Spectroscopy-based tracing of sediment sources in a large heterogeneous catchment with different geologies of the Pampa Biome (Ibirapuitã River, Southern Brazil)

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Study catchments

This study will be carried out in two tributary catchments of the Uruguay River, representative of two contrasting geomorphological regions of the southern region of Brazil. The Uruguay River is one of the largest rivers in the southern region of South America, draining an area of approximately 242,000 km² (considering the Salto Grande Hydroelectric Plant as the outlet of the catchment), with 70% of its area located in southern Brazil, 20% in Argentina and 10% in Uruguay (Figure 1.1). On the one hand, land use in the catchment is predominantly agricultural, with intensive agricultural production in the states of Rio Grande do Sul (RS-Brazil) and Santa Catarina (SC-Brazil), where small farms (<50 ha) predominate in the more undulating relief regions (with dairy crops, intensive poultry and pig farming, natural and cultivated forests), while larger properties (mostly <100 ha) are located in flatter areas (with dominant soybean, corn and wheat productions). Similar conditions are also observed in the northernmost portion of the Argentinian side, in the Atlantic biome, where land use is predominantly occupied by plantations of mate, tea, and also eucalyptus plantations. On the other hand, in the southwestern regions of RS and northern Uruguay, large properties are predominant (>100 ha). In general, in these regions, most of the areas are occupied with pasture for extensive ranching and irrigated rice production in the plains, with an increased soybean production in the region during the last few years. The southernmost part of the Argentinian side comprises the flooded grasslands and savannahs, an extremely flat region, with hydromorphic soils where pastures and planted forests for the extraction of wood and cellulose are predominant.
Along the course of the Uruguay River and its tributaries, there are approximately 44 dams, which are predominantly used for electric power generation, while some reservoirs are used for water abstraction for basic sanitation. These dams are located in the upper portion of the Uruguay River basin, where the relief is more mountainous and allows for a more effective use of the potential river energy, with the exception of the Salto Grande Hydroelectric Power Plant dam, which is the most downstream dam of the basin, located in the flattest portion of the basin. Therefore, the two study sites were chosen because they represent two very contrasting regions and the main geomorphological features of the Uruguay River basin, which have a high environmental, economic and social significance.

Conceição River catchment

The Conceição River catchment is located on the southern plateau in the state of Rio Grande do Sul, with an area of approximately 804.3 km² (Figure 5.1). The catchment outlet is located next to the monitoring point number 75200000 of the National Water Agency (ANA) (28°27’22 » S, 53°58’24 » O) in the municipality of Ijuí. According to the Köppen classification, the climate is Cfa type, humid subtropical without a defined dry season, with an average precipitation of 1900 to 2200 mm per year and an average temperature of approximately 18oC (Alvares et al., 2013). This catchment is representative of the basalt plateau region of the Serra Geral Formation, where the main soil classes found are the Ferralsols (80%), Nitosols (18%) and Acrisols (2%), rich in iron oxides and kaolinite, with the Ferralsols being the most widespread soil type in the catchment (Figure 5.1a). Small areas from the Tupanciretã Formation, which are remnants of the Botucatu Formation amidst the volcanic spills of the Serra Geral Formation, are also found. The relief of the region is characterized by gentle slopes (6-9%) at the top and moderate or steep slopes (10-14%) near the drainage channels, with altitude ranging from 270 to 480 m a.s.l. In this catchment, the main land use is cropland for grain production and dairy farming, where inadequate soil management has resulted in high erosion rates (Didoné et al., 2015b). The main land use is cropland (82%) mainly cultivated with soybean (Glycine max) under no-tillage system in the summer and wheat (Triticum aestivum) for grain production, oat (Avena sativa and Avena strigosa) and ryegrass (Lolium multiflorum) for dairy cattle feed or soil cover during the winter. However, inadequate soil management in these areas has resulted in high erosion rates in the last decades (Didoné et al., 2019, 2015a). Pastures (grassland, pasture and mosaics of agriculture and pasture, according to the Mapbiomas classification), mainly used for cattle raising, cover 12% of the total surface area, whereas forest is found on only 5% of the surface (Figure 5.1b). In the Conceição River catchment, the pasture area obtained from Mapbiomas, includes both perennial and temporary pastures, although only perennial pastures were sampled in this study site.

Ibirapuitã River catchment

The Ibirapuitã River catchment is located in the extreme south of Brazil and is representative of the southern grassland region, typical of the Pampa biome, covering approximately 7975 km². The outlet of the studied catchment is located next to monitoring point number 76750000 of ANA (28°27’22 » S, 53°58’24 » O) in the municipality of Alegrete (5943 km²), representing 75% of the total area of the Ibirapuitã catchment. According to the Köppen classification, the climate is Cfa type, humid subtropical without a defined dry season, with an average precipitation of 1,600 to 1,900
mm per year and an average temperature of 17oC (Alvares et al., 2013). The altitude of the catchment is comprised between 70 and 370 m a.s.l., and the elevations above 280 meters are located in the headwaters of the catchment, near the border between Brazil and Uruguay and represent less than 15% of the catchment area. Approximately 90% of the catchment area has slopes of less than 15%, and the slopes decrease in the northern direction, varying from 2 to 5% in the lower Ibirapuitã region. Land use is predominantly occupied by native grasslands with extensive livestock activity (81%) (Figure 5.2), although as in the whole Pampa region, it tends to be increasingly occupied by soybean production areas. The catchment is composed of three sub-catchments. The (i) Ibirapuitã Environmental Protection Area subcatchment (EPA), which is an environmental protection area controlled by the Chico Mendes Institution of Biodiversity Conservation (ICMBio) from the National Ministry of the Environment, where native grasslands (85%) and natural forests (10%) predominate. Located in the central portion of the Ibirapuitã River catchment, the EPA subcatchment has an area of 3196 km², where the main soil types are Regosols in the upper half and Acrisols in the lower half, from basalts of the Serra Geral formation (Fácies Alegrete) and sandstones/silts of the Botucatu formation (Fácies Gramado, Caxias and Guará), respectively. The (ii) Pai-Passo Stream subcatchment covers approximately 1043 km², and it is mainly occupied by native grassland (83%) areas with extensive livestock on shallow Regosols originating from basalt (Fácies Alegrete), and paddy fields for irrigated rice production (10%) located in the lower and flat positions of the landscape, where Planosols and Vertisols occur. The
(iii) Caverá Stream subcatchment covers approximately 1455 km², and it is the tributary catchment with the higher percentage of cropland (irrigated rice and soybean) and cultivated pastures (15%), where the native grassland (73%) has been converted into cropland on deeper soils, predominantly Acrisols originating from sandstones of the Botucatu formation (Figure 5.3).

Sediment source sampling

The potential sediment sources were chosen during reconnaissance campaigns, where the main components of the landscape showing potential for sediment contribution were identified. For the Conceição River catchment, sediment source samples collected by Tiecher (2015) in 2012 were used in this study, distributed among croplands (n = 77), unpaved roads (n = 38), stream banks (n = 34), gullies (n = 14) and permanent pastures (n = 24).
In the Ibirapuitã River catchment, soil samples from potential sources were collected at representative areas that showed active erosion and that were connected to the drainage network, following the same principle adopted by Tiecher (2015). Care was taken to avoid those sites that have accumulated sediment originating from other sources, not to collect transiting material. In the surface sources, soil from the upper 0-2 cm layer was collected, as this layer is the most likely to be eroded and transported to the waterways. In the subsurface sources, samples were collected at the edge of the drainage channels at exposed sites sensitive to erosion. For each composite source sample, around 10 sub-samples were collected within a radius of approximately 50 meters, mixed in a bucket and approximately 500 grams of material were stored. The source samples sites were selected in order to cover all the soil types and the variability in slope positions, as well as the three main tributary catchments. Samples were taken during the winter period in 2018 under the following land uses and erosion features: croplands and cultivated pastures (n = 28), paddy fields (n = 8), native grasslands (n = 31), unpaved roads (n = 31), stream banks (n = 18) and erosion channels (n = 16).

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Sediment sampling

For sediment characterization and quantification of suspended sediment fluxes, sediment samples were collected using different strategies, in order to have enough material for subsequent analyses. Three methods were used:
i. Time integrated suspended sediment sampler (TISS) – The sampler designed by Phillips et al. (2000) consists in a plastic tube of 75 mm of diameter and 80 cm length, which has a small inlet and outlet tubes (4 mm of diameter) in the extreme edges, which allows the suspended sediment to enter, reducing the flow velocity and allow the sediment to deposit inside the tube based on the principle of sedimentation. The equipment is submerged for a certain period of time integrating the sediments from different rainfall events, in which the eroded material of the catchment is mobilized under different conditions of transport and energy, and consequently consists of contrasted physical, chemical and mineralogical characteristics. The samples were collected during intervals of 2 to 3 months, varying according to the records of rainfall events.
ii. Storm event samples (Event) – Samples collected during storm events were collected at the outlet, where a large volume of water (50 to 200 litters) was collected at different stages of the rainfall-runoff event to evaporate the water and have enough material to perform the analyses.
iii. Fine bed sediment samples (FBS) – in the Conceição River, samples were collected with a suction device in the bottom of the river. Multiple samples collected in different positions of the river bed close to the outlet composed each individual sample. According to Horowitz et al. (2012), fine sediment (<63 µm) deposited in the first centimetres of the river bed can be used as a surrogate to quantify the concentration of chemicals in suspended sediment.
iv. Flood deposits (FD) – sediment deposited in the flooding area after a storm event were collected along the Ibirapuitã River. Care was taken to sample only material that was deposited by the previous major rainfall event.
For the Conceição River catchment, sediment samples already collected by Tiecher (2015) were used, where more information related to the samples can be found. Sediment samples collected by TISS, Event and FBS methods were taken at the outlet of the catchment in the period of March of 2011 to January of 2013. Samples were also collected during the period of 2017 to 2019, however the samples could not be used in this thesis due to the different analytical methods used.
In the Ibirapuitã River catchment, suspended sediment samples were collected during rainfall events that resulted in increased water discharge and sediment yield at the main outlet. Samples were collected in duplicates, where one was used for sediment concentration analysis and the other, with a greater volume, was collected to accumulate enough sediment for subsequent physical and chemical analyses. A second method used was the TISS that were installed in the three main sub-catchments of the Ibirapuitã River (Pai-Passo Stream, Caverá Stream and the Ibirapuitã-EPA catchment) and at the main outlet (Figure 5.4).
Figure 5.5. Suspended sediment sampling to determine the concentration during a monitoring campaign made in collaboration with the National Water Agency service provider, the Brazilian Geological Service (CPRM) (a); Event sampling in the Ibirapuitã River during a storm event, sample taken with a bucket to obtain larger volumes (b) and for suspended sediment concentration collected with a US-D49 sampler (d); Suspended sediment sampling in the Conceição River before a storm event using a US-D49 on a bridge (c). Source: the author (a, c), Paulo C. Ramon (b), Antônio A. Marquez (d).
In addition to the samples collected in the two tributaries (Conceição and Ibirapuitã), samples were collected from a sediment profile located on an island of the Uruguay River, situated downstream of the municipality of Uruguaiana, on the border with Argentina. Samples were collected in 5 cm layers down to a depth of 50 cm, and at 10 cm intervals down to a depth of 1 meter. These samples were used to make a first characterization of the sediment samples from the Uruguay River and compare them to the samples from the tributaries.

Soils and sediment analysis

Sample preparation

Source and sediment samples were oven-dried at 45°C and gently disaggregated using pestle and mortar and sieved with a 2.0 mm mesh to remove gravels and coarse material. Soil source and sediment samples were divided into two parts, one preserved as <2.0 mm and the other sieved to 63m prior to the laboratory analysis in order to compare similar grain-size fractions for conducting the sediment fingerprinting approach (Koiter et al., 2013b; Laceby et al., 2017).

Table of contents :

2.1 The context of soil erosion and its on-site and off-site impacts
2.2 Sediment discharge monitoring to understand the erosion processes in the catchment scale
2.3 Challenges associated with the sediment fingerprinting technique under different environmental conditions
2.4 References
4.1 Aim
4.2 Objectives
5.1 Study catchments
5.2 Conceição River catchment
5.3 Ibirapuitã River catchment
5.4 Sediment source sampling
5.5 Sediment sampling
5.6 Soils and sediment analysis
5.6.1 Sample preparation
5.6.2 Biogeochemical analysis
5.6.3 Magnetic susceptibility analysis
5.6.4 Radionuclide analyses
5.6.5 Ultra-violet-visible, near and mid infrared diffuse reflectance analysis Ultra-violet-visible diffuse reflectance analysis Fourier Transform Infrared Spectroscopy analysis
5.7 Sediment source discrimination and apportionment
5.8 References
6 Chapter 1. Sediment fingerprinting using geochemical tracers: a global meta-analysis
6.1 Introduction
6.2 Methodology
6.2.1 Review strategy
6.2.2 Data base and analysis
6.3 Results and discussion
6.3.1 Literature overview
6.3.2 Methodological aspects
6.3.3 Geochemical elements as sediment source tracers
6.4 Conclusions
6.5 References
7 Chapter 2. Combining spectroscopy and magnetism with geochemical tracers to improve the discrimination of sediment sources in a homogeneous subtropical catchment
7.1 Introduction
7.2 Methodology
7.2.1 Study site
7.2.2 Source and sediment sampling
7.2.3 Source and sediment analyses Geochemical properties Magnetic properties Ultraviolet-visible analysis and parameters calculation
7.2.4 Sediment source discrimination and apportionment
7.3 Results
7.3.1 Selection of sediment tracers
7.3.2 Model results for each approach
7.4 Discussion
7.4.1 Tracer selection and discrimination between sources
7.4.2 Mixing model results
7.5 Conclusions
7.6 References
8 Chapter 3. Sediment sources tracing using different spectral ranges, multivariate models and spectral pre-processing techniques in a homogeneous subtropical catchment in Sou
8.1 Introduction
8.2 Methodology
8.2.1 Study site
8.2.2 Source and sediment sampling
8.2.3 Artificial mixtures and sediment analyses Artificial mixtures of sediment sources Spectral analyses
8.2.4 Spectral pre-processing techniques and multivariate model calibration and validation
8.2.5 Building spectroscopy models for different sediment sources
8.3 Results and Discussion
8.3.1 Multivariate model calibration Effect of multivariate models Effect of pre-processing techniques Effect of spectral ranges Effect of number of sediment source considered
8.3.2 Sediment source contributions
8.4 Conclusions
8.5 References
9 Chapter 4. The conversion of native grassland into cropland in the Pampa biome (Southern Brazil) is increasing suspended sediment supply to river systems
9.1 Introduction
9.2 Methodology
9.2.1 Study site
9.2.2 Sediment source sampling
9.2.3 Sediment sampling
9.2.4 Source and sediment analysis Total organic matter composition Ultra-violet-visible derived parameters Fallout radionuclides analysis Sediment source discrimination and apportionment
9.3 Results
9.3.1 Monitoring results and suspended sediment characteristics
9.3.2 Source and sediment properties Organic matter composition Fallout radionuclide activities UV derived parameters
9.3.3 Selection of sediment tracers
9.3.4 Sediment source apportionment
9.4 Discussion
9.4.1 Source and sediment samples composition
9.4.2 Sediment source contribution
9.5 Conclusions
9.6 References
10 Chapter 5. Spectroscopy-based tracing of sediment sources in a large heterogeneous catchment with different geologies of the Pampa Biome (Ibirapuitã River, Southern Brazil)
10.1 Introduction
10.2 Methodology
10.2.1 Study site
10.2.2 Source and sediment sampling
10.2.3 Artificial mixtures and sediment analyses Artificial mixtures of sediment sources Spectral analyses
10.2.4 Spectral pre-processing techniques and multivariate model calibration and validation
10.2.5 Building spectroscopy models for different sediment sources
10.3 Results and Discussion
10.3.1 Multivariate model calibration Effect of multivariate models Effect of pre-processing techniques Effect of spectral ranges Effect of number of sediment source considered
10.3.2 Sediment source contributions
10.4 Conclusions
10.5 References
11 Chapter 6. Sediment fingerprinting in the Uruguay River basin – General Discussion
11.1 Introduction
11.2 Conceição River catchment
11.3 Ibirapuitã River catchment
11.4 Uruguay River basin
11.4.1 Methodology Source and sediment analyses Magnetic susceptibility Sediment source discrimination and apportionment
11.4.2 Results and Discussion
11.5 Final conclusions and perspectives for future studies
11.6 References


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