Theory and Experiments on Subsurface Contaminant Sorption Systems

Abstract

Subsurface sorption systems (SSS) are zones created within aquifers to enhance the contaminant sorptive capabilities of the porous media. These systems could reduce contaminant mobility by as much as three orders of magnitude, thus delaying contaminant arrival and reducing maximum concentrations downgradient. Reduced contaminant mobility may also allow time for microbial and abiotic degradation reactions to occur. A few investigators explored the use of cationic surfactants in sorption zones. There are limitations to the use of these surfactants, namely reliance upon cation exchange sites, limited contaminant sorption ability, and their biocidal activity. The possible use of nontoxic, water‐insoluble, nonaqueous phase liquids (NAPL) to form SSSs is examined. Experimental results are presented for a bench‐scale system constructed with sand that is partially saturated with a sorbing NAPL (SNAPL). Residual SNAPL saturations, aqueous phase relative permeabilities, and capture efficiency for a hydrophobic organic contaminant (1‐methylnaphthalene) are determined for the sorption zone. An effective SSS is established in the experimental system. In addition to experimental work, this paper introduces the concept of “effective retardation” as a design aid. An application of the concept produces parametric expressions of system performance in terms of residual SNAPL saturation, partition coefficients, and permeabilities.