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

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Introduction

Urbanization in the Western United States is occurring rapidly with projected population growth of about 46 percent between 2000 and 2030 (U.S. Census Bureau, 2005). The greatest increase in population (45.8 percent) is expected to occur in the Western United States compared to other regions in the United States, with an additional 29 million people expected during this period (U.S. Census Bureau, 2005). Nevada and Arizona are expected to undergo the greatest population growth in the country, with estimated growth rates of 114 and 109 percent, respectively. Throughout the country, land is being converted from pastures, agricultural fields, forests, and desert to commercial and residential communities. With increasing population and changing land use, demand for water resources will increase for use in agriculture (including irrigation, grazing, and stock watering), industry, and domestic supply.

In most ground-water basins, the quality of shallow ground water is influenced over relatively short time scales by near-surface activities and, therefore, can be used as an indicator of land-use stresses on shallow aquifers (Barbash and Resek, 2000). Although shallow aquifers are not typically used as municipal drinking-water supplies, concern arises where these shallow systems may be hydrologically linked to deep aquifers which are, or could be, used as sources of potable water. National studies have indicated that nonpoint chemical contamination of ground water in urban and agricultural land-use settings is occurring (Miller and Hamilton, 2001).

Natural factors that influence general ground-water quality include evapotranspiration and water-rock reactions. Although considerable variability existed among areas sampled, Walvoord and others (2003) determined that substantial subsoil nitrate reserves are present in desert environments. These reserves may pose a threat to ground-water resources through leaching of nitrate through the vadose zone (Walvoord and others, 2003). Numerous human factors affect ground-water quality, including infiltration of agricultural and urban irrigation water, irrigation with reclaimed wastewater, application of pesticides and fertilizers, artificial recharge, mining activities, leakage from septic tanks and sewer systems, and spills of chemicals during household, commercial, and industrial use. As a consequence of the increasing use of ground water in urban areas for drinking water and lawn irrigation, ground-water systems have become increasingly engineered through human activities such as artificial-recharge operations (Edmonds and Gellenbeck, 2002; Thiros and Manning, 2004; Hamlin and others, 2005).

Squillace and others (1999) estimated that untreated ground water in approximately 15 percent of the contiguous United States contains volatile organic compounds (VOCs) and that untreated ground water in urban areas was four times more likely to exceed a drinking-water criterion than untreated ground water in rural areas (Squillace and others, 1999). In agricultural and urban areas throughout the United States, about 15 percent of shallow ground water samples exceeded the U.S. Environmental Protection Agency (USEPA) drinking water standard for nitrate concentrations (U.S. Environmental Protection Agency, 2006) and moderate to high levels of herbicide contamination were detected in the samples (U.S. Geological Survey, 1999).

Since 1991, the National Water-Quality Assessment (NAWQA) program of the U.S. Geological Survey (USGS) has determined water-quality status and trends in major aquifer systems throughout the United States (Gilliom and others, 1995). Pilot efforts were undertaken as early as 1986 in some areas in the United States (Hirsch and others, 1988). Part of the NAWQA sampling effort included sampling of shallow aquifers in Arizona, California, Colorado, Nevada, New Mexico, and Utah. These shallow aquifers were included in this study because they are primarily alluvial aquifers and represent major metropolitan and agricultural areas of the southwestern United States. Water-quality data used in this study were collected to assess the effects of primary land use and hydrologic conditions on the concentration and distribution of anthropogenic compounds in shallow ground water within individual NAWQA study units (Gilliom and others 1995).

Purpose and Scope

The objective of this investigation was to evaluate the effect(s) of agricultural and urban land uses on shallow ground-water quality in the arid to semiarid Western United States. Ground-water-quality data collected from 1993 to 2004 from 273 agricultural and 181 urban wells in Arizona, California, Colorado, Nevada, New Mexico, and Utah were used. This report summarizes water-quality data for nutrients, pesticides and their degradation products, and VOCs. Potential explanatory factors influencing shallow ground-water quality such as aquifer oxidation-reduction (redox) condition, land use, general soil characteristics, and irrigation practices are identified.

Previous Studies

Shallow aquifers unaffected by human activities typically contain less than 2 mg/L nitrate (Mueller and Helsel, 1996; Edmonds and Gellenbeck, 2002), but can contain much higher concentrations. The occurrence of elevated nitrate concentrations in shallow ground water is caused by population density, surficial geology, depth to water, and ground-water chemistry (Nolan, 2001; Eckhardt and Stackelberg, 1995; Tesoriero and Voss, 1997). In the Western United States, the use of commercial fertilizers was determined to be the predominant source of nitrogen in agricultural areas (Puckett, 1994) and, in some cases, in urban areas (Hamlin and others, 2002). Seepage of irrigation drainage containing anthropogenic nitrate to ground water has been identified as an important source of nutrients in agricultural areas (Edmonds and Gellenbeck, 2002, p. 9). In forested watersheds and remote headwater areas with no apparent anthropogenic sources of nitrogen, atmospheric deposition was determined to be the dominant source of nitrogen (Puckett 1994). Secondary recharge from septic systems and lawn irrigation has been shown to elevate nitrate concentrations in urban areas (Thiros, 2003a; Shipley and Rosen, 2005; Maurer and Thodal, 2000).

Between 1992 and 2001, approximately one billion pounds of pesticides were used annually within the United States. It was estimated that in 2001, approximately 76 percent of registered pesticide use was for agricultural purposes (Gilliom and others, 2006). Pesticides frequently were detected in shallow ground-water samples from both agricultural and urban areas. Fumigants were more commonly detected in agricultural areas, insecticides were more common in urbanized areas, and herbicides were detected in both (Kolpin and others, 2000). NAWQA ground-water investigations determined that at least one or more detection of a pesticide or pesticide degradate were present in more than 50 percent of shallow and 33 percent of deep aquifers in areas with mixed land uses (Gilliom and others, 2006). Nationally, human-health based benchmarks established for pesticides were exceeded in about 5 percent of shallow ground-water samples from urban areas and 1 percent of ground-water samples from agricultural areas (Gilliom and others, 2006).

In samples of ambient ground water in the United States collected between 1985 and 1995, VOCs were detected more frequently in ground water collected from urban areas than rural areas (Squillace and others 1999). The VOCs most commonly detected in ground-water samples from urban areas were chloroform, methyl tert-butyl ether (MTBE), tetrachloroethene (PCE), and trichloroethene (TCE). Dissolved-oxygen concentration, source of ground-water recharge, and type of land use were the most important factors associated with the distribution of VOCs in shallow ground water in newly developed urban areas (Squillace and others, 2004; Hamlin and others, 2002). Other studies determined well depth, depth-to-screened interval, and population density also to be important explanatory variables affecting the detection of VOCs (Shelton, 2005; Hamlin and others, 2002; Squillace and others, 1999).

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