Landfill Monitoring

Keene Landfill Water Quality Monitoring

The City of Keene closed its municipal landfill in July 1999 after over 30 years of operation. In accordance with New Hampshire Department of Environmental Services (NHDES) requirements all closed landfills are required to monitor water quality around the site. Water quality monitoring within the groundwater management zone (GMZ) is conducted in accordance with the groundwater management permit (GWP#199009001-K-001) during post-closure. The groundwater permit is renewed every 5 years and requires monitoring of water quality within the GMZ for a 30-year period or until 2030. The GMZ consists of the approximately 20-acre landfill and area of approximately 210 acres of land around the landfill to the north, south, east and west.

City of Keene personnel collect groundwater and surface water samples tri-annually in the months of April, July and November each year. The permit requires sampling and analysis of 9 overburden and bedrock groundwater monitoring wells and 4 surface water locations within the GMZ. Groundwater and surface water samples are tested for routine landfill sampling parameters, which include: pH, specific conductance, chemical oxygen demand, chloride, nitrate as nitrogen, total kjeldahl nitrogen, iron, manganese, volatile organic compounds by EPA Method 8260B. In addition, groundwater and surface water samples are analyzed for metals, which include arsenic, barium, cadmium, total chromium, lead, mercury, selenium and silver, every 5 years. All sampling is conducted in accordance with United States Environmental Protection Agency (USEPA) procedures.

Monitoring results are submitted to the NHDES after each tri-annual sampling round. An annual summary report which includes a tabular summary of all monitoring data collected to date, an evaluation of trends in the data, is submitted to the DES in the month of January each year. Contaminant concentrations detected in groundwater and surface water in downgradient locations (deemed affected by landfill leachate) are compared to "background" water quality (locations deemed unaffected by landfill leachate). In addition, contaminant concentrations detected in groundwater and surface water are compared to applicable state water quality standards. Groundwater concentration in exceedance of state water quality standards at or outside the GMZ boundary constitute a "violation" of the permit, which requires notification to the DES within 30 days of discovery and a re-evaluation and revision of the Remedial Action Plan (or RAP) within 60 days of discovery. No water quality standard violations were identified in 2001.

Historically, the primary contaminants of concern detected in groundwater and surface water include low-level volatile organic compounds and iron and manganese. Landfill closure and construction of the cap is expected to reverse this condition and reduce leachate generation (and its associated contaminants) to the surrounding groundwater and surface water over time.



Keene Landfill Gas Monitoring

In accordance with NHDES and USEPA post-closure gas control requirements, operations at the landfill cannot not cause the concentration of methane and other explosive gases to:

  • Exceed 25% of the lower explosive limit (LEL) for gases in structures on or off-site, excluding leachate collection and gas control and recovery components; and,
  • Exceed 50% of the LEL for the gases at and beyond the property boundary within the soil.

In order to assess these conditions the City of Keene contracted with Weston & Sampson Engineers to monitor landfill gas on a quarterly basis in the spring, summer, fall and winter months. The constituents of landfill gas monitored include: methane, carbon dioxide, oxygen, hydrogen sulfide, and non-methane total volatile organic compounds total non-methane volatile organic compounds. These gases are measured to monitor for 1) subsurface landfill fires, 2) explosions, and, 3) nuisance odors. Since completion of the landfill cap in December 2000, a total of 4 quarterly rounds of landfill/soil gas monitoring have been conducted. Landfill gas is monitored at 12 passive venting wells and 17 gas extraction wells located within the boundaries of the landfill cap. The passive vents are designed to relieve landfill gas buildup beneath the cap and emit gases to the atmosphere. The gas extraction wells collect the gas and direct it to a gas collection piping system. The gas flows to an on-site "gas-to-energy" facility, where the gas is combusted to produce electricity. The electricity produced is used to power the three on-site buildings including the recycling center.

Landfill gas is also monitored in 8 soil gas monitoring wells to assess horizontal migration, if any, of landfill gas via the unsaturated subsurface soil. The soil gas monitoring wells are located along the landfill's northern perimeter outside the landfill cap boundary. Landfill gas is also monitored in ambient air (at an up-wind location) and inside the three on-site buildings: the transfer station building, the hazardous waste storage shed, and the scale house.