In a 2,700 km2 study area in the eastern San Joaquin Valley near Modesto, California water chemistry data from multiple sources were used to determine the distribution of reduction-oxidation (redox) conditions and their relations to explanatory variables, and to evaluate the relationship between changes in nitrate (NO3) concentrations over time and redox and other factors. Groundwater is predominantly oxic but some zones have anoxic or mixed redox conditions. Anoxic conditions were correlated with proximity to the valley trough and decreased depth to water table, and were co-located with areas of historical groundwater discharge. Anoxic conditions in areas with shallow depths to water table occur in both modern recharge affected by interactions of the shallow water table with soils, and pre-modern groundwater discharge. NO3 concentrations were significantly lower in anoxic than oxic or mixed redox groundwater, primarily because decreasing NO3 and dissolved oxygen are correlated with increasing groundwater age. Dissolved N2 gas data indicated that denitrification caused NO3 concentrations to decrease from above to below the drinking water threshold in only 1 of 39 wells; thus, denitrification does not currently protect many wells from NO3 contamination. Changes in NO3 concentrations over time are primarily controlled by loading from agricultural land use rather than redox.