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September 17, 2000 To: Jim Dillie From: Harry Posey Subject: Technical Revision 30; BMRI San Luis; M-88-] 12 The captioned TR proposes to dispose of Fluid from the concentrate stream of the water treatment plant into the tailings facility. This proposed change in operations provides a means of maintaining the treatment plant using the existing ponds and transfer facilities. In earlier discussions with the Operator about the potential need for this proposed TR, the Division asked whether the concentrate might lead to precipitation of mineral(s) within the sub- drain leading from the tailings pond to the recovery pond and whether the high sodium water might affect re-vegetation. The proposed TR-30 evaluates these questions adequately. Potential Effects on Re-vegetation. The evaluation includes a response letter from [he NRCS which, in effect, indicates that the composition of the concentration should not adversely affect the re-vegetation success. The only element of concern might be sodium. The local atea is noted already for relatively high sodium soils. According to the NRCS, the only plant of significant local interest that might be affected is alfalfa. The tailings is to be reclaimed with mostly native grasses, and those are more tolerant of Na than alfalfa. Alfalfa will not be grown on the reclaimed tailings. Thus, the concentrate would appear not to offer a significant deterrent to re-vegetation efforts. Potential Effects on Pond Draindown. To evaluate whether minerals might precipita[E in the tails, HSI Geotrans conducted geochemical modeling [o determine what, if any, mineral species might precipitate and what volumes might be anticipated. The inputs to the model appear to be appropriate and assumptions are conservative. The inputs included the treated water, make up water, and water existing in the tails. Reaction with atmospheric gases was presumed. It was assumed that the treated water mixes and equilibrates with atmospheric CO, prior to being mixed with the existing pond water. Given that the ponds are shallow and the discharge is somewhat agitated upon mixing, this seems appropriate; CO, was the only atmospheric component of any significance to the reactions that might take place in these solutions, so other atmospheric gases were not considered.