Stable Isotopes 18O Research Articles

The hydrological characterisation and water budget of a South African rehabilitated headwater wetland system

This paper presents a synopsis of the findings of a valley bottom wetland monitoring study in which dominant hydrological processes maintaining the system are quantitatively defined. The Craigieburn-Manalana is a wetland subjected to technical rehabilitation, at the headwaters of the Sand River in the lowveld savanna region of South Africa. Findings include the identification of a rapid water delivery mechanism from the surrounding hillslopes to the wetland following a threshold-exceeding precipitation event, when hillslope-toe soil matric potential is close to 0, leading to a raising of the wetland water table by >0.7 m within 3 h. A summary of quantified fluxes and associated water budget of the wetland and its contributing catchment is developed. It is revealed that this wetland does not necessarily conform to the typical assumptions that wetlands augment low flows. Surface layer scintillometry shows actual wetland evapotranspiration to dominate the water budget during the dry season (2.3–3.5 mm/d) compared to its contributing catchment (0.9–2.2 mm/d), whilst stream discharge had ceased. Hydrograph separation, based on stable isotopes (18O), revealed that the wetland does not attenuate peak flows during the summer rains when the wetlands soil moisture deficit is close to 0, since more than 66% of stream discharge comprised event water. These results are discussed within the context of current hydrological understanding of southern African headwater wetlands, such as dambos. Keywords: hillslope processes, hydro-geomorphology, water budget, dambos, rehabilitation, wetlands

Hydrochemical and isotopic investigation of atmospheric precipitation in Beijing, China

Precipitation water samples were collected at an urban site in Beijing in a hydrological cycle (July 2008–July 2009), and analyzed for TDS, total alkalinity, total hardness, free CO2, soluble SiO2, bromide, sulfide, phosphate, major ions (K+, Na+, Ca2+, Mg2+, NH4+, HCO3−, Cl−, SO42−, NO3−), trace elements (CO32−, Mn, Sr2+, Fe2+, Fe3+, Al, F−, NO2−), stable isotopes (2H and 18O), and radioactive isotope (3H). In addition, available published hydrochemical and isotopic data of precipitation of Beijing in the past were also collected and conjointly analyzed. Most of the parameters of samples tested varied considerably in the hydrological cycle. In general, HCO3− and SO42−, and Ca2+ and NH4+ are the dominant anions and cations, respectively. Using Na+ as an indicator of marine origin, and Al for the terrestrial inputs, the proportions of major elements from sea salt and terrestrial sources were estimated by using the combination of statistical analysis methods and analogy method. More than 70.1% of Cl−, 98.1% of Ca2+, and 93.6% of K+ were non-sea-salt origin, while more than 98.4% of Na+ was from marine sources. The LMWL (Local Meteoric Water Line) was obtained with an equation of δ2H=7.0181δ18O+3.5231 (‰, R2=0.86, n=36), which was similar to GMWL (Global Meteoric Water Line). δ2H, δ18O and Δ-excess changed radically with month and season, but had no apparent seasonal effect, precipitation amount effect, and temperature effect. The annual mean values of Δ-excess for 1979 (16.5‰) and 1980 (16.3‰) were much bigger than that for 2007 (7.2‰), 2008 (2.1‰) and 2009 (4.5‰). The composition of 2H and 18O was probably intrinsically determined by the sources of water vapor and the physical and chemical processes occurred along the migration paths of water vapor from sources to the aimed precipitation area. Temporal change of 3H was only influenced by the nuclear testing in the early 1960s worldwide and the natural yield of 3H in the upper atmosphere intrinsically. 3H had nearly approached to the natural levels, which would bring difficulty if not invalidation to groundwater dating using 3H technique.

Deciphering the interaction between quaternary and continental Sabkhas aquifers in Central Tunisia using hydrochemical and isotopic tools

Climate aridity and intensive exploitation due to uncontrolled pumping for irrigation have caused a drastic decrease in the piezometric level of the shallow aquifer of Chougafiya plain, central Tunisia, and have seriously degraded groundwater quality. According to the hydrochemical data (Cl−, SO42−, NO3−, HCO3−, Br−, Na+, Mg2+, K+, Ca2+, Sr2+) and the stable isotopes (18O and 2H content), groundwater salinization in the investigated aquifer is caused by four main processes: (1) evaporite dissolution (2) cation exchange reactions (3) evaporation processes and (4) mixing with Sabkhas salt water causing salinity to increase in the central and southern parts of the basin. The radiogenic (3H) isotope data provided insight into the presence of significant contemporaneous recharge waters in the western part of the shallow aquifer. The movement of the tritiated water may have occurred according to the general flow path (NW–SE). When tritium was used in conjunction with the stable isotopes and chloride, the mixing process could be clearly identified, especially in the central part of the study area.

Accessing Long-Lived Nuclear Spin Order by Isotope-Induced Symmetry Breaking

Nuclear singlet states are nonmagnetic states of nuclear spin-1/2 pairs that may exhibit lifetimes much slower than the relaxation of the component spins in isolation. This feature makes them attractive vehicles for conveying nuclear hyperpolarization in NMR spectroscopy and magnetic resonance imaging experiments and for reducing signal losses in other NMR experiments caused by undesirably fast nuclear spin relaxation. Here we show access to (13)C(2) singlet states in a symmetrical oxalate molecule by substituting one or more (16)O nuclei by the stable nonmagnetic isotope (18)O. The singlet relaxation time of the (13)C(2) pair in [1-(18)O,(13)C(2)]-oxalate is 2-3 times longer than the spin-lattice relaxation time T(1).

Access to Any Site Directed Stable Isotope (2H, 13C, 15N, 17O and 18O) in Genetically Encoded Amino Acids

Proteins and peptides play a preeminent role in the processes of living cells. The only way to study structure-function relationships of a protein at the atomic level without any perturbation is by using non-invasive isotope sensitive techniques with site-directed stable isotope incorporation at a predetermined amino acid residue in the protein chain. The method can be extended to study the protein chain tagged with stable isotope enriched amino acid residues at any position or combinations of positions in the system. In order to access these studies synthetic methods to prepare any possible isotopologue and isotopomer of the 22 genetically encoded amino acids have to be available. In this paper the synthetic schemes and the stable isotope enriched building blocks that are available via commercially available stable isotope enriched starting materials are described.

Temporal and spatial variation of water stable isotopes (18O and2H) in the Kaidu River basin, Northwestern China

Water resources are the most critical factors to ecology and society in arid basins, such as Kaidu River basin. Isotope technique was convenient to trace this process and reveal the influence from the environment. In this paper, we try to investigate the temporal and spatial characteristics in stable isotope (18O and 2H) of surface water and groundwater in Kaidu River. Through the water stable isotope composition measurement, spatial and temporal characteristics of deuterium (δ2H) and oxygen 18 (δ18O) were analysed. It is revealed that (1) comparing the stream water line with the groundwater line and local meteorological water line of Urumqi City, it is found that the contribution of precipitation to surface water in stream runoff is the main source, whereas the surface water is the main source of groundwater. Groundwater is mainly drainage of surface runoff in the river; (2) in the main stream of Kaidu River, the spatial variability of river water showed a ‘heavier-lighter-heavier’ change along with the main stream for δ18O, and temporal variability showed higher in summer and lower in winter; (3) the δ18O and δ2H values of groundwater samples ranged from −11.36 to −7.97‰ and −73.45 to −60.05‰, respectively. There is an increasing trend of isotopic values along the groundwater flow path. The seasonal fluctuation of δ18O is not clear in most samples. Copyright © 2012 John Wiley & Sons, Ltd.

Estimating groundwater discharge to rivers from river chemistry surveys

AbstractEnvironmental tracer methods have been used to quantify groundwater discharge to rivers for the past few decades. A number of different tracers have been used in these studies, including individual ion concentrations, electrical conductivity, stable isotopes 2H and 18O, and the dissolved gases helium, chlorofluorocarbons and radon. This paper discusses the assumptions of the method, as well as its resolution and accuracy. The method will be most accurate when the tracer concentration in groundwater is very distinct from that in the river. On the basis of typical parameters, groundwater inflow rates as low as 5 mm/day can usually be estimated with electrical conductivity and ion tracers. A lower limit of resolution of approximately 2 mm/day is usually possible with radon, principally because the ratio of the river concentration to the groundwater concentration will be higher. However, hyporheic exchange can also contribute radon to the river. Where this process is significant, it is more difficult to estimate groundwater inflow from radon activities in the river, thus reducing the accuracy of the method. For CFCs, the lower limit of resolution is approximately 30 mm/day. Helium has not been widely used but can potentially be very accurate if the groundwater is old.The method assumes steady‐state conditions and so can only be applied when river flows are stable. Sampling resolution is also particularly important for dissolved gases, and uncertainty in where groundwater inflow occurs between sampling points can cause large uncertainty in inflow rates if the distance between sample locations is large. Poor mixing of solutes within the river can limit the method if the river is wide and shallow. When correctly applied, however, the environmental tracer method is able to provide robust estimates of groundwater discharge at a scale and accuracy that is not possible with most other methods. Copyright © 2012 John Wiley & Sons, Ltd.

Hydrodynamic study of karstic and intergranular aquifers using isotope geochemistry in Basara basin, Sulaimani, North-Eastern Iraq

The study area is located in the border of the high and low folded zone having a semi-arid climate area. This study initiated in 2009–2010, deals essentially with the investigation of the stable isotope (18O and deuterium 2H) as well as radioactive isotope represented by tritium 3H in rain, carbonate springs, intergranular aquifer, and surface water in order to investigate the source and relative ages of the groundwater and to show the influence of the altitude variations of the isotope composition. In this study and for the first time in the area, a local meteoric water line have been drawn with an empirical formula δ2H = 7.7δ18O + 14.4. The δ18O–altitude effect was determined using isotopic data for several samples taken from groundwater and surface water; it was approximated at −0.79 ‰/100 m for the entire area of the study. The output of the tritium concentrations in the springs and water well samples revealed to the conclusion that the values closely resembles to the present time tritium concentration in precipitation.

Determination of Oxygen Diffusion Coefficient in Oxidation Films of the AISI 439 Ferritic Stainless Steel

In order to investigate the role of oxygen diffusion in the oxidation process of the AISI 439 ferritic stainless steel, oxygen ion diffusion coefficients were determined, for the first time, in oxide films formed by the oxidation of this steel. Steel samples were firstly oxidized from 750o C to 900o C, in synthetic air, in order to grow oxide films mainly made up of chrome oxide; the oxygen diffusion experiments were then performed using the stable isotope 18O as oxygen tracer. The introduction of the 18O in the oxide film was performed by means of the gas-solid isotopic exchange method, in the temperature range of 750-900o C, in Ar+21%18O2 atmosphere. The 18O diffusion profiles were established by secondary ion mass spectrometry (SIMS). Parabolic oxidation constants calculated by means of Wagner´s theory, using the oxygen ion diffusion coefficient determined by our experimental process, are greater than oxidation constants previously determined in oxidation experiments from 850 to 950º C, in air, which indicates that the oxygen ion diffusion is large enough to assure the growth rate of the oxide film formed by the oxidation of the AISI 439 steel in these temperatures.

Determination of groundwater recharge regime and flowpath in the Lower Heihe River basin in an arid area of Northwest China by using environmental tracers: Implications for vegetation degradation in the Ejina Oasis

Abstract Environmental tracers (CFCs, stable isotopes 18O, 2H, and 3H) and major ions were employed to study river infiltration and groundwater recharge in the aquifer system in the basin of the Lower Heihe River, Northwest China. Three groups of waters have been recognized: (1) young groundwater, connected to the river, with large variation of CFC apparent ages ranging from 40 a, and being depleted in 18O and 2H compared with the river water; and (3) groundwater in Gurinai, a grassland located about 100 km from the river, in which the predominant discharge is from the Badain Jaran desert, with CFC apparent ages ranging from 25 to >50 a and being enriched in 18O and 2H compared to the river water. The groundwater along the river contains CFCs and 3H down to depths of about 120 m, and the shallow groundwater exhibits CFC apparent ages in a wide range which are not dependent on the well depth. Groundwaters along the river show a similar trend of enrichment in 18O and 2H as the river water whereas groundwaters in depression cones are depleted in heavier isotopes, and have low CFC and 3H concentrations. The CFC apparent age of the groundwater increases with increasing distance downstream, indicating that the dominant part of the groundwater is from infiltration of river water in the upper reaches. Modifications of groundwater recharge are reflected in variations of stable isotope compositions, as well as CFC and 3H concentrations in the groundwater that was recharged from the river over the last decades. Despite recharging from river water, groundwater abstraction has induced a water balance deficit. The riparian ecosystem in the Ejina Oasis is constrained by both decreased river flow and increased groundwater abstraction. The vegetation degradation in the Ejina Oasis is controlled not only by natural aridification but also worsened by heavy groundwater abstraction and decreased river flow.

Stable isotope composition of subfossil Cerastoderma glaucum shells from the Szczecin Bay brackish deposits and its palaeogeographical implications (South Baltic Coast, Poland)

The environmental conditions of the Szczecin Bay, which existed prior to Szczecin Lagoon, have been reconstructed on the basis of the stable carbon and oxygen isotope (18O and 13C) analysis and radiocarbon dates obtained for subfossil shells of Cerastoderma (Cardium) glaucum. The shells in the collected core were well preserved in their life positions, representing a geochemical record of past temperature variation over the middle Holocene. Three major periods with different thermal conditions have been distinguished in the interval ~6000–4300calyr BP, when the important Littorina regional transgression took place. During the first period, 6000–5250calyr BP, water temperature decreased by 1.4°C, and then remained constant over the second period (5250–4750calyr BP). In contrast, during the third period (4750–4300calyr BP) both δ-values were highly variable and the mean summer temperature (March–November) increased by about 3.5°C. During first two periods, δ18O and δ13C were significantly correlated, indicating stability of the environmental conditions.

Groundwater residence time downgradient of Trench No. 22 at the Chernobyl Pilot Site: Constraints on hydrogeological aquifer functioning

Following the explosion of reactor 4 at the Chernobyl power plant in northern Ukraine in 1986, contaminated soil and vegetation were buried in shallow trenches dug directly on-site in an Aeolian sand deposit. These trenches are sources of radionuclide (RN) pollution. The objective of the present study is to provide constraints for the Chernobyl flow and RN transport models by characterising groundwater residence time. A radiochronometer 3H/3He method (t1/2 = 12.3 a) and anthropogenic tracers including CFC and SF6 are investigated along with the water mass natural tracers Na, Cl, 18O and 2H. The groundwater is stratified, as evidenced by Na and Cl concentrations and stable isotopes (18O, 2H). In the upper aeolian layer, the Na–Cl relationship corresponds to evapotranspiration of precipitation, while in the underlying alluvial layer, an increase in Na and Cl with depth suggests both water–rock interactions and mixing processes. The 3H/3He and CFC apparent groundwater ages increase with depth, ranging from ‘recent’ (1–3 a) at a 2 m depth below the groundwater table to much higher apparent ages of 50–60 a at 27 m depth below the groundwater table. Discrepancies in 3H/3He and CFC apparent ages (20–25 a and 3–10 a, respectively) were observed during the 2008 campaign at an intermediate depth immediately below the aeolian/alluvial sand limit, which were attributed to the complex water transfer processes. Extremely high SF6 concentrations, well above equilibrium with the atmosphere and up to 1112 pptv, are attributed to significant contamination of the soils following the nuclear reactor explosion in 1986. The SF6 concentration vs. the apparent groundwater ages agrees with this interpretation, as the high SF6 concentrations are all more recent than 1985. The persistence of the SF6 concentration suggests that SF6 was introduced in the soil atmosphere and slowly integrated in the groundwater moving along the hydraulic gradient. The apparent age distribution in the lumped parameter models suggests an exponential or piston flow model in the upper geological section, followed by more pronounced mixing processes in the lower section.

Application of stable isotopes and hydrochemical analysis in groundwater aquifers of Argolis Peninsula (Greece)

The present study examines the isotopic and hydrochemical composition of 18 inland spring waters and 3 coastal karstic spring waters, covering the period between October 2005 and March 2008. The stable isotopes (18O, 2H) processing has revealed the absence of significant evaporation phenomena and that the origin of fresh water samples is meteoric. Using 18O values in rainfall waters, an average line of isotopic depletion with altitude has been constructed, extracting a rate of−0.45‰/100 m as typical for the study area. Furthermore, the mean altitude of recharge of the springs has been estimated by plotting the groundwater sampling points on a δ18O versus altitude diagram. Hydrochemistry results have shown that the dissolution of carbonate, flysch and ophiolitic formations defines the hydrochemical characteristics of groundwater. Moreover, seawater intrusion in the coastal area is significantly high, causing the water in the three karstic springs to be brackish.

Geochemical and isotopic investigation of groundwater mineralization process in the Zeroud basin, central Tunisia

The determination of the origin of the salinity in the Zeroud aquifer (central of Tunisia) and the understanding of its hydrogeological and geochemical behaviors related to severe natural and anthropogenic constraints were approached by the combined survey of major element concentrations and stable isotopes (18O, 2H). The hydrogeochemical investigations of the Zeroud aquifer show that groundwaters can be classified into two major water facies: Ca–SO4–Cl and Na–Cl. The main hydrogeochemical processes that typically control the groundwater mineralization are dissolution of evaporates (halite, gypsum and anhydrite), dedolomitization, cation exchange and mixing processes related to the return flow of irrigation waters and the artificial recharge operations. The stable isotope signatures reveal that the first significant recharge origin is related to the direct infiltration of actual meteoric water particularly observed in the foot of hills bordering the basin and in the vicinity of the Zeroud Wady. A main part of groundwater is significantly affected by evaporation, giving evidence of a recharge mostly from the dam water during the artificial recharge operations. Moderated evaporation of waters was observed in the southern part of the basin, especially in the agricultural areas.

Updated geochemical and isotopic data from the Continental Intercalaire aquifer in the Great Occidental Erg sub-basin (south-western Algeria)

Due to its size and to its strategic importance as a huge transboundary aquifer, the Continental Intercalaire (C.I.) has been widely studied, making use of the most classical groundwater research approaches as well as the latest techniques. Investigations focused more on the shared parts and neglected the Great Occidental Erg sub-basin occurring exclusively in Algeria. The present research deals with recent hydrochemical and isotope results obtained for that sub-basin. The latter has seen major developments and an increase in water demand during the last two decades. The Occidental sub-basin occurs in an arid region limited by the Atlas Mountains (north), Tademaït plateau (south), Saoura valley (west) and the M’Zab uplift (east). Rainfall is relatively high only in the mountainous recharge area (Saharan Atlas). Rainfall decreases southward with decreasing altitude, from ∼200 mm to 20 mm yearly in the discharge area. The sub-basin is characterized by a dense run-off network (wadis flowing southward from the southern Atlas foothills). The C.I. reservoir is contained in the continental formations of the lower Cretaceous. The aquifer is mostly unconfined, unlike the eastern part. Its depth increases from north to south. It has a shape of a depression that disappears further south where it becomes confined. TDS increases southward along the recognized flowpath. Sample points align parallel to the halite dissolution line on a Cl–Na plot, confirming the origin of those elements. Major elements increase with the increase of TDS. Moreover, a good correlation between calcium and sulfates suggests gypsum and anhydrite leaching as water flows underground. Isotopically speaking, groundwater are enriched and exhibit large variations in stable isotopes (18O and 2H). They are distributed along an evaporation line corresponding to a mixing of two end-members: the Erg aquifer that feeds the C.I. in the north, and unaffected C.I. groundwater in the south. In the west, Saoura valley alluvial aquifer samples align in the continuation of the evaporated C.I. line, mirroring the intermingling of two aquifers during wadi Saoura flooding. Radiocarbon data showed that appreciable amount of 14C in the C.I. is traceable only up to few distances from the aquifer exposure zones up north (Atlas escarpments). The confined part of the reservoir shows depleted stable isotopes data and low 14C content, reflecting the absence of modern water at the discharge zone.

Assessing impact of irrigation water on groundwater recharge and quality in arid environment using CFCs, tritium and stable isotopes, in the Zhangye Basin, Northwest China

Environmental tracers CFCs, 18O, 2H and tritium were used to determine the natural groundwater recharge and the impact of irrigation activity on the groundwater system in the semi-arid Zhangye Basin of China. Groundwaters in the irrigated areas have been identified as mixtures containing fractions recharged in different periods of time. The CFC and 3H data show that the oldest fraction in the groundwater was recharged before 1950, whereas the younger fractions were recharged in different periods of time since 1950. Stable isotope (18O, 2H), CFC and electrical conductivity data show that most of the samples can be regarded as binary mixtures with the river/irrigation water presents the younger fraction and the regional groundwater presents the older fraction. Binary mixing model is used to estimate the age and fraction of the younger component. Most of the younger fraction was recharged after 1980s, in response to the increasing irrigation activities. Compared to local precipitation surface water plays a major role in recharging the aquifer in the irrigated area. The irrigation activity had more impact on the aquifer under thin unsaturated zone (<10 m), due to short travel times and high amounts of recharge, whereas it had less impact on the aquifer under thick unsaturated zone (tens of meters). CFCs are useful in identifying regions of different impact of irrigation return flow. The positive correlation between nitrate and CFC data show that contaminants are transported to the saturated zone by irrigation water. This study shows that in this semi-arid basin due to strong evaporation of infiltrating surface water and regional groundwater, δ18O and EC values, in contrast to CFCs, do not show simple relationship with NO3- concentration in groundwater. Combined with a proper mixing model, however, they can provide evidences that the CFCs found in groundwater were introduced by infiltrating irrigation return flow and, therefore, reveal that human activities can produce a much localized water circulation and influence groundwater vulnerability.

Oxygen isotopic signature of CO&amp;lt;sub&amp;gt;2&amp;lt;/sub&amp;gt; from combustion processes

Abstract. For a comprehensive understanding of the global carbon cycle precise knowledge of all processes is necessary. Stable isotope (13C and 18O) abundances provide information for the qualification and the quantification of the diverse source and sink processes. This study focuses on the δ18O signature of CO2 from combustion processes, which are widely present both naturally (wild fires), and human induced (fossil fuel combustion, biomass burning) in the carbon cycle. All these combustion processes use atmospheric oxygen, of which the isotopic signature is assumed to be constant with time throughout the whole atmosphere. The combustion is generally presumed to take place at high temperatures, thus minimizing isotopic fractionation. Therefore it is generally supposed that the 18O signature of the produced CO2 is equal to that of the atmospheric oxygen. This study, however, reveals that the situation is much more complicated and that important fractionation effects do occur. From laboratory studies fractionation effects on the order of up to 26%permil; became obvious in the derived CO2 from combustion of different kinds of material, a clear differentiation of about 7‰ was also found in car exhausts which were sampled directly under ambient atmospheric conditions. We investigated a wide range of materials (both different raw materials and similar materials with different inherent 18O signature), sample geometries (e.g. texture and surface-volume ratios) and combustion circumstances. We found that the main factor influencing the specific isotopic signatures of the combustion-derived CO2 and of the concomitantly released oxygen-containing side products, is the case-specific rate of combustion. This points firmly into the direction of (diffusive) transport of oxygen to the reaction zone as the cause of the isotope fractionation. The original total 18O signature of the material appeared to have little influence, however, a contribution of specific bio-chemical compounds to individual combustion products released from the involved material became obvious.

Seasonal evolution of water contributions to discharge from a Greenland outlet glacier: insight from a new isotope-mixing model

AbstractThe Greenland ice sheet (GrIS) subglacial hydrological system may undergo a seasonal evolution, with significant geophysical and biogeochemical implications. We present results from a new isotope-mixing model to quantify the relative contributions of surface snow, glacial ice and delayed flow to the bulk meltwater discharge from a small (∼5 km2) land-terminating GrIS outlet glacier during melt onset (May) and at peak melt (July). We use radioactive (222Rn) and stable isotopes (18O, deuterium) to differentiate the water source contributions. Atmospherically derived 7Be further constrains meltwater transit time from the glacier surface to the ice margin. We show that (1) 222Rn is a promising tracer for glacial waters stored at the bed and (2) a quantitative chemical mixing model can be constructed by combining 222Rn and the stable water isotopes. Applying this model to the bulk subglacial outflow from our study area, we find a constant delayed-flow (stored) component from melt onset through peak melt. This component is diluted first by snowmelt and then by increasing glacial ice melt as the season progresses. Results from this pilot study are consistent with the hypothesis that subglacial drainage beneath land-terminating sections of the GrIS undergoes a seasonal evolution from a distributed to a channelized system.

Origin of flooding water through hydrogeochemical identification, the Buenos Aires plain, Argentina

Hydrogeochemical behaviour of samples of surface and groundwater collected on a cross-section from Mendoza to the Buenos Aires provinces was studied based on chemical trends, mass balance and water mixing. Hydrogeochemical modelling included major, minor elements as well as stable isotopes (deuterium and 18-O). The area investigated is located in the “Medanos Longitudinales” (longitudinal dunes) of the northwest of Buenos Aires province, Argentina. The study area is subject to alternating flooding and drought. Rainfall and surface water subsequently transferred by rivers, canals and lagoons have been usually considered responsible for local flooding. For this study, origins of excess water were investigated using physical and chemical characteristics of the water involved. The prevalence of groundwater inflow to rainfall events was proposed based on data interpretation. Groundwater influence of flows of local and intermediate nature were defined and the importance of recharge, transit and discharge zones was highlighted. Lagoon floodwater, as well as groundwater from observation wells and production boreholes, show components of intermediate origin. Regional recharge water was identified in Mendoza and San Luis provinces. Their discharge zone was inferred to be located beyond the Buenos Aires province.

An approach to identify urban groundwater recharge

Abstract. Evaluating the proportion in which waters from different origins are mixed in a given water sample is relevant for many hydrogeological problems, such as quantifying total recharge, assessing groundwater pollution risks, or managing water resources. Our work is motivated by urban hydrogeology, where waters with different chemical signature can be identified (losses from water supply and sewage networks, infiltration from surface runoff and other water bodies, lateral aquifers inflows, ...). The relative contribution of different sources to total recharge can be quantified by means of solute mass balances, but application is hindered by the large number of potential origins. Hence, the need to incorporate data from a large number of conservative species, the uncertainty in sources concentrations and measurement errors. We present a methodology to compute mixing ratios and end-members composition, which consists of (i) Identification of potential recharge sources, (ii) Selection of tracers, (iii) Characterization of the hydrochemical composition of potential recharge sources and mixed water samples, and (iv) Computation of mixing ratios and reevaluation of end-members. The analysis performed in a data set from samples of the Barcelona city aquifers suggests that the main contributors to total recharge are the water supply network losses (22%), the sewage network losses (30%), rainfall, concentrated in the non-urbanized areas (17%), from runoff infiltration (20%), and the Besòs River (11%). Regarding species, halogens (chloride, fluoride and bromide), sulfate, total nitrogen, and stable isotopes (18O, 2H, and 34S) behaved quite conservatively. Boron, residual alkalinity, EDTA and Zn did not. Yet, including these species in the computations did not affect significantly the proportion estimations.