2013-01-29_PERMIT FILE - C1981010A

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water, but their predicted concentrations, in general, are over estimated. The concentrations of water-quality constituents in the springs derived from the spoil aquifers following mining are expected to initially be higher than baseline levels and gradually, over a period of time potentially extending centuries, decline to near baseline values. The sulfate and TDS concentrations in the backfill aquifers are likely related to the amount of pyrite contained in the backfill material. The average percent pyritic sulfur in the sampled Trapper pits varies from a low of 0.1 % in the F Pit/E. Pyeatt drainage to a high of 0.3% in the A Pit/Johnson drainage. These values were derived directly from overburden core quality data contained in Volume V-A, Appendix I, Sections 3 and 4. The average percent pyritic sulfur content calculated from all core sample values is 0.2% . Several processes influence the geochemical evolution of water quality in backfill aquifers. The primary processes as identified by the USGS in a study conducted at the Seneca Mine (Water Resources Investigations Report 92-4187) include the introduction of waters containing dissolved CO2 and OZ gases into the system (rainfall and snowmelt), the dissolution and precipitation of carbonate and sulfate minerals, the oxidation of pyrite, and cation exchange reactions. These chemical reactions are influenced by the prevailing pH. At Trapper, It is typical in spoil aquifers derived from sedimentary rock dominated groundwater systems for carbonate dissolution to effectively buffer the production of acids and thereby maintain near neutral pH levels. The groundwater developing in these aquifers is effectively buffered and therefore the potential for trace metals to be mobilized is low. The subsequent dilution of spoil-influenced groundwater by unoxygenated groundwater, or the influence of microbial activity can also affect equilibrium water quality. The TDS and sulfate concentrations in Johnson Gulch backfill well GF-7 are noticeably higher than the TDS and sulfate levels in backfill well GF-5 in the No Name drainage. The water quality characteristics of backfill wells GD-3 and GF-11 fall between these two extremes. Pyrite oxidation has been determined to be an important contributing source of increased TDS and sulfate levels in spoil water. Variation in the percent of pyrite contained in the spoil is expected to result in variation in the TDS and sulfate concentrations observed in the backfill aquifers, at least initially. This type of variation was noted in the USGS study at Seneca and may contribute to the variation observed to date at Trapper. Over time, groundwaters can be anticipated to accumulate sulfate and other dissolved constituents to the point of saturation given the prevailing geochemical conditions. The maximum TDS concentration estimated by the USGS at Seneca based on undiluted lysimeter samples was approximately 5000 mg/l. This level 4-238d PRy ° (Zllag