Abstract
Metal contaminants have become a major issue in today’s environment, and increasing research is being done on methods to monitor and determine point sources of these contaminants. Passive sampling devices are one such method that has become a major topic of research. Passive sampling devices come in a variety of different forms, with granule, bottle and gel forms all being used to determine contaminants in aquatic systems. Diffusive gradients in thin-films (DGT) are one example of a gel based passive sampling device that has been used to monitor metal contaminants in these aquatic systems.
The design and assessment of the two different sampler housing units for the determination of baseflow and event concentrations was a major aspect of the project. The baseflow unit deployed at Shadbolt Park had been used in a prior study at the University of Otago, and was again successful in facilitating the use of DGTs to determine baseflow concentrations. The housing unit deployed in the Pakuranga was designed to allow DGTs to obtain event mean concentrations. This involved the design of a sampler tray to hold the DGTs in a storage solution before a storm event occurred.
In the present project, DGTs were used in two scenarios to determine metal concentrations in aquatic systems. Stage 1 of sampling involved DGTs being used to monitor Ni, Cu, Zn, Pb and Cd in the baseflow at two upstream and one down stream site at Shadbolt Park in the Whau Catchment, Auckland. This site was selected due to the park being a closed landfill, which had been repurposed into a park. Upstream and downstream sites were compared to determine if there was any contribution of metal contaminants from the landfill. Three DGT deployments were undertaken at Shadbolt Park, with the data being compared to grab samples taken throughout each deployment.
Results indicated that there was a contribution of metal from landfill leachate, although this contribution was not significant for most metals sampled. The comparison between the DGT and grab sample contributions showed a significant difference for all metals of interest, although this was expected due to DGT collecting labile metal concentrations, whereas the grab samples were analysed for dissolved metal concentrations.
Stage 2 of the project involved the determination of event mean concentration (EMC) of metals at a stormwater outflow site in the Pakuranga Catchment, Auckland. A sampler housing unit was designed in order to allow DGTs to only monitor during events, rather than baseflow and events as was the case in Stage 1. EMC was successfully obtained for two rainfall events in the Pakuranga Catchment, with these events being analysed for Ni, Cu, Zn, Pb and Cd. Event 1 was found to have higher metal concentrations than Event 2, which can be explained by the longer dry period before Event 1. In order to determine the success and accuracy of the DGTs at obtaining EMC, the data was compared to automated samples obtained by NIWA at the same site during each event. The comparison between the DGT and automated samples showed differing results due to measuring different species, but still showed similar trends.