Sources and mechanisms of metal bioaccumulation in the Lagaip/Strickland Rivers, Papua New Guinea

The Porgera Joint Venture (PJV) gold mine, located in Enga Province, Papua New Guinea (PNG), discharges approximately 15,000 tonnes of treated tailings per day to an erodible dump and flows to the receiving river system. As a condition of the permit to discharge tailings, PJV undertakes routine chemical and biological monitoring of the downstream system. Indigenous prawns of the genus Macrobrachium demonstrate significant differences in bioaccumulated metal concentrations (predominantly cadmium) between populations exposed to mine-derived materials compared to those in reference tributaries. Chemical analyses of the downstream system was unable to explain why these differences in bioaccumulated metals exist.

This PhD study investigated the sources and mechanisms of metal bioaccumulation by prawns in the Strickland River. The project was divided into two main components, a chemical survey of the river system aimed at identifying potential sources of bioavailable metals in the dissolved and solid phases, and laboratory-based bioaccumulation assays using stable and radiolabelled metals. The second component was intended to provide a greater understanding of the significance of specific metal sources to the prawns.

The chemical survey of the river system identified the particulate matter as the main source of metal loading to the downstream system. Dilute-acid extractions of particulate materials, aimed at targeting the potentially bioavailable pool of carbonate and hydroxide-bound metals, also indicated no significant difference in extractable metal concentrations between sites. The surveys demonstrated the difficulties in using chemistry alone to describing metal bioaccumulation patterns when metal concentrations had been diluted to near background levels.

The prawn bioaccumulation assays were conducted using a surrogate species of Macrobrachium (M. australiense) due to issues involved with obtaining and holding the PNG species in Australia. Cadmium bioaccumulation by M. australiense was mostly attributed to uptake from solution when exposed to mine tailings, whereas lead and arsenic appeared to be bioaccumulated mostly from fine sediment ingestion. Results from radioisotope studies were used to develop a steady-state biokinetic model of cadmium bioaccumulation by M. australiense, which was in turn used to determine the relevant contributions of dietary and dissolved phases to the total body cadmium loading. The model predicted that the contribution of dietary cadmium would account for approximately 80% of total body cadmium when exposed to typical cadmium concentrations found in the Strickland River, with the remaining 20% coming from dissolved cadmium.

A final study investigated the applicability of the biokinetic model developed for M. australiense to Macrobrachium species found in the studied system. The experiments included a dissolved cadmium exposure and mine tailings exposure using M. australiense, M. rosenbergii and M. latidactylus (the latter two species were tested in laboratories at the PJV mine site). The species comparison study suggested that the biokinetic model would most likely over-predict cadmium concentrations in the two PNG Macrobrachium species (but only by a factor of 2 to 4). The study also demonstrated the issues with using surrogate species of the same genus to describe metal bioaccumulation, even in controlled environments.

Recommendations for routine monitoring of the river system and future studies are presented.