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 toinfants. Water from the alluvial aquifer generally is of suitable quality forother uses.