isotope abundance N 0 = abundance at time of recharge  = decay time constant t = time N 0 dependent on reactive and transport processes Variation in source concentration Dispersion/mixing/dilution Phase changes Half-Life =

10. > 1 for hydrogen and oxygen isotopes SMOW Evaporation Rain-out GMWL Mean Annual Precipitation Isotopic values controlled by temperature  Latitude  Elevation  Inland distance Groundwater reflects mean annual precipitation values Climate Change

11. Paleo- Recharge Modern- Recharge Climate Change Recharge during last glacial maximum (~10kyr ago) likely had lower isotopic values Groundwater values significantly lower than mean annual precipitation (except in karst) No plausible higher elevation recharge sources No plausible surface water recharge sources with low isotopic values Must make hydrologic sense

12. Water Table elevation – Sacramento Valley

13. Groundwater Oxygen-18 Values – Sacramento Valley Potential Sources Rain/Snow  Low elevation  High elevation Rivers Agricultural irrigation  Local sources  Imported sources Urban landscaping

14. Age-dating groundwater older than human occupation Radiocarbon (14 C/ 12 C) std is an oxalic acid whose radiocarbon abundance is equal to the abundance of atmospheric CO 2 in 1950 Radiocarbon dating typifies challenges in age-dating methods Where carbon comprises significant amount of aquifer matrix, water-rock rxn dominates over radioactive decay Volcanoes are another source of dissolved carbon absent in 14 C

15. Closed System Rxn: 14 CO 2 + H 2 O + M 12 CO 3 H 14 CO 3 + H 12 CO 3 + M ++ Open System Rxn: 14 CO 2 + H 2 O H 2 14 CO 3 + H 12 CO 3 H 2 12 CO 3 + H 14 CO 3 fastslow Saturated Flow: H 14 CO 3 + M 12 CO 3 H 12 CO 3 + M 14 CO 3 10 -8  10 -10 /cm 2 s < 1yr

16. Possible Correction Method Establish all plausible initial 14 C content of recharge Draw reaction lines (straight lines) toward 14 C-absent source material Compute horizontal off-set of measured values from reaction lines Subtract off-set from one and compute age

17. 4 He Dissolved 4 He concentration increases Natural uranium and thorium decay Steady-state 4 He flux from crust ~1e 9 atoms/cm 2 -yr Rate dependent on  Regional uranium-thorium concentrations in crust  Localized geologic faulting Uncertainties factor of two or more Good for only groundwater >1000 years old Helium-4 Accumulation in Age-Dating

18. Castro et al., 2000 Carrizo Aquifer, TX

19. Age-dating groundwater since human occupation Impacts of engineered systems Land Use How groundwater recharge is affected Arid ClimateWetter Climate Agriculture Significantly enhances recharge; depletes and often contaminates groundwater Modest changes in natural recharge rates; nutrient contaminants Urbanization Significantly reduces natural recharge; petroleum and solvent contamination Modest changes in natural recharge rates; petroleum and solvent contamination Seawater intrusion Common in coastal environments Common in coastal environments using groundwater Surface water management Changes where recharge occursReduces river recharge

20. Young groundwater age-dating Chlorflourocarbons (CFCs)Krypton-85 ( 85 Kr) NO NATURAL SOURCES Age = mol/L in air = mol/L in water x H air-water H = Henry’s Law partitioning coefficient f (mean soil temperature) CFCs Drawbacks Reducing conditions Point sources (e.g. landfills) However: CFC-113/CFC-111 ratios verify conservation 85 Kr Drawbacks Point sources (e.g. nuclear sites) Not many labs measure it

21. Tritium ( 3 H) Numerous studies since the 1960s Part of the water molecule Useful half-life (12.4 years) Atmosphere is sole source Point source contamination rare Atmospheric concentration has large variation 3 H alone is excellent post-1950 age indicator

22. 3 He meas = 3 He trit + 3 He equil + 3 He excess + 3 He rad 4 He meas = 4 He equil + 4 He excess + 4 He rad 22 Ne meas = 22 Ne equil + 22 Ne excess Over determined system allows the calculation of 3 He trit Noble Gas Mass Spectrometry

25. Chemically suitable for potable supplies Conservative behavior Water soluble and measureable over large dynamic range Inexpensive Common Tracers Sulfur-hexafluoride Noble gases (He, Xe) Dyes (Rhodamine) Artificial Tracers

26. High degree of accuracy Discriminate individual flow paths Track contaminant fate Evaluate health risks

27. Selected Reading Craig, H., 1961, Isotopic variations in meteoric water. Science, 133, 1702-1703. Dansgaard W., Stable isotopes in precipitation. Tellus XVI 4, 436-468, 1964. Handbook of Environmental Isotope Geochemistry. Elsevier: New York, Fritz, P., Fontes, J.Ch. (eds.); 1980. Heaton T.H.E. and Vogel J.C., 1981, "Excess air" in groundwater. J. Hydrol., 50, 210-216. Ian D. Clark, Peter Fritz, 1997, Environmental Isotopes in Hydrogeology. CRC Press; 352 pgs Ingraham, N.L., Taylor, B.E., Light stable isotope systematics of large-scale hydrologic regimes in California and Nevada, Water Resour. Res., 27, 77-90, 1991. Mazor, E., 1991, Applied Chemical and Isotopic Groundwater Hydrology. Halsted Press: New York, 274 pgs. Poreda, R.J., Cerling, T.E., Solomon, D.K., 1988, Tritium and helium-isotopes as hydrologic tracers in a shallow unconfined aquifer. J Hydrol. 103, 1-9. Schlosser, P. Stute, M., Dorr, H., Sonntag, C., Munnich, O., 1988, Tritium/3He dating of shallow groundwater. Earth, Planet. Sci. Lett., 89, 353-362. Schlosser, P. Stute, M., Sonntag, C., Munnich, O., 1989, Tritiogenic 3He in shallow groundwater. Earth, Planet. Sci. Lett., 94, 245-256.