Geochemical stability of chromium in sediments from the lower Hackensack River, New Jersey

Elevated levels of chromium, partly attributable to historical disposal of chromite ore processing residue, are present in sediment along the eastern shore of the lower Hackensack River near the confluence with Newark Bay. Due to anaerobic conditions in the sediment, the chromium is in the form of Cr(III), which poses no unacceptable risks to human health or to the river ecology. However, as water quality conditions have improved since the 1970s, aerobic conditions have become increasingly prevalent in the overlying water column. If these conditions result in oxidation of Cr(III) to Cr(VI), either under quiescent conditions or during severe weather or anthropogenic scouring events, the potential for adverse ecological effects due to biological exposures to Cr(VI) is possible, though the reaction kinetics associated with oxidation of Cr(III) to Cr(VI) are unfavorable. To investigate the stability of Cr(III) in Hackensack River sediments exposed to oxic conditions, sediment suspension and oxidation experiments and intertidal sediment exposure experiments that exposed the sediments to oxic conditions were conducted. Results revealed no detectable concentrations of Cr(VI), and thus no measurable potential for total chromium oxidation to Cr(VI). Furthermore, total chromium released from sediment to elutriate water in the oxidation and suspension experiments ranged from below detection (<0.01 mg/L) to 0.18 mg/L, below the freshwater National Recommended Water Quality Criteria (NRWQC) of 0.57 mg/L for Cr(III). These results support conclusions of a stable, in situ geochemical environment in sediments in the lower Hackensack River with respect to chromium. Results showed that chemicals other than Cr(VI), including copper, lead, mercury, zinc, and PCBs, were released at levels that may pose a potential for adverse ecological effects.