2023-08-03_PERMIT FILE - M2023025 (32)

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Bernhardt Pit Groundwater Evaluation May 2023 Page 12 of 31 A2). This effectively widened the alluvium approximately 400 ft (two model cells) further west than Robson. Water Level Fluctuations In 2014, at the request of the State legislature,the Colorado Geologic Survey conducted a study to evaluate observed increases in water levels in the Gilcrest/LaSalle area. The nearest well (SB00406619CBS) that was part of the study is located approximately '/z mile east of the pit, across the SPR river, in Section 19, T4N, R66W (Barkmann and Others, 2014, Fig. E4). The published data only extends from 1966 to 1975 and it shows a seasonal fluctuation of approximately 1 to 2 feet. The purpose of J&T's routine measurements of Bernhardt pit monitoring wells is to evaluate site specific seasonal fluctuations to determine if there will be any future issue. Saturated Aquifer Thickness We calculated the aquifer saturated thickness as the difference between the water table (Figure A3) and the top of bedrock (Figure A4). Figure A5 shows how the saturated aquifer thickens from zero on the west side of the alluvium (at the Pit boundary) to approximately 25 - 30 feet at the SPR. The saturated aquifer thickness increases to over 50 feet in the eastern portion of the study area. Well Yield Well yield is promm to the aquifer saturated thickness. We plotted reported well yields extracted from the State's well database on Figure A6. Well yields from domestic wells are less than 15 gpm. Agricultural wells are scattered throughout the study area and typically range from 800 (yellow squares) to over 2000 gpm (pink squares). There is one 800 gpm agricultural well located on the south side of the Bernhardt pit(permit 85-RF)that will likely see some effect of pit mounding(See Figure A10). The yield of the well will likely not be effected because of the slight increase in water level. Aquifer Transmissivity Figure A7 shows the contoured aquifer transmissivity (T) which is the product of the aquifer hydraulic conductivity (a measure of permeability) multiplied by the saturated thickness (Figure A5). The resulting transmissivity distribution used in the model is consistent with Barkmann (2014,Fig.E9)who used refined aquifer properties from the SPDSS model(CDM,2006,Fig. 7a). The computed transmissivity ranges from 0 to 150,000 gpd/ft west to east across the pit area (in the S24,T5N R67W),and up to 250,000 gpd/ft in the southeastern corner of the model. Since the transmissivity is equal to the hydraulic conductivity(a measure of permeability)multiplied by the saturated thickness (Figure A5),we used a constant hydraulic conductivity value of 600 ft/day to best duplicate Barkmann's transmissivity distribution(note: 600 ft/day x 33 ft thick x 7.48 gal/ft^3 =—150,000 gpd/ft^2). A hydraulic conductivity of 600 ft/day his consistent with other models we constructed for gravel pit impact analyses along the SPR(MWE, 2015, 2019, 2020, and 2022). MODELING We used the Groundwater Vistas(GV)pre and post processor(version 6.85 Build 11,Rumbaugh, Environmental Solutions, Inc.) to construct and run a MODFLOW2000 (Harbaugh et al, 2000) McGrane Water Engineering,LLC 1669 Apple Valley Rd. 0 Lyons,CO 80540 0 Phone:(303)917-1247;email:dennis@mcgranewater.com