Freshwater sediment source fingerprinting using compound-specific isotope analysis: Suitability and application

Freshwater systems across the globe are experiencing increased sediment delivery. Anthropogenic activities including intensification of land management are primary causes of increased sediment delivery to freshwater systems. Excessive input of suspended sediments in freshwater systems can substantially alter physical, chemical, and biological properties of the water bodies, which can have detrimental effects on the aquatic flora and fauna. Changing climate scenarios are expected to further increase soil erosion and associated increase of sediment load to freshwater systems due to frequent extreme weather events and changing land use practices. Hence, information on the sources of the suspended sediments is invaluable for efficient, focused, and cost-effective implementation of land management strategies.
Recently introduced sediment source fingerprinting technique using compound-specific isotope analysis (CSIA) can potentially identify major erosion sources and quantify their contribution to the stream suspended sediments based on the land uses in the catchment. Compound-specific stable isotope (CSSI) values of carbon (δ13C values) and hydrogen (δ2H values) of plant lipid components such as long-chain fatty acids (LCFAs) and n-alkanes are primarily used as tracers in the CSIA-based sediment source fingerprinting. However, a rigorous evaluation of tracer conservativeness in terms of stability of isotopic signatures during detachment and transport of soil during erosion is essential to validate suitability of the fingerprinting method. Potential fractionation and shifts in the isotopic signatures of long-chain fatty acids and n-alkanes during early degradation in soils after their production by plants was studied to assess the stability of the isotopic tracers during early degradation. Increasing degradation stages across the plant–soil systems (fresh plant biomass–organic horizons–mineral soil) from forests with different vegetation, soil, climatic zones, and humus forms were sampled. A clear increase in δ13C values for both compound classes from aboveground plant biomass to the O horizon overlaying the mineral soil was observed; however only slight or no further changes were observed during the further degradation in the mineral soil. The latter was in clear contrast to bulk δ13C values, which continued to become enriched in 13C through all the degradation stages from fresh plant biomass to mineral soil. This finding emphasizes the suitability of studied compounds as tracers used in CSIA-based sediment source fingerprinting.
Analysis of the effect of particle size on the isotopic composition of LCFAs and n-alkanes is crucial to validate conservative behaviour of isotopic tracers during soil erosion processes. Compound-specific carbon isotope values were compared between the two particle sizes which are frequently used for characterizing source tracer signatures from soils (< 63 µm and < 2 mm). δ13C values of both compound classes were not significantly different between the two particle sizes, implying the absence of particle-size-dependent variability in the isotopic tracers. To understand the particle sorting effect during sediment transport, we compared two frequently used suspended sediment sampling techniques. Although the particle size distributions of collected sediments were different between the sampling methods, no significant particle-size-dependent fractionation was observed in δ13C values of long-chain fatty acids from the collected sediments. The results above further strengthen the necessary requirement of the conservativeness of tracer signatures during erosion processes, for their application in CSIA-based sediment source fingerprinting.
A comprehensive study was designed to apply the CSIA technique to identify and quantify the major land use source in an intensively managed Tarland catchment (74 km2) in NE Scotland, with increased the risk of sediment pollution and associated freshwater quality degradation. A balance between robust source characterization and cost-effectiveness of the CSIA technique was achieved using a nested soil and sediment sampling approach. Stream suspended sediments were sampled from the streams of different orders for a period of fourteen months for spatial and temporal analysis of suspended sediment sources. Compound-specific δ13C and δ2H compositions of LCFAs were successfully used to differentiate the suspended sediment sources based on the land uses present in the catchment. δ13C variability between different land uses was able to differentiate between forest, heather moorland, permanent grassland, and arable land use. Plant growth form was the primary control on the source soils LCFAs δ2H variability, while the isotopic composition of source water in the catchment had no significant control. Hence, sediments originating from the land uses covered with dicotyledonous and gymnosperm species were differentiated from sediments originating from the land uses covered with monocotyledonous species.
A common finding from the sediment source fingerprinting using CSIA of carbon and hydrogen isotope values from LCFAs suggested agriculture as the main stressor to freshwater suspended sediment load in the Tarland catchment. Specifically, δ13C-based sediment source fingerprinting showed permanent grassland as the major source of suspended sediments across the Tarland catchment. This underlines the threat from intensively managed grasslands to the freshwater systems. Monocot-based land use (e.g., cereal crops, grassland) was identified as the major source of suspended sediments throughout the Tarland catchment using δ2H-based sediment source fingerprinting. δ2H-based sediment source fingerprinting additionally identified forest and heather moorland contribution during autumn and early winter, capturing changes in the connectivity due to hydrological events, or possible riparian vegetation contribution. The study demonstrates the differences in source characterization with two isotope systems associated with the difference in their fractionation mechanisms. It also underlines the usefulness of the complementary information gathered from the two isotope systems for characterizing and quantifying freshwater suspended sediment sources.
This study strongly contributes to further advancement of the CSIA-based sediment source fingerprinting by addressing some of the open questions around the suitability of the method. The study further applied the method to identify major land uses causing sediment pollution and to quantify land-use-based suspended sediment contributions in an intensively managed mesoscale catchment.