5.4. SR 05-17-2010 - Laserfiche WebLink

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Page Q of 6 Letter to Mr. John Elks & Mr. Matt Ledvina RE: Revised SDA T~Iydrogeologic Evaluation Report Elk Rivcr Landfill, Inc. approximately 2/3 mile downgradient of the site. The potential for impacts to this well is very small considering the well cased to a depth of nearly 300feet, however ERL should consider the inclusion of this well in the residential well monitoring program. WMI currently monitors the Ridges of Rice Lalce community water supply well annually on a voluntarily basis. Page I5, Continuous Till Layer: A discussion is provided regarding the fine-grained soil deposits (till) separating the "upper outwash"unit from the "lower outwash" unit. It is stated that the thickness of till deposits varies from S to mop°e than SO feet at the Site. Figure IS presents a Till Isopach Map which shows areas of the site where the till thickness is less than ten feet. Although it is not stated within the text of the report or on the map, it is implied that the till layer is continuous throughout the site. The available boring data suggest that the till layer is continuous, however, the borings in the west central portion of the site do not extend to an adequate depth to definitely matte this conclusion. It should be noted that borings 9, 16(x), 24, 25 to do not provide evidence that the till is continuous. It should be further noted that the geologic cross- sections which depict a continuous till layer for this portion of the site are interpretive. There exists a possibility that the till is discontinuous at the site and the upper and lower outwash units are in hydraulic connection. It should be noted that the absence of the till layer is the situation which is present for the existing landfill area in the vicinity of the ERL shop. Discontinuity of the layer has implications for establishing an effective groundwater monitoring system at the site. Geologic data from the recently advanced soil borings, SB51 and SB52, suggest the till is continuous across the site. `~,~- •,, -- ~~rei4~:~:a 3. Page 20, Groundwater I?ischarge: The report states that groundwater in the north central portion of the site flows northwest and is interpreted to discharge to Rice Lake/Tibbits Broolc It is understood that WMI has conducted additional site investigation related to the groundwater impacts from the existing waste deposit. No data has yet been presented to support this groundwater discharge conclusion. Results of the second and final phase of the Rice Lake/Tibbits Brook Area groundwater investigation will be submitted shortly. Results from the second phase of investigation support the groundwater discharge conceptual model. Page 22, Groundwater Illonitoring: WMI proposes the addition of one nested piezometer in the lower outwash to evaluate the hydraulic gradient across the till layer. Given the concerns regarding the continuity of till layer; the piezometer should be utilized as a monitoring well and included in the proposed monitoring workplan. A second nested monitoring well (P322C location suggested) should be installed along the western portion of the south boundary for similar gradient and water quality monitoring purposes.