Quantifying sediment sources in a lowland agricultural catchment pond using 137Cs activities and radiogenic 87Sr/86Sr ratios

Abstract

Soil erosion often supplies high sediment loads to rivers, degrading water quality and contributing to the siltation of reservoirs and lowland river channels. These impacts are exacerbated in agricultural catchments where modifications in land management and agricultural practices were shown to accelerate sediment supply. In this study, sediment sources were identified with a novel tracing approach combining cesium ((137)Cs) and strontium isotopes ((87)Sr/(86)Sr) in the Louroux pond, at the outlet of a lowland cultivated catchment (24km(2), Loire River basin, France) representative of drained agricultural areas of Northwestern Europe. Surface soil (n=36) and subsurface channel bank (n=17) samples were collected to characterize potential sources. Deposited sediment (n=41) was sampled across the entire surface of the pond to examine spatial variation in sediment deposits. In addition, a 1.10m sediment core was sampled in the middle of the pond to reconstruct source variations throughout time. (137)Cs was used to discriminate between surface and subsurface sources, whereas (87)Sr/(86)Sr ratios discriminated between lithological sources. A distribution modeling approach quantified the relative contribution of these sources to the sampled sediment. Results indicate that surface sources contributed to the majority of pond (μ 82%, σ 1%) and core (μ 88%, σ 2%) sediment with elevated subsurface contributions modeled near specific sites close to the banks of the Louroux pond. Contributions of the lithological sources were well mixed in surface sediment across the pond (i.e., carbonate sediment contribution, μ 48%, σ 1% and non-carbonate sediment contribution, μ 52%, σ 3%) although there were significant variations of these source contributions modeled for the sediment core between 1955 and 2013. These fluctuations reflect both the progressive implementation of land consolidation schemes in the catchment and the eutrophication of the pond. This original sediment fingerprinting study demonstrates the potential of combining radionuclide and strontium isotopic geochemistry measurements to quantify sediment sources in cultivated catchments.

Quantification of bank erosion in a drained agricultural lowland catchment

“…A previous fingerprinting approach study based on the 137 Cs activity performed in this catchment estimated that the subsurface source (bank erosion) contributed 18% (σ 1%) of the pond sediment over the last 10 years (Le Gall et al, ). This result is slightly different than the contribution estimated in this study (27% to 30%) but still on the same order of magnitude.…”

“…Globally, streams are commonly being recognized as primary sources of sediment to streams and rivers (e.g., Laceby, ). Recent studies in an intensively farmed watershed have found that human‐altered stream channels are a minor component of net sediment delivery to a 52 ha pond (Foucher et al, Le Gall et al, ). Based on historic channel cross section surveys, the volume of material eroded from stream banks is quantified at different spatial and time scales: The annual volume of material eroded from stream banks along a small headwater stream (1,400‐m long) during one hydrological year has been estimated using erosion pin data. Long‐term bank erosion has been estimated by comparing elevation data obtained in 2013 from differential global positioning system (DGPS) acquisition combined with aerial LiDAR survey to three‐dimensional historical plans dating back to the stream redesign in 1944.…”

“…Globally, streams are commonly being recognized as primary sources of sediment to streams and rivers (e.g., Laceby, 2012). Recent studies in an intensively farmed watershed have found that human-altered stream channels are a minor component of net sediment delivery to a 52 ha pond Le Gall et al, 2016. Based on historic channel cross section surveys, the volume of material eroded from stream banks is quantified at different spatial and time scales:…”

Tracing Sediment Sources in a Subtropical Agricultural Catchment of Southern Brazil Cultivated With Conventional and Conservation Farming Practices

“…A distribution modelling approach was used to quantify the relative contribution of each source to sediment through the minimization of the mixing model difference (MMD) (Laceby & Olley, ; Le Gall et al ., ): Where: C i is the concentration of element i in the suspended sediment, m is the number of sources in the catchment; S si is the concentration of element i in source ( s ); C r is the strontium isotopic ratio ( r ) in the suspended sediment; S sr is the strontium isotopic ratio ( r ) in source ( s ); W s is the strontium concentration in source (s), which is used to weight the strontium isotopic ratios; MMD is the mixing model difference; and x s is the proportional contribution of source ( s ), which is modelled as a truncated normal distribution (0 ≤ x s ≤ 1) with a mixture mean ( μ m ) and a standard deviation (SD) ( σ m ). Absolute values ( ABS ) are summed in Equation .…”

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

“…Three combinations of particulate matter parameters were modelled: 1) TN and d 15 N (the N model); 2) TOC and d 13 C (the C model); 3) all 4 properties (the C þ N model). These combinations were used to quantify relative source contributions with a concentration weighted distribution modelling approach (Laceby et al, 2015b;Le Gall et al, 2016):…”