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Abstract

The exaggeration of light nonaqueous phase liquids (LNAPL), typically hydrocarbon, by monitoring wells is a well-known problem that introduces significant errors in the estimation of recoverable hydrocarbon. Farr et al. (1990) and Lenhard and Parker (1990) show that significantly different volumes of hydrocarbon may produce the same thickness of hydrocarbon in a monitoring well, due to differences in the capillary characteristics between soil types. The purpose of our investigation was to evaluate the influence of local sediment variability on estimation of hydrocarbon volumes. Sediment samples from two sites underlain by a relatively homogeneous sandy deposit were collected within a small area. Capillary characteristic curves were determined for 10 samples from one site and 41 samples from the second site using a pressure plate. Grain-size analysis was performed on all samples for which pressure plate data were available.

The results show significant variability, even for small sites. For example, a hydrocarbon volume of three cm3/cm2 could produce anywhere between 45 and 200 cm of hydrocarbon within an observation well. This suggests that use of an “average” soil sample to characterize hydrocarbon exaggeration, within even a very small site, can lead to substantial errors. It also suggests that maps of apparent hydrocarbon thickness can be extremely misleading, leading hydrologists to place remediation wells in areas of greatly exaggerated thicknesses produced by fine-grained materials.

Comparison between characteristic curves calculated from grain-size analysis using the approach of Mishra et al. (1989) to those measured using the pressure plate shows poor correlation that introduces large errors into the estimated hydrocarbon volumes.