When is Landfill Gas a Source of Low-Level Volatiles in Groundwater?

With the advent of Subtitle D-mandated detection monitoring programs, more and more municipal solid waste landfills are encountering the presence of low-level volatiles in the monitored aquifers. Often the source of these volatiles is not clear, and further study at the site is needed.

Because landfill gas (LFG) is produced at most solid waste sites and because it contains trace levels of certain volatile organic compounds (VOCs) in the gas phase, some researchers and consultants theorized that landfill gas may serve as a source of low-level VOC contamination of groundwater. This is a recent issue that in the future may have an impact on the way landfills are designed, operated, and closed.

LFG as a source of contamination has been offered in certain situations where conventional explanations do not seem to apply:

• The presence of volatiles in groundwater is upgradient of the waste site.
• Other indicator parameters do not suggest a leachate source.
• Subsurface gas migration has occurred.
• Similar volatiles are present in both the gas and groundwater.
• A bias exists to attribute groundwater contamination to a source other than leachate.

It should be noted that this theory has not been verified in the field, in part due to limitations in sampling and analytical methods at low levels (less than 20 parts per billion [ppb] for soil and groundwater), the matrix of VOCs considered, and the absence of long-term monitoring programs designed to examine the relationship between LFG, leachate and groundwater.

However, empirical data at numerous sites suggest this mechanism is taking place. At these sites, the installation and operation of LFG control systems appear to reduce the levels of volatiles in localized groundwater.

  Vocs and Transport Mechanisms

Low-level VOCs found in LFG and LFG condensate are sometimes found in off-site gas and groundwater monitoring wells. Table 1 lists some of the more commonly identified VOCs. Detection levels range from the low ppb to low parts per million (ppm) levels.

TABLE 1 – Commonly Identified VOCs

Benzene

Dichlorodifluoromethane

1,1-Dichloroethane

1,2-Dichloroethane

Dichloromethane

Tetrachloroethylene

Trichloroethylene

1,1,-Trichloroethane

Toluene

Vinyl Chloride

Xylene

  Diffusion and Advection

LFG transport is by two methods—diffusion and advection. Advection transport is a function of barometric variations and landfill pressure gradients caused by the internal generation of LFG. Advection is the key transport mechanism with regard to emissions and migration control strategies.

While diffusional flow likely is minimal compared to advection, this mechanism is associated with the ultimate transfer of compounds into air, soil, and liquid media. Diffusional transport is compound-specific and is affected by the following parameters, many of which are difficult to measure outside a laboratory:

• Henry’s Law (solubility, vapor pressure);
• Porosity;
• Bulk density;
• Organic carbon fraction;
• Soil moisture content;
• Seasonal water table fluctuations; and
• Concentration gradients at air/soil/water interfaces.

Few studies have examined VOC diffusion transport, in part because compound interactions/relationships are not well defined at the ppb level.

  Defining Sources of Contamination

The issue of LFG contaminating groundwater has arisen because of a failure of classic leachate generation models to account for the presence of volatiles in groundwater upgradient of a landfill.

When groundwater contamination in a monitoring well is found upgradient from a landfill, LFG may be a possible source. This is not an unreasonable conclusion, particularly where LFG migration has been observed, the level of contamination is in the 1 to 100 ppb range, and the matrix of VOCs in the gas is similar to that in the groundwater.

  Regulatory Considerations

At this time, no federal or state regulatory levels exist for soil gas VOCs, and regulatory programs to monitor LFG are limited mostly to methane content. Subtitle D calls for methane contents in soil that cannot exceed 5 percent (by volume in air) at the landfill boundary. On the other hand, groundwater standards and associated monitoring programs are established for most of the VOCs typically found at landfills and in leachates. Groundwater standards generally are at the low ppb level and are not limited to the landfill boundary. As a result, landfill owners/operators with VOC exceedances in groundwater monitoring wells may not be tracking the actual source of contamination.

The source of contamination can have a significant impact on the costs for control and remediation. Usually, costs for LFG control are less than for groundwater remediation. In addition, Clean Air Act regulations may require collection systems to be installed anyway, making LFG collection systems a required cost at many solid waste facilities across the nation.

  Groundwater Remediation

Some landfill owners/operators have petitioned state regulatory agencies to provide them time to explore the remediation of LFG migration in advance of implementing traditional groundwater clean-ups, such as pump and treat. Generally, costs for LFG control are less than for groundwater remediation, particularly where LFG collection systems already exist (or are planned).

LFG control systems which may be applicable to groundwater remediation include active well extraction, either in soil or in refuse, horizontal collection, air injection or cut-off trenches. LFG typically is flared or used for energy recovery purposes.

SCS Engineers is the nation’s leader in the design, construction, and operation of LFG migration control systems. The use of such systems to reduce or mitigate groundwater contamination may be appropriate for your site.

Technical Bulletins are prepared by SCS Engineers to alert clients to potential issues which can affect their businesses. For more information, contact Eric Peterson, P.E., project manager, or W. Gregory Vogt, vice president, at 703/471-6150.