2016-05-04_REVISION - C1982056 (12)

Home Browse Search

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 water is applied in excess of what is necessary for the species grown, and this excess leaches the soil profile, thus making the assumed relationship between soil salinity and irrigation water salinity a worst case assumption. The applicant proposes that no decrease in production of any of the species currently grown on the Trout Creek AVF would be expected unless the conductivity of the irrigation water were to exceed 1 mmhos/cm. Based on species composition data submitted, the applicant predicts that significant production decreases (>3% for a farm unit) would not occur unless irrigation water salinity were to exceed 2.0 mmhos/cm. Given the factors involved, the Division finds it is appropriate to propose an irrigation water material damage suspect level of 1.5 mmhos/cm for Trout Creek, from the confluence of Middle Creek to the Yampa River. Establishing the suspect level at 1.5 mmhos/cm acknowledges the evidence submitted by the applicant while maintaining a reasonable suspect level established through research and application. As a result of TR -32, the applicant received approval of a plan to discharge mine water only when the stream has the dilution capacity to maintain waters at or below the discharge points on Fish Creek and Trout Creek to 1.5 mmhos/cm. In summary, projected flood irrigation water quality is 1.230 mmhos/cm and material Page 48 of 52