National Water-Quality Assessment (NAWQA) Project
Results from an analysis of nitrate and pesticides in groundwater in the eastern San Joaquin Valley at the local and regional scale demonstrate that anthropogenic chemicals from nonpoint source inputs are being recharged to the groundwater system and are moving downward with time as a response to pumping and irrigation recharge (Burow and others, 2008). Two local-scale well networks (44 wells) and five regional-scale well networks were evaluated in the SANJ. The regional scale networks include three agricultural land-use networks (90 wells), one regional aquifer network (30 wells), and one historical data network (6,342 wells). Sampling results from monitoring, domestic, irrigation, public-supply, and other well types were investigated to maximize the use of available water-quality data.
Location of the San Joaquin (SANJ) Study Area (from Burow and others, 2008).
Concentrations of nitrate, the nematocide 1,2-dibromo-3-chloropropane (DBCP), and the herbicide simazine were generally highest in the shallow part of the aquifer and decreased with depth. Concentrations of nitrate were highest beneath agricultural land, with concentrations in the shallow part of the aquifer as high as 75 milligrams per liter (mg/L). Nitrate concentrations in deep wells representing pre-development conditions were generally less than background concentrations of about 2 mg/L. Concentrations were greater than the USEPA drinking-water standards for nitrate or DBCP in 44 percent of domestic wells sampled during 2001–03, indicating that domestic drinking water supplies have been significantly affected by inputs of nonpoint source agricultural chemicals over time (Burow and others, 2008). In contrast to nitrate and DBCP, concentrations of simazine were low, ranging from less than 0.005 to 0.16 micrograms per liter, more than an order of magnitude below the drinking-water standard.
Concentrations of nitrate, DBCP, and simazine have increased since the 1950s throughout the aquifer system. Nitrate, DBCP, and simazine in groundwater are moving downward from the shallow portions of the aquifer into deeper parts of the aquifer in response to well pumping and irrigation recharge. Because of the time of travel between the shallow and the deeper part of the aquifer system, current concentrations in water from deeper wells, such as public-supply wells, likely reflect the effects of land use practices 40 or 50 years ago (Burow and others, 2008). Therefore, concentrations of nitrate and detections of pesticides will likely increase as the shallow groundwater migrates into the deeper portions of the aquifer. However, it is difficult to predict the length of time for water from public-supply wells to reach concentrations of concern without additional monitoring and analysis of the age of groundwater reaching wells screened in the deep part of the aquifer.