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"'-~ ~ r'- ;-. :) C'".:"' c.:;. 15 ( 6. GROUND WATER (; 6.1 Re~ional Ground Water The following description of the regional ground water system is based on Lohman's (1965) work in the Grand Junction area. While his study did not reach as far west as the Contour Ponds Site, the geologic environment is very similar; Grand Junction and the Contour Ponds Site both lie on the flanks of the Uncompaghre arch and have the same sedimentary sequence underlying them. The ground water aquifers in the region can be divided into two systems; the bedrock system and the alluvial/surface system. The bedrock is characterized by semi-separated~flow systems within the more permeable units, with pressures increasing with depth. The major aquifers and the quality of the contained waters, in order of increasing depth, have been described by Lohman (1965) as follows: o Dakota Sandstone and Burro Canyon Formation--these units yield small supplies of water to a few wells. The water is generally salty and contains local pockets of natural gas. Generally, the water is under low artesian pressure. { o Salt Wash Member of Morrison Formation--one or more of the sandstone lenses in the lower part yield small amounts of soft water to a few wells. The water is under moderate artesian pressure. ( o Entrada Sandstone--this unit yields small quantities of soft water to many wells in the area. The water is under moderate to high artesian pressure except in areas of concentrated development. o Win~ate Sandstone--small quantities of soft water are produced from this unit by several wells that also obtain water from the Entrada sandstone. The water is under moderate to high artesian pressure. As can be surmised from these descriptions, the Entrada is the most important aquifer, followed by the Wingate, Salt Wash,and Dakota. The Dakota is the least important because of small prOduction of poor quality water. l. In general, because of increasing artesian pressure with depth, the tendency is for upward flow from the lower aquifers to the upper, and from the Dakota to the surface. Thus recharge of the bedrock system from the alluvial/surface system is not likely. Lohman (1965) proposes the crest and flank of the Uncompahgre arch as the primary recharge area with discharge occurring via upward flow through the broad expanse of surface in the down-dip direction, the upward flow being removed by evaporation. c