Antimony Flux and Transport Dynamics in a Mining-Impacted River Is Linked to Catchment Hydrodynamics and Climate Oscillations

We investigate how seasonal flow variations and a climatic regime that is dominated by the El Niño–Southern Oscillation (ENSO) influence Sb flux dynamics in an Australian river impacted by mining. Sampling (n = 496) spans a hydrologically complex 7-year period of drought, bushfires and floods from 2016 to 2023, during which 17% of samples exceeded the Sb drinking water guideline concentration (3 μg L⁻¹). Aqueous Sb (Sb_Aq) concentration–discharge (C–Q) relationships are non-continuous/non-linear across the flow range, with chemodynamic behaviour at moderate flows reflecting hydrological connection to the primary Sb-source area combined with variable dilution. In contrast chemostatic behaviour occurred at extreme low and high flows, reflecting hydrological disconnection from the source area and persistent dilution, respectively. Sb_Aq was significantly positively correlated (p < 0.01, Spearman's ρ = 0.58) with a Q index representing the proportional contribution of sub-catchment flow from the mineral-field area, suggesting sufficient localised rainfall in the Sb mining-impacted sub-catchment contributes to downstream peaks in Sb_Aq concentrations. Aqueous and particulate Sb (Sb_P) annual loads (L_a) during the study period spanned 24–5174 and 1.2–2820 kg, respectively and were strongly flow dependant with extreme interannual variability reflecting dry and wet years. We extrapolate daily load-daily discharge (L_d–Q_d) relationships for Sb_Aq and Sb_P to estimate L_d over a 53-year period (1970–2023) of continuous Q data (mean total Sb L_a = 1865 kg ± [SE] 247). Positive correlations between the annual Southern Oscillation Index and both Sb L_a (p < 0.05) and proportional Sb_P L_a over 53 years suggests ENSO fluctuations influence annual Sb transport dynamics. Upstream Sb_P load estimates correspond with downstream estimates of coastal floodplain sedimentary Sb mass, with approximately 10%–45% of the estimated Sb_P exported downstream since approximately 1880 accumulated on the Macleay coastal floodplain. Data suggest at current rates of export, complete flushing-leaching of mine tailings-derived Sb from the upper Macleay catchment may take in the order approximately 600–1000 years.