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WATER QUALITY EVA—uATIONS AT STATION A72 ON THE ANIA,,-ta RIVER USING <br /> CONTROL CHARTS <br /> confidence limit for the 95% envelope and seven degrees of freedom has a critical t <br /> value of 2.365. For a mean of 336 ug/l and a standard deviation of 49 ug/l, the <br /> upper control limit for (100 cfs <Q < 300 cfs) would be 452 ug/l. <br /> 6. Table 3 presents and analyzes Zn data for the High-Flow (Q > 300 cfs) data set. <br /> Given that the population of Zn concentrations in the Animas River at A72 can be <br /> described by Student's-t distribution, the upper confidence limit for the 95% <br /> envelope and ten degrees of freedom has a critical t value of 2.2281. For a mean <br /> of 259 ug/1 and a standard deviation of 73 ug/l, the control limit for (Q > 300 cfs) <br /> would be 421 ug/1. <br /> Method for Evaluating Maintenance of Water Quality Over Time <br /> 1. As water-quality monitoring proceeds, a body of data will accumulate that can be <br /> compared to the baseline set and the statistical "reference water quality" that it <br /> represents. The new data can be compared with the old data to determine whether <br /> there is a statistically discernible change in water quality. <br /> 2. Several statistical tests are available to evaluate the status of water quality, among <br /> which control charts are perhaps the most powerful because the X-chart shows <br /> trends in water quality. A mirror image of the upper control limit can be plotted, <br /> and, if values fall consistently between the upper and lower control limits, then the <br /> data would demonstrate that the current water quality has been maintained. <br /> Geochimica, Inc. 5 ANIMAS RIVER/9-Jun-95 <br />