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

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