2012-02-28_REVISION - M1980244 (90)

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Cresson Project Hvdrogeochemistry AdrianBrown overlies the Diatreme was constructed in 2002 If this portion of the liner prevented the natural 6 inches of infiltration per year on the Diatreme, this would constitute a permanent seepage flow reduction of approximately 80 acre -feet per year (50 gpm) 2. Reduction in precipitation. There has been a 0.4 inch per year reduction in precipitation over the last 8 years. At the pre -2002 diatremal infiltration rate of 30 %, the reduction in seepage would be 0.12 inches per year. Over the entire 7 square mile diatremal area, a 0.12 inch per year reduction in infiltration would result in approximately 60 acre -feet per year reduction (35 gpm) in Carlton Tunnel flow. Because infiltration is governed by high precipitation events which are less frequent and less extreme in low precipitation years, a greater reduction in seepage would be expected. 3. Sequestration in overburden produced by prior surface mining. Infiltrating water cannot pass through overburden until it reaches its field moisture capacity, which has been measured to be 7.5% by volume in column tests on crushed ore (CC &V, 2000). When the field capacity is reached, exfiltration from the base of the overburden approximately equals infiltration. The total area covered by overburden in 2011 was approximately 600 acres. If the natural infiltration of 6 inches per year was sequestered by this overburden, this would constitute a (temporary) Carlton Tunnel flow reduction of 300 acre -feet per year ( -180 gpm). This does not appear to be occurring, as at most 160 acre -feet per year (100 gpm) of the reduction is unaccounted for by changes in the above two categories. Further, the flow reduction due to sequestration is not observed to have increased over time, as would be expected with the increase in overburden produced. What is occurring is likely more complex: sequestration is occurring in newer overburden, but the effect on Carlton Tunnel flow is being compensated for by exfiltration from overburden and mine backfill areas where field capacity has already been reached, and flow of infiltrating water has resumed. The result is that net sequestration has reached approximate steady state, and is not expected to change with future mine extension, including MLE2. Taken together, the reduction in Carlton Tunnel flow reduction since 2002 is 10% is due to the recent reduction in precipitation, 25% is due to covering of diatremal catchment by the AGVLF, and the remaining 65% is due to the net effects of sequestration. The character of the regional ground water system flow from the Carlton Tunnel portal has also changed since 2002. The variation of flow rate as a result of precipitation is much more muted, with little correlation between precipitation peaks and tunnel outflow since that time. This is attributed to the closure by the current surface mining activities of most of the direct points of access for precipitation and storm water flow into the Diatreme via underground mine openings (mainly shafts, and subsidence and collapse areas). The infiltration and storm water flow that formerly reached the Carlton Tunnel had more potential direct infiltration routes, resulting in a portion of the flow arriving shortly after the 4 The AGVLF was expanded to approximately 400 acres in 2002, with the expansion for the first time covering a portion of the Diatreme. It was further expanded to 514 acres in 2011, with essentially all of the expansion occurring in locations underlain by diatremal rock. There has been too little time to see if this latest change has further reduced flow at the Carlton Tunnel portal. 5 "Permanent" meaning continuing until the AGVLF liner system is breached at closure. 1385L.20120125 7