Arkansas Basin_Water Supply and Needs

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Section 7 Availability of Existing Water Supplies in the Arkansas Basin I 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 1 O-square-mile area. Figures 7-2 and 7-3 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 Metro 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 within its service area. Conjunctive use of available surface water supplies would reduce this cost and, more importantly, decrease the annual demand on the O:\SHAWN\ARKANSAS\S7 _ARKANSAS.DOC aquifers by approximately 50 percent (Black and Veatch 2004). As water levels continue to decline, water pressure will drop and the possibility exists that the saturated rocks will no longer be able to support the weight of the overlying strata. Compaction will occur and, if significant enough, could lead to subsidence that propagates upwards to the land surface. This phenomenon has been seen in many urban areas where groundwater pumping is concentrated and can lead to considerable damage to existing streets, buildings, and infrastructure. The available supply in the Denver Basin bedrock aquifers is further governed by the legal availability of the water. The legal availability is determined in part by the location in the basin and in part by the well age. Approximately the eastern half of the Denver Basin aquifers are part of one of four designated basins (Kiowa-Bijou, Lost Creek, Upper Big Sandy, and Upper Black Squirrel). In the western half of the Denver Basin, wells that have been permitted since 1973 and do not have an affect on the overlying surface streams are considered non-tributary and have been allowed to withd raw 1 percent of the water per year based on how much water is underlying the land owned or controlled by the appropriator, thus providing for at least a 100-year life for the aquifer. 7.2.3 Designated Groundwater Basins Designated basin groundwater is located in eight specified areas in eastern Colorado, as shown in Figure 7-4. Designated basin groundwater is administered by the Colorado Groundwater Commission, with daily management typically given to the Ground Water Management District or districts within the basin. Rules governing usage differ by basin but typically distinguish between tributary and non-tributary aquifers, if both are present, and permit usage based on aquifer volume within an allowed radius and a specified annual rate of aquifer depletion. 7.2.3.1 Designated Basins Other Than the High Plains There are six designated basins in this category, including four that comprise the eastern part of the Denver Basin geologic region (Lost Creek, Kiowa-Bijou, Upper Big Sandy, and Upper Black Squirrel) and two CDIVI 7-3