1997-06-13_REVISION - M1981302 (60)

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Page 15 federal and Colorado laws. As stated above, the Division has determined that these requirements will be complied with by the operator. Comment: "...the City of Boulder does not believe it is appropriate to approve the proposed amendment to the existing reclamation plan for the Deepe Farm Pit until Western Mobile conducts a thorough evaluation of the potential ground water and irrigation system impacts for adjacent Open Space lands. Specifically, this evaluation should determine what affect the revision from 38.1 acres of surface water to 4.2 acres will have on ground water levels and irrigation flows within the adjacent Open Space lands. The City of Boulder wishes to ensure that the reductions in surface water which the proposed reclamation plan amendment incorporates do not result in adverse impacts to ground water levels and irrigation flows on the adjacent Open Space properties, threatening the habitat for Ute ladies' tresses orchid or the Preble's meadow jumping mouse." Response: The reclamation plan proposed in the amendment application includes a 1.9-acre groundwater-fed pond near the center of the permit area. There has been a groundwater- fed pond near this location for several decades. The size and shape of this pond has varied somewhat over time, depending on excavation and dewatering operations at the site. The proposed reclamation plan also includes a 2.3-acre groundwater-fed pond at the north end of the permit area. There is currently a pond at this location that has been created by relatively recent mining at the site (since 1988). Groundwater-fed ponds such as the two ponds at the Deepe Farm Pit create a slight cone of depression in the groundwater table. The effect of groundwater table depression caused by the existing ponds on the lands surrounding the permit area would be negligible. A quantitative analysis that forms the basis of the Division's opinions on groundwater table elevations is provided here. On December 7, 1989, the operator of the Deepe Farm Pit provided the Division with a report titled "HYDROGEOLOGIC IMPACT ANALYSIS OF DEWATERING FLATIRON SAND AND GRAVEL STAGE 4 AND 6 AT DEEPE PIT" prepared by Leonard Rice Consulting Water Engineers, Inc. The statement is made in that report that the hydraulic conductivity of the alluvial gravel mined at the Deepe Farm Pit ranges from 80-1200 gpd/fe and averages about 200 gpd/ft'. Using the average value for hydraulic conductivity,the capture zones for the dewatering trenches were analyzed and platted in the Leonard Rice report. These plats show that the capture zones extend no more than 260 feet from the deep dewatering trenches. The presence of only a 260-foot lateral extent for the groundwater capture zone during aggressive pit dewatering leads to the conclusion that the capture zone of evaporating groundwater lakes would cause only a slight cone of depression resulting in negligible impacts on the elevation of the ground water table.