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Modeling the dynamic penetration depth of post-1950s water in unconfined aquifers using environmental tracers: Central Valley, California

This study contributes to the USGS long-term scientific knowledge base of how water quality can constrain water-supply reliability. Anyone can also use the accompanying web tool to explore these data and results for themselves.

Modern, shallow groundwater in California’s Central Valley can contain elevated concentrations of nitrate and other anthropogenic contaminants. As that water moves deeper into the aquifer, it moves into the zone tapped by wells that supply drinking water. A new USGS model of the Central Valley allows users to predict how this front of modern water will move deeper into the aquifer across the whole basin.

The model identifies the depth of water that was recharged before and after 1950 using multiple geochemical tracers collected from many domestic and public drinking-water-supply wells. Outputs map how the modern/ premodern interface moves in three dimensions through time.

The left map (Figure 6a in paper) shows the modeled depth of the modern/premodern interface in 2005. Warmer colors indicate the modern/premodern interface is deeper. The right map (Figure 8a in paper) shows the modeled change in depth of the modern/premodern interface in 2025. The darkest blue indicates locations where the penetration rate of the modern/ premodern interface is fastest.

Results from this work (Faulkner and others, 2023) can be used by groundwater basin managers to estimate where and when groundwater quality may be affected by modern water moving downwards.

This conceptual diagram shows how the downward-moving modern water (deep purple) and pre-/post-1950 dividing line (thick, black, horizontal line) might look like at four hypothetical wells during a 20-year period. Managing degrading source water quality is likely to increase costs for domestic users and public drinking-water suppliers in the future.

This study is a product of the statewide Groundwater Ambient Monitoring and Assessment Program-Priority Basin Project (GAMA-PBP), a cooperative partnership between the USGS and the California State Water Resources Control Board. Funding was provided by the State and USGS Cooperative Matching Funds. The work was made possible by multiple USGS laboratories.

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Modeling the dynamic penetration depth of post-1950s water in unconfined aquifers using environmental tracers

Faulkner, K.E ., Jurgens, B.C., Voss, S.A., Dupuy, D.I., and Levy, Z.F., Central Valley California. Journal of Hydrology, v. 616, article 128818


Central Valley Aquifer Age Dating (1st ed.)

Dupuy, D.I., Nguyen, D.H., and Jurgens, B.C., U.S. Geological Survey website