Sediment source tracing with stratified sampling and weightings based on spatial gradients in soil erosion

Abstract

Purpose The results of sediment source tracing in large river catchments depend on defined sources being adequately represented by the sampling and in subsequent numerical analysis. We hypothesise that surface soil concentrations of fallout radionuclides caesium-137 ( 137 Cs) and lead-210 excess ( 210 Pb ex ) are smaller at locations with higher soil erosion rate and that if this is not accounted for, then spatially random sampling gives a biased representation of surface soil delivered to rivers and biased source contribution estimates. Materials and methods Surface soil was sampled across the Burdekin River basin in northeast Australia at 90 locations stratified by three classes of modelled soil erosion rate and analysed by gamma spectroscopy. Separate probability distributions (density functions) were fitted to the 137 Cs concentrations of samples of each erosion class, of subsurface soil and of river sediment. Surface soil distributions were aggregated by weighting in proportion to the upstream area and mean erosion rate of each erosion class, so that the high erosion class contributed disproportionately to the tracer properties of the surface soil source. Source contributions were estimated using a Monte Carlo mixing model. Results and discussion The mean surface soil concentrations of 137 Cs and 210 Pb ex were significantly different between soil erosion classes as hypothesised. Weighting surface soil from the high erosion class more heavily increased the estimated proportion of river sediment contributed from surface soil, by 35 % larger than if surface soil sampling was confined to low erosion areas. Stratified sampling and weighting by erosion rate is of greater importance in river basins with large gradients in soil erosion and where surface soil contributes substantially to river sediment. Surface soil contributed 6 % to fine sediment at the basin outlet and 0-14 % in major tributaries, which was somewhat lower than in a prior study probably due to recent above-average rainfall increasing vegetation ground cover. Conclusions Surface soil sampling for source tracing using fallout radionuclides should be stratified by erosion rate. The tracer properties of high erosion areas should be weighted more heavily than low erosion areas in source mixing models. If comprehensive sampling cannot be afforded, then sampling should be biased towards more highly eroding areas. The approach should be considered for other source tracers whose properties may co-vary with soil erosion rate. Fine sediment delivered from the Burdekin River basin to the Great Barrier Reef lagoon in recent decades was predominantly derived from gully erosion, streambank erosion and rilled and scalded areas on hillslopes.

Suspended sediment transport dynamics in rivers: Multi-scale drivers of temporal variation

“…While the use of fallout radionuclides in sediment fingerprinting has helped to assess the time passed since the sediment eroded (residence time) (e.g. Palazón et al, 2015;Smith and Blake, 2014;Wilkinson et al, 2015), it remains a challenge to interpret patterns in sediment source contributions in terms of catchment erosion processes.…”

Testing the Use of 210Pbex to Study Sediment Connectivity in a Mediterranean Mountain Basin with Badlands

“…A ref in equation reaches equilibrium after 4–6 half‐lives of lead‐210 (about 90–140 years). However, 210 Pb ex areal activity density for sites subjected to intense soil erosion, such as badlands, is far from equilibrium (Hancock et al, ; Wilkinson et al, ). On the assumption that each erosion event removes a layer of soil, and that depth distribution of 210 Pb ex mass activity (Bq kg −1 ) approaches a negative exponential form (Walling, ; Benmansour et al, ; Porto et al, ), the remaining 210 Pb ex areal activity density of a site after the effects of an erosion event ( A(Er) , Bq m −2 ) can be calculated as (Walling et al, ):…”

“…Particularly, the fallout component of the lead‐210 radioisotope (generally termed unsupported or excess lead‐210, 210 Pb ex ) has been very valuable in a variety of applications during the last decades. 210 Pb ex applications have included the exploration of sedimentation rates in lakes, reservoirs, floodplains and other sedimentary systems (Appleby & Oldfield, ; Foster et al, ; Du & Walling, ), the analysis of soil erosion patterns (Mabit et al, ; Benmansour et al, ; Porto et al, ) and spatial analysis of the processes of sediment transport and redistribution, including sediment fingerprinting techniques (Collins et al, ; Walling, ; Hancock et al, ; Wilkinson et al, ; Estrany et al, ). Fallout lead‐210 has a relatively long half‐life (22 years).…”

Do forests represent a long-term source of contaminated particulate matter in the Fukushima Prefecture?

“…Source properties that were non-normal were fit with alternative distributions with Oracle's Crystal Ball Software that uses maximum likelihood estimation calculations to select the best fit of 14 potential distributions with Anderson-Darling, KolmogorovSmirnoff and Chi-Square tests. For instream particulate matter samples, normal distributions were fit around each parameter, using analytical uncertainty to act as the particulate matter parameter standard deviation similarly to Wilkinson et al (2015) and Evrard et al (2016).…”