The role of colloids in tracemetal transport and adsorption behavior in New Jersey Pinelands streams

Dissolved and colloidal concentrations of Al, Fe, Mn, Cu, Zn, Cd, and Pb we re determined in the Mullica River, New Jersey, and its tributaries on nine occasions between August 1994 and September 1995. Cross-flow filtration (C FF) at a nominal 10 kDa cutoff was used to separate total dissolved (<0.45 mu m) riverine metals into colloidal and truly dissolved fractions. Reprodu cibility tests on full procedural replicates demonstrated CFF's precision a nd indicated that temporal or spatial differences exceeding 10-15% in collo idal fraction can be resolved using these techniques. For the whole data se t, mean colloidal fractions of 0.45-mu m filtrates were 65% for Al, 82% for Fe, 29% for Mn, 66% for Cu, 31% for Zn, 46% for Cd, and 88% for Pb. The ma gnitude of the colloidal fraction followed an elemental order that was larg ely consistent from site to site and among different sampling dates, despit e large variations in total dissolved metal concentrations. The order was v ery similar to that observed previously in rivers of varying geochemical pr operties, suggesting that the degree of colloidal association is driven by a metal's affinity for humic materials and is relatively insensitive to oth er riverine properties such as pH, dissolved organic carbon, and suspended mass. The constancy of this elemental pattern in Pinelands streams suggests that the relative fates of metals during estuarine colloid flocculation ma y be predictable and temporally stable. Iron behavior was distinct from tha t of the other elements: colloidal Fe fraction was positively correlated wi th pH (range, 4.3-7.1) and the colloidal Fe/Al ratio showed clear seasonal variation. The other metals did not exhibit these behaviors, which suggests that their colloidal association was distinct from that of Fe. Colloidal m etal/Al ratio for Fe, Pb, and Zn was clearly different from that of >0.45-m u m particles; that finding is consistent with the distinct compositions an d sources for colloidal and particulate fractions. The distribution coeffic ient (K-d) for Pb was positively correlated with suspended particulate mass (<10-fold variation) over time at one representative site. This inverse pa rticle concentration effect runs contrary to recent observations in other f reshwater and marine systems and suggests that colloidal and particulate si ze fractions vary independently over the course of the year.