Trace metal speciation during dry and wet weather flows in the Tama River, Japan, by using diffusive gradients in thin films (DGT)

Abstract

The labile species of Ni, Cu, Zn, Cd, and Pb in the Tama River, an urban river in Tokyo, Japan, were measured using diffusive gradients in thin films (DGT) method under dry and wet weather conditions, and the results were compared with total dissolved concentrations in hourly samples collected in parallel. A total of 10 DGT deployments were made and 251 hourly samples were collected during 2 rounds of sampling, conducted between August and October, 2006. Two types of diffusive gradients in thin films (DGT) devices—DGT-RG for labile inorganic and DGT-APA for total (inorganic and organic) labile species—were applied throughout the samplings. The proportions of metals measured by DGT, compared with the dissolved metal concentrations (filtered using a membrane of 0.5 µm pore size), were 38 ± 5% (RG) and 45 ± 8% (APA) for Ni and 45 ± 22% (RG) and 53 ± 23% (APA) for Zn. No labile Cu was detected throughout the sampling; Cu was assumed to be in stable complexed forms. Labile Pb was detected in 3 out of 10 deployments only; the rest were lower than the detection limit. Dissolved and labile Cd concentrations were below the detection limits. Three rain events encountered during the sampling periods were evaluated. Rains brought considerably higher loads of metals in dissolved form, and DGT measurements indicated that labile metal loads also increased. Selected DGT measurements were compared with the WHAM 6 speciation model and found to be similar to the model-computed results.