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COLORADO DEPARTMENT OF HEALTH, Water Quality Control Division Rationale -Page 4 Permlt No. CO-0045675 should be performed as total metals III. RECEIVING STREAM A. Identification, Classification and Standards (Cont.) Table Value Standards (NS) refers to numerical criteria initially set /orth in Tables I, ll and 111 o(the @3.51! Standards and Methodoloeies for SuAace Water. The criteria or NS may be adopted as standards for a receiving stream. The standards are listed by segment in the Gfascincations and Numeric St~ndardc /or Ih~ Rio Grande Basin. Where a standard for a parameter is given as NS in the Classifications and Numedc Standards. it conesponds only to those criteria or NS in the Basic Standards that are based upon the protoction of the aquatic life use class cation, as explained in section 36 o(the lac i( ationc and Num ri tandardc. The aquatic life use standards consist of both an acute and chronic standard and are generally more stringent than those for the protection of either agricultural or drinking water uses. Therefore, they are usually considered protective of all three uses. In cases where a standard is based on something other than the acute and chronic NS criteda, a numeric value rather than the NS designation has been specified in the Class cations and Numeric Standards Forhardness-dependent metals standards, the numerical value o(the aquatic life use based standard will depend upon the combined enluenUreceiving stream hardness below the discharge point. The hardness values to be used are those which correspond to the lower 95th percent confidence IimR of the mean haniness value at the periodic low-now criteria, as determined from a regression analysis o(site-speck data. If data for such an analysis is not available, or if a regression analysis is not appropriate, other methods maybe used. In this case, there is paired now/hardness data available as well as mean data (50^ percentile) at various locations in the Rito Seco for Sampling Stations RS-1, RS-2, and RS-5. The Division has selected the 5L^ percentile value of 64 mg/I as CaCO~ for total hardness as being representative for conditions in the Rito Seco; this is based upon station RS-2 for the period o(record from 1-11-1990 through 7-20-1999, which was the data submitted at the time of the permit application. This value was used to calculate the hardnessdependent metal water quality standards. B. Receiving Water Data 1. Quality: Instream water quality data is available from severel locations in the Rito Seco which have been routinely sampled by the permittee as part of their CDMG mining permit. These sampling stations include RS- 1, RS-2, and RS-5, which are located in the Rico Seco and which are shown in ngure 3 0/the permit. Upstream data is based upon data from RS-1 from 1-1988 through 7-20.1999. Baseline loading levels for the Rilo Seco are based upon the 85 ^ peroentile of the data from station RS-5 for the period from 1-i 1-1990 through 8-16-1998, which is the period from pre-mining up to the Ome just before an increase was observed (or certain parameters when the impact to ground water and the seep occurred. The 50 ^ percentile value that is used /or the upstream M1 concentration is included in Table V!-3 of the rationale. 2. Quantity. Acute and Chmnic Low Flows: The stream low nows which are used in wafer quality standard- based mass balance equation determinations to calculate the acute and chronic enluent limitations are the one day in three year low now (1E3) and the 30 day in three year low now (30E3) respectively. These nows have been determined /or the Rifo Seco above the Battle Mountain Resources Inc. (BMRI) facility discharge locations by the Water Qualify Control Division, and are as follows: Table 111-1 -Acute and Chronic Low Flows TYPE FLOW Acute (1E3) 1.3 cIs (or 0.84 MGD) Chronic (30E3) 1.8 cfs (or 1.2 MGD) 3. facts on Downstream Wafer Sypplies: 7hare are no known surtace water supply Intakes located within 5