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RMW - May 2017

22 | ROCKY MOUNTAIN WATER MAY 2017 HOT TOPICS Considerations for Subsurface Water Storage in Colorado By Ralf Topper, C.P.G., and Kevin G. Rein, P.E . A RECENT SUBSURFACE WATER STORAGE SYMPO-sium held at Colorado State University in Fort Collins highlighted a renewed interest in this alternative water management strategy. Increasing storage is an integral theme of Colorado’s 2016 Water Plan. Subsurface water storage in aquifers can significantly reduce the financial, permitting, envi-ronmental, security, and socio-economic hurdles associated with construction of new surface water reservoirs. Aquifer storage, as implemented through aquifer recharge, is designed to introduce water into and store water in underlying aquifers. Aquifer storage and recovery (ASR) adds the extraction component to the water being stored and is typically implemented through wells or, since most groundwater formations in Colorado interact with the stream system, the water may also be taken through accretions to a surface stream. In 2004, the Colorado Geological Survey (CGS) published a reconnaissance level, statewide assessment of available storage capacities in the state’s alluvial and bedrock aquifers (Topper et al., 2004). CGS identified 13 unconsolidated alluvial aquifer sys-tems and 24 consolidated bedrock aquifer systems throughout the state with potential storage capacities in excess of 100,000 acre-feet each. In 2006, the Colorado General Assembly autho-rized an underground water storage study that focused on the alluvial and bedrock aquifer systems on the eastern plains (CWCB, 2007). The SB-193 study incorporated 10 evaluation criteria for hydrogeologic, environmental, and implementation considerations. This study concluded that numerous areas for potential underground water storage exist in both alluvial and bedrock aquifers in the South Platte and Arkansas River basins and that available underground storage capacities are on the order of tens to hundreds of thousands of acre-feet. Aquifer recharge projects can meet numerous objectives including: water supply management, satisfying legal obliga-tions, managing water quality, aquifer restoration, and envi-ronmental protection (Figure 1). Colorado water users have been very effective in operating projects—predominantly in the South Platte and Arkansas river basins—that introduce water into alluvial aquifers so it is delivered in a timed fashion to mitigate impacts to the stream caused by diversion through junior water rights (wells). This is a legal obligation to prevent injury to senior water rights. Recharge facilities have also been constructed to protect or enhance wetland habitat, largely for waterfowl, and to help meet downstream endangered species obligations. These projects, however, do not have a typical stor-age objective. A brief discussion of the basic legal classifications of water is nec-essary to understand the difficulty of long-term storage in Colo-rado’s aquifers (Hobbs, 2015). Of the four legal classifications of groundwater: tributary, nontributary, Denver Basin, and designat-ed, it is helpful to first consider tributary and nontributary. Tribu-tary groundwater is hydrologically connected to a surface stream. Nontributary groundwater is groundwater whose connection to any surface stream is so insignificant that it is considered isolated from the surface water for water rights administration purposes. Nontributary groundwater is quantitatively defined in section 37-90-103 (10.5) of the Colorado Revised Statutes (CRS). Unfortunately, long-term storage is impractical in a tributary aquifer. Any water introduced into the aquifer will naturally migrate down gradient and discharge to surface water, quickly moving downstream. Recovery of water placed into a tributary aquifer would very likely cause an impact to senior surface water rights. The Prior Appropriation Doctrine, established by Colo-rado’s Constitution, gives the framework for regulating the use of surface water and tributary groundwater, and protects senior South Platte River, Colorado


RMW - May 2017
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