Reported here are results from new flume experiments examining deposition and entrainment of inert, silt-sized particles (with spherical diameters in the range from 20 to 60 lm) to and from planar, impermeable and initially starved beds underlying channel flows. Bed surfaces comprised smooth or fixed sandsize granular roughness and provided hydraulically smooth to transitionally rough boundaries. Results of these experiments were analysed with a simple model that describes the evolution of vertically averaged concentration of suspended sediment and accommodates the simultaneous delivery to and entrainment of grains from the bed. The rate of particle arrival to a bed diminishes linearly, and the rate of particle entrainment increases by the 5/2 power, as the value of the dimensionless Saffman parameter S = u * 3 /g'm approaches a threshold value of order unity, where u * is the conventional friction velocity of the turbulent channel flow, g' is the acceleration due to gravity adjusted for the submerged buoyancy of individual particles and m is the kinematic viscosity of the transporting fluid. This transport behaviour is consistent with the notion that non-cohesive, silt-sized particles can neither reach nor remain on an impermeable bed under flow conditions where mean lift imposed on stationary particles in the viscous sublayer equals or exceeds the submerged weight of individual particles. Within the size range of particles used in these experiments, particle size and the characteristic size of granular roughness, up to that of medium sand, did not affect rates of dimensionless arrival or entrainment to a significant degree. Instead, a new but consistent picture of fine-particle transport is emerging. Silt-sized material, at least, is subject to potentially significant interaction with the bed during intermittent suspension transport at intermediate flow speeds greater than the value required for initiation of transport (ca 20 cm sec )1 ) but less than the value (ca 50 cm sec )1 ) required by the Saffman criterion ensuring transport in fully passive suspension or, equivalently, 'wash-load'.

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Isotopes of iodine play significant environmental roles, including a limiting micronutrient ( 127 I), an acute radiotoxin ( 131 I), and a geochemical tracer ( 129 I). But the cycling of iodine through terrestrial ecosystems is poorly understood, due to its complex environmental chemistry and low natural abundance. To better understand iodine transport and fate in a terrestrial ecosystem, we traced fallout 131 iodine throughout a small temperate catchment following contamination by the 11 March 2011 failure of the Fukushima Daiichi nuclear power facility. We find that radioiodine fallout is actively and efficiently scavenged by the soil system, where it is continuously focused to surface soils over a period of weeks following deposition. Mobilization of historic (pre-Fukushima) 137 cesium observed concurrently in these soils suggests that the focusing of iodine to surface soils may be biologically mediated. Atmospherically deposited iodine is subsequently redistributed from the soil system via fluvial processes in a manner analogous to that of the particle-reactive tracer 7 beryllium, a consequence of the radionuclides’ shared sorption affinity for fine, particulate organic matter. These processes of surficial redistribution create iodine hotspots in the terrestrial environment where fine, particulate organic matter accumulates, and in this manner regulate the delivery of iodine nutrients and toxins alike from small catchments to larger river systems, lakes and estuaries.

[1] We summarize the results of flume experiments examining the transport behavior of dilute suspensions of silt-sized particles carried in an open channel flow over three separate bed types including porous open mesh, glass beads, and small cobbles. In these experiments the time-varying concentration of particles in suspension transport was measured as a function of suspended-particle size and bed shear stress, and analyzed using a 1-D model incorporating simultaneous deposition and entrainment. Rates of fine-particle deposition to the bed are found to approach Stoke's settling velocity in slow flows, but diminish systematically with increasing bed shear stress and mean flow speed. No discernible re-entrainment from the porous beds was observed, indicating that such beds act as an effective sink for fine particles. When our new results are compared to those of related, previously reported experiments examining fine-particle transport over smooth impermeable beds, silt-sized particles display similar behavior with regard to systematic reduction in deposition velocity independent of suspended-silt-particle size or bed porosity. This behavior is tentatively interpreted to reflect the effects of lift in a linear shear flow in excess of submerged weight of individual particles. Our findings compare favorably with values of effective settling velocity of fine particulate organic matter in natural channel flows reported elsewhere.

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