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Wes! Elk Mine the short duration over which the discharges occur (i.e., less than two days to empty both • ponds at their maximum discharge rates), or while the North Fork flows are at the 7-day, 10- year (7Q10) low flow conditions. This is important since TDS is a parameter that is of concern only when the concentration is elevated for significant periods of time (i.e., months to years). Additionally, the primary concern with elevated TDS is for agriculmral uses. The 7Q 10 low flow is most likely to occur during the winter months, when irrigation is not occurring. There would be no material damage to irrigated agricultural operations as a result of the short- term increases in salinity to 670 mg/L as shown in Table 54. While this value is above the 500 mg/L level, the increased TDS levels would occur for a duration of less than one week. Since adverse salinity impact to crops result from an accumulation of salts in the soil, a minor, short- term increase would have no effect on the crops. The 500 mg/L level at which there may be limited restrictions for irrigation is applicable to the average concentration of the source irrigation water. The instantaneous concentration of the source water can exceed 500 mg/L without adverse impact to irrigated crops. The more relevant analysis is to consider the annual salinity loading to the North Fork, which is presented in the CHIA, and remains valid. To protect agricultural and municipal uses in the lower Colorado River Basin, Colorado participates in the multi-state, basin-wide approach for managing salinity (TDS) in the Colorado River. There are no Colorado stream standards for TDS based on stream classifications. TDS is controlled to maintain the following flow-weighted annual values for • TDS: Below Hoover Dam: 723 mg/L; below Parker Dam: 747 mg/L; at Imperial Dam: 879 mg/L. The Colorado regulation recognized that periodic increases above the criteria are in conformance with the standards, provided that, when conditions return to normal, concentrations are expected to be at or below the criteria level. The normal range for TDS in irrigation water is between 0 to 2,000 mg/L. Continued and prolonged irrigation with water having a TDS concentration between 500 mg/L and 2,000 mg/L may impose slight to moderate requirements for management practices (leaching and drainage) or crop selection (use of less sensitive crops) in order to avoid crop yield reductions. Because crops are affected only when TDS in root-zone water builds up to excessive levels, occasional excursions of TDS in irrigation water above 500 mg/L are generally not significant, as long as the average annual value is lower. Dr. Robert Weiner of WWE has evaluated the concentrations of the individual constituents which collectively constitute TDS in mine discharges. Dr. Weiner has determined that the pond discharge water is predominantly sodium carbonate/sodium bicarbonate in nature. Aside from high concentrations of sodium, none of the other parameters, including the heavy metals, are found in significant concentrations. As for sodium, there are no applicable numeric standards or criteria. Elevated sodium levels can have negative implications for irrigated soils that aze clayey in • nature. This is because high sodium can cause such soils to "seal." The sodium adsorption ratio 2.05-201 Revised Jun. 1995 PR06: Revised Nov. 1998 TR80: l/98 PR08