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3. [n Table 9, SCC presents surface water agricultural standards from various <br />references. In our AHR review letter to SCC dated October I5, 1998, the Division <br />recommended using the Basic Standards and Methodologies for Surface Water for <br />comparison with surface water samples from the Seneca lI-W Mine. Upon review of <br />these standards and the corresponding Classifications and Numeric Standards for the <br />Upper Colorado River Basin, it is the Division's understanding that there are no metals <br />standards for Segment 12 of the Yampa River Sub-basin. This was also verified in <br />Section III of SCC's Colorado Discharge Permit System permit. It is commendable that <br />SCC has attempted to identify applicable water quality standards for comparison with <br />analytical results and for the sake of discussion. However, please check the referenced <br />metals standards and other parameters in Table 9 for accuracy and applicability. It may <br />be appropriate to rename Table 9 as "Use-Suitability Standards". <br />4. Information presented in the 1998 AHR indicates that predictions for Hubberson <br />Gulch alluvium at Well WHAL7-2 have been exceeded, when comparing 1998 average <br />TDS values to predicted TDS values. Please provide an explanation for this exceedance <br />of the PHC prediction. <br />5. For Well WOV 17 in the Wadge overburden, the predicted TDS concentration of <br />4295 mg/I has been significantly exceeded by an observed 1998 value of 8860 mg/l. The <br />Division notes that Well WOV 17 exhibited a premining value of 8043 mg/I, and the <br />predicted value (4295) was predicated on a premining value of 4072 mg/l. However, the <br />original PHC prediction estimated a 5.5%TDS increase in the overburden below the <br />center pit. The observed 1998 TDS concentration is an increase of approximately 9.2 % <br />over the baseline value for Well WOV 17, which appears to be an exceedance of the <br />estimated 5.5%o TDS increase. Please provide any available information on the source of <br />the high initial baseline TDS levels at this well location, and provide an explanation for <br />the apparent exceedance of the PHC prediction. <br />6. With the exception of surface water site SW-S2W-FG4, observed 1998 TDS <br />values for surface water locations were all below the predicted TDS concentrations <br />presented in the PHC. The predicted value for TDS at site SW-S2W-FG4 on Sage Creek <br />is 464 mg/l. The observed 1998 value was 655 mg/1. Please provide any available <br />information on the source of the elevated TDS level, and provide an explanation for the <br />exceedance of the PHC prediction. <br />If you have any questions concerning the review of the 1998 AHR, please call Mike <br />Boulay or me. <br />Sincerely, <br />Sandra L. Brown <br />Environmental Protection Specialist <br />