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Mr. Douglas M. Ikenberry, P.E. <br />Page 3 <br />October 27, 2012 <br />that the closest off -site well is approximately one mile from the former mine limits. The <br />well is completed in shallow bedrock and is used for stock watering. The well is generally <br />downgradient of the facility and is downgradient of monitoring well DH -96. There is no <br />residential development and essentially no use of surface water in the general area. <br />The key site characteristics from the point of view of groundwater monitoring frequency <br />are as follows. <br />1. Groundwater flow from the site to off -site areas is extremely slow (about 0.5 feet <br />per year). This means that releases (if any) would only travel a fraction of a foot <br />farther between monitoring events if the frequency is reduced. <br />2. The geologic materials are only sparingly permeable and, as a result, have not <br />been developed as a groundwater resource. Therefore, there are essentially no <br />receptors in the vicinity of the site and risks to receptors are not increased by a <br />reduced monitoring frequency. <br />GROUNDWATER MONITORING RESULTS <br />Doty & Associates, LLC <br />Groundwater monitoring began at the site prior to the commencement of mining in 1981 <br />to support baseline environmental characterization. Although groundwater monitoring was <br />on -going in accordance with the Division of Reclamation, Mining & Safety (DRMS) <br />regulations, a revised groundwater monitoring plan was developed through negotiation <br />with your Division in 1998 (Doty, 1998) such that it met both the requirements of your <br />regulations and those of DRMS. Since that time, we have been statistically evaluating <br />the groundwater monitoring results on a quarterly basis using a database beginning in <br />March 1993 for five wells (79 samples each) and in December 1999 for a newer well (51 <br />samples). A total of 446 quarterly samples have been collected from the six wells since <br />March 1993 (almost 20 years of quarterly samples). <br />During this period, the chemistry of the groundwater samples has been quite consistent. <br />There have been only two significant increases indicated by the tolerance limit based <br />statistical evaluations. The first involved dissolved iron in two wells in 2001 that we <br />suggested was caused by changed laboratory procedures and reporting limits (Doty, <br />2001). Although you did not agree with our reasoning at the time, we believe our position <br />is supported by the fact that iron has since been essentially non - detectable in these wells, <br />albeit at a higher reporting limit. The other significant increase involved manganese in <br />a well completed in the pit backfill (well SMW -2). Between March 1993 and September <br />2012, manganese in the SMW -2 samples has increased from about 0.3 to 0.6 milligrams <br />per liter and a statistically significant increase was indicated in 2004. In Doty (2005), we <br />demonstrated that the increased manganese most likely was caused by dissolution of <br />manganese from the backfilled overburden as it gradually resaturates following cessation <br />of mining and not by a release from the ash backfill. <br />