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West Pit Backfill Wells: <br />BF-1 BF-2 BF-3 BF-4 BF-5 BF-6 <br />BF-6 BF-7 BF-8 <br />For each proposed water-quality parameter (e.g., TDS, sulfate, manganese, etc.), the water- <br />quality condition for a given groundwater sampling event in the performance-monitoring zone <br />would be taken as the arithmetic average of values from the MW-wells. If one or more well in <br />the set does not yield sufficient water for analysis, the arithmetic average of the remaining wells <br />in the performance-monitoring set would be used. <br />Because the purpose of the TR-15 response period is to return water quality to a steady-state <br />hydrologic and geochemical condition, progress towazd that goal during the response period <br />would be evaluated using the following approach: <br />• Water-quality data for the performance monitoring wells in the Rito Seco alluvia] <br />aquifer would be compiled for the time period between December 1998 and Apri] <br />1999. <br />• For each sampling event, the arithmetic average of the eleven wells would be <br />computed for each designated water quality pazameter. <br />• For this set of sampling data, the individual average values would be used to establish <br />a test statistic. The test statistic would be computed as the arithmetic mean and the <br />upper 95% threshold limit for the set of averages using Student's t-distribution. (A <br />lower 95% threshold limit would be used for pH.) <br />• Average water-quality values for each monitoring pazameter would be tracked as a <br />time series using a Shewart control chart that identifies the baseline time period for <br />this monitoring program. In addition, the monitoring program would calculate (and <br />graph) the CUSUM statistic, using the sample mean over the initial test period as the <br />tazget level, to evaluate progress at improving water quality through the corrective <br />actions proposed by BMRI. [Note: For technical reasons having to do with the <br />definition of CUSUM, after pazameter values fall outside either the upper or the lower <br />control limit in a Shewart chart, it is no longer useful to continue to monitor CUSUM, <br />the purpose of which is to identify trends among time-series samples that lie near the <br />target-value before values approach control limits.] <br /> <br />Battle Mountain Resourcer, /nc. <br />p.l/002671reporulmnrchrptl~rl6wvmngtmarch.doc 7~l Mnrch 12. 1999 <br />