Sorption of Aromatic Hydrocarbons by Materials Used in Construction of Ground-Water Sampling Wells

Sorption of dissolved constituents from water within a sampling well onto well-casing materials or sampling equipment is frequently cited as a potential source of bias in ground-water sampling programs. This study examined the sorption of six monoaromatic hydrocarbons (benzene, toluene, ethylbenzene, and m-, o-, and p-xylene) onto seven sampling well materials. The materials included stainless steel, rigid polyvinyl chloride (PVC), polytetrafluoroethylene (PTFE), polyvinylidene fluoride, epoxy-impregnated fiberglass, flexible polyvinyl chloride (PVC), and polyethylene. Samples of the test materials were exposed to aqueous solutions containing the six organic compounds for time periods ranging from 5 min to 8 weeks. The concentration remaining in solution, at the end of the exposure period relative to the initial concentration, was taken as a measure of the degree of sorption. No uptake of any compound onto stainless steel was noted, while some degree of sorption was observed for all compounds on all polymer well-casing materials. Sorption increased with increasing hydrophobicity of the organic compounds. The sorption increased in this order benzene, toluene, o-xylene, m-xylene, ethylbenzene, and p-xylene. Of the polymer materials, rigid PVC showed the least sorption, followed by fiberglass and polyvinylidene fluoride (which were similar) and PTFE. The flexible tubing, polyethylene, and flexible PVC showed the highest rates of uptake, with significant losses from solution by the first sampling time (5 min). The experimental data were shown to follow a diffusion model closely, with effective diffusion coefficients corresponding to different rates of uptake from solution. The model was used to extend the experimental results to monitoring wells of different diameter. Stainless steel and PTFE are commonly recommended as preferred materials for constructing monitoring wells, particularly if organic contamination is expected. Under the conditions of this study, and based only on their relative sorption characteristics, several polymer materials, including rigid PVC, are as advantageous and possibly more advantageous than PTFE.