Air emissions from exposed, contaminated sediments and dredged materials 1. Experimental data in laboratory microcosms and mathematical modelling

The air emissions of two polycyclic aromatic hydrocarbons (pyrene and phenanthrene) and one heterocyclic compound (dibenzofuran) from an 'aged' contaminated sediment (Rouge River, Michigan) were studied in an experimental microcosm. The sediment-to-air flux of the above semivolatile organic compounds (SOCs) was obtained from an initially water-saturated sediment. The magnitude of flux varied as dibenzofuran > phenanthrene > pyrene. At a low air flow rate (10 mlmin^-1) the flux was low and stable and air-phase resistance controled. At a larger flow rate (100 mlmin^-1) the flux was high initially and then declined, indicating the effects of sediment-side diffusion on mass transfer to air. The flux was also sensitive to the relative humidity (RH) of the air flowing above the sediment. It was observed that the flux, though high initially, declined rapidly as dry air (0% RH) was passed over the wet sediment. Loss of sediment moisture which increased the sorptive capacity of the sediment for the contaminants was also noted. Subsequent exposure of the dry sediment to humid air increased the flux. The steady state flux was consistently high when humid air was passed over dry sediment. The experimental data were analyzed using a mathematical model which estimated the air emission from an exposed sediment layer under both sediment-side and air-side resistance controlled conditions. The model incorporated an advancing 'drying front' in the direction of airflow for water evaporation in accordance with our observations. The sharp decrease in contaminant flux was attributed to the drying-out of the sediment and a consequent increase in its sorptive capacity for the contaminant. The framework of the theory underlying the air emission modelling from exposed sediment, dredged and placed in a confined disposal facility (CDF) is summarized.