Compilation of data for isotope mapping of groundwater in the Central Valley of California, 1993-1995

A major stable isotope mapping project is underway that will provide important baseline information to the State of California in management of their groundwater resources. The results represent a new technological application using isotope hydrology to better understand and predict the sustainability of California's groundwater supply for the future. This project is driven by the fact that Californians inhabit a semi-arid region of seasonal precipitation, but have created a lifestyle and economic infrastructure requiring a sub-tropical climate. They have accomplished this by engineering systems that store and divert alpine runoff, and by utilizing a large, productive alluvial aquifer. In the past, both of these resources appeared to be unlimited. Today, water shortages are recognized, regardless of drought conditions. Because Californians maintain their current practices of prolific water use, the deep-seated competition between agricultural users and urban consumers has been amplified. This has been aggravated by the acquisition of one-third of the available surface water resources for maintenance of aquatic habitats. The State of California accepts and encourages the use of groundwater to supplement these diverse water demands. Stable isotope imaging of the groundwater resources has proven to be the most economical and effective means to diagnose the health of the giant alluvial aquifer of the Central Valley. Augmented by radiocarbon analysis and nitrate determinations, stable isotope data can be used to clearly distinguish groundwater recharged from natural or anthropogenic sources. Isotope maps delineate (1) the geographic distribution of various groundwater masses and of preferential recharge zones, (2) the sources and extent of non-point source pollution, and (3) the locations and rates of lateral flow channels. Different recharge rates of natural and modem groundwater bodies can be used to characterize safe yield parameters for aquifers