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Gilcrest/LaSalle Pilot Project <br />Hydrogeologic Characterization Report <br />The bedrock geology underlying the alluvial aquifer consists of Laramie Formation and Fox Hills <br />Sandstone subcrops dipping gently to the southeast. This subcrop pattern is based on a projection <br />from the subsurface using geophysical logs combined with surface outcrop patterns (Dechesne and <br />others, 2011). The subcropping Fox Hills Sandstone appears to correspond to the scalloped buried <br />bench along the southern perimeter of the alluvial valley which may have been a resistant bluff at the <br />time of deepest incision (Figure 7). Using a Fox Hills Sandstone average thickness of 200 feet, the <br />paleochannel should not extend into the underlying Pierre Shale in the Study Area. It is possible, <br />however, that the Fox Hills Sandstone includes shale layers (Dechesne and others, 2011). Shale <br />layers exposed within the buried paleochannels could have created a very landslide -prone <br />topography at the time of maximum incision resulting in the presence of very localized bedrock highs <br />in the paleovalley. Elements of the buried topography suggest the possibility of landslide scarps and <br />hummocky deposits further complicating the bedrock surface geometry. The very irregular buried <br />topography is significant in that the irregularity may affect groundwater movement through the <br />alluvial aquifer. <br />Bedrock aquifer permeability is two to three orders or magnitude lower than the overlying alluvial <br />aquifer. The influence of groundwater flow between the bedrock and alluvial aquifers is expected to <br />be insignificant with respect to the Pilot Project goals and will not be addressed in this study. <br />Alluvial Aquifer Depositional Settin <br />Figure 4 provides alluvial aquifer cross sections, Figure 8 shows the unconsolidated Quaternary <br />deposit thickness, and Figure 9 shows the saturated alluvial aquifer thickness. The alluvial aquifer <br />consists of a package of unconsolidated alluvial sediments filling a broad paleovalley incised into a <br />highly irregular bedrock surface, as described above. Locally the unconsolidated alluvial sediment <br />thickness can exceed 100 feet, however, most of the Quaternary sediments are between 45 and 85 <br />feet thick (Figure 8). Alluvial aquifer saturated thickness (Figure 9) was determined by subtracting <br />the bedrock surface (Figure 7) from the water table surface determined using the spring 2012 <br />groundwater level (Figure 10) raster map using GIS. An important aspect of the alluvial aquifer <br />depositional setting is the relationship of a historic main river channel fed by small tributaries and <br />slope wash from uplands along the sides. The river deposited bed load sediments dominated by <br />sand and gravel from distant sources, while the small ephemeral tributaries and slope wash carried <br />much finer grained material derived from shale and mudstone outcrops and eolian deposits above <br />the alluvial valley. Cross -Section B -B' (Figure 5) illustrates the geometry of deposits produced in this <br />environment. Coarser -grained sand and gravel prevail in the central part of the aquifer while finer - <br />grained silt and clay dominate the sides, lapping down from the adjacent slopes and inter -fingering <br />with the coarser grained central alluvium. Layers of silt and clay in this deeper part of the alluvium <br />11 <br />