Open Cut » Environment
Release of mine water into streams in the Fitzroy catchment to remove surplus water accumulated during extreme wet seasons raised concerns about the impacts of releases on aquatic biota. The regulator has subsequently imposed stringent monitoring conditions that include a suite of 17 metals and arsenic. In many cases metal concentrations have been close to or below the analytical detection limit and well below guideline trigger values. In some cases elevated metal concentrations occur at upstream and downstream sampling sites due to natural processes. In other instances it is difficult to know whether a spike in the concentration of a single metal is due to natural or anthropogenic causes or simply contamination during sampling.
This study investigated the use of a novel in-situ method for determining metal and metalloid concentrations. The method, called Diffusive Gradients in Thin Film technique, measures metal and metalloid concentrations by uptake onto a resin. The units were tested in both laboratory and field trials. The metals and metalloids chosen for analysis in this work were based on a review of water quality data obtained from the Fitzroy Partnership for River Health. The laboratory trials exposed DGT units to filtered river water for a week. This solution was periodically spiked and diluted to imitate mine water releases and runoff events. Comparison of the DGT concentrations with the dissolved concentrations showed that the concentrations of each metal determined by DGT was usually lower than the average dissolved concentration. The relative proportion of DGT determined metals compared to dissolved concentrations varied between metals. This variation likely represents differences in the elemental properties of each metal.
In the field trials DGT units were deployed at four REMP monitoring sites in the Isaac River catchment and in two mine water dams. The sites were chosen to take advantage of several years of water quality monitoring undertaken as part of the EA conditions. The DGT units were deployed to capture metals and arsenic concentrations during periods with and without mine water releases. The concentration of metals and metalloids in the mine water dams were generally higher than in the rivers corresponding with higher salinity. Results showed that the concentration of Ag, Cd, Co, Cr, Pb, V and Mo were below the LOR for grab samples and DGT in-situ measurements at all sites. Of the remaining metals and metalloids the bioavailable concentrations of most of metals in this study were significantly lower than the respective dissolved concentrations with clear differences in these relationships depending on the metal/metalloid. Results from the river trial also showed that most of the As and Zn concentrations determined by grab samples were below the LOR. By virtue of the integrative nature of the DGT technique the bioavailable concentrations of these metals could be determined. The concentration of all metals and metalloids using the DGT technique were all less than the guideline trigger values for these sites.
By design, the units provide an average concentration integrated over the period of deployment. Thus, one DGT unit could potentially replace numerous water samples and will provide a far more representative view of in stream concentrations over a deployment period. In addition, the uptake of metals and arsenic on the gel is known to mimic biological uptake of these constituents thereby providing a more robust indication of toxicity to aquatic organisms. The in-situ gel capture of metals and arsenic also reduces the possibility of contamination of water samples during collection that can easily occur in low concentrations. Because the units can remain in-situ during high stream flow periods and are deployed and recovered during low stream flow, the method can provide metal concentrations during high stream-flow periods without compromising the safety of personnel required to undertake water monitoring.
A period of longer deployments and manual water sampling across a wider range of water quality is recommended to better calibrate the DGT method against dissolved concentrations. Once fully calibrated, we expect that this technique could replace a considerable amount of the manual water sampling for metals and metalloids in routine compliance and other monitoring. In addition, incorporation of DGT units in standardised toxicity tests with a variety of taxa could provide insights into developing trigger values for bioavailable metal and metalloid fractions.