A Quantitative Approach for Passive Removal of LNAPL from Groundwater

Persistent light non-aqueous phase liquids (LNAPLs) are one of the more problematic challenges for obtaining site closure or no further action status at remediation sites. The source of the LNAPLs can be varied, ranging from leaking underground petroleum storage tanks to manufacturing facilities where long-term oil loss from equipment creates LNAPL accumulations beneath factory floors. Active recovery using pumping or periodic vacuum recovery from wells or sumps typically are employed as a remedial action, but usually have disappointing results because the LNAPL re-accumulates to thicknesses that exceed the 0.01 foot action level recognized by many states shortly after active recovery ceases. This paper presents a simple passive approach to removing persistent LNAPL using non-woven hydrophobic oil absorbing fabrics. The method was explored in a laboratory setting to assess physical properties of the sorbent cloth. Parameters that included sorptive capacity, buoyancy and LNAPL wicking were measured and observed. From the observations it was determined that the cloth could be rolled and secured with cable ties for placement in the wells/sumps. Two sorbent placement designs were developed; one where the rolled sorbent freely floated on the well/sump fluid surface and a second where the sorbent roll was placed in the fluid column at a fixed depth. The rolled sorbents were then applied at two manufacturing facilities that have had persistent LNAPLs present for over a decade. In both instances, LNAPLs were reduced to thicknesses below the action level in less than two months. At both locations, the sorbents were removed and LNAPL thicknesses were gauged one month later. In most wells LNAPL did not re-accumulate; where it did re-accumulate, it was less than 50% of the original thickness. The application of this approach to quantify mobility and migration analysis for site closure will be presented.