1977-09-29_PERMIT FILE - M1977315

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1 1 body which will move toward the Cache la Poudre River at ' a speed defined by the slope of the intervening water table ' and by the permeability of the water-bearing formation. The result of this migration of the .ground water depression twill be a depletion of the streamflow of the Cache la Poudre River which is exactly equal to the volume of water pumped from the gravel pit. Whereas the actual timing of pumping from the pit may be erratic and subject to both day-to-day and seasonal fluctua- tions , it would be expected that the impact on the river will ' be approximately uniform throughout the year and equal to fthe daily average discharge from the pit. Consequently the streamflow depletion may be approximately uniform throughout I' the year. ll Compensating for the depletion to the river will be the drainage water which is discharged from the pit. The net impact on streamflow will depend upon which of the two alternative procedures is chosen for disposition of the pit € drainage water: 1. Discharge to storm sewer. While subject to much more I1erratic fluctuation than the depletion effect on the river, it may be expected that on an annual basis , the positive effect of the drainage water discharged to the river will be approximately equal to the negative effect of the depletion L� brought about by dewatering of the pit in prior years. The combined effects of both streamflow depletion and streamflow gain are illustrated schematically on Figure 1 where, for Lt convenience, both the natural streamflow of the river and the ' rate of stream depletion due to dewatering of. the pit have Lbeen assumed to be constant. It may be observed from Figure 1 that the only time at which a possible adverse impact on the Lriver may be manifested will be during the periods in which dry mining of the surface deposits is taking place, and no pit drainage water is being discharged to the River. The l manner in which the negative streamflow impacts 1 1 -5-