Ground-water withdrawals in the San Joaquin Valley totaled 64 million m³ /day (19 million ac-ft) in 2000, supplying about 45% of agricultural irrigation demand and about 80% of municipal supply (Hutson et al., 2004). Most of the population and ground-water use are in the eastern San Joaquin Valley, where reliance on ground water is expected to increase as a result of rapid population growth and limited surface water supplies. Protection of ground-water quality for future use requires monitoring and understanding the mechanisms controlling the long-term quality of ground water in the regional aquifer system.

Nitrate has been widely detected above background concentrations in ground water in the eastern San Joaquin Valley. Nitrate concentrations (reported as nitrogen in this paper) were above the MCL of 10 mg/L in 24% of domestic wells screened in the shallow part of the aquifer that were sampled during 1993–95 (Dubrovsky et al., 1998) and the Central Valley is one of the top three regions in the state in terms of the number of public drinking-water wells exceeding the MCL for nitrate (California State Water Resources Control Board, 2002).

To assess spatial and temporal trends in nitrate concentrations in the eastern San Joaquin Valley and to evaluate the long-term effects of nitrogen fertilizer use on ground-water quality in this region, data were evaluated at multiple spatial scales. Data from regional-scale monitoring networks were used to map the regional occurrence of nitrate and to determine whether shallow ground water containing elevated nitrate is migrating to deeper parts of the aquifer system. At the local scale, mean ground-water ages from analysis of age-dating tracers were combined with concentrations of nitrate to reconstruct nitrate inputs in recharge through time and to compare with estimated nitrogen applications. Ground-water flow and transport simulations of a typical public-supply well screened from about 100 to 400 ft below the water table were used to evaluate long-term concentrations beneath agricultural areas under different nitrogen management scenarios.