Modeling time scale of integration in equilibrium passive sampling.

Abstract

Passive samplers (PSs) deployed in the field for several months provide a time-averaged measurement of the freely dissolved concentration of pollutants, which is important for assessing ecological exposure and estimating pollutant loads. A comprehensive theoretical modeling assessment of the sampling time scale of integration (TSI) of an equilibrium PS is required to correctly interpret the results. We address this knowledge gap by modeling exchange kinetics of polychlorinated biphenyl congeners in low-density polyethylene (PE) PS based on diffusive transport and first-order kinetics. We evaluate the sampling TSI by analyzing the response of the PS to simulated pulsed concentration increases in the water column that lasted for 1 day in a total sampling period of 90 days. More hydrophobic compounds experience slower transfer into the sampler and show a longer TSI compared with less hydrophobic compounds. Similarly, a thick sampler shows longer TSI than a thinner sampler. The sampling TSI for a typical 25.4 μm PE sheet ranged widely from 14-15 days for a dichlorobiphenyl to 43-45 days for a hexachlorobiphenyl. We show that strategic deployment of a thick and thin passive sampler can be used to narrow the range of TSIs for all congeners and used to simultaneously capture episodic events along with long-term averages.