Combined use of geochemistry and compound-specific stable isotopes for sediment fingerprinting and tracing

Abstract

Sediment fingerprinting has emerged as a valuable tool for elucidating soil erosion processes and assessing the sources of sediment and particle-bound chemicals. Due to its upward trend in popularity and the parallel advances in analytical methods, different types of tracers such as Compound-Specific Stable Isotopes (CSSIs) have been incorporated to identify the potential sources. However, the physical processes of CSSIs, usually characterised by the ratio of two stable isotopes, also depends on the isotopic content requiring specific fingerprinting models. For this reason, isotopic tracers have not been combined with classical tracers such as the elemental composition. In this context, we i) propose a simple physical model describing the mixing of isotopic tracers, ii) derive the conservative balance (CB) governing the isotopic ratio of the mixture, iii) demonstrate that previous models for isotopic tracers are approximations of the proposed formulation, and iv) obtain an exact transformation combining the isotopic ratio and the isotopic content into a virtual elemental tracer. The CB model was successfully validated and tested in three fingerprint datasets from published isotopic tracer studies, demonstrating the equivalence of the proposed transformation. Compared to previously reported methods, the CB has two advantages: the model enables the analysis of isotopic tracers using classical unmixing models and, at the same time, allows the combined use of isotopic and elemental tracers. Within the CB framework the Consensus and the Consistent Tracer Selection (CTS) methods can also be applied to isotopic tracers, which is critical for a correct fingerprinting analysis. The effect of combining both types of tracers is illustrated in a dataset composed of elemental and CSSI tracers. The results support its compatibility and reveal an increase in the discriminant capacity and better-defined results for the channel bank source when combining elemental and CSSI tracers.

Keywords: Conservative balance; Consistent tracer selection; Isotopic content; Isotopic ratio; Tracer combination; Virtual elemental tracer.