REV11416 - Laserfiche WebLink

Home Browse Search

Memo to Matthew S. Hayes - 3 - May 17, 1989 The response to adverse surface water flow impacts will consist of using the corrugated pipe to span subsidence features to route the water over them. This surface use of corrugated pipe seems workable. If major protions of surface waters are diverted underground by subsidence, this water will be intercepted underground, a hole from the surface will be drilled and cased and the diverted water pumped back to the surface. The underground diversion plan appears to be complicated by potential delays in obtaining a drill on short notice, drilling and casing in a subsiding area, and maintaining the well's functionability in this unstable ground. A review of WECC's water balance and water rights has indicated that WECC has sufficent water to mitigate most adverse impacts on surface water rights. If, however, due to subsidence, significant volumes of surface waters are diverted underground, there is some doubt that the proposed mitigation plan would quickly (within a week) restore this water to surface water flow. However, under normal conditions no significant quantities of surface waters are expected to be diverted underground. There are thirteen decreed surface water rights on or near the permit area. Twenty-nine surface water rights are located downstream of the life of mine area. Table 2.8.5A gives an inventory of water rights owned by WECC. During 1982 the conditional right to withdraw water from the North Fork was deemed absolute. During average precipitation years, this water right is expected to be out of priority from July to September. 10 acre feet of water will be stored in each of two ponds (one not yet built) to supply water when the water right is out of priority. To date this system has reportedly functioned well. From 1983 through 1987 Annual Mine Water Balance reports show an average consumptive rate of 13.96 acre feet (AF). The high was in 1986 and was 32.85 AF. The low water consumption year was in 1982 when 2.93 AF was used. Table 2.8.5A shows water available to WECC for augmentation purposes through its ditch and North Fork water rights. These adjudicated water rights far exceed the mine's consumption. Based upon the precautions that WECC has committed to minimize adverse impacts on surface flow these water rights appear more than adequate to protect downstream water users. Probable Hydrologic Consequences. )/~ n~b In April, 1987 TR-54 allowed a reduction in the water monitoring program at h' /~ the Mt. Gunnison No. 1 Mine. Baseline data has been accumulated since the ~ L/~ late 1970's. This inactivated monitoring will be gradually reactivated one year before mining reaches each site. The present Mt. Gunnison hydrologic monitoring plan is found in Volume 1, Section 4.9.1 (July 16, 1987), Section 2 pages 2-481 & 2 - 482 and in Section 4, pages 4-67 - 72b. Exhibit 2.8.1 C shows the general hydrologic system and monitoring sites. During mining the expected hydrologic consequences to the surface system is limited to localized drainage of springs in areas where mining is near to the surface. The relative impermeability of the overburden sandstone -shale formation will most likely limit such drainage to these immediate areas. This is best characterized by the underground water inflows in the Sylvester Gulch area near the fan portal. At this time in the mining cycle overburden levels are in the hundreds of feet making surface impacts of subsidence much more unlikely. With plans to begin mining the "B" Seam, ultimately using long wall methods, potential subsidence impacts on the surface must be carefully considered, especially beneath the Minnesota Creek Drainage System.