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All of the mines within the Sunday Mines Group were permitted prior to January 31, <br />1994. Therefore, the ambient groundwater quality is defined as the quality present on <br />January 31, 1994. As discussed in Section 8.2, the BSGW clarify that "data generated <br />subsequent to January 31, 1994 shall be presumed to be representative of existing <br />quality as of January 31, 1994, if there have been no new or increased sources of <br />groundwater contamination initiated in the area in question." Although a limited <br />amount of mining has occurred briefly on two occasions at the Sunday Mines <br />Complex since January 31, 1994, the mining activities were essentially the same as <br />conducted prior to January 31, 1994 (although with little impact to areas below the <br />water table), and the water quality measured in the underground well nest (see <br />Section 8) is representative of ambient conditions in close proximity to the <br />underground workings. <br />However, the ambient groundwater quality at the underground well nest may not be <br />representative of ambient groundwater quality at the compliance wells. Spatial <br />variability in groundwater quality is expected to be significant based on the following <br />considerations: <br />• Groundwater is in geochemical equilibrium with the rocks hosting the aquifer, <br />and these rocks contain highly variable concentrations of uranium, vanadium, <br />and other trace metals and metalloids associated with the natural uranium and <br />vanadium concentrations. Sedimentary rocks hosting the aquifer also contain <br />variable concentrations of evaporate minerals such as gypsum and halite, <br />which lead to elevated TDS concentrations in groundwater. These evaporate <br />mineral concentrations vary laterally and vertically in relation to natural <br />stratigraphic facies changes. <br />• Groundwater moves very slowly (estimated groundwater velocity is only 3.3- <br />6.2 feet per year), which makes it likely that groundwater will equilibrate with <br />the rock along its flow path leading to variations in concentrations of uranium, <br />vanadium and other trace constituents in groundwater. <br />• Activities inherent to the underground mining process such as mine <br />ventilation may lead to local changes in groundwater quality in close <br />proximity to the underground workings. <br />As a result of this spatial variability in groundwater quality, the ambient groundwater <br />quality measured in close proximity to the undergound mine may not be <br />representative of the ambient groundwater quality at the intermediate and <br />compliance wells. Further, upgradient wells are not likely to be representative of <br />downgradient conditions given the expected spatial variability of groundwater <br />quality. <br />The hydrogeologic and geochemical conditions present at the Sunday Mines Group <br />are, therefore, well- suited to use of an intrawell statistical approach based on EPA <br />guidance for statistical analysis of groundwater monitoring data (EPA 2009). <br />Important considerations in selecting an intrawell statistical approach include: <br />C \Users\cwoodwardfDesk top \Sundays\FINAL - Environmental Protection Plan Sunday Mines Revised January 2012 doco <br />11 -6 <br />