Evaluation of the diffusive gradient in a thin film technique for monitoring trace metal concentrations in estuarine waters.

Monitoring trace metal concentrations in dynamic estuarine waters is not straightforward. This study demonstrated that important information could be obtained from intensive sampling of physicochemical parameters and trace metal concentrations, in the Gold Coast Broadwater, Australia. A regular pattern of variation in Cu and Ni concentrations was related to the movement of water passed point sources with tidal flows, rather than due to conventional estuarine mixing of end-member waters. However, this approach was logistically demanding and expensive. The diffusive gradients in a thin film (DGT) technique was used as an alternative method due to its continual time-integrated response to changes in trace metal concentrations. Significant correlations were found between 24 h DGT-labile measurements and 0.45-microm filterable measurements, on time-averaged composite samples (grab samples combined every 4 h for 24 h), for Cu (n = 24, r = 0.965, p < 0.001), Pb (n = 24, r = 0.799, p < 0.001), Zn (n = 17, r = 0.909, p < 0.001), and Ni (n = 23, r = 0.916, p < 0.001). DGT-labile measurements as a fraction of 0.45 microm-filterable concentrations were 21 +/- 2% for Cu, 29 +/- 11% for Pb, 28 +/- 5% for Zn, and 27 +/- 12% for Ni, demonstrating the speciation capabilities of DGT. Although DGT measurements were confirmed as being highly operationally defined, DGT was still found to be very promising as a monitoring approach, particularly for dynamic estuarine waters.