Gasoline that spills on a pervious surface or leaks from an underground storage tank will infiltrate downward until it encounters the groundwater table. Since gasoline is less dense than water, it floats as a light non-aqueous phase liquid (LNAPL) plume. In a boring at an active service station, our client encountered floating gasoline, which caused concern about the possibility of LNAPL contamination beneath the site. The client needed to know the plume footprint to properly clean up the site, and hired RETTEW to conduct a novel geophysical survey to detect and delineate the extent of this LNAPL.
In a variation of the traditional mise à la masse electrical resistivity method, developed and offered exclusively by RETTEW, a current electrode was placed in the earth near the boring, and another in the water beneath the LNAPL in the boring. Since the LNAPL is an electrical resistor, the current between the electrodes must flow out to the edge of the plume and “spill up” around it. Using mobile potential electrodes, we saw where the current was rising and mapped the plume footprint. Based on the shape of the plume and the known location of the gasoline release, RETTEW instructed the client where to place recovery wells in its path. Using our exclusive mise à la masse technique, RETTEW helped this client delineate and properly remediate LNAPL contamination at the site.