4.3. SR 10-18-1993 - Laserfiche WebLink

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e e e Ms. Amy Hadiaris Minnesota Pollution Control Agency September 28, 1993 Page 2 wells downgradient of these wells, along with continued monitoring, needs to be performed. Screen placement within downgradient wells, in relation to elevations of detected upgradient contaminants, is not considered adequate for effective contaminant monitoring. Prior to additional data collection, additional nested monitoring wells are requested. These wells should be screened at sufficient depth to monitor contaminants that are similar to or higher in density than water. These monitoring wells should be located downgradient of three locations: 1) 18-0WA, 24-0WA and P-43-B to the southwest; 2) P-17-C and 26-0WA to the west; and 3) 32-0WA and 33-0WA to the north. 2. At this time, the vertical gradients and surface recharge zones within the wetlands located west and southwest of ERL are not fully understood. However, some recharge zones north and northwest of ERL have been identified. It is therefore possible that surface recharge zones exist within the wetlands. Also of concern are potential further downgradient effects upon Rice Lake. Therefore, additional groundwater and surface water monitoring within the wetlands together with monitoring of Rice Lake is requested. Because background data is also missing from the wetlands study presented in the CAIR., locations that would produce background analytical information for comparison with wetlands sample data should be described. Proposed monitoring locations and methods, i.e. monitoring wells, surface grab samples, bomb sampler etc., should be defined in a workplan addendum to the CAIR. 3. A single monitoring well network, as opposed to separate Environmental Monitoring System (EMS) and Remedial Investigation (RI) networks now used, needs to be established. This network should be determined based upon existing hydrogeologic and analytical information and should be reduced from a combined EMS and RI monitoring well network. For example, the criterion of an MPCA established total suspended solids (TSS) concentration limit of 500 ppm for groundwater within monitoring wells, could be used for eliminating wells screened solely within the till. Other criteria that could be used for eliminating monitoring wells from this network include well materials and construction techniques and screen placement in relation to strata to be monitored. 4. Additional monitoring should be conducted at ERL in conjunction with determination of remedial alternatives. Three rounds of monitoring well and wetlands sampling should be conducted in order to verify existing analytical data and to initiate a data base of chemical analytical information that can be statistically analyzed for trends and migrational relationships. 5. Subsequent to additional monitoring at ERL, an addendum to the Corrective Action Investigation Report (CAIR.) should be submitted that would discuss hydrogeological and contaminant migration issues. However, in order to accurately address these issues,