Copper toxicity to fishes and an extremely sensitive shrimp in relation to a potential Australian tropical mining‐waste seep

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Abstract
Ephemeral or seasonal streams tributaries to Magela Creek (Alligator Rivers Uranium province, Northern Territory), containing perennial pools, lie adjacent to a 1 km2 tailings storage dam, still being filled, whose partial seepage collecting system contains an increasing concentration of copper. Acute copper toxicity to local fauna was determined in a flow‐through apparatus at 27°C and pH 6. The 4‐day LC50 values (μg L‐1) for total copper were: rainbowfish Melanotaenia splendida inornata 170–190, pennyfish Denariusa bandata 75, and atyid shrimp Caridina sp. 3–4, and the 3‐day LC50 of the catfish Porochilus rendahli 85. Free ionic copper approximated the total copper. Test water quality was as predicted for seepage‐affected streams during a brief critical period in some years: of low hardness and alkalinity but enriched in Mg2+ and SO4 2‐. The relative LC50 values correspond to the relative occurrence of similar species among zones of the metals‐affected Finniss River. Magela Creek system waters often had naturally low pH and ion‐concentrations (pH < 6, conductivity < 20 μS cm‐1); the total copper concentration has repeatedly been similar to the free ionic copper LC50 of Caridina sp. The abundance and distribution of Caridina sp. are likely already influenced by local natural copper concentrations. Seepage total copper has increased to the fishes’ free ionic copper LC50.The hydraulic head of the wet tailings will increase further from 15 to 20 m.