Investigating the use of fallout and geogenic radionuclides as potential tracing properties to quantify the sources of suspended sediment in a mining catchment in New Caledonia, South Pacific

Purpose New Caledonia, a French island located in the south-west Pacific Ocean, has vast nickel resources. Open-cast mining has strongly increased soil erosion and the subsequent downstream transfer of sediments in river systems resulting in fundamental morphological changes (e.g., hyper-sedimentation, over-burden). Understanding the sediment source contributions from mining activities is therefore important to guide the implementation of effective management measures. Materials and methods A pilot sediment tracing study was conducted in the Thio River (400 km(2)) catchment draining the island's first mine. Sediment deposited during the February 25, 2015, and April 10, 2017, flood events was collected with a tributary tracing design, including main stem (n = 19) and tributary (n = 24) samples. The tributaries were classified into two source types: sub-catchments draining mining sites and sub-catchments devoid of mining activities. Three sets of potential tracing parameters (i.e., fallout radionuclides, geogenic radionuclides, and elemental geochemistry) were examined for their potential to model the contributions of sediment from mining versus non-mining tributaries to sediment collected on the Thio River. Results and discussion The very low fallout radionuclide activities (Cs-137 and Pb-210(xs)) found in the source and sediment samples demonstrate that most material transiting the river network is derived from subsoil sources. Geogenic radionuclides and elemental geochemistry were therefore utilized in the tributary tracing approach. Accordingly, U and K were selected as the optimal tracers of the two main lithological regions supplying sediment to the main stem of the Thio River. Model results demonstrated that tributaries with mining activity dominated the sediment supply to the Thio River with mean sediment contributions of 68% (SD 28%) for the 2015 flood and 86% (SD 7%) for the 2017 event. Conclusions Geogenic radionuclides and elemental geochemistry were more effective at discriminating between tributaries with and without mines than fallout radionuclides. The similarity of the model results from tracing with geogenic radionuclides and elemental geochemistry illustrates their potential to investigate sediment source contributions in similar catchments across the South Pacific Islands affected by mining activities.