Age-dating of Groundwater Lecture at Washington University, St. Louis April 11, 2007

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

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

4. Distance Can 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 K Cannot be measured in the field Difficult to measure in the laboratory Sensitive to geographic and depth scale Source of most uncertainty in hydrogeologic analysis • 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. DEMAND • Agriculture • Urban • Recreation • Environmental SUPPLY 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 Large elevation change Distance Groundwater Travels Increases Small elevation change Tropical Arid

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

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 N0 = abundance at time of recharge l = decay time constant t = time • N0 dependent on reactive and transport processes • Variation in source concentration • Dispersion/mixing/dilution • Phase changes Half-Life =

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

11. Paleo-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 Modern-Recharge

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 (14C/12C)std is an oxalic acid whose radiocarbon abundance is equal to the abundance of atmospheric CO2 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 14C

15. Closed System Rxn:14CO2 + H2O + M12CO3 H14CO3 + H12CO3 + M++ Open System Rxn:14CO2 + H2O H214CO3 + H12CO3 H212CO3 + H14CO3 < 1yr fast slow Saturated Flow:H14CO3 + M12CO3 H12CO3 + M14CO3 10-8-10-10/cm2s

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

17. Helium-4 Accumulation in Age-Dating • Steady-state 4He flux from crust ~1e9 atoms/cm2-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 Dissolved 4He concentration increases 4He 4He 4He Natural uranium and thorium decay

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

19. Age-dating groundwater since human occupation Impacts of engineered systems

20. Young groundwater age-dating Chlorflourocarbons (CFCs) Krypton-85 (85Kr) NO NATURAL SOURCES Age = mol/Lin air = mol/Lin water x Hair-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 • 85Kr Drawbacks • Point sources (e.g. nuclear sites) • Not many labs measure it

21. Tritium (3H) • 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 • 3H alone is excellent post-1950 age indicator

22. Noble Gas Mass Spectrometry 3Hemeas = 3Hetrit + 3Heequil + 3Heexcess + 3Herad 4Hemeas = 4Heequil + 4Heexcess + 4Herad 22Nemeas = 22Neequil + 22Neexcess Over determined system allows the calculation of 3Hetrit

25. Artificial Tracers • 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)

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.