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

Abstract

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 (137 Cs and 210 Pbxs) 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 and perspectives. 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.

Reconstructing the impact of nickel mining activities on sediment supply to the rivers and the lagoon of South Pacific Islands: Lessons learnt from the Thio early mining site (New Caledonia)

“…Particle size sorting may be at the origin of the nonconservation of some tracer properties (Laceby et al, 2017). Nevertheless, the previous results obtained in Sellier et al (2020) demonstrated that 11 geochemical tracers analyzed in the current research were not significantly impacted by particle size effect. According to the range test results, all properties (except Fe) were found to be conservative.…”

“…In New Caledonia, a sediment fingerprinting approach had previously been developed to quantify sediment source contributions over two cyclonic events (Sellier et al, 2020). In this approach, the use of geochemical elements such as K to trace mining versus non-mining tributary contributions is supported by the characteristics of the lithological material where both source types are found.…”

“…A recent pilot sediment tracing study was conducted in the Thio River (397 km²) catchment draining the archipelago's first mine (Sellier et al, 2020) to estimate the contributions of sediment sources during two recent cyclonic events (i.e. Cyclone Marcia in 2015 and Cyclone Cook in 2017).…”

A global review of sediment source fingerprinting research incorporating fallout radiocesium (137Cs)

“…One exception was a mining catchment of New Caledonia, South Pacific Islands, where the very low 137 Cs activities measured in mining and non-mining tributaries indicated the dominance of subsurface sources in the sediment contribution in this region. Given much higher activities were measured at similar latitudes in Australia, these low activities likely reflect the intensity of soil erosion processes and the dominance of subsoil erosion on this island (Sellier et al, 2019).…”

Sediment source fingerprinting: benchmarking recent outputs, remaining challenges and emerging themes

“…Indeed, there has been a recent emphasis to justify the parameters selected by the three-step statistical procedure (e.g. Mukundan et al 2010 ; Vale et al 2016 ; Sellier et al 2020 ) and investigators are encouraged to adopt this level of scrutiny, rather than relying on statistical solutions alone (Collins et al 2017 ).…”