Abstract. Over the last century, human activities have induced significant land-cover changes that have accelerated soil erosion processes around the world. In New Caledonia, a French island located in the south-west Pacific Ocean, open-cast nickel mining has raised many concerns regarding its impact on riverine systems (i.e. hyper-sedimentation, overburden) and the island's ecosystems (i.e. flooding, lagoon siltation, water pollution). A sediment tracing study has been conducted to quantify the contribution of mining versus non-mining sub-catchments in one of the first areas exploited for nickel mining, the Thio River catchment (397 km2). Sediment deposited during two cyclonic events (i.e. 2015 and 2017) was collected following a tributary design approach. Source (n = 24) and river sediment (n = 19) samples were analyzed by X-ray fluorescence and spectroscopy in the visible spectra (i.e. 365–735 nm). Four fingerprinting approaches based on (1) colour parameters, (2) geochemical properties, (3) colour parameters coupled with geochemical properties and (4) the entire visible spectrum were tested to estimate sediment source contributions. The results demonstrated that the individual sediment tracing methods based on spectroscopy measurements (i.e. (1) and (4)) did not provide sufficient discrimination between sources. However, the inclusion of colour properties in addition to geochemical parameters (3) provided the highest discrimination between sources (i.e. 92.6 % of source variance explained). Although with a slightly lower discrimination potential (i.e. 83.1 % of variance explained in sources), the geochemical approach (2) provided similar results to those obtained with the colour coupled with geochemical approach (3). In addition, mixed linear models associated with these two approaches have been experimentally validated with artificial mixture samples. The results obtained with model (3) showed that mining source contributions strongly dominated the sediments inputs with a mean contribution of 68 % (SD 25 %) for the 2015 flood event and 88 % (SD 8 %) for the 2017 flood event. These results suggest that catchment management should focus on the contributions of mining tributaries to reduce sediment inputs in the river systems. Therefore, the use of these approaches based on geochemical properties individually (2) and coupled to colour parameters (3) could be extended to other mining catchments of New Caledonia but also to other similar nickel mining catchments around the world (e.g. Australia, Brazil, Dominican Republic, Cuba) to estimate sediment source apportionment.
Combining colour parameters and geochemical tracers to improve sediment source discrimination in a mining catchment (New Caledonia, South Pacific Islands)
