Freely dissolved concentration in sediment pore water is a critical measurement that is useful in assessing fate, transport, and bioavailability of hydrophobic pollutants in sediment (Mayer, 2014). Accurate measurement of low aqueous concentrations of hydrophobic compounds is challenging due to the association with colloidal and dissolved organic matter in pore water which has led to the development of passive sampling approaches using well characterized polymeric materials. When the polymeric material is able to reach equilibrium with the sediment pore water, such as in a well-stirred laboratory measurement, the estimation of freely dissolved pore water concentration (Cfree) becomes a trivial exercise based on the known partition constant of the polymeric medium (Mayer, 2003; Ghosh, 2014). However, in several situations, an in-situ measurement in sediment is desired, and such measurements have been challenged by the difficulty in reaching equilibrium between pore water and the polymer as mass transfer through the static depletion layer outside the polymer becomes limiting in the absence of active mixing (Lampert, 2010). It has been shown that for strongly hydrophobic compounds equilibrium may not be achieved in the field even after one year (Lohmann, 2011). Several researchers have adopted the use of performance reference compounds (PRCs) dosed in the polymer to assess the kinetics of mass transfer and correct for non-equilibrium (Huckins, 2006; Huckins, 2002; Fernandez, 2009; Booij, 2010; Booij, 2003; Fernandez, 2014; Oen, 2011; Tomaszewski, 2008; Apell, 2014). While corrections based on PRC loss work reasonably well for compounds with low to midrange hydrophobicity, the corrections become increasingly erroneous for strongly hydrophobic compounds when the departure from equilibrium increases (Apell, 2014). Several approaches for calibration using PRC data have been suggested (Huckins, 2006; Huckins, 2002; Fernandez, 2009; Booij, 2010; Booij, 2003). In all of these approaches, the uncertainties introduced by the PRC correction are larger when the extent of equilibrium is low, which is the case for strongly hydrophobic compounds in the field.
A primary uncertainty in the PRC correction arises from the fact that nearly always it is the sediment side mass transfer in the immediate vicinity of the passive sampler that controls kinetics and as such, is dependent on the site-specific sorption characteristics of the sediment which can vary across orders of magnitude. For example Hawthorne (Hawthorne, 2006) reported a 3-4 orders of magnitude range for site-specific KocS. Thus, to be able to correct for non-equilibrium and estimate in-situ pore water concentrations we need to first have an estimate of site-specific partitioning of the analytes of interest. The loss of a few PRC compounds and adsorption into the sediment matrix then have to be used to infer the desorption behavior of a large range of analyte compounds from sediment (Fernandez, 2009).
Thinner polymeric materials can be used to increase the surface area to volume ratio and reduce the depletion per unit area. However, even with some of the thinnest polymers practically deployable in the field (e.g., 25 μm thick polyethylene), sediment-side mass transfer limitation can be significant. Making the polymers too thin makes them prone to damage during deployment in sediment, reduces the total mass of polymer sampling material (impacting detection limits), and also poses a physical challenge of insertion in sediments if the area is very large.
To address these challenges, the present inventors manipulated the external depletion layer in the sediment side of a passive sampler deployment, which overcomes the slow approach to equilibrium for hydrophobic organic compounds in static sediments. Specifically, the present invention relates to an apparatus and method to mechanically disrupt the static depletion layer outside the polymer surface using periodic vibration performed in-situ. Using the apparatus and method to mechanically disrupt the static depletion layer using periodic vibration, the present invention further relates to a method of determining the freely dissolved concentration of analyte in the porewater of sediment.