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and compared the antidegradation-based limits for each baseline loading 4as is summarized in Appendix C for the 250 gpm discharge flow, along with a comparison with the calculated water quality standard-based limitationsl. Based upon this evaluation, the Division has concluded that the baseline loading will still be based upon Station RS- 5, which overall results in more stringent and protective limitations in the permit. Aluminum: Based upon the concentrations of aluminum that have been detected in the pit backfill monitoring wastewater source, the Division finds that there is a reasonable potential for aluminum to occur in the untreated ground water source. Thus, limitations and monitoring have been added for potentially dissolved aluminum for the 001 outfalls. Since there should be no discharge from the West Pit to 002, there are no limits or routine monitoring for aluminum at this location. However, if seeps should be identified within the historically defined area as shown in figure 2 of the permit, parameters such as potentially dissolved aluminum, dissolved iron, and dissolved manganese must be analyzed in samples collected. Radioactive Constituents: With respect to the radioactive constituents that are associated with Gross Alpha Activity, discharge permits in Colorado have specifically identified that this is primarily associated with radium (226+228) and uranium. Where radioactive constituents are limited for other mining discharge permits, limits and monitoring are specified for these two constituents, and these limits are based upon the statewide standards for all surface waters. The BMRI discharge permit explicitly sets limits on the concentrations of uranium and radium discharged from the water treatment facility. Time Periods: In the original public noticed draft permit, there was reference to an initial time period that included a higher discharge flow rate (400 gpml, with a later time period when discharge flows would be expected to decrease. Thus, tiered limitations in the permit were developed and these limits along with discharge flow rate limits are based upon discharge flow values of 400, 250, and 100 gallons per minute Igpml• With the revisions and updates made for this facility's permit, there is no longer any defined periods for initial and later discharge flows. Thus, previous comments related to flows are no longer applicable. The tiered-discharge approach for Discharge Point 001 provides the flexibility necessary to reasonably represent the probable range of ilows that are expected to occur during the extraction of groundwater from the West Pit. For the purposes of the CDPS permit, BMRI will pump the backfilled West Pit as necessary, up to a maximum flow rate of 400 gpm 130 day averagel, to maintain a reversed hydraulic gradient from the Rito Seco alluvial aquifer to the West Pit. Comments: Issues related to outfall 002: - Better definitions for the location and requirements for outfall 002 - Utilizing monitoring wells to define outfall 002 - Addressing specific flow and other limitations and monitoring requirements for outfall 002 - Identifying seepage contributions as part of the mass balance water quality standard-based calculations - Not referencing the CDMG requirements for TR-26 and TR-28 in association with the requirements for outfall 002 in the CDPS discharge permit - Concerns related to short-term West Pit groundwater discharge issues and long-term water groundwater discharge issues (defining a window far identification of ground water flowing through a restricted zone as points of compliance; utilization of additional ground water monitoring wells; installation of a slurry wall) - Concerns regarding the Best Management Practices requirements described far outfall 002 Response: There were a number of comments related to outfall 002. In response to these comments, several major revisions were made relating to outfall 002. Descriptive language, as well as figure 2 of the permit, have been revised to define the window and boundary for 002. Also, a limitation that is associated with this outfall has been•added in the permit which defines 002 with respect to maintaining a required water level depth in the West Pit that is below the water level of the Rito Seco to ensure that the hydraulic gradient is reversed resulting in permanent elimination of flow from the West Pit to the seeps on the bank oT the Rito Seco. A compliance schedule has also been incorporated and the Best Management Practices requirements for this outfall have been revised in the permit to further address the existing situation. The compliance schedule will address requirements to follow in case any seeps should occur in the window /boundary of the 002 outfall. (See permit languagel. With regard to the installation of a slurry wall, BMRI, WOCD, and CDMG have reviewed the potential merits of the slurry wall and all parties have concluded that the long-term consequences of the slurry wall outweigh the short- term benefits as perceived by the reviewers. The flushing action provided by the inflow of groundwater from the Rito Seco alluvial aquifer is a fundamental component of the water management plan because the inflow of groundwater with lower concentrations of constituents more effectively flushes the secondary sulfate salts associated with the backfill material. When the water management program is completed, the water quality in the West Pit is expected to no longer contain elevated concentrations of constituents. Thus, the issue of "keeping clean water clean" will have been addressed. Additionally, the installation of a slurry wall would further disturb the hydrologic system beyond the changes induced as a result of the historical mining activity with no corresponding benefits.