Research papersApplying geochemical and colour properties to quantify sediment sources in a Brazilian semiarid ephemeral river system
Graphical abstract
Introduction
Climate change projections underline the urgency of creating integrated strategies for protecting water resources and guaranteeing the continued development of dry regions (IPCC, 2014). In Brazil, the semiarid region, the most densely populated dry region in the world (Marengo, 2008), covers 982,563 km2 of the country and is mainly concentrated in the Northeast region. The high spatial and temporal variability of rainfall and the high air temperatures and evapotranspiration rates have led to an historical problem of water availability in this region (Correia et al., 2011). These background problems draw attention to the necessity of tackling soil erosion and sediment transfer within semiarid catchments since the excessive accumulation of fine-grained sediment threatens water quality and decreases reservoir storage capacity (Ayrault et al., 2014, Simplício et al., 2020). In recent decades, the intense deforestation of native vegetation in Brazilian drylands, triggered by agricultural activities, overgrazing, and coal production, has led to an increase in soil erosion (Medeiros and de Araújo, 2014, Tomasella et al., 2018, Neto et al., 2020). Therefore, understanding sediment source apportionment at the catchment scale is crucial for implementing targeted strategies to control the transfer of sediments and associated pollutants to surface waters. The sediment fingerprinting approach has been the most widely used direct means of investigating sediment provenance (Collins et al., 2020). Although this approach has been successfully used in other Brazilian rivers in the subtropics (Minella et al., 2017, Tiecher et al., 2014, Ramon et al., 2020), its application for sediment source apportionment in a Brazilian semiarid ephemeral river system has not been reported yet, representing a significant research gap (Silva et al., 2018a, Collins et al., 2020).
Various properties have been used to achieve sediment source apportionment, such as geochemical elements (Collins et al., 1997, Batista et al., 2019), radionuclides (Walling and Woodward, 1992, Ben Slimane et al., 2016), C and N stable isotopes (Garzon-Garcia et al., 2017, Collins et al., 2019) and spectroscopic properties (Tiecher et al., 2016, Amorim et al., 2021). However, the time and resources required to analyse some of these tracers frequently determine the tracer selection and the ongoing lack of standardization among source fingerprinting procedures continues to hinder the wider application of the fingerprinting approach around the world (Collins et al., 2020). For instance, the use of colour tracers is relatively simple, less time-consuming and associated with a lower cost than other properties. Accordingly, colour has been used as an alternative sediment source tracer in many catchments under humid climates (Martínez-Carreras et al., 2010, Tiecher et al., 2015, Amorim et al., 2021, Pulley and Collins, 2021). In contrast, studies reporting the applicability of colour tracers in dry environments remain scarce for fluvial sediment fingerprinting and incipient for aeolian sediments (Pulley and Rowntree, 2016, Nosrati et al., 2020, Nosrati et al., 2021a).
The Ipojuca River is one of the most polluted catchments in Brazil and is largely (∼70 %) semiarid in nature. Sediment-associated metal concentrations and fluxes in this river are comparable to those observed in drainage systems heavily impacted by mining activities (Silva et al., 2015, Silva et al., 2018b). Bed sediments in the upper catchment (semiarid) exhibit, for instance, high levels of Ni and Zn (Silva et al., 2017). Battery factories, textile industries and some municipalities directly discharge non-treated wastewater into the main watercourse (Lima Barros et al., 2013). In the above context, the current research quantified source contributions to suspended sediments (SS) and bed sediments (BS) using geochemical and colour tracers in this semiarid catchment. In doing so, we explored the applicability of colour parameters as an alternative to geochemical tracers, which can be particularly problematic to use in catchments potentially impacted by substantial anthropogenic pollution.
Section snippets
Study area
The semiarid part of the Ipojuca River basin (Fig. 1) comprises an area of approximately 2500 km2 (CONDEPE/FIDEM, 2005). The average annual rainfall ranges from 600 to 900 mm with high spatial and temporal variability, leading to an ephemeral and perennial fluvial regime in the upper and middle portions of the catchment, respectively (CONDEPE/FIDEM, 2005). Soil parent materials are mainly metalluminous granitoids, orthogneisses and biotite-muscovite gneisses (Silva et al., 2015) (Fig. 1). Soils
Selection of colour and geochemical tracers
The results of the range test for tracer conservation are presented in Fig. 2, Fig. 3. Approximately 82 % (regional sources) and 64 % (land uses) of the geochemical elements passed this test. Non-conservative geochemical tracers were removed from the final discrimination and apportionment modelling for both regional sources (Ce, La, Th, and Zr) and land use sources (Ce, La, Nd, Pr, Sm, Y, Th, and Zr). Among the colour tracers, only HRGB, h*, Hcol and λd (nm) for the regional sources and R, HRGB
Discrimination of regional sources and land uses
In general, the geochemical tracers propagated lower errors for sediment source discrimination and apportionment. This was most notable for the regional sediment source scheme and possibly reflects the exclusive geochemical properties for each part of the study catchment. The average Al, Ba, and Ti concentrations showed the main contrasts between these sources and followed typically the sequence: upper catchment > lower catchment > middle catchment. In the middle portion of the study catchment,
Conclusions
The recovery and conservation of the vegetation in the Caatinga biome and the stabilization of channel banks, especially in the lower catchment, are fundamental for controlling the sediment problem in the investigated semiarid study catchment. Poor conservative behaviour is one of the challenges for the application of colour tracers to quantifying sediment sources in this ephemeral river. Nevertheless, such tracers were fundamental for obtaining the most reliable sediment source apportionment
CRediT authorship contribution statement
Rennan Cabral Nascimento: Conceptualization, Data curation, Formal analysis, Investigation, Methodology, Project administration, Writing - original draft. Angelo Jamil Maia: Investigation, Writing - review & editing. Ygor Jacques Agra Bezerra da Silva: Investigation, Writing - review & editing. Fábio Farias Amorim: Data curation. Clístenes Williams Araújo do Nascimento: Resources, Writing - review & editing. Tales Tiecher: Writing – review & editing. Olivier Evrard: Writing – review & editing.
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgements
Y.J.A.B. Silva are grateful to the National Council for Scientific and Technological Development – CNPq for research productivity scholarship (Process Number: 303221/2019-4). The contribution to this manuscript by ALC was funded by UKRI-BBSRC (UK Research and Innovation-Biotechnology and Biological Sciences Research Council) grant award BBS/E/C/000I0330. This study was financed in part by the Coordenação de Aperfeiçoamento de Pessoal de Nível Superior - Brasil (CAPES) - Finance Code 001.
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