The Laramie-Fox Hills aquifer is one of the four statutory aquifers contained in the geologic formations (Dawson, Denver, Arapahoe, Laramie, and the Fox Hills Sandstone in top-down order) that are the most widely utilized fresh water sources within the Denver Basin. The Denver Basin is a structural basin that extends roughly from Greeley to Colorado Springs (north to south), and Fort Morgan and Limon to the Foothills (east to west). The Denver Basin is part of a larger structural basin, which is often referred to as the Denver-Julesburg basin or Denver-Cheyenne Basin, where the Denver Basin and Cheyenne Basin are separated by a structural arch. This is often referred to as the Greeley Arch, lying in an east-west orientation near the City of Greeley Colorado. An arch is identified by an upward structural fold in the geologic strata, with downward slopes on each side of the arch.
The most widely recognized base of the Denver Basin aquifers in the Denver-Cheyenne Basin is the impermeable Pierre Formation, which can reach thicknesses of 7,000 feet (Topper, et al., 2003). The Greeley Arch, which separates the Denver and Cheyenne Basins, is composed of the Pierre Formation. Two of the Denver Basin Formations are present in the Cheyenne Basin; the Laramie Formation and the Fox Hills Formation. There is discussion that suggests that the Laramie Formation in the Colorado portion of the Cheyenne Basin is more similar to the Lance Formation in Wyoming. However, as the Laramie Formation and the Lance Formation have similar depositional environments (coastal plain), we will consider the Laramie Formation in the Denver Basin to be the same as the Laramie Formation in the Cheyenne Basin.
Extensive research, discussion, and modeling have been performed on the Denver Basin aquifers, which resulted in Senate Bill 5 (SB-5), enacted on June 6, 1985. As part of SB-5, maps were developed which show where modeling resulted in a nontributary determination for each of the Denver Basin aquifers, including the Laramie-Fox Hills aquifer. A nontributary determination is where the surface stream system is depleted at less than 1/10th of one percent in one hundred years as the result of well pumping. These depletions to the surface stream system are considered di minimis and as a result, wells that withdraw water from nontributary aquifers are exempt from surface stream administration. A large portion of the Laramie-Fox Hills aquifer in the Denver Basin has been determined as nontributary and is therefore exempt from surface water administration.
The Cheyenne Basin has not experienced the same level of research, discussion, and modeling as the Denver Basin. Excepting for the Upper Crow Creek Designated Basin, which is the only Designated Ground Water Basin in the Cheyenne Basin, there has not been any rulemaking in the Cheyenne Basin regarding the locations of the nontributary and tributary portions of the Laramie-Fox Hills aquifer. Bedrock aquifers in Designated Basins fall under Colorado Groundwater Commission Rules and Regulations, while the remainder of the Cheyenne Basin falls under the Statewide Nontributary Ground Water Rules for nontributary determinations.
Under the Statewide Nontributary Ground Water Rules, the Applicant may apply for a well permit to withdraw nontributary groundwater. However, lacking rules defining the boundaries of the nontributary portions of an aquifer, it is the Applicant’s burden of proof to show that the bedrock aquifer for which the Applicant is seeking a well permit meets the nontributary criteria. As a result, any Applicant seeking a Laramie-Fox Hills aquifer well permit in the Cheyenne Basin through a nontributary determination must demonstrate that the Laramie-Fox Hills aquifer meets the nontributary criteria set forth in State statute.