STABLE ISOTOPE IMAGING OF A DYNAMIC GROUNDWATER SYSTEM IN THE SOUTHWESTERN SACRAMENTO VALLEY, CALIFORNIA, USA

A dynamic image of a shallow (45-160 m below the surface) aquifer in a 25km2 area in the southwestern Sacramento Valley, California has been obtained by O-18/O-16 and D/H determinations of groundwater from municipal wells. The regular summertime drawdown of the water table is strongly correlated with municipal-wide increases in the deltaO-18 values, particularly in wells with shallow perforation levels, thus demonstrating that the high O-18 water resides in the upper reaches of the aquifer and penetrates more deeply with increased discharge rates. These 1.5 parts per thousand enrichments in the deltaO-18 values are primarily attributed to infiltration of irrigation water that has penetrated to depths of at least 80 m. The increased deltaO-18 values correlate with increased deviation from the meteoric water line and with increased nitrate levels, indicating that the high O-18 groundwater component represents local meteoric water that has been enriched by soil evaporation processes in nitrogen-fertilized, flood-irrigated cropland. Isotopic gradients provide a new method for qualitatively determining subsurface permeability. Zones of low isotopic gradients, interpreted as high O-18 plumes, probably follow coarse-textured deposits of ancient stream channels. The O-18 gradients are primarily controlled by subsurface permeability, although additional influences may include: (1) the heterogeneous distribution of the high O-18 groundwater; (2) isotopic contributions from deeper groundwater; (3) variable perforation depths of the wells; (4) local differences in the water table level. Deeper, sodium bicarbonate groundwaters (300-650 m below the surface) have lower deltaO-18 values (to -9.4 parts per thousand) than the more shallow magnesium bicarbonate groundwaters, suggesting derivation from O-18 depleted meteoric groundwaters of the Sierra Nevada. Minor mixing of the deeper groundwater into the municipal discharge occurs, even though previous workers considered the deeper groundwater to be confined.