The Mountain Geologist - Laserfiche WebLink

Home Browse Search

Glenn Graham and George Vanslyke was confined to alluvial groundwater that was to be regu- lated as surface water. Bedrock - derived groundwater was looked upon as a resource to be used at will. Evaluation of the Denver Basin bedrock aquifers grew out of a larger effort to address water availability issues for the entire Front Range area. In the late 1970s, at the urging of the Denver Regional Council of Governments and the Colorado General Assembly, a groundwater task group was formed and charged with the preparation of a report including recommendations for the development of groundwater from the bedrock aquifers. The task group concluded that: • The Denver basin bedrock aquifers contain large amounts of groundwater that could supplement surface water supplies. • Wells dependent primarily on potentiometric head, and which do not fully penetrate the aquifer, will run dry as the potentiometric heads are drawn down. • Users of these depleting wells will have to re -drill and deepen their wells or secure alternate sources of water. • Wells drilled at the edge of the Denver Basin where the water - saturated parts of the aquifers are thin may also experience early depletions. • As the potentiometric head decreases, serious social and economic problems may arise. This however, should not prevent the development of Denver basin bedrock aquifers. • Assuming wells can be drilled to the bottom of the aquifers in the thicker sections of the Denver Basin, economics (well construction and pumping costs) will be the limiting factor in determining the useful life of the groundwater resource. • Properly studied and managed, the bedrock groundwa- ter could be an asset in meeting total water supply requirements of the metropolitan area. The task group recommended an extensive study of the groundwater resources of the Denver Basin, including technical, legal, and administrative actions, and also the potential environmental consequences at various rates of utilization and depletion as a result of reliance on this resource. During the late 1960s and early 1970s geologists of the CDWR and the USGS began to study the bedrock of the Denver Basin and to map the distribution of the aquifers. The first aquifer to be identified and mapped was the Laramie -Fox Hills in 1971 (Romero and Hampton, 1972). This aquifer consisted of the Fox Hills Sandstone and the lower -most sandstone beds in the Laramie Formation. The three aquifers above the Laramie -Fox Hills were lumped The Rocky Mountain Association of Geologists 156 into the "Dawson Group" or "Dawson arkose" aquifer without subdivision (Fig. 2). Since it was widely known that different subunits such as the Arapahoe aquifer existed within the group, work was begun to further sub- divide and separate aquifers. Because tributary groundwater was now to be adminis- tered in the priority system, the value of a right to non- tributary groundwater, or groundwater not hydraulically connected to the surface water system, was significant, as that water was not subject to administration in the priority system, and could be claimed only by the owner, or by permission of the owner of the overlying land. Prior to the effective date of the Denver Basin Rules, all groundwater in Colorado was presumed to be tributary groundwater. Claims for nontributary groundwater were evaluated on a case -by -case basis. Clear and convincing hydrologic and geologic data to substantiate a nontributary claim had to be provided by a claimant. However, the existence of non - tributary groundwater was formally recognized in the law known as Senate Bill 73 -213. This law stated nontributary groundwater would be administered on the basis of own- ership of the overlying land, and maintenance of a 100 - year aquifer life. A landowner who was able to prove to the satisfaction of the State Engineer that the groundwater underlying his land was nontributary groundwater could divert only that quantity of water calculated to be available beneath his lands. In addition, in any single year, the landowner could withdraw only one percent of the water determined to be available, in order to maintain a mini- mum useful aquifer life of 100 years. The 100 -year life begins at the issuance of a well permit, or a decree of the water court. The allowed annual withdrawal of nontribu- tary groundwater is subject to a banking provision that permits groundwater left in the aquifer to be pumped at greater than the one percent of the total volume calculated to be available, e.g. a landowner receives a well permit or a court decree, but does not pump any of the water avail- able to him for 5 years. In the fifth year, he can pump a total of five percent of the total volume of water available. The authors presume that most of the legislators involved in drafting the legislation felt that additional sources of water would be developed to replace the nontributary groundwater being mined. In late 1983 a groundwater legislation committee was formed to propose legislation to more efficiently regulate the development of groundwater in bedrock aquifers. The use of nontributary groundwater was a major considera- tion, given the vagueness of existing statutes and the ambi- guities in case law. The legislation committee was able to agree on some assumptions around which proposed legislation would be crafted. Some of the key assumptions were: • The definition of nontributary groundwater should be clarified.