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1 The City of Everett The Floyd & Snider Team Everett Landfill/Tire Fire Site • A summary of groundwater and surface water elevation data, including a discussion of any changes in groundwater flow, direction, or discharge rates to surface water. • Recommendations for modifications to the sampling and analysis plan and a list of wells, sampling frequency, and analytes for Performance Monitoring. • Recommendation and rationale for selection of appropriate statistical analysis methodology. 4.4.3.1 Contaminants of Concern If a compound is not detected in any of the deep wells more than once in the ten scheduled Evaluation Monitoring events, it will be considered insignificant and dropped from the list of potential analytes for subsequent Performance Monitoring. Remaining compounds detected in at least two of the ten Evaluation Monitoring sampling events will be evaluated for selection as a contaminant of concern (COC) for Performance Monitoring. COC's are those compounds detected during Evaluation Monitoring at either shallow or deep monitoring wells where: • The average concentrations at any well during Evaluation Monitoring exceeded the cleanup levels, • A single detected concentration of the compound exceeded the cleanup levels by a factor of 2, or • Two or more single detections of compounds at a well exceeded the cleanup levels. Average concentrations will be determined using statistical methods appropriate for the data set conforming with the requirements of MTCA (WAC 173-340-720), Statistical Guidance for Ecology Site Managers (Ecology Pub. 92-54) and Statistical Methods for Ground Water Monitoring at Municipal Solid Waste Facilities (WAC 173-351-420). 4.5 PERFORMANCE MONfTORING The objective of Performance Monitoring as defined by MTCA is to confirm that a cleanup action has attained performance and cleanup standards [WAC 173-340-410(1)(b)]. Results from Evaluation Monitoring will establish baseline conditions for groundwater pathway Performance Monitoring. Performance Monitoring will be used to evaluate the effectiveness of site remedial actions, detect deep aquifer migration of site contaminants towards surface water, assess the potential impact of pile installation on the deep aquifer, and establish the site's regulatory compliance with respect to surface water quality protection. Shallow aquifer Performance Monitoring may consist only of water level monitoring of shallow wells and leachate trench water to demonstrate that hydraulic gradients are toward the leachate collection system. 4.5.1 PERFORMANCE MONITORING PLAN As described in 4.4.1.1, the groundwater monitoring well network will be improved prior to commencement of Performance Monitoring. Results of Evaluation Monitoring will be used to select appropriate Performance Monitoring wells. Deep monitoring wells that are found to be useful for area background and/or compliance point monitoring will be included in the F: ro cts\COEvBR Everett Landfill\PDF maker - final '� Compliance Monitoring and Contingency Plan dots\Compliance Monitoring PIan1CMCP 11-10-OO.doc FINAL - March 2001 Page 4-7