Investigation factors that control trace metal toxicity in coastal systems : a temporal and spatial analysis of kinetically inert copper in Boston Harbor

Abstract

(cont.) form of inert Cu. Correlations between Cu concentrations and total suspended solids at one sampling location suggested that sediment resuspension may be a source of inert Cu to coastal waters. Consistently significant inert Cu concentrations at a site near a salt marsh implied that the presence of sulfides may result in formation of inert Cu. No correlation between inert Cu and chlorophyll a was observed, suggesting that biological formation of inert Cu compounds in situ did not occur in this system. Whether Cu is added to surface waters as existing inert Cu compounds or as inert Cu that is formed when exchangeable Cu reacts with certain constituents in surface waters remains a subject for future work.

Understanding how Cu reacts with constituents in coastal waters and interacts with biota is important for predicting Cu toxicity. It was recently shown that some of the Cu in coastal waters is present in kinetically inert forms that are not released from strongly bound compounds and do not bind to a strong synthetic ligand, remaining undetectable by competitive ligand exchange / adsorptive cathodic stripping voltammetry (CLE-ACSV) on time scales relevant to natural processes (48 hours). Inert Cu seems to be irreversibly bound to strong ligands or physically sequestered within organic and inorganic matrices such that it is likely non-toxic. The primary objective of this work was to collect an extensive data set on one site, Boston Harbor, in order to examine the sources and sinks of inert Cu in this coastal system. Surface water samples were collected and filtered (0.2 [micro]m) using trace-metal clean techniques from eleven locations in Boston Harbor every two to three weeks from June to November 2002. A strong ligand (salicylaldoxime) and increments of Cu were added to each sample to determine exchangeable Cu using CSV-ACSV. UV-treated samples were analyzed by CLE-ACSV to determine total dissolved Cu concentrations. Inert Cu concentrations were deduced by subtracting exchangeable from total Cu. An average of 46.1% of the total dissolved Cu in Boston Harbor was determined to be inert Cu. Exchangeable Cu concentrations averaged 10.8 nM ([sigma]=5.5 nM); total Cu, 20.6 nM ([sigma]=5.7 nM); inert Cu, 10.7 nM ([sigma]=3.1 nM). Although the fraction of inert Cu was variable among different sites and sampling dates, the concentration of exchangeable Cu remained within a smaller range (5-15 nM), implying that Cu inputs above this level mainly took the