The role of wind-wave related processes in redistributing river-derived terrigenous sediments in Lake Turkana: A modelling study

A complete annual cycle of the dynamics of fine-grained sediment supplied by the Omo and smaller rivers is simulated for Lake Turkana, one of the world’s large lakes, with the hydrodynamic, wave and sediment transport model Delft3D. The model is forced with river liquid and solid discharge and wind data in order to simulate cohesive sediment transport and resuspension. It simulates stratification due to salinity, wave generation and dissipation, and sediment advection and resuspension by waves and currents, with multiple cohesive sediment fractions. A comparison of the simulation results with remotely-sensed imagery and with available in-situ sediment deposition rates validates the model. By devising simulation scenarios in which certain processes were switched on or off, we investigated the contribution of waves, wind-induced surface and bottom currents, salinity-induced stratification and river jet, in resuspending and transporting fine sediments in the lake basin. With only the wind or river influence, most of the sediment deposition occurs in the first 10 km off the Omo River mouth and at a depth < 10 m. When waves are switched on, increased bed shear stresses resuspend most of the fine sediments, that are then deposited further and deeper in the first 30 km, in water depths > 30 m. This study sheds new light on sediment transport in Lake Turkana and in great lakes in general, favouring the view that wind-waves can be the main agent that transports sediment away from river mouths and to deeper areas, as opposed to river-plume or gravity-driven transport.     

A novel coupled biokinetic-equilibrium model to capture oyster metal bioaccumulation in a contaminated estuary (Sydney estuary,Australia)

Enriched concentrations of metal contaminants have been reported in surficial sediments and tissues of aquatic organisms in Sydney estuary, New South Wales, Australia. Dietary ingestion of contaminated, suspended sediments is potentially a major route of metal exposure to the filter-feeder Saccostrea glomerata. A dynamically coupled biokinetic-equilibrium bioaccumulation model was developed to explore sediment-oyster-metal uptake interactions. The biokinetic component simulated the sediment dynamics and oyster uptake and loss kinetics while the ion equilibrium model accounted for the metal speciation reactions. Results of a laboratory-based mesocosm experiment as well as data from the literature were used to parameterise the model. The model demonstrated a good fit of the experimental data and indicated that dissolved and particulate organic matter were important determinants of metal bioavailability to this species. The model served also as a unique tool to formulate testable hypotheses and help to better explain the bioaccumulation patterns observed from the experiment.     

Resuspension of carbon dust collected in Tore Supra and exposed to turbulent airflow: Controlled experiments and comparison with model

This work presents the results of experiments conducted with carbon microparticles collected in the tokamak Tore Supra in order to characterize their resuspension behaviour from a stainless-steel substrate when exposed to turbulent airflow. Experiments were conducted in a wind tunnel with controlled velocity profiles and monitored environmental conditions. A consequent amount of dust has been collected in the vessel of the tokamak and a bimodal particle size distribution of samples is first demonstrated. Comparison with resuspension of alumina powders with equivalent particle size distributions under turbulent airflow is also discussed. Results for both carbon and alumina microparticles are then compared to a theoretical resuspension model. Data reveal that exposing multilayer deposits with bimodal particle size distributions to low-speed flows (i.e. 3–10 m/s) induces a significant reduction of the mobilized fractions compared to what was predicted by the model. In addition, results helped to highlight some limitations in the model to physically describe changes in the adhesive strength that can occur with a polydisperse deposit.     

A comparative study of walking‐induced dust resuspension using a consistent test mechanism

Human walking influences indoor air quality mainly by resuspending dust particles settled on the floor. This study characterized walking‐induced particle resuspension as a function of flooring type, relative humidity (RH), surface dust loading, and particle size using a consistent resuspension mechanism. Five types of flooring, including hardwood, vinyl, high‐density cut pile carpet, low‐density cut pile carpet, and high‐density loop carpet, were tested with two levels of RH (40% and 70%) and surface dust loading (2 and 8 g/m²), respectively. Resuspension fraction r_a (fraction of surface dust resuspended per step) for house dust was found to be varied from 10^?7 to 10^?4 (particle size: 0.4–10 µm). Results showed that for particles at 0.4–3.0 µm, the difference in resuspension fraction between carpets and hard floorings was not significant. For particles at 3.0–10.0 µm, carpets exhibited higher resuspension fractions compared with hard floorings. Increased RH level enhanced resuspension on high‐density cut pile carpet, whereas the opposite effect was observed on hard floorings. Higher surface dust loading was associated with lower resuspension fractions on carpets, while on hard floorings the effect of surface dust loading varied with different RH levels.     

Chamber bioaerosol study: human emissions of size‐resolved fluorescent biological aerosol particles

Humans are a prominent source of airborne biological particles in occupied indoor spaces, but few studies have quantified human bioaerosol emissions. The chamber investigation reported here employs a fluorescence‐based technique to evaluate bioaerosols with high temporal and particle size resolution. In a 75‐m³ chamber, occupant emission rates of coarse (2.5–10 μm) fluorescent biological aerosol particles (FBAPs) under seated, simulated office‐work conditions averaged 0.9 ± 0.3 million particles per person‐h. Walking was associated with a 5–6× increase in the emission rate. During both walking and sitting, 60–70% or more of emissions originated from the floor. The increase in emissions during walking (vs. while sitting) was mainly attributable to release of particles from the floor; the associated increased vigor of upper body movements also contributed. Clothing, or its frictional interaction with human skin, was demonstrated to be a source of coarse particles, and especially of the highly fluorescent fraction. Emission rates of FBAPs previously reported for lecture classes were well bounded by the experimental results obtained in this chamber study. In both settings, the size distribution of occupant FBAP emissions had a dominant mode in the 3–5 μm diameter range.     

Modeling sediment impact on the transport of fecal bacteria

Sediment storage of fecal bacteria has been observed for a long time. However, limited modeling efforts have been conducted to dynamically and explicitly study the impact of sediment on the transport of fecal bacteria. In this paper, a modeling framework based on the Environmental Fluid Dynamics Code (EFDC) model has been proposed. Free and particle-associated fecal bacteria can be modeled explicitly using the developed model. A linear adsorption relationship is incorporated into the model to obtain a sediment concentration dependent attachment ratio. The deposition and resuspension fluxes of fecal bacteria across the sediment bed–water interface are calculated coupled with the sediment deposition and resuspension flux. The model was applied to hypothetical scenarios to evaluate the sediment impacts under ideal settling and resuspension conditions. A case study was provided to test the resuspension calculation of fecal bacteria under an artificial flooding condition. Using this model, the contributions of fecal bacteria from sediment bed and from the watershed can be modeled separately and explicitly.     

Resuspension and settling of helminth eggs in water: Interactions with cohesive sediments

Helminth parasite eggs in low quality water represent main food safety and health hazards and are therefore important indicators used to determine whether such water can be used for irrigation. Through sedimentation helminth eggs accumulate in the sediment, however resuspension of deposited helminth eggs will lead to increased concentration of suspended eggs in the water. Our study aimed to determine the erodibility (erosion rate and erosion threshold) and settling velocity of Ascaris and Trichuris eggs as well as cohesive sediment at different time points after incorporation into the sediment. Cohesive sediment collected from a freshwater stream was used to prepare a sediment bed onto which helminth eggs were allowed to settle. The erodibility of both sediment and helminth eggs was found to decrease over time indicating that the eggs were incorporated into the surface material of the bed and that this material was stabilized through time. This interaction between eggs and bulk sediment was further manifested in an increased settling velocity of suspended eggs when sediment was present in the suspension as compared to a situation with settling in clean water. The incorporation into the sediment bed and the aggregation with sediment particles decrease the mobility of both helminth egg types. Our findings document that helminth eggs should not be viewed as single entities in water systems when modelling the distribution of eggs since both erodibility and settling velocity of eggs are determined by mobility of the sediment present in the water stream. Recalculation of the erosion threshold for helminth eggs and sediment showed that even at relatively low current velocities i.e. 0.07-0.12 m s⁻¹ newly deposited eggs will be mobile in open irrigation channels. These environmental factors affecting resuspension must be taken into account when developing models for sedimentation of helminth eggs in different water systems.     

A model for predicting resuspension of Escherichia coli from streambed sediments

To improve the modeling of water quality in watersheds, a model is developed to predict resuspension of Escherichia coli from sediment beds in streams. The resuspension rate is expressed as the product of the concentration of E. coli attached to sediment particles and an erosion rate adapted from work on sediment transport. The model uses parameter values mostly taken from previous work, and it accounts for properties of the flow through the bottom shear stress and properties of the sediment through the critical shear stresses for cohesive and non-cohesive sediment. Predictions were compared to resuspension rates inferred from a steady mass balance applied to measurements at sixteen locations in a watershed. The model’s predictions matched the inferred rates well, especially when the diameter of particles to which E. coli attach was allowed to depend on the bottom shear stress. The model’s sensitivity to the parameters depends on the contributions of particle packing and binding effects of clay to the critical shear stress. For the current data set, the uncertainty in the predictions is controlled by the concentration of E. coli attached to sediment particles and the slope used to estimate the bottom shear stress.     

Use of a robotic sampling platform to assess young children's exposure to indoor bioaerosols

Indoor exposures to allergens, mold spores, and endotoxin have been suggested as etiological agents of asthma; therefore, accurate determination of those exposures, especially in young children (6-36 months), is important for understanding the development of asthma. Because use of personal sampling equipment in this population is difficult, and in children <1 year of age impossible, we developed a personal sampling surrogate: the Pretoddler Inhalable Particulate Environmental Robotic (PIPER) sampler to better estimate their exposures. During sampling, PIPER simulates the activity patterns, speed of motion, and the height of the breathing zones of young children, and mechanically resuspends the deposited dust just as a young child does during running and crawling. The concentrations of allergens, mold spores, and endotoxin measured by PIPER were compared to those measured using traditional stationary air sampling method in 75 homes in central New Jersey, United States. Endotoxin was detected in all homes with median concentrations of 1.0 and 0.55 EU/m(3) for PIPER and stationary sampler, respectively. The difference in median concentrations obtained using the two methods was statistically significant for homes with carpeted floors (P = 0.0001) in the heating season. For such homes, the average ratio of endotoxin concentration measured by PIPER to the stationary sampler was 2.96 (95% CI 2.29-3.63). Fungal spores were detected in all homes, with median fungal concentrations of 316 and 380 spores/m(3) for PIPER and stationary sampler, respectively. For fungi, the difference between the two sampling methods was not statistically significant. For both sampling methods, the total airborne mold levels were statistically significantly higher in the non-heating season than in the heating season. Allergens were detected in ~15% of investigated homes. The data indicate that the traditional stationary air-sampling methods may substantially underestimate personal exposures to endotoxin, especially due to resuspension of dust from carpeted floor surfaces. A personal sampling surrogate, such as PIPER, is a feasible approach to estimate personal exposures in young children. PIPER should be seriously considered as the sampling platform for future exposure studies in young children. PRACTICAL IMPLICATIONS: This study investigated potential indoor bioaerosol exposure of young children using a Pretoddler Inhalable Particulate Environmental Robotic (PIPER) sampler platform. The results show that the traditional stationary air-sampling methods can substantially underestimate personal exposures to resuspended material, and that a personal sampling surrogate, such as PIPER, offers a feasible surrogate for measuring personal inhalation exposures of young children.     

Decomposition of high protein aquaculture feed under variable oxic conditions

The microbial decomposition of nitrogen-rich organic matter in aquaculture ponds is affected by the oxic-anoxic conditions gradient at the soil-water interface as well as by resuspension practices. To investigate these interactions, the decomposition of a 49% protein fish feed was analyzed in 10 marine lab-scale systems with different exposures to aerobic and anaerobic conditions. The degree of coupling between oxic and anoxic conditions in the system had a strong effect on product accumulation and loss from the culture system. Pure oxic or anoxic conditions proved to be less favorable than mixed aerobic-anaerobic systems with respect to the metabolites accumulated in the system. Short 15-min resuspension events and a continuous alternation of oxic and anoxic conditions at 12-h time intervals proved to be the best options to minimize the accumulation of organic matter in the systems. The correct coupling of aerobic-anaerobic conditions in space and time is a key to maintain a good water quality condition for the cultured animals and also to improve the decomposition and recycling of organic matter, reducing the environmental impact from the effluents.