Light Non-Aqueous Phase Liquids (LNAPLs) are present at many active petroleum release sites. LNAPLs are defined as organic liquids such as gasoline, diesel and other petroleum hydrocarbon products that are immiscible with water and less dense than water. LNAPL also can be referred to as “free product” since it is essentially product (e.g., gasoline, diesel, fuel oil, etc.) that remains undiluted in the subsurface.

Free product released from a tank or product piping flows downward via gravity through the pore spaces in subsurface soils eventually reaching the saturated zone. The LNAPL will be held up in the unsaturated zone and trapped in the pore spaces between grains. If a limited volume is released, it is possible that much of the LNAPL will be trapped in the unsaturated soils. However, if a larger volume of LNAPL is released to soils, the LNAPL will migrate through the unsaturated zone and accumulate at and above the water table as it displaces some of the water in the pore spaces.

Determining how much product has been released and the overall dimensions of the LNAPL plume can be challenging. Typically, the volume of LNAPL is evaluated by measuring the thickness of free product in monitoring wells. However, it has been shown that using LNAPL thickness measurements from monitoring wells will result in overestimating the amount of free product. The typical model used to evaluate free product (e.g., “pancake model”) results in overestimation of LNAPL because it assumes the pore spaces are 100 percent filled with free product. Instead, LNAPL occurs with water in the pore spaces in the aquifer materials. Studies have shown that LNAPL saturation is typically 10-50 percent, and is dependent on the type of product, history of the release(s), soil lithology, fluid properties and the overall volume of LNAPL released.

In addition, the free product thickness in a well is determined not only by the characteristics of the geologic formation and the contaminants, but also by the size of the well (a smaller diameter well will result in a thicker product layer). Based on the above factors, free product thickness measurements in monitoring wells can be used to estimate LNAPL volume, but those estimates will be biased high.

Once the physical properties of the formation are understood and the nature and extent of LNAPL is identified, remediation options can be evaluated. Typically, remedial approaches to remove LNAPL include:

• Hydraulic methods such as skimmers and down-hole product pumping
• Volatilization or combined volatilization/hydraulic approaches, such as soil vapor extraction, air sparging or dual phase extraction (e.g., high vacuum pumps that remove water, product and soil gas)

Several other enhancements to these approaches are available, such as adding steam, hot/cold water or surfactants to the formation to facilitate removal of LNAPL. In addition, LNAPL impacted soils can be excavated, but this will generally be limited to soils above the water table. Chemical oxidation and other destructive technologies are becoming more common and accepted for treatment of LNAPL.

Prior to evaluating remedial technologies, it is important to assess whether the selected approach will be able to recover enough free product to make it an economically viable option. For systems relying on hydraulic methods, it is important to ensure a system is technically feasible based on the site conditions, and the installation of a system will result in enough product recovery to make it worthwhile and cost-effective. The factors that affect the volume that can be recovered include the heterogeneity of the media, the amount of saturated LNAPL “trapped” in the soil/aquifer matrix and the physical properties of the LNAPL. None of the common remediation techniques for LNAPL will remove 100 percent of the LNAPL. Due to the inherent complexities in investigation and remediating LNAPL, pilot testing is recommended prior to installing any LNAPL treatment system.

LNAPL presents special challenges for investigation and remediation. Determining the overall volume and extent of free product is not as straight-forward as once believed, and none of the common remediation techniques for LNAPL will remove the entire product due to the inability to access smaller pore spaces that contain LNAPL.