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equivalent to sulfate concentrations in WR-1 and "as such, sulfate concentrations at NPDES <br />#006 are estimated to average 2230 mg/L". It is not clear whether this average was based on <br />data from NPDES #006 or inferred from WR-1. If data are available to show similarity <br />between the two monitoring points, can an average for NPDES #006 not be directly calculated <br />from the data? <br />How were the predicted short term sulfate concentrations of 652 mg/L for the Moffat azea and <br />587 mg/L for the West Ridge area derived? Clarification of the steps taken after the <br />calculation of S(t) on page 2.5-B-4 needs to be provided. <br />Long term predictions still need to be quantified. <br />DMG Item 7: <br />This concern has not been addressed, but will be delayed as requested until all pending <br />revisions are approved. <br />Following aze additional concerns with the revised text, and with the original submittal that <br />were not previously noted in our January 4, 1993 letter, <br />8. Figure la on Page 2.SA-4 shows a "Predicted Moffat Background" value for TDS of 572 <br />mg/1. How was the value derived. Is it background, as in based on historic data, or is <br />it conjecture for lack of data? <br />9. Past surface water monitoring data have indicated that collection of flow data at sites <br />TR-a and TR-b has been inconsistent due to a number of factors. Flow measurements <br />are significant information for evaluating the relative concentrations of substances <br />monitored. Please include in the "Description of Data" section (page 2.5-A-1) that there <br />is insufficient flow data for surface water sites over the 1981-1991 period. Please also <br />include, in Appendix 2.5-A, a tabular or graphical summary of available flow data for <br />sites TR-a and TR-d. <br />10. On pages 2,5-A-10 and -11, P & M asserts that comparisons of sample data to predicted <br />values for that data are invalid. If this were the case, the PHC in the mine permit <br />would be of no value. The Division does not agree with the conclusions presented in <br />Appendix 2.5-A. <br />P & M suggests that the conditions and terms of predicted and measured values must <br />be the same in order for comparisons to be valid. The discussion on page 2.5-A-10 <br />raises two points to indicate that the comparisons are not valid. They are: <br />1) conditions of PHC predictions are post-mining while monitoring data <br />represents conditions during Honing, and <br />ever n ontare 4 cto er 1 , 1 <br />