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i~ ' 3.3 GEOCHEMISTRY ' The geochemical behavior of the backfill material was studied by BMRI and the Division prior to full approval of TR-18. As discussed in Section 2.4 above, the Santa Fe material was not ' acid generating and the gniess showed none to low acid generation potential. There were low levels of sulfur present in the rock material, but it was generally unreactive. Because of the low levels of sulfur in the rock and the lack of acid generation in any of the ' geochemical testing performed on the San Luis rock materials, the elevated TDS and sulfate in the backfilled waste rock is believed to be a result of "flushing" of oxidized sulfates from the rock and does not represent an acid rock drainage threat: The e]evated sulfate ' concentrations represent the dissolution of soluble, secondary salts on the surfaces of the backfill solids as the ground water rose through the backfill to its equilibrium elevation. There ' have been no changes in pH, nor would they be expected. In a memo from Geochmica to Harry Posey of the Division in 1995 discussing TR-18, Geochimica predicted the potential for ' some flushing of sulfate, although the exact levels were not predicted. This information is discussed in detail in a recent memo from Geochimica presented in Appendix B. ' This temporary elevation of sulfate concentrations due to flushing is not uncommon at mining operations and Appendix B also references several papers which have been written describing ' this occurrence. The "flushing" ceases once the material is under water and the sulfate and other constituents which have temporarily become elevated will generally experience reductions as successive pore volumes of water are passed through the material. Column testing performed as part of the TR-18 review demonstrated this occurrence. The Geochimica ' memo in Appendix B provides a full discussion of the expected change in sulfhte levels over successive pore volume movement. ' In conclusion, the elevated levels are believed to be a short-term occurrence due to the ' formation and dissolution of secondary sulfate minerals within the backfill material. The material is not acid generating and these elevated levels do not represent a long term water quality problem. Rather, the secondary sulfate minerals will be flushed through the system ' as successive pore volumes of groundwater move through the backfill and sulffate levels will drop over time. 14