COLLABORATIVE RESEARCH: Radiogenic Helium as a Chronologic Tracer for Younger Groundwater

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9628627 Solomon The concentration of radiogenic 4He in groundwater that results from U/Th decay has been used as a dating tool over time scales of 103 to 106 years. Recent measurements indicate that aquifer solids in many shallow flow systems may be releasing 4He at rates that are much greater than U/Th decay, making it possible to use 4He as a groundwater dating tool over time scales as short as 101 years. This proposal is to investigate the mechanisms leading to and the geographical extent of aquifers that are releasing 4He at rates greater than U/Th decay. The primary objective of the proposed research is to evaluate the use of 4He as a groundwater tracer over time scales of 10 to several hundred years. Specific research questions include the following: (1) What are the mechanisms controlling the exchange of 4He between aquifer solids and groundwater? Can this exchange rate be predicted and if so what is the uncertainty? (2) How spatially variable are He release rates? Are spatial variations correlated with variations in basic hydraulic properties (porosity, permeability) and/or geochemical properties (mineralogy, sedimentary facies, diagenesis)? (3) What is the geographical extent of shallow aquifers that are releasing 4He at rates that are significantly greater than U/Th decay? Is it possible to predict the occurrence of high 4He release rates based on general geologic concepts? (4) What are 4He release rates in carbonate and other fractured rock systems? (5) To what extent does the total 4He content of aquifer solids represent the paleohydrology of the aquifer protolith? Can total 4He contents be used to reconstruct the hydrologic history of an aquifer? Direct measurements of the 4He release from aquifer solids will be compared with release rates based on radiogenic 4He concentrations in groundwater and direct determinations of groundwater age using 3H/3He and chlorofluorocarbons. An evaluation of release mechanisms including potential spatial varia tions in release rates will be used to assess the viability of dating groundwater with 4He over the time scale of 50 to 500; existing groundwater dating methods are not accurate over this environmentally important time scale. Evaluating the potential impacts of anthropogenic practices on the quality of groundwater resources depends on an accurate measure of fluid velocities and travel times. However, traditional hydraulic approaches are problematic due to extreme variability in permeability. The utility of groundwater dating methods results from the fact that the groundwater age itself, at any point in a flow system, is an integration of the velocity field along the entire upstream flow path. Thus, a single measurement of age contains memory and embodies much more information than a point measurement of velocity (or permeability). Methods that provide accurate measures of groundwater travel times over time scales of 10 to hundreds of years may thus allow a characterization of flow systems at a level of detail that is presently not available and yet is needed to solve water quality as well as water supply problems. ??