ECAD Case Study: Groundwater Plume Source Investigation in Minnesota Landfill

The Problem

DrillingVolatile organic compound (VOC) concentrations exceeded regulatory standards in groundwater at the property lines of a municipal solid waste (MSW) landfill in West Central Minnesota. This situation required additional investigation and possible Corrective Action (CA).

The source of the VOCs appeared to be a 23-acre unlined section of the landfill.  Preliminary corrective action discussions considered a complete excavation of the waste material within the unlined section of the landfill and relocation to a new lined space area that could be built at the site. Initial cost estimates to complete this work were $3-4 million.

The Minnesota Pollution Control Agency (MPCA) required the landfill owner to develop and implement a plan that would define plume limits and establish an effective long-term groundwater monitoring system.

Background

Landfill records indicated that a significant volume of paint waste had been disposed of in the unlined section of the landfill between 1970 and 1980.  The unlined section was capped from 1989 to 1995.  It was unknown if contaminants in the unlined capped landfill made any significant contributions to the groundwater contamination.

Lined space development plans for permitting required the building of new space on top of the existing closed 23-acre unlined space and knowing whether groundwater remediation plans should include contaminated waste excavation.

The impacted aquifer is a semi-confined water table aquifer in outwash sand and gravel with discontinuous clay units. The saturated thickness is 90 feet in some areas and reaches depths of 120 feet to the significant aquitard bottom.

Groundwater monitoring on-site indicated that elevated concentrations of vinyl chloride, along with other VOCs typical of landfills, had migrated more than 600 feet beyond the facility’s compliance boundary to the property boundary.  Nested wells used for monitoring indicated that the vertical profile of the plume was not clearly defined, and horizontal limits were unknown.  Landfill contaminants at private wells adjacent to the site were not historically reported.

Work Scope

The investigation was tailored to help the facility owner determine long-term development needs for additional lined space and delineate groundwater plume limits to meet environmental regulatory requirements.

Based on the review of available project information and the costs associated with the potential need to relocate waste from the unlined landfill, a source investigation was completed to determine if any VOC “hot spots” may be present within the unlined landfill to warrant excavation.  The groundwater plume delineation was performed on a parallel track to define the horizontal and vertical extent of the groundwater plume.

A description of vertical profiling techniques required to achieve QA/QC standards was described in a Request for Proposals (RFP) sent to drillers.  Following some preliminary testing of experimental techniques, a unique method of sampling groundwater at discrete 5-foot intervals was developed that could meet real-time reporting requirements for this site.

Procedures Taken

Standard procedures approved by the MPCA and Minnesota Department of Health (MDH) were used to evaluate in-place landfill waste material, soil below the old scale house/paint storage area, soil between the bottom of the landfill and top of the water table, and groundwater at the water table directly beneath the landfill waste material.

  • Standard hollow-stem auger (HSA), mud rotary, and geo-probe sampling techniques were used.
  • Completed borings were sealed and surveyed as necessary and classified according to ASTM D2488, Visual and Manual Procedure.
  • Landfill vapor samples from 20 existing deep vertical landfill gas vents (DV1-DV20) within the 23-acre unlined fill area were collected with a portable blower and tested for volatile organic compounds (VOCs).
  • Representative groundwater samples at the water table, below the bottom of the landfill, were collected by pounding a drive-point assembly ahead of a drill auger into the undisturbed material.

The Results

An evaluation of the reported results from the source investigation indicated the following:

  • The highest levels of Tetrachloroethene and Vinyl Chloride within the unlined landfill area were discovered at DV9 and DV12 (Figure 1 – Soil Vapor Extraction). Water table samples from below the unlined closed and capped landfill had no vinyl chloride, and low levels of PCE reported. The depth of mixed solid waste ranged from 42 to 69 feet below the top of the landfill.

The results indicated an excellent correlation between the two different analytical laboratories and the samples obtained from the Rotosonic groundwater profiling work. An evaluation of the reported results also indicates the following:

  • The leading edge of the plume migrated approximately 1 mile from the landfill, west-northwest.
  • The highest levels of VOC concentrations in the groundwater were found approximately 2,000 feet downgradient from the source area.
  • The impacted aquifer ranges in thickness from approximately 90 feet along the northern edge of the unlined source area (RS12) to approximately 30 feet within the southern half. of that area (PH1-8).

A video with 3D illustration of the groundwater plume delineation project is presented online at: http://ECADengineering.com/#video

Conclusion

The success of the project was driven by a cooperative effort between the regulators, the owner, drilling contractors, and the consultant.  The project required meticulous planning, data collection, and effective regulatory communications, in dynamic conditions.  Cost-effective options were considered, and the project was completed in a timely manner, meeting facility permit and lined space development needs.

The landfill owner was able to obtain a new Solid Waste permit in 2011.  Based on results of the source investigation, the owner was permitted to build a new lined space cell on top of the old unlined facility in 2012 as the excavation of waste from the unlined facility was not required, saving the owner millions of dollars.

Finally, the facility owner, local drillers, the MDH, and property owners who may be impacted, were notified of the inferred vinyl chloride plume limits (Figure 3). They were also informed that drilling in the area may require special considerations with MDH and/or MPCA approval.

 

This is an excerpt of the original case study created by Principal Engineer/Scientist Terry Kaiser titled “2010-2012 Groundwater Plume Source Investigation In Capped MSW Landfill & Chlorinated Solvent Deep Vertical Profiling”

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