Laserfiche WebLink
Daniel Arnold, Esq. January 25, 2011 <br />Denver Water Page 7 of 21 <br />14.0 inches in 2008, or 24 percent less than average. Overall, the precipitation data and <br />patterns of uranium concentrations in alluvial groundwater suggest that precipitation <br />patterns are unlikely to be a major factor for the increased uranium concentrations. <br />3.3 Alluvial Groundwater <br />EPP Finding: The alluvial groundwater and creek are hydraulically connected. <br />This finding is supported by the available information and data. An example of this <br />connection in the EPP is a graph (Figure 8 -8 in the EPP) of flow from the sumps and flow <br />in the creek. The graph shows that flow from the sumps increases as flow from the creek <br />increases. However, the EPP does not elaborate on the degree of connection which is <br />critical to the conceptual understanding of where water is moving and how this affects <br />remedial or environmental protection strategies. <br />The EPP describes the bedrock constriction at the downstream end of the mine near <br />Sump 1 (Figure 1). The effect of this constriction is that the alluvial aquifer is "pinched" <br />and alluvial groundwater is forced up and into the creek. Not only is the alluvial aquifer <br />constricted laterally, but it is also constricted from below because the bedrock surface <br />rises resulting in a "thinning" of the alluvial aquifer. Review of borehole logs shows that <br />the depth to bedrock in the central mine area averages about 20 feet and the depth <br />decreases to approximately 8 feet near the downstream end of the mine. This pinching <br />and thinning of the alluvial aquifer is such that the majority of groundwater discharges to <br />the creek in the vicinity of Sump 1. <br />The following example documents the degree of communication between the alluvial <br />aquifer and creek by first estimating the flow through the aquifer and comparing that flow <br />to measured pumping rates from the sumps. The flow through the alluvial aquifer was <br />estimated by ARCADIS /Malcolm Pirnie using Darcy's Law, which is based on the <br />hydraulic conductivity of a porous media, groundwater hydraulic gradient, and cross <br />sectional area perpendicular to the flow direction. Based on results of pumping tests <br />conducted in 1999 and 1982, the hydraulic conductivity of the alluvium is reported to <br />range from approximately 1 to 48 feet /day. The average groundwater hydraulic gradient <br />through the central mine was approximated by the ground surface slope between wells <br />MW -1 and MW -6 and estimated to be 0.036 feet /foot. The cross sectional area in the <br />central mine area was estimated based on an aquifer width of 200 feet and average <br />saturated thickness of 15 feet. Using the high -end value for hydraulic conductivity as a <br />conservative approach, the groundwater flow through the alluvial aquifer is estimated to <br />be 27 gallons per minute (gpm). From 1995 to 2002, the EPP reports that the sump flow <br />ranged from 35 to 404 gpm, averaging 104 gpm. Therefore, on average, the sumps <br />removed about three times the amount of water that the alluvial aquifer can support and <br />the excess water is from the creek. This should not be surprising as some of the sumps are <br />only 50 feet from the creek. A detriment of this connection is that creek water with lower <br />uranium concentrations mixes with alluvial groundwater with higher uranium <br />