5.4. SR 05-17-2010 - Laserfiche WebLink

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v ReviewComments on the Revised Hydrogen Report (Phase Z and II) and Phase ITZPhase Iv work Plan.t~ down- radient portions of the site. All three of the borings show,the occurrence of sand ~ayers, previously not shown on cross-sections at these locations, which are present within the till. These sand layers lead to questions regarding the ' continuity of the till layer and potential connection between the upper and lower outwash units. cross-section A-A' Additional data from P318~ indicates within the till. The upper sand layer shown for sB~32. The lower sand layer lower till contact elevations for s6-31 the two sand layers to be in connection of the site. the occurrence of two significant sand layers (913 to 919) correlates with a sand layer (889-905) is 16 feet thick, eased on the and Ss-33, there is a potential to interpret with the upper outwash unit for this portion cross-section s-B' The additional data collected for P318~ adds significance to the 1ithblogic interpretation for borings sB15, 5616 and ss-17 which shows a significant sand layer roughly between 890 and 916 for these borings. The sand layer dips down to the east and west of sa16 with a trend that could potentially intersect the lower outwash unit. This sand layer may be connected to the sand layers shown in cross-section A-A' with the potential that the upper and lower outwash units are cannected to farm a groundwater flow pathway. Cross-Section G-G` The litholo~ic data for 317E shows the occurrence of two sand seams (907-912 and $$7-897) within the till. A significant sand layer (893-902) is shown for sa35 further to the south. There is a strong potential for these sand layers to be connected although the cross-section does not show this. There is also a potential that the ~ower sand seam could be connected to the lower outhwash unit. The information from the additional deep borings shows the complexity of the geology and potential lack of continuity in the till confining layer in the south and west portions of the Site. Hydraulic connection between the upper and lower outwash units is not supported by water level monitoring information far nested wells. The water level data show a vertical head difference of approximately 4.5 feet between the outwash units at all locations for this area of the site. Cross-section H-H' has been redrawn to show discontinuity in the outwash and a groundwater flow boundary located between Sa-2 and sB~9. Figures 17, 19 and 20 should be revised to show the extension of the flow boundary further to the east in the northwest area of the s~A. section 5.0 discusses the manitoring wells proposed for the compliance boundary of the s~A. City of Elk River comments of May 11, 2004 identified the concern regarding the proposed distance of •the wells from the fill limit during the early years. of operation. The City suggested that a sequenced approach be taken for the compliance boundary and the placement of compliance boundary wells coinciding with the 10-year fill limit. MPCA correspondence dated July,26, 2005, from aohn Elks to Terry 7ohnson of wMZ indicates that a phased approach will be used for the compliance boundary. The city supports the MPCA in relocating proposed monitoring page 2