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The upper Black Squirrel Creek basin in eastern El Paso County, Colorado, is underlain by an alluvial aquifer and four bedrock aquifers. The climate of the area is semiarid, and streamflow is irregular. The alluvial aquifer has supplied water to wells since the late 1800's when ranchers first pumped water from shallow wells to grow grass hay for livestock. Ground-water pumpage from the alluvial aquifer has increased since the mid-1950's, and water-level declines have been substantial; the bedrock aquifers virtually are undeveloped. Ground-water pumpage for domestic, stock, agricultural, and municipal uses has exceeded recharge for the past 25 years. The present extent of the effect of pumpage on the alluvial aquifer was evaluated, and a ground-water flow model was used to simulate the future effect of continued pumpage on the aquifer. Measured water-level declines from 1974 through 1984 were as much as 30 feet in an area north of Ellicott, Colorado. On the basis of the simulations, water-level declines from October 1984 to April 1999 north of Ellicott might be as much as 20 to 30 feet and as much as 1 to 10 feet in most of the aquifer. Flow from the bedrock aquifers to the alluvial aquifer may account for a substantial volume of the recharge to the alluvial aquifer. The ground-water flow models provided a means of evaluating the importance of ground-water evapotranspiration at various stages of aquifer development. Simulated ground-water evapotranspiration decreased from 1949 to 1984; prior to 1950 ground-water evapotranspiration was about 43.5 percent of the total outflow, but in 1984 it was less than 3 percent. Thirty-six ground-water samples were collected during 1984. Chemical analyses indicated that concentrations of dissolved nitrite plus nitrate as nitrogen generally were large. Samples from 5 of the 36 wells had concentrations of dissolved nitrite plus nitrate as nitrogen that exceeded drinking-water standards; these concentrations could pose a health threat to infants. Water from the alluvial aquifer generally is of suitable quality for other uses. INTRODUCTION The upper Black Squirrel Creek basin is located about 25 mi east of Colorado Springs (fig. 1) and has an area of about 353 mi 2 . Streams in the area flow in response to thunderstorms, snowmelt, and prolonged rainfall. All streams are ephemeral and do not provide a dependable source of water. The upper Black Squirrel Creek basin is underlain by an alluvial aquifer and four bedrock aquifers. Most of the water used in the basin is pumped from the alluvial aquifer; the bedrock aquifers virtually are undeveloped. Since the late 1800's, the alluvial aquifer has provided a dependable source of suitable quality water for domestic, agricultural, and municipal uses. Water from the alluvial aquifer is exported to suburbs east of Colorado Springs and to the Falcon Air Force Station by the Cherokee Water District and the Pikes Peak Water Company. Water from the alluvial aquifer also is used extensively within the basin for irrigation and other agricultural purposes. The study area shown in figure 1 approximately corresponds to the part of the upper Black Squirrel Creek basin underlain by the alluvial aquifer. Since the mid-1950's, after extensive development of the alluvial aquifer, pumpage of water from storage in the aquifer has resulted in large declines of the water table. Because of the declining water levels and increasing demand for ground water in the basin and adjacent areas, the U.S. Geological Survey in cooperation with the Cherokee Water District began a study to: (1) Define the geohydrologic system in the upper Black Squirrel Creek basin, emphasizing the alluvial aquifer; and (2) determine the quality of water in the alluvial aquifer. Purpose and Scope This report describes the results of a study of the geohydrologic system of the upper Black Squirrel Creek basin. Because almost all ground water pumped in the basin is from the alluvial aquifer, this aquifer is the focus of the report. Effects of water withdrawals from the alluvial aquifer on water levels during spring 1964 to spring 1984 and from October 1984 to April 1999 are discussed. A ground-water flow model of the alluvial aquifer was used to estimate effects of continued pumpage on water levels in the alluvial aquifer. The chemical quality of water from the alluvial aquifer, based on analyses of samples from 36 wells, also is described. Data collected during the study (water-level measurements and water-quality analyses of chemical constituents in ground-water samples) are included in the report. Methods of Study A literature search of reports pertaining to the geohydrology and water quality of the upper Black Squirrel Creek basin was made early in this study; selected references at the end of the report include most of the published information about the area. Many of the data used to evaluate the aquifer were collected during previous investigations and as part of ongoing datacollection programs. Water-level measurements were made and used to define the configuration and altitude of the spring 1984 water table. Development of a ground-water flow model involved the use of independent estimates of model parameters, such as the thickness of the alluvial aquifer, altitude of the water table, aquifer boundaries, hydraulic properties, rates, sources, and distribution of recharge, and rates of ground-water underflow, ground-water evapotranspiration, and ground-water pumpage. These estimates were developed using precipitation data, geologic maps, drillers logs of wells and other existing information on the area.

water-resources investigations report

Geohydrology, water quality, and preliminary simulations of ground-water flow of the alluvial aquifer in the upper Black Squirrel Creek basin, El Paso County, Colorado