UNITED STATES ENVIRONMENTAL PROTECTION AGENCY NEW ENGLAND - REGION 1

5 POST OFFICE SQUARE, SUITE 100 BOSTON, MASSACHUSETTS 02109-3912

July 1, 2016

Mr. Thor Helgason De Maximis 135 Beaver Street Waltham, MA 02452

Re: Review of the Operable Unit Three Monitoring Program Report, and the Recommended Modification Letter, both dated December 21, 2015, and prepared by Tetra Tech; W. R. Grace (Acton Plant) Superfund site, Acton & Concord , Massachusetts

Dear Thor,

The Environmental Protection Agency (EPA) and the Massachusetts Department of Environmental Protection (MassDEP) have reviewed the above referenced report (the "Report") and cover letter ("Recommended Modification Letter").

The following are combined comments by EPA and MassDEP. In drafting these comments, we also considered letters provided by the Town of Acton, the Acton Water District (AWD), and Green Acton, attached here.

GENERAL COMMENTS

1. Report Section 3, Groundwater Quality Sampling, General. Groundwater sampling is mentioned throughout Section 3 of the Report (e.g., Sections 3.1.1, 3.2, 3.3, and 3.4). In some cases, reference is made to the Field Sampling Plan ("FSP") to "describe" specific FSP procedures; in other cases, limited information is included. Please add text to each section that adequately describes the sampling method(s) used. For example, in addition to citing the FSP, the text should somehow convey the following (as annotated): samples are collected using [state which] methods except where [PDBs, other methods] are used; that field parameters [list them] are monitored during purging; that samples for dissolved metals analysis are prepared by [state how- filtration, filter opening size]; that samples for 1,4­dioxane analysis are collected using [clarify method(s) used in PDB and non­PDB wells]; etc. It is not necessary to include more specific detail, since reference to the FSP is adequate for that type of information.

2. Report Section 3.4, 1,4-Dioxane Sampling, General. Since the distribution of 1,4-dioxane is currently a topic of considerable interest at this Site as well

as at the Nuclear Metals (NMI) Site, these reports will likely now be more widely circulated. As a result, please provide more specific descriptive information in Section 3 regarding locations of the different areas of the Site. For example, in the Assabet River Area section (3.4.4), the word: "area" is used to refer to both the entire area as well as to areas within the Assabet River Area. Although Agency reviewers are familiar with the Site and context of references and nomenclature, in order to avoid misunderstanding by other readers, please distinguish between the six major Site areas by capitalizing "A" for these areas, and contrast those major areas from internal subareas by using a small "a."

SPECIFIC COMMENTS

1. Recommended Modification Letter, Proposed Changes to voe Sampling, p. 1.

Well G-3A

Per your Recommended Modification Letter, one of the proposed changes to voe sampling shown in Table 2 is the elimination of G-3A, which has reportedly shown concentrations of all three primary voes below the IGe L for three years or more. According to Table 4-1, the concentrations of vinyl chloride in G-3A show a downward trend; however, the concentration in 2015 (1.5 ug/L) represents an increase from 2014 and is close to the IGe L of 2.0 ug/L. We request that G-3A be retained in the sampling program, at least through next year, to continue to monitor for any potential rjse in the concentration of vinyl chloride in this area of the Site.

Well AR-1481

It is also requested that AR-1481 be sampled for voes in 2016, in response to Acton Water District (AWD) concerns regarding water quality near the eastern edge of the Assabet wellfield capture zone. When last sampled in 2001 , the concentrations of voe, vinyl chloride, and benzene at AR-1481 were <2 ug/L. Sampling in 2016 will provide insight into whether or not the downward trends for voes at LF-180 and LF-200 are also occurring at AR­1481.

2. Recommended Modification Letter, 1,4-Dioxane Sampling, p. 1.

Well LF-180

Per your Recommended Modifications Letter, one of the proposed changes to the 1,4-dioxane sampling shown in Table 3 is the elimination of LF-180, a well in the Assabet River Area. We do not agree with elimination of Well LF­180 at this time. According to Table 4-1, the concentrations of all three primary voes at this well show downward trends; however, the

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concentration of 1 ,4-dioxane in 2015 was 8.4 ug/L, which is relatively high for a well that may not have been downgradient of the Landfill. Even though LF­180 appears to be outside the capture zone of the Assabet wellfield, it is recommended that annual sampling for 1,4-dioxane continue, at least through 2018, to ascertain if the concentrations of 1,4-dioxane are also trending downward (based on non-statistical analysis). Note that since the 2006 sampling result for 1,4-dioxane at this well is similar to the more recent sampling done in 2015 (7.9 ug/L), it appears that a downward trend may not yet have started for 1 ,4-dioxane at this well.

Well LF-200

In addition to continuing the sampling for 1,4-dioxane at LF-180, it is recommended that LF-200 also be sampled, in 2016, for 1 ,4-dioxane (in addition to the already planned sampling for VOCs). The samples that have been collected over the last decade in the Southwest and Assabet River Areas indicate that, with the exceptions of LF-180 and PT-0361 (which are located in the NMI plume), most or all results for 1,4-dioxane have been between approximately 0.5 and 2.1 ug/L. It is recommended that LF-200 be sampled in 2016 to confirm that the groundwater between LF-180 and Assabet-1A is with in that range for 1 ,4-dioxane.

We/19-78 and well clusters AR-02, AR-15, and 8-09

In their recent letter (attached), the AWD expressed concern about 1 ,4­dioxane concentrations at Well 9-78 and at well clusters AR-02, AR-15, and B-09. These wells are generally in the eastern part of the Assabet well field Zone II. While the recent and past results from nearby clusters (and from B­09 prior to 2013) suggest that these wells lie within the area where 1,4­dioxane concentrations are generally between 0.5 and 2.1 ug/L, we request that the following actions be taken as soon as possible to confirm that extrapolation: These four wells/clusters should be inspected (preferably before the field sampling for the 2016 sampling round, but no later than the end of that field event) to determine which wells/Barcads are, or appear to be, accessible and functional. Within three weeks of that inspection , a memorandum should be submitted to EPA that describes the status of each well/Barcad, and proposes a plan for sampling at least one well at each of these four locations (assuming the presence of at least one potentially functional sampling point) either during the 2016 sampling event, or during a supplemental fall 2016 sampling event, to be completed no later than October 28, 2016.

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Well B-0383

Per your Recommended Modification Letter, another proposed change to the 1,4-dioxane sampling shown in Table 3 is elimination of B-0383, a well in the Southwest Landfill Area. We disagree with this recommendation. The concentrations of 1,4-dioxane at B-0383 in 2006 (2.2 ug/L) and in 2015 (2.1 ug/L) were essentially identical and similar to other wells farther west of the landfill. Since this well is quite close to the landfill, the relatively low concentrations of 1,4-dioxane suggest that the western edge of the landfill plume (with concentrations in the range of 4 to 7 ug/L) is being kept east of the B-03 cluster, possibly due to effective capture by the nearby extraction wells. We therefore recommend that this well continue to be sampled for 1,4-dioxane, to confirm the limited western extent of the landfill plume.

Well OSA-138

Per your Recommended Modification Letter, another proposed change to the 1,4-dioxane sampling shown in Table 3 is elimination of OSA-13B, a well in the Former Lagoon Area. We disagree with this recommendation. In recent years, OSA-13B has exhibited extreme fluctuations in the concentrations of VDC and vinyl chloride; therefore, it is recommended that it continue to be sampled annually for 1,4-dioxane to determine if the concentrations of that compound show similar variability.

Well AR-1182

Per your Recommended Modification Letter, another proposed change to the 1,4-dioxane sampling shown in Table 3 is elimination of AR-11 B2, a well in the SE Landfill Area. We disagree with this recommendation. The concentration of 1,4-dioxane (17 ug/L) detected in 2015 was much higher than that detected (7.5 ug/L) in 2006, possibly due to the shutdown of extraction well ELF. Due to the increase in concentration, this well should be sampled again in 2016 and in the future, possibly at a decreased frequency, if warranted, to help characterize 1,4-dioxane concentrations outside the capture zone of the landfill area extraction wells.

Wells AR-20 and AR-1481

Two other proposed changes recommended in your letter with respect to the 1,4-dioxane sampling shown in Table 3, are elimination of AR-20, in the SW Landfill Area, and AR-14B1 in the Assabet River Area. We disagree with th is recommendation . At AR-20, the 1,4-dioxane concentration (3.9 ug/L) was higher in 2015 than it was in 2006 (2.2 ug/L), showing a net general upward trend in concentration over time. This well should be sampled for two more rounds, to try to determine whether or not this upward trend is continuing.

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AR-14B1 , as stated above in reference to VOC sampling, is of concern to the AWD due to its position relative to the Assabet well field capture zone. It is recommended that both of these wells be sampled in the next two annual rounds.

Sampling of Upgradient Wells

We also recommend that two or more wells upgradient of the Site be sampled for 1 ,4-dioxane. Since on-Site wastewater disposal systems are a possible source of this contaminant, it may aid the understanding of the overall conceptual site model to determine a range of background concentrations in upgradient areas of the Site. It is recommended that well AR-07P and AR-330 or -340 be considered; however, the selection of two or more specific wells should be based on an evaluation of the most suitable locations by your consultant(s).

Summary of the Comments Above with respect to 1, 4-dioxane

The following table summarizes the wells in each area of the Site at which we recommend sampling for 1,4-dioxane in 2016:

WELLS TO BE SAMPLED FOR 1,4-DIOXANE IN 2016 Area of Site (Total # Wells Proposed in 12/21 /15 Additional wells to be Sampled letter from Grace/TetraTech Wells in Area) Recommended

to be Samoled Northeast Area (16 AR-28S, AR-290, AR-29SBR, Upgradient well wells) AR-300*, AR-30SBR, AR­

310*, AR-31S, MW-06B, MW­078, PS-22A, PS-22B, PS-29B, Scribner*, Lawsbrook*, Christofferson*

Former Lagoon Area None OSA-13B (1 well) Southwest Area (10 AR-03P, AR-03B1 , B-0584, B- Upgradient well wells) 06B5,

PT-03B1*, R-2, R-2A*, Assabet 1A*, Assabet 2A*

Assabet River Area (3 None LF-180, LF-wells) 200, AR-14B1 Southwest Landfill MLF*, WLF*, SWLF-2* B-0383, AR-20 Area (5 wells) Southeast Landfill B-08B, LF-06C, SELF-1 *, AR-11 B2 Area (5 wells) SELF-2* *Multiple Samples

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3. Report Section 3.4, 1,4-Dioxane Sampling, p. 3-9. In the first paragraph, in addition to referencing the MassOEP GW-1 standard, please include the most recent (June 2016) USEPA risk-based concentration range of 0.46 ppb to 46.0 ppb. Also please include a statement that, as of June 2016, there is no Massachusetts Maximum Contaminant Level (MMCL). This information will supplement the statement, already included, that there is no federal drinking water standard for 1,4-dioxane.

4. Report Section 3.4.4, Assabet River Area, and Section 3.4.5, Southeast and Southwest Landfill Areas, pp. 3-11 to 3-12. In both of these sections, the text states that there are no private wells within or downgradient of the subject area(s). This statement should be qualified by noting the date of the most recently-conducted search for private wells and briefly explaining what the search entailed. Also, since these areas end at the Assabet River, there is theoretically no downgradient area; the area to which the text is referring should be clarified, particularly if it is the area southeast of the Assabet River (beyond the downgradient boundary of these Areas).

5. Report Section 3.4.5, Southeast and Southwest Landfill Areas, p. 3-12. The text at the end of this section states that there is no reason that Monitored Natural Attenuation (MNA) would not be an equally appropriate remedy for 1,4­dioxane present in this area beyond the capture zone, as it is for VDC, vinyl chloride, benzene, and arsenic. There is limited information to support the conclusion that 1,4-dioxane naturally degrades. The recalcitrance of 1,4­dioxane to degradation suggests that it may attenuate much more slowly than VOC, vinyl chloride, or benzene. Future reports should include a brief discussion of the current thinking regarding natural attenuation of 1 ,4-dioxane.

6. Report Section 4.2, Site Evaluation, p. 4-3. The six Areas of the Site are formally introduced and listed in this section of the Report but are used to specifically describe the distribution of 1,4-dioxane in Section 3.4. In future reports, please consider moving the introduction of the six Areas to an earlier part of the report.

7. Report Section 4.2.1.1 , Site Evaluation, p. 4-5. Regarding your discussion about the extra sampling that was done in the area surrounding OSA-13B, the statement that the VOC concentrations do not extend downgradient to the B-04 cluster may not be accurate, since the 8-04 cluster does not appear to be downgradient of the OSA-13 cluster. (Note that the groundwater elevation in B­04P, the only measurable well at the B-04 cluster, was not the same as the groundwater elevation in OSA-13 cluster, but about a foot higher.) Stating that the voe was not detected in a downgradient well implies that it is not migrating, which is probably not the case. The reference to the 8-04 wells as "nearby" wells in the previous sentence seems more appropriate and should be used in future reports.

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Another limitation of relying on results from nearby wells to assess the extent and possible migration of the contamination found at OSA-138 is the difference in well depths between 8-04 and OSA-138. The screened interval of OSA-138, which is the intermediate-depth well in that cluster, is from an elevation of 105 to 115 feet. A review of the screened intervals of the wells/Barcads at the surrounding clusters/wells (OSA-14, OSA-15, 8-04, EL­3, SLGP-R) shows that no other well/8arcad has any part of its screened interval located between elevations 123 feet and 94 feet; therefore, there is a vertical "gap" of about 30 feet of overburden at which depth no well other than OSA-138 is screened.

It appears, however, that the significantly lower concentrations of contamination in 0SA-13A and 0SA-13e (in 2012) support a conclusion that the most highly contaminated zone is likely restricted to the intermediate depths. Since the data from 2012 shows that this contamination seems fairly persistent at such depths, we request that a conceptual plan for investigating the extent of contamination be developed, with the goal of defining the extent of contamination and determining if anything can be done to expedite the attainment of IGeLs in this area.

8. Report, Table 4-1, Statistically Significant Concentration Trends Based on Mann-Kendall Trend Test for Small Sample Size. Monitoring well LF-06N was removed from the list of sampled wells after the 2014 round (with the concurrence of EPA/MassOEP). The graph of concentrations was removed from the report (Attachment E) in 2015, but the well was not deleted from Table 4-1. In future reports, please delete LF-06N from Table 4-1, or somehow indicate that the trends are based on data through 2014.

9. Report, Table A-1, voe Concentrations in Groundwater. The footnotes for this table seem to be out-of-date and no longer applicable to the current data; for example, 1) several vinyl chloride results for extraction wells are followed by an asterisk, but the footnotes indicate that an asterisk refers to the diffusion bag depths in SWLF-1; the qualifier "F1" after the voe result for the August sample at OSA-138 is not explained; and 3) while the "A" symbol that follows two of the results for voe in OSA-138 is explained in the report text, it would be preferable to identify it in the table footnotes as well. In future reports , please update the table footnotes.

1O. Report, Table A-1, voe Concentrations in Groundwater. LF-12 is sampled annually for voes and geochemical parameters. The heading for LF-12 in Table A-1 indicates that it is sampled by P08, but the graph in Appendix E has no note to indicate that a P08 is used or was used in the past. In future reports, please eliminate this inconsistency.

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11. Report, Table A-1, voe Concentrations in Groundwater. For the second set of samples collected from OSA-13B, the resu lts for all VOCs, except for VDC, are shown as "NA" (not applicable). Since benzene and vinyl chloride concentrations also exceed the IGCLs at this well, it would be of interest to know if the concentrations of other VOCs were also significantly higher in the second sample and its duplicate. Please explain why the results for the other VOCs are not presented (e.g., not requested from the lab, rejected due to the lab quality control .problem referred to in the VDC result discussion , or other relevant explanation).

If you have any questions, you may contact me at 617-918-1448.

errick Golden Remedial Project Manager Office of Remediation and Restoration Environmental Protection Agency

cc: Bob Cianciarulo - EPA Cynthia Lewis - EPA Melissa Taylor - EPA Jennifer Mcweeney - MassDEP Barbara Weir - AECOM Chris Allen -AWD Matt Mostoller - AWD Acton Board ofHealth Steve Ledoux - Town of Acton Jane Ceraso - Green Acton File- EPA

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