Combining colour parameters and geochemical tracers to improve sediment source discrimination in a mining catchment (New Caledonia, South Pacific Islands)

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.

Comparing the discrimination power of contrasted sediment tracing techniques to quantify the impact of nickel mining on river and lagoon siltation in New Caledonia

<p>Open-cast mining has strongly increased soil erosion and the subsequent downstream transfer of sediments in river systems. New Caledonia, a French island located in the south-west Pacific Ocean and currently the world's 6th largest nickel producer, is confronted in particular to unprecedented sediment pollution of river systems: hyper-sedimentation. A significant fraction of this sediment is likely originating from tributaries draining nickel mining sites. Nevertheless, the contribution of this sediment source has not been quantified and this estimation is required to guide the implementation of efficient management measurements to mitigate fine sediment supply to New Caledonian river systems and lagoons. To this end, a pilot sediment tracing study has been conducted in one of the first areas exploited for nickel mining, the 397-km² Thio River catchment.</p><p>Sediment deposits were collected after two major floods (~10 yr return period): the tropical depression of February 25, 2015 and Cyclone Cook on April 10, 2017. Sources (n=25) were sampled on (i) tributaries draining mines, and (ii) tributaries draining ‘natural’ areas affected by landslides occurring frequently in the region, and sediment (n=19) on (iii) the main stem of the Thio River. In addition, (iv) a 1.60 m long sediment core was collected in the Thio river deltaic floodplain in April 2016. Six sediment tracing techniques were tested based on the following properties: fallout radionuclides, geogenic radionuclides, elemental geochemistry, colorimetric parameters and reflectance spectra.</p><p>Several of these methods were identified as relevant to the New Caledonian context and allowed to estimate the contributions of both mining and non-mining sources according to their variations both in space and time. In particular, the sedimentary contributions of mining sources were estimated between 65-68 % for the 2015 flood and 83-88 % for the 2017 flood. The impact of the spatial variability of precipitation was highlighted to explain the variations in the spatial contributions of the sources. The temporal variations in the contributions of the sources deduced from the analysis of the sediment core were interpreted at the light of the mining history in the Thio River catchment (pre-mechanization, mechanization, post-mechanization of mining activity). The contributions of mining sources were again dominant with an average contribution along the sedimentary profile of 74 ± 13 %.</p><p>In the future, similar studies should be carried out in other catchments draining mines in New Caledonia and potentially across similar South Pacific and other tropical islands.</p>

Determining tributary sources of increased sedimentation in East-African Rift Lakes using a combination of sediment tracing and radioactive dating.

<p>Temporal and spatial sediment dynamics in an East-African Rift Lake (Lake Manyara, Tanzania), and its river inputs, have been evaluated via a combination of sediment tracing and radioactive dating. Changes in sedimentation rate were assessed using radioactive dating of sediment cores in combination with geochemical profile analysis of allogenic and autogenic elements. Geochemical fingerprinting of riverine and lake sediment was integrated within a Bayesian mixing model framework, including spatial factors, to establish which tributary sources were the main contributors to recent lake sedimentation. The novel application of Bayesian source attribution on sediment cores and subsequent integration with sedimentation data permitted the coupling of changes in the rate of lake sedimentation with variations in sediment delivery from the tributaries. These complimentary evidence bases demonstrated that Lake Manyara has experienced an overall upward trajectory in sedimentation rates over the last 120 years with distinct maxima in the 1960s and in 2010. Sedimentation rates are largely a result of a complex interaction between increased upstream sediment delivery following changes in land cover and natural rainfall fluctuations. Modelling results identified two specific tributaries as responsible for elevated sedimentation rates, contributing 58% and 38% of the recently deposited lake sediment. However, the effects of sedimentation were shown to be spatially distinct given the domination of different tributaries in various areas of Lake Manyara. The application of source-tracing techniques constrained sedimentation problems in Lake Manyara to specific tributary sources and established a link between upstream land degradation and downstream ecosystem health. This novel application provides a solid foundation for targeted land and water management strategies to safeguard water security and environmental health in Lake Manyara and has potential application to fill knowledge gaps on sediment dynamics in other East-African Rift Lakes.</p>