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

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Summary and Conclusions

The primary nutrients detected in shallow ground water in agricultural and urban areas in the arid to semiarid Western United States were nitrate and orthophosphorus. Generally, nitrate concentrations were greater in agricultural areas than in urban areas. Currently (2007), there is no drinking-water criterion for phosphate; however, the U.S. Environmental Protection Agency (USEPA) maximum contaminant level (MCL) for nitrate is 10 mg/L. This MCL was exceeded in every study unit included in this investigation. In agricultural and urban areas, the nitrate MCL was exceeded in water from 25 and 10 percent of the wells, respectively. Nationally, 15 percent of shallow ground water sampled in agricultural and urban areas exceeded the USEPA nitrate criterion. The probability of exceeding the nitrate standard in agricultural areas can be estimated by the interaction of three primary parameters: irrigation practices, fertilizer use, and aquifer redox condition. Fertilizer application rate and sprinkler irrigation practices were positively correlated to the exceedance of the nitrate standard. High iron concentration, typically associated with low dissolved-oxygen concentration and considered indicative of reducing environments, showed a negative relation to the exceedance of the nitrate MCL. The lowest median nitrate concentrations occurred in areas with the lowest median dissolved-oxygen concentrations.

The triazines were the predominant class of pesticide detected in both agricultural and urban areas. Simazine was detected more frequently in ground water beneath agricultural areas, whereas atrazine and prometon were more commonly associated with urbanized areas. The second most commonly detected class of pesticide was the ureas. Of the urea herbicides, diuron was associated with agriculture and tebuthiuron with urbanization. Because of the relatively low detection frequencies of pesticides in shallow ground water in both agricultural and urban areas included in this investigation, sufficiently strong predictive models for the detection of pesticides in these systems could not be determined. However, in general, aquifer redox condition, temperature, general soil-permeability characteristics, and depth to screened interval were determined to be important factors to consider when examining environmental controls on pesticide occurrence. All pesticide concentrations were below established USEPA drinking-water criteria.

Volatile-organic compounds (VOCs) were more commonly associated with ground water underlying urban areas than agricultural areas. The most commonly detected VOCs in this investigation belonged to the disinfection by-product, trihalomethane (THM), category. Of the THMs, the most prevalent was chloroform. The solvents, PCE and TCE, also were commonly detected in shallow ground-water samples collected in urbanized areas. Of the gasoline-related VOCs, BTEX (benzene, toluene, ethylbenzene, xylene) and MTBE, MTBE was detected most frequently. Chloroform, PCE, and MTBE detection frequencies were influenced by aquifer redox condition. Chloroform and PCE detection frequencies increased with increasing dissolved-oxygen concentrations, whereas MTBE was detected more frequently when dissolved-oxygen concentration was low. Other important explanatory factors influencing the detection of VOCs in shallow ground water included pH and percent industrial land use. The MCL for total THMs (80 µg/L) was not exceeded in any sample included in this data analysis. However, the MCL for PCE (5 µg/L), TCE (5 µg/L), and the drinking-water advisory for MTBE (20 – 40 µg/L) was exceeded in 3.5, 1.2, and 1.2 percent of the urban samples included in this investigation, respectively.

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