Fundamental concepts of exchange and transport time scales in a coastal sea

Concepts of age, residence time, transit time, and turn-over time are summarized which are useful for describing the exchange and transport of water or materials in a coastal sea. The age of a particle is defined as a time which has elapsed since it entered the reservoir, and the residence time is defined as a time which will be taken for a particle to reach the outlet. Time scales based on the age are simply related with those based on the residence time. It is shown that a suitable time scale for representing the exchange characteristics is the average residence time and not the turnover time, which has often been used as the exchange time scale. Further, the ‘remnant function’ which describes the phenomena of exchange or transport is introduced, and is related to the residence time. Exchange and transport time scales in a coastal sea are discussed on the basis of the residence time which can be applied to not only steady-state cases, but also the cases where material is injected instantaneously. The average residence time in a one-dimensional channel and bay is obtained from the solutions of the advection-diffusion equation. If we know a flow speed and diffusion coefficient in a channel or bay regarded as one-dimensional, we can translate them into the average residence time. As an example, the average residence time of the Seto Inland Sea is discussed.     

Influence of tide and waves on water renewal in Óbidos Lagoon, Portugal

The role of oceanic tide, wind stress, freshwater river inflows, and waves in the long-term circulation and residence time in óbidos Lagoon is investigated using a sensitivity analysis carried out by means of a two-dimensional model. MOHID modeling system coupled to Steady-State Spectral Wave model for simulate óbidos Lagoon circulation were implemented. For residence time calculus, a Lagrangian transport model was used. Tidal forcing is shown to be the dominant forcing, although storm waves must be considered to simulate accurately the long-term circulation. Tidal forcing enhances a spatial distribution in water residence time. Renewal time scales varies from values of 2 days in the near-ocean areas and 3 weeks in the inner areas. Freshwater river inflows decrease the residence time, while waves increase. In heavy rain periods, the water residence time decreases by about 40% in the upper lagoon. When wave forcing is considered, the residence time increases between 10% and 50% depending on lagoon area. The increase in residence time is explained by the sea level rise within lagoon (~1 m above average lagoon sea level) during storm wave periods. Average residence time is 16 days for tidal forcing, 9 days when the rivers are included (wet period), and 18 days when the waves are considered.     

Utilization of decadal tritium variation for assessing the residence time of base flow

An iterative algorithm is presented that allows the user to model the subsurface residence time of shallow ground water comprising stream base flow based on decadal scale variation of tritium concentrations. The algorithm accounts for the effects of radioactive decay, the shallow subsurface mixing of ground water with precipitation, and ground water flux. The inverse of the best-fitting modeled flux through the saturated zone is equivalent to the residence time. The data required for this model include at least two measurements of tritium in base flow for a given stream location made at least a decade apart and the long-term tritium input in precipitation for the region of interest. The model is sensitive to relatively small changes in tritium concentrations and is limited by analytic uncertainties to an accuracy of approximately +/-5 years. The algorithm was applied to stream base flow for several basins in the Piedmont Province of Georgia in which tritium concentrations were measured during the early 1990s and again in the 2000s. The model results produced highly concordant residence times for three hydrogeologically similar basins in the Upper Ocmulgee Basin in North Central Georgia. The best estimate of the average residence time for ground water comprising base flow in this Piedmont basin using this new method is between approximately 14 and 18 years. These results are generally consistent with calculations made in previous studies, and these relatively long residence times can be attributed to the storage of water in the clay soils that dominate Piedmont Province watersheds.     

Deep water circulation, residence time, and chemistry in a karst complex

We investigated the hydrochemistry of a complex karst hydrosystem made of two carbonate units along a coastal lagoon. Ground water emerges on the lagoon floor from a submarine spring. In addition, thermal waters circulate through the limestone and mix with karst water near the lagoon shore. A distinction between the water from the two carbonate units is related to marine influences and human activities. In one of the massifs, the data show an incongruent dissolution of dolomite with time. In the other system, a slight contamination by saline fluids from the thermal reservoir has led to high calcium and magnesium concentrations. 36Cl, 14C, and 3H data constrain the residence time of the water, and allow for the distinguishing of four circulation types: (1) shallow surface circulation (primarily above sea level) in the karstic units with short residence times (<20 years); (2) shallow subsurface circulation (approximately 0 to -50 m) below the karstic units with residence time in the order of 50 years; (3) deep circulation at depth of 700 to 1500 m in the Jurassic limestones below thick sedimentary cover, with residence time of several thousand years for a part of the water; and (4) deep circulation at a depth of approximately 2500 m, which represents the thermal reservoir in the Jurassic units with residence time of approximately 100,000 years. An interpretative hydrogeological framework is based on the constraints of the geochemical analyses of the deep thermal system, and by water flow from the surface to the deep parts of the carbonate formations.     

Quantitative estimation of water storage and residence time in the epikarst with time‐lapse refraction seismic

The hydrodynamic characterization of the epikarst, the shallow part of the unsaturated zone in karstic systems, has always been challenging for geophysical methods. This work investigates the feasibility of coupling time‐lapse refraction seismic data with petrophysical and hydrologic models for the quantitative determination of water storage and residence time at shallow depth in carbonate rocks. The Biot–Gassmann fluid substitution model describing the seismic velocity variations with water saturation at low frequencies needs to be modified for this lithology. I propose to include a saturation‐dependent rock‐frame weakening to take into account water–rock interactions. A Bayesian inversion workflow is presented to estimate the water content from seismic velocities measured at variable saturations. The procedure is tested first with already published laboratory measurements on core samples, and the results show that it is possible to estimate the water content and its uncertainty. The validated procedure is then applied to a time‐lapse seismic study to locate and quantify seasonal water storage at shallow depth along a seismic profile. The residence time of the water in the shallow layers is estimated by coupling the time‐lapse seismic measurements with rainfall chronicles, simple flow equations, and the petrophysical model. The daily water input computed from the chronicles is used to constraint the inversion of seismic velocities for the daily saturation state and the hydrodynamic parameters of the flow model. The workflow is applied to a real monitoring case, and the results show that the average residence time of the water in the epikarst is generally around three months, but it is only 18 days near an infiltration pathway. During the winter season, the residence times are three times shorter in response to the increase in the effective rainfall.     

Residence times in river basins as determined by analysis of long-term tritium records

The US Geological Survey has maintained a network of stations to collect samples for the measurement of tritium concentrations in precipitation and streamflow since the early 1960s. Tritium data from outflow waters of river basins draining 4500–75000 km² are used to determine average residence times of water within the basins. The basins studied are the Colorado River above Cisco, Utah; the Kissimmee River above Lake Okeechobee, Florida; the Mississippi River above Anoka, Minnesota; the Neuse River above Streets Ferry Bridge near Vanceboro, North Carolina; the Potomac River above Point of Rocks, Maryland; the Sacramento River above Sacramento, California; the Susquehanna River above Harrisburg, Pennsylvania. The basins are modeled with the assumption that the outflow in the river comes from two sources—prompt (within-year) runoff from precipitation, and flow from the long-term reservoirs of the basin. Tritium concentration in the outflow water of the basin is dependent on three factors: (1) tritium concentration in runoff from the long-term reservoir, which depends on the residence time for the reservoir and historical tritium concentrations in precipitation; (2) tritium concentrations in precipitation (the within-year runoff component); (3) relative contributions of flow from the long-term and within-year components. Predicted tritium concentrations for the outflow water in the river basins were calculated for different residence times and for different relative contributions from the two reservoirs. A box model was used to calculate tritium concentrations in the long-term reservoir. Calculated values of outflow tritium concentrations for the basin were regressed against the measured data to obtain a slope as close as possible to 1. These regressions assumed an intercept of zero and were carried out for different values of residence time and reservoir contribution to maximize the fit of modeled versus actual data for all the above rivers. The final slopes of the fitted regression lines ranged from 0.95 to 1.01 (correlation coefficient > 0.96) for the basins studied. Values for the residence time of waters within the basins and average relative contributions of the within-year and long-term reservoirs to outflow were obtained. Values for river basin residence times ranged from 2 years for the Kissimmee River basin to 20 years for the Potomac River basin. The residence times indicate the time scale in which the basin responds to anthropogenic inputs. The modeled tritium concentrations for the basins also furnish input data for urban and agricultural settings where these river waters are used.     

鄱阳湖换水周期与示踪剂传输时间特征的数值模拟

在复杂湖泊水动力环境作用下,换水周期和传输时间变化直接影响着污染物的迁移和转化.本文运用数值模拟方法,定量研究了季节水情动态下鄱阳湖换水周期和示踪剂传输时间的空间分布.结果表明,不同季节下鄱阳湖换水周期均具有较高的空间异质性,贯穿整个湖区的主河道换水周期约10 d,大多湖湾区的换水周期则长达300多天.尽管不同季节下换水周期空间分布格局几乎相似,但受鄱阳湖水动力场的季节变化影响,夏、秋季的换水周期要明显大于春、冬季.基于换水周期频率分布曲线的统计表明,80%的鄱阳湖区的换水周期约30 d,其余湖区换水周期为几十天至几百天,表明鄱阳湖应该更加确切地描述为一个快速换水和慢速换水同时共存的湖泊系统.鄱阳湖示踪剂传输时间介于4~32 d,夏、秋季的传输时间(11~32 d)约为春、冬季(4~8 d)的4倍,主要与鄱阳湖季节性水情特征及示踪剂的迁移路径有关.本文所获取的换水周期和示踪剂传输时间的时空分布信息可为今后鄱阳湖水质、水环境和生态系统管理和维护等方面提供重要科学参考.     

Mean residence time of lagoons in shallow vegetated floodplains

Lagoons interspersed within wetlands are expected to increase the residence time of the flow in the system which, in turn, will lead to enhanced pollutant removal thus ensuring a good ecological status of the ecosystem. In this study, lagoons interspersed in vegetated wetlands have been mimicked in the laboratory to develop a theoretical model to establish the impact three major driving parameters (the vegetation density surrounding a lagoon, the depth aspect ratio [length vs. depth] of the lagoon and the circulating flow – through the Reynolds number) have on determining the residence time of the flow in the lagoon. The results indicate that, according to the maximum free available area of the flow, the presence of vegetation (Juncus maritimus) decreases the residence time. In addition, an increase in the Reynolds number of the circulating flow in the wetlands also resulted in a decrease in the lagoon residence time. Nevertheless, lagoon residence times were found to depend on the depth of the lagoon, with deeper lagoons having higher residence times. The length of the lagoon, however, was found not to affect the residence time. High lagoon residence times in either natural or constructed wetlands are desirable because they enhance pollutant removal from the water. Although, if the residence times are too long, this may lead to anoxic water conditions that could in fact threaten the wetland's ecosystem.     

Soil water residence times and solute uptake on a dolomite bedrock—preliminary results

Calcium and magnesium levels have been monitored in slope foot drainage waters on a dolomite bedrock. Both calcium and magnesium rich pulses occur. Short term dissolution experiments demonstrate high calcium levels in solution while other authors have suggested that long residence time groundwater has relatively high levels of magnesium due to calcite precipitation. Patterns of field fluctuations in Ca: Mg ratios can thus be tentatively interpreted in terms of short residence time water of high calcium content mixing with long residence time water of high magnesium content. Fluorometric dye tracing has been used to indicate the orders of magnitude of soil water residence times, suggesting that quickflow components are resident in the system for a few hours to a few days. Further work is in progress.     

Modeling decadal timescale interactions between surface water and ground water in the central Everglades, Florida, USA

Surface-water and ground-water flow are coupled in the central Everglades, although the remoteness of this system has hindered many previous attempts to quantify interactions between surface water and ground water. We modeled flow through a 43,000 ha basin in the central Everglades called Water Conservation Area 2A. The purpose of the model was to quantify recharge and discharge in the basin's vast interior areas. The presence and distribution of tritium in ground water was the principal constraint on the modeling, based on measurements in 25 research wells ranging in depth from 2 to 37 m. In addition to average characteristics of surface-water flow, the model parameters included depth of the layer of ‘interactive’ ground water that is actively exchanged with surface water, average residence time of interactive ground water, and the associated recharge and discharge fluxes across the wetland ground surface. Results indicated that only a relatively thin (8 m) layer of the 60 m deep surfical aquifer actively exchanges surface water and ground water on a decadal timescale. The calculated storage depth of interactive ground water was 3.1 m after adjustment for the porosity of peat and sandy limestone. Modeling of the tritium data yielded an average residence time of 90 years in interactive ground water, with associated recharge and discharge fluxes equal to 0.01 cm d⁻¹. ³H/³He isotopic ratio measurements (which correct for effects of vertical mixing in the aquifer with deeper, tritium-dead water) were available from several wells, and these indicated an average residence time of 25 years, suggesting that residence time was overestimated using tritium measurements alone. Indeed, both residence time and storage depth would be expected to be overestimated due to vertical mixing. The estimate of recharge and discharge (0.01 cm d⁻¹) that resulted from tritium modeling therefore is still considered reliable, because the ratio of residence time and storage depth (used to calculated recharge and discharge) is much less sensitive to vertical mixing compared with residence time alone. We conclude that a small but potentially significant component of flow through the Everglades is recharged to the aquifer and stored there for years to decades before discharged back to surface water. Long-term storage of water and solutes in the ground-water system beneath the wetlands has implications for restoration of Everglades water quality.     

Water temperature and conductivity variability as indicators of groundwater behaviour in complex aquifer systems in the south‐east of Spain

Groundwater temperature and electrical conductivity logs were applied in this study to gain an understanding of the hydrogeological behaviour of a complex system. This system, compartmentalized by a large number of fractures, which may even cause geothermal anomalies, is that of the Campo de Dalías in the south‐east of the Iberian Peninsula. In addition to this complexity one must consider the different levels with aquifer properties and their varying lithologies, which in some cases are in direct contact with the sea. Vertical sections based upon the various logs allow lateral correlation to be made between the anomalies detected, and provide information on the existence of both ascending and descending vertical flows, and the inflow of sea water into the system. Analysis of environmental isotopes (¹⁸O and D) indicate the areas of recharge with the greatest flow velocity. No seasonal variation in the isotopes was observed, which suggests that the residence time of the groundwater in the aquifer is several years. Radiocarbon dating gave estimates of generally long residence times, although in some sectors it is possible to detect the current recharge based on its tritium content, from rainwater infiltration or seawater intrusion. Copyright © 2002 John Wiley & Sons, Ltd.     

Modelling water fluxes for the analysis of diffuse pollution at the river basin scale

Diffuse pollution is a significant and sometimes even major component of surface water pollution. Diffuse inputs of pollutants to the surface water are related to runoff of precipitation. This means that the analysis of diffuse pollutant fluxes from the land surface to the surface water requires an analysis of water fluxes. In this paper we have modelled the average long‐term total runoff, groundwater recharge index and groundwater residence times for two large European river basins (Rhine and Elbe). We applied and compared two independently developed and recently published methods. We found that with the available large‐scale databases and methods we could simulate successfully the regional patterns of the average long‐term total runoff. The reported groundwater recharge indices and groundwater residence times should be interpreted as estimates based on available knowledge and databases. They do not represent absolute values, but illustrate the possible travel times and spatial patterns of the different runoff components that have to be taken into account for the analysis of diffuse pollution at large regional and temporal scales. Copyright © 2000 John Wiley & Sons, Ltd.     

The frequency response of the Seto Inland Sea to several time-varying inputs

The frequency response characteristics of the Seto Inland Sea to three types of input of heat or materials are investigated. The types are uniform input to the entire inland sea, input from the rivers and input from the open ocean. The frequency response functions are obtained by a Fourier transform of the remnant functions, which are a kind of impulse response function and obtained by the hydraulic model experiments. The characteristics of the frequency response functions obtained are fairly different from one another, especially that for the input from the open ocean is different from the other two. Variations of the average water temperature and average salinity of the inland sea are discussed by using these results, and contributions of the main factors to their annual variations are revealed.     

An assessment of residence times of land-sourced contaminants in the Great Barrier Reef lagoon and the implications for management and reef recovery

We argue that the residence times of key pollutants exported to the Great Barrier Reef (GBR) are greater in the GBR lagoon than those of the water itself, in contradiction to some previous assumptions. Adverse effects of the pollutant discharge will be greater and longer lasting than previously considered, in turn requiring stronger or more urgent action to remediate land practices. Residence times of fine sediments, nitrogen and phosphorus, pesticides and trace metals are suggested to be from years to decades in the GBR lagoon and highly likely to be greater than the residence time of water, estimated at around 15-365days. The recovery of corals and seagrass in the central region of the GBR following current land-use remediation in the catchment depends on the residence time of these contaminants. Ecohydrological modeling suggests that this recovery may take decades even with adequate levels of improved land management practices.     

Modelling travel and residence times in the eastern Irish Sea

The Irish Sea, which lies between 51 degrees N-56 degrees N and 2 degrees 50'W-7 degrees W, provides a sheltered environment to exploit valuable fisheries resource. Anthropogenic activity is a real threat to its water quality. The majority of freshwater input down rivers flows into the eastern Irish Sea. The structure of the water circulation was not well understood during the planning of Sellafield nuclear plant outfall site in the eastern Irish Sea. A three-dimensional primitive equation numerical model was applied to the Irish Sea to simulate both barotropic and baroclinic circulation within the region. High accuracy was achieved with regard to the prediction of both tidal circulation and surface and nearbed water temperatures across the region. The model properly represented the Western Irish Sea Gyre, induced by thermal stratification and not known during planning Sellafield. Passive tracer simulations based on the developed hydrodynamic model were used to deliver residence times of the eastern Irish Sea region for various times of the year as well as travel times from the Sellafield outfall site to various locations within the Irish Sea. The results indicate a strong seasonal variability of travel times from Sellafield to the examined locations. Travel time to the Clyde Sea is the shortest for the autumnal tracer release (90 days); it takes almost a year for the tracer to arrive at the same location if it is released in January. Travel times from Sellafield to Dublin Bay fall within the range of 180-360 days. The average residence time of the entire eastern Irish Sea is around 7 months. The areas surrounding the Isle of Man are initially flushed due to a predominant northward flow; a backwater is formed in Liverpool Bay. Thus, elevated tracer concentrations are predicted in Liverpool Bay in the case of accidental spills at the Sellafield outfall site.     

Windscale radiocaesium in the Clyde sea area

Radiocaesium, introduced into coastal waters in the effluent from the B.N.F.L. nuclear fuel reprocessing plant at Windscale, is present at significant levels throughout the Clyde Sea Area. Concentrations reached maximum in April, 1977, and have recently declined. Around 30% of Windscale radiocaesium output passes through the Clyde Sea Area but less than 0.3% remains in sediments. There appears to be no major radiological safety hazard. The radiocaesium is, however, an excellent tracer species for both waters and sediments. The Irish Sea-derived component comprises over 90% of Clyde seawater and may well introduce major inventories of pollutants. The average water residence time in the Clyde system is 4.5 months, although significantly longer entrainment is evident in northern sea lochs. Study of radiocaesium variations has enabled improved assessment of the Clyde's response to marine pollution.     

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.     

Drivers of phytoplankton,bacterioplankton, and zooplankton carbon biomass in tropical hydroelectric reservoirs

Studies of carbon sources in plankton communities are important because carbon content has become the main currency used in functional studies of aquatic ecosystems. We evaluated the contribution to the total organic carbon pool from different plankton communities (phytoplankton, bacterioplankton, and zooplankton – C-biota) and its drivers in eight tropical hydroelectric reservoirs with different trophic and hydrological status and different physical features. Our systems were separated into three groups based on trophic status and water residence time: (i) mesotrophic with low residence time (ML); (ii) mesotrophic with high residence time (MH); and (iii) eutrophic with low residence time (EL). Our hypothesis that reservoirs with low water residence times and low nutrient concentrations would show the lowest C-biota was supported. Phytoplankton carbon (C-phy) showed the highest concentrations in the EL, followed by MH and ML systems. The EL group also showed significantly higher zooplankton carbon (C-zoo). No significant difference was observed for bacteria carbon (C-bac) among the three system groups. In addition to trophic status and water residence time, regression analyses revealed that water temperature, light, pH, and dissolved organic carbon concentrations were the main drivers of plankton communities in these large tropical hydroelectric reservoirs.     

Tracer Method to Determine Residence Time in a Permeable Reactive Barrier

A method is presented to evaluate ground water residence time in a zero‐valent iron (ZVI) permeable reactive barrier (PRB) using radon‐222 (²²²Rn) as a radioactive tracer. Residence time is a useful indicator of PRB hydraulic performance, with application to estimating the volumetric rate of ground water flow through a PRB, identifying flow heterogeneity, and characterizing flow conditions over time as a PRB matures. The tracer method relies on monitoring the decay of naturally occurring aqueous ²²²Rn as ground water flows through a PRB. Application of the method at a PRB site near Monticello, Utah, shows that after 8 years of operation, residence times in the ZVI range from 80 to 486 h and correlate well with chemical parameters (pH, Ca, SO₄, and Fe) that indicate the relative residence time. Residence times in this case study are determined directly from the first‐order decay equation because we show no significant emanation of ²²²Rn within the PRB and no measurable loss of ²²²Rn other than by radioactive decay.     

Ground water flow parameterization of an Appalachian coal mine complex

We examined a large (240 km2) northern Appalachian bituminous coal basin (Irwin Syncline, Westmoreland County, Pennsylvania) comprising 27 mine complexes with nine major (> 2.5 x 10(3) L/min) discharges. The synclinal basin was divided into seven subbasins based on equilibrium hydraulic relationships established during the past 25 years. Recharge rates, mine pool velocity, and residence times respond to hydraulic changes in the overburden induced by mine subsidence. The estimated maximum depth for subsidence fractures is 60 m (30 times mined thickness) with recharge rates decreasing significantly in subbasins with thicker overburden (> 75 m). Calculated subbasin recharge rates range from 2 to 6 x 10(-4) L/min/m2 and are significantly lower than the previously used rate for the basin. Residence time of ground water in the Irwin subbasins calculated using average linear velocity ranged from one to five years and were more consistent with field observations than estimates obtained using discharge and basin volume area. A positive correlation (r2 = 0.80) exists between net alkalinity of the mine water-impacted discharges and residence time in the mine pools. Our results for the Irwin coal basin suggest that use of a subbasin approach incorporating overburden depth, mining methodology, and the extent of postmining inundation will lead to improved determination of ground water flow parameters in mined watersheds in northern Appalachia and elsewhere.