Influence of aeration on hydrophobic organic contaminant distribution and diffusive flux in estuarine sediments

Dredging operations, resuspension events during storms, and bioturbation alter the oxic state of estuarine sediments and induce changes in the composition of dissolved and particle-associated natural organic matter. These changes may alter the distribution of hydrophobic organic chemicals (HOCs) in sediments and their diffusive flux across the sediment-water interface. In this study, the impact of aerating anoxic sediments on the distribution and diffusive flux of a model HOC, 2,2',4,4'-tetrachlorobiphenyl (TeCB), was investigated. Anoxic estuarine sediments collected from three sites along a salinity gradient were used to determine site-specific apparent sorption coefficients for porewater dissolved organic carbon (Kpwdoc) and sediment organic carbon (Koc) under anoxic and oxic conditions. A two-compartment sediment flux model was employed to examine the diffusive flux of TeCB under both oxic states. Aeration of anoxic porewaters resulted in significant decreases in porewater dissolved organic matter (DOMpw) aromaticity as indicated by declines in molar absorptivity at 254 nm (p < 0.005). Aeration also resulted in a 9-13% decrease in DOMpw concentration (p < 0.005) at the two sites exhibiting lower ionic strengths; the high ionic strength site did not exhibit a significant change in DOMpw concentration (p > 0.10). The impact of aeration on TeCB distribution and diffusive flux appeared to be site-specific. Aeration of anoxic sediments induced a significant 1.4 log unit reduction in Kpwdoc at the lowest ionic strength site (p < 0.0005), while sediments from the intermediate ionic strength site exhibited a significant 0.6 log unit increase (p < 0.005). No significant change in sorption to DOMpw was observed for the high ionic strength site (p > 0.10). The sediment displaying the drop in Kpwdoc also exhibited a significant 0.4 log unit drop in Koc (p < 0.01), while the other two sites did not exhibit significant aeration-induced changes in sorption to particle-associated organic matter (p > 0.10). No significant change in diffusive flux was observed for two sites (p > 0.10), while a significant 89-110 mg m(-2) yr(-1) increase in diffusive flux was observed at the low ionic strength site (p < 0.10). This latter result represented approximately a doubling in diffusive flux. In the systems studied, facilitation of TeCB transport across the sediment-water interface by organic colloids did not appear important.