5.4. SR 05-17-2010 - Laserfiche WebLink

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Page 2 of 6 Letter to Mr. John Elks & Mr. Matt Ledvina RE: Revised 5DA Hydrogeologic Evaluation Report Ellc River Landfill, Inc. characterization of the lower outwash sand is needed. The direction and rate of groundwater flow of the lower outwash sand shall be documented. Two soil borings and four piezometers were installed between November 2004 and April 2005 to evaluate the hydraulic relationship between the upper and lower outwash units. The four piezometers were completed in the lower outwash unit as deeper pairs to existing monitoring wells 315-OWA, 316-OWA, 317-OWA, and P-318C. Hydraulic head data from the well pairs yields the following information: ® This additional work indicates that the Superior till units form a continuous confining layer between the u~ er and lower. outwash; o The lowex outwash unit i.s fully saturated; • The potentiometric surface in the lower outwash is approximately four feet lower than the hydraulic head in the upper outwash indicating a downward potential through the till that separates the two outwash deposits; ® The direction of groundwater flow in the lower outwash is to the southwest; and • The average linear groundwater velocity through the lower outwash unit is approximately 13 feet per year. Discussion regarding the hydraulic relationship between the upper and lower outwash units is included in Section 4.2.5. 2. How was the till isopach map generated? Please provide, in the narrative, a description of how the thickness of the till unit was determined, and give examples from represe~ztative borings. ~n the cross sections, pleas show dashed contacts were its thickness is estimated. Additionally, is the till unit continuous across the site? If not, the upper and lower outwash could be hydraulically connected, particularly in the area of soil borings SB-9, 513-24 and SB-,25. Geologic data from the two recently advanced soil borings, SB51 and SB52, suggest the till is continuous across the 5DA. A discussion of how the till thickness map was generated is included in Section 4.1.2. Additionally, the estimated till thickness is illustrated on the cross section figures. 3. How do current groundwater elevations at the South Development Area compare historically with groundwater elevations measured at the existing facility? Also, please document the relationship between current groundwater elevations and the proposed liner elevation, A hydrograph illustrating water levels from the SDA and the existing facility is attached as Figure 16. The hydrograph indicates that the SDA water level data has been collected during a period of average to high water table elevations at the existing facility. The water levels measured at the existing facility during 2004 are consistent with those measured from 1996 to 1998 and are approximately four feet higher than