Duxbury Resident James Begley shared the following professional review of the Landfill Assessment Report for the Former Duxbury Landfill/McNeil Dump and Duxbury Municipal Landfill prepared by Weston & Sampson.
Mr. Begley is a certified Licensed Site Professional with an expertise in water quality, and has presented to Duxbury’s Selectboard on PFAS contaminants this past year.
Hi Amy (McNab, Duxbury Selectboard),
I reviewed the Landfill Assessment Report for the Former Duxbury Landfill Inc. / McNeil Dump and Duxbury Municipal Landfill dated October 5, 2022, prepared by Weston & Sampson for the Town, and offer the following summary, comments, and recommendations.
Weston & Sampson reported the following assessment activities for the McNeil Dump:
- review of municipal records, previous reports, and historic documents
- observation and documentation of test pit excavations to evaluate the extent of waste
- installation of groundwater monitoring wells and soil gas wells
- soil gas testing and one groundwater monitoring event testing for a range of contaminants including 1,4-dioxane and PFAS.
Groundwater monitoring was also reported for Duxbury Municipal Landfill, which is adjacent to McNeil dump. Both are located within a Current Drinking Water Source Area as delineated by the Zone II for one or more public water supply wells and by the presence of private drinking water wells.
The Weston & Sampson report also included a historical report (Attachment B) prepared by the consulting firm IEP for the Town in 1988 that was particularly informative. The IEP report was titled Hydrogeologic Contaminant Assessment of the Duxbury Landfills.
IEP had been tasked to:
- investigate groundwater contamination in the vicinity of the landfills,
- develop a detailed understanding of groundwater flow in the area,
- evaluate existing water quality, and
- determine the current and future impact of the past landfill activity on the Town’s water resources.
Through subsurface investigation including monitoring well installation and sampling, IEP identified landfill leachate contaminated groundwater extending downgradient from the landfills as far as the Depot Street well and provided figures with both maps and cross sections of the groundwater leachate plume.
The contaminants identified in 1988 were mainly indicators of landfill leachate e.g., specific conductivity, alkalinity, sodium, chloride etc. Volatile organic compounds were not detected above applicable standards. Except for elevated sodium concentrations contaminant concentrations did not then exceed applicable groundwater standards at the Depot Street well. 1,4-dioxane and PFAS were in use but not generally known as contaminants of concern at that time and were not targeted for analyses. IEP concluded that the landfill plume had reached the Depot Street well and that there was a distinct probability that the landfill plume would also impact the Partridge Street well.
Weston & Sampson Investigation Findings:
According to Weston & Sampson the results of the groundwater elevation survey and gauging event indicated overall groundwater flow at the McNeil Dump Site and the Duxbury Municipal Landfill is to the east.
The volatile organic compound 1,4-dioxane was detected above the applicable groundwater regulatory standard of 0.3 micrograms per liter (µg/L) in groundwater samples collected from two monitoring wells in the vicinity of the Duxbury Landfill with concentrations of 1,4-dioxane up to 1.6 times higher than the standard. In addition, PFAS6 (the total concentration of six PFAS compounds that are compared to the groundwater regulatory standard) was detected above the standard of 20 nanograms per liter (ng/L) or 0.02 micrograms per liter (µg/L) in samples collected from nine monitoring wells in the vicinity of the Duxbury landfill with concentrations up to 15 times higher than the standard. Groundwater test results for one monitoring well located in the McNeil Dump area identified total PFAS6 at up to 3.5 times the standard.
Based on the groundwater investigation findings and the results of subsurface soil investigations including test pits at the McNeil Dump Site, Weston & Sampson recommended the following additional assessment activities:
For McNeil Dump
- Additional test pit excavations along the western and northwestern boundaries of the property to define the limits of waste off property.
- Installation of additional groundwater monitoring wells / piezometers west of the property to evaluate the extent of PFAS impact to groundwater.
- Additional seasonal groundwater monitoring at the dump and downgradient of the property.
For the Duxbury Municipal Landfill
- Additional monitoring to evaluate the nature and extent of 1,4 dioxane in groundwater.
- Additional seasonal groundwater monitoring using the existing monitoring well network
downgradient of the landfill, which has not been sampled, to further evaluate the nature and extent of the leachate plume and assess potential PFAS impacts to downgradient public water resources (e.g., Depot Street PWS).
- A comprehensive field (elevation) survey to update elevations for the entire monitoring well network and further evaluate groundwater flow patterns.
Comments and Recommendations: The presence of these contaminants in groundwater at the landfills indicates they have been released from the landfills and likely have been, and continue to be, transported significant distances by groundwater flow impacting drinking water sources. Accordingly, a process to develop contingencies for implementation of carbon treatment at all potentially affected Town wells should be started now.
While all of Weston & Sampson’s recommended activities have merit, the Weston & Sampson report does not include a comprehensive set of objectives or rationale for the additional investigation. Also missing is a detailed understanding of the setting for the work including 1) current information on the regional groundwater flow system, 2) the fate and transport characteristics of the contaminants of concern, and 3) a full picture of the potential impacts of groundwater contaminants migrating from the landfills on public health including impacted or threatened water supplies and the environment. These elements are all necessary to put the proposed work in context.
The proposed work seems to focus exclusively on groundwater east of the landfills which appears to be the predominant groundwater flow direction. There is no evaluation of the vertical extent of groundwater contamination and how that relates to the existing monitoring well network and perceived contaminant distribution. Additional groundwater monitoring data should also be collected from existing wells to assess groundwater elevation, flow directions, and water quality west of the landfills (in the direction of the Evergreen well), along with areas north and south of the landfills not addressed in the current report. The proposed work should consider using existing groundwater models to trace particle tracks (the flow path) in groundwater from the landfill source to potential receptor wells and ultimate discharge locations under pumping and non-pumping conditions. This modeling effort could result in a significant reduction in the number of monitoring wells needed in the downgradient areas, reducing investigation costs, and providing a timelier basis for recommending response actions.
A work plan for additional assessment activities should be developed by Weston & Sampson that specifies the objectives of the work, provides a conceptual understanding of conditions and concerns, and includes necessary phasing of the investigation to be most efficient along with the foreseeable cost for the additional work and a specific schedule to complete the work.
Jim Begley, Elm Street Duxbury