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The only potential for the diminution of the quantity of water supplied to the Trout Creek <br />AVF would be as a result of undermining of Fish Creek and Foidel Creek, which aze tributary <br />to Trout Creek. Dewatering of the streams as a result of undermining is not considered to be a <br />possibility for reasons explained in the Fish Creek and Foidel Creek AVF Findings section of <br />this document. <br />Rule 2.06.8(5)(c) of the Colorado Regulations specifically refers to potential increases in <br />electrical conductivity of water supplying AVFs to levels above threshold value at which crop <br />yields decrease as constituting material damage. A reference initially used is a paper by E.V. <br />Maas and G.J. Hoffman of the U.S. Salinity Laboratory published in the Journal of the <br />Imgation and Drainage Diversion, ASCE., June, 1977. In their paper, Maas and Hoffman list <br />research-derived salinity threshold levels and response rates for selected agricultural species, <br />and group those species as well as a number of additional crops into four categories based on <br />relative tolerance to salinity (sensitive, moderately sensitive, moderately tolerant and <br />tolerant). <br />The Division has taken the approach that a positive material damage finding would be made <br />if projected salinity increases resulting from mining would result in significant decreases in <br />production or shifts in species composition based on the Maas and Hoffman paper. <br />On the Trout Creek AVF, the applicant sampled vegetative cover within a number of farmed <br />fields as denoted on Map No. 1 of Exhibit 42 of the PAP. Vegetative production, as well as <br />cover was sampled in one field (Pasture 11). A majority of the species on the AVF would be <br />considered moderately tolerant, and on an acreage weighted basis only 18.4% of the cover <br />was made up of moderately sensitive species (salinity threshold reached at root zone soil <br />saturated paste extract electrical conductivity of 1.5 mmhos/cm). A number of the species <br />encountered have not been ranked by Maas and Hoffman, and other references were <br />consulted. In general, Aveneae tribe grasses and Trifolium species were considered to be <br />moderately sensitive, while Triticeae, Stipeae, and Poeae tribe grasses (with the exception of <br />Dactylis} were considered to be moderately tolerant. Carex and Juncus species were also <br />considered to be moderately tolerant. <br />Estimates submitted by the applicant project mining will impact Trout Creek water salinity, <br />producing EC levels in the vicinity of 1.23 mmhos/cm (1230 umhos/cm X .73 ratio = 922 <br />mg/1 TDS). Assuming that a root zone saturated extract soil salinity of 1.845 mmhos/cm <br />would result from flood imgation water with a specific conductance of 1.23 mmhos/cm, <br />some decrease in crop production would be expected. The assumed relationship between <br />irrigation water salinity and root zone soil salinity is within the typical range reported in <br />USDA Handbook 60. Based on leaching calculations included in the TCC permit application, <br />and on soil sampling conducted on the Fish Creek AVF by the Division, the ].5:1 <br />relationship (soil salinity/irrigation water salinity) is considered conservative. However, the <br />applicant previously submitted vegetation data for irrigated areas on Trout Creek (Exhibit 42) <br />that demonstrate that moderately sensitive species comprise a very small portion of the total <br />vegetative production on those azeas irrigated downstream of the Foidel Creek mine. The <br />applicant also points out that imgation water is applied in excess of what is necessary for the <br />36 <br />