Mapping Of Bedrock Aquifers In The Denver Basin Using Borehole Geophysical Logs

Abstract

Ground water is a natural resource that is present in bedrock aquifers that underlie much of the Front Range Urban Corridor in Colorado. In 1996, mapping of bedrock-aquifer outcrop and subcrop areas along the western margin of the Denver Basin was undertaken by the U.S. Geological Survey in cooperation with the Colorado Department of Natural Resources, Division of Water Resources, and the Colorado Water Conservation Board. Detailed mapping of outcrop and subcrop areas of the Denver Basin bedrock aquifers was done to better define areas where the aquifers receive recharge directly from precipitation at the ground surface and where they are in direct hydraulic connection with alluvial aquifers. Geophysical logs from oil, gas, and water wells and lithologic logs from exploration borings were used to map the subsurface structure and thickness of the bedrock aquifers. Structural contour maps of the bedrock aquifers then were used in combination with topographic maps of the land surface, information from field observations, and published geologic maps to determine where bedrock-aquifer outcrop and subcrop areas occur. The results of the study indicate that 1) the bedrock aquifers along the western margin of the Denver Basin generally dip gently eastward into the basin and have steepening dips in the vicinity of the mountains of the Front Range and 2) the width of bedrock-aquifer recharge areas varies greatly as a function of dip. The presence of deltaplain paleochannels was inferred from a thickness map of one of the bedrock aquifers. The study demonstrates that geophysical logs can be a useful tool in mapping the subsurface structure of bedrock aquifers, especially in areas where it is difficult to distinguish between formations on the basis of lithology and in areas where bedrock is covered by a thick mantle of unconsolidated sediments.