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concentrations that exceed the table values will be calculated based on statistical <br />analysis of ambient groundwater quality at the compliance wells. <br />Neither the BSGW nor the HRMMR define a statistical approach for calculation of <br />permit limits based on ambient groundwater quality data. A tolerance interval <br />approach as recommend by EPA (2009) is planned to be used to define the interim <br />permit limits. The tolerance interval is a concentration range designed to contain a <br />specified proportion of the underlying population from which the groundwater <br />quality samples are drawn. The interim permit limits applicable at the compliance <br />wells for these constituents will be calculated as follows: <br />IPL = x + r(n,.95,.95)s <br />Where IPL is the interim permit limit, x is the mean of the water quality samples from <br />each compliance well, r is the tolerance limit based on sample size with 95 percent <br />confidence that 95 percent of the underlying population is contained within the <br />tolerance interval, and s is the sample standard deviation. Because this is an intrawell <br />approach, different permit limits will be calculated for each compliance well. These <br />permit limits will apply to the dissolved portion of the sample as defined by filtration <br />at 0.45 microns, and will be applicable at the compliance wells until final permit limits <br />are established in accordance with this EPP. The statistical analyses will be conducted <br />by Denison in consultation with DRMS, and will be provided to DRMS for review and <br />approval. If actual results indicate that the foregoing statistical analysis would not be <br />appropriate, then a comparable statistical approach will be used consistent with EPA <br />guidance (EPA 2009). Although permit limits will not be set at the upgradient and <br />intermediate wells, the same statistical intrawell approach will be used to determine <br />baseline conditions at those wells. <br />Final Permit Limits <br />Groundwater quality data collected from the compliance wells will be used to <br />develop final permit limits after sufficient samples have been collected to adequately <br />characterize background /ambient water quality for each well on an intrawell basis. <br />The BSGW do not specify the number of samples necessary to characterize <br />background /ambient groundwater quality. EPA guidance recommends that at least <br />8 -10 independent samples be collected to adequately characterize <br />background /ambient groundwater quality (EPA 2009). Based on the quarterly <br />sampling frequency set forth in Section 11.2.2, additional statistical analysis of <br />background /ambient groundwater quality data will be completed to establish final <br />permit limits approximately 2 years after approval of this EPP and installation of the <br />monitoring wells. <br />Although more frequent sampling of the monitoring wells could theoretically <br />expedite this process, it is critical that independent samples are collected to support <br />valid statistical analysis (EPA 2009). Given the relatively low hydraulic conductivity <br />of the Salt Wash Member, the slow estimated groundwater velocity of 3.3 to 6.2 feet <br />per year, and the negligible recharge rate to the Salt Wash aquifer caused by the <br />C \ Users \cwoodwarddDesIdop\Sundays \FINAL - Environmental Protection Plan Sunday Mines Revised January 2012 docx <br />11 -8 <br />