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Comparing model-computed turnover rates with isotopic ages for sustainable groundwater management
The Sustainable Groundwater Management Act (SGMA) provides an opportunity for innovative approaches of assessing groundwater models and water budgets. Advanced models calibrated with groundwater elevation alone may not accurately reflect residence times or aquifer storage turnover times, as uncertainty remains over recharge and seepage rates to deep aquifers. This study compares groundwater residence times based on isotopic signatures with those based on output from the Butte Basin Groundwater Model (BBGM). Comparison of these independent observational and numerical estimates allows for assessment and refinement of the model’s reliability for sustainable management of the groundwater basin. This study focuses on 210 wells located in Butte County, with an emphasis on 45 production and monitoring wells screened over the heavily pumped Tuscan Formation. Groundwater with an apparent age of less than 70 years is dated using the 3H/3He method, and longer residence time estimates are based on radiogenic 4He concentrations. Samples from nested monitoring wells provide greater resolution with depth than samples from long-screened production wells, which typically reflect a broad mixture of ages. Independent estimates of residence times are based on output from the BBGM. The Z-budget subroutine was applied to model output to examine the water budget over specific hydrostratigraphic layers, including the Tuscan Formation. The output budget from the model gives the volume of water in storage in each aquifer layer and the inflow and outflow rates; therefore, the mean residence time is readily calculated. The isotopic data indicate a wide range of groundwater ages and recharge sources with stream water recharging younger water (water table and modern water) and foothill and mixed precipitation recharging older water (pre-modern and fossil water). Of the 45 samples from the Tuscan Formations, half contain a portion of modern water with an average age of 31 years and show evidence of substantial dilution of modern water with pre-modern water. Eighty-seven percent of Tuscan wells contain pre-modern and fossil water, with an average age of ten thousand years. The mean residence time from the model for the targeted Tuscan layers in the Vina and Durham subregions from the model is 25 years. The model outputs a single turnover time, analogous to ‘piston’ flow. If we more appropriately assume an exponential age distribution for the same mean residence time, this translates to only 6 % of all water being older than 70 years, much less than determined from isotopic ages (87 %). More importantly, the exponential age distribution predicts no water older than 500 years, whereas the isotopic data indicate fossil water in 58 % of the Tuscan wells. While the model captures the residence time of the younger portion of groundwater ages, the difference in model and isotope-derived residence time for older groundwater provides an opportunity to improve the model calibration to include these fossil waters. By neglecting this older component of water, recharge rate estimates may be unrealistically high, which could allow unsustainable pumping under some management scenarios.