A recent study by the U.S. Geological Survey (USGS) under the National Water-Quality Assessment (NAWQA) program characterized the groundwater quality in a part of the Denver, Colorado, metropolitan area. The study provides an assessment of water-quality conditions in an alluvial aquifer that drains into the South Platte River. Thirty wells randomly distributed in residential, commercial, and industrial land-use settings were sampled once in 1993 for a broad range of compounds. Nutrients, pesticides, and volatile organic compounds (VOC's), all of which are generally associated with human activities, frequently were detected in the urban wells sampled. Nutrients and VOC\"s occasionally exceeded drinking-water standards.
FS-106-95 (PDF Version - 90Kb)
Colorado currently is one of the fastest growing states in the country, and the largest population center in the state is the Denver metropolitan area, which also is experiencing rapid growth. The continued urbanization of the Denver area might affect the quality of the groundwater resource that it overlies. It is difficult, however, to measure specific effects of the urban land-use setting without an assessment of backgroundwater-quality conditions. The alluvial aquifer beneath Denver is particularly vulnerable to human factors affecting the quality of the groundwater resource because of its shallow water table and high permeability. A decrease in the quality of water in the alluvial aquifer could have important consequences to local users. Figure 1
In 1993, the USGS sampled 30 existing wells randomly distributed throughout the Denver metropolitan area, all of which were completed in the unconsolidated sands and gravels that fill the river and creek valleys. Most water pumped from these alluvial deposits is for industrial and commercial uses; a small amount is withdrawn for public consumption (after treatment and dilution) and for the irrigation of lawns and gardens
Nutrients, in particular nitrogen and phosphorus compounds, often are associated with human and animal wastes, fertilizers, and detergents. These compounds can come from natural sources; however, the natural contribution from the sediments in the alluvial-aquifer system probably is very small. The concentrations of nutrients detected in this study, therefore, are considered to be derived primarily from the urban land-use practices.
Nitrate, the most common nitrogen compound, has a U.S. Environmental Protection Agency (USEPA) maximum contaminant level (MCL) for drinking water of 10 milligrams per liter (mg/L) as nitrogen. In this study, only 3 of the 30 groundwater samples exceeded this level. The concentrations averaged 4 mg/L and ranged from nondetectable (less than 0.05 mg/L) to 24 mg/L. Higher nitrate concentrations were detected in residential and commercial areas; the industrial areas had lower concentrations or no nitrate detections. Figure 2
Dissolved phosphorus concentrations generally were low. The average concentration was 0.3 mg/L, and the range was from nondetectable (less than 0.01 mg/L) to 7 mg/L. Eighteen of the 30 samples were at or below the detection limits for phosphorus. Elevated phosphorus concentrations in groundwater often have been associated with human activities, but phosphorus does not seem to be a groundwater quality concern at this time.
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