Tidal Effect on LNAPL Mobility

The tidal influence on LNAPL movement is a subject poorly studied with limited literature available. Tidal fluctuations along shorelines create a unique condition where the migration of LNAPLs is buffered and often mitigated from discharging into an open water body. A site investigation has been conducted in an area with presence of LNAPL located close to a major tidal river. The objective of the study was to assess the influence of the tide upon the distribution and mobility of a separated LNAPL body migrating toward the river. The Investigation area is strongly tidalinfluenced and in a continuous transient state, and therefore the vertical equilibrium is never reached. Baildown tests were performed at high and low tide in wells were LNAPL was present for assessing the physical characteristics of the LNAPL body. The baildown tests showed in general a low to very low recovery, generally not very affected by tidal fluctuation, since the recovery time is in general much longer than the tidal cycle. Tidal influence appears to have a limited impact on measured NAPL thickness in the wells over time, but affects strongly the gradients, increasing locally the potential for NAPL mobility. The LNAPL body appears to be slowly migrating towards the river.