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Ryegrass Limited Purpose Landfill and Balefill Closed Landfill <br />Groundwater and Leachate Sampling and Analysis Plan <br />911 itm Cuunty Wid Waste <br />representative constituents thought to be reliable indicators of a contaminant release, that exhibit a <br />large concentration contrast between leachate and groundwater and are relatively mobile In <br />groundwater. In accordance with Chapter 173-350 WAC, selection of parameters for formal <br />statistical analysis wilt initially Include pH, specific conductance, chloride, iron, manganese, nitrate, <br />sulfate, ammonia, and total dissolved solids. If leachate data are available in the future, the subset <br />of parameters will be updated based on a comparison of the ratio of average concentrations In <br />leachate to upgradient groundwater. <br />3.1.1.2 Control Charts <br />Shewhart-CUSUM control charts are more sensitive to gradual releases than UPLs but are somewhat <br />less sensitive to immediate releases. The combined Shewhart-CUSUM control chart method involves <br />plotting parametric statistical representations of concentrations over time. Upper control limits are <br />computed and a data result that falls outside the control limits provides evidence of potential <br />contamination. <br />The control charts will show the cumulative sum (CUSUM) and the compliance data since the <br />background period. These will be compared to the respective control limit (h) and upper Shewhart <br />control limit (SCL) calculated using background data, to determine whether a statistically significant <br />increase has occurred. The use of control charts assumes the data are independent and normally <br />distributed. Additionally, Shewhart-CUSUM control charts cannot be calculated when the percentages <br />of non -detected values data are greater than 50 percent. <br />3.1.1.3 updates to Statlsticai Analysis <br />A new monitoring well is proposed that is anticipated to be upgradlent of both landfill's. The statistical <br />approach will be re-evaluated following at least eight quarters of groundwater monitoring at new well <br />B8 to evaluate whether interwell comparisons would be more suitable than the existing intrawell <br />evaluation. Similarly, additional laboratory analyses of leachate will be used to reevaluate site - <br />specific parameters for statistical comparisons that have a high contrast with groundwater, <br />3.1.2 Quarterly Evaluation <br />Quarterly data evaluation will include preparingtime series plots to graphically represent all previous <br />and current groundwater monitoring data and comparing data for the current quarter to calculated <br />UPLs. <br />3.1, 3 Annual Evaluation <br />3A.3.1 Upda'Unid Backgtound Data Sets <br />In accordance with the Unified Guidance, with intrawell tests using UPLs or control charts, updating <br />background data sets Is performed both to enlarge initially small well -specific background data sets <br />and to ensure that more recent compliance measurements are not already impacted by a potential <br />release (even if not triggered by the formal detection monitoring tests). The Unified Guidance adopts <br />the general principle that updating should occur when enough new measurements have been <br />collected to allow a two -sample statistical comparison between the existing background data set and <br />a potential set of newer data. Updating intrawell background data sets should not occur until at least <br />four to eight new compliance observations have been collected. <br />Further, a potential update is predicated on there being no statistically significant increase [SSI] <br />recorded for that well constituent, including since the last update. Then a t-test or Wilcoxon rank -sum <br />April 2024 1 553-1763-010 3-2 <br />Revised May 2024 <br />