Formatted Title:

Equilibrium Passive Sampling Demonstration for Short-Term Surface Water Measurements of PCBs

Background/Objectives:

Short-term surface water measurements are often critical for assessing loadings from episodic events for which equilibrium passive sampling is currently less explored. There is a lack of mechanistic understanding of how polyethylene (PE) sheet samplers may behave during short-term deployments in high flow conditions. A theoretical understanding of the influence of water flow velocity on the exchange kinetics of PCBs in an equilibrium passive sampler is key to designing samplers that accurately measure freely dissolved concentrations when deployed for a short term.

Approach/Activities:

In this study this knowledge gap is filled by evaluating the behavior and optimizing the designs of two short-term polymeric samplers (PE sheet and PDMS fibers) using a combination of mathematical modeling, lab-scale experiments, and field demonstration. The mathematical model was built using one-dimensional Fick’s diffusion equation and the exchange kinetics of PCB molecules through PE sheet was calculated for various flow scenarios. A lab-scale artificial flow chamber (AFC) was designed that could simulate a range of flow conditions (unmixed, river-like flow velocities and well-mixed). A field demonstration of short-term passive sampling was performed in the Anacostia River (Washington, DC) to measure PCB concentrations through a storm event.

Results/Lessons Learned:

Desorption of performance reference compounds (PRCs) from the samplers exposed to simulated flow velocities between 6 and 25 cm/s within a smooth pipe corresponded to modeled water boundary layer (WBL) thicknesses between 50 mm and 10 mm associated with flexible films. The WBL was observed to vary with water velocity (u), with a proportionality that was close to 1 (ranging from 0.6 to 1 for ¹³C-PCB PRCs 37, 47, 54) that was unique for flexible films like PE. In well-mixed experiments, the fractional approach to equilibrium for a 25 mm PE varied from 80% for the tri-¹³C PCB PRC 54 to 20% for the hepta-¹³C PCB PRC 178 within 24 hours of exposure. Field applications of 18 mm PEs were conducted at the Anacostia River, Washington, DC, through a storm event that lasted for 42 hours with an average water flow velocity of 14.5 cm/s. The measured surface water concentration during the storm showed that the concentrations were not diluted by the high flow event and that the loading during the storm event was a substantial contribution to the river. We also demonstrate here that the sampler accumulates PCBs at a higher rate during high flow conditions enabling a passive approach for flow-weighted sampling across changing discharge conditions in a river.

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Louis Cheung