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• Spatial Variability: If background concentrations are expected to change <br />substantially from one well to the next due to natural <br />hydrogeologic /geochemical conditions, an intrawell approach provides the <br />most accurate baseline for use in statistical comparisons. <br />• Definable Hydraulic Gradient: Where the groundwater flow is not in a <br />definable pathway from upgradient to downgradient wells, an interwell <br />approach may not be valid and an intrawell approach is appropriate to <br />consider. <br />• Flow Velocity: Interwell approaches (e.g., upgradient to downgradient well <br />comparisons) assume that groundwater flows at a sufficient velocity <br />downgradient from the site so that the same groundwater observed at <br />upgradient well locations is subsequently monitored at downgradient wells <br />during the course of an evaluation period (e.g., six months to a year). If <br />groundwater flow is much slower, measurements from upgradient and <br />downgradient wells may be more akin to samples from two separate aquifers <br />and an intrawell approach is appropriate. <br />Based on the slow groundwater velocity, the likely inward gradient of groundwater <br />towards the underground mine caused by historic mine activity, and the negligible <br />recharge rate, it is unlikely that groundwater at the compliance wells has been <br />affected by past mining activities. Accordingly, background and ambient <br />groundwater quality as defined by the BSGW and the HRMMR at the location of the <br />compliance wells are essentially the same, because it is unlikely that groundwater at <br />the compliance wells has been affected by mining activities. Further, in the unlikely <br />event that the compliance wells have been impacted by historic mine activities, it is <br />even more unlikely, given the slow rate of groundwater flow and the distance from <br />the underground mine workings to the compliance well locations, that any such <br />impacts could be caused by mining activities after January 31, 1994. <br />Based on the site - specific conditions, permit limits applicable at the compliance <br />monitoring wells will, therefore, be based on intrawell statistical analysis of data <br />collected from the groundwater compliance wells. The intrawell approach is not <br />affected by spatial variations in groundwater quality. Permit limits based on the <br />intrawell approach at the compliance monitoring wells will be the best indication of <br />background groundwater quality at these locations. <br />Interim Permit Limits <br />Groundwater at the Sunday Mines Group is not classified. Therefore, the narrative <br />standard of BSGW §41.5(C)(6)(a) applies, which requires that groundwater meet <br />either the "existing ambient quality as of January 31, 1994, or that quality which meets <br />the most stringent criteria set forth in Tables 1 through 4 of the BSGW, whichever is <br />less stringent ". After five quarters of data have been collected from the compliance <br />wells, constituents that are present in the groundwater at concentrations lower than <br />the most - stringent criteria of Tables 1 through 4 of the BSGW will be identified. The <br />most - stringent criteria set forth in Tables 1 through 4 of the BSGW will be used to <br />define permit limits for those constituents. Permit limits for constituents that occur at <br />C \Users \cwoodwardfDesktop \Sundays \FINAL - Environmental Protection Plan Sunday Mines Revised January 2012 docx <br />11 -7 <br />