THE IMPORTANCE OF ACCURATE GEOLOGIC MAPPING FOR AQUIFER DELINEATION AND CALCULATED SURFACE WATER DEPLETIONS, NORTHERN WELD COUNTY, COLORADO

Based on geologic mapping circa 1978, it was determined that significant discrepancies existed between the geologic descriptions and known aquifer boundaries at a property located in north central Colorado. The geology of the area was refined using surface mapping, a review of more recent literature, analysis of subsurface geophysical logs, analysis of water well lithology logs, and discussions with regulatory geologists. Published geologic mapping of the area indicates Laramie Formation is located at the surface of the majority of the site. However, it was determined that the White River Group exists at the surface and extends several miles beyond the southern boundary of the Property, significantly further than the 1978 mapping suggested. A large paleo-stream channel deposit, the ‘natural fort’ often pondered by geologist passing between Wellington, Colorado and Cheyenne, Wyoming along Interstate 25, was also mapped through the area as the Chadron Formation. The remainder of the White River Group, predominately tuffaceous claystone and siltstone, is reinterpreted as the Brule Formation member. The White River Group formations have been determined to be regional aquicludes, and perched water from the Ogallala Formation is easily observed in outcrop. Based on the remapping of these geologic formations, it was determined that the Upper Laramie aquifer at the property is not in contact with the local surface water, which in part helped secure a nontributary groundwater determination from the State of Colorado.