GeoInsighter Summer 2000 Newsletter Volume 5 Number 2 Apparent vs. Actual LNAPL Thickness Return to the Newsletter Index A large percentage of disposal sites in New England contain Light Non-Aqueous Phase Liquids (LNAPLs) such as fuel oil and gasoline. Because these liquids have densities less than water, they will float on the capillary fringe and the water table as a separate, immiscible phase (if a large enough release occurs) and migrate in the general direction of ground water flow. Estimating the thickness of LNAPLs in site characterization studies is critical because many regulations contain thickness thresholds for volume of LNAPL is necessary to evaluate volumes released and appropriate active or passive remediation programs. Coated measuring tapes, oil/water interface probes, and clear bailers can be used to measure LNAPL thickness in monitoring wells. However, the thickness of LNAPL in a monitoring well typically exceeds the thickness of LNAPL in the subsurface by a factor estimated to range between 2 and 10. Due to this difference, the LNAPL thickness measured in a monitoring well is commonly referred to as the "apparent thickness" and is not an accurate measurement of the LNAPL thickness in the subsurface. The difference in LNAPL thickness is primarily caused by LNAPL floating on the capillary fringe (water held above the water table in soil pore spaces by capillary forces), which is not present in the monitoring well, and the weight of the LNAPL (see the attached figure). The absence of the capillary fringe in the monitoring well causes the well to act as a low point into which LNAPL will drain. When LNAPL accumulates in the well, the weight of the LNAPL will depress the water table in the well resulting in additional LNAPL drainage into the well. The formation and well will be greater for heavier oils, such as No. 4 or 6 oils, due to the higher densities of these oils further depressing the water table in the well. In addition, the difference in LNAPL thickness increases with decreasing formation grain size due to the presence of a higher capillary fringe in the smaller grain size formation. Therefore, silty soils will produce a greater thickness difference than coarse-grained sand. Many studies have been performed to correlate LNAPL thickness in a monitoring well to actual LNAPL thickness. These studies have produced correlations that can be used to estimate the actual LNAPL thickness from apparent LNAPL thickness measured in a well. However, these correlations may not be accurate under a variety of field conditions and typically produce only order-of-magnitude estimates. Other methods for estimating LNAPL thickness include boring programs and the use of a cone penetrometer. Multiple borings with continuous sampling can be used to define the thickness of LNAPL from visual observations; however, it is difficult to visually distinguish between mobile LNAPL and immobile LNAPL trapped in the pore spaces by capillary forces. The cone penetrometer is a geotechnical tool that is capable of rapidly producing information regarding subsurface stratigraphy and LNAPL distribution. A cone penetrometer typically consists of a truck-mounted hydraulic ram that pushes a cone-shaped instrument through the soil. A signal is sent back to the rig that is translated into thicknesses of various materials. Some cone penetrometers are now equipped with devices to sample ground water, LNAPL, and soil during cone advancement. However, this method is more expensive than conventional drilling methods and, also, does not accurately distinguish between mobile and immobile LNAPL. The method selected to estimate LNAFL thickness is generally dependent upon the degree of certainty needed in characterizing LNAPL distribution. Order-of-magnitude estimates obtained from monitoring wells are generally acceptable for evaluating remedial approaches. In addition, many of the regulations that have LNAPL thickness thresholds are for LNAPL measured in a well, thus eliminating the need for more accurate measurements when applying these regulations to the site. It should be recognized that, if LNAPL is detected in a monitoring well, it is unlikely to be as bad as it looks and that estimates of release volumes derived from measurements in wells should take this LNAPL relationship into consideration. Richard J. Wozmak, P.E., P.H., L.S.P. info@geoinc.com Return to the Newsletter Index