2009-03-06_PERMIT FILE - C1981012A (6)

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COLORADO DEPARTMENT OF PUBLIC HEALTH AND ENVIRONMENT, Water Quality Control Division Rationale - Page 3, Permit No. CO-0000906 Because oj'the location of'Pond 6 adjacent to the Middle Fork of the Purgatoire River, the groundwater in the alluvium • is expected to be at or near the surface. Furthermore, due to the locations of Pond 6 and the other ponds (Ponds 1, 3, 4, 7 and 8) adjacent to the Middle Fork of the Purgatoire River, the potential for a hydrological connection exists between the seepage from these pond and surface waters. In sum, discharges to surface water may actually be occurring that is not being regulated. For this reason, the permit contains a compliance schedule for the performance of a hydrological connection study. 2. Classification: The ground waters in the area are not classified. Until such time as use classifications and numerical standards are adopted for ground water on a site-specific basis throughout the state, the "Interim Narrative Standard" will apply as discussed in Section 41.5(C)(6)(a) of The Basic Standards for Ground Water. 3. Numeric Standards: The "Interim Narrative Standard" as set forth in Regulation 41, Section 41.5. (C)(6)(b)(i), indicates that the Division is required to implement the less restrictive of (1) existing ambient quality as of January 31, 1994, or (2) that quality which meets the most stringent criteria set forth in Tables I through 4 of The Basic Standards for Ground Water (Regulation No. 41). There are no ambient groundwater data that can be used in determining existing ambient quality as of January 31, 1994. Thus, the Division is utilizing Tables I through 4 of The Basic Standards for Ground Water in order to develop groundwater standards. These are summarized in Table III-1. • C? Table III-I: Ground Water Standards Physical and Biological pH=6.5-8.5su Total Coliform = 2.21100 ml (30-day average), Multiple Tube Fermentation Method Total Coliform = 1/100 ml (30-day average), Membrane Filter Technique Total Coli orm = 231100 ml (max in 30 days) Inorganic Dissolved Nitrate =10 mg11 Dissolved Nitrite =1 mg11 Total Nitrate plus Nitrite as N =10 mg/l Dissolved Chloride = 250 mg/l Dissolved Sulfate =250 mg11 Metals Dissolved Arsenic (As) = 0.05 mg/l Dissolved Cadmium (Cd) = 0.005 mg11 Dissolved Chromium (Cr) = 0.1 mg/1(combined trivalent and hexavalent forms) Dissolved Copper (Cu) = 0.2 mg11 Free Cyanide (CN) = 0.2 mg11 Dissolved Iron (Fe) = 0.3 mg11 Dissolved Lead (Pb) = 0.05 mg11 Dissolved Manganese (Mn) = 0.05 mg/l Dissolved Mercury (Hg) = 0.05 mg11 Dissolved Nickel (Ni) = 0.1 mg11 Dissolved Selenium (Se) = 0.05 mg11 Dissolved Silver (Ag = 0.05 mg11 Dissolved Zinc (Zn) = 2 mg11 TDS • For background concentrations of 0 - 500 mg11, 400 mg11 or 1.25 times the background level, whichever is least restrictive • For background concentrations of 501 - 10,000 mg11, 1.25 times the background value • For background concentrations of 10,001 mg/1 or reater, no limit. Because no data are currently available to calculate the background concentration for TDS, the exact TDS standard has not yet been established.