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Sediment Fingerprinting in the Nile Nyabarongo Upper Catchment, Rwanda
Abstract and Figures
Water resources management in Rwanda faces a major challenge in relation to the sedimentation of rivers and lakes due to soil erosion from both agricultural and industrial (mining) practices. Remedial measures necessitates knowing the precise sources of the sediment, which is difficult in the extensive mountainous region. This study identifies the regions in the Upper Nyabarongo catchment that are possibly the largest contributors of sediment to the highly turbid headwaters of the Nile. Elemental composition of soils in all the various geological types were obtained using mass spectrometry via laser ablation. River sediment elemental composition was similarly obtained and a mass balance approach used to infer percentages of source sediments. Maps of the catchment were created from the results with a color scheme indicating the proportions of sediment being sourced from.
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Purpose
The goal of this study was to develop a “green chemistry” alternative sample preparation method for elemental analysis of sediments using laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) to avoid the need for acid digestions. The LA-ICP-PS data are used for sediment fingerprinting (sediment source determination) in order to locate sources of soil erosion and sediment transport into the Ruvu River Basin, the domestic water supply source for East Africa’s largest city, Dar es Salaam, in the United Republic of Tanzania.
Materials and methods
Suspended sediments were collected via filtration at multiple locations along the river for comparison to soil collected from potential sources throughout the watershed. The pellet method commonly used for soil requires approximately 0.5 g (Musil et al. Spectrochim Acta, Pt A: At Spectrosc 55:1747–1758, 2000; Bustamante et al. Spectrochim Acta, Pt A: At Spectrosc 57:303–309, 2002; Mikolas et al. Anal Bioanal Chem 374:244–250, 2002; Lee et al. Spectrochim Acta, Pt A: At Spectrosc 58:523–530Q1, 2003; Hassan et al. Spectrochim Acta, Pt A: At Spectrosc 63:1225–1229, 2008; Jantzi et al. Spectrochim Acta, Pt A: At Spectrosc 115:52–63, 2016). However, on average, less than 0.1 g of sediment was non-uniformly distributed and could not be separated from the filters on which they were collected. Therefore, a novel “filter pellet” method was developed in order to homogenize and pelletize these filter-bound specimens, requiring only 50 mg of sediment on an 86-mg cellulose nitrate membrane filter. A quantitative multi-element LA-ICP-MS analysis method was optimized for the filter pellets. A mixing model was then utilized to apportion the primary sources of sediment composing the suspended sediment samples.
Results and discussion
The analytical performance of the filter pellets was similar to that achieved with the soil pellets. An element menu of 17 elements was obtained with recoveries between 75 and 125%, precision of 10% or less (relative standard deviation), repeatability of 10% or less for duplicate analyses, and limits of detection in the ppb to low ppm range. A significant source of sediments was determined to be coming from an area of particular concern within the Mgeta Catchment of the Ruvu River Basin—the meta-anorthosite complex, which comprises less than 3% of the basin yet contributes up to 10% of the suspended sediment load in the Ruvu River.
Conclusions
A novel sample preparation strategy was developed in which sediments bound to filters were processed into pellets for analysis by LA-ICP-MS (and potentially LIBS). Quantitative multi-element LA-ICP-MS data were generated, of sufficient quality for use in sediment fingerprinting applications without the need for acid digestions. This paper describes the method development and LA-ICP-MS results of the filter pellet method and a brief exploration of the sediment fingerprinting results.
Purpose
Sediment fingerprinting with elemental tracers is widely used to identify sources of sediment to rivers. However, due to the need to isolate large amounts of suspended sediment, this approach can be difficult to implement in remote locations, such as the Mara River in Kenya, where high (and increasing) sediment loads are of concern.
Materials and methods
We report several innovations that allowed us to carry out sediment fingerprinting in a portion (>6,500 km2) of the Mara River Basin. First, we utilized sediment-laden filters (sediment mass ∼0.1 g) for our river samples, rather than the traditional approach of extracting >1 g of sediment from large volumes of water. This allowed us to easily collect flow-weighted samples, and to process and analyze samples without access to centrifugation equipment. We carried out extensive quality control tests to ensure that we could reproducibly measure elemental concentrations of sediment trapped on filters. Second, we modified a readily available Bayesian inference mixing model (Stable Isotope Analysis in R) to create source signatures and to apportion downstream samples to sources. Third, we included hippo feces as a potential source, given the critical role that large wildlife plays in this ecosystem.
Results and discussion
We found that: (1) sediment captured by filtration can be digested and analyzed reproducibly and used in sediment fingerprinting; (2) our four sources (three geographic categories and hippo feces) were reasonably well-separated in their signatures; (3) the three sub-basins all contributed substantially to sediment loading in the Mara; and (4) hippo feces contributed a small, but measurable, proportion of sediment in this system.
Conclusions
Sediment-laden filters can be used successfully in identifying sediment sources through fingerprinting. The modified method of sediment fingerprinting should prove useful in other remote river basins. Our results support the hypothesis that the Upper Mara is important in supplying sediments to the river, while also highlighting the Talek sub-basin as a major contributor.
Purpose
Understanding hydro-sedimentary dynamics at the catchment scale requires high temporal resolution data on suspended sediments such as their origin, in addition to the common measurements of sediment concentrations and discharges. Some rapid and low-cost fingerprinting methods based on spectroscopy have recently been developed. We investigated how visible spectra could be used to predict the proportion of various source materials in suspended sediment samples, paying particular attention to the potential alteration of spectrocolorimetric signatures between soils and suspended sediments during transport.
Materials and methods
The 22-km2 Galabre catchment, France, is composed of black marls, limestones, molasses, undifferentiated deposits and gypsum. Forty-eight source materials were sampled and 328 suspended sediment samples were collected at the outlet during 23 runoff events. Measurements were taken with a diffuse reflectance spectrophotometer on dried samples. As the erosion processes are particle size selective, five particle size fractions of source material were measured in order to assess the potential alteration of the fingerprint signatures. As the biogeochemical processes occurring in the river could also affect the signatures, source materials were immersed in the river for durations ranging from 1 to 63 days and subsequently measured. Finally, partial least-squares regression models were constructed on 81 artificial laboratory mixtures to predict the proportions of source materials.
Results and discussion
The spectrocolorimetric measurements discriminated the primary source materials but not the Quaternary deposits. As the gypsum was not conservative, only the black marls, molasses and limestones were used in the fingerprinting procedure. The construction of the partial least-squares regression models led to a median absolute error of 1.1%. This error increased to 3.9% when the models were applied to source samples with: (1) different particle sizes; (2) different durations spent in the river; or (3) different origins than those used for their construction. The effect of particle size on the fingerprinting procedure was larger than the effect of biogeochemical reactions or the spatial variability of the spectrocolorimetric signatures. Half of the 23 runoff events analysed exhibited huge variations in the source proportions from one sediment sample to another.
Conclusions
The spectrocolorimetric fingerprinting approach was able to quantify routinely the proportion of primary source materials in all suspended sediment samples collected during runoff events. The high temporal resolution of the predicted proportions revealed that only analysing three or four suspended sediment samples during a runoff event could lead to a misunderstanding of the hydro-sedimentary processes for more than half of the investigated runoff events.
There is an increasing need for information on the sources of the suspended sediment transported by rivers, but such information is diffi cult to obtain using traditional monitoring techniques. The fingerprinting technique offers an alternative approach, although it is important to select appropriate fingerprinting properties. The use of several such properties or tracers with contrasting behavior was used to assist in discriminating be tween several potential sources and in providing more definitive and reli able results. Examples of a multi-parameter approach to fingerprintin g suspended sediment source in two small catchments in Devon, United Kingdom, are presented. Cluster analysis was used to distinguish statistical ly significant source groupings and a mixing model was employed to esti mate the relative contributions from these sources to the overall suspended sediment load.
Silt and clay carried in suspension by a mountain river in southwestern Montana were examined by X-ray diffraction. Suspended sediment samples were collected from the river and its tributaries during spring snowmelt. In general the mineralogy of the sediment was related to geology of the drainage area but also differed between streams from similar geologic materials. Quartz gave the dominant diffraction peak in silts and smectite in clays from most of the streams. One tributary, Taylor Fork, was identified as the the chief source of suspended solids. The mineralogy of its silt and clay resembled that carried by the river on 8 of 10 sampling dates. Erosion of alluvial materials from the lower valley produced sediment with mineralogy similar to that presently carried by the river suggesting that the source of sediment has not changed much in recent geologic time. Branches of the tributary contributing the most sediment and of one other large tributary were examined. The branch contributing the most silt and clay to each could be identified from mineralogical measurements. An attempt to use crystallinity to determine if clays were eroded from surface soils was only partly successful.
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The river discharge and supply (accumulation) in river waters and suspended sediments are characterized by some global latitudinal peculiarities. The maximum discharge and supply (38.5%) of river waters falls within equatorial belts (0 ± 10°). The maximum quantity of wash-down of river waters and especially their supply (accumulation) per unit land area of the 20°-latitude belts falls within the equatorial belts (0 ± 10°) as well as high-latitude boreal (50 70°N and especially 70–85°N) and antiboreal (30–35°S) belts of the Earth, which are the latitudinal foci (centres) of maximum absorption of land waters.The maximum discharge of suspended sediments (41.2%) and their supply (42.3%) to the basins of sediment accumulation is shifted to the north (10–30° N) as compared to the maximum river discharge; the statistical median of this shift for the Earth as a whole is about 800 km along the meridian.Between the western and eastern hemispheres of the Earth exists a certain asymmetry in the river discharge and wash-down of suspended sediments. According to the median of statistic research these discharge and supply are shifted 2100–2400 km to the north in the eastern hemisphere as compared to the western. Also there have been established some other peculiarities of global distribution of river discharge and sediments on the Earth. At the end of the paper the latitudinal data on the wash-down of river waters and suspended sediments are integrated for the main regions of the Earth (continents and Oceania).
The current population pressure, inappropriate cultivation practices, forest removal and high grazing intensities on forests, wetlands, rangelands and marginal agricultural lands leads to unwanted sediment and stream flow changes that mainly impacts the downstream human and natural communities. Forests and bush are cleared, and wetlands are encroached to create space for human settlement, roads construction and to satisfy wood fuel energy demands. Similarly, pastoral areas are subjected to growing human and livestock populations, leading to land degradation, soil erosion and to an increase in the load of non-point pollutants. Landscape disturbance over many decades, and the resulting increase in soil erosion and sedimentation is the dominant cause of the ongoing eutrophication in many of the lakes in eastern Africa. Increased sedimentation in the rivers and lakes has many impacts. For example, it has altered some aquatic habitats and communities, contributed to increasing eutrophication, abetted the proliferation of algal blooms and water hyacinth reduced the amount of dissolved oxygen, etc. This paper outlines some of the problems created by increased sedimentation within the East African Great Lakes basin, and provides some possible solutions to the mitigation of sediment flux through integrated sediment management approaches.
Sediment fingerprinting has been developed by researchers over the past three decades for watershed sediment transport research. Sediment fingerprinting is a method to allocate sediment nonpoint source pollutants in a watershed through the use of natural tracer technology with a combination of field data collection, laboratory analyses of sediments, and statistical modeling techniques. The method offers a valuable tool for total maximum daily load assessment to aid in developing efficient remediation strategies for pollution in watersheds. We review the methodological steps of sediment fingerprinting including classification of sediment sources in a watershed, identification of unique tracers for each sediment source, representation of sediment sources and sinks using field sampling, accounting for sediment and tracer fate during transport from source to sink, and utilization of an unmixing model to allocate sediment sources. This review places additional emphasis upon tracers used to discriminate sediment sources during past studies performed on different continents and across different physiogeographic regions. Review and analysis of tracer dependence upon watershed variables provides an additional resource for tracer selection to the community. Finally, future improvements needed for sediment fingerprinting are discussed in order to practically apply the technology for sediment nonpoint source pollution allocation within the context of total maximum daily load assessments.
s u m m a r y In mountainous catchments, large quantities of sediment are exported within very short periods leading to numerous environmental problems (e.g. reservoir siltation). The origin of suspended sediment during two distinct floods was determined by conducting an original fingerprinting method coupling Diffuse Reflectance Infrared Fourier Transform Spectroscopy (DRIFTS) and a chemometric technique (i.e. Partial Least Squares – PLS-analysis). Samples of the potential sediment sources were collected in badland areas developed on various substrates (i.e. molasse, marly limestones, black marls and gypsum) in the Galabre 20 km 2 -catchment located in the French Southern Alps. DRIFTS spectra provided a way to discriminate between the different potential sediment sources. Furthermore, the use of mid-infrared spectra allowed the direct quantification of the gypsum proportion in sediment. This contribution was systematically null at the catchment outlet because of the rapid dissolution of gypsum in the river. A PLS model was then constructed to estimate the contribution of the three other potential sources to the sediment flux during the floods. This model was developed and validated using a set of 45 ''experimental'' samples that were prepared in the laboratory in order to contain various proportions of the three remaining sources. By introducing DRIFTS spectra into the PLS model, we could predict the proportions of those sources in the mixed 'experimental' samples with a confidence interval of ca. ±10%. The model was then applied to the sediment collected during the two selected floods in order to outline their origin. Black marls pro-vided the highest contribution of sediment during both events, but the analysis also revealed a significant contribution of molasse. Results also showed the remobilisation of sediment originated from molassic substrates that deposited on the riverbed during a preceding event. Opportunities for improvement and further use of this method as an alternative or rapid complementary sediment fingerprinting tech-nique are finally discussed.