MOBILITY OF SUBSURFACE LNAPL IN A TIDAL ZONE: A CASE STUDY

Characterization of a subsurface LNAPL's mobility is central to determining the nature of and need for remedial actions at a site. To assess LNAPL mobility in the subsurface and the scale of any mobility, it is necessary to synthesize a number of data sources including measured chemical concentrations, field observations, the nature and timing of LNAPL releases, soil and LNAPL characteristics, and water table dynamics. We consider all of these factors in a case study to evaluate subsurface LNAPL mobility in a tidally influenced aquifer.

At the site, which is a former above ground petroleum tank farm, LNAPL is present in a surficial aquifer bounded by a tidally and seasonally fluctuating surface water body. We evaluated LNAPL mobility on two scales – local (i.e., into and out of wells as influenced by changes in water level) and regional (i.e., persistent travel along a given trajectory).

Temporal fluctuations in LNAPL thickness in wells – ranging from several feet to zero thickness – indicate areas where LNAPL is locally mobile. Concentration measurements identify additional areas of potentially mobile LNAPL by comparison to saturation benchmarks. Properties such as LNAPL density, LNAPL viscosity, and the hydraulic gradient allow us to define a critical LNAPL thickness for mobility in the formation. Overall, these evaluations delineate an area where LNAPL is locally mobile and may potentially be regionally mobile.

Site measurements reveal a number of factors that affect regional LNAPL mobility. Groundwater elevation fluctuations that increase with proximity to the surface water body create an expanded residual smear zone and tidal buffer zone, attenuating the regional mobility of LNAPL. Moreover, seasonal and tidal groundwater flow reversals prevent any locally mobile LNAPL from sustained travel in any one direction. This is indicated by their strong influence on the temporal fluctuation in observed LNAPL (or lack thereof) in wells at the downgradient edge of the LNAPL area. Other site measurements and observations (including numerous bank surveys during which no LNAPL seeps were observed) confirm that water table dynamics are significant in preventing regional LNAPL transport.

In summary, our mobility analysis indicates that despite local LNAPL mobility at this site, there is no regional LNAPL transport to the adjacent surface water body.