Nitrate in Ground Water of the San Joaquin Valley, California: Evolution of Understanding

Since 1991, the National Water Quality Assessment (NAWQA) Program of the U.S. Geological Survey has used studies at a variety of scales - regional aquifer surveys, surveys of shallow ground water under specific crops, and well-transect studies - to better understand nitrate transport and trends in ground water of the San Joaquin Valley, California. Nitrate concentrations in shallow ground water used for domestic consumption exceeded the federal maximum contaminant level in 29% of the wells sampled during 2001-2002, and the median nitrate concentration increased by a factor of close to 4 from the 1950's. Nitrate concentrations in deeper ground water used for public supply increased by approximately a factor of 2 during the same time period. These increases are consistent with increased application rates of commercial fertilizer, as well as recent conversion to high value, nitrogen-intensive crops such as almonds. In contrast, nitrate concentrations have decreased at the water table where agricultural land has been urbanized. Data for the unsaturated zone below almond orchards in the eastern San Joaquin Valley show that as much as 60% of the applied nitrate can reach the water table during a growing season. An oxygenated unsaturated zone and sandy soils with little organic matter resulted in very little denitrification and facilitated the transport of nitrate in that region. Both shallow and deeper zones of the aquifers are oxic, even water 1000's of years in age, suggesting limited potential for nitrate attenuation by denitrification. Accordingly, concentrations at depths typical of public-supply wells will likely continue to increase for decades. In contrast, denitrification can be effective in streambed sediments as indicated by recent studies of hyporheic processes. However, the overall effectiveness of the riparian zones to attenuate nitrate is still poorly understood. Further research is needed on how changes in management practices may decrease nitrate transport.