Effect of groundwater table fluctuations on LNAPL thickness in monitoring wells

In this paper we present a model for Light Non-Aqueous Phase Liquid (LNAPL) thickness fluctuations in a monitoring well due to transient conditions in an unconfined aquifer. The model is based on volumetric equilibrium and multi-phase Darcian flow principles, which include capillary effects both at the water/LNAPL interface and the LNAPL/air interface. First, the proposed model is validated by data obtained from laboratory experiments. The model is then used to analyze several cases that characterize LNAPL thickness variations in a monitoring well in response to groundwater table oscillations. Our findings indicate that groundwater table fluctuations and capillary pressures at the water/LNAPL interface and the LNAPL/air interface influence the LNAPL thickness in a monitoring well. The transient groundwater table effects on the LNAPL thickness in the well are a dominant factor for non-cohesive soils. The study leads to the conclusion that, under fluctuating groundwater table conditions, if LNAPL volume estimates in an aquifer are based on monitoring well measurements made at one point in time, or if these estimates are based on a sequence of measurements made over a period of time, which are then evaluated independently for each temporal measurement, assuming mutually independent static equilibrium conditions between the LNAPL thickness in the monitoring well and the formation, then the volume estimate calculations will be in significant error. In this paper, the relation between LNAPL thickness in a monitoring well and LNAPL thickness in a formation is discussed primarily based on numerical studies.