Laserfiche WebLink
<br />rapidly heal as alluvial materials collapse in the void and stresses <br />are relieved due to adjacent subsidence movements. <br />Surface water quality should not be impacted by subsidence. A alight <br />increase in sediment load may occur as the stream channel slope is <br />slightly modified due to localized channel slope changes. This <br />increase is within normal sediment load increases due to spring runoff <br />and normal volume increases along Fiah Creek. No change due to <br />potential degradation of surface water through groundwater changes <br />since only a very small portion of the streamflow comes from adjacent <br />bedrock groundwater discharge. <br />Some increases in permeability of the aquifer units above the subsided <br />area can be expected up to 600 feet above the Foidel Creek workings. <br />However, after completion of mining, the permeability of the disturbed <br />strata should return to close to pre-mining levels. <br />The Fiah Creek alluvial aquifer should not be affected due to its <br />fine-grained nature and the nearly 1000 feet of separation from the <br />mine workings. <br />No impact to groundwater quality is predicted. Subsidence could cause <br />inter-communication of aquifers. However, the two aquifers of <br />significance above the workings are the Fish Creek alluvial aquifer <br />and the Twentymile Sandstone aquifer. No interaction between these <br />aquifers is predicted due to their separation. <br />Water quality impacts to the stream/alluvial aquifer system of Fish <br />Creek could result from two additional processes, mine water discharge <br />through the Fish Creek borehole/shaft during mining and contamination <br />of tributary ground water through seepage from the underground <br />workings after mining has ceased and the workings have flooded. <br />Impacts from mine water discharge during operation will be discussed <br />first. <br />Discharge of mine water having a projected electrical conductivity of <br />1 mmhos/cm at a projected discharge rate of .41 cfs would cause a <br />measurable increase in salinity downstream on Fish Creek, but the <br />increased stream salinity would not materially damage crop production <br />when based on irrigation season average flow during low water years. <br />As was the case on Trout Creek, the majority of the plant species <br />identified in surveys conducted by the applicant and summarized in the <br />supplemental package of September 22, 1986 are rated moderately <br />tolerant to salinity. Only 3.7} of the relative vegetative cover on <br />Fish Creek was made up of moderately sensitive species. No production <br />sampling was conducted on Fiah Creek, and so a ratio of 4.5 obtained <br />from sampled fields at the Foidel Creek/Middle Creek confluence was <br />used to convert relative cover of 3.7k to assumed relative production <br />of 16.6ir for moderately sensitive species. <br />As shown in Table 17, projected Fish Creek water quality for flood <br />irrigation is approximately .9 mmhos/cm. Projected root zone soil <br />salinity would be less than 1.5 mmhos/cm and therefore no decrease in <br />crap production would occur. Moderately sensitive species would be <br />expected to exhibit some decline in productivity if irrigation water <br />conductivity were to exceed 1.0 mmhos/cm. Due to the relatively small <br />component of moderately sensitive species material damage would not <br />occur unless flood irrigation water quality were to exceed 2.0 <br />mmhos/cm (which is assumed to correspond to a root zone conductivity <br />of 3.0 mmhos/cm). <br />As stated previously in the Trout Creek AVF Findings the 1.5:1 <br />relationship between soil salinity and irrigation water salinity is <br />23 <br />