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Appendix D, Comment Letters and Public Meeting: EPA Comments (Cont.): Page 6 !n the vicinity of the West Pit, and at the outfall of the water treatment facility, the Rito Seco is deeply incised into unconsolidated alluvial material drat is comprised of weathered remnants of the underlying Santa Fe Fonnatian and Precambrian bedrock. As such, the naturally occurring hydrologic and geochemical processes influencing the baseline Hater quality of the Rito Seco, for purposes ojestablishing antidegradation-based effluent standards, are more appropriately evaluated using historical water quality data obtained from the dowmgradient water quality monitoring stations RS-2 or RS-5, the latter of which integrates the combined hydrologic and geochemical processes of the Rito Seco upgradient of that location. In later meetings involving the Division and EPA staff; both parties reached an agreement that another surface rater station other [hart RS-1 (i.e., station RS-2) x~ould be appropriate jot the antidegrodation baseline loading assessment. As an alternate evaluation, data from station RS-2, x'/rich is downgradient of the backfilled West Pit, was considered. The Division [Wade an alternate antidegradation baseline determination using data from station RS-2. This comparison between the two stations (RS-5 and RS-2) has been added as Appendix C of Jhe rationale. !n comparing the anridegradation baselines and antidegradation- based lisnitarions using data from each station, many ojtlte limits are the same because the baseline loading was found to be the same value for both stations. Since the baseline loading x~as loner for a nttrnber of parameters ar station RS-2, this resulted in higher antidegradation-based limits for many snore parameters (arsenic, copper, dissolved iron, total recoverable iron, dissolved manganese, and zinc) than the RS-5 station. Thus, the Division has determined that use of the RS-5 data jot the baseline loading determinaton is appropriate, since this rill result in more protective limitations jot the permit than from the RS-2 station determination. Once again, the Division has derennined t/tat the antidegradation-based limitations in this permit are still applicable, since the discharge front 001 a rill be occurring for a longer term (more than one year), and this is not a temporary activin' As pan of this evaluation, the Division found that the previously calculated limit for copper in the permit (0.053 mg/I jot the 30- day average) was a typographical error, and this limit has been corrected to 0.0053 sng/l ((or ouffall 0016). Also, of/rer corrections were made in the calculations ojAppendix B (although this /ms not changed the permit limits). EPA has asserted that the inactive "Earth Science Pit"may contribute metals loading to the Rito Seco in the vicinity of sample location RS-3. The Division is uwware ojany x•ater gtta[iry data that would suggest [trot the historical activities conducted by Earth Sciences, /rte. (ESq have resulted in continuing effects on the water quality ojthe Rito Seco. The only pit associated xeth misting conducted by ESl was completely subsumed during the mining and backfilling of the East Pit. As part of the TR-0!5 Response Plan, BMRJ installed a backf~ll groundx~ater monitoring we// lBF-8) in the backfilled East Pit that has never recorded a measurable amount of groundwater in the well bore. Thus, the "Earth Science Pit "could not be a source of metal loading to the Rito Seco because there is no groundwater in the East Pit. ESI operated a heap leach system in an area on the south side ojtlte Rito Seco (on the orlrer side ojthe Rito Seco from the mining activities conducted 6y BMR/). The heap leach materials were removed by BMR/ and beneficiated through the mill circuit with resultant tailings deposited in the lined, zero-discharge tailings basin. After removing the heap leach material, the liner was removed and the heap leach area x~as reclaimed by the addition of topsoil and vegeta(ion. Historical water quality dam have been collected at the surface water station RS-3 that is located upgradient ojthe historic ES/ heap leach area. The rater quality data collected at RS-3 show the effects ojnatttral geologic influences on the concentration of manganese. The naturally elevated concentrations of manganese recorded at RS-3 are consistent with elevated concentrations of manganese recorded in groundwater rnoni[oring wells (M-26 and M-30) that have not been affected by the historic Jlow of groundwater from the West Pit. A seepage face located along the south bank of the Rito Seco, downgradient from the historic ES/heap leach area, was not sampled during a recent (June 8, 2000) USEPA NPDES site inspection because the EPA personnel derennined that the pooled water associated with the seepage face did not constitute a representative sampling location. Ahhouglr a sample from the location was subsequently collected during a sire visit by an EPA representative of the Emergency Response Program, the sample x~as collected x•ith reservations regarding the ability to collect a representative sample noting similar conditions as were identified by the NPDES sampling team. Comment: l!. Baseline Hardness Value: Response: As noted above, the low /tardness at RS-1 reflects the fats that the water quality in the Rito Seco, as it flows through snetanrorphic bedrock, is not the same as tl:e baseline xa[er quality found naturally in t/te Rito Seco as it flows through alluvium in the dox~nstream reach that are more characteristic of flows in the vicinity of the West Pit. 77re issue of representative wafer quality collected at RS-/, as affected by the fundasnentalty different geological environment, x~as addressed in the previous response.