PROGRESS IN DEVELOPING A METAL ISOTOPE TOOLKIT FOR IDENTIFYING METAL SOURCES IN THE ENVIRONMENT
Stormwater Conference 2024
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J. Gadd (NIWA), C. Stirling, M. Druce & M. Devakumar (Department of Geology, University of Otago), M. Ellwood (Australian National University)
ABSTRACT
Metals are common contaminants in the aquatic environment, particularly within urban areas. As they do not degrade, metals can accumulate over time and reach concentrations that are toxic to freshwater and marine biota. The metals copper, cadmium, lead and zinc have multiple industrial, agricultural and household uses – leading to multiple sources to the environment. It can be very difficult to distinguish the key source, as required to target reductions.
Recent advancements in the analytical technology required for metal isotope analysis mean that it is now possible to distinguish between different stable and radiogenic isotopes of metals even at low concentrations (as found in environmental samples). This research project aims to identify metal isotope fingerprints of known pollution sources and use this information to identify the key sources of metals in environmental samples of waters, soils and sediments.
Source materials have been investigated from several categories including:
The lead isotope data indicates that many of the sources are indistinguishable from each other, and likely share a common source of the original lead ore (Broken Hill Australia). However, the lead wheel weight appears to have a different isotope signature, indicating a different lead ore source. This means that it could be challenging to distinguish between most lead sources in environmental samples, with the exception of lead wheel weights.
Copper isotope data showed variation between sources, but also within different sources. For example, three antifouling paints demonstrated different isotopic signatures, which overlapped with the signatures of vehicle brake pads and sediments collected from stormwater catchpits.
The zinc isotope signatures were consistent across the roofing materials tested (ZnAl, colour steel, galvanised steel), and different to transport-related products (wheel weights, brake pads).
Cadmium isotope signatures varied considerably across different samples of phosphate fertilisers (a recognized source of cadmium in the agricultural sector), but a very different signature for items associated with road transport.
There are additional source materials (tyre wear particles, carpark runoff) that will be analysed in a second round of isotope analyses. We expect that the observed differences in isotope signatures between different sources will be useful to distinguish between many, but not all, of the metal sources in environmental samples.
The next steps of the project are to analyse metal isotopes in environmental samples including:
From these analyses, we aim to identify key sources of metals, using the isotope signatures. Any available results from those next steps will be shared at the conference.