NONAQUEOUS PHASE HYDROCARBON IN A FINE-GRAINED SANDSTONE .2. EFFECT OF LOCAL SEDIMENT VARIABILITY ON THE ESTIMATION OF HYDROCARBON VOLUMES

The exaggeration of light nonaqueous phase liquids (LNAPL), typically hydrocarbon, by monitoring wells is a well-known problem that introduc es significant errors in the estimation of recoverable hydrocarbon. Fa rr et al. (1990) and Lenhard and Parker (1990) show that significantly different volumes of hydrocarbon may produce the same thickness of hy drocarbon in a monitoring well, due to differences in the capillary ch aracteristics 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 a rea. Capillary characteristic curves were determined for 10 samples fr om 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, ev en for small sites. For example, a hydrocarbon volume of three cm(3)/c m(2) could produce anywhere between 45 and 200 cm of hydrocarbon withi n an observation well. This suggests that use of an ''average'' soil s ample to characterize hydrocarbon exaggeration, within even a very sma ll site, can lead to substantial errors. It also suggests that maps of apparent hydrocarbon thickness can be extremely misleading, leading h ydrologists to place remediation wells in areas of greatly exaggerated thicknesses produced by fine-grained materials. Comparison between ch aracteristic curves calculated from grain-size analysis using the appr oach of Mishra et al. (1989) to those measured using the pressure plat e shows poor correlation that introduces large errors into the estimat ed hydrocarbon volumes.