Nutrient trends in the Sacramento and San Joaquin basins: a comparison to state and regional water quality policies

Non-point source (NPS) control strategies were initiated in California in the late 1980’s under the authority of the Porter-Cologne Act and eventually for the development of total maximum daily load (TMDL) plans. Most of the NPS TMDLs developed for California’s Central Valley region (CV) were related to pesticides. Efforts to reduce pesticide loads and concentrations began in earnest around 1990. The NPS control strategies either encouraged or mandated the use of management practices (MPs). Although TMDLs were largely developed for pesticides, the resultant MPs would likely have affected the runoff of other potential pollutants (specifically, nutrients). This study evaluates the effectiveness of agricultural NPS control strategies implemented in California’s CV between 1990 and 2013 by comparing surface-water nutrient concentration and load trends between two periods. In general, use of MPs were encouraged during a “voluntary” period (1990 to 2004) and mandated during an “enforcement” period (2004 to 2013). Nutrient water-quality data were obtained from U.S. Geological Survey National Water Inventory System (NWIS) and the California Environmental Data Exchange Network (CEDEN). Nutrient concentrations, loads, and trends were estimated by using the recently developed Weighted Regressions on Time Discharge and Season (WRTDS) model. Sufficient total phosphorus (TP), total nitrogen (TN) and nitrate (NO3) data were available to compare the voluntary and enforcement periods for seven sites within the lower Sacramento and San Joaquin basins. For six of the seven sites, flow-normalized mean annual concentrations of TP and NO3 decreased at a faster rate during the enforcement period than during the voluntary period. Concentration changes during similar years and ranges of flow conditions suggest that MPs designed for pesticides, reduced nutrient loads. Results show that enforceable NPS policies, and accelerated MP implementation, limits NPS pollution, and may control runoff of non-targeted constituents such as nutrients.