Isotopic imaging of surface water/groundwater interactions, Sacramento Valley, California

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Groundwater isotope data across the Sacramento Valley, California establish two types of groundwater mining: (1) overdraft of ancient groundwater with limited recharge by surface waters, producing cones of depression; (2) ancient groundwater withdrawal followed by rapid recharge of irrigation water, reducing groundwater quality. The first type occurs in the Sacramento metropolitan area, where meteoric runoff is unnaturally high and 40 years of pumping have depressed water levels to 25 m below sea-level, inducing recharge from losing reaches of the Sacramento and American rivers. Lateral migration rates are quantified by the binary mixing between river water (δ18O = −10.8) and natural groundwater (δ18O = −7.0). The second type of mining occurs in agricultural regions to the west, where 14C ages indicate that irrigation waters constitute more than 80% of modern recharge. This recharge has several characteristics of evaporated irrigation water, including: (1) high δ18O values (to > −6.0) that define closed contour patterns; (2) elevated NO3 concentrations (to 100 ppm); (3) low 14C ages of less than 500 years.Stable isotope contours, augmented by 14C data, provide dynamic recharge patterns in this profoundly disturbed, giant alluvial aquifer. On a large scale (> 100 km2), the lateral permeabilities of alluvial aquifers are essentially isotropic, whereas on a smaller scale (< 25 km2), anisotropy is evident and isotope values can be geographically complex and seasonally transient. Groundwater flow patterns implied by the isotope data can differ substantially from steady-state models based on head measurements.