Comparison of direct benthic flux to ebullition-facilitated flux of polycyclic aromatic hydrocarbons and heavy metals measured in the field

Abstract

No studies to date have compared directly the ebullition-facilitated organic and metal contaminant fluxes to direct benthic contaminant fluxes measured in the field. To address this knowledge gap, we measured benthic organic and metal contaminant fluxes at the sediment-water interface and compared them to simultaneously measured ebullition-facilitated contaminant fluxes at nine sites in a contaminated waterway in the Chicago River, IL, USA, to determine the relative importance of each transport mechanism to total sediment release. Two benthic flux chambers with integrated gas ebullition samplers were built to measure both gas ebullition and in situ fluxes of polycyclic aromatic hydrocarbons (PAHs), metals, sulfide, ammonia, nitrate, dissolved organic carbon (DOC), total filtered phosphorus (TFP), and dissolved oxygen (DO) across the sediment-water interface over time. Aqueous pH and oxidation-reduction potential (ORP) within the chamber were also monitored. The gas ebullition collection system trapped heavy metals and the 16 United States Environmental Protection Agency (USEPA) priority-pollutant PAHs (Σ16PAH) transported with gas bubbles through sorption and particle entrainment on a pre-combusted glass wool trap as in our previous studies. The results demonstrate that gas ebullition contaminant transport is a significant source of pollution release to the water column, as great as or greater than direct benthic transport. In several sites, release rates of Σ16PAHs and some metals from the sediment by gas ebullition were more than an order of magnitude higher than from direct benthic release. The average total PAH ebullition-facilitated release of all sites is predicted to be nearly 0.5 g m−2 on an annual basis. Gas ebullition is an important pathway for release of PAHs and heavy metal pollutants to the water column and monitoring of this important release pathway should be performed to adequately address the true environmental impacts of polluted sediments.