Groundwater Vulnerability and the Meaning of Groundwater Age Dates

   Dr. Graham Fogg
         Hydrologist and Professor of Hydrogeology
Department of Land, Air and Water
 Resources, University of California at Davis

Regional-scale analyses of flow and transport in two groundwater basins demonstrate hydrogeologic approaches for characterizing the vulnerability of groundwater to contamination and lead to important implications about the meaning of groundwater age dates and the sustainability of groundwater quality. The approach combines detailed models of three-dimensional hydrostratigraphy with a backward-time solution of the advection-dispersion equation (ADE). Results for the Salinas Valley, California show that, owing to heterogeneities represented through geostatistical modeling, the simulated regional spatial patterns of nitrate occurrence agree closely with field data from wells. The transport simulations suggest time lags of more than 30 yr for breakthrough at well screens 55 m below the water table, implying that nitrate groundwater contamination detects, which began emerging in the 1970's, originated from land use practices of circa 1940's. The random-walk solution of the backward-time ADE also produces estimates of the age distributions of groundwater pumped by individual wells (i.e., ages of individual water "particles" reaching the well screen). The simulated groundwater ages typically range from decades to over 100 years within individual water "samples," suggesting that if nitrate-loading rates do not decline appreciably, historical breakthroughs of contaminants at wells merely represent the beginning of gradual deterioration in groundwater quality.

A more detailed analysis of both hydrostratigraphy and transport in the eastern San Joaquin Valley, California leads to refinements in the vulnerability mapping approach and further investigation of age distributions in water samples. A new approach to modeling alluvial fan heterogeneity uses geostatistical simulation in a sequence stratigraphic framework in which paleosols form sequence boundaries and semi-confining beds. This model of multi-scale heterogeneity produces detailed maps of vulnerability that more accurately reflect the geology, including nonstationary heterogeneity. Simulated CFC-11 and -12 age dates agree closely with field measured CFC-11 and -12 ages. Importantly, however, the distributions of water age reaching the well screens in the model at any instant in time are both broad (many decades) and skewed, except at shallow (<10 m) wells. Significant dispersion due to heterogeneity causes mixing of relatively old water (> 40 yr) with young water in most wells, even when the well screen is short (<1 m). Consequently, the "true" average groundwater age differs significantly from the CFC age. These results suggest that groundwater age date measurements alone are inadequate for estimating vulnerability to contamination in systems with typical alluvial heterogeneity.