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MEMORANDUM B B A <br /> Water Consultants, Inc. <br /> To: Joe Lamanna <br /> From: Timothy A. Crawford <br /> Subject: Brannan Sand and Gravel Company LLC — Hydrogeologic Investigation of the <br /> Morton Lakes Site Stage 7A and 713 Gravel Pit Lining <br /> Job: 0430.08 <br /> Date: December 21, 2022 <br /> This memorandum presents a hydrogeologic investigation of the impact of mining at the Stage <br /> 7A and 7B gravel pits at the Morton Lakes site generally located in Sections 1 and 12, Township <br /> 1 North, Range 67 West of the 6th P.M., and Sections 6 and 7, Township 1 North, Range 66 West <br /> of the 6th P.M, as shown in Figure 1. Specifically, this investigation focuses on the potential <br /> impact to ground water elevations in the South Platte River alluvial aquifer resulting from the <br /> lining of the Stage 7A and 7B gravel pits and whether subsurface ground water drains may be <br /> necessary to allow ground water to continue to flow towards and interact with the South Platte <br /> River. To analyze the potential water table elevation changes and impacts to ground water flow <br /> paths, a previously prepared MODFLOW model was utilized to model the existing and proposed <br /> gravel pit liners at the Morton Lakes site. <br /> Background <br /> Available geologic mapping indicates the presence of a shallow alluvium at the Morton Lakes <br /> site that consists of quaternary-aged, clay, silt and gravel. The geologic mapping indicates that <br /> the shallow alluvium overlies deeper alluvial material identified as quaternary-aged river gravel <br /> which consists of small to large pebbles and cobbles in coarse sand. Approximate terrace <br /> equivalents for this river gravel are the Broadway and Louviers alluviums. The gravel contained <br /> in these alluvial units is the target of the gravel pit mining at the Morton Lakes site. Based on <br /> geologic mapping, the bedrock below the alluvial aquifer at the site consists mainly of the <br /> Dawson Formation, a siltstone/ sandstone unit of the Denver Basin. <br /> The alluvium at the site is saturated with a general ground water gradient from south to north. <br /> The alluvium interacts with the South Platte River, Big Dry Creek and Little Dry Creek. The <br /> Stage 7A and 7B gravel pits at the Morton Lakes site will be lined similar to several of the other <br /> existing gravel pits at the site to address these saturated conditions. The gravel pits directly to <br /> the south and southwest of Stage 7A are unlined gravel pit ponds. <br /> Available USGS geologic mapping, State Engineer well permit database information, test <br /> drilling data and South Platte Decision Support System (SPDSS) mapping and well data were <br /> utilized to identify the extent and occurrence of the South Platte River alluvium for this effort. <br /> Available SPDSS mapping reviewed for this effort included mapping of bedrock elevations, <br /> water level elevations, surface elevations and transmissivity for the general area. <br />