Search

• Two contrasting years of monitoring of an agricultural tile-drained catchment. • Water and suspended particle transfers displayed high temporal variability. • Two transfer pathways to tile drains were observed. • Suspended particle dynamics in tile drains may be controlled by sediment storage and exhaustion. Soil erosion, runoff and sediment connectivity are strongly impacted by anthropogenic features in lowland agricultural catchments. Among these landscape features, the role played by tile drainage on water and sediment transfers and hillslope-to-river connectivity in drained catchments remains poorly understood. This study quantified water and sediment transfers in a tile drained catchment of central France by combining high frequency rainfall, discharge and sediment concentration measurements at the outlet of a set of 10 tile drained plots (34 ha) and at the medium-sized (120 km2) catchment scale. Over the monitoring period, including a dry and a wet year compared to average conditions (one year with 112% of the mean annual rainfall and one year with 64% of the mean annual rainfall), 36 rainfall-flood events were recorded and analyzed. The high frequency analysis of water and sediment transfers in tile-drained plots showed a high seasonal variability and the occurrence of two transfer pathways in the soil column including the slow drainage of saturated soils and the occurrence of preferential flow pathways through the soil column. Indeed, 13 of the 36 recorded flood events showed hydrographs with two components, reflecting these two pathways: slow transfers in the soil columns and fast transfers through soil macropores and/or cracks. Indeed, at the beginning of the flood event, a high-magnitude peak overlaid on the hydrograph. On average, this fast peak contributed 15% of the water and sediment fluxes. The sediment dynamics in tile drains was suggested to depend on sediment storage and exhaustion processes occurring in the tile drain network. [ABSTRACT FROM AUTHOR]

The geochemical distribution of U and associated major and trace elements (As, Li, Pb, Sr, Zn) was studied at the former Rophin U mine (Puy-de-Dôme, France). Three zones of contrasting radiological settings were identified and sampled: a background area (<200 nSv·h−1), a streambed flowing downstream of the former mine (radiation level between 200 and 800 nSv·h−1) and a wetland soil (>1000 nSv·h−1). Sediment and water samples were collected both in a streambed located in the background area and in the streambed downslope the former mining area, and a soil sample was collected in the wetland downslope. Using both sequential and selective chemical extractions, and quantifying also the chemical reservoirs of As, Li, Pb, Sr, and Zn, it could be concluded that, in the streams, U was mainly bounded to primary ore minerals (phosphates) that were transported through particulate transport. It was also bound, to a lower extent, to clays, Mn oxyhydroxides and organic matter, certainly due to the sorption of aqueous U originating from partial dissolution or leaching of primary ore materials. Ore minerals remaining stable in the stream sediments were certainly included in a quartz matrix and hence were not in accessible for dissolution. In the wetland soil, selective extractions evidenced that U was about evenly distributed between humid/fulvic acids and organic matter unaffected by NH 4 OH 1M. [Display omitted] • Investigation of the radiological and geochemical settings downstream a former U mine. • In streams, U migration dictated by interplay between sorption of aqueous U and particulate transport. • In stream sediments, U primarily bound to ore material and to lesser extent clay, Fe/Mn hydroxides, and organic matter. • In a wetland soil, U bound to humic/fulvic acids and possibly lignin, clue for U uptake by plants. [ABSTRACT FROM AUTHOR]

A major challenge in geomorphology is reconciling the disparity between runoff coefficients and erosion rates from the field to the catchment scale. In the European loess belt, a decrease of runoff coefficients and erosion rates occurs with increasing spatial scales. Indeed, it is important to understand the scale dependency of run-off and erosion to manage the off-site impacts of accelerated soil erosion. Accordingly, a continuous simulation of the scale dependency of runoff and erosion, from the field scale to the catchment scale (100 – 1000 km²) was conducted with the Water and Sediment (WaterSed) model for two catchments (Andelle-756 km², and Austreberthe-214 km²) over 12 years (>1000 events). Scale effects were evident with a 100-fold decrease in runoff coefficients and sediment delivery ratios between the field scale and the catchment scale. In spite of a low variability of the annual rainfall (19%), the inter-annual variability of the runoff volume (37%) and erosion rates (92%) at catchment outlets were high. The inter-annual variability of runoff and erosion was closely linked to the number of extreme events per year and their distribution through the year, in particular during periods with highly crusted soil surface states. For these high magnitude events, a complex distributed modelling approach was not necessary as the ability of the soil surface and of the landscape to retain overland flows are largely exceeded. The seasonality of soil surface characteristics also affected the scale dependency of runoff and erosion, from the field scale to the catchment scale. However, this is only observed up to a certain spatial extent, i.e. where hillslope erosion processes are governed by hortonian overland flow. When saturation flows are generated, topography and soil depth become the dominant factors. Understanding runoff and sediment response at different scales should focus on the location and amount of runoff and sediment production within the catchment and the capacity of the downstream flow path to retain or transfer overland flow and sediment. Furthermore, this study demonstrates the need to implement model that represent both hortonian and saturation flows when simulating erosion events at the catchment scale. [ABSTRACT FROM AUTHOR]