Impacts of tracer type, tracer selection, and source dominance on source apportionment with sediment fingerprinting

Using multi-proxy analysis to determine the long-term impacts of catchment dynamics on water reservoirs - A case from a tropical reservoir (Ruiru Basin Kenya)

Globally, siltation of water reservoirs is a major risk and cost to the provision of fresh water. Therefore, managing reservoir sedimentation is a significant task for water management agencies. In Kenya, the Ruiru water reservoir, one of four water reservoirs supplying Nairobi with drinking water has experienced a significant loss of volume since construction in 1949. However, there have been no studies characterizing the long-term catchment dynamics such as erosion, sedimentation and accumulation in the reservoir. A detailed understanding of the sediment dynamics such as identification of sediment source area; processes driving soil erosion in the catchment and accumulation in the reservoir, and identification of flood layers are necessary for the reservoir and landscape management. The accumulated sediment has not been characterized; therefore, long-term land-use and climate change impacts on the catchment on the reservoir are not documented. The study aims to identify the historical land use and climate events in the catchment impacting the reservoir through a multiproxy sediment characterization of the sediment accumulating in the Ruiru reservoir. An undisturbed 1-m sediment core retrieved in 2017 from Ruiru Reservoir was dated using 210Pb and 137Cs and particle size distribution and chemical element profiles analyzed. The accumulated sediment is a predominantly fine-grained red silt, with a particle size end-member analysis identifying four possible sources or processes of sediment accumulation. The multi-proxy analysis reveals six periods of significant accumulation, 1949, 1963/64, 1974/75, 1982/83, 1997/98 and 2013/14. The peaks coincide with high rainfall events and two are attributed to significant land-use changes in the catchment. The study identifies the catchment dynamics with a significant sediment input into the reservoir. This highlights the importance of reservoirs as environmental archives documenting 20th century land -use and climate events while providing a long-term perspective for management of critical water infrastructure.

Isotopic analysis (δ13C and δ2H) of lignin methoxy groups in forest soils to identify and quantify lignin sources

The relative apportion of above and below ground carbon sources is known to be an important factor in soil organic matter formation. Although lignin is the most abundant aromatic plant material in the terrestrial biosphere, our understanding of lignin source contributions to soil organic matter (SOM) is limited due to the complex molecular structure and analysis of lignin. In this study, we novelly apply the dual isotopic analysis (δ13C and δ2H values) of lignin methoxy groups (LMeO) with the Bayesian mixing model, MixSIAR, to apportion lignin sources in two contrasting soil types, a podzol and a stagnosol. Results of the isotopic analysis of LMeO demonstrate the ability of δ2H LMeO values to discriminate between above and below ground lignin sources, while δ13C LMeO values discriminated between photosynthesising and non-photosynthesising tissues. In the stagnosol subsurface horizons, a decreasing proportion of the leaf litter lignin was observed with increasing organic matter degradation, cumulating in the Ah horizon being dominated by lignin from roots. The podzol sites indicated a similar reduction in leaf litter lignin with an increase in organic matter degradation and depth. However, the Ah horizon was shown to accumulate lignin from the above ground woody material. Furthermore, given the significant abundance of LMeO groups in the terrestrial biosphere and the extremely depleted δ13C LMeO values in leaf litter, we employed a mass balance approach to determine the extent in which the 13C bulk enrichment generally associated with isotopic fractionation during organic matter decomposition can be attributed to the shift in lignin sources. Analysis reveals that 14 % and 11 % of bulk 13C enrichment can be attributed to the transition in LMeO sources from leaf litter to roots in the stagnosol and podzol, respectively. Thus, models relying on 13C enrichment with depth as an indicator of carbon turnover may be partially overestimating rates.

Quantification of the provenance contribution and sedimentary mixing effect of sediments in the Yellow River Basin, China

Quantifying the source contributions of sediments in large fluvial systems with active wind erosion problems has crucial implications for understanding morphological evolution and ecological progression in the Earth system. Much effort have been focused on characterizing sediments of the Yellow River, but quantitation of the sediment source proportions at the basin-wide scale is lacking. To this end, the research aims to quantitatively elucidate the potential source contributions of sediments in the Yellow River based on geochemical characteristics and sediment fingerprinting technique, in order to identify sedimentary mixing effect and propose sustainable development strategies. In total, samples of four source groups (n = 107) and target floodplain sediments (n = 61) were collected and tested for elemental composition, grain size, magnetic susceptibility, and quartz grain microtextures. The results indicated that the optimal tracer combination was determined as P, Zn, and Ca. The average contributions of the "Tibetan Plateau", "Sandy deserts-Loess Plateau", "Loess Plateau", and "Loess Plateau-Qinling Mountains" source groups to the target sediments were 23.0 %, 21.5 %, 31.6 %, and 23.9 %, respectively. The accuracy of source apportionments was supported by the goodness of fit (GOF) and virtual mixtures tests. Meanwhile, large amounts of debris from surrounding mountains was transported to the Loess Plateau through fluvial processes and ultimately mixed with aeolian deposits, leading to sedimentary mixing effect. To maintain water balance and minimize erosion risk, the drought-resistant perennial planting and moderate grazing were recommended. The findings are instrumental in promoting soil and water conservation and disclosing fluvial and aeolian interaction on a global scale.

13C dicarboxylic acid signatures indicate temporal shifts in catchment sediment sources in response to extreme winter rainfall

Rainfall and land-use interactions drive temporal shifts in suspended sediment sources, yet the magnitude of such changes remains poorly understood due to the lack of land-use specific source tracers. We investigated α,ω-dicarboxylic fatty acid root-specific biomarkers, as diagnostic tracers for apportioning sources of time-integrated suspended sediment samples collected from a grassland dominated agricultural catchment in the southwest of England during the wet winter period. Applying fatty acids-specific stable carbon isotope analysis and a Bayesian isotope mixing model, we show that stream banks contributed most of the sediment in the early winter, i.e. October-December, while winter cereal-dominated arable land contributed more than half of the sediment during the late winter, i.e. January-March. The dominant sediment source shifted in conjunction with a period of prolonged consecutive rainfall days in the later period suggesting that intervention required to mitigate soil erosion and sediment delivery should adapt to changing rainfall patterns. Our novel findings demonstrate that isotopic signatures of α,ω-dicarboxylic fatty acids are promising tracers for understanding the resistance of agricultural soils to water erosion and quantifying the interactive effects of extreme rainfall and land use on catchment sediment source dynamics.

Supplementary Information

The online version contains supplementary material available at 10.1007/s10311-023-01684-1.

The influence of erosion sources on sediment-related water quality attributes

Suspended fine sediment has a significant impact on freshwater quality variables such as visual clarity (VC). However, freshwater quality is related to the attributes of the catchment sources contributing fine sediment to the stream network. Here, the extent to which an array of sources defined spatially according to erosion process and geological parent material may be discriminated and classified based on sediment-related water quality (SRWQ) attributes that potentially affect VC was examined. Erosion sources were sampled across two New Zealand catchments representing six types of erosion and eight parent materials. Erosion source measurements focused on particle size, organic matter content, and light beam attenuation (which is convertible to VC). The source data were analysed to: 1) evaluate source variability using a combination of Kruskal-Wallis and principal component analysis; 2) reclassify sources using a Random Forest model; and 3) demonstrate how erosion source affects VC for a range of theoretical sediment concentrations (SC) using a simple empirical model. The results indicate that SRWQ attributes show significant variation across erosion sources. The extent to which attributes differed between sources often related to whether there was a strong association between a specific erosion process and parent material. The 19 a priori source classifications were reduced to 5 distinct sources that combined erosion process and parent material (i.e., bank erosion-alluvium; mass movement-ancient volcanics; mass movement-sedimentary; surficial erosion; gully-unconsolidated sandstone). At low SC, the impact of erosion source on VC became most evident ranging from 2.6 to 5.6 m at SC of 5 g m^-3. These findings show how catchment sources of sediment, in addition to sediment concentration, influence VC, and highlight the need to consider quality as well as quantity of material supplied to stream networks when planning erosion control.