URS completed the Phase I Hydrogeologic Characterization under contract with the Garfield County Board of County Commissioners and published the final report in March 2006. The project area is approximately 110 square miles in size and is located south of the cities of Rifle and Silt in western Colorado. The scope of work for the hydrogeologic characterization was developed by Garfield County, the Colorado Oil and Gas Conservation Commission (COGCC), the Grand Valley Citizens Alliance, and the Western Colorado Congress. The primary objectives of the study are to address vulnerability of surface water and groundwater resources in the area to impacts from gas well development and other human activities, and to evaluate if a relationship exists between water quality variations and lithology type (i.e. sandstone, mudstone, or alluvium). The study area comprises the majority of the Mamm Creek gas field in the southeastern portion of the Piceance Basin.