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Section 7 Availability of Existing Water Supplies the amount of groundwater in storage and its availability will have a larger amount of uncertainty associated with them than will estimates of surface water availability. The groundwater resources in each basin have been characterized based on published reports and data for the major aquifer systems. 7.2.2 Denver Basin Bedrock Aquifers The Denver Basin contains four major aquifer units. These cover an area of approximately 6,700 square miles extending from Greeley south to Colorado Springs and from Limon west to the edge of the foothills. The aquifers consist of layers of sedimentary rocks that are, from youngest to oldest, the Dawson, Denver, Arapahoe, and Laramie-Fox Hills. Figure 7-2 shows a cross-section of the aquifer through the center of the basin. There have been several estimates of the available water in storage. One of the early estimates of water availability was from the USGS (Robson 1987). In this study, information from driller's logs, laboratory tests of core samples, aquifer pumping tests, water level measurements, and groundwater flow modeling were used to delineate the configuration and storage coefficients of each aquifer, from which estimates of the available volume were made. The USGS study concluded that approximately 467 million AF of water existed in the Denver Basin aquifers, and of this approximately 269 million AF of water could be recovered. In 1985 the Colorado General Assembly promulgated Senate Bill 5, which set forth criteria for management of these bedrock aquifers. As part of this Bill, the storage coefficient was determined for each aquifer. The total amount of recoverable groundwater was estimated to be 295 million AF. The aquifer storage coefficient has a strong influence on the estimated volume of water contained in an aquifer. Detailed studies conducted on core samples from a borehole located in the center of the basin near Kiowa (Lapey 2003) indicated that the storage coefficient might be as much as 30 percent lower than previously thought. This translates into a possible 30 percent reduction in the amount of recoverable water in storage, to approximately 206 million AF. Even the lower estimates of the amount of available water in storage in the Denver Basin aquifers are quite large. Unfortunately, the sediments that make up each of the aquifers tend to be relatively fine grained and include many interlayered clay and shale units that have very low permeability. As a result, the water-bearing ability of the Denver Basin bedrock aquifers is relatively low. This leads to large drawdown in water levels from pumping of these aquifers. In the Arapahoe aquifer, water levels have declined by as much as 30 feet per year. In addition, some areas of the Denver Basin aquifers have declined by over 250 feet and this decline has been seen over a 10-square-mile area. Figures 7-3 and 7-4 show recent groundwater level trends for the significantly impacted aquifers (Arapahoe and Laramie-Fox Hills). Non-tributary groundwater rights and withdrawal volumes are linked to the surface land area ownership. Thus, the amount recoverable may be less and the cost of recovery increased than previously estimated. Water levels are still above the physical top of each aquifer in most parts of the Denver Basin, thus exhibiting confined aquifer conditions. As water levels continue to drop, there are concerns about loss in well yield, increases in pumping costs, and aquifer subsidence. Well yield will likely decrease as the height of water in an aquifer declines. There are also concerns about a loss in well yield if water levels drop below the top of existing well screens. Air would then enter the system and cause minerals to precipitate and possibly bacteria to form on the well screens. Pumping costs are likely to increase because, with declining water levels, there is a greater pump lift required so existing pumps must run longer or more powerful pumps will be needed. Eventually, wells would need to be deepened or replaced with deeper wells. Higher pumping costs are also likely when, due to declining yields, there will be a need to install and operate more wells to achieve the same production rates. In the South Metra Denver area, it is anticipated that aquifer production will decline by 40 to 85 percent by the year 2050, and that municipal wells in this part of the Denver Basin that can produce even 100 gpm will be considered to be a good producing well. Current production rates average 540 gpm for the Arapahoe aquifer and 120 gpm for the Lower Dawson. To maintain current production, an increase in number of wells would be needed. It is estimated that it will cost $2.7 to $4 billion for infrastructure by 2050 for supplies provided by the non-tributary groundwater source ~ $~ole'ri~ice Wo~e' $upplY Initia~ive ~~ S:\REPORT\WORD PROCESSING\REPORT\S7 11-10.04.DOC 7-~J