SITE INFORMATION AND CORRESPONDENCE

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• Exhibit 7 • Considerations for Ground Water Quality Sampling and Analysis Select sampling locations,sampling techniques,analytical methods,and sampled constituents based on the goals of the system(e.g.,sampling from the body of the plume may not be required if plume containment is the only system goal). Note piezometer and monitoring well integrity and condition. Routine redevelopment or cleaning may be necessary. Use consistent sampling techniques and analytical methods;report any inconsistencies if they do occur. Select sampling techniques that are appropriate for the site: D Consider dedicated sampling equipment when cost-effective and appropriate. D Utilize low-flow sampling when appropriate(e.g.,reduced sampling time,more accurate measure of dissolved metals concentrations, less purge water,etc.). D Consider traditional purge and sample techniques if parameters,such as turbidity,do not stabilize in a reasonable time frame during low-flow sampling. Data validation is a methodical process of checking precision,accuracy,and completeness of laboratory data quality and utility. Such validation is merited during initial investigations and at other decision points of the remedy but should be avoided for most routine sampling events during O&M. Interpret ground water samples from each event with respect to the site conceptual model and site goals. Sample for appropriate natural attenuation parameters if natural attenuation is or will be considered as a site remedy. Reconsider sampling frequency and locations if current amount of data and interpretation is either insufficient or excessive with respect to the system goals and site conceptual model. The monitoring frequency for water levels and water quality need not be the same. Note.Inaccurate or insufficient data can lead to poor management decisions,and excessive data are not cost-effective. New plume maps,potentiometric surfaces,and data Increased or constant concentrations, or even trend plots do not always require immediate submission decreased concentrations that remain above the site in individual reports. In some cases, it may be more standards, in downgradient or sentinel wells may appropriate to collect data and generate plots for indicate inadequate capture by the extraction system. several events prior to interpreting the combined results In such cases, the capture zone should be analyzed, in a single O&M report(e.g., generating an annual and the extraction system may require augmentation. report that discusses four quarters of ground water monitoring). Increases or constant, but elevated, concentrations in a well may indicate the presence of a continuing source The frequencies of monitoring events, data analysis, of contamination from the vadose zone or from NAPL. and reporting should each be commensurate with the If such sources are not addressed, the P&T system will time frame for site decision making and consistent with likely operate indefinitely. appropriate regulations. Relatively new systems, where the system and the site conditions are in a state of flux, Decreases in contaminant concentrations at wells may may require more frequent monitoring, data processing, indicate success of the remedy,but they may also and reporting than relatively mature, stable systems. A indicate dispersion or contaminant transport to review of historical trends in the data may help a site downgradient areas. Reviewing water quality data at team determine if a change in monitoring frequency is other locations or ground water flow patterns may merited. help confirm which of the above is occurring. Contaminant levels may also decrease and then Evaluate Concentration Trends At Monitor Wells plateau above cleanup levels or may rebound after pumping has stopped. The trends in concentrations at each monitoring well and groups of monitoring wells should also be studied. 10