Factors influencing source apportionment accuracy using sediment fingerprinting: observations from New Zealand

Sediment source fingerprinting is a technique for determining proportional contributions from different catchment sources to sediments in downstream receiving environments. The technique involves a) selecting tracers that discriminate sources based on their biogeochemical or isotopic properties and b) applying statistical mixing models to quantitatively determine source contributions. Tracer suitability varies depending on the characteristics of the study catchment and the source property or erosion processes being targeted and can include geochemical, fallout radionuclides (FRNs), or compound specific stable isotopes (CSSIs). For instance, the spatial variation in soil geochemical properties is largely determined by underlying geological and pedogenic processes, whereas CSSIs utilise δ¹³C isotopic properties of fatty acid biomarkers that bind to soils and vary based on plant communities associated with each land cover.

While the environmental basis for sediment fingerprinting is increasingly understood, methodological challenges continue to present limitations that may hinder wider catchment applications. Here, we draw from recent research in New Zealand to highlight some of the challenges to source apportionment accuracy using numerical mixture testing and catchment studies to represent a range of tracers and sources. Tracers include bulk geochemistry, fallout radionuclides (FRNs), and compound specific stable isotopes (CSSIs) and sources are defined by parent material, erosion processes, and land cover. We focus on the influence of source dominance and source discrimination by different tracer types on source apportionment accuracy, as well as uncertainties introduced from post-unmixing transformations associated with CSSIs.