2002-10-25_REVISION - M1986015 (5)

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Mr. Mark Heffner May 31, 2002 Page 4 Ground Water Modeling Analysis In order to estimate water level impacts to nearby wells, we developed a ground water flow model using USGS MODFLOW. The model simulates the Arkansas River and boundary conditions at the aquifer boundaries; and it was calibrated to the monitoring program water level data. The model allows us to simulate various mining scenarios, including simulations with various levels of pit dewatering and various mechanisms in place to mitigate the water level impacts. In general the model shows that water level drawdowns are greatest very neaz the pit and decrease further from the pit. The Arkansas River buffers many of the water level impacts; accordingly, drawdowns are greatest near the pit and the northern boundary of the aquifer. Near the River, impacts aze minimal to none due to the buffering effects of the River. Our modeling analysis included a simulation of a worst case scenario in which the entire Phase 1 area is dewatered with nothing in place to mitigate water level impacts. This simulation indicated that during mining, water level impacts may be great enough in the Martin well to interrupt the wells' ability to produce the pumping entitlement. Water level impacts to the Tilley well are not expected to be great enough to impair the well's ability to produce its pumping entitlement. We note that the possible interruption of service to the Martin well will be temporary and alternative water supplies can be provided during the period of interruption. Alternative water supplies may include hauling water, pumping water from the gravel pit to the well users for irrigation, or other water supplies. Upon completion of mining, a slurry wall will be installed along the perimeter of the pit to allow for lined reservoir storage. This scenazio was also simulated in the model and indicated that water level impacts following mining will be very minimal. There will be a slight bulge and shadow effect above and below the pit, respectively, but the water level changes will not be enough to impair any wells from pumping their entitlements. Summary and Conclusions Our findings indicate that there is potential for periods of interruption to nearby water supply wells, but these periods of interruption will be temporary, and Transit Mix of Pueblo is prepared to provide alternative water supplies during periods of interruption. The well most directly impacted by the mining activities (Helmsing well) will be replaced or an alternative water supply provided. The water level and water quality monitoring program will provide an early warning system if water levels in nearby wells are adversely impacted. With the information provided from the monitoring program, Transit Mix of Pueblo will be able to mitigate or minimize impacts before any wells or well users aze adversely impacted. i Bishop-Brogden Associates, lnc.