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<br />base. The maximum manganese (Dissolved) concentration observed in WCMW-6-234 over 20 <br />measurements was 0.268 mg/1. The manganese data set for well WCMW-6-234, consisting of <br />20 measurements, is not normally distributed. It meets the criteria for log normal distribution. <br />The log normal distribution parameters are mean = 0.161 mg/I and standard deviation (as a <br />multiplier) = 1.181. The highest data point, recorded in August 1997, was 0.268 mg/I, which <br />is 3.1 standard deviations above the log-normal mean. Because the higher data point appears <br />to be a valid measurement of background, it would be reasonable to expect similar <br />concentrations in the future, as well an occasional higher value. <br />Again in the case of well WCMW-6-234, there are 10 additional wells at upgradient locations <br />to this well, all of which have generated data reported to the OMLR. This provides a unique <br />opportunity to observe the distribution of manganese upgradient of the monitoring well. The <br />maximum manganese (D) concentrations for each of these wells over the period of record are <br />0.46, 0.72, 0.26, 1.2, <0.05, 0.076, 0.838, 0.35, 0.30, and 0.64 mg/I'. Given the isolation <br />of CC&V's ongoing operations from the watershed in which these wells are located and given <br />the fact that CC&V's operations were initiated in the adjacent watershed well after these wells <br />began to generate data, all of these maximums represent ambient or "baseline" conditions. <br />Because every one of the wells is located upgradient of WCMW-6-234, these maximum <br />concentrations might be observed at WCMW-6-234 as water migrates toward this well. A limit <br />for manganese (D) should take these ambient upgradient concentrations into account. In fact, <br />a trigger level of 1.0 mg/1 would properly reflect background concentrations based only on these <br />wells. <br />If the maximum manganese concentrations observed in the eleven monitoring wells in Bateman <br />Creek are considered, the mean maximum lognormal value is 0.335 mg/I and the standard <br />deviation is a multiplier of 2.65. The 99 percent confidence limit for these maximums thus <br />becomes, on a lognormal basis, 3.25 mg/l. <br />Nonetheless, to adhere as much as possible to the initial values issued for comment, CC&V has <br />re-proposed a manganese trigger level of 0.5 mg/1 for WCMW-6-234, provided, of course, that <br />the record shows the higher values described above also would be ambient concentrations. <br />Sulfate (SO,) trigger levels are of similar concern. First, any level set for sulfate (in this case <br />250 mg/I was proposed) must be a guidance level rather than an absolute limit because there is <br />not ahealth-based sulfate limit. Of course, the SO4 criterion proposed by the OMLR of 250 <br />mg/1 is based on a secondary guide level that indicates, when exceeded, that one should examine <br />alternative water supplies if these are available. Second, the range in maximum values observed <br />in the 10 monitoring wells located upgradient of WCMW-6-234 are 9, 258, 44, 600, 100, 90, <br />140, 130, 221, and 260 mg/I. A trigger level of 250 mg/I would not be suited for these levels <br />should ground water from any of these locations flow to WCMW-6-234. CC&V is re-proposing <br />These data have been reported to the OMLR as part of the ground-water monitoring program for Amendment <br />No. 6. <br />7 <br />