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N, = (x, - Rd /sr where <br /> N, is the index number; <br /> x, is the observed value of dissolved Zn for the reporting period or the average of <br /> dissolved Zn valves if more than one sample is taken in the reporting period; <br /> xf is the mean value of dissolved Zn in the reference water-quality set for the <br /> flow class to which the observed value belongs; and <br /> sf is the standard deviation of dissolved Zn in the reference water-quality set for <br /> the flow class to which the observed value belongs. <br /> The index numbers will be plotted against the sampling <br /> begins with the number 1 as the first sampling event of the Project pen* (which <br /> graph which has the index number(called "normalized residual")as the ordinate <br /> and the sampling period number as the abscissa_ An example of such a graph is <br /> shown on Figure I a. <br /> SGC will compute a 12-month moving average <br /> Beginning with the twelfth 12th month of �theCtindex numbers, as follows. <br /> the mean of the index numbers for the 12-month period, SGC will compute <br /> Period <br /> month, the oldest index number will be dropped <br /> . In each subsequent <br /> added to the set of 12 ft I <br /> rom which the mean inde number is be g c mputed. <br /> uer <br /> The moving-average index number also will be plotted over time, as shown in the <br /> example of Figure 1 b. <br /> (d) Evaluation of the normalized data will be performed as follows: <br /> If a single observation has an index number greater than+2.0(i.e., that <br /> observation has a concentration of dissolved Zn greater than 2 standard deviations <br /> above the mean value of dissolved zinc for the relevant flow class in the data of <br /> the reference water-quality set), SGC will have a repeat analysis done on the <br /> sample. <br /> If two out of three consecutive observations have index values greater than +2.0. <br /> then SGC will report by the 28th day of the month following receipt of the second <br /> of those observations: <br /> 37J/2-/ Z112196 <br /> 3_a <br />