2010-06-04_GENERAL DOCUMENTS - C1982056

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which are tributary to Trout Creek. Dewatering of the streams as a result of undermining is not considered to be a possibility for reasons explained in the Fish Creek and Foidel Creek AVF Findings section of this document. Rule 2.06.8(5)(c) of the Colorado Regulations specifically refers to potential increases in electrical conductivity of water supplying AVFs to levels above threshold value at which crop yields decrease as constituting material damage. A reference initially used is a paper by E.V. Maas and G.J. Hoffrnan of the U.S. Salinity Laboratory published in the Journal of the Irrigation and Drainage Diversion, ASCE., June, 1977. In their paper, Maas and Hoffman list research- derived salinity threshold levels and response rates for selected agricultural species, and group those species as well as a number of additional crops into four categories based on relative tolerance to salinity (sensitive, moderately sensitive, moderately tolerant and tolerant). The Division has taken the approach that a positive material damage finding would be made if projected salinity increases resulting from mining would result in significant decreases in production or shifts in species composition based on the Maas and Hoffman paper. On the Trout Creek AVF, the applicant sampled vegetative cover within a number of farmed fields as denoted on Map No. 1 of Exhibit 42. Vegetative production, as well as cover was sampled in one field (Pasture 11). A majority of the species on the AVF would be considered moderately tolerant, and on an acreage weighted basis only 18.4% of the cover was made up of moderately sensitive species (salinity threshold reached at root zone soil saturated paste extract electrical conductivity of 1.5 mmhos/cm). A number of the species encountered have not been ranked by Maas and Hoffman, and other references were consulted. In general, Aveneae tribe grasses and Trifolium species were considered to be moderately sensitive, while Triticeae, Stipeae, and Poeae tribe grasses (with the exception of Dactylis) were considered to be moderately tolerant. Carex and Juncus species were also considered to be moderately tolerant. Estimates submitted by the applicant project mining will impact Trout Creek water salinity, producing EC levels in the vicinity of 1.23 mmhos/cm (1230 umhos/cm X .73 ratio = 922 mg/l TDS). Assuming that a root zone saturated extract soil salinity of 1.845 mmhos/cm would result from flood irrigation water with a specific conductance of 1.23 mmhos/cm, some decrease in crop production would be expected. The assumed relationship between irrigation water salinity and root zone soil salinity is within the typical range reported in USDA Handbook 60. Based on leaching calculations included in the TCC permit application, and on soil sampling conducted on the Fish Creek AVF by the Division, the 1.5:1 relationship (soil salinity/irrigation water salinity) is considered conservative. However, the applicant previously submitted vegetation data for irrigated areas on Trout Creek (Exhibit 42) that demonstrate that moderately sensitive species comprise a very small portion of the total vegetative production on those areas irrigated downstream of the Foidel Creek mine. The applicant also points out that irrigation Foidel Creek Mine 44 June 4, 2010