Combining isotopic and elemental tracers for enhanced sediment source partitioning in complex catchments

Abstract

Sediment fingerprinting quantifies soil erosion processes by tracing sediment origins in water bodies. In this regard, several types of tracers have been used to discriminate different sediment sources. Stable isotopic composition of fatty acids is associated with land use/vegetation cover, while elemental composition is related to different mineralogy. Isotopic tracers are characterised by their isotopic ratio and total content, requiring specific fingerprinting models. Consequently, this has led to few studies combining elemental and isotopic tracers.

In this context, our analysis explores merging isotopic and elemental tracers to identify sediment sources in an ungauged Mediterranean mountain catchment. This catchment features consistent lithology, seasonal streams, and a history of land use changes, shifting from rangelands to croplands for increased agricultural production and later reverting to natural revegetation due to mid-20th-century land abandonment. Achieving effective source discrimination in these intricate landscapes remains a significant challenge, emphasising the need for diverse tracer integration.

To explore this possibility, we collected composite source samples from three distinct land uses: cropland, Mediterranean forest, and pine forest, as well as two geomorphic features: highly disturbed areas such as exposed subsoil and channel banks. By considering these diverse sampling locations, we aim to capture the variability in sediment sources within the catchment. Our dataset spans one full hydrological year, allowing us to analyse sediment dynamics throughout different seasons and hydrological events.

First, the Conservative Balance (CB) method was applied to integrate isotopic ratio and total content of each fatty acid into a single weighted tracer. Combining the weighted fatty acid (WFA) with elemental composition tracers significantly enhanced source discrimination, underlining the utility of employing diverse tracers for challenging source discrimination.

The apportionment results revealed that agriculture, channel banks, and subsoils were the primary contributors, accounting for an average contribution of 29%, 39%, and 30% across most seasons. In contrast, sediment sources characterised by more permanent vegetation cover and minimal human influence, such as pine afforestation and Mediterranean forest, exhibited negligible contributions. The findings indicate the substantial impact of agricultural practices in Mediterranean agroecosystems, especially during storm events, on catchment hydrodynamics and sediment export. Additionally, despite their limited coverage, degraded areas significantly contributed to overall sediment dynamics through severe soil loss.

By unravelling the hydrological implications of these sediment sources, our study provides valuable insights into the interplay between land use, hydrological processes, and sediment dynamics in Mediterranean mountain catchments.