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GROUNDWATER PUMPING <br /> Key Points <br /> • The SB06-193 study conducted by the Colorado Water Conservation Board(CWCB) <br /> estimated 10 MAF of water is stored in the S. Platte alluvial aquifer. <br /> • Prior to 2003, approximately 8,200 high capacity wells pumped on average nearly 500,000 <br /> AF/yr from the alluvial aquifer. There are now approximately 6,500 high capacity wells in <br /> the alluvial aquifer, and total annual groundwater pumping in the basin is now closer to <br /> 450,000 AF/yr,with agricultural pumping estimated in the 400,000 AF/yr range. <br /> • In 2005, 60% of the 830,000 irrigated acres in the basin were watered solely with surface <br /> water, 18% solely with groundwater, and 22%with a mix of surface and groundwater. <br /> • Most of the irrigation wells in adjudicated augmentation plans now have full or near full <br /> allocations in most years, however, the Central Colorado Water Conservancy District has <br /> approximately 1,200 wells in the WAS and GMS plans that are on a restricted quota and <br /> not able to pump 100% of full crop ET. <br /> • The greatest groundwater pumping occurs in Water District 1,which correlates with the <br /> large amount of acreage served only by groundwater. The greatest percentage reduction in <br /> pumping has occurred in Water District 2 as Central WAS and GMS and other <br /> augmentation plans work to develop reliable augmentation supplies. Water District 64 has <br /> the most recharge and surface augmentation sources due to their downstream position in <br /> the basin. Pumping amounts have returned to previous levels in District 64. <br /> • Groundwater pumping has shown a rebound in Water Districts 2, 1, and 64 since 2009 as <br /> additional augmentation supplies have been acquired and adjudicated. <br /> • Groundwater pumping and consumptive use estimates were developed for the HB 1278 <br /> jstudy by the Wilson Water Group in collaboration with Leonard Rice Engineers. The well <br /> metering rules enacted in 2013 will enhance future pumping analyses. <br /> Description of the South Platte Alluvial Aquifer <br /> The S. Platte alluvial aquifer consists primarily of silt, sand, and gravel deposits of alluvial and <br /> aeolian origin that cover an area of over 4,000 square miles. Drilling logs indicate the deposits <br /> near the base of the alluvium are coarsest and become finer towards the surface, with <br /> considerable heterogeneity in the aquifer materials,particularly with respect to clay and silt. Clay <br /> layers are common throughout the basin, both laterally and vertically, and although clay layers <br /> may not be laterally continuous over great distances, they can affect pathways of groundwater <br /> movement. In addition, the aquifer grades from coarsest material in the west to finer material in <br /> the east. The ancient S. Platte River and its tributaries, swollen with snowmelt at the end of the <br /> last ice age (Pleistocene), left extensive alluvial deposits ranging in width from two to six miles <br /> wide and up to 200 feet deep in the main river channel. The flood plain of the S. Platte River east <br /> 41 <br />