2008-09-17_REVISION - M1980244 (28)

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The water quality conditions at GVMW-8A and GVMW-8B (Tables C16 and C17 of the • Water Management Consultants, Inc. (WMC) report, MLE Permit Application, Volume II, Appendix 2 - included in Attachment 10) differ from other compliance wells (PGMW-2, SGMW-3A, SGMW-3B, VIN-2A, and VIN-2B) due to the general absence of historic mine activities in the GVMW-8A and GVMW-8B area. Historic mine disturbance in Squaw Gulch and Vindicator Valley have affected the water quality of the shallow "B" monitor wells. Typically, elevated sulfate, total dissolved solids, nitrate, and selected cations (Tables A24 and A27 of the WMC report) are associated with the historic mine disturbance. In Squaw Gulch and Vindicator Valley, the deeper "A" monitor wells (Tables A23 and A28 of the WMC report) have been dry since installation. The dry conditions are consistent with the downward hydraulic gradient in the diatreme. The Poverty Gulch monitor well has been dry (at 218 feet) since completion. The dry conditions in PGMW-2 also are consistent with the downward hydraulic gradient in the diatreme. The water quality data for GVMW-8A and GVMW-8B show relatively good conditions. The water quality sources for GVMW-8A and GVMW-8B reflect surface water recharge from upgradient in Grassy Valley and natural water: rock interaction along the vertical infiltration path within the diatreme. Although the capture area of monitor wells has some historic water quality impacted areas (old tailings in Grassy Creek), the runoff and infiltration from undisturbed areas on the northern slopes of Grassy Valley appear to have a greater influence on the water quality conditions at GVMW-8A and GVMW-8B. The field data for GVMW-8A. and GVMW-8B also suggest a moderate downward hydraulic gradient • is present in the diatreme in this area. H. If The Grassy Valley is within the diatreme, the hydraulic gradient at depth is expected to be essentially opposite to that of the shallow alluvium (Fig.4.9). It is difficult to comprehend that all the infiltration will be captured by the deeper hydraulic gradient before being transported down Grassy Valley and off site by the shallow gradients, especially since the water management report indicates the rate of transmissivity to be higher in the alluvium than in the deeper bed rock. This possibly could lead to metal loading to the Grassy Valley alluvium and points down gradient. The proposed mitigation plan in the event of this occurrence appears satisfactory. However, since CC&V is going to place a potentially higher sulfur content overburden in ECOSA, the DBMS proposes all the additional monitoring wells in the area in addition to the current compliance wells GVMW-8A &B be considered as compliance wells, with the same reporting criteria as the current ground water monitoring approved plan. In addition, one or two down gradient wells should be added in both the shallow alluvium aquifer and the bed rock aquifer. It is not very clear from the discussions in the water management report if the aquifers are perched or not? Please explain. Response: The hydraulic gradient beneath the ECOSA is expected to be vertical, until the water reaches the saturated portion of the diatremal rocks at considerable depth (elevation 9,300 ft). There is not expected to be any lateral downgradient flow in the overlying colluvium because it is of • considerably lower hydraulic conductivity than the diatremal bedrock. Thus, water from the ECOSA would infiltrate into the clayey colluvium and would proceed vertically downwards. No water is expected to appear at, or to flow beneath, the toe of the ECOSA. 35