NAWQA San Joaquin - Tulare Basins Study Unit: Abstract

Nitrate and Pesticides in Ground Water in the Eastern San Joaquin Valley, California: Occurrence and Trends

By Karen R. Burow, Sylvia V. Stork, and Neil M. Dubrovsky

U.S. Geological Survey Water-Resource Investigations Report 98-4040a

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Abstract

The occurrence of nitrate and pesticides in ground water in California¹s eastern San Joaquin Valley may be greatly influenced by the long history of intensive farming and irrigation and the generally permeable sediments. This study, which is part of the U.S. Geological Survey National Water-Quality Assessment Program, was done to assess the quality of the ground water and to do a preliminary evaluation of the temporal trends in nitrate and pesticides in the alluvial fans of the eastern San Joaquin Valley. Ground-water samples were collected from 30 domestic wells in 1995 (each well was sampled once during 1995). The results of the analyses of these samples were related to various physical and chemical factors in an attempt to understand the processes that control the occurrence and the concentrations of nitrate and pesticides. A preliminary evaluation of the temporal trends in the occurrence and the concentration of nitrate and pesticides was done by comparing the results of the analyses of the 1995 ground-water samples with the results of the analyses of the samples collected in 198687 as part of the U.S. Geological Survey Regional Aquifer-System Analysis Program.

Nitrate concentrations (dissolved nitrate plus nitrite, as nitrogen) in ground water sampled in 1995 ranged from less than 0.05 to 34 milligrams per liter, with a median concentration of 4.6 milligrams per liter. Nitrate concentrations exceeded the maximum contaminant level of 10 milligrams per liter (as nitrogen) in 5 of the 30 ground-water samples (17 percent), whereas 12 of the 30 samples (40 percent) had nitrate concentrations less than 3.0 milligrams per liter. The high nitrate concentrations were associated with recently recharged, well-oxygenated ground water that has been affected by agriculture (indicated by the positive correlations between nitrate, dissolved-oxygen, tritium, and specific conductance).

Twelve pesticides were detected in 21 of the 30 ground-water samples (70 percent) in 1995, although only 5 pesticides were detected in more than 10 percent of the ground-water samples. All 12 pesticides were detected at concentrations below the maximum contaminant levels, except the banned soil fumigants 1,2-dibromo-3-chloropropane (3 detections) and 1,2-dibromoethane (1 detection). Atrazine and desethyl atrazine (a transformation product of atrazine) were the most frequently detected pesticides; they were detected in 11 ground-water samples. The frequent detections of atrazine and desethyl atrazine may be related either to past applications of atrazine or to recent application on rights-of-way. Simazine was detected in 10 ground-water samples and diuron was detected in 4 ground-water samples. The detections of simazine and diuron are generally consistent with their reported applications on the crops near the wells where they were detected. 1,2,3-trichloropropane, a manufacturing by-product of 1,2-dichloropropane and 1,3-dichloropropene formulations, was detected in 4 ground-water samples. The occurrence of 1,2,3-trichloropropane, 1,2-dibromo-3-chloropropane, and 1,2-dibromoethane is probably related to past use. Similar to nitrate concentrations, pesticide occurrence was positively correlated to dissolved-oxygen concentrations, indicating that areas with high dissolved-oxygen concentrations may be vulnerable to contamination by nitrate and pesticides. High dissolved-oxygen concentrations may be associated with water that has been rapidly recharged.

A comparison of the concentrations and the occurrence of nitrate and pesticides between 198687 and 1995 indicates that nitrate concentrations may pose a greater threat to the quality of the ground-water resource in this region than pesticides, in the context of current drinking-water standards. Nitrate concentrations were significantly higher in the 1995 ground-water samples than in the 198687 samples collected from the same wells. Although the number of pesticide detections in 1995 is higher than the number of pesticide detections in 198687, the difference in detections is attributed to the lower detection limits that have resulted from improvements in analytical methods. When the data are censored at the highest detection or reporting limit, the number of pesticide detections between the 198687 and the 1995 samples did not increase. Furthermore, the concentrations of the detected pesticides may have decreased. The difference in temporal trends between the occurrence and the concentrations of nitrate and pesticides may be related to the large spatial variability in the amounts and the locations of the pesticide applications or to the difference in chemical properties of the pesticides; however, the results of this comparison may also be affected by the relatively small data set.

Contents
Abstract
Introduction
    Background
    Purpose and Scope
    Description of Study Area
    Acknowledgements
Study Design and Methods
    Well Network
    Water-Quality Data Collection and Analysis
    Quality-Control Data
        Nitrate
        Pesticides
        Volatile Organic Compounds
    Determination of Local-Scale Nitrogen Fertilizer Applications
    Statistical Methods
Occurrence of Nitrate
    Relation Between Nitrate and Fertilizer Applications
    Physical and Chemical Factors Related to Occurrence of Nitrate
Occurrence of Pesticides
    Pesticide Use
    Physical and Chemical Factors Related to Occurrence of Pesticides
Relation Between Nitrate and Pesticides
Trends in Nitrate and Pesticides
    Nitrate Trends
    Pesticide Trends
Summary and Conclusions
References Cited

URL http://water.wr.usgs.gov/sanj_nawqa/pub/usgs/wrir98-4040a/wrir98-4040a.html

Contact: jmgronbe@usgs.gov

Last modification: Fri, April 17, 1998