National Water-Quality Assessment (NAWQA) Program
Temporal monitoring of the pesticide 1,2-dibromo-3-chloropropane (DBCP) and nitrate and groundwater age were used to evaluate the transport and fate of agricultural chemicals in groundwater and to predict the long-term effects in the regional aquifer system in the eastern San Joaquin Valley, California (Burow and others, 2007). Twenty monitoring wells were installed on a transect along an approximate groundwater flow path. The monitoring wells were installed at multiple depths representing a range of groundwater ages. The groundwater ages, determined from chlorofluorocarbon (CFC) concentrations, were used to estimate the concentration of DBCP and nitrate in recharge through time. Samples were collected during 1994–1995 and 2003.
DBCP, a soil fumigant used to control nematodes, was applied to crops nationwide beginning in the 1950s. In 1977, agricultural use of DBCP was suspended in California in response to concern about the potential hazardous effects of DBCP on human health; however, DBCP persists in groundwater in this region at concentrations above the US Environmental Protection Agency (USEPA) maximum contaminant level (MCL) of 0.2 micrograms per liter (µg/L), posing a threat to drinking-water supplies more than 25 years after it was banned from use.
DBCP concentrations increased in samples from deep wells along the transect as DBCP moved deeper in the aquifer. Using the changes in concentration of DBCP and the difference in mean groundwater age between samples collected in 1994–1995 and 2003, an estimated half-life of 2–6 years with a median of 4 years was determined. This half-life is in the range of earlier analysis; contaminant transport modeling results indicating a half-life of about 6 years.
Nitrate concentrations above the MCL of 10 milligrams per liter (mg/L) reached depths of more than 100 feet (30 meters) below the water table. Nitrate concentrations increased from 8 to 23 mg/L in the shallow part of the aquifer between 1994–1995 and 2003. Estimated initial concentrations of nitrate in recharge indicate an overall increase in nitrate concentrations during the last 50 years, which is generally consistent with increases in nitrogen fertilizer applications. Elevated nitrate concentrations resulting from pumping groundwater with high nitrate concentrations and reapplying it as irrigation water may explain elevated concentrations in later years.
Because of the intensive pumping and irrigation recharge, vertical flow paths are dominant. High concentrations (above MCLs) in the shallow part of the regional aquifer system will likely move deeper in the system, affecting both domestic and public-supply wells. The large fraction of old water (unaffected by agricultural chemicals) in deep monitoring wells suggests that it could take decades for concentrations to reach MCLs in deep, long-screened public-supply wells.