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support the contention that a 1.5 conversion factor is applicable for the <br />irrigated land downstream of the Foidel Creek Mine nor that moderately <br />sensitive species dominate the area. <br />CMLRD has used a 1.5 conversion factor to describe the salinity of the <br />soil after the application of irrigated waters of a particular salinity. For <br />example, applying irrigation waters of 2000 umhoslcm could result in soil <br />salinity of 3000 umhos/cm. <br />As discussed under Rule 2.06.8 (61 of the Twentymile Coal Company Life- <br />of-Mine Permit submittal (revised 5/22/86), the conversion factor is not <br />appurtenant for the irrigated land downstream of the Foidel Creek Mine. The <br />amount of irrigated waters applied to the pastures is more than sufficient to <br />supply both the consumptive demands and the amount of water necessary to <br />effectively leach the soil profile. Sufficient leaching exists to maintain <br />the electrical conductivity of the soil at the level of the applied water. <br />For example, if the applied water EC was 1000 umhos/cm, the soil profile EC <br />would be maintained at 1000 umhos/cm. <br />The vegetative data collected by TCC does not support the assumption <br />that moderately sensitive species dominate the irrigated lands of the <br />alluvial valley floors. In the Middle Creek region, the percentage of cover <br />of moderately sensitive plants is 5.6%. The Trout Creek moderately sensitive <br />species cover is 15.3% and the cover of Fish Creek is 3.1%. <br />Based on site specific conditions, Twentymile Coal Company contends a <br />material damage standard of 2000 umhos/cm is applicable and reasonable for <br />the areas downstream of the Foidel Creek Mine. <br />Projected yield decreases were calculated for Pastures 2 and 11. As <br />indicated by the data of Pasture 2, the vegetative cover of moderately <br />sensitive grasses and clovers is 4.2%. Moderately sensitive grasses <br />comprise 11.2 grams of the production (24.0%) and clovers supply .6 grams <br />(1.4%). Using the relative yield calculations as presented in the article of <br />Maas and Hoffman <br />Y= 100-B (EC -A) <br />Y = expected relative yield <br />A = salinity threshold (millimhos per centimeter) <br />B = percent yield decrease per unit salinity increase <br />ECe = specific conductance of saturated soil extract <br />the expected yield decrease at an EC of 2.0 mmhos/cm (2000 umhos/cm) for <br />the production of Pasture 2 would onl,~ be .8%. The published expected yield <br />decrease of orchard grass (6.2% at 1.5 mmhos/cm) was used to represent the <br />grasses and the decrease of clover (12.0% at 1.5 mmhos/cm) was used for the <br />genera of clover. Thus, concerning Pasture 2, the moderately sensitive <br />grasses might decrease .7°~ of 24.0% and clovers might decrease .1% of 1.40°,6 <br />or a total decrease of .8% might be experienced with a soil salinity of 2.0 <br />mmhos/cm. <br />Concerning the Trout Creek alluvial region, the percent vegetative cover <br />of moderately sensitive species is 15.3%. Within Pasture 11, the cover of <br />moderately sensitive grasses is 19.2% (21.9% relative composition) and the <br />