Hydrologic effects of pumpage from the Denver basin bedrock aquifers of northern El Paso County, Colorado

The Denver groundwater basin underlies a 6,700 sq-mi area in eastern Colorado. To assess current conditions of the four bedrock aquifers in the basin, water levels, streamflow gain and loss, and other data were collected. Current aquifer conditions in the southern part of the basin and likely response to various 100-year pumping scenarios were analyzed using a digital finite-difference model. Simulated predevelopment flow through the bedrock aquifers was about 59 cu ft/sec. Water level changes between 1978 and 1985, likely caused by variations in precipitation and in pumping and by lowering of the water table in the overlying Black Squirrel Creek alluvial aquifer, ranged from rises of more than 40 ft to declines of as much as 80 ft. In 1985, pumping from the bedrock aquifers was about 56 cu ft/sec. Simulations indicate that 43% of the pumpage came from a decrease in volume of groundwater in storage; 37% came from induced recharge and captured discharge. The remaining 20% came from a transient high rate of recharge from precipitation. A baseline 100-yr simulation, beginning in 1985, indicated minimal drawdowns for constant pumping at 1985 rates in the southern part of the basin. Other simulations indicated that the pumpage required to supply the needs of the projected population would be accompanied by drawdowns of as much as 1,300 ft and by large decreases in amount of groundwater in storage. Pumpage from a hypothetical well field, located where the aquifers are thickest , and from the aquifers underlying Colorado Springs also was simulated. (USGS)