Estimated Residence Time Research Articles

Groundwater recharge and evolution of water quality in China’s Jilantai Basin based on hydrogeochemical and isotopic evidence

We investigated major ions, stable isotopes, and radiocarbon dates in a Quaternary aquifer in semi-arid northwestern China to gain insights into groundwater recharge and evolution. Most deep and shallow groundwater in the Helan Mountains was fresh, with total dissolved solids <1,000 mg L−1 and Cl− <250 mg L−1. The relationships of major ions with Cl− suggest strong dissolution of evaporites. However, dissolution of carbonates, albite weathering, and ion exchange are also the major groundwater process in Jilantai basin. The shallow desert groundwater is enriched in δ18O and intercepts the local meteoric water line at δ18O = −13.4 ‰, indicating that direct infiltration is a minor recharge source. The isotope compositions in intermediate confined aquifers resemble those of shallow unconfined groundwater, revealing that upward recharge from intermediate formations is a major source of shallow groundwater in the plains and desert. The estimated residence time of 10.0 kyr at one desert site, indicating that some replenishment of desert aquifers occurred in the late Pleistocene and early Holocene with a wetter and colder climate than at present.

Source, transport and fluxes of Amazon River particulate organic carbon: Insights from river sediment depth-profiles

In order to reveal particulate organic carbon (POC) source and mode of transport in the largest river basin on Earth, we sampled the main sediment-laden tributaries of the Amazon system (Solimões, Madeira and Amazon) during two sampling campaigns, following vertical depth-profiles. This sampling technique takes advantage of hydrodynamic sorting to access the full range of solid erosion products transported by the river. Using the Al/Si ratio of the river sediments as a proxy for grain size, we find a general increase in POC content with Al/Si, as sediments become finer. However, the sample set shows marked variability in the POC content for a given Al/Si ratio, with the Madeira River having lower POC content across the measured range in Al/Si. The POC content is not strongly related to the specific surface area (SSA) of the suspended load, and bed sediments have a much lower POC/SSA ratio. These data suggest that SSA exerts a significant, yet partial, control on POC transport in Amazon River suspended sediment. We suggest that the role of clay mineralogy, discrete POC particles and rock-derived POC warrant further attention in order to fully understand POC transport in large rivers.To examine the source of POC in the Amazon Basin in more detail, we use radiocarbon (14C) content and the stable isotope composition (δ13C) of POC. 14C activity demonstrates that rock-derived POC is a significant component of river bed sediments and contributes to POC across the river depth-profiles of suspended sediments. We estimate that the flux of rock-derived POC may reach ∼10% of the total POC export by the Amazon River. After correcting for rock-derived POC input, we find that POC from the terrestrial biosphere (biospheric POC) is mostly sourced from C3-plants. Higher biospheric POC δ13C values in the Madeira River (by ∼0.5–1‰) are best explained by a small (<5%) contribution of C4-grasses from Bolivian savannas. Finally, we use 14C to estimate the mean age of biospheric POC exported from the Amazon Basin. The data show that biospheric POC is younger in the Solimões River (1120years) than in the Madeira River (2850 years). These ages are much younger than the corresponding estimates of sediment residence time in the basin, suggesting that lowland areas and/or young POC from above-ground biomass may contribute disproportionately to the biomarker signals in terrigenous sediments of the Amazon delta.

Understanding and modelling dissolved gas transport in the bedrock of three Fennoscandian sites

• Gas fluxes at three Fennoscandian sites have been estimated using Fick’s law. • Data have been reinterpreted to account for in situ gas production. • A new analytical solution allows different sources of gas to be discriminated. • The new solution has been used to estimate the groundwater residence time at Olkiluoto. The origin and transport of dissolved gases in the geosphere is of interest for assessment studies of nuclear waste repositories. In this paper, we analyse available field measurements of helium, methane and hydrogen at three Fennoscandian sites: Forsmark and Laxemar in Sweden and Olkiluoto in Finland. The field data are interpreted using different analytical models all based on the one-dimensional diffusion equation. The results of the different models provide estimates about the amount of deep gas flux, the in situ production and the groundwater residence time of the considered sites. The computed helium fluxes, which fall within the lower range of crustal degassing fluxes reported by Torgersen (2010) , are strictly related with the high tightness of the considered fracture media. The very high estimates of groundwater residence time indicate that, at the considered depths, there are only very few flowing fractures while in the rest of the fractured domain groundwater has been almost motionless during a whole glacial cycle.

Investigation of young water inflow in karst aquifers using SF6–CFC–3H/He–85Kr–39Ar and stable isotope components

Karst aquifers are known for their wide distribution of water transfer velocities. From this observation, a multiple geochemical tracer approach seems to be particularly well suited to provide a significant assessment of groundwater flows, but the choice of adapted tracers is essential. In this study, several common tracers in karst aquifers such as physicochemical parameters, major ions, stable isotopes, and δ13C to more specific tracers such as dating tracers – 14C, 3H, 3H–3He, CFC-12, SF6 and 85Kr, and 39Ar – were used, in a fractured karstic carbonated aquifer located in Burgundy (France). The information carried by each tracer and the best sampling strategy are compared on the basis of geochemical monitoring done during several recharge events and over longer time periods (months to years).This study’s results demonstrate that at the seasonal and recharge event time scale, the variability of concentrations is low for most tracers due to the broad spectrum of groundwater mixings. The tracers used traditionally for the study of karst aquifers, i.e., physicochemical parameters and major ions, efficiently describe hydrological processes such as the direct and differed recharge, but require being monitored at short time steps during recharge events to be maximized. From stable isotopes, tritium, and Cl− contents, the proportion of the fast direct recharge by the largest porosity was estimated using a binary mixing model. The use of tracers such as CFC-12, SF6, and 85Kr in karst aquifers provides additional information, notably an estimation of apparent age, but they require good preliminary knowledge of the karst system to interpret the results suitably. The CFC-12 and SF6 methods efficiently determine the apparent age of baseflow, but it is preferable to sample the groundwater during the recharge event. Furthermore, these methods are based on different assumptions such as regional enrichment in atmospheric SF6, excess air, and flow models among others. 85Kr and 39Ar concentrations can potentially provide a more direct estimation of groundwater residence time. Conversely, the 3H–3He method is inefficient in the karst aquifer for dating due to 3He degassing.

Residence Times and Decay Rates of Downed Woody Debris Biomass/Carbon in Eastern US Forests

A key component in describing forest carbon (C) dynamics is the change in downed dead wood biomass through time. Specifically, there is a dearth of information regarding the residence time of downed woody debris (DWD), which may be reflected in the diversity of wood (for example, species, size, and stage of decay) and site attributes (for example, climate) across the study region of eastern US forests. The empirical assessment of DWD rate of decay and residence time is complicated by the decay process itself, as decomposing logs undergo not only a reduction in wood density over time but also reductions in biomass, shape, and size. Using DWD repeated measurements coupled with models to estimate durations in various stages of decay, estimates of DWD half-life (THALF), residence time (TRES), and decay rate (k constants) were developed for 36 tree species common to eastern US forests. Results indicate that estimates for THALF averaged 18 and 10 years for conifers and hardwoods, respectively. Species that exhibited shorter THALF tended to display a shorter TRES and larger k constants. Averages of TRES ranged from 57 to 124 years for conifers and from 46 to 71 years for hardwoods, depending on the species and methodology for estimating DWD decomposition considered. Decay rate constants (k) increased with increasing temperature of climate zones and ranged from 0.024 to 0.040 for conifers and from 0.043 to 0.064 for hardwoods. These estimates could be incorporated into dynamic global vegetation models to elucidate the role of DWD in forest C dynamics.

Prediction of Residence Time Distribution of Solid Wastes in a Rotary Dryer

The residence time distribution (RTD) of heterogeneous citrus waste particles was determined in a semi-pilot-scale rotary dryer with concurrent flow under several operational conditions. The experimental methodology was based on the stimulus-response technique, which consisted of injecting pulse-like tracers in the dryer feed stream. Measurements of RTD were performed to build up experimental curves that were numerically integrated to provide the mean residence time. A perfectly-stirred-tank in series model and a neural network model were derived. In addition, empirical and semi-empirical correlations from the literature were used to estimate residence time and the influence of operating conditions on this variable was investigated.

Impact of Data Density and Geostatistical Simulation Technique on the Estimation of Residence Times in a Synthetic Two-dimensional Aquifer

Connectivity patterns of heterogeneous porous media are important in the estimation of groundwater residence time distributions (RTDs). Understanding the connectivity patterns of a hydraulic conductivity ( $$K$$ ) field often requires knowledge of the entire aquifer, which is not practical. As such, the method used to estimate unknown $$K$$ values using known $$K$$ values is important. This study investigates how varying levels of conditioning data and four simulation techniques, one multi-Gaussian and three multi-point, are able to recreate key $$K$$ field features and connectivity patterns of a synthetic two-dimensional bimodal distributed ln( $$K$$ ) field with highly connected high $$K$$ features. These techniques are then assessed in the context of RTD estimation. It was found that the multi-Gaussian technique presented a bias towards earlier travel times with increased conditioning data. This was due to the inability of the method to recreate multiple scales of connecting features. Of the multi-point methods investigated, the facies method was unable to predict early arrival times. The use of a continuous variable training image produced good fits to the observed residence time distribution with a high number of conditioning points. The ability of the methods to predict the shape of residence time distributions appears to be related to their ability to reproduce the connection patterns of higher $$K$$ features.

Process inhomogeneity leads to rapid side product turnover in cultivation of Corynebacterium glutamicum

BackgroundCorynebacterium glutamicum has large scale industrial applications in the production of amino acids and the potential to serve as a platform organism for new products. This means the demand for industrial process development is likely to increase. However, large scale cultivation conditions differ from laboratory bioreactors, mostly due to the formation of concentration gradients at the industrial scale. This leads to an oscillating supply of oxygen and nutrients for microorganisms with uncertain impact on metabolism. Scale-down bioreactors can be applied to study robustness and physiological reactions to oscillating conditions at a laboratory scale.ResultsIn this study, C. glutamicum ATCC13032 was cultivated by glucose limited fed-batch cultivation in a two-compartment bioreactor consisting of an aerobic stirred tank and a connected non-aerated plug flow reactor with optional feeding. Continuous flow through both compartments generated oscillating profiles with estimated residence times of 45 and 87 seconds in the non-aerated plug flow compartment. Oscillation of oxygen supply conditions at substrate excess and oscillation of both substrate and dissolved oxygen concentration were compared to homogeneous reference cultivations. The dynamic metabolic response of cells within the anaerobic plug flow compartment was monitored throughout the processes, detecting high turnover of substrate into metabolic side products and acidification within oxygen depleted zones. It was shown that anaerobic secretion of lactate into the extracellular culture broth, with subsequent reabsorption in the aerobic glucose-limited environment, leads to mixed-substrate growth in fed-batch processes. Apart from this, the oscillations had only a minor impact on growth and intracellular metabolite characteristics.ConclusionsCarbon metabolism of C. glutamicum changes at oscillating oxygen supply conditions, leading to a futile cycle over extracellular side products and back into oxidative pathways. This phenomenon facilitates a dynamic and flexible shift of oxygen uptake at inhomogeneous process conditions. There is no loss of process characteristics at oscillation times in the minute range, which emphasizes the robustness of C. glutamicum in comparison to other industrial microorganisms. Therefore, the metabolic phenotype of C. glutamicum seems to be particularly well-suited for cultivation at inhomogeneous process conditions for large-scale fed-batch application, which is in good accordance with the respective industrial experiences.

Assimilation of preexisting Pleistocene intrusions at Long Valley by periodic magma recharge accelerates rhyolite generation: rethinking the remelting model

Rhyolite flows and tuffs from the Long Valley area of California, which were erupted over a two-million-year time period, exhibit systematic trends in Nd, Hf, and Pb isotopes, trace element composition, erupted volume, and inferred magma residence time that provide evidence for a new model for the production of large volumes of silica-rich magma. Key constraints come from geochronology of zircon crystal populations combined with a refined eruption chronology from Ar–Ar geochronology; together these data give better estimates of magma residence time that can be evaluated in the context of changing magma compositions. Here, we report Hf, Nd, and Sr isotopes, major and trace element compositions, 40Ar/39Ar ages, and U–Pb zircon ages that combined with existing data suggest that the chronology and geochemistry of Long Valley rhyolites can be explained by a dynamic interaction of crustal and mantle-derived magma. The large volume Bishop Tuff represents the culmination of a period of increased mantle-derived magma input to the Long Valley volcanic system; the effect of this input continued into earliest postcaldera time. As the postcaldera evolution of the system continued, new and less primitive crustal-derived magmas dominated the system. A mixture of varying amounts of more mafic mantle-derived and felsic crustal-derived magmas with recently crystallized granitic plutonic materials offers the best explanation for the observed chronology, secular shifts in Hf and Nd isotopes, and the apparently low zircon crystallization and saturation temperatures as compared to Fe–Ti oxide eruption temperatures. This scenario in which transient crustal magma bodies remained molten for varying time periods, fed eruptions before solidification, and were then remelted by fresh recharge provides a realistic conceptual framework that can explain the isotopic and geochemical evidence. General relationships between crustal residence times and magma sources are that: (1) precaldera rhyolites had long crustal magma residence times and high crustal affinity, (2) the caldera-related Bishop Tuff and early postcaldera rhyolites have lower crustal affinity and short magma residence times, and (3) later postcaldera rhyolites again have stronger crustal signatures and longer magma residence times.

Isotopic approach to trace water and nutrient dynamics in forest soils

Forest soil fertility is generally estimated by studying the water and nutrient cycles essential to living organisms (i.e. biogeochemical cycles). Use of geochemical and/or isotopic tracers is an innovative, complementary approach to more traditional studies. Experimental procedures and results of some recent studies in forest ecosystems using natural tracers (e.g. 18O, 13C, 26Mg) or artificially supplied tracers (e.g. enrichment in Sr, Rb, 15N, 44Ca, 26Mg, 32P) are presented. Results are discussed in terms of relevance for using these tools to identify sources, to estimate residence times of water and elements, and to trace nutrient fluxes, of both organic and mineral origins, that are internal or external to the ecosystem.

Molecular transformation and degradation of refractory dissolved organic matter in the Atlantic and Southern Ocean

More than 90% of the global ocean dissolved organic carbon (DOC) is refractory, has an average age of 4000–6000years and a lifespan from months to millennia. The fraction of dissolved organic matter (DOM) that is resistant to degradation is a long-term buffer in the global carbon cycle but its chemical composition, structure, and biochemical formation and degradation mechanisms are still unresolved. We have compiled the most comprehensive molecular dataset of 197 Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR MS) analyses from solid-phase extracted marine DOM covering two major oceans, the Atlantic sector of the Southern Ocean and the East Atlantic Ocean (ranging from 50°N to 70°S). Molecular trends and radiocarbon dating of 34 DOM samples (comprising Δ14C values from −229‰ to −495‰) were combined to model an integrated degradation rate for bulk DOC resulting in a predicted age of >24ka for the most persistent DOM fraction. First order kinetic degradation rates for 1557 mass peaks indicate that numerous DOM molecules cycle on timescales much longer than the turnover of the bulk DOC pool (estimated residence times of up to ~100ka) and the range of validity of radiocarbon dating. Changes in elemental composition were determined by assigning molecular formulae to the detected mass peaks. The combination of residence times with molecular information enabled modelling of the average elemental composition of the slowest degrading fraction of the DOM pool. In our dataset, a group of 361 molecular formulae represented the most stable composition in the oceanic environment (“island of stability”). These most persistent compounds encompass only a narrow range of the molecular elemental ratios H/C (average of 1.17±0.13), and O/C (average of 0.52±0.10) and molecular masses (360±28 and 497±51Da). In the Weddell Sea DOC concentrations in the surface waters were low (46.3±3.3μM) while the organic radiocarbon was significantly more depleted than that of the East Atlantic, representing average surface water DOM ages of 4920±180a. These results are in accordance with a highly degraded DOM in the Weddell Sea surface water as also shown by the molecular degradation index IDEG obtained from FT-ICR MS data. Further, we identified 339 molecular formulae which probably contribute to an increased DOC concentration in the Southern Ocean and potentially reflect an accumulation or enhanced sequestration of refractory DOC in the Weddell Sea. These results will contribute to a better understanding of the persistent nature of marine DOM and its role as an oceanic carbon buffer in a changing climate.

Measurements of nitrite production in and around the primary nitrite maximum in the central California Current

Abstract. Nitrite (NO2−) is a substrate for both oxidative and reductive microbial metabolism. NO2− accumulates at the base of the euphotic zone in oxygenated, stratified open-ocean water columns, forming a feature known as the primary nitrite maximum (PNM). Potential pathways of NO2− production include the oxidation of ammonia (NH3) by ammonia-oxidizing bacteria and archaea as well as assimilatory nitrate (NO3−) reduction by phytoplankton and heterotrophic bacteria. Measurements of NH3 oxidation and NO3− reduction to NO2− were conducted at two stations in the central California Current in the eastern North Pacific to determine the relative contributions of these processes to NO2− production in the PNM. Sensitive (&lt; 10 nmol L−1), precise measurements of [NH4+] and [NO2−] indicated a persistent NH4+ maximum overlying the PNM at every station, with concentrations as high as 1.5 μmol L−1. Within and just below the PNM, NH3 oxidation was the dominant NO2− producing process, with rates of NH3 oxidation to NO2− of up to 31 nmol L−1 d−1, coinciding with high abundances of ammonia-oxidizing archaea. Though little NO2− production from NO3− was detected, potentially nitrate-reducing phytoplankton (photosynthetic picoeukaryotes, Synechococcus, and Prochlorococcus) were present at the depth of the PNM. Rates of NO2− production from NO3− were highest within the upper mixed layer (4.6 nmol L−1 d−1) but were either below detection limits or 10 times lower than NH3 oxidation rates around the PNM. One-dimensional modeling of water column NO2− production agreed with production determined from 15N bottle incubations within the PNM, but a modeled net biological sink for NO2− just below the PNM was not captured in the incubations. Residence time estimates of NO2− within the PNM ranged from 18 to 470 days at the mesotrophic station and was 40 days at the oligotrophic station. Our results suggest the PNM is a dynamic, rather than relict, feature with a source term dominated by ammonia oxidation.

Investigation of hydraulics transport time scales within the Arvand River estuary, Iran

Transport time scales are key parameters for understanding the hydrodynamic and biochemical processes within estuaries. In this study, the flushing and residence times within the Arvand River estuary have been estimated using a two-dimensional hydrodynamic model called CE-QUAL-W2. The model has been calibrated and verified by two different sets of field data and using the k-ε vertical eddy diffusivity scheme. Flushing time has been estimated using different methods such as the tidal prism and fraction of freshwater methods. Moreover, residence times have been investigated using pulse residence time, estuarine residence time and remnant function approaches. The results have shown that different methods yield different time scales, and freshwater inflow has the greatest impact upon estimation of residence time, whereas tidal circulation hardly contributes to residence time at all. It has also been shown that the neap-spring circulation and start phase of simulations have negligible effects on the Arvand's time scales. The investigation of bathymetry showed that two sills of the estuary tend to significantly increase residence time. Understanding the applicability of these time scales and their estimation approaches helps us to evaluate the water quality management of estuaries. Copyright © 2013 John Wiley & Sons, Ltd.

Escape rate: a Lagrangian measure of particle deposition from the atmosphere

Abstract. Due to rising or descending air and due to gravity, aerosol particles carry out a complicated, chaotic motion and move downwards on average. We simulate the motion of aerosol particles with an atmospheric dispersion model called the Real Particle Lagrangian Trajectory (RePLaT) model, i.e., by solving Newton's equation and by taking into account the impacts of precipitation and turbulent diffusion where necessary, particularly in the planetary boundary layer. Particles reaching the surface are considered to have escaped from the atmosphere. The number of non-escaped particles decreases with time. The short-term and long-term decay are found to be exponential and are characterized by escape rates. The reciprocal values of the short-term and long-term escape rates provide estimates of the average residence time of typical particles, and of exceptional ones that become convected or remain in the free atmosphere for an extremely long time, respectively. The escape rates of particles of different sizes are determined and found to vary in a broad range. The increase is roughly exponential with the particle size. These investigations provide a Lagrangian foundation for the concept of deposition rates.

Chylomicron metabolism in rats: kinetic modeling indicates that the particles remain at endothelial sites for minutes

Chylomicrons labeled in vivo with (14)C-oleic acid (primarily in triglycerides, providing a tracer for lipolysis) and (3)H-retinol (primarily in ester form, providing a tracer for the core lipids) were injected into rats. Radioactivity in tissues was followed at a series of times up to 40 min and the data were analyzed by compartmental modeling. For heart-like tissues it was necessary to allow the chylomicrons to enter into a compartment where lipolysis is rapid and then transfer to a second compartment where lipolysis is slower. The particles remained in these compartments for minutes and when they returned to blood they had reduced affinity for binding in the tissue. In contrast, the data for liver could readily be fitted with a single compartment for native and lipolyzed chylomicrons in blood, and there was no need for a pathway back to blood. A composite model was built from the individual tissue models. This whole-body model could simultaneously fit all data for both fed and fasted rats and allowed estimation of fluxes and residence times in the four compartments; native and lipolyzed chylomicrons ("remnants") in blood, and particles in the tissue compartments where lipolysis is rapid and slow, respectively.

Contaminant exchange rates in estuaries – New formulae accounting for advection and dispersion

The transport timescales of water in an estuary are important measurements for monitoring pollution threats to the estuarine ecosystem. In this study we re-evaluated the application of simple analytical solutions to estimate these timescales and found that the Land Ocean Interaction Costal Zone model (LOICZ) uses similar equation as from the fresh water fraction model, and thus often resulting in shortened transport timescales. Therefore, the LOICZ model is neither based upon the well-known Knudsen relation nor Fischer formulation. Three transport timescales, namely water renewal, residence time and exposure time were calculated using analytical solutions for a range of estuaries worldwide. The analytical results were compared with available estimates of residence times from numerical models. The theoretical formulation from the LOICZ, the fresh water fraction model, and a newly proposed modified LOICZ model were used to calculate water renewal. Residence times and exposure times were calculated using the Constituent-oriented Age and Residence time Theory (CART). The modified LOICZ model was found to be the most comparable to residence times from numerical models, with r2∼0.7. In addition to the proposed modified LOICZ model (which uses Fischer formulation), we have developed an advection–dispersion timescale diagram. This graphic conceptual model provides a visual representation of the relative contribution of advective and dispersive processes to water renewal for different estuaries. Estuaries can be categorized as either dominated by dispersion, dominated by advection, or having dispersion and advection of similar magnitude.

Modeling environmental controls on the transport and fate of early life stages of Antarctic krill (Euphausia superba) on the western Antarctic Peninsula continental shelf

A one-dimensional, temperature-dependent model was used to simulate the descent–ascent cycle of the embryos and early larval stages of Antarctic krill to determine which regions of the western Antarctic Peninsula (wAP) continental shelf support successful completion of this cycle under present environmental conditions and those projected to occur as a result of climate change. The transport and fate of the embryo and larva under present and modified conditions was investigated with Lagrangian particle tracking simulations. The two modeling studies were implemented using temperature and density (embryo–larva model) and circulation distributions (Lagrangian particle tracking) obtained from a high resolution version of the Regional Ocean Modeling System configured for the wAP shelf region. Additional simulations used temperature and circulation distributions obtained from simulations that were forced with increased wind speed and increased transport of the Antarctic Circumpolar Current (ACC), both projected to possibly occur with climate change in the wAP region. Simulations using present conditions showed that successful completion of the descent–ascent cycle occurred along the outer shelf and on the shelf in regions with bottom depths of 600–700m. Estimated residence times for the shelf regions that support success of the embryo and larva were 20–30 days. Thus, krill spawned in the mid and inner shelf regions can be retained in these regions through development to the first feeding stage (calyptopis 1). Increased winds and ACC transport resulted in more onshelf transport of Circumpolar Deep Water (CDW), which increased the volume of warm (1–1.5°C) water at depth. These conditions supported a moderate increase in success of the krill embryo and larva, but only for limited areas of the shelf where hatching depths decreased by 10–30m (<5%) and development time to the calyptopis 1 stage decreased by 15–20%. The modified circulation conditions also supported increased advection of krill larvae into areas of the shelf that would experience the largest reduction of sea ice, especially in winter. Projected changes in wind strength and CDW transport may potentially enhance larval survival and advection onto the wAP shelf, but recruitment may be decreased by modifications to local sea ice distributions that would impede survival of Antarctic krill that overwinter on the shelf.

Study of the hydrodynamic characteristics of a free flowing liquid film in open inclined microchannels

A widely used device for continuous contacting in microdevices is the Falling Film Micro-Reactor (FFMR). The liquid films that are formed in the channels of a FFMR are quite stable and have a typical thickness of less than 100μm. In the present work, a μ-PIV system was used for measuring velocity profiles and for estimating the local liquid film thickness in three open inclined microchannels (with width of 1200, 600 and 300μm). The film thickness measurements were utilized for reconstructing the shape of the interface, while the effect of various fluid parameters (e.g. physical properties, flow rate) as well as geometrical characteristics of the conduit and the inclination angle of the microchannels, on the geometrical characteristics of the liquid phase has been also experimentally investigated. As the correlations that are valid in the macroscale are generally not applicable in the microscale, the aim of the present study is to develop a means for predicting the geometrical characteristics (i.e. liquid film thickness, shape of the gas/liquid interface) of the liquid phase, as well as its mean residence time. Generalized correlations for predicting film thickness and meniscus shape are proposed (accuracy±15%) while, by combining these correlations an estimation of the mean residence time can be also made.

A fluid-mechanics based classification scheme for surface transient storage in riverine environments: quantitatively separating surface from hyporheic transient storage

Abstract. Surface transient storage (STS) and hyporheic transient storage (HTS) have functional significance in stream ecology and hydrology. Currently, tracer techniques couple STS and HTS effects on stream nutrient cycling; however, STS resides in localized areas of the surface stream and HTS resides in the hyporheic zone. These contrasting environments result in different storage and exchange mechanisms with the surface stream, which can yield contrasting results when comparing transient storage effects among morphologically diverse streams. We propose a fluid mechanics approach to quantitatively separate STS from HTS that involves classifying and studying different types of STS. As a starting point, a classification scheme is needed. This paper introduces a classification scheme that categorizes different STS in riverine systems based on their flow structure. Eight STS types are identified and some are subcategorized based on characteristic mean flow structure: (1) lateral cavities (emergent and submerged); (2) protruding in-channel flow obstructions (backward- and forward-facing step); (3) isolated in-channel flow obstructions (emergent and submerged); (4) cascades and riffles; (5) aquatic vegetation (emergent and submerged); (6) pools (vertically submerged cavity, closed cavity, and recirculating reservoir); (7) meander bends; and (8) confluence of streams. The long-term goal is to use the classification scheme presented to develop predictive mean residence times for different STS using field-measurable hydromorphic parameters and obtain an effective STS mean residence time. The effective STS mean residence time can then be deconvolved from the transient storage residence time distribution (measured from a tracer test) to obtain an estimate of HTS mean residence time.

Radium Sampling Methods and Residence Times in St. Andrew Bay, Florida

Activity ratios (AR) of radium isotopes have been used with success to constrain estimates of water ages and to approximate residence times in coastal waters. We compared two common radium sampling methods (grab sampling and stationary moorings) to estimate water ages and the residence time of St. Andrew Bay waters in northwest Florida, USA. Both sampling methods utilize manganese dioxide fibers (“Mn fibers”) to adsorb dissolved radium from the water column. Grab samples capture radium activities at a discrete time while moorings integrate radium activities over longer deployments. The two methods yielded similar results in this study and thus both approaches are useful for water age comparisons and residence time approximations. However, since radium often varies as a function of tidal stage, deploying moorings over a complete tidal cycle is the preferred approach. An estimated residence time for North Bay and West Bay of 8–11 days was approximated using ARs for both ex224Ra/223Ra and ex224Ra/228Ra. Some complications were introduced as St. Andrew Bay is a tidally dominated, rather than a river-dominated bay system where this method has previously been applied. The largest freshwater source to this bay system is from a man-made reservoir, with an average freshwater flow of only 20 m3 s−1. The activity concentrations and ARs measured by both sampling methods suggest that while the reservoir is the prominent radium source, it is not the only radium source. Nonetheless, a tidal mixing model applied to the western half of the system yielded an approximate flushing time of 10–12 days, similar to that derived from our radium-based water age approach.