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The species of chromium that is analyzed by ACZ is trivalent chromium. <br /> Any hexavalent chromium present in the sampled aliquots is converted to <br /> the trivalent chromium species when acid is added to the aliquot during <br /> the sample preservation process. <br /> Peabody recognizes that no established standards have been promulgated <br /> for silver and chromium for the applicable receiving stream (Tuttle <br /> Draw) . Because of CMLRD's concern over the detection limits used in <br /> analyses of these two chemical parameters, Peabody can only assume that <br /> CMLRD is in fact judging our mine effluent against stream standards <br /> established for the main stem of the San Miguel River (refer to Table <br /> 7-32, Tab 7, Nucla Permit Revision) . It is not clear where along the <br /> main stem of the San Miguel the chemical data was obtained to develop <br /> these standards. It also appears, particularly in the case of the <br /> silver standard, that the limits are based on the most sensitive aquatic <br /> species. <br /> Peabody is proposing an approach that will provide CMLRD a means of <br /> determining whether Peabody should continue monitoring for silver and <br /> chromium. At least two composite water quality samples from <br /> representative verticals of a stream cross section on the San Miguel <br /> immediately above the confluence with Tuttle Draw will be analyzed for <br /> the complete suite of surface water parameters including silver and <br /> chromium (see Table 7-24, Tab 7, Nucla Permit Revision) . Peabody has <br /> analyzed for silver and chromium in water samples collected at the <br /> ground and surface water sites at Nucla during December of 1986. <br /> Analyses for silver and chromium will be repeated on samples collected <br /> at these sites during the first quarter of 1987 (see revised pages 7-79 <br /> Tab 7, Nucla Permit Revision and page 15 of the Nucla 1986 AHR) . <br /> Based on what concentrations of silver and chromium are determined for <br /> the site specific reach of the San Miguel River, Peabody will request <br /> permission to discontinue analyzing for silver and chromium at the Nucla <br /> Mine ground and surface water monitoring sites unless measurable <br /> concentrations of the two constituents are determined in relation to <br /> existing concentrations in the San Miguel River. <br /> 9 <br />