Dispersion of Water Age in Aquifer Systems and in Groundwater Samples

University Of California-Davis, Davis CA

Investigators

Abstract

0208492 Fogg Groundwater age dates estimated using various environmental tracer methods is being increasingly called upon to address groundwater quality problems and to understand many other aspects of subsurface systems. However, systematic analysis of the meaning of groundwater age dates is lacking, which may limit the application, or result in serious misinterpretations. The PI's recent work indicates that common interpretations of estimated groundwater ages are dependent on assumptions about dispersion and mixing that are not appropriate in many, perhaps most, cases. The PI finds that, while most age dates are thought to be representative of all the water molecules in the water sample if the well screen is not too long, actual ages within that sample can vary significantly (from IO0 to 102+ yr.) in typically heterogeneous systems. Although some subsurface hydrologists already anticipated this, the scientific basis for evaluating groundwater age dates remains undeveloped and vague. This in turn has led to an extreme range of interpretations and assumptions by both water managers and researchers. Herein the PI proposes a theoretically accurate, tested simulation approach to provide the first ever quantitative evaluation of the roles of heterogeneity, well-bore mixing, well screen depth and length, aquifer parameters, and boundary conditions on groundwater age variations in space and time in geologically realistic alluvial aquifer systems. In particular, the research will investigate how ages of water "particles" can be expected to vary within individual water samples in space and time in subsurface with different degrees of heterogeneity. Through both modeling and field-testing the PI also proposes to explore a new hypothesis that variation in age dates during long-term pumping of a well provides a signal indicative of aquifer heterogeneity and vulnerability . Understanding of the main factors controlling groundwater ages and age distributions is a prerequisite for any application of tracer-based age-dating data. Fluctuations in groundwater age as a function of space and time may be used to explore subsurface hydrologic processes far beyond the traditional static groundwater age dating method. Proposed testing of the above hypothesis holds out the possibility that groundwater age dating approaches can be transformed from qualitiative procedures dependent on vague, untested assumptions to powerful characterization tools for quantitatively (or semiquantitatively) assessing groundwater conditions in previously unanticipated ways. The PI shows abundant preliminary simulation results to support this problem statement and to demonstrate feasibility of the general approach. Key words: groundwater, contamination, transport, aquifer vulnerability, modeling, age Date.