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Annual runoff of the South Platte River varies widely, from less than 75,000 acre -feet per <br />year to over 800,000 acre -feet per year. During wet runoff years, water is anticipated to <br />be available for injection into the Denver Basin aquifers, while in the dry years or <br />drought periods, some or all of the water that was previously injected would be available <br />for use. This would take some of the strain off of the Denver Water surface supply <br />system, which may be operating below capacity due to reduced water levels in its <br />reservoir system. <br />The availability of water on a seasonal basis fluctuates as well. Peak flows on the South <br />Platte River typically occur in June, with base flows occurring over approximately <br />seven months of the year. These data indicate that water would likely be available for <br />injection during the winter time base flow period (when there are low municipal <br />demands and municipal treatment facilities are either idle or at low capacity) and during <br />the peak flows, when the reservoir system would otherwise be spilling. During peak <br />flows, the availability of treatment capacity to accommodate injection would have to be <br />evaluated. <br />Project Description <br />The demonstration project used potable surface water supplies provided by Denver Water <br />from its water rights holdings. The water used was diverted at Denver Water's terminal <br />reservoir at Strontia Springs, treated at the Foothills Water Treatment Plant, and delivered <br />through Denver Water's transmission system to the vicinity of Willows. An additional <br />water line was installed as part of this project to connect the Denver Water system to the <br />Willows system. <br />Willows' production well A -6A, a well recently completed into the Arapahoe aquifer, <br />functioned as the recharge facility. The production well was modified to accommodate <br />both a production string and an injection line, so the same well can be used to inject the <br />water and subsequently extract the water for use within the municipal water supply <br />system. Figure 1 shows a schematic of injection well retrofit. <br />A monitoring well, approximately 110 feet from well A -6A, provided a remote point to <br />observe water level, water temperature, and water quality data. This well was installed <br />before injection started and was used throughout the project as an additional data point. <br />Arapahoe Aquifer Hydrogeology <br />The target aquifer for the demonstration project, the Arapahoe aquifer, is part of the <br />Denver Basin, which covers approximately 6,700 square miles along the Front Range. <br />The principal water- bearing units of the Denver Basin, in stratigraphic succession from <br />the lowest to the highest, include the Laramie -Fox Hills sandstone of Late Cretaceous <br />Age, the Arapahoe Formation of Late Cretaceous Age, the Denver Formation of Late <br />Cretaceous and Early Tertiary Ages, and the Dawson Formation of Tertiary Agee <br />The Arapahoe aquifer, contained within the Arapahoe Formation, generally consists of a <br />400- to 700 -foot thick sequence of interbedded sandstones, siltstones, and shale. The <br />Arapahoe aquifer typically may have 7 to 10 distinct sandstone units that can produce <br />Program Summary Report Part 111 — Demonstration Project Summary Reports <br />