Geochemical factors controlling free Cu ion concentrations in river water

Copper speciation was determined monthly at seven sites on four rivers in s outhern New England to understand which geochemical factors control free me tal ion concentrations in river water. Samples were conventionally filtered (<0.45 mu m) and then ultrafiltered (3.000 molecular weight cut-off) to de termine Cu speciation in the truly dissolved size fraction. Differential pu lse anodic stripping voltammetry (DPASV) was used to quantify natural organ ic complexation and cathodic stripping square wave voltammetry (CSSWV) to m easure directly both Cu sulfide complexes and total EDTA concentrations. Th e results showed both dissolved organic matter (DOM) and sulfide complexati on dominate Cu speciation and control the concentrations of free ion. Free Cu2+ was calculated to be in the subnanomolar range for the majority of the year. Only in the winter months, when concentrations of DOM and metal sulf ides complexes were at a minimum were free metal ions directly measurable b y DPASV at low nanomolar concentrations. The extent of sulfide complexation appears to be dominated by the size of headwater marshes (upstream samplin g sites) and by the amount of sewage treatment plant effluent (downstream s ites). DOM complexation was related to the organic matter composition and f ollowed model organic ligands. Indirect evidence suggests variations in riv er water pH and Ca2+ (metal competition) has only a minor role in Cu comple xation. Measured concentrations of total EDTA suggest this synthetic ligand can control Ca speciation in some highly developed watersheds; however, co mpetition from Ni (and possibly Fe) limits the extent of this complexation. Copyright (C) 1999 Elsevier Science Ltd.