Evaluation of the Ground-Water Quality in the Upper Colorado River Basin study unit, Colorado and Utah
By Lori Apodaca, U.S. Geological Survey, Box 25046, MS 415, Denver Federal Center, Lakewood, CO, 80225
As part of the U.S. Geological Survey's National Water Quality Assessment (NAWQA) Program, ground-water quality in the Upper Colorado River Basin is being evaluated, including an examination of historical ground-water data. An objective of NAWQA is to determine the occurrence and variability in concentrations of a broad array of chemical constituents from particular hydrologic units. The Upper Colorado River Basin study unit of Colorado and Utah has a drainage area of 17,800 square miles; all but 100 square miles are located in Colorado. In the basin, most of the water is obtained from unconsolidated alluvial aquifers for use in rural communities. Groundwater quality in the basin is affected by natural and human factors such as the physiographic, geologic, hydrologic, land-use, and water-use characteristics. Natural sources account for the high dissolved-solids (greater than 500 milligrams per liter) and high selenium concentrations (greater than 10 micrograms per liter) in the groundwater. Non-point- and point-agricultural sources can account for increases with time in nutrient, pesticide, and trace- element concentrations in the groundwater. Increasing urbanization in the basin can account for changes in nutrient and organic-compound concentrations. The median concentration for nitrate plus nitrite, as nitrogen was 0.8 and 0.6 milligrams per liter for agricultural and urban land-use settings, respectively. Concentrations of nitrate plus nitrite, as nitrogen exceeded 2.0 milligrams per liter in the agricultural setting. Past and present metal mining activities in the headwaters of the basin result in the presence of acidic waters and metal contamination in localized alluvial aquifers. High concentrations of copper, iron, lead, and zinc are present in the groundwater.