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U.S. GEOLOGICAL SURVEY
Scientific Investigations Report 2007–5179

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Abstract

Within the Western United States, agricultural and rural lands are being developed into commercial and residential areas. With changes in land use and increasing population, greater demands are placed on water resources for agricultural, industrial, and domestic supplies. Many areas in the Western United States rely exclusively on ground water as their source of drinking water. Areas that use surface-water resources often need to supplement this supply with ground water.

Generally, shallow ground water is susceptible to fluctuating water quality within relatively short time scales and therefore can be used as an indicator of land-use stresses that may, in time, affect deep aquifer systems. This regional study examines data on shallow ground-water quality collected from 1993 to 2004 from 273 agricultural and 181 urban wells from 7 U.S. Geological Survey National Water-Quality Assessment study units in Arizona, California, Nevada, New Mexico, south-central Colorado, and Utah. This report determines important influences that land-use practices may have on the quality of recently recharged ground water, which may ultimately affect deep water supplies within the region.

The results of this investigation show that nitrate, the principal species of nitrogen present in ground water, exceeds the U.S. Environmental Protection Agency (USEPA) maximum contaminant level (MCL) of 10 milligrams per liter in water from more than 25 percent of agricultural wells and 10 percent of urban wells. In agricultural areas, the probability of exceeding the USEPA MCL for nitrate is influenced primarily by three factors: fertilizer use, irrigation, and aquifer oxidation-reduction (redox) conditions. At the study-unit level, differences in nutrient concentrations between agricultural and urban land use likely are influenced by ground-water redox conditions within respective aquifer systems.

The most commonly detected pesticides belonged to the triazine, urea, amide, and carbamate classes. The triazine herbicides were the predominant pesticides present in both agricultural and urban areas. Simazine and diuron were most frequently detected in shallow ground water within agricultural areas, whereas atrazine, prometon, and tebuthiruon were more commonly detected in urban areas. The carbamate pesticides and the amide, metolachlor, were infrequently detected. Pesticide concentrations in shallow ground water within agricultural or urban land-use settings did not exceed established USEPA MCLs for drinking water. Generally, factors that influenced the detection of pesticides in shallow ground water in the regional area were dissolved oxygen, general soil permeability characteristics, temperature, and depth to screened interval.

The most commonly detected volatile organic compounds (VOCs) within urban areas were the trihalomethanes (THMs), solvents, and the fuel oxygenate, methyl tert-butyl-ether (MTBE). The most frequently detected THM was chloroform and the primary detected solvents were tetrachloroethene (PCE) and trichloroethene (TCE). Soil fumigants were detected almost entirely within an agricultural area in the Central Valley of California. The predominant fumigant detected was dibromochloropropane (DBCP), and its occurrence may be a result of its persistence and past use as a soil fumigant. THM concentrations did not exceed the USEPA MCL of 80 micrograms per liter (µg/L) for total THMs in any sample. Two ground-water samples collected within urban areas in Nevada exceeded the USEPA MCL for TCE (5 µg/L) and the drinking-water advisory for MTBE (20-40 µg/L). The PCE concentration in one sample from the urban area in Utah and five samples from urban areas within Nevada exceeded the 5 µg/L MCL. An important factor affecting the detection frequencies of VOCs was the ground-water redox condition. Chloroform and PCE were detected more frequently in well-oxygenated ground water, whereas MTBE was detected more frequently in less-oxygenated water. Multivariate logistic-regression showed that dissolved oxygen, pH, and the percentage of industrial land use played important roles in the detection frequency of a number of VOCs present in ground-water samples.

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