USGS Publications Warehouse

Abstract

The processes that affect nitrate and pesticide occurrence may be better understood by relating ground-water quality to natural and human factors in the context of distinct, regionally extensive, land- use settings. This study assesses nitrate and pesticide occurrence in ground water beneath three agricultural land-use settings in the eastern San Joaquin Valley, California. Water samples were collected from 60 domestic wells in vineyard, almond, and a crop grouping of corn, alfalfa, and vegetable land-use settings. Each well was sampled once during 1993?1995. This study is one element of the U.S. Geological Survey?s National Water-Quality Assessment Program, which is designed to assess the status of, and trends in, the quality of the nation?s ground- and surface-water resources and to link the status and trends with an understanding of the natural and human factors that affect the quality of water. The concentrations and occurrence of nitrate and pesticides in ground-water samples from domestic wells in the eastern alluvial fan physiographic region were related to differences in chemical applica- tions and to the physical and biogeochemical processes that charac- terize each of the three land-use settings. Ground water beneath the vineyard and almond land-use settings on the coarse-grained, upper and middle parts of the alluvial fans is more vulnerable to nonpoint- source agricultural contamination than is the ground water beneath the corn, alfalfa, and vegetable land-use setting on the lower part of the fans, near the basin physiographic region. Nitrate concentrations ranged from less than 0.05 to 55 milligrams per liter, as nitrogen. Nitrate concentrations were significantly higher in the almond land-use setting than in the vineyard land-use setting, whereas concentrations in the corn, alfalfa, and vegetable land-use setting were intermediate. Nitrate concentrations exceeded the maximum contaminant level in eight samples from the almond land- use setting (40 percent), in seven samples from the corn, alfalfa, and vegetable land-use setting (35 percent), and in three samples from the vineyard land-use setting (15 percent). The physical and chemical characteristics of the vineyard and the almond land-use settings are similar, characterized by coarse-grained sediments and high dissolved- oxygen concentrations, reflecting processes that promote rapid infiltration of water and solutes. The high nitrate concentrations in the almond land-use setting reflect the high amount of nitrogen appli- cations in this setting, whereas the low nitrate concentrations in the vineyard land-use setting reflect relatively low nitrogen applications. In the corn, alfalfa, and vegetable land-use setting, the relatively fine-grained sediments, and low dissolved-oxygen concentrations, reflect processes that result in slow infiltration rates and longer ground-water residence times. The intermediate nitrate concentrations in the corn, alfalfa, and vegetable land-use setting are a result of these physical and chemical characteristics, combined with generally high (but variable) nitrogen applications. Twenty-three different pesticides were detected in 41 of 60 ground- water samples (68 percent). Eighty percent of the ground-water samples from the vineyard land-use setting had at least one pesticide detection, followed by 70 percent in the almond land-use setting, and 55 percent in the corn, alfalfa, and vegetable land-use setting. All concentra- tions were less than state or federal maximum contaminant levels only 5 of the detected pesticides have established maximum contaminant levels) with the exception of 1,2-dibromo-3-chloropropane, which exceeded the maximum contaminant level of 0.2 micrograms per liter in 10 ground-water samples from vineyard land-use wells and in 5 ground- water samples from almond land-use wells. Simazine was detected most often, occurring in 50 percent of the ground-water samples from the vineyard land-use wells and in 30 percent of the ground-water samples from the almond and the corn, alfalfa, and vegetable land-use wells. Atrazine (or desethyl atrazine) was detected in 40 percent of ground- water samples from the vineyard and almond land-use wells and in 25 percent of the ground-water samples from the corn, alfalfa, and vegetable land-use wells. Diuron was detected most often in the corn, alfalfa, and vegetable land-use setting (35 percent). The occurrence of simazine, 1,2-dibromo-3-chloropropane, and diuron in the three land-use settings was fairly consistent with recent or historical use of these pesticides. The occurrence of atrazine was not directly related to use on these crops; its occurrence may be linked to rights-of-way applications. Five pairs of monitoring wells were installed near a subset of five domestic wells in each of the three land-use settings to define whether ground-water samples from shallow domestic wells were repre- sentative of recently recharged ground water beneath the targeted agricultural land uses. Although the number of samples is small, the occurrence of nitrate and pesticides was not significantly different (at a significance level of 0.05) between the domestic well ground- water samples and the ground-water samples from shallow monitoring wells screened near the water table. Nitrate concentrations were significantly higher in the ground-water samples from the domestic wells than in the samples from monitoring wells screened at the same depth. Conversely, simazine and 1,2-dibromo-3-chloropropane concen- trations appeared to be higher in ground-water samples from these monitoring wells, though the sample size was too small to evaluate statistically.