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TECHNICAL MEMORANDUM To: Bill Lyle Date: January 3, 2004 Page 3 i I L outcrop, exploration boreholes and ground water monitoring wells drilled in this area exhibited artesian pressures with flows reported in the hundreds of gallons per minute. A conceptual model of ground water flow in the mineralized area can be developed using the available hydrologic and geologic information. Figure 6 shows a cross section extending from the north (bedrock) to the south (Rico Creek/fault line). The topography in the mineralized area is such that large hydraulic heads can be generated in the ground water system so that there is plenty of driving force to induce ground water flow. The unconfined to confined transition zone where the fault gouge crops out is interpreted to act as a pressure relief zone where deep ground water flow preferentially migrates. Again, the combination of lower hydraulic pressure near the fault gouge outcrop and hydraulic conductivity contrasts would result in preferential flow of ground water through the shallow part of the bedrock aquifer as depicted in Figure 6. Although most of the bedrock ground water flow would probably occur in the upper parts of the bedrock aquifer (and through the fault gouge), some ground water leakage would also occur at depth as evidenced by isolated iron-cementation of Santa Fe Formation i^ the West Pit area (Benson, 1997). As noted in the previous section, the Santa Fe Formation was completed eroded in the area just east of the current West Pit. As a result, there is direct contact between the bedrock aquifer and alluvium in that area. Thus, the preferential ground water flow path in pre- mining conditions was probably similar to the current preferential flow path. That is, ground water flows more easily in the bedrock and alluvium than in the Santa Fe Formation, which would mean that the geologic contact between bedrock and alluvium was a point of discharge of bedrock ground water. There is evidence from iron-stained seeps in the vicinity of the contact between bedrock and alluvium that ground water did flow as described (Figure 5). West Pit water chemistry has previously been characterized with the original Mine Permit application and then again with the review and approval of Technical Revision 26. Ground water chemistry was chazacterized prior to mining and the water quality data collected provided an unprecedented view of pre-mining water chemistry conditions in the mineralized azea of the San Luis Mine (Figure 5). Ground water monitoring wells were installed in mineralized and non-mineralized zones of the bedrock aquifer. Results of the baseline water quality monitoring for the bedrock aquifer were presented in Exhibit G of the Mine Permit application. These water chemistry data were used in the current evaluation. Ground water well chazacteristics for locations used in this analysis are presented in Table 1. Some of the ground water wells used to gather baseline water quality data were mined out during development of the West Pit (e.g., 87-13, 87-25, and 87-86). Bedrock aquifer baseline well D87-24 is the only ground water well that remains from the mining and reclamation activities (Figure 5). nb~ iPadll2.nercmongamJuisp~-wfuitvr-zxxmq~nNreminr rexrpicdoa