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6.0 Surrounding Wells and Local Aquifers <br /> There are two aquifers in the area that are typically used for water supply. They <br /> are the alluvial aquifer of St. Vrain Creek and the Dakota sandstone. Both of <br /> these aquifers are isolated from C-Pit by 800-1000 feet of relatively impermeable <br />' sedimentary rocks. <br /> Figure 4 shows a plot of wells in the area based on data from the Colorado <br /> Division of Water Resources (permitted wells database). The list of wells from <br /> the Colorado Division of Water Resources is included in Appendix B. Wells are <br /> shown by quarter -quarter section. It should be noted that many of the wells are <br />' <br /> piezometers and monitoring wells adjacent to Southdown <br />s property in the sand <br /> and gravel alluvium. <br />^ Recent water quality data is available from wells in both the Dakota sandstone <br /> and in the alluvial aquifer. Water quality in both aquifers is generally good with <br /> no indication of any impact from CKD disposal. <br /> A well completed in the Dakota sandstone (located in the extreme southeast <br /> corner of section 28) was sampled in April, 1999 by personnel from Grant <br /> Environmental, Inc. The laboratory results (Table 6.1} indicate that the water <br /> quality is in compliance with almost all applicable standards. Results of water <br />' quality analysis in A-Pit Reservoir are also shown in Table 6.1. <br />The pH {which would serve as a key indicator of impacts due to CKD disposal) of <br />' Dakota Well water is 8.28, within the natural, expected range. Table 6.1 shows <br /> the measured water quality in this well compared to the groundwater standards <br /> suggested by the DMG. <br /> <br /> Eight wells completed in the alluvial aquifer have been recently sampled and <br /> analyzed for a variety of water quality parameters. These results are presented <br /> in Appendix C and they indicate good water quality that meets the various <br /> groundwater quality standards. There is no indication of elevated pH, which is <br /> evidence that the water in the alluvial aquifer is not affected by CKD disposal. <br /> Figure 1 shows the location of the Dakota well and the eight alluvial wells. <br />' <br /> 7.0 Fate and Transport of Certain Constituents of Concern in <br /> Groundwater <br /> <br /> 7.1 Introduction <br />The hypothetical groundwater quality impacts resulting from the disposal of CKD <br />in C-Pit are the formation and migration of leachate from the disposed CKD to <br />groundwater. Leachate forms as precipitation infiltration percolates through the <br />backfitled CKD and dissolves soluble constituents present in the CKD. The <br />amount and composition of leachate is governed by three considerations: 1) the <br />' 14 <br />