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Water in Fish Creek could see a light increase in sediment load as subsidence creates slight changes to <br />the stream channel gradient and minor erosion occurs at the head cuts in the stream. This increase in <br />sediment load is anticipated to be no more than the sediment increase observed during spring runoff <br />and after large rainstorms. An increased sediment load was not detected during past subsidence of <br />Fish Creek, Foidel Creek, or Middle Creek. <br />No impact to groundwater quality is predicted. Ground water could flow between aquifers through <br />subsidence fractures. The Fish Creek alluvium and the Twentymile Sandstone have the greatest <br />potential for such interstitial flow, but the stratigraphic separation of several hundred feet between the <br />two units would prevent any significant amount of flow between them. <br />Water quality impacts to the stream/alluvial aquifer system of Fish Creek could result from two <br />additional processes: mine water discharge through the Fish Creek borehole or Fish Creek Vent Shaft <br />during mining and contamination of tributary ground water through seepage from the underground <br />workings after mining has ceased and the workings have flooded. Impacts from mine water discharge <br />during operation will be discussed first. <br />Originally, mine water discharge was projected to have an electrical conductivity of 1 mmhos/cm at a <br />projected discharge rate of 0.41 cfs. Actual data from numerous years of operation reveals mine water <br />discharge values of approximately 0.20 cfs but an electrical conductivity of nearly 3 mmhos/cm. <br />Discharge and conductivity in this range would cause a measurable increase in salinity downstream on <br />Fish Creek. Stream flow modeling conducted during the CHIA produced EC values in the area of .750 <br />mmhos/cm, given a conservative TDS/EC ratio of .8, for areas downstream of the Fish Creek <br />dewatering borehole. Electrical conductivity values at this level would not materially damage crop <br />production from either flood irrigation or sub-irrigation. <br />As was the case on Trout Creek, the majority of the plant species identified in surveys conducted by <br />the applicant and summarized in the supplemental package of September 22, 1986, are rated <br />moderately tolerant to salinity. Only 3.7% of the relative vegetative cover on Fish Creek was made up <br />of moderately sensitive species. No production sampling was conducted on Fish Creek, and so a ratio <br />of 4.5 obtained from sampled fields at the Foidel Creek/Middle Creek confluence was used to convert <br />relative cover of 3.7% to assumed relative production of 16.6% for moderately sensitive species. <br />Cover and productivity data were collected in 1997 in and adjacent to the Fish Creek AVF. AVF <br />species are dominated by Smooth bromegrass, Kentucky bluegrass, Common yarrow, sage, Artemesia <br />cana, and Timothy. <br />Projected Fish Creek water quality for flood irrigation is conservatively estimated at 0.9 mmhos/cm <br />electrical conductivity. Projected root zone soil salinity would be less than 1.5 mmhos/cm and <br />therefore no decrease in crop production would occur. Moderately sensitive species would be <br />expected to exhibit some decline in productivity if irrigation water conductivity were to exceed 1.0 <br />mmhos/cm. Due to the relatively small component of moderately sensitive species, material damage <br />would not occur unless flood irrigation water conductivity were to exceed 2.0 mmhos/cm (which is <br />assumed to correspond to a root zone conductivity of 3.0 mmhos/cm). <br />A 1.5:1 relationship between soil salinity and irrigation water salinity is considered to be a <br />conservative assumption (the actual relationship may be less than 1.5:1). Data collected by the <br />Division on August 15, 1986, showed root zone soil conductivity to be slightly lower than Fish Creek <br />surface water conductivity. <br />30