Progress on Remediation of VOCs at WR Grace Superfund Site

Since 1985, groundwater at the WR Grace Superfund Site in southeast Acton has been extracted and treated for volatile organic compounds (VOCs). Data from the 2018 annual monitoring report suggest that this remediation is making progress.

The manufacturing activities carried out by WR Grace and earlier companies in southeast Acton left a witch’s brew of more than 20 contaminants in the soil and water. Of particular concern from the point of view of groundwater quality are a suite of volatile organic compounds (VOCs), including:

  • VDC, also known as vinylidene chloride or 1,1-dichloroethene
  • vinyl chloride
  • benzene

VOCs can cause a variety of impacts. Exposure can irritate the skin or mucous membranes. Animals exposed to VDC may experience problems with lungs, kidneys, or liver. Vinyl chloride is classified by the federal Environmental Protection Agency (EPA) as a known human carcinogen, as is benzene.

These organic contaminants were first detected in Acton Water District (AWD) municipal wells Assabet 1 and Assabet 2 in 1978 (sources of all dates: EPA timeline, ACES chronology). As part of an agreement between the AWD and WR Grace, a treatment system to remove VOCs was added to the public water supply in 1984. Since then, the AWD’s treatment system has been successfully keeping the VOC content of Town water below 1 microgram per liter for any single VOC and below 5 micrograms per liter for any mixture of VOCs — a more stringent standard than EPA’s.

Beginning in 1985, the EPA operated an “Aquifer Restoration System” to treat contaminated groundwater and redirect it away from the Assabet wells (which are south of the Grace site). However, another plume was detected traveling northeastward from the site, toward the AWD’s School Street well field (north of the Grace site). Annual groundwater monitoring began in 2000, resulting in better delineation of this northeasterly plume.

In 2005, EPA finalized a clean up plan (“Record of Decision”) for groundwater contamination at the Grace site (referred to as Operable Unit 3, or OU3). The plan called for extracting and treating polluted groundwater from two parts of the site, called the Northeast Area and the Landfill Area (see map below), and long-term monitoring of groundwater, surface water, and sediment.

Site map from EPA’s February 2013 Site Update, showing the Northeast Area and the Landfill Area, where treatment began in 2012. Note also the School Street municipal water supply well field in the northeast corner of the map.

After many years of study and planning, construction of the treatment systems began in 2009, and they began pumping and treating water in the 2010–2011 time frame. We now have at least six years of data from these systems, and some progress is apparent.

The pair of maps below contrast the VDC concentration in the northern section of the Grace site (north of the train tracks) in 2012 and 2018, from the 2012 annual monitoring report and the draft 2018 annual monitoring report. These reports were prepared for WR Grace by consultants from the Littleton office of TetraTech GEO.

Concentration of VDC (also known as vinylidene chloride or 1,1-dichloroethene) in the northern section of the WR Grace site. The areas underlain by the highest concentrations of VDC shrank from 2012 (left) to 2018 (right).

The colors are based on the maximum concentration of VDC in samples from wells, regardless of the depth in the well. Wells are shown as dots, except for the three AWD municipal water wells, which are highlighted with stars. Collectively, the Christofferson, Scribner, and Lawsbrook wells are also known as the “School Street Wells.”

One notable change is that the gray area has retreated back from most of the oval area of houses along the western edge of the maps (the Lexington Drive loop). Gray marks the area in which the concentration of VDC was measured as being between 7 and 30 micrograms per liter. EPA’s maximum contaminant level (MCL) for VDC is 7 micrograms per liter of drinking water (you may also see this as 0.007 mg/L or 7 ppb), so the gray/white contact marks the transition between areas that exceed and do not exceed the MCL. However, the maps note that “concentration boundaries are approximate.”

Note also that the yellow blob (signifying >30 to 60 micrograms per liter) in the middle of the 2012 map has disappeared by 2018, and the orange blob (signifying >60 to 100 micrograms per liter) between the Lawsbrook and Scribner wells on the 2012 map has disappeared by 2018.

The next pair of maps show the concentration of vinyl chloride in 2012 and 2018. On these maps, the view is of the area south of the train tracks and Independence Road. This southern area had already been substantially remediated by the 1980s vintage Aquifer Restoration System.

Concentrations of vinyl chloride in the groundwater in the southern part of the WR Grace Superfund Site. Areas with high concentrations of this contaminant shrank from 2012 (left) to 2018 (right).

As with the VDC maps, the vinyl chloride data maps also show a decrease over time in the areas with high concentrations of contaminant. EPA’s maximum contaminant level (MCL) for vinyl chloride in drinking water is 2 micrograms per liter, coinciding with the boundary between white and the lightest colored shading on each map.

The benzene data tell a similar story. Benzene is reported from a small area (appproximately 600′ x 1,500′) south of the Landfill Area, spanning the Acton/Concord town line. In 2012, a cluster of wells exceeded 30 micrograms per liter, and the highest mapped values were above 100. By 2018, the highest value measured was 23 — much better, but still above the EPA’s MCL of 5 microgram/L.

In addition to many more maps, the 2018 annual monitoring report also includes time series graphs and statistical trend analyses for individual wells for VDC, vinyl chloride, benzene, and many other contaminants. For some wells, the time series go back as far as the 1980s. These graphs and analyses confirm what the maps above summarize: that the trend over time is toward lower levels of all three of these volatile organic compounds.

In summary, there’s a long way to go, but the remediation for VOCs in the groundwater at the WR Grace site seems to be working.

Thanks to Matt Mostoller of the Acton Water District for introducing the author to the water quality status of the School Street Wells, and to Chris Smith of the EPA for providing a digital copy of the draft 2018 annual monitoring report. (A paper copy can be viewed at the Acton Memorial Library; ask the Reference Librarian for access to the WR Grace Collection.)

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