Field evidence for copper mobilization by dissolved organic matter

Abstract

Fluxes of copper from the sediments to the water column in response to the presence of dissolved organic matter were measured in field enclosures placed on the floor of the Contrary Creek arm of Lake Anna, Virginia. Experimental chambers received a spike input of either sodium humate or phytoplankton; unamended chambers served as controls. Over the subsequent five days, water samples were withdrawn from the chambers and analyzed for total and dissolved copper, total and dissolved organic carbon (DOC) Eh, pH, dissolved oxygen, dissolved sulfides and heterotrophic microbial activity. Chambers amended with phytoplankton did not experience a rise in dissolved copper concentrations. Furthermore, the concentrations of dissolved copper in these chambers were not well-correlated with DOC concentrations (r = 0.1175, P = 0.642). Dissolved copper concentrations in chambers amended with sodium humate rose approximately 500% and remained elevated for the duration of the experimental trial (5 days). In this treatment, dissolved copper concentrations were well-correlated with DOC concentrations (r = 0.798, P < 0.005). The increase in dissolved copper in the chambers amended with sodium humate was observed to persist despite reducing conditions which were confirmed by redox measurements and the occurrence of dissolved sulfides. The addition of sodium humate to the chamber water resulted in a net mobilization of copper to the water column from the sediments. In contrast, dissolved organic carbon generated by the decomposition of phytoplankton did not appear to mobilize copper over the time interval examined. None of the changes in soluble copper concentrations could be related to Eh or pH effects. Supplementary laboratory experiments corroborated the field trials. The results suggest that long term (but not short term) products of decomposition processes may enhance the concentration of dissolved copper in the water column by the formation of soluble organometallic complexes.