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July 13, 1990 File No. 4054.15-C,PC

4054.32-C

Mr. James K. RogersSilresim Site Trust Suite 1129 83 Parkhurst Road P.O. Box 2100 Chelmsford, Massachusetts 01824

Re: Revised Deliverable FS-2 Silresim Feasibility Study

Dear Mr. Rogers:

Goldberg-Zoino & Associates, Inc. (GZA) has revised Deliverable FS-2 of the Silresim Feasibility (FS) to address comments presentedby the US EPA in their February 1, 1990 letter. Ccpies of the revised document are enclosed. We feel that the majority of EPA's concerns have been adequately addressed in the revised document; however, we identified a number of comments which have not been fully resolved or which require additional explanation. This letter is intended to address these issues.

In the cover letter to their 12-page commentary, EPA identified the need to evaluate aggressive removal of Dense Non Aqueous Phase Liquids (DNAPL) during the FS. We did not directly address this requirement in the revised FS-2; it is our understanding that the EPA will be evaluating DNAPL extraction strategies as part of their pending FS addendum. (This issue is also raised in EPA's Comment 218.)

Specific comments which we feel require further discussion are outlined b low:

Comment 7 - In lieu of developing specific cleanup goals for other exposure pathways (East Pond water, basem nt air, etc.), we used groundwater ingestion cleanup values to demonstrate the degree of protectiveness tor these other pathways. Risk evaluations ar summariz d i n App ndix B and Table 2-6.

Copyright• 1990 Goldb r~-Zoino ' Associ a, Inc.

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I Silresim Site Trust July 13, 1990

File No. 4054.15 Page 2

Comment 9 Cand others> - We have included the three potentialgroundwater receptors (on-site, off-site, and River Meadow Brook) throughout the evaluation of cleanup goals,preserving a range of values as requested. However, we have retained statements concerning the potential use of groundwater in the Silresim study area, identified as our opinions.

comment 11 - Aside from River Meadow Brook, the primary receptorsof Silresim site groundwater were identified as sewer lines (84 inch, Canada Street, and Maple Street sewers) in the RI. We did not feel it was appropriate to applyingestion based cleanup goals to the sewer lines; accordingly, we did not expand the receptor scenario.

Comment 16 - A risk-based cleanup goal (2.9 ppm) was d~veloped for arsenic as requested by EPA; however, we do not feel it is appropriate. This value is substantially below average background levels of arsenic in soils in the Eastern United States.

Comment 20 - our discussion of site-specific factors related to calculation of voc- and arsenic-contaminated soil volumes was intended to convey limitations in the method we used, not justification for not addressing these contaminants. Elevated arsenic levels appear to extend off-site to the north and south along the railroad lines. The precise extent of this contamination is unknown, so we could not estimate quantities. Similarly, soil voc contamination above the lower bound cleanup goals (generally equivalent to detection limits) probably exists over the entire groundwater contaminant plume area. However, data is insufficient to map the extent of this "secondary" VOC contamination arising from volatilization and gaseous diffusion of these constituents.

Comment 21A - We attempted to address EPA's required 3a-year treatment duration for groundwater at the site. As outlined in Section 3.20, however, we do not believe it is feasible to treat groundwater to ingestion-based cleanup goals within 30 years. We feel that the 20 to 30 gallons per minute value we estimated for the high rate extraction system is a reasonable upper bound for proposed treatm nt scenarios. Pending data on deepgroundwater flow could change this estimate, however. This issue will b further evaluated during FS-3.

Silresim Site Trust July 13, 1990 File No. 4054.15 Page 3

Comment 218 Removal of DNAPL collected during proposedgroundwater pump and treat alternatives has been included in these options. We are not proposing a specific program for locating and extracting deep DNAPL, however. We have assumed that this issue will be further addressed upon completion of EPA's DNAPL studies.

comment 23A - In response to EPA's request for more supporting data for cost screening, we have provided unit cost ranges for the various source control remedial technologies.However, unit costs are not readily available for groundwater treatment options (groundwater treatment technologies are components of all Management of Migration options except for the No Action alternative).Further cost evaluations of groundwater treatment technologies have been deferred until FS-3.

comment 44 - Calculation of the dil1Jtion factor ·for off-site groundwater (using a hypothetical water supply well in the vicinity of the 84-inch sewer as a potentialreceptor) assumed dilution of contaminated groundwateremanating from the site by clean infiltration. A detailed discussion of the solute transport analysis is provided in Section 6.43 of the Final Draft RI. No other forms of attenuation (biodegradation, volatilization, adsorption, etc.) were accounted for in this analysis.For the off-site receptor, a percentage of the area used for calculating diluting infiltration is underlain bycontaminated unsaturated zone soils. Using this area in the dilution calculation may slightly influence solute transport results; however, it is our opinion that the analysis is not significantly affected by this approach.We feel that the dilution factor employed and the resultant cleanup goals calculated are valid.

It is also noted that the other receptor scenarios (on­site groundwater and River Meadow Brook) do not consider solute transport dilution calculations. Cleanup goals tor these scenarios would not be affected bymodifications to the solut transport analysis.

comments 43 and 468 - In your May 22, 1990 letter, you indicated that leaching of metals would not have to be separatelyaddressed in evaluating cleanup goals.

Comment 48 - We provided a refer nc for the MacKay model as request d. Advantages, disadvantages, and limitations ot the thre odels we us d were discussed in Appendix 8, Section 2 .4. we were not c rt in as to what additional information EPA n ds.

h Silresim Site Trust July 13, 1990 File No. 4054.15 Page 4

COmment 50 - Note 6 in Table B-4 (formerly Table B-3) presents the equation used to adjust solids concentrations to total contaminant levels. This adjustment is discussed within the text of Appendix B. Leaching models derive soil (solid) concentrations (Cs) based on a selected groundwater cleanup goal. Analytical methods measure total mass concentrations, however, including mass within ·soil moisture and soil pore gas. Accordingly, Cs values must be adjusted to yield actual measurable soil cleanup goals. The equation in note 6 is an expression ot these phase relationships.

We trust these explanations, along with the FS-2 revisions, will address EPA's concerns. We would be pleased to further discuss any ot these issues, . if desired.

Very truly yours,

GOLDBERG-ZOINO & ASSOCIATES, INC.

~4~~ Charles A. Lindberg Project Manager

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