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Mayer Well&Pond Inspection <br /> Platteville,Colorado <br /> Page 2 <br /> 5. Pump type: Berkley turbine. <br /> A pump test conducted on July 7, 1989, documented a continuous pumping rate of 900 gallons per <br /> minute (gpm) with a pumping water level of 49 feet bgs, or a drawdown of 29.5 feet. The CDWR <br /> records are presented as Attachment A. <br /> Mr. Pyeatt indicated that the irrigation well was used only as a backup and that the pond storage <br /> was the primary source of irrigation water. A flow restrictor is present on the irrigation well and it <br /> was indicated that flows in excess of 300 to 600 gpm was causing the well to surge. Water level data <br /> obtained from a monitoring well located just to the southeast of the irrigation well (Figure 2) <br /> indicate that the depth to water in the irrigation well was between 9 and 10 feet bgs at the time of <br /> my inspection. It should be noted that the monitoring and irrigation wells were surveyed for vertical <br /> and horizontal locations prior to this letter report. <br /> A numerical groundwater model was used to predict the drawdown associated with dewatering at <br /> the Pit 122 mine. Based on the model outputs the predicted drawdown in the vicinity of the <br /> irrigation well could be on the order of 20 feet. The maximum drawdown would only occur during <br /> the mining of Cell B, which is adjacent to the Mayer pond and well.The irrigation well currently has <br /> approximately 43.5 feet of water column and at maximum drawdown associated with the Pit 122 <br /> dewatering the well would have a water column of over 23 feet. Based on well test data from 1989, <br /> the predicted well yield with a 23 foot water column is 440 gpm. However, given that the well is <br /> surging at flow rates of half its permitted use it is assumed that excessive well losses occur during <br /> pumping and well yields will be much less than predicted by past pumping data. <br /> Based on available water level data it is estimated that the water table elevation is less than two <br /> feet below the base of the pond. Infiltration from the pond will cause the water table to mound and <br /> intersect the pond bottom. Under these conditions seepage from the pond occurs horizontally and <br /> the seepage area and hydraulic gradient are dramatically less than what occurs under free (vertical) <br /> drainage conditions. As such, once that water table is lowered beneath the pond infiltration rates <br /> are expected to increase. However, if the permeability of the pond bottom is sufficiently low the <br /> increase in infiltration may be insignificant relative to the inflow rate and retention volume and <br /> mitigation may not be required. <br /> AwIEs 4809 Four Star Court, Fort Collins, CO 80524 970-590-3807 <br />