The Burdekin catchment, with an area of 133,600 km², is in tropical North Queensland and is the largest source of suspended sediment released to the Great Barrier Reef. During the wet season, flooding dislodges and transports material containing metals and/or metalloids downstream to the estuary. In addition–almost annually–during high-volume flooding events, overspill occurs at the Burdekin Falls Dam (BFD), resulting in finer suspended sediment fractions surpassing the dam wall and travelling downstream. Mercury (Hg) is a toxic heavy metal that can bioaccumulate and bio-magnify across food chains, posing a risk even at low concentrations.

Our study examined Hg transport in the Burdekin River system and the implications upon Hg bioavailability to aquatic species by quantifying Hg concentrations in the different fractions of the water column. Hg concentrations in the finer fractions, more specifically the colloidal (filterable <0.45 µm) and labile (dissolved and truly bioavailable fraction, <0.01 µm) fractions, are more bioavailable to aquatic species. A total of 60 samples were taken throughout the upper and lower Burdekin sub-catchment, utilising acid wash bottles for the total (unfiltered) and filtered (<0.45 µm) size fractions, and the Hg-DGT (Diffusive Gradient Thin Films) for the labile fraction. Samples were collected during the dry season over a period of four sampling campaigns: 2016, 2018, 2019 and 2020. We measured a mean of 0.015 μg/L ± 0.001 of total Hg (THg, or unfiltered fraction), a mean of 0.015 μg/L ± 0.002 in the colloidal fraction, and 0.001 μg/L ± 0.0004 of the labile fraction. This study suggests that 91% of the observed Hg concentrations reside in the colloidal fraction, and 9% are in the labile (or the bioavailable fraction). This also suggests that Hg concentrations are included in the primary size fraction that surpasses the dam wall during flooding events. Mercury concentrations are below ANZG (2018) guidelines for 99% protection of freshwater and marine aquatic species (0.06 and 0.1 respectively), suggesting low aquatic toxicity risk.

Geochemical modelling of water quality and Hg concentration data reveals that fulvic acids containing either carboxylic (FA1) or phenolic groups (FA) (>99% FA2-Hg and <1% FA1-Hg) are likely to occur as Hg speciation forms at all sites above the estuary and minor (<5%) amounts of Hg Chloride (Cl) with FA2-Hg at one site, where chloride concentrations had increased. In contrast, in the estuary and seawater Hg speciation is mainly HgCl₄^2- with dissolved inorganic species of Hg (II) ^–and neutral HgCl₂, which, due to its high lipid solubility, penetrates cell membranes 107 times faster than the free metal ion, Hg₂⁺. This suggests that Hg is more bioavailable in seawater than freshwater Hg speciation forms.

Pb Isotope ratios were determined, and the obtained data was used to identify the source of Hg contaminants. It was found that water and sediment samples were younger than Pb isotope ratios associated with Charters Towers and Ravenswood regions, suggesting Hg concentrations measured during the dry season were unrelated to legacy gold mining.

Mercury’s preliminary ecological risk assessment in this study demonstrates a low risk of exposure to aquatic species, and no further action is required. From our study, we can expand upon the mechanisms involved in Hg transport in river systems and identify its implications on aquatic toxicity. This study was part of a major collaborative research initiative between UQ and Queensland Health Forensic and Scientific Services to evaluate rare earth and heavy metals transfer along the Burdekin River.

Burdekin Estuary 9 May 2024

 

Authors:Fiona H M Henderson^a, Barry N Noller^a, Tatiana Komarova^b

a. Sustainable Minerals Institute, The University of Queensland, QLD 4072;

b. Inorganic Chemistry, Forensic and Scientific Services (FSS), Queensland Health, Coopers Plains QLD 4018