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water, but their predicted concentrations, in general, are over estimated. <br />The concentrations of water-quality constituents in the springs derived from the spoil aquifers following <br />mining are expected to initially be higher than baseline levels and gradually, over a period of time <br />potentially extending centuries, decline to near baseline values. The sulfate and TDS concentrations in <br />the backfill aquifers are likely related to the amount of pyrite contained in the backfill material. The <br />average percent pyritic sulfur in the sampled Trapper pits varies from a low of 0.1 % in the F Pit/E. Pyeatt <br />drainage to a high of 0.3% in the A Pit/Johnson drainage. These values were derived directly from <br />overburden core quality data contained in Volume V-A, Appendix I, Sections 3 and 4. The average <br />percent pyritic sulfur content calculated from all core sample values is 0.2% . <br />Several processes influence the geochemical evolution of water quality in backfill aquifers. The primary <br />processes as identified by the USGS in a study conducted at the Seneca Mine (Water Resources <br />Investigations Report 92-4187) include the introduction of waters containing dissolved CO2 and O2 gases <br />into the system (rainfall and snowmelt), the dissolution and precipitation of carbonate and sulfate <br />minerals, the oxidation of pyrite, and cation exchange reactions. These chemical reactions are influenced <br />by the prevailing pH. At Trapper, It is typical in spoil aquifers derived from sedimentary rock dominated <br />groundwater systems for carbonate dissolution to effectively buffer the production of acids and thereby <br />maintain near neutral pH levels. The groundwater developing in these aquifers is effectively buffered and <br />therefore the potential for trace metals to be mobilized is low. The subsequent dilution of spoil-influenced <br />groundwater by unoxygenated groundwater, or the influence of microbial activity can also affect <br />equilibrium water quality. <br />The TDS and sulfate concentrations in Johnson Gulch backfill well GF-7 are noticeably higher than the <br />TDS and sulfate levels in backfill well GF-5 in the No Name drainage. The water quality characteristics of <br />backfill wells GD-3 and GF-11 fall between these two extremes. Pyrite oxidation has been determined to <br />be an important contributing source of increased TDS and sulfate levels in spoil water. Variation in the <br />percent of pyrite contained in the spoil is expected to result in variation in the TDS and sulfate <br />concentrations observed in the backfill aquifers, at least initially. This type of variation was noted in the <br />USGS study at Seneca and may contribute to the variation observed to date at Trapper. Over time, <br />groundwaters can be anticipated to accumulate sulfate and other dissolved constituents to the point of <br />saturation given the prevailing geochemical conditions. The maximum TDS concentration estimated by <br />the USGS at Seneca based on undiluted lysimeter samples was approximately 5000 mg/l. This level <br />4-238d