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' ~~ _ i ~; ~~,- During the reclamation phase of the Hayden Gulch t4ine, there will be ttiro quantitative impacts to the surface water regime. Drainage structures at the .mine site will change the hydrographs of peak flow within the watershed and contributions from the ground water regime at the mine site will increase flows in Hayden Gulch. Temporary and permanent diversions in the backfilled pit area will increase the rate of flow across the mine site, but detention time in the sediment pond will slow discharges and minimize peaks from snowmelt and storm events into the drainage. In addition, runoff from the permanent overburden pile, which is in the Dry Creek watershed, is diverted into the Hayden Gulch drainage. The amount of area removed from the Dry Creek watershed as a result of the diversion is insignificant; it is approximately 0.002% of the total watershed. The area affected by the loadout in Dry Creek is insignificant with respect to the Dry Creek drainage; approximately 0.0006 of the total watershed. Runoff from the area will be contained by a system of ditches, culverts and sediment ponds. Any water contained within the system will be required to meet applicable effluent standards upon discharge. Surface water will be monitored for quantity and quality downstream on Dry Creek. As mentioned in the previous section, springs from backfilled spoils and the excess overburden pile will result in an increase in surface water flow in the drainage. The applicant predicts that spoils spring discharges will average 255 gpm (0.57cfs) during peak flow periods and that base flow spoils spring discharges will average 10.5 gpm (0.02 cfs). The Division predicts that the extremes of peak spring flow could be as low as ll7 gpm (0.26 cfs) and as igh as 408 gpm (0.91 cfs). The extremes of low flow could range from 13.5 to _2.5 gpm (0.03 to 0.05 cfs). The discrepancy between the applicant's and the Division's predictions of base flow is caused by differences in distributing the discharge of spoils flow between peak and low flow periods. A worst case projection of spoils spring impacts on the surface water regime can be made by adding the predicted peak spoils flow during a low flow year to fl:;.rs at i;SGS site 09240D00 on the East Fork of the Williams Fork near Pagoda, Colorado (TABLE 6). This calculation assumes that spoils spring flow is the only contribution from flow in Hayden Gulch (there is no dilution from surface runoff) and that this flow actually reaches the Williams Fork. During periods of average flow, salt loading would increase less than 3%. During the categorical 7 day - 10 year low flow event, salt loading could increase 2%, and electrical conductivity levels of 0.255 mmhos/cm, far below the material damage standard of 1 mmho/cm. • -20-