SIMULATION OF APPARENT GROUNDWATER AGE AND MIXING IN A HETEROGENEOUS SHALLOW AQUIFER UNDERGOING AGRICULTURAL PUMPING AND RECHARGE

A shallow (< 25 m deep) aquifer in the San Joaquin Valley, California is pumped for crop irrigation. The aquifer is recharged primarily by canal leakage and irrigation return flow. Tritium-helium measurements indicate depth-dependent trends of increasing groundwater age and mixing of old (>50 yr) groundwater. Understandably, interpretation of tritium-helium ratio is complicated by uncertainties related to bomb source input, diffusivity, heterogeneity, and transient flow. Considering that groundwater “age” is initialized when a water molecule reaches the saturated zone, additional complications arise from vadose zone processes and water table fluctuation.

This modeling study assimilates complexities of groundwater age interpretation by including important processes affecting bomb source tritium-helium ratio in groundwater. Aquifer heterogeneity is explicitly modeled using transition probability geostatistics applied to lithologies inferred from cone-penetrometer data. Rather than simulating age directly, apparent age is inferred from separate simulations of tracer transport with and without decay to account for dispersive and mixing effects. Localized tracers distinguish different recharge sources. Using a variably-saturated flow model, recharge processes are initiated at the ground surface, while time-dependent bomb source tritium concentration is conditioned within the unsaturated zone. Water table fluctuation is based on real measurements since the 1940s. Agricultural pumping dominates outflow and, thus, causes mixing zones to develop along the well screen. Despite considerable heterogeneity and transient flow, the simulations of apparent groundwater age show relatively one-dimensional increase with depth. Tritium-helium data is shown useful for flow model calibration through sensitivity to uncertain model parameters including irrigation rate and clay permeability.

The Groundwater Ambient Monitoring and Assessment program is sponsored by the California State Water Resources Control Board and carried out in cooperation with the U.S. Geological Survey. This work was performed under the auspices of the U.S. Department of Energy by the University of California, Lawrence Livermore National Laboratory under contract No. W-7405-ENG-48.