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(1) an aquifer or its portion; <br />(i) which supplies any public water system; or <br />(ii) which contains a sufficient quantity of <br />groundwater to supply a public water system; and <br />(A) currently supplies drinking water for human consumption; or <br />(B) contains fewer than 10,000 mg/] total dissolved solids; and <br />(2) which is not an exempt aquifer. <br />In this area, the principal aquifer system used for public and domestic and other uses is the <br />alluvial aquifer. The alluvial aquifers aze mainly located in the valley flood plains along the <br />Colorado River, Parachute Creek, Roan Creek and other streams and tributaries. The Wasatch <br />Formation also contains water bearing units which are defined as USDWs. Some of the shallow <br />sand units in the Wasatch are used for domestic and stock watering supplies. There are also <br />records from deep test wells in the basin that contain water with a TDS of azound 7,000 mg/ liter <br />making these units USDWs. Although the chemical quality of ground water vanes within and <br />between the various aquifers, EPA considers the aquifers that overlie the Mesaverde group to be <br />USDWs in this azea. <br />The upper confining zone is a tongue of the Mancos shale approximately 54 feet thick <br />layer with a top at about 3,810 feet BGS. The main part of the Mancos underlies the Corcoran <br />Sandstone at about 4,280 feet BGS. The Mancos is an interbedded mudstone and marine shale <br />with some sand zones. Several geologic studies, mainly related to the potential production of oil <br />and gas from various geologic units, have been carried out in the basin. Papers summarizing the <br />results of these studies were submitted by American Soda as part of their application and are part <br />of the administrative record. These papers have been reviewed by EPA, and they indicate that the <br />Mancos is a very effective confining zone. <br />Injection Zone: The proposed injection zone is composed of two sandstone intervals that <br />aze separated by a shale interval: the Cozzette and Corcoran Members of the Iles Formation. The <br />top of the perforations is located at a depth of 3,911 feet BGS. The Cozzette Sand interval is <br />perforated from 3,911 to 3,927 feet and the Corcoran Sand interval is perforated from 4,077 to <br />4,241 feet. The Cozzette sandstone is underlain by a shale interval that makes up the lower <br />portion of the Cozzette Member. A pressure falloff test was nm on the Getty well to assess <br />reservoir suitability for injection starting on May 11, 2001, and ending on May 21, 2001. The test <br />indicated that the injection zone has good permeability (1-2 millidazcies) and has no boundaries or <br />limits within the present area of influence (300 feet) of the well. <br />As indicated above, the disposal of the reject solution mining fluid will be into the <br />Cozzette and Corcoran Sandstones that do not meet the definition of a USDW. The Getty No.l- <br />7LW was sampled and analyzed (2/17/83) prior to conversion to a Class II injection well and the <br />reservoir fluid contained about 20,000 mg/liter total dissolved solids (TDS). Samples from <br />adjacent Corcoran-Cozzette producers contained fluids with a TDS generally ranging from 16,000 <br />to 29,000 mg/liter with a low outlier of 10,020 mg/liter (Well USA 1-1 LW) and two wells with <br />Statement of Basis for EPA Permit No. CO 10932-04664 Page 4 of 18 <br />