Application of Solid Phase Extraction (SPE) Media Rods to Assess Degree of NAPL Encapsulation in In Situ Deposited NAPL Sediments

Abstract

ABSTRACT Oil-particle aggregates (OPAs) develop from solid particles attaching to and/or penetrating into the surface of a non-aqueous phase liquid (NAPL) bead suspended in the water column. In situ deposited NAPL (IDN) sediments result from the deposition of OPAs on the sediment bed at the bottom of the water body. The extent of solid particle coverage on the surface of the oil bead defines the degree of encapsulation of the OPA (i.e., fully or partially encapsulated). Since important properties such as NAPL mobility and contaminant flux are controlled by the degree of OPA encapsulation, the ability to measure this physical property is critical in characterizing NAPL fate and transport in IDN sediments. This paper describes a semi-quantitative method to measure the degree of encapsulation within IDN sediment using the fluorescence response from rods coated with solid phase extraction (SPE) media. Specifically, Direct Analysis in Real Time (DART®) technology, which was developed and is distributed by Dakota Technologies, was applied to measure the amount of contact between the SPE material coated on a vertical rod and the PAHs contained in the NAPL found in the surrounding IDN sediment. Based on laboratory results in this paper, the magnitude of the fluorescence response correlates with the degree of encapsulation. For the single NAPL and various sediment combinations prepared in the laboratory for this study, fully (or nearly fully) encapsulated IDN sediments produced a DART® response below 15% reference emitter (%RE), a defined internal standard, whereas partially encapsulated IDN sediments produced a DART® response that was generally above 30%RE and as high as several thousand %RE. This difference in fluorescence response enables DART® technology to be applied as a line of evidence to determine the degree of NAPL encapsulation in an IDN sediment in field screening programs.