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nonparametric tolerance limit method. However, this method requires a large number of samples to achieve a false positive rate of 1 percent or less,which is required by CCR Title 23, Chapter 15, Article 5 guidelines. Thus,when the alpha level was higher than 1 percent,the concentration limit was not established and data was compared to established limits in the Waste Discharge Requirements. Nonstatistical Approach When the background data set contains 100 percent nondetect values, Sanitas applies the nonstatistical approach. This approach takes into consideration that if historically a constituent has not been detected, any detection of that constituent would be considered an exceedence. Thus, in these situations (100 percent nondetected values),the tolerance limit is set at"detect",meaning that if a value is detected above its method detection limit, this value is an exceedance of the concentration limit. Intrawell Rank Sum When Intrawell Tolerance Limit analysis is unable to normalize the data and thus a nonparametric analysis is performed, the false positive rate becomes greater than five percent. When this happens the data is compared to the concentration limit listed in the WDRs, if any. In addition, an Intrawell Rank Sum analysis is performed. This is a nonparametric procedure where the sums of ranked data sets are compared. Subsequent sample data are compared with sampling data from the initial monitoring period of the same well. It is assumed that during the initial monitoring period the well has shown no evidence of contamination nor an increasing trend. This test procedure is used to evaluate whether the historical (background data) and the compliance data have the same median constituent concentration. Trend Analysis Sen's Slope measures the change in constituent concentrations per unit time. Sen's method is not greatly affected by outliers, and the slope can be computed when data are missing. Sen's estimator is closely related to the Mann-Kendall test,which is a nonparametric rank correlation test for trend. The test uses only the relative magnitudes of the data rather than their actual values;therefore, missing values are allowed. Sen's Slope and the Mann-Kendall tests are described in"Statistical Methods for Environmental Pollution Monitoring,"Richard O. Gilbert,Van Nostrand Reinhold, New York, 1987. The Mann-Kendall test is recommended in"An Evaluation of Trend Detection Techniques for Use in Water Quality Monitoring Programs,"Jim Loflis, et al., (USEPA), 1989. Only data detected at least four times above its PQL are evaluated. Trace values used are the estimated values from the certified analytical reports. A trend analysis was conducted only on data collected from the point data was detected above the PQL. For the Corral Hollow Sanitary Landfill, the null hypothesis is"no statistically significant trend in constituent concentrations." The null hypothesis was rejected if trends in the monitoring data were significant at alpha=0.05,which is the probability of rejecting the null hypothesis of no trend when it is true (i.e. a Type 1 error). The County uses Sanitas to calculate both the Sen's Slope and the Mann-Kendall test for trends. Corral Hollow Landfill D-2 Department of Public Works/Solid Waste 4T"Qtr/Annual 2003 Groundwater Monitoring County of San Joaquin-January 28,2004