BACKGROUND AND STATEMENT OF ISSUES
The Bemidji Public School System is currently considering construction of a high school on Anne Street, approximately 600 feet south of the former Kummer Landfill in Bemidji, Minnesota (Figure 1). As a precaution, the Superintendent of the Bemidji Public Schools requested the Minnesota Department of Health (MDH) conduct an assessment to determine if students and staff could potentially be exposed to landfill contaminants. MDH reviewed Minnesota Pollution Control Agency (MPCA) files and consulted with MPCA and University of Minnesota (U of MN) staff.
There are two potential exposure pathways of concern for students and staff of the proposed high school. The first is from gas produced by the landfill moving laterally within the soil toward the proposed site of the school. A large portion of this gas is made up of methane which can be explosive at high concentrations. Gas in the soil can enter structures through foundations and accumulate to explosive levels. This danger of explosion may represent an acute safety hazard and it was one of the problems first addressed by the MPCA during cleanup of the landfill.
The MPCA constructed a passive landfill gas venting system over the landfill to ensure landfill soil gas does not move to nearby structures. This system should effectively control the landfill gas migrating off-site via soil towards the proposed high school location. In addition, there are monitoring vents south of the landfill along Anne Street. Monitoring results from these vents indicate that soil gas does not move south of Anne Street to the proposed school location (MPCA 1997a).
While the passive landfill gas venting system appears to have effectively addressed concerns over the explosive hazards, it raises a secondary health concern. A small portion of the gas produced by the landfill (typically less than 1%) is made up of non-methane organic compounds (NMOCs). These chemicals can represent a health concern when people are exposed to even relatively small amounts over long periods of time. This health consultation will address this chemical exposure pathway in relation to the proposal to construct a new high school near the landfill.
Estimated Ambient Air Concentrations
MDH reviewed data estimating both the concentrations of NMOCs in the landfill gas from the Kummer Landfill and the flow rates from the passive vents on the landfill (Maier and Tam 1994, MPCA 1993a, MPCA 1993b). Estimates of landfill emissions were calculated from this work and entered into a U.S. Environmental Protection Agency (USEPA) screening model to predict what landfill derived pollutant air concentrations might be in the area of the proposed school (USEPA 1995a). The estimated air concentrations were then compared with health based criteria. Contaminants of greatest concern in the area of the proposed school site are listed in Table 1. Further information about how these estimates were derived can be found in Appendix I.
|Pollutant||Health Based Criteria Used with Screening Model (mg/m3)5||Concentration Estimated at the Proposed School Site (mg/m3)|
|Ethylene Dibromide||0.05 2||0.02|
|Vinyl Chloride||0.1 1||0.03|
1. U.S. EPA Health Effects Assessment Summary Tables (Heast) USEPA 1995b.
2. MDH Draft Health Risk Values (HRV). September 1997.
3. U.S. EPA Integrated Risk Information System. USEPA 1997.
4. MDH Draft HRV. September 1997.5. (mg/m3) - micrograms per cubic meter
The health criteria for all the pollutants in Table 1 are based on carcinogenicity. At air concentration less than or equal to these criteria, MDH considers the cancer risk to be negligible when an individual is exposed for a lifetime. MDH considers a lifetime risk of 1 in 100,000 to be negligible. This means that if 100,000 people were exposed to one of these contaminants at their health based criteria over a lifetime, no more than 1 individual would be expected to develop cancer as a result. To keep this in perspective, approximately half of all Minnesotans are expected to develop cancer in their lifetime. The total risk cancer risk from a lifetime exposure to all four pollutants at the concentrations listed in Table 1 is 0.8 in a 100,000, which is considered negligible.
These health based criteria are based on conservative assumptions and are meant to ensure that the public health is protected. Even if individuals were exposed to concentrations of carcinogens greater than the health based criteria, it is very unlikely that they would experience adverse health effects. Nevertheless, MDH does not recommend that individuals be exposed to these contaminants at levels above the draft Health Risks Values (HRVs).
The health criteria for ethylene dibromide (EDB) and benzene are draft HRVs established by MDH. The HRVs are currently in a process leading to promulgation in Minnesota Rules. The health criteria for chloroform and vinyl chloride are based on USEPA documents (USEPA 1995b and 1998).
Uncertainty Associated with the Results from Air Modeling
Assuming some confidence in the model inputs, the USEPA screening model provides conservative results. Air concentrations are likely to be no higher than those estimated by the model. Levels could be lower than the estimates, but in order to determine levels more accurately, more refined modeling must be done.
There is a considerable amount of uncertainty surrounding the estimates in Table 1 because the NMOC emission rates inputted into the model are rough estimates. The estimated NMOCs emissions from the landfill are derived from estimates of landfill gas flow rates through the passive vents located on-site (MPCA 1993a and 1993b) and VOCs detected in the landfill gas during a single sampling event conducted in August 1992 (Maier and Tam 1994).
MDH assumed a flow rate of 791 liters per minute (L/min) from the each landfill vent when determining landfill NMOC emissions rates for the USEPA screening model. This was based on sampling done at the 23 gas vents located on-site on May 26, 1993 (MPCA 1993a). MDH believes this is the most reasonable flow rate data available. However, the flow rates will vary over time depending on variables like wind speed, barometric pressure, and microbiological activity occurring within the landfill. It is also generally accepted that flow rates through the passive vents will gradually decline over many years as the landfill waste is degraded.
MDH believes that flow rates are unlikely to be greater than 150% of the rates used for the screening model. This is based on data that estimated an average flow rate of 1,327 L/min from each landfill vent during measurements collected immediately after new vents were installed in April 1993 (MPCA 1993b). The analogy of "popping a balloon" helps describe what happens when a venting well is initially drilled into a landfill. A significant amount of pressure is built up in a landfill when landfill gas is produced. When the venting well is installed, it releases this pressure. After this initial release, a lower flow rate is established between the landfill gas and the ambient air. Therefore, it is unlikely that vent flow rates would ever reach the initial average flow rate of 1,327 L/min detected immediately after the installation of the vents.
VOC Landfill Gas Concentrations
Estimates of landfill gas emissions can also vary depending on the assumed NMOC concentrations within the landfill gas. NMOC landfill gas concentrations are known to vary significantly from landfill to landfill (USEPA 1991, MPCA 1992). In addition, VOC landfill gas concentrations have been shown to vary over time at the Kummer Landfill. Table 2 list the variability in concentrations of two NMOCs, vinyl chloride and EDB detected in landfill gas from the Kummer Landfill. These data indicate that NMOC concentrations can vary by orders of magnitude. Table 2 indicates NMOC concentrations in the landfill gas will vary widely over time adding uncertainty to the screening model results.
|Contaminant||Date Sampled or Analyzed||Highest Concentrations Detected (µ/m3)||Number of Samples Analyzed||Average Concentrations Detected (µ/m3)|
|Vinyl chloride||7/30/92 (date sampled)||23,716||6||2,754|
|Vinyl chloride||8/18/92 (date sampled)||1,118||5||211|
|Vinyl chloride||10/16/92 (date sampled)||390||6||121|
|Vinyl chloride||12/10/93 (date analyzed)||22.5||3||12.5|
|Vinyl chloride||9/20/94 (date sampled)||17,160||1||17,160|
|Vinyl chloride||9/26/94 (date analyzed)||3,167||2||3,039|
|Ethylene dibromide||7/30/92 (date sampled)||2,356||6||1,331|
|Ethylene dibromide||8/18/92 (date sampled)||2,356||5||1,312|
|Ethylene dibromide||10/16/92 (date sampled)||4,314||6||1,206|
|Ethylene dibromide||12/10/93 (date analyzed)||730||3||417|
|Ethylene dibromide||9/20/94 (date sampled)||N/D||1||N/D|
|Ethylene dibromide||9/26/94 (date analyzed)||N/D||2||N/D|
* Data from the MPCA's Kummer Landfill File
The NMOC concentrations for vinyl chloride and EDB used with the USEPA screening model were 211 µ/m3 and 1,312 µ/m3 respectively. Significantly higher average concentrations of vinyl chloride are reported in Table 2.