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The only potential for the diminution of the quantity of water supplied to the Trout <br /> Creek AVF would be as a result of undermining of Fish Creek and Foidel Creek, <br /> which are tributary to Trout Creek. Dewatering of the streams as a result of <br /> undermining is not considered to be a possibility for reasons explained in the Fish <br /> Creek and Foidel Creek AVF Findings section of this document. <br /> Rule 2.06.8(5)(c) of the Colorado Regulations specifically refers to potential <br /> increases in electrical conductivity of water supplying AVFs to levels above <br /> threshold value at which crop yields decrease as constituting material damage. A <br /> reference initially used is a paper by E.V. Maas and G.J. Hoffman of the U.S. <br /> Salinity Laboratory published in the Journal of the Irrigation and Drainage <br /> Diversion, ASCE, June, 1977. In their paper, Maas and Hoffman list research- <br /> derived salinity threshold levels and response rates for selected agricultural <br /> species, and group those species as well as a number of additional crops into four <br /> categories based on relative tolerance to salinity (sensitive, moderately sensitive, <br /> moderately tolerant and tolerant). <br /> The Division has taken the approach that a positive (adverse) material damage <br /> finding would be made if projected salinity increases resulting from mining would <br /> result in significant decreases in production or shifts in species composition based <br /> on the Maas and Hoffman paper. <br /> On the Trout Creek AVF, the applicant sampled vegetative cover within a number <br /> of farmed fields as denoted on Map No. 1 of Exhibit 42. Vegetative production, as <br /> well as cover, was sampled in one field (Pasture 11). A majority of the species on <br /> the AVF would be considered moderately tolerant, and on an acreage weighted <br /> basis only 18.4% of the cover was made up of moderately sensitive species <br /> (salinity threshold reached at root zone soil saturated paste extract electrical <br /> conductivity of 1.5 mmhos/cm). A number of the species encountered have not <br /> been ranked by Maas and Hoffman, and other references were consulted. In <br /> general, Aveneae tribe grasses and Trifolium species were considered to be <br /> moderately sensitive, while Triticeae, Stipeae, and Poeae tribe grasses (with the <br /> exception of Dactylis)were considered to be moderately tolerant. Carex and <br /> Juncus species were also considered to be moderately tolerant. <br /> Estimates submitted by the applicant project mining will impact Trout Creek water <br /> salinity, producing EC levels in the vicinity of 1.23 mmhos/cm (1230 umhos/cm X <br /> .73 ratio= 922 mg/l TDS). Assuming that a root zone saturated extract soil <br /> salinity of 1.845 mmhos/cm would result from flood irrigation water with a <br /> specific conductance of 1.23 mmhos/cm, some decrease in crop production would <br /> be expected. The assumed relationship between irrigation water salinity and root <br /> zone soil salinity is within the typical range reported in USDA Handbook 60. <br /> Based on leaching calculations included in the TCC permit application, and on soil <br /> sampling conducted on the Fish Creek AVF by the Division, the 1.5:1 relationship <br /> (soil salinity/irrigation water salinity) is considered conservative. However, the <br /> applicant previously submitted vegetation data for irrigated areas on Trout Creek <br /> Foidel Creek Mine 43 August 4,2023 <br />