HYDROLOGIC EFFECT OF FAULTS IN SOUTH PARK, COLORADO

Folds and faults pervade mountainous regions in the western US and complicate the path of groundwater travel. Faults can act as conduits, barriers, and conduits/barriers for groundwater flow. South Park Basin in Colorado is used to study fault’s hydrogeologic behavior. The geology of the basin consists of Precambrian crystalline bedrock; Cretaceous sandstones, shales, and limestones; and Tertiary volcanics and sandstones. In general, the crystalline bedrock is located on the east side of the basin. The older formation has been thrust over the younger sediments. A low-angle, east dipping reverse fault, known as the Elkhorn Fault, marks the easternmost boundary of the Precambrian material in the study area. The purpose of this study is to gain a preliminary understanding of the fault’s hydrologic behavior by analyzing the groundwater levels on both sides of the fault.

Monthly water levels were recorded by the USGS for 32 wells located within the basin from July 1997 to June 2001. The wells were installed in crystalline and sandstone aquifers and ranged in depth from 9 to 1950 feet below ground surface. Groundwater elevations were derived from these measurements and used to develop potentiometric surfaces.

Preliminary analysis of the comparison of the time series data between the wells located on both sides of the fault suggests the possible difference between the infiltration rates and change in storativity of the aquifers. Greater oscillations occur in the wells located in the crystalline aquifers in the hanging wall than the wells in the sedimentary aquifers in the footwall. The patterns of water level rise and fall are similar in the wells located within the same aquifer. However, the water level fluctuations in the crystalline aquifer wells are out of phase with those in the sedimentary aquifer wells. Groundwater flow typically follows the topography in the study area. Groundwater flow did not vary drastically throughout the study period except during short-term responses to precipitation events. When the fault crosses higher relief areas the hydraulic gradient is relatively steep. Accordingly, the hydraulic gradient is gentler when the fault encounters lower relief areas. Initial review of the data suggests that the fault is not directly promoting or impeding the regional flow of groundwater in South Park Basin.