Development and Optimization of Passive Sampling Methods for Surface Water and Sediment Porewater Measurements of Freely Dissolved Concentrations of Hydrophobic Organic Contaminant

Abstract

Polychlorinated biphenyls (PCBs) are hydrophobic organic contaminants (HOCs) that have persisted in the environment for decades after production was banned by the USEPA in 1979. PCBs are known carcinogens and bioaccumulate into organisms through the aquatic food web, thereby posing threats to ecological and human health even at trace concentrations in the environment. The freely dissolved concentration in the aqueous phase (Cfree) is a useful indicator of chemical activity and is critical to determine exchange between environmental phases and bioaccumulation in organisms. Polymeric passive sampling (PS) provides time- integrated Cfree measurements in surface waters and sediment porewaters, unlike grab sampling that provides a snapshot of the concentration at one point in time. The primary objectives of this research were: (1) to understand the true nature of the time-integrative property of equilibrium PS under fluctuating ambient surface water concentrations, (2) to design PS devices (by manipulating their major rate-limiting zones for mass transfer so that approach to equilibrium is expedited) and develop field demonstrated PS prototypes that can perform (i) short-term measurements of HOCs in surface water during a storm event or (ii) address the challenges associated with Cfree porewater measurements for strongly hydrophobic compounds, (3) to develop standardized methods for impregnating performance reference compounds (PRCs) into PS (that helps determining the extent of a sampler’s approach to equilibrium) and calculating the cost of preparing PRC impregnated low density polyethylene (LDPE) samplers. The theoretical and modeling framework of exchange kinetics in PS developed in this study, was utilized to calculate sampling time scale of integration (TSI) and sensitivity (%) for various chemical-sampler pairs. This was used to develop and optimize the designs of PS devices for long- and short-term deployments in the field for accurate pore-water and surface water Cfree measurements. In high flow conditions, when diffusion is controlled by the sampler side, thin sheet PS (18 ?m PE) were used to perform short-term Cfree measurements over a stormflow event lasting for 42 hours in a PCB contaminated tributary to an urban river in Washington DC. Results indicated evidence of the tributary as a major source of PCB loading to the main river during stormflow conditions. In compact sediments, diffusion is controlled by the water boundary layer (WBL) external to the sampler. Mechanical disruption of the WBL outside the sampler by introducing periodic vibration on field demonstrated sampling devices showed that within compact sediments, PS reached at least 20% equilibrium in 8 days even for the more hydrophobic PCBs and dioxins/furans, as demonstrated by the depletion of PRCs from the samplers. Accurate measurements of Cfree in sediment porewater was possible by performing PRC corrections. Non homogenous PRC loading can lead to error prone estimations of fractional PRC losses used for correcting for non-equilibrium conditions. PRC stock solutions in nonane when spiked in methanol-water systems, homogenous impregnations of LDPE with overall RSD 4% was observed. The material-cost of preparing 1 g of LDPE at 200 ng/g from 80:20 methanol-water solutions was approximately $8.