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COLORADO DEPARTMENT OF HEALTH, Water Qunlity Control Division Rationale -Page 3 Permit No. CO-0045675 /ll. RECEIVING STREAM A. /denhfication, Classification and Standards (Cont.) Table Value Standards (TVS) refers to numerical criteria initially set fonlr in Tables 1, I! and !// of the Basic Standards and Methodologies for Surface Water. The criteria or TVS may be adopted as standards jar a receiving stream. The standards are listed by segrrrent in the Classifications and Numeric Standards jar the Rio Grande Basin. Where a standard jar a parameter is given as TVS in the Classifications and Numeric Standards, it corresponds only to those criteria or TVS in the Basic Standards that are based upon the protection of the aquatic life use classification, as erplained in section 36 ojthe ClassiJcations and Numeric Standards. The aquatic life use smrtdards consist of both an acute and chronic standard and are generally more srrirrgent t/tan those for the protection of either agricultural or drinking rater uses. Therefore, they are usually considered protective of all three uses. In cases where a standard is based on sonnet/ring other than the acute and chronic TVS criteria, a numeric i•alue rather than the TVS designation has been specified in the Classifications and Numeric Standards. For hardness-dependent metals standards, the numerical value of the aquatic life use based standard will depend upon the combined effluent/receiving stream hardness below the discharge point. The hardness values to be used are those which correspond to the lower 95th percent confidence limit of the mean hardness value at the periodic low-flow criteria, as determined from a regression analysis of site-specific data. if data for sac/r an analysis is nor available, or if a regression analysis is not appropriate, other methods tray be used. /n this case, there is paired flow/hardness data available as x•e[l as mean data (50" percentile) at various locations in the Rito Seco jar Sampling Stations RS-1, RS-2, and RS-5. The Division has selected the 50" percentile value oj64 mg/l as CaCO~ jar total hardness as being representative for conditions in the Rito Seco; this is based upon station RS- 2 for the period aj record from 1-11-1990 through 7-20-1999, w/rich was !/re data submitted at the time of the permit application. This value was used to calculate the hardness-dependent metal water quality standards. B. Receiving Water Data uali lnstream water quality data is available from several locations in the Rito Seco wlrich have been routinely sampled by the permittee as pan of their COMG mining permit. These sampling stations include RS-1, RS-2, and RS- 5, x~hich are located in the Rito Seco and w/rich are shorn in figure 3 of the permit. Upstream data is based upon data from RS-I from 1-1988 through 7-20-1999. Baseline loading levels for the Rito Seco are based upon t/te 85'" percentile of the data from station RS-5 jar the period from 1-11-1990 through 6-16-1998, which is the period from pre-mining up to the nine just bejore an increase was observed for certain parameters when the impact to ground water and the seep occurred. Tl:e 50'" percentile slue that is used for the upstream MI concentra[ion is included in Table VI-3 of the rationale. Quantity. Acute and Chronic Law Flox~s: The stream low flows which are used in water quality standard-based mass balance equation determinations to calculate the acute and chronic effluent limitations are the one day in three year low flow (IE3) and the 30 day in three year low flow (30E3) respectively. These flows have been determined jar t/re Rita Seco above the Battle Mountain Resources Inc. (BMRI) facility discharge locations by the Water Qtwlity Control Division, oral are as follows: Table !/1-1 -- Acttte and Chronic Low Flows TYPE FLOW Acute (1 E3) 1.3 cjs (or 0.84 MGD) Chronic (30E3) 1.8 cjs (or 1.2 MGD) 3. _Lnpacls on Downstream Water Supplies: There are no known surface water supply intakes located within 5 miles downstream in the Rito Seco Fran !Iris facility. Witlt compliance of permit limimtians, the BMRI facility discharge should not impact downstremn classified water uses in segment 28. Public notice comments also indicated concerns regarding impacts to downgradient alluvial/groundwater supplies jar irrigation and domestic rater supply uses. BMR! is required to comply in attaining applicable groundwater standards under their COMG mining permit.