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O BRIEN E GERE : J&&2. = = ENGINEERS, INC.

November 6, 1998

Mr. David O. Lederer Remedial Project Manager United States Environmental Protection Agency Massachusetts Superfund Section Waste Management Division - HRS - CAN3 JFK Federal Building Boston, Massachusetts 02203-2211

Re: Sullivan's Ledge Superfund Site Slurry Wall Construction Status

File: 5509.005

Dear Dave:

On behalf of the Sullivan's Ledge Site Group, this letter presents a summary of the status of the construction of the slurry wall. This letter also addresses issue 3 in your letter dated November 6, 1998.

Background

The Statement of Work for the Sullivan's Ledge Superfund Site indicates that ground water collection and treatment will be implemented at the site as part of the overall remedy. As described in the SOW, ground water collection consists of both an active system ( recovery ) and a passive system (collection on the bedrock surface through the overburden).

A boring program was completed in 1996 along the layout of the shallow collection trench to design the collection trench profile. This boring program identified an area along Hathaway , approximately 200 ft long, where the top of bedrock was up to 50 feet below the ground surface. This low portion of the bedrock profile has been referred to as the "bedrock window."

A report entitled Technical Memorandum Shallow Collection Trench (June 1996) evaluated alternatives to augment the passive collection system in the area of bedrock window. This evaluation included the completion of three-dimensional ground water flow modeling to assist in the evaluation of alternatives. As a result of this evaluation, it was recommended that a slurry wall be installed in the area of the bedrock window. This alternative would mitigate ground water discharge from the site through the area of the bedrock window.

As described in the attached letter and profile from HLA, and as we discussed on October 9, 1998, due to debris encountered during excavation, the slurry wall was limited to a depth of approximately 20 to 25 feet below ground surface along its entire length. This limitation has resulted in a gap between the bottom of the slurry wall and the top of the bedrock surface of up to 20 to 25 feet. The slurry wall was also constructed with a width of 6 to 16 feet, versus the design width of 3 feet.

The slurry wall being constructed shallower and wider than designed requires that the effectiveness of shallow collection as as long-term geotechnical stability of the area be evaluated.

6ntjn % Oort"1 Fnui(i^-jr^ I i Mr. David O. Lederer November 6, 1998 Page 2

This letter presents:

1. A description of the design revisions which have been recommended to date to accommodate the wider slurry wall;

2. Adescription of the actions planned to assess the long-term stability of the area and an assessment of potential short-term impacts or requirements; and

3. A evaluation n of the relative effectiveness of the slurry wall as designed, the slurry wall as-built, and the slurry wall augmented with .

Design Revisions

Based on the width of the slurry wall as constructed and its horizontal location, the following revisions will be made to the design:

1. Collection Trench. To accommodate the width of the slurry wall, the east / west leg of the shallow collection trench will be moved south 20 feet (i.e., manholes 1, 2, and 3 will be moved south 20 feet). Manhole 4, which was installed prior to completion of the slurry wall, will not be relocated. Some east / west field adjustment to the locations of manholes 1, 2, and 3 is anticipated based on the actual bedrock profile in the new trench location and the presence of other design components (e.g., MW-17). Although the invert elevations of manholes 1, 2, and 3 will not be revised, the top of manholes 1, 2, and 3 will be extended to meet final grades.

2. Cap Access Road. The east/west access road be moved approximately 20 ft south, centered on the collection trench (same as north south leg.) Because of the new location of the access road, it was possible to raise the elevation of the access road in the northeast corner of the site, and install culverts beneath it. Culverts have been sized to handle the 24-hr, 100 year storm. The inlet and outlet of the culverts will be lined with rip-rap.

3. Run-off Collection Swale. The east/west run-off collection swale will also be moved south, by approximately 20 or 30 feet. To avoid relocating the shallow collection trench pump station, the swale will pass south of it. In addition, a second small swale will be provided parallel and adjacent to Hathaway Road, to intercept run-off between the primary collection swale and the site boundary. This swale will not be lined with rip-rap.

A draft of this approach was presented to USEPA by fax on October 23, 1998. The revisions described above address comments on that draft made by Metcalf & Eddy by letter on October 26. These revisions were shown on sheets G-6, G-9, and G-20 of the Contract Drawings, which were provided to USEPA on November 5, 1998. Based on discussions with USEPA on November 5, 1998, the following additional revisions will be made:

Gas monitoring wells GM-13 and GM-15 will be relocated to either side of the slurry wall (approximately 10 feet from each end) and will remain within the site fence line. This will provide well spacing in this area of approximately 270 feet. Gas monitoring well GM-14 will be eliminated.

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Curbing or some other drainage control feature will be provided to prevent storm water from Hathaway Road from running onto the site.

O'Brien & Gere requests that EPA provide written approval of the above described modifications so that onsite work may proceed.

From a design perspective, the following issues will be resolved after additional information is obtained:

The location of the peizometers down gradient of the slurry wall.

The configuration of slurry wall / cap interface at the northern boundary of site. (Preliminary indications are that the slurry wall extends past the site fence line.)

Measures required, if any, to stabilize the vicinity of the slurry wall. (See discussion below.)

Site Stability

As discussed on November 5, 1998, the effect that the slurry wall as constructed could have on the long term stability of the area is being evaluated. Issues being evaluated include:

The stability of the cap

The stability of Hathaway Road (Some cracking on the edge of the road adjacent to the slurry wall has been observed.)

The following activities are underway:

The area is being surveyed and as-built drawings of the slurry wall will be prepared. The survey drawings will include in plan view the slurry wall, the former site fence line, the traveled edge of Hathaway Road, the sanitary sewer line and manholes, the cracking observed on the edge of Hathaway Road, and spot elevations on either side of the cracking.

Geotechnical engineers from O'Brien & Gere observed the slurry wall and the adjacent portion of Hathaway Road on November 5, 1998. Although the focus of their observations was to assess long-term stability, they also assessed the requirement for immediate actions with respect to road stability. Based on their observations, additional immediate actions in this area beyond those already taken by Harding Lawson are not recommended at this time.

A 5-gallon sample of - will be collected for potential bench scale stabilization testing.

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Based on the above, an assessment will be made concerning the need for additional actions in this area. It is anticipated that preliminary results of the assessment will be made to USEPA by November 12, 1998.

Effectiveness Evaluation

Ground water flow modeling was used to evaluate the relative effectiveness of the slurry wall as designed, the slurry wall as built, and the slurry wall augmented with pressure grouting. The model code, assumptions, and files used in the 1996 evaluation were used for this evaluation. Model simulations considered the drain elevation set at 70 ft elevation and 67 ft elevation. These drain elevations were used in the 1996 simulations. The focus of the current simulation^ was an evaluation of the percentage and rate of ground water flow which would pass under the slurry wall in the area of the bedrock window and flow off-site.

The model assumed that the slurry wall was installed to an elevation of 60ft MSL. This is a conservative assumption because although the bottom of the slurry wall excavation ranged from 55 to 60 ft MSL, it averaged 57 ft MSL. Thus, the slurry wall as constructed covers more of the bedrock window than modeled.

The scenario of pressure grouting below the slurry wall was simulated using a wall with uniform . The uniform hydraulic conductivity was based upon two grout curtain conditions. Both conditions assumed that the grout had a hydraulic conductivity of 10"6 cm/sec. For one condition it was assumed that the grout filled 80% of the pore spaces. For this assumption the equivalent uniform hydraulic conductivity was the same a the slurry wall, 7 x 10"6 cm/sec. The second condition assumed that the grout filled 60% of the pore spaces. For this assumption the equivalent uniform hydraulic conductivity was 3.5 x 10"5 cm/sec.

The results of the effectiveness evaluation are presented in the table below:

Options Drain Bypass Recovery in Recovery in Total Elevation Volume Vicinity of Remainder of Collection Slurry Wall Collection Trench Trench Recovery

Wall as Designed 70ft < 1 gpm 29 gpm 12 gpm 41 gpm

67ft < 1 gpm 35 gpm 12gpm 47 gpm

Wall as Built 70ft 7 gpm 23 gpm 12 gpm 35 gpm

67ft 2 gpm 33 gpm 12 gpm 45 gpm

Wall as Built w/ 80% Grout Curtain 70ft < 1 gpm 29 gpm 12 gpm 41 gpm

67ft < 1 gpm 35 gpm 12 gpm 47 gpm

Partial Wall w/ 60% Grout Curtain 70ft 2 gpm 27 gpm 12 gpm 39 gpm

67ft < 1 gpm 35 gpm 12 gpm 47 gpm

I \BOSTON\PROJECTS\5509005\2 CORRES\LEDER24 WPD Mr. David O. Lederer November 6, 1998 Page 5

This evaluation indicates that the elevation of the groundwater in the collection trench is an important factor in controlling off site migration for the current configuration of the slurry wall. At an elevation of 67 ft, the groundwater in the collection trench is at a similar elevation as off site ground water and there is limited head difference to cause the ground water to flow off site.

As discussed with USEPA, recommendations concerning future actions with respect to the gap beneath the slurry wall until the appropriate parties have had a chance to meet to discuss the situation. We do note that the design modifications described above have resulted in most cap appurtenances be relocated from the foot print of the slurry wall. As such, the gap is not an immediate concern, since additional action, if required, can occur in the future with minimal re-work of the cap.

Summary

This letter presents the status of our assessment of this matter. Although some of the issues associated with this matter may require time and additional information to resolve, the following short-term action items are discussed in this letter:

Date Entity Action

Nov 6 Group Continue regular observations of the surface cracking in Hathaway Road in areas adjacent to the slurry wall.

Nov 10 EPA Approve design modifications recommended to date.

Nov 12 Group Provide preliminary assessment with respect to long-term stability of area.

Nov 13 Group Make recommendations concerning location of

The Sullivan's Ledge Site Group looks forward to working with EPA, MADEP, and the City of New Bedford towards the development of a prudent resolution of this matter. Please contact Steve Wood or me if you have any recommendations concerning the above approach.

Very truly yours,

O'BRIEN & GERE ENGINEERS, INC.

James M. O'Loughlin, P.E. Senior Project Engineer cc:

Edgard Bertaut David Buckley Jerry Johnson Bob Bowers Gary Kirsch Robert Connors Don Dwight Wayne Hoagland Karin Kohl-Dyson Tom Sauro Stephen Wood Molly Cote Steve Anagnost Dave Farber Guy Swenson

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