Nitrate is a major concern to water-resource managers in the eastern San Joaquin Valley, and there are extensive regulatory programs in place that seek to reduce nonpoint-source loading and eventually reduce concentrations in groundwater. One recurring question is whether and to what extent natural denitrification processes affect groundwater concentrations. This GAMA-PBP study was conducted to answer that question.

Landon and others (2011) focused on a well-studied area around Modesto (Figure 1) and concluded that changes in nitrate concentrations (Figure 2) were only affected a minor degree by aquifer conditions. In addition, the study confirmed that observed spatial and temporal trends of nitrate are primarily controlled by water fluxes and nitrate use.