by: Stewart J. Smith, M.S.

Chlorinated volatile organic compounds (CVOCs) in groundwater are often found associated with municipal solid waste landfills in arid to semi-arid environments. Common characteristics of these landfills include: 1) a deep water table, (usually greater than 100 feet below land surface), 2) absence of low–permeability covers or liners, 3) vapor–phase CVOC concentrations of tens to thousands of micrograms per liter in soil beneath the landfills, 4) much lower CVOC concentrations within the landfills, and 5) low soil moisture content beneath the landfills. The landfills may not be an obvious source of the CVOCs in ground-water because the CVOC concentrations increase with depth beneath the landfills, the landfills themselves contain only low CVOC concentrations, and in some cases, the CVOCs are found in upgradient groundwater. Furthermore, the low moisture content of the vadose zone suggests that leachate did not carry or is not carrying CVOCs from the landfill to the groundwater.

Our analysis of several postulated CVOC transport mechanisms have shown, however, that the landfills are likely to be sources of the vadose zone and groundwater CVOC contamination. Using the computer model TRACRN¹, we have shown that CVOCs can migrate rapidly downward from a landfill into the vadose soils and subsequently dissolve in the groundwater. The CVOCs are carried by landfill gas (LFG) that creates a gas pressure gradient downward into underlying vadose soils. Although most of the LFG CVOCs may migrate upward through the soil cover, sufficient CVOCs can be transported downward by advection and diffusion to result in groundwater contamination. After 20 or 30 years, continuing LFG generation and anaerobic biodegradation strips the landfill of most of the CVOCs, reversing the concentration gradient, and leading to CVOC concentrations that increase with depth and distance from the original source area. Figure 1 shows both the measured and simulated soil gas and groundwater tetrachloroethene (PCE) concentrations beneath a closed landfill in Tucson, AZ.

Unless removed, the high CVOC concentrations in soils above the water table represent a continuing source to groundwater. Because the remaining CVOC source is within the deep vadose zone, and typically covers a large area, soil vapor extraction (SVE) is the preferred technique for source removal. SVE systems installed beneath landfills must be carefully designed, however, to meet CVOC source removal objectives without changing the anaerobic character of the overlying landfill.

Hydro Geo Chem has successfully designed SVE systems that address these concerns. The systems are designed to remove CVOCs from deep vadose soils to protect groundwater, minimize the migration of CVOCs to the water table, and minimize air intrusion into the landfill. Figure 2 illustrates the SVE removal of LFG (containing CVOC) from deep vadose soils at a closed desert landfill site.

For more information, contact Stewart Smith at (520) 293-1500 ext. 122 (stewarts@hgcinc.com).

¹Travis, B. and K. Birdsall. 1988. A 3-dimensional finite-difference computer code capable of simulating liquid and gas movement and solute transport within variably saturated porous media.