Mean Residence Time Of Water Research Articles

Interactive effects of catchment mean water residence time and agricultural area on water physico-chemical variables and GHG saturations in headwater streams

Greenhouse gas emissions from headwater streams are linked to multiple sources influenced by terrestrial land use and hydrology, yet partitioning these sources at catchment scales remains highly unexplored. To address this gap, we sampled year-long stable water isotopes (δ18O and δ2H) from 17 headwater streams differing in catchment agricultural areas. We calculated mean residence times (MRT) and young water fractions (YWF) based on the seasonality of δ18O signals and linked these hydrological measures to catchment characteristics, mean annual water physico-chemical variables, and GHG % saturations. The MRT and the YWF ranged from 0.25 to 4.77 years and 3 to 53%, respectively. The MRT of stream water was significantly negatively correlated with stream slope (r2 = 0.58) but showed no relationship with the catchment area. Streams in agriculture-dominated catchments were annual hotspots of GHG oversaturation, which we attributed to precipitation-driven terrestrial inputs of dissolved GHGs for streams with shorter MRTs and nutrients and GHG inflows from groundwater for streams with longer MRTs. Based on our findings, future research should also consider water mean residence time estimates as indicators of integrated hydrological processes linking discharge and land use effects on annual GHG dynamics in headwater streams.

A novel approach for accurate quantification of lake residence time — Lake Kinneret as a case study

Water residence time, which is affected by increasing water demands and climate change, plays a crucial role in lakes and reservoirs since it influences many natural physical and ecological processes that eventually impact the water quality of the waterbody. Thus, accurate quantification of the water residence time and its distribution is an important tool in lake management. In this study we present a novel approach for assessing the residence time in lakes and reservoirs. The approach is based on the Leslie matrix model that was originally developed for the analysis of age-structured biological population dynamics. In this approach the water in the lake is divided into different age classes each representing the time since the “parcel” of water entered the lake and provides an overall picture of the water age structure. The traditional approach for calculating residence times, which relies only on the lake volume and annual inflow or outflow volumes thereby disregarding any previous information, is very sensitive to large interannual variation. While the proposed approach produces the fraction and volume distribution curves of all age classes within the lake for each simulated timestep. Thus, in addition to mean residence time, the fraction of young water (FYW), quantifying the “young” fraction of water in the lake can be analyzed. The same is true for any other age class of water.The approach was applied to Lake Kinneret (Sea of Galilee) historical data collected over 32 years (1987-2018) and for prediction of long-term time series based on several future scenarios (inflows and outflows). It offers a more accurate quantification of the mean residence time of water in a lake and can easily be adapted to other waterbodies. Comparison of simulation results may serve as basis for determining the lake's management policy, by controlling the inflows and outflows, that will affect both the mean residence time and the fraction of “young/old” age classes of water.

Long term response of water and nitrogen fluxes to Good Agricultural Practices at field and catchment scales

Facing the nitrate pollution problem, the European Union has encouraged a code of “Good Agricultural Practices” (GAP) in order to recover a good chemical and ecological status of waterbodies. In this study, we hypothesized that the systematic application of GAP in time and space would allow to meet the EU standard of nitrate concentration (50 mg NO3 L−1) under arable cropping systems. Water and nitrogen fluxes were determined in an agricultural catchment (187 ha) during 22 years after GAP implementation, at field and catchment scales. The aquifer outlet is a set of springs which were monitored for water flow and nitrate concentration. GAP management mainly consisted in adjusting N fertilization rates and establishing catch crops. Crop N uptake, soil water and mineral N were measured respectively two and three times per year on 36 sites representing soil variability. These data were used to initialize the STICS model which simulated the fluxes of infiltrated water and nitrate leached below the rooting zone at field scale. The elementary fluxes (calculated for each site-year) were then used as independent inputs of the hydrological model MODCOU which made the integration at catchment scale. Simulations of agricultural scenarios allowed to calculate the cost/efficiency ratio of GAP implementation per soil type. The mean amounts of infiltrated water and N leached calculated below rooting depth in agricultural fields were 179 mm yr−1 and 19 kg N ha−1 yr−1, respectively, yielding a mean weighted nitrate concentration of 41 mg NO3 L−1 during 22 years over the whole catchment. The mean residence time of water in the catchment was estimated at 17–22 years using tritium and CFC tracers. The observed nitrate concentration in the main spring declined 11 years after GAP implementation and levelled off to 49 mg NO3 L−1. The agro-hydrological model satisfactorily predicted water flow and nitrate concentration in springs but overestimated their response time. It predicted a positive impact of GAP application on water quality. GAP management appeared to be efficient on the long term with a low cost/efficiency ratio. A more flexible and motivating management could consist in including GAP as a first step in a loop progress towards agro-ecological systems.

Predicting benthic macroalgal abundance in shallow coastal lagoons from geomorphology and hydrologic flow patterns

AbstractMacroalgae structure coastal ecosystems affecting metabolism, nutrient dynamics, and food webs. Spatially explicit prediction of macroalgal abundance is critical for understanding coastal ecosystems and trajectories. However, models of macroalgal distribution tend to be mechanistic and generalize poorly, or biogeographic and too coarse to use over spatial scales most appropriate to ecosystem research and management (1–100 km2). Our objective was to develop spatial distribution models for benthic macroalgae in soft‐sediment environments. We compared macroalgal abundance quantified as percent cover, with environmental drivers on 1 ha intertidal flats in a > 900 km2 lagoon system along the Atlantic Coast of Virginia, U.S.A. Physical drivers of macroalgae (e.g., depth‐mediated light availability, exposure to waves) are related to bed morphology. We developed a novel topographic index (τ) to determine whether bed morphology predicts macroalgal abundance. This topographic index described variation in elevation occurring over spatial scales relevant to macroalgae, ranging from smooth to hummocky (τ = 0.01–1.07). Models tested τ along with mean elevation, fetch, and water residence time as predictors of macroalgal abundance. τ, and the interaction with water residence time, were most strongly related to macroalgal abundance. Hummocky flats accumulated less macroalgae than smoother flats, but exceptions occurred with short residence times. Model error (root mean square error) was low, varying between 8% and 18% across models. These models, based on readily measured physical features, are a useful approach for assessing macroalgal abundance in relation to shoreline hardening, species invasions, sea‐level rise, and changing sedimentation affecting coastal ecosystems.

Tidal Effect on Water Export Rate in the Eastern Shelf Seas of China

AbstractWater export rate of shelf seas is a pivotal factor impacting the global carbon cycle. Tides have important impacts on shelf hydrodynamics but are excluded in many climate models. To assess the effect of tides on export rates of shelf water, this study used a regional hydrodynamic model and a water residence time (WRT) adjoint model and examined model runs with and without tides for the eastern shelf seas of China. The results show that the average WRTs in the Bohai, Yellow, and East China seas were 11.60, 4.95, and 0.39 years, respectively. When tides were excluded, the WRTs decreased by >70% in the Bohai and Yellow seas and by ~10% in the East China Sea, indicating a significant acceleration in the shelf water export due to the absence of tides. The tidal effect has spatial variability associated with the water depth. Sensitivity experiments suggest that the tidal effect on the mean WRT was stronger than the effect of other dynamical factors (winds, rivers, and boundary currents). In the model with tides, tides weakened the wind‐driven coastal current by intensifying the bottom resistance and thus slowed water export in the inner and middle portions of the shelf, compared to the model without tides. Parameterization of the tidal bottom friction in the model without tides could significantly improve the WRT result. This study highlights the crucial role of tides on the long‐term transport of shelf seas and the significance of parameterizing the effect of tidal friction in climate models.

Distinct stores and the routing of water in the deep critical zone of a snow-dominated volcanic catchment

Abstract. This study combines major ion and isotope chemistry, age tracers, fracture density characterizations, and physical hydrology measurements to understand how the structure of the critical zone (CZ) influences its function, including water routing, storage, mean water residence times, and hydrologic response. In a high elevation rhyolitic tuff catchment in the Jemez River Basin Critical Zone Observatory (JRB-CZO) within the Valles Caldera National Preserve (VCNP) of northern New Mexico, a periodic precipitation pattern creates different hydrologic flow regimes during spring snowmelt, summer monsoon rain, and fall storms. Hydrometric, geochemical, and isotopic analyses of surface water and groundwater from distinct stores, most notably shallow groundwater that is likely a perched aquifer in consolidated collapse breccia and deeper groundwater in a fractured tuff aquifer system, enabled us to untangle the interactions of these groundwater stores and their contribution to streamflow across 1 complete water year (WY). Despite seasonal differences in groundwater response due to water partitioning, major ion chemistry indicates that deep groundwater from the highly fractured site is more representative of groundwater contributing to streamflow across the entire water year. Additionally, the comparison of streamflow and groundwater hydrographs indicates a hydraulic connection between the fractured welded tuff aquifer system and streamflow, while the shallow aquifer within the collapse breccia deposit does not show this same connection. Furthermore, analysis of age tracers and oxygen (δ18O) and stable hydrogen (δ2H) isotopes of water indicates that groundwater is a mix of modern and older waters recharged from snowmelt, and downhole neutron probe surveys suggest that water moves through the vadose zone both by vertical infiltration and subsurface lateral flow, depending on the lithology. We find that in complex geologic terrain like that of the JRB-CZO, differences in the CZ architecture of two hillslopes within a headwater catchment control water stores and routing through the subsurface and suggest that shallow groundwater does not contribute significantly to streams, while deep fractured aquifer systems contribute most to streamflow.

The atmospheric carbon sequestration potential of man-made tidal lagoons

Understanding sequestration of carbon by coastal ecosystems is central to addressing the role they play in climate change mitigation. To quantify this process, accurate measurements of CO2 fluctuation, coupled with variations in residence time of coastal water-bodies are required. Nearshore ecosystems, including coastal lagoons, may provide an effective sink for atmospheric carbon dioxide, particularly those containing productive biota such as seagrass. However, the rate and pattern of carbon sequestration in seagrass meadows across a range of environmental settings is still poorly constrained. In this study, we utilize a robust physical tidal model, along with biogeochemical dissolved inorganic carbon (DIC) assessment, to estimate water residence time and net sequestration of atmospheric CO2 in an intertidal lagoon containing a seagrass (Zostera noltii) meadow. Total alkalinity and pH measurements taken from advected water mass exchanged with the open ocean at inlet boundaries are used to calculate DIC and pCO2. A predictive model of hydrodynamics provides good approximation of mean water residence time to within 6 h (±3 s.d). Results indicate that during the daytime study period the lagoon is a sink for carbon, having a mean net ecosystem productivity (NEP) of 3.0 ± 0.4 mmol C m−2 hr−1. An equivalent diel NEP range of between 15.23 and −9.24 mmol C m−2 d−1 is calculated based on reported shallow water pelagic respiration rates. Moreover, approximately 4% of DIC availability occurs from atmospheric CO2 transfer to lagoon water. However, a negative diel rate of −82 ± 81 mmol C m−2 d−1 is found, assuming overnight respiration ascertained from converted Zostera noltii O2 utilization. We hypothesize that analogous regional nearshore ecosystems provide baseline study sites suitable to elucidate the carbon capture potential of planned, nearby tidal range energy schemes.

Karst flow processes explored through analysis of long-term unsaturated-zone discharge hydrochemistry: a 10-year study in Rustrel, France

The unsaturated zone of karst aquifers influences the dynamics and the chemistry of water. Because of a lack of direct access, other than via caves, flows in the aquifer matrix and the smallest conduits remain poorly characterized. The few artificial underground structures in the unsaturated karst provide a rare opportunity to study the variety of flow processes. At the low noise underground research laboratory (Laboratoire Souterrain a Bas Bruit, LSBB) in Rustrel (France), 12 variables (temperature, pH, electrical conductivity, alkalinity, major anions and cations, total organic carbon) have been monitored on 12 perennial or temporary flows and leakages over a 10-year period covering contrasting climatic periods. This unique dataset of 1,135 samples has been used to discriminate, identify, and rank the processes associated with the hydrochemical variability of these different types of flows. A principal component analysis and a hierarchical cluster analysis, using mean values and standard deviation of the flow along the principal components, were performed. The results indicate that seasonal variability, mean water residence time, and the depth of acquisition of the chemical characteristics are the main factors of the variability of chemistry at the monitored flow points. Distinguished clusters highlight the great diversity of flows and processes occurring in the fine pathways that may be neighboring the large and structured fractures and conduits. Long-term monitoring with various climatic conditions appears to be a useful tool for assessing this diversity.

Scoured or suffocated: Urban stream ecosystems oscillate between hydrologic and dissolved oxygen extremes

AbstractHeadwater streams draining urbanized watersheds are subject to frequent and intense storm flows. These floods can disrupt metabolic processes occurring in benthic biofilms via the removal of biomass (i.e., scouring flows, bed mobilization) or light attenuation due to turbidity. Furthermore, channel incision caused by frequent hydraulic disturbance alters the geomorphology of streams, indirectly changing the flow and light regimes experienced by benthic biofilms. We measured dissolved oxygen (DO) and modeled whole‐stream metabolism for 18 months in six urban headwater streams in the North Carolina Piedmont, U.S.A. All streams were heterotrophic and had low rates of productivity despite relatively high streamwater nutrient concentrations. Light availability at the channel surface explained more of the day to day variation in gross primary productivity within each stream than did hydrologic disturbance. Yet among streams, the explanatory power of light declined with increasing hydrologic flashiness. We found a surprisingly wide range in DO regimes, which ranged from frequent hypoxia to near constant saturation. Hypoxia was more common in streams with lower channel gradients where bedrock outcroppings and culverts create rapid slope transitions between pools. We hypothesize this geomorphic change increases the susceptibility of benthic biota to perturbation during storms and the mean water residence time during baseflow. Increased water residence times together with elevated organic matter and nutrient inputs can set up ideal conditions for hypoxia at baseflows punctuated by frequent scouring storm flows. As a result, benthic biota are caught between hydrologic and chemical extremes that constrain their productivity.

Free energy study of H2O, N2O5, SO2, and O3 gas sorption by water droplets/slabs

Understanding gas sorption by water in the atmosphere is an active research area because the gases can significantly alter the radiation and chemical properties of the atmosphere. We attempt to elucidate the molecular details of the gas sorption of water and three common atmospheric gases (N2O5, SO2, and O3) by water droplets/slabs in molecular dynamics simulations. The system size effects are investigated, and we show that the calculated solvation free energy decreases linearly as a function of the reciprocal of the number of water molecules from 1/215 to 1/1000 in both the slab and the droplet systems. By analyzing the infinitely large system size limit by extrapolation, we find that all our droplet results are more accurate than the slab results when compared to the experimental values. We also show how the choice of restraints in umbrella sampling can affect the sampling efficiency for the droplet systems. The free energy changes were decomposed into the energetic ΔU and entropic -TΔS contributions to reveal the molecular details of the gas sorption processes. By further decomposing ΔU into Lennard-Jones and Coulombic interactions, we observe that the ΔU trends are primarily determined by local effects due to the size of the gas molecule, charge distribution, and solvation structure around the gas molecule. Moreover, we find that there is a strong correlation between the change in the entropic contribution and the mean residence time of water, which is spatially nonlocal and related to the mobility of water.

Electric Field Induced Polarization Effects Measured by in Situ Neutron Spectroscopy

Despite the success of electrical stimulation in many areas, including clay or sludge dewatering, extraction of juices from fruit pulp, fracture healing and targeted drug delivery, the induced transport mechanisms are controlled by unknown factors. While electroosmotic dewatering of clays particles <10 μm is well known, understanding of how tightly bound water molecules are removed from within the clay interlayers is still incomplete. By performing quasielastic neutron scattering experiments with in-situ electric field stimuli on calcium montmorillonite (Ca-Mt) pre-hydrated at relative humidity (RH) of 58 and 85%, we observed an increase in the water mean residence time, suggesting that hydrogen bonding lifetimes are prolonged under electric field. Assuming that at these RH most all water present resides within the interlayer space of the Ca-Mt, this result indicates strong polarization of the water by the interlayer cation. This electroosmotic induced reorganization has important implications in terms of...

Circulation and oxygenation of the glacial South China Sea

Degree of oxygenation in intermediate water modulates the downward transferring efficiency of primary productivity (PP) from surface water to deep water for carbon sequestration, consequently, the storage of nutrients versus the delivery and sedimentary burial fluxes of organic matter and associated biomarkers. To better decipher the PP history of the South China Sea (SCS), appreciation about the glacial-interglacial variation of the Luzon Strait (LS) throughflow, which determines the mean residence time and oxygenation of water mass in the SCS interior, is required. Based on a well-established physical model, we conducted a 3-D modeling exercise to quantify the effects of sea level drop and monsoon wind intensity on glacial circulation pattern, thus, to evaluate effects of productivity and circulation-induced oxygenation on the burial of organic matter. Under modern climatology wind conditions, a 135m sea-level drop results in a greater basin closeness and a ∼24% of reduction in the LS intermediate westward throughflow, consequently, an increase in the mean water residence time (from 19.0 to 23.0years). However, when the wind intensity was doubled during glacial low sea-level conditon, the throughflow restored largely to reach a similar residence time (18.4years) as today regardless its closeness. Comparing with present day SCS, surface circulation pattern in glacial model exhibits (1) stronger upwelling at the west off Luzon Island, and (2) an intensified southwestward jet current along the western boundary of the SCS basin. Superimposed hypothetically by stronger monsoon wind, the glacial SCS conditions facilitate greater primary productivity in the northern part. Manganese, a redox sensitive indicator, in IMAGES core MD972142 at southeastern SCS revealed a relatively reducing environment in glacial periods. Considering the similarity in the mean water residence time between modern and glacial cases, the reducing environment of the glacial southeastern SCS was thus ascribed to a productivity-induced rather than ventilation-induced consequence.

Use of mean residence time of water, flowrate, and equilibrium temperature indicated by water geothermometers to rank geothermal resources. Application to the thermal water circuits of Northern Calabria

A new hydro-geochemical survey was carried out on the thermal sites of Northern Calabria. By use of new and historical data, it was possible to improve the knowledge on the deep systems of interest. First, thermal waters were characterized by means of a classical approach, including water classifications, use of the triangular diagram of Cl-Li-B, and geothermometry through suitably selected functions. Then, the tritium-based mean residence time and flowrate of the thermal waters of Northern Calabria were used to evaluate the volume of the geothermal reservoir and its component terms. Based on these results, the heat stored in the geothermal reservoir was computed by means of the volume method. Their natural heat discharge was also calculated. These parameters as well as the reservoir temperature indicated by water geothermometers were used to rank the geothermal resources of Northern Calabria. Sambiase resulted to be the thermal circuit of highest geothermal potential.

Evaluation of the effect of transcytolemmal water exchange analysis for therapeutic response assessment using DCE-MRI: a comparison study

This study compares the shutter-speed (SS) and the Tofts models as used in assessing therapeutic response in a longitudinal DCE-MRI experiment. Sixteen nu/nu mice with implanted colorectal adenocarcinoma cell line (LS-174T) were randomly assigned into treatment/control groups (n = 8/group) and received bevacizumab/saline twice weekly (Day1/Day4/Day8). All mice were scanned at one pre- (Day0) and two post-treatment (Day2/Day9) time points using a high spatiotemporal resolution DCE-MRI pulse sequence. The CA extravasation rate constant from the Tofts/SS model and the mean intracellular water residence time from the SS model were analyzed. A biological subvolume (BV) within the tumor was identified based on the intensity distribution, and the SS model parameters within the BV ( and ) were analyzed. It is found that and have a similar spatial distribution in the tumor volume. The Bayesian information criterion results show that the SS model was a better fit for all scans. At Day9, the treatment group had significantly higher tumor mean (p = 0.021), (p = 0.021) and (p = 0.045). When BV from transcytolemmal water exchange analysis was adopted, the treatment group had higher mean at both Day2 (p = 0.038) and Day9 (p = 0.007). Additionally, at Day9, the treatment group had higher mean (p = 0.045) and higher spatial heterogeneity indices (Rényi dimensions) d1 (p = 0.010) and d2 (p = 0.021). When mean and its coefficient of variation (CV) were used to separate treatment/control group samples using supporting vector machine, the accuracy of treatment/control classification was 68.8% at Day2 and 87.5% at Day9; in contrast, the Day2/Day9 accuracy were 62.5%/87.5% using tumor mean and its CV and were 50.0%/87.5% using tumor mean and its CV, respectively. These results suggest that the SS model parameters outperformed the Tofts model parameters in terms of capturing bevacizumab therapeutic effect in this longitudinal experiment.

Validity and reliability of a 7‐day fluid diary to estimate average daily water intake

Water is the most abundant molecule in the human body that undergoes continuous recycling, but often an overlooked nutrient. One of the main reasons that even in large scale studies water intake is not recorded is the lack of validated tools to assess water intake.OBJECTIVETo assess the validity and reliability of a seven‐day fluid diary (7DFD) by comparing it to total water turnover assessed via deuterium oxide.METHODSOne hundred one healthy males and females (41±14 y, 76.6±16.9 kg, 1.70±0.09 m, 26.4±5.5 kg·m−2, 30±11% body fat) from the North West Arkansas were enrolled. Subject participated in the study for four consecutive weeks where the 7DFD was recorded on week two and four. Three doses of D2O (0.10 g, 0.05 and 0.08·kg lean body mass−1, assessed by DXA) were ingested at the beginning of weeks 1, 2 and 3. Urine samples were collected immediately before ingestion of D2O, the following day and at the end of each week. Urine D/H, expressed in ppm vs the V‐SMOW standard (155.76 ppm) was measured by mass spectrometry using the H2‐water equilibration method. Water turnover (WTO) was calculated as total body water (TBW, L)/mean residence time of water in the body water pool. Average daily WTO was also estimated from the diary as the sum of: 1) participant entries in 7DFD, 2) water intake from solid foods calculated from a traditional 7 day food record, and, 3) calculated daily metabolic water production.RESULTSWTO assessed either via D2O (W2: 3.64±1.20, W4: 3.68±1.31 L/day) or 7DFD (W2: 3.51±1.30, W4: 3.47±1.23 L/day) was not different between weeks 2 and 4 with both methods of estimation (P&gt;0.05). Water turnover from D2O vs the 7DFD was linearly related during week 2 (F[1,95]=144.2, R2 = 0.61, p &lt;0.001), and week 4 (F[1,85]=92.2, R2 = 0.52, p &lt; 0.001). Intraclass correlations (95% CI) were 0.77 (0.68–0.84), and 0.74(0.63–0.83) for weeks 2 and 4, respectively. Lastly, the Bland‐Altman analysis for combined mean difference between WTO for D2O and 7DFD was 0.167±0.876 L·d−1, a 95% CI of −1.886–1.552 L·d−1, with no apparent bias (F[1,180]= 1.0, R2=0.005, p = 0.325).CONCLUSIONThe seven day fluid diary is a reliable and valid tool to estimate water from fluids in adults.Support or Funding InformationFunded by Danone Research

Towards zero liquid discharge in the presence of silica: Stable 98% recovery in nanofiltration and reverse osmosis

Silica scaling can be a limiting factor for membrane treatment at very high recoveries. In this work the feasibility of reaching very high recoveries (⩾98%) was investigated in a pilot plant consisting of cation exchange pretreatment, nanofiltration (NF) and reverse osmosis (RO), with pretreated ground water as feed water, with no aluminum detected. Experiments were carried out at total recovery (NF+RO) of 98% and 99% and with the addition of two different antiscalants at 99% recovery. 98% recovery was possible during 23days of operation with only a minor decrease in the membrane permeability. At 99% recovery SEM–EDX analysis showed that silica and ironsilicate scaling occurred in the RO membrane, and strongly decreased the membrane permeability. The addition of antiscalants to the pilot plant did not prevent the occurrence of scaling. The laboratory beaker experiments did not prove the efficiency of the antiscalants either. The mean residence time of water in the pilot plant was estimated at 1h. The occurrence of scaling in the presence of antiscalants can be attributed to factors such as the long residence time in the pilot plant, which is probably longer than the induction time, and the relatively high concentration of particulate iron in the feed water.

Characterisation of stable isotopes to identify residence times and runoff components in two meso-scale catchments in the Abay/Upper Blue Nile basin, Ethiopia

Abstract. Measurements of the stable isotopes oxygen-18 (18O) and deuterium (2H) were carried out in two meso-scale catchments, Chemoga (358 km2) and Jedeb (296 km2) south of Lake Tana, Abay/Upper Blue Nile basin, Ethiopia. The region is of paramount importance for the water resources in the Nile basin, as more than 70% of total Nile water flow originates from the Ethiopian highlands. Stable isotope compositions in precipitation, spring water and streamflow were analysed (i) to characterise the spatial and temporal variations of water fluxes; (ii) to estimate the mean residence time of water using a sine wave regression approach; and (iii) to identify runoff components using classical two-component hydrograph separations on a seasonal timescale. The results show that the isotopic composition of precipitation exhibits marked seasonal variations, which suggests different sources of moisture generation for the rainfall in the study area. The Atlantic–Indian Ocean, Congo basin, Upper White Nile and the Sudd swamps are the potential moisture source areas during the main rainy (summer) season, while the Indian–Arabian and Mediterranean Sea moisture source areas during little rain (spring) and dry (winter) seasons. The spatial variation in the isotopic composition is influenced by the amount effect as depicted by moderate coefficients of determination on a monthly timescale (R2 varies from 0.38 to 0.68) and weak regression coefficients (R2 varies from 0.18 to 0.58) for the altitude and temperature effects. A mean altitude effect accounting for −0.12‰/100 m for 18O and −0.58‰/100 m for 2H was discernible in precipitation isotope composition. Results from the hydrograph separation on a seasonal timescale indicate the dominance of event water, with an average of 71 and 64% of the total runoff during the wet season in the Chemoga and Jedeb catchments, respectively. Moreover, the stable isotope compositions of streamflow samples were damped compared to the input function of precipitation for both catchments. This damping was used to estimate mean residence times of stream water of 4.1 and 6.0 months at the Chemoga and Jedeb catchment outlets, respectively. Short mean residence times and high fractions of event water components recommend catchment management measures aiming at reduction of overland flow/soil erosion and increasing of soil water retention and recharge to enable sustainable development in these agriculturally dominated catchments.

International cross-validation of a BOD5 surrogate.

BOD5 dates back to 1912 when the Royal Commission decided to use the mean residence time of water in the rivers of England, 5 days, as a standard to measure the biochemical oxygen demand. Initially designed to protect the quality of river waters from extensive sewage discharge, the use of BOD5 has been quickly extended to waste water treatment plants (WWTPs) to monitor their efficiency on a daily basis. The measurement has been automatized but remains a tedious, time- and resource-consuming analysis. We have cross-validated a surrogate BOD5 method on two sites in France and in the USA with a total of 109 samples. This method uses a fluorescent redox indicator on a 96-well microplate to measure microbial catabolic activity for a large number of samples simultaneously. Three statistical tests were used to compare surrogate and reference methods and showed robust equivalence.

Geochemistry and residence time estimation of groundwater from Miocene–Pliocene and Upper Cretaceous aquifers of Southern Tunisia

Miocene–Pliocene and Upper Cretaceous formations in Tunisia contain one of the most productive artesian aquifers in the country. They consist of Mio-Pliocene sands and Senonian/Turonian carbonates. Intensive pumping in Kébili and Nefzaoua regions over the past decades resulted in gradual lowering of water table at the rate of approximately 0.85 m y−1. Forty-two groundwater samples have been collected from different sectors of the studied aquifers for chemical and isotopic analyses. The observed large spatial variability of chemical composition of groundwater in the study area is most probably linked to two processes: (i) dissolution of dolomite and gypsum, combined with calcite precipitation (dedolomitisation), and (ii) partial evaporation of water. The first process plays an important role in the study area due to abundance of evaporites. Partial evaporation occurs in the upper part of the unsaturated zone during infiltration, especially for groundwater sampled in the Kebili and Djerid regions. Apart from these processes, there are others which influence the salinity of the aquifers. In the Mio-Pliocene aquifer, which behaves as an open system to gases and which receives inputs of CO2 gas derived from intensive tectonic activity in the area, the interaction of carbon dioxide with carbonate matrix of the aquifer produces an increase in the alkalinity of water. In the Senonian and Turonian aquifers, the process of dedolomitisation evolves in a closed system with respect to CO2 gas. Ca2+/Na+ cation exchange and halite dissolution processes are also important. Stable isotope composition of water (δ18O, δ2H) indicates that the recharge occurs from the Dahar upland. The 14C activity varies between 89.5 (±1.5) and 3.7 (±2.1) pmc. The 13C content in the total dissolved inorganic carbon (TDIC) range between −13.9 and −3.6‰. The calculated concentrations of 13C in the CO2 gas in equilibrium with the TDIC vary between −22 and −11‰, indicating two sources of carbon in the solution: carbonate matrix (δ13C = −2‰) and soil CO2 (δ13C from −25 to −21‰ for the cultivated areas). Mean residence times of water have been determined after correction of the initial 14C activities for 14C-dilution processes including carbonate dissolution, calcite precipitation and cation-exchange. The dilution processes were quantified on the basis of geochemical and 13C mass balance equations. The calculated mean residence times of water confirm modern recharge from Dahar upland and the mountains surrounding depressions, and indicate the presence of paleowaters in the east and south-west region, and in the discharge zone.

Stable isotope characterization of the Vermigliana catchment

Summary Characterizing the hydrological behavior of streams in small Alpine catchments spanning a wide range of elevations is a difficult task, often hampered by the intrinsic variability of streamwater sources. Stable isotope ratios of oxygen and hydrogen have been sampled monthly in order to determine the spatial and temporal hydrological behavior and the mean residence time of water in the Vermigliana catchment, North-Eastern Italy. This study aims at separating contributions to streamflow originating from Presena and Presanella glaciers, both exerting a strong control on the hydrologic budget of the study site. The isotopic signature of precipitation has been collected at two locations at different altitudes (1176 m a.s.l. and 2731 m a.s.l.), while stream water was sampled at 11 locations, 8 along the main course of the Vermigliana creek and 3 along the two tributaries of the Vermigliana creek: the Presanella and Presena creeks. Groundwater was sampled monthly in a single location, whilst the waters of two small lakes, Capanna Presena and Cantiere, both located in the proximity of the Presena glacier, were sampled during summer, when the sites were accessible. Isotope analysis evidenced that Presena and Presanella creeks are the main contributors to the Vermigliana creek. The contribution of the Presanella creek is 44% of the total flow at the confluence with the Vermigliana, while the contribution of the Presena creek rises to 75% of the total flow immediately after the confluence. The mean residence times computed for the Vermigliana and the tributaries vary between 7 and 5 months, respectively. This work allows us to investigate the main components in the hydrological cycle of the Vermigliana catchment and constitutes the basis for future modeling and climate change impact studies on this important Alpine catchment. The methodology can be exported to other sites with the aim to provide additional data, with respect to streamflow at the catchment outlet, to reduce epistemic uncertainty of hydrological models.