Challenge

Since 2010, GZA has performed investigation and post-remedial action monitoring on behalf of the New Hampshire Department of Environmental Services (NHDES) at the New Hampshire Plating Company (NHPC) Superfund Site on the bank of the Merrimack River. NHPC had operated an electroplating facility there from 1962-1985 resulting in groundwater contamination that included metals, cyanide, and chlorinated organic solvents. The remedy for contaminated groundwater, as developed by U.S. Environmental Protection Agency (EPA) earlier in the project, involved natural attenuation and long-term monitoring, which GZA performed annually when it joined the project team. After several years, GZA determined that the conceptual site model was not supported by long-term data trends being produced and a Supplemental Remedial Investigation was warranted.


Solution

A number of the investigations conducted by GZA for the NHDES and EPA throughout the project exceeded the standard monitoring scope of work. The investigative impetus was sometimes directed by NHDES and EPA as the clients: using passive diffusion bag interval sampling at wells with screens greater than 10 feet and co-located porewater, groundwater, sediment, and surface water sampling for a sediment toxicity analysis. Upon joining the project team, GZA made additional investigative recommendations to address potential deficiencies in the monitoring program and understanding of the site, including: installation of overburden and bedrock wells using rotosonic drilling techniques, a hydrogeologic study to evaluate the effect of a nearby production well on Site groundwater flow and contaminant fate, and a geochemical assessment of bedrock groundwater to evaluate whether the detected concentrations of arsenic in bedrock groundwater at a downgradient/off-site monitoring location were site-related. 

At the request of NHDES, GZA also designed a preliminary groundwater screening plan and collected groundwater samples for laboratory analysis of per- and polyfluoroalkyl substances (PFAS) at the site. Data from the initial screening resulted in supplemental sampling of additional locations to better refine our understanding of the PFAS plume at NHPC. GZA provided an initial opinion on whether detected PFAS concentrations were likely site-related or potentially associated with the St. Gobain Performance Plastics facility located approximately 2.5 miles north of the site in Merrimack, NH. As GZA assisted EPA with preparation of the second Five-Year Review for the Site, it brought to the regulators’ attention the discrepancy between expected data and the existing conceptual site model. 

GZA was then asked to perform a Supplemental Remedial Investigation (SRI). This included a high-resolution groundwater investigation to assess the extent of the trichloroethene (TCE) plume and further evaluate the mechanism responsible for ongoing contamination using Waterloo™ high resolution profiling techniques combined with an on-site mobile laboratory and direct push methodologies for groundwater and soil sample collection. Other activities performed during the SRI included a three-dimensional compound-specific isotope analysis (CSIA) evaluation of the potential for a separate TCE source; a one-dimensional CSIA assessment for evidence of TCE degradation; collection of soil samples using anoxic methodology for assessment of the abiotic natural attenuation potential of Site soil; and a natural attenuation assessment using pQCR bacteria testing and a geochemical assessment of groundwater conditions. GZA assessed data obtained during the profiling investigation using three-dimensional visualization software and prepared an SRI report.  GZA is preparing a Focused Feasibility Study for EPA.

TCE was detected at concentrations exceeding EPA and NHDES standards, prompting additional assessment and development of remedial alternatives to address the potential for vapor intrusion within a building on an adjacent property.  
 


Benefit

GZA’s keen understanding of the investigatory process and data trends led to an SRI that revealed more accurate information about contaminants and sources impacting the Site. GZA’s SRI findings prompted reassessment of the remedial strategy and, ultimately, recommendations for a supplemental remedy. 

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