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According to a letter provided to the Division on February 27, 2008, the City of Thornton has a major <br />raw water reservoir immediately adjacent to the proposed Priola Pit. This municipal water supply <br />reservoir provides water to a major part of the City. This City stated that the reservoir includes the <br />following elements that could be impacted by the proposed project: <br />• Raw water storage reservoir <br />• Perimeter embankment dam <br />• Soil-bentonite cutoff wall below the dam <br />• Concrete surface water diversion ditch <br />• Groundwater drain (horizontal drain permit No 6F3647) <br />The City of Thornton listed the following summary of the potential adverse impacts of the Pit to the <br />City's infrastructure and water supply: <br />(1) According to Rule 5.10.1.2 of Rules and Regulations for Dam Safety and Dam <br />Construction of the Colorado Division of Water Resources "Borrow area shall be located at <br />least 200 feet from the toe of the dam." The excavation for the Pit is within 200 feet of the <br />toe of the dam and could impact the stability and/or foundation seepage. <br />(2) It is unclear in the permit application if the Pit will be dewatered during mining If the Pit is <br />dewatered during mining, the dewatering will have severe impacts on the groundwater drain <br />the City uses as part of their raw water supply. The Pit is located upgradient of a horizontal <br />groundwater drain and the dewatering operation for the mining will remove the water prior <br />to it entering the groundwater drain. <br />(3) The permit application on Page 9 states the Pit will be backfilled with "inert material;" <br />however inert material may contain asphalt or other contaminants which over time could <br />leach hydrocarbons or other chemicals into the soil and eventually into the groundwater <br />drain that flows into the reservoir. <br />(4) Currently, groundwater flows east to west through the areas of the proposed Pit and into the <br />City's groundwater drain. If the Pit is excavated to bedrock and backfilled with "inert <br />material," it could be assumed the hydraulic conductivity of the inert material is several <br />orders of magnitude less permeable than the existing sand and gravel. This could greatly <br />diminish the flow into the groundwater drain and reduce the City's water supply from the <br />groundwater drain. <br />(5) A second affect of the "inert material" being several orders of magnitude less permeable <br />than the existing sand and gravel is that it could cause groundwater to mound below <br />properties to the east of the Pit. This mounding could cause water to rise to the ground <br />surface and impact drainage facilities along I-76 and the I-76frontage road and, in an <br />extreme case, 1-76 and the 1-76 frontage road. <br />2