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FW Elk River Landfill - Review of Phase ITI Report for MSW Expansion.tx•t <br />will not be affected by the development of the site. The engineering design for <br />surface water routin will result in a significant change in the hydraulics for the <br />wetland area. The a~fects on the wetland are unknown at this time. The <br />significance of localized groundwater recharge from infiltration at the <br />sedimentation ponds is unknown at this time. zt may be appropriate to assess <br />impacts to the wetland area and groundwater using a technical approach. <br />pa e 10, Water Well Survey: A comprehensive updated water well survey.is presented. <br />The Ridges of Rice wake community water supply well is located approximately 2/3 <br />mile downgrad7ent of the site. The ppotential for impacts to this well is very small <br />considering the well cased to a depth of nearly 300,feet, however GRL should <br />consider the inclusion of this well in the residential well monitoring program. <br />page 15, Continuous Till layer: A discussion is pprovided regarding the fine-grained <br />soil deposits (till) separating the "upper outwash" unit from the lower outwash" <br />unit. Tt is stated that the thickness of the till deposits varies from 5 to more <br />than 50 feet at the Site. Figure 15 presents a Till zsopach Map which shows areas <br />of the site where the till thickness is less than ten feet. Although ~t is not <br />stated within the text of the report or on the map, it is implied that the till <br />la er is continuous throughout the site. The available boring data suggest that the <br />till layer is continuous, however, the borings in the west central portion of the <br />site do not extend to an adequate de th to definitively make this conclusion. ~t <br />should be noted that borings 9, 16 (~), 24, 25 to do not provide evidence that the <br />till is continuous. rt should be further noted that the geologie cross-sections <br />which depict a cantinuous till layer for this portion of the site are interpretive. <br />There exists a possibility that the till is discontinuous at the site and the upper <br />and lower outwash units are in hydraulic connection. It should be noted that the <br />absence of the till layer is the situation which is present for the existing <br />landfill area in the vicinity of the ERL shop. Discontinuity of the till layer has <br />implications for establishing an effective groundwater monitoring system at the <br />site. <br />Page 20, Groundwater Discharge: The report states that groundwater in the north <br />central pportion of the site flows northwest and is interpreted to discharge to Rice <br />`ake/Tibbits Brook. zt is understood that wMZ.has conducted additional site <br />investigation related to the groundwater impacts from the existing waste deposit. <br />No data has yet been presented to support this groundwater discharge conclusion. <br />page 22, Groundwater Monitoring: wMZ proposes the addition of one nested piezometer <br />in the lower outwash to evaluate the hydraulic gradient across •the till layer. <br />Given the concerns regarding the continuity of till layer, the piezometer should be <br />utilized as a monitoring well and included in the proposed monitoring work plan. A <br />second nested monitoring well (p322C location suggested) should be installed along <br />the western portion of the south boundary for similar gradient and water quality <br />monitoring purposes. <br />A maximum spacing of 400 feet between downgradient compliance boundary monitoring <br />wells has be used for the existing landfill. zt is suggested that this maximum <br />spacing be maintained for the landfill expansion. This implies that several <br />additional upper outwash monitoring wells would be included in the manitoring <br />network. <br />page 22, well installation schedule: The report states that the current network of <br />monitoring wells is sufficient to monitor the expansion of the fi17 area through <br />cell 20. The southerly limit of fill of Cell 20 would correspond to plan stat7on <br />246,100 based on available enggineering plans for site development. From the <br />conceptual Hydrogeologic Model, Figure 17, a southwesterly component to groundwater <br />flow from portions of the proposed fill area is apparent.. There are no monitoring <br />wells existing or planned within a reasonably close distance that would intercept <br />these groundwater flow lines. The suitability of the existing network of wells to <br />monitor the development of the site through cell 20 is questionable. <br />Page 2 <br />