NYSDEC letter re: Con-Edison release report at 7 WTC substation,

NYC 9/11 Public Portal Document Fax; Nov 5 1599 13:15 p. Q? NOV 21 '01 eS:01Pn R & D P.4

MONITORING WELL LOCATIONS

Well Number

1 Western side of Ramp J (along W, Broadway) - only if product is present in MW-2 2 Ramp J (Point “Z") 3 Comer of Ramp J and Tub Wall (inside 7WTC Building footprint 4 Within 7WTC Footprint (between Con Edison Manholes) 5 Within 7WTC Footprint (Southwest comer of former building) 6 West side of former Transformer 8 7 Within 7WTC Footprint (Northeast comer, within Transformer 6 vault - Off crane walk)

*See attached map for approximate wcU locations.

MONITORING WELL LOCATION RATIONALE

The monitoring wells will be used to identify the presence of free product, to anempl to prevent off-site migration, and to optimize product recovery, The proposed locations of the monitoring wells have been selected based upon the following rationale;

• The “tub wall” along Vesey Street will form the southern boundary of the 7WTC site while Verizon's Barclay Street and Washington Street trenches will form the northern and western boundaries of the 7WTC site.

• The concrete retaining wall between Ramp J and 7WTC is a minimum of 15 feet below ground surface (BGS), as measured at Point “Z”. Despite a review of the structural drawings for the site, the depth of the wall could not be determined. Because depth to groundwater at the site is estimated at 15 feet BGS, it is assumed that the concrete retaining wall is constructed below the groundwater table and thus forms a harrier to oil-site eastern migration. Monitoring wells MW-1 and MW-2 will be installed to delineate the eastern boundary. (Note: MW-l will be installed only if free product is observed in MW-2.) MW-3 will bo installed to detennine whether there is free product on the western side of the retaining wall at the southeastern comer of the 7WTC footprint.

• Free product has been observed in the two accessible Con Edison manholes that are located within the 7WTC footprint (along the scuthem boundary of the former Con Edison substation). MW-4 will be installed in order to optimize product recovery at this location.

• Assuming that the “tub wall" intersects with Ramp J, the probable direction of product flow would be to the west-BOUthwest, MW-5 will be installed at the southwestern comer of the 7WTC footprint as a product recovery location and

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