Transport Time Scales Research Articles

Relating land surface, water table, and weathering fronts with a conceptual valve model for headwater catchments

AbstractKnowing little about how porosity and permeability are distributed at depth, we commonly develop models of groundwater by treating the subsurface as a homogeneous black box even though porosity and permeability vary with depth. One reason for this depth variation is that infiltrating meteoric water reacts with minerals to affect porosity in localized zones called reaction fronts. We are beginning to learn to map and model these fronts beneath headwater catchments and show how they are distributed. The subsurface landscapes defined by these fronts lie subparallel to the soil‐air interface but with lower relief. They can be situated above, below, or at the water table. These subsurface landscapes of reaction are important because porosity developed from weathering can control subsurface water storage. In addition, porosity often changes at the weathering fronts, and when this affects permeability significantly, the front can act like a valve that re‐orients water flowing through the subsurface. We explore controls on the positions of reaction fronts under headwater landscapes by accounting for the timescales of erosion, chemical equilibration, and solute transport. One strong control on the landscape of subsurface reaction is the land surface geometry, which is in turn a function of the erosion rate. In addition, the reaction fronts, like the water table, are strongly affected by the lithology and water infiltration rate. We hypothesize that relationships among the land surface, reaction fronts, and the water table are controlled by feedbacks that can push landscapes towards an ‘ideal hill’. In this steady state, reaction‐front valves partition water volumes into shallow and deep flowpaths. These flows dissolve low‐ and high‐solubility minerals, respectively, allowing their reaction fronts to advance at the erosion rate. This conceptualization could inform better models of subsurface porosity and permeability, replacing the black box.

Diverse responses of hydrodynamics, nutrients and algal biomass to water diversion in a eutrophic shallow lake

Water diversion has been increasingly applied to accelerate lake water exchange and alleviate urgent water crisis. However, effects of water diversion on water exchange and water quality for eutrophic lakes remain controversial. In this study, a three-dimensional hydrodynamic-water quality-sediment diagenesis model has been developed to assess effects of water diversion on hydrodynamics and water quality in eutrophic shallow Lake Wanghu. Results suggested that water diversion could dramatically promote water exchange and reduce residence time in most lake regions but its influence on water quality was diverse. A water transferring flow rate of 20–30 m3/s could reduce water age to 40–58 days during regular water diversion operation, whereas a high transferring flow rate of 100 m3/s was the best for emergency operation in late spring before the wet season. Moreover, nutrients and Chlorophyll-a exhibited notable spatial heterogeneity in improvement efficiency. Nutrients level in the donating system was a prerequisite to the relationship among water transport time scales, nutrients, and algal biomass in this eutrophic lake. During a clean water diversion, nutrients and algal biomass were positively associated with water age. However, when the donating system contained high level of nutrients, accumulated nutrients in the lake may still trigger algal bloom after a temporary relief due to flushing effect. Therefore, these water diversion strategies could be applied to guide a sustainable management of eutrophic Lake Wanghu in terms of transferring flow rate, wind fields, water quality in the donating system, transferring operation, and water diversion route.

Impact of Lagrangian transport on lower-stratospheric transport timescales in a climate model

Abstract. We investigate the impact of model trace gas transport schemes on the representation of transport processes in the upper troposphere and lower stratosphere. Towards this end, the Chemical Lagrangian Model of the Stratosphere (CLaMS) was coupled to the ECHAM/MESSy Atmospheric Chemistry (EMAC) model and results from the two transport schemes (Lagrangian critical Lyapunov scheme and flux-form semi-Lagrangian, respectively) were compared. Advection in CLaMS was driven by the EMAC simulation winds, and thereby the only differences in transport between the two sets of results were caused by differences in the transport schemes. To analyze the timescales of large-scale transport, multiple tropical-surface-emitted tracer pulses were performed to calculate age of air spectra, while smaller-scale transport was analyzed via idealized, radioactively decaying tracers emitted in smaller regions (nine grid cells) within the stratosphere. The results show that stratospheric transport barriers are significantly stronger for Lagrangian EMAC-CLaMS transport due to reduced numerical diffusion. In particular, stronger tracer gradients emerge around the polar vortex, at the subtropical jets, and at the edge of the tropical pipe. Inside the polar vortex, the more diffusive EMAC flux-form semi-Lagrangian transport scheme results in a substantially higher amount of air with ages from 0 to 2 years (up to a factor of 5 higher). In the lowermost stratosphere, mean age of air is much smaller in EMAC, owing to stronger diffusive cross-tropopause transport. Conversely, EMAC-CLaMS shows a summertime lowermost stratosphere age inversion – a layer of older air residing below younger air (an “eave”). This pattern is caused by strong poleward transport above the subtropical jet and is entirely blurred by diffusive cross-tropopause transport in EMAC. Potential consequences from the choice of the transport scheme on chemistry–climate and geoengineering simulations are discussed.

Phase-Dependent Surfactant Transport on the Microscale: Interfacial Tension and Droplet Coalescence.

Liquid-liquid emulsion systems are usually stabilized by additives, known as surfactants, which can be observed in various environments and applications such as oily bilgewater, water-entrained diesel fuel, oil production, food processing, cosmetics, and pharmaceuticals. One important factor that stabilizes emulsions is the lowered interfacial tension (IFT) between the fluid phases due to surfactants, inhibiting the coalescence. Many studies have investigated the surfactant transport behavior that leads to corresponding time-dependent lowering of the IFT. For example, the rate of IFT decay depends on the phase in which the surfactant is added (dispersed vs continuous) due in part to differences in the near-surface depletion depth. Other key factors, such as the viscosity ratio between the dispersed and continuous phases and Marangoni stress, will also have an impact on surfactant transport and therefore the coalescence and emulsion stability. In this feature article, the measurement techniques for dynamic IFT are first reviewed due to their importance in characterizing surfactant transport, with a specific focus on macroscale versus microscale techniques. Next, equilibrium isotherm models as well as dynamic diffusion and kinetic equations are discussed to characterize the surfactant and the time scale of the surfactant transport. Furthermore, recent studies are highlighted showing the different IFT decay rates and its long-time equilibrium value depending on the phase into which the surfactant is added, particularly on the microscale. Finally, recent experiments using a hydrodynamic Stokes trap to investigate the impact of interfacial surfactant transport, or "mobility", and the phase containing the surfactant on film drainage and droplet coalescence will be presented.

A method for studying pico to microsecond time-resolved core-level spectroscopy used to investigate electron dynamics in quantum dots

Time-resolved photoelectron spectroscopy can give insights into carrier dynamics and offers the possibility of element and site-specific information through the measurements of core levels. In this paper, we demonstrate that this method can access electrons dynamics in PbS quantum dots over a wide time window spanning from pico- to microseconds in a single experiment carried out at the synchrotron facility BESSY II. The method is sensitive to small changes in core level positions. Fast measurements at low pump fluences are enabled by the use of a pump laser at a lower repetition frequency than the repetition frequency of the X-ray pulses used to probe the core level electrons: Through the use of a time-resolved spectrometer, time-dependent analysis of data from all synchrotron pulses is possible. Furthermore, by picosecond control of the pump laser arrival at the sample relative to the X-ray pulses, a time-resolution limited only by the length of the X-ray pulses is achieved. Using this method, we studied the charge dynamics in thin film samples of PbS quantum dots on n-type MgZnO substrates through time-resolved measurements of the Pb 5d core level. We found a time-resolved core level shift, which we could assign to electron injection and charge accumulation at the MgZnO/PbS quantum dots interface. This assignment was confirmed through the measurement of PbS films with different thicknesses. Our results therefore give insight into the magnitude of the photovoltage generated specifically at the MgZnO/PbS interface and into the timescale of charge transport and electron injection, as well as into the timescale of charge recombination at this interface. It is a unique feature of our method that the timescale of both these processes can be accessed in a single experiment and investigated for a specific interface.

Strong hydroclimatic controls on vulnerability to subsurface nitrate contamination across Europe

Subsurface contamination due to excessive nutrient surpluses is a persistent and widespread problem in agricultural areas across Europe. The vulnerability of a particular location to pollution from reactive solutes, such as nitrate, is determined by the interplay between hydrologic transport and biogeochemical transformations. Current studies on the controls of subsurface vulnerability do not consider the transient behaviour of transport dynamics in the root zone. Here, using state-of-the-art hydrologic simulations driven by observed hydroclimatic forcing, we demonstrate the strong spatiotemporal heterogeneity of hydrologic transport dynamics and reveal that these dynamics are primarily controlled by the hydroclimatic gradient of the aridity index across Europe. Contrasting the space-time dynamics of transport times with reactive timescales of denitrification in soil indicate that ~75% of the cultivated areas across Europe are potentially vulnerable to nitrate leaching for at least one-third of the year. We find that neglecting the transient nature of transport and reaction timescale results in a great underestimation of the extent of vulnerable regions by almost 50%. Therefore, future vulnerability and risk assessment studies must account for the transient behaviour of transport and biogeochemical transformation processes.

A Modeling Study on the Influence of Sea-Level Rise and Channel Deepening on Estuarine Circulation and Dissolved Oxygen Levels in the Tidal James River, Virginia, USA

The impact of channel deepening and sea-level rise on the environmental integrity of an estuary is investigated using a three-dimensional hydrodynamic-eutrophication model. The model results show that dissolved oxygen (DO) only experienced minor changes, even when the deep channel was deepened by 3 m in the mesohaline and polyhaline regions of the James River. We found that vertical stratification decreased DO aeration while the estuarine gravitational circulation increased bottom DO exchange. The interactions between these two processes play an important role in modulating DO. The minor change in DO due to channel deepening indicates that the James River is unique as compared with other estuaries. To understand the impact of the hydrodynamic changes on DO, both vertical and horizontal transport timescales represented by water age were used to quantify the changes in hydrodynamic conditions and DO variation, in addition to traditional measures of stratification and circulation. The model results showed that channel deepening led to an increase in both gravitational circulation strength and vertical stratification. Saltwater age decreased and vertical exchange time increased with increases in channel depth. However, these two physical processes can compensate each other, resulting in minor changes in DO. A comparison of the impact of a sea-level rise of 1.0 m with channel deepening scenarios was conducted. As the sea level rises, the vertical transport time decreases slightly while the strength of gravitational circulation weakens due to an increase in mean water depth. Consequently, DO in the estuary experiences a moderate decrease.

Estimation of riverine sediment fate and transport timescales in a wide estuary with multiple sources

Sediment retention time in estuaries plays a significant role in the morphological evolution and estuarine ecosystem. In this study, we used a well calibrated numerical model of the Pearl River Estuary (PRE) and examined its sediment budget, transport processes, and transport timescales as regards to sediment discharge from four outlets. The sediment transport timescales are characterized by sediment age, which means the time elapsed since sediment particles enter into the estuary. The wide PRE (with a Kelvin number = 1.4–7.8) allows river plumes to develop inside the estuary, which is a controlling factor on sediment dispersion. Sediments from the estuary head are initially dispersed downstream when flushed from the outlets. However, a large portion of them are carried to the low-salinity region owing to the development of freshwater plume. These sediments therefore have a larger sediment age than the remainder, which are transported by energetic tidal currents via the high-salinity region. Most riverine sediments from the lateral outlets are constrained in the low-salinity region on the West Shoal. The sediments deposit near the mouths of the lateral outlets because of weak dynamics on the shoal. An additional time is therefore needed for the sediments from the lateral outlets to be transported outside of the estuary. Because of the sediment transport pattern in the estuary, a large portion (58%) of the total riverine sediments ultimately accumulates on the West Shoal. The sediment settling velocity greatly affects the sediment age which ranges between 10 days for sediment with a settling velocity of 0.05 mm/s to approximately 110 days for sediment with a settling velocity of 0.2 mm/s. These findings regarding the fate and transport timescales of riverine sediments have implications for ecosystem managements.

Evaluating Domestic Well Vulnerability to Contamination From Unconventional Oil and Gas Development Sites

AbstractThe rapid expansion of unconventional oil and gas development (UD), made possible by horizontal drilling and hydraulic fracturing, has triggered concerns over groundwater contamination and public health risks. To improve our understanding of the risks posed by UD, we develop a physically based, spatially explicit framework for evaluating groundwater well vulnerability to aqueous phase contaminants released from surface spills and leaks at UD well pad locations. The proposed framework utilizes the concept of capture probability and incorporates decision‐relevant planning horizons and acceptable risks to support goal‐oriented modeling for groundwater protection. We illustrate the approach in northeastern Pennsylvania, where a high intensity of UD activity overlaps with local dependence on domestic groundwater wells. Using two alternative models of the bedrock aquifer and a precautionary paradigm to integrate their results, we found that most domestic wells in the domain had low vulnerability as the extent of their modeled probabilistic capture zones were smaller than distances to the nearest existing UD well pad. We also found that simulated capture probability and vulnerability were most sensitive to the model parameters of matrix hydraulic conductivity, porosity, pumping rate, and the ratio of fracture to matrix conductivity. Our analysis demonstrated the potential inadequacy of current state‐mandated setback distances that allow UD within the boundaries of delineated capture zones. The proposed framework, while limited to aqueous phase contamination, emphasizes the need to incorporate information on flow paths and transport timescales into policies aiming to protect groundwater from contamination by UD.

Molecular Origins of the Barriers to Proton Transport in Acidic Aqueous Solutions.

The self-consistent iterative multistate empirical valence bond (SCI-MS-EVB) method is used to analyze the structure, thermodynamics, and dynamics of hydrochloric acid solutions. The reorientation time scales of irreversible proton transport are elucidated by simulating 0.43, 0.85, 1.68, and 3.26 M HCl solutions at 270, 285, 300, 315, and 330 K. The results indicate increased counterion pairing with increasing concentration, which manifests itself via a reduced hydronium oxygen-chloride (O*-Cl) structuring in the radial distribution functions. Increasing ionic concentration also reduces the diffusion of the hydrated excess protons, principally by reducing the contribution of the Grotthuss proton hopping (shuttling) mechanism to the overall diffusion process. In agreement with prior experimental findings, a decrease in the activation energy of reorientation time scales was also observed, which is explicitly explained by using activated rate theory and an energy-entropy decomposition of the state-averaged radial distribution functions. These results provide atomistic verification of suggestions from recent two-dimensional infrared spectroscopy experiments that chloride anions (as opposed to hydrated excess protons) create entropic barriers to proton transport.

Direct characterization of solute transport in unsaturated porous media using fast X-ray synchrotron microtomography

Solute transport in unsaturated porous materials is a complex process, which exhibits some distinct features differentiating it from transport under saturated conditions. These features emerge mostly due to the different transport time scales at different regions of the flow network, which can be classified into flowing and stagnant regions, predominantly controlled by advection and diffusion, respectively. Under unsaturated conditions, the solute breakthrough curves show early arrivals and very long tails, and this type of transport is usually referred to as non-Fickian. This study directly characterizes transport through an unsaturated porous medium in three spatial dimensions at the resolution of 3.25 μm and the time resolution of 6 s. Using advanced high-speed, high-spatial resolution, synchrotron-based X-ray computed microtomography (sCT) we obtained detailed information on solute transport through a glass bead packing at different saturations. A large experimental dataset (>50 TB) was produced, while imaging the evolution of the solute concentration with time at any given point within the field of view. We show that the fluids' topology has a critical signature on the non-Fickian transport, which yet needs to be included in the Darcy-scale solute transport models. The three-dimensional (3D) results show that the fully mixing assumption at the pore scale is not valid, and even after injection of several pore volumes the concentration field at the pore scale is not uniform. Additionally, results demonstrate that dispersivity is changing with saturation, being twofold larger at the saturation of 0.52 compared to that at the fully saturated domain.

Investigations of positive streamers as quasi-steady structures using reduced order models

Single streamers are currently well simulated using detailed computational models. Most of these models are inhibitively complex to use for modelling many-streamer interactions in a streamer corona. This work develops reduced order models of single positive streamers in atmospheric pressure air that replicate the core macroscopic behaviour of detailed models while using a simpler physics representation. Models are developed using the 1.5D framework, with emphasis placed on solving the equations of motion in the streamer reference frame. The solution in this quasi-steady frame is shown to be a good representation of the instantaneous state of the streamer. Finally, a method of uniquely characterizing the instantaneous state of a streamer using its macroscopic parameters (velocity, radius, tip electric field and channel electric field) is developed. This characterization is interpreted graphically, with streamers treated as quasi-steady structures which evolve in time at a rate much slower than the time scale of electron transport. Previous work in the literature is shown to be well captured by this interpretation.

Modelling Study of Transport Time Scales for a Hyper-Tidal Estuary

This paper presents a study of two transport timescales (TTS), i.e., the residence time and exposure time, of a hyper-tidal estuary using a widely used numerical model. The numerical model was calibrated against field measured data for various tidal conditions. The model simulated current speeds and directions generally agreed well with the field data. The model was then further developed and applied to study the two transport timescales, namely the exposure time and residence time for the hyper-tidal Severn Estuary. The numerical model predictions showed that the inflow from the River Severn under high flow conditions reduced the residence and exposure times by 1.5 to 3.5% for different tidal ranges and tracer release times. For spring tide conditions, releasing a tracer at high water reduced the residence time and exposure time by 49.0% and 11.9%, respectively, compared to releasing the tracer at low water. For neap tide conditions, releasing at high water reduced the residence time and exposure time by 31.6% and 8.0%, respectively, compared to releasing the tracer at low water level. The return coefficient was found to be vary between 0.75 and 0.88 for the different tidal conditions, which indicates that the returning water effects for different tidal ranges and release times are all relatively high. For all flow and tide conditions, the exposure times were significantly greater than the residence times, which demonstrated that there was a high possibility for water and/or pollutants to re-enter the Severn Estuary after leaving it on an ebb tide. The fractions of water and/or pollutants re-entering the estuary for spring and neap tide conditions were found to be very high, giving 0.75–0.81 for neap tides, and 0.79–0.88 for spring tides. For both the spring and neap tides, the residence and exposure times were lower for high water level release. Spring tide conditions gave significantly lower residence and exposure times. The spatial distribution of exposure and residence times showed that the flow from the River Severn only had a local effect on the upstream part of the estuary, for both the residence and exposure time.

Observed Hemispheric Asymmetry in Stratospheric Transport Trends From 1994 to 2018

AbstractTotal columns of the trace gases nitric acid (HNO3) and hydrogen chloride (HCl) are sensitive to variations in the lower stratospheric age of air, a quantity that describes transport time scales in the stratosphere. Analyses of HNO3 and HCl columns from the Network for the Detection of Atmospheric Composition Change panning 77°S to 79°N have detected changes in the extratropical stratospheric transport circulation from 1994 to 2018. The HNO3 and HCl analyses combined with the age of air from a simulation using the MERRA2 reanalysis show that the Southern Hemisphere lower stratosphere has become 1 month/decade younger relative to the Northern Hemisphere, largely driven by the Southern Hemisphere transport circulation. The analyses reveal multiyear anomalies with a 5‐ to 7‐year period driven by interactions between the circulation and the quasi‐biennial oscillation in tropical winds. This hitherto unrecognized variability is large relative to hemispheric transport trends and may bias ozone trend regressions.

The legacy and drivers of groundwater nutrients and pesticides in an agriculturally impacted Quaternary aquifer system

Nutrient and pesticide pollution are among the major threats to groundwater quality in agriculturally impacted aquifers. Understanding their legacy effects and drivers are important to protect aquifers from exposures to contamination. However, the complexities of groundwater flowpaths make it difficult to predict the time-scales of groundwater flow and contaminant transport. To determine these controls of groundwater nutrient and pesticides in an aquifer system underlying an intensive agricultural area in the Great Barrier Reef catchment, Australia, we sampled tritium (3H) to estimate groundwater-age, nutrient and pesticide concentrations to investigate groundwater contamination, and nitrogen (ẟ15N-NO3−) and oxygen (ẟ18O-NO3−) isotopes to determine groundwater nitrate dynamics. We, then, constructed high-resolution 3D geological and groundwater flow models of the aquifer system to determine the role of the geologic heterogeneity on the observed nutrient and pesticide concentrations. Groundwater 3H derived ages, and nutrient and pesticide concentrations did not follow distinct spatial trends. ẟ15N-NO3− and ẟ18O-NO3− values indicated that nitrification and denitrification processes influenced nitrate dynamics in the aquifer system; however, they were not solely able to explain the entire 3D variability. The 3D geologic modelling identified possible preferential flowpaths and perched systems, which helped to explain the observed groundwater-age, nutrient and pesticide variabilities. Old-groundwater (~100-years) was found in shallow depths (<15 m) where perched systems were identified. In areas with preferential flowpaths, young-groundwater (⁓1-year) with significant nitrate (~12 mg-N/L) and pesticides (up to 315 ng/L) concentrations were detected at deeper depths (>25 m), below perched and locally confined systems. Downward increasing groundwater-age, and decreasing nutrient and pesticide concentrations were detected in the unconfined aquifer, while old-groundwater (~160-years) and lower nitrate (<3 mg-N/L) and pesticides (<2 ng/L) concentrations were detected in the confined systems. This study demonstrates the importance of understanding both the geology and the hydrogeology of an area before deploying monitoring studies and/or making conclusions from tritium, nutrient and pesticide data alone.

Power balance investigation in long-pulse high-performance discharges with ITER-like tungsten divertor on EAST

The Experimental Advanced Superconducting Tokamak (EAST) research program concentrates on demonstrating steady-state high-performance H-mode operations with ITER-like tungsten divertor. Calorimetry was applied to actively water-cool the plasma facing components (PFCs) by increasing the water temperature for power balance investigation. Considering the energy balance of EAST long-pulse high-performance discharges with upper single null (USN) configuration, thus far, approximately 78% of the injected energy could be accounted for. The method of estimation of heat flux on upper tungsten divertor target with a high time- and spatial-resolved infrared camera has been developed, and the sum of its heat load was found to be significantly consistent with that measured through calorimetry. The record longest steady-state H-mode plasma #73 999 was sustained for up to 101.2 s with net injected energy exceeding ∼0.25 GJ in the USN configuration. Heat load analysis of this discharge using calorimetric measurement indicates that the modification of heat load distribution was observed and this was induced by a slight change in the magnetic configuration. Not all temperature increments in the five cooling water modules reached the saturated state for the 100 s level discharge, which means that 100 s timescales are insufficient as compared to the thermal transport timescale in the targeted PFCs. The heat load on the tungsten divertor targets is not evenly distributed with the ratio of ∼2 in favour of the outer divertor.The experimental results and analysis of the physics involved in these USN configuration discharges are reported and discussed.

Multiscale Simulation Reveals Passive Proton Transport Through SERCA on the Microsecond Timescale

The sarcoplasmic reticulum Ca2+-ATPase (SERCA) transports two Ca2+ ions from the cytoplasm to the reticulum lumen at the expense of ATP hydrolysis. In addition to transporting Ca2+, SERCA facilitates bidirectional proton transport across the sarcoplasmic reticulum to maintain the charge balance of the transport sites and to balance the charge deficit generated by the exchange of Ca2+. Previous studies have shown the existence of a transient water-filled pore in SERCA that connects the Ca2+ binding sites with the lumen, but the capacity of this pathway to sustain passive proton transport has remained unknown. In this study, we used the multiscale reactive molecular dynamics method and free energy sampling to quantify the free energy profile and timescale of the proton transport across this pathway while also explicitly accounting for the dynamically coupled hydration changes of the pore. We find that proton transport from the central binding site to the lumen has a microsecond timescale, revealing a novel passive cytoplasm-to-lumen proton flow beside the well-known inverse proton countertransport occurring in active Ca2+ transport. We propose that this proton transport mechanism is operational and serves as a functional conduit for passive proton transport across the sarcoplasmic reticulum.

A Comprehensive Lagrangian Transport Study in A Long-Narrow Bay, Xiangshan Bay, China

Study on the transport and mixing in coastal waters is of great concern to the ocean resources exploitation and ecological system protection. Lagrangian methods are direct and effective of researching mass transport. Two Lagrangian tools were adopted and combined to describe water transport in a long-narrow bay, Xiangshan Bay, China. Based on the fields of tidal velocity simulated from the 3-D hydrodynamic model, Lagrangian Coherent Structures (LCSs) and synoptic Lagrangian maps (SLMs) were calculated in the study area. Through comparison of the results, the features and relation of the two tools were discussed. The results show that the LCSs act as the separatrix of the water regions with different transport characteristics and can identify the water areas with different transport time scales. The comprehensive application of the Lagrangian tools is helpful to obtain more insight into the water transport process.

Numerical study of sediment transport time scales in an ebb-dominated waterway

Understanding the transport time scales for sediment in an estuary is important to study the retention of pollutants, such as heavy metals, that adsorb to sediment. In this study, a three-dimensional hydrodynamic and sediment transport model was developed to investigate the transport time scales of sediment in the Modaomen Waterway (MW), China. The results show that approximately 100 and 240 days are required for fine-grained (settling velocity, ωs = 0.2 mm/s) and coarse-grained sediment (ωs = 0.5 mm/s) to transport from the headwater to the estuary mouth, respectively. Unlike dissolved materials, sediment undergoes intermittent settling and resuspension, which are key processes that determine the sediment age. The sediment stays longer at the seabed than suspended in the water column. The sediment has more potential to be eroded from the seabed in the freshwater region because of the strong bottom shear stress. These eroded sediments can be rapidly diffused to the upper water column and advected seaward. In contrast, sediment tends to deposit within the turbidity maximum zone due to low bottom shear stress and weak turbulent mixing. Downstream of the turbidity maximum, turbulent mixing is further suppressed by density stratification and the eroded sediment is rarely diffused to the upper water column. As a result, sediment travels more rapidly in the freshwater region than in the brackish zone. The sediment in the upper water column is mostly advected from upstream, and the difference in the sediment ages near the surface and at the bottom can reach 40 days for the coarse-grained sediment. The sediment age has an effect on water quality because a long sediment retention time facilitates the accumulation of pollutants in the MW.

Timescales of transport through Lower Green Bay

Contaminated sediments, poor water quality, and lost or altered habitat in Lower Green Bay and Fox River in the 1980s led to its listing as an Area of Concern by the International Joint Commission. Previous studies on the geophysics and health of the bay demonstrated the need for estimates of transport timescales. Such estimates can contribute to improved understanding of the transport and fate of nutrients, contaminants, and biogeochemical processes in the bay. This study reviews definitions of residence time and flushing time, estimates the temporal distribution of flushing time, the spatial and temporal distributions of residence time, and horizontal diffusivities. The study used a previously developed hydrodynamic model, a drifter experiment, a lake particle transport model, and methods appropriate for the estimation of transport timescales. The estimated residence time for lower Green Bay, 56 ± 16 days, is smaller than a previous estimate of 190 days for the whole bay, and had values similar to the estimated flushing times. The closeness between those timescales demonstrates the important role of water exchange across the Chambers Island transect. Flushing time and residence time do not follow the same trends in monthly variability because the former depends directly on water exchange across Chambers Island, while the later depends, in addition, on tributary inflows and the circulation patterns in the bay. The study includes a discussion of the relations between the timescales of transport and the previous studies of biogeochemical processes, such as trophic conditions, spatial distribution of cyanobacteria, cold-water intrusions, and hypoxia.