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Miller Groundwater Engineering, LLC <br /> Consulting and Contracting <br /> 2302 Tarragon Lane• Fort Collins,CO 80521•970.492.5710 <br /> MEMORANDUM <br /> TO: JARED RAINS, PE <br /> APPLEGATE GROUP, INC. <br /> FROM: CALVIN D.MILLER,PE,PHD <br /> MILLER GROUNDWATER ENGINEERING, LLC <br /> SUBJECT: MOUNDING AND SHADOWING AROUND DUCKWORTH GRAVEL PIT LINER <br /> DATE: NOVEMBER 21,2016 <br /> This memo presents results of groundwater model simulations we recently performed at your request <br /> to estimate potential mounding(increases)and shadowing(decreases)of groundwater levels around <br /> the Duckworth Gravel Pits caused by lining those pits with an impermeable barrier. As you know, by <br /> causing the natural groundwater flow in the aquifer to flow around the lined pits, lining the pits can <br /> increase water levels on the up-gradient side(south side in this case)and decrease levels on the down- <br /> gradient side(north side). That is the scenario we have examined and present here. <br /> Method <br /> To maintain a degree of consistency with past modeling work done for the subject gravel pit,we used a <br /> MODFLOW model roughly similar to the one described in the March 2004 report by Applegate titled <br /> "Duckworth Pit Groundwater Study" (AG Project 04-105). In fact,we started with the same MODFLOW <br /> files you provided, but made a few changes. First,we rescaled the horizontal dimensions of the model. <br /> This was necessary due to an error either in the original model or that was created in transferring the <br /> MODFLOW files from the GMS GUI to the Groundwater Vistas GUI. Next,due to having re-scaled the <br /> model,we had to adjust the peripheral boundary conditions to re-create the target groundwater <br /> gradient. For that we changed the specified-flux boundaries on the south,east,and north sides to <br /> constant head boundaries(CHB)and we adjusted the CHB heads until the model simulated a <br /> groundwater gradient that was similar in magnitude and direction as in the 2004 model(before pits). <br /> We also added to the model the four unlined,water-filled gravel pits located immediately to the south <br /> of the Duckworth Pit. As was done in the 2004 modeling for the pit to the north of the Duckworth Pit, <br /> the south unlined pits were simulated as high hydraulic conductivity(K)zones with K set at a nominal <br /> 20,000 ft/day. The presence and proximity of these pits will affect the shape and distribution of <br />