Transport and Retention of Hydrophobic Organic Micropollutants in Estuaries: Implications of the Particle Concentration Effect

Abstract

An increase in the particle–water distribution coefficient with decreasing particle concentration, or particle concentration effect (PCE), is a common phenomenon observed in both laboratory and field studies of many hydrophobic organic micropollutants (HOMs), and is demonstrated in this work by14C-labelled benzo[a]pyrene sorption to estuarine sediment. While the focus of most studies has been the precise cause of the PCE, a more immediate requirement from a regulatory and impact assessment standpoint is its empirical definition and incorporation into any pollution modelling or management tool.A compilation of literature data on the sorption of HOMs to natural particles indicates that the relationship between distribution coefficient, KD, and suspended particle concentration, SPM, is of the form: KD=aSPM−b. The constants a and b are compound- and site-specific and are dependent to some extent on the inadequacy of particle-aqueous phase separation (or the concentration of colloids or non-settling particles encompassed by the aqueous phase). It is proposed that, for chemical flux calculations and pollution transport considerations, a PCE algorithm of the same form is appropriate:KD ′=a′SPMs−b′. Here, a transport distribution coefficient represents the ratio of HOM concentrations discriminated according to contaminant transport properties (i.e. contaminant bound to particles which are subject to gravitational settlement, and contaminant in an aqueous phase which embraces non-settling particles), SPMsis the concentration of particles which are subject to gravitational settlement, and a′ and b′ are empirical constants which define the PCE derived in this way. The implications of this algorithm for the transport and fate of HOMs in estuaries are examined, and a simple model for the estuarine retention of HOMs, incorporating the PCE and accounting for compound degradation in the aqueous phase, is presented. Estuarine retention increases with increasing magnitude of a′, the particle concentration-normalized partitioning or ‘hydrophobicity index’, and increasing degradation half-life. For b′<1, retention increases with increasing particle concentration; for b′>1, the reduction in KD′ more than offsets the accompanying increase in particle concentration and a decrease in retention with increasing particle concentration is predicted.In the absence of necessary site-specific sorption and degradation data, and hydrological and sediment transport parameters, these calculations are largely hypothetical. Nevertheless, they demonstrate the factors that affect the overall retention of HOMs by estuaries and highlight the requirement for incorporation of the PCE in any contaminant modelling framework and environmental impact evaluation.