A study of the interrelation between surface water and groundwater using isotopes and chlorofluorocarbons in Sanjiang plain,Northeast China

Surface water and groundwater are the main water resources used for drinking and production. Assessments of the relationship between surface water and groundwater provide information for water resource management in Sanjiang plain, Northeast China. The surface water (river, lake, and wetland) and groundwater were sampled and analyzed for stable isotopic (δD, δ ¹⁸O) composition, tritium, and chlorofluorocarbons concentrations. The local meteoric water line is δD = 7.3δ ¹⁸O–6.7. The tritium (T) and chlorofluorocarbon (CFC) contents in groundwater were analyzed to determine the groundwater ages. Most groundwater were modern water with the ages <50 years. The groundwaters in mountain area and near rivers were younger than in the central plain. The oxygen isotope (δ ¹⁸O) was used to quantify the relationship between surface water and groundwater. The Songhua, Heilongjiang, and Wusuli rivers were gaining rivers, but the shallow groundwater recharged from rivers at the confluence area of rivers. At the confluence of Songhua and Heilongjiang rivers, 88 % of the shallow groundwater recharged from Songhua river. The combination of stable isotopes, tritium, and CFCs was an effectively method to study the groundwater ages and interrelation between surface water and groundwater. Practically, the farmlands near the river and under foot of the mountain could be cultivated, but the farmlands in the central plain should be controlled.     

The interaction between surface water and groundwater and its effect on water quality in the Second Songhua River basin,northeast China

The relationship between surface water and groundwater not only influences the water quantity, but also affects the water quality. The stable isotopes (δD, δ ¹⁸O) and hydrochemical compositions in water samples were analysed in the Second Songhua River basin. The deep groundwater is mainly recharged from shallow groundwater in the middle and upper reaches. The shallow groundwater is discharged to rivers in the downstream. The runoff from upper reaches mainly contributed the river flow in the downstream. The CCME WQI indicated that the quality of surface water and groundwater was ‘Fair’. The mixing process between surface water and groundwater was simulated by the PHREEQC code with the results from the stable isotopes. The interaction between surface water and groundwater influences the composition of ions in the mixing water, and further affects the water quality with other factors.     

The impact of hydrological conditions on salinisation and nitrate concentration in the coastal Velez River aquifer (southern Spain)

This study reports the impact of hydrological conditions on salinisation and nitrate concentrations of a coastal aquifer located at the Mediterranean Sea, southern Spain. Eighty-two samples of ground- and surface water taken during two extreme hydrological events between 1994 and 1996 at 25 different wells were evaluated with regard to hydrochemistry, focusing on nitrate concentrations and salinisation, which constitute the main hazard of this aquifer. Furthermore, hydrochemical data were analysed by principal component analysis (PCA). Additionally, in 2007 13 ground- and surface water samples taken at 12 different locations were analysed for stable isotopes of D/¹⁸O, and one sample was analysed for ¹⁵N. Since 1993 until present saltwater intrusion was observed only during dry hydrological conditions in 1994; it showed an irregular salinisation pattern probably related to locally elevated hydraulic conductivities. Nitrate concentrations increase significantly during wet hydrologic conditions owing to uptake of nitrate by rising groundwater. Stable isotopes of groundwater reveal an Atlantic origin of the precipitation that recharges the aquifer and a minor amount of groundwater recharge by the water coming from the La Viñuela reservoir, which is used for irrigation over the aquifer. ¹⁵N isotopes point to a considerable input of nitrates derived from organic fertilisers.     

Groundwater recharge and evolution in the Dunhuang Basin,northwestern China

Groundwater recharge and evolution in the Quaternary aquifer beneath the Dunhuang Basin was investigated using chemical indicators, stable isotopes, and radiocarbon data to provide guidance for regional water management. The quality of groundwater and surface water is generally good with low salinity and it is unpolluted. The dissolution of halite and sylvite from fine-grained sediments controls concentrations of Na⁺ and K⁺ in the groundwater, but Na⁺/Cl⁻ molar ratios >1 in all samples are also indicative of weathering of feldspar contributing to excess Na⁺. The dissolution of carbonate minerals yields Ca²⁺ to the groundwater, thereby exerting a strong influence on groundwater salinity. The δ¹⁸O and δ²H values in unconfined groundwater are enriched along the groundwater flow path from SW to NE. In contrast, confined groundwater was depleted in heavy isotopes, with mean values of −10.4‰ δ¹⁸O and −74.4‰ δ²H. Compared with the precipitation values, all of the groundwater samples were strongly depleted in heavy isotopes, indicating that modern direct recharge to the groundwater aquifers in the plains area is quite limited. The unconfined water is generally young with radiocarbon values of 64.9–79.6 pmc. In the northern basin, radiocarbon content in the confined groundwater is less than 15 pmc and an uncorrected age of ∼15 ka, indicates that this groundwater was recharged during a humid climatic phases of the late Pleistocence or early Holocene. The results have important implications for inter-basin water allocation programmes and groundwater management in the Dunhuang Basin.     

Hydrogeochemical and Isotopic Study of Groundwater in a Semi-arid Region: Yeniceoba Plain (Cihanbeyli-KONYA), Central Anatolia,Turkey

Groundwater is the most important source of water supply in the Yeniceoba Plain in Central Anatolia,Turkey.An understanding of the geochemical evolution of groundwater is important for the sustainable development of water resources in this region.A hydrogeochemical investigation was conducted in the Plio-Quaternary aquifer system using stable isotopes(δ~(18)O andδD),tritium(~3H),major and minor elements(Ca,Na,K,Mg,Cl,SO_4,NO_3,HCO_3 and Br)in order to identify groundwater chemistry patterns and the processes affecting groundwater mineralization in this system.The chemical data reveal that the chemical composition of groundwater in this aquifer system is mainly controlled by rock/water interactions including dissolution of evaporitic minerals,weathering of silicates,precipitation/dissolution of carbonates,ion exchange,and evaporation.Based on the values of Cl/Br ratio(300 mg/l)in the Plio-Quaternary groundwater,dissolution of evaporitic minerals in aquifer contributes significantly to the high mineralization.The stable isotope analyses indicate that the groundwater in the system was influenced by evaporation of rainfall during infiltration.Low tritium values(generally1 tritium units)of groundwater reflect a minor contribution of recent recharge and groundwater residence times of more than three or four decades.     

Hydrogeological and hydrochemical investigation of groundwater using environmental isotopes (18O, 2H, 3H, 14C) and chemical tracers: a case study of the intermediate aquifer,Sfax, southeastern Tunisia

Major element concentrations and stable (δ¹⁸O and δ²H) and radiogenic (³H and ¹⁴C) isotopes in groundwater have proved useful tracers for understanding the geochemical processes that control groundwater mineralization and for identifying recharge sources in the semi-arid region of Sfax (southeastern Tunisia). Major-ion chemical data indicate that the origins of the salinity in the groundwater are the water–rock interactions, mainly the dissolution of evaporitic minerals, as well as the cation exchange with clay minerals. The δ¹⁸O and δ²H relationships suggest variations in groundwater recharge mechanisms. Strong evaporation during recharge with limited rapid water infiltration is evident in the groundwater of the intermediate aquifer. The mixing with old groundwater in some areas explains the low stable isotope values of some groundwater samples. Groundwaters from the intermediate aquifer are classified into two main water types: Ca-Na-SO₄ and Ca-Na-Cl-SO₄. The high nitrate concentrations suggest an anthropogenic source of nitrogen contamination caused by intensive agricultural activities in the area. The stable isotopic signatures reveal three water groups: non-evaporated waters that indicate recharge by recent infiltrated water; evaporated waters that are characterized by relatively enriched δ¹⁸O and δ²H contents; and mixed groundwater (old/recent) or ancient groundwater, characterized by their depleted isotopic composition. Tritium data support the existence of recent limited recharge; however, other low tritium values are indicative of pre-nuclear recharge and/or mixing between pre-nuclear and contemporaneous recharge. The carbon-14 activities indicate that the groundwaters were mostly recharged under different climatic conditions during the cooler periods of the late Pleistocene and Holocene.

     

Oxygen, hydrogen, and helium isotopes for investigating groundwater systems of the Cape Verde Islands, West Africa

Stable isotopes (δ¹⁸O, δ²H), tritium (³H), and helium isotopes (³He, ⁴He) were used for evaluating groundwater recharge sources, flow paths, and residence times of three watersheds in the Cape Verde Islands (West Africa). Stable isotopes indicate the predominance of high-elevation precipitation that undergoes little evaporation prior to groundwater recharge. In contrast to other active oceanic hotspots, environmental tracers show that deep geothermal circulation does not strongly affect groundwater. Low tritium concentrations at seven groundwater sites indicate groundwater residence times of more than 50 years. Higher tritium values at other sites suggest some recent recharge. High ⁴He and ³He/⁴He ratios precluded ³H/³He dating at six sites. These high ³He/⁴He ratios (R/R_a values of up to 8.3) are consistent with reported mantle derived helium of oceanic island basalts in Cape Verde and provided end-member constraints for improved dating at seven other locations. Tritium and ³H/³He dating shows that São Nicolau Island’s Ribeira Fajã Basin has groundwater residence times of more than 50 years, whereas Fogo Island’s Mosteiros Basin and Santo Antão Island’s Ribeira Paul Basin contain a mixture of young and old groundwater. Young ages at selected sites within these two basins indicate local recharge and potential groundwater susceptibility to surface contamination and/or salt-water intrusion.     

Environmental isotopic and hydrochemical study of water in the karst aquifer and submarine springs of the Syrian coast

The groundwater of major karst systems and submarine springs in the coastal limestone aquifer of Syria has been investigated using chemical and isotopic techniques. The δ¹⁸O values of groundwater range from ?6.8 to ?5.05‰, while those for submarine springs vary from ?6.34 to +1.08‰ (eastern Mediterranean seawater samples have a mean of +1.7‰). Groundwater originates from the direct infiltration of atmospheric water. Stable isotopes show that the elevation of the recharge zones feeding the Banyas area (400–600 m a.s.l.) is higher than that feeding the Amrit area (100–300 m a.s.l.). The ¹⁸O_extracted (¹⁸O content of the seawater contribution) for the major submarine springs suggests a mean recharge area elevation of 600–700 m a.s.l., and lower than 400 m a.s.l. for the spring close to Amrit. Based on the measured velocity and the percentage of fresh water at the submarine springs outlet, the estimated discharge rate is 350 million m³/year. The tritium concentrations in groundwater (1.6–5.9 TU) are low and very close to the current rainfall values (2.9–5.6 TU). Adopting a model with exponential time distribution, the mean turnover time of groundwater in the Al-sen spring was evaluated to be 60 years. A value of about 3.7 billion m³ was obtained for the maximum groundwater reservoir size.     

Using hydrogen,oxygen, and tritium isotopes to identify the hydrological factors contributing to landslides in a mountainous area,central Taiwan

Over a 1-year period, 343 samples, including precipitation, creek, pond, and groundwater, were collected from June 2003 to May 2004. Analyses were performed for stable oxygen and hydrogen isotope compositions. Selected samples were also analyzed for tritium. The goal was to identify possible hydrologic factors contributing to a severe landslide in the Li-Shan area, central Taiwan. The isotope characteristics indicate that groundwater from Fu-Shou-Shan farm located up-slope from the landslide area is a major source for slope groundwater, in addition to precipitation. The groundwater is mainly recharged by pond water at Fu-Shou-Shan farm. According to the calculation of a two-end member equation with δ ¹⁸O, the contribution of farm groundwater to slope groundwater is significantly higher than that of precipitation, up to a factor of five. The estimated drainage efficiency of the existing system is only 23%. Draining off the slope groundwater in the up-slope region to decrease farm groundwater flow into the slope area is a feasible strategy to effectively reduce the risk of landslide.     

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

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 <10 a to 40 a, and δ¹⁸O and δ²H values which are similar to the river water; (2) regional background water, unaffected by the river, having CFC apparent ages >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.     

Impacts of agricultural irrigation recharge on groundwater quality in a basalt aquifer system (Washington, USA): a multi-tracer approach

Irrigation in semi-arid agricultural regions can have profound effects on recharge rates and the quality of shallow groundwater. This study coupled stable isotopes (²??, ¹⁸O), age-tracers (³H, CFCs, ¹⁴C), ⁸⁷Sr/⁸⁶Sr ratios, and elemental chemistry to determine the sources, residence times, and flowpaths of groundwater and agricultural contaminants (e.g. NO ₃ ^?C ) in the Saddle Mountains Basalt Aquifer in central Washington, USA, where over 80% of the population depend on groundwater for domestic use. Results demonstrate the presence of two distinct types of water: contaminated irrigation water and pristine regional groundwater. Contaminated irrigation water has high NO ₃ ^?C concentrations (11?C116? mg/l), ⁸⁷Sr/⁸⁶Sr ratios (0.70659?C0.71078) within range of nitrogen-based fertilizers, detectable tritium (2.8?C13.4 TU), CFC ages 20?C40?years, high ??¹⁸O values (?16.9 to ?13.5??), and ??100 percent modern ¹⁴C. Pristine regional groundwater has low NO ₃ ^?C concentrations (1?C5? mg/l), no detectable tritium (??0.8 TU), low ??¹⁸O values (?18.9 to ?17.3??) and ¹⁴C ages from ??15 to 33?ky BP. Nitrogen and oxygen isotopes of NO ₃ ^?C , combined with high dissolved oxygen values, show that denitrification is not an important process in the organic-poor basalt aquifers resulting in transport of high NO ₃ ^?C irrigation water to depths greater than 40?m in less than 30? years.     

Tracing stable isotope values from meteoric water to groundwater in the southwestern part of the Chad basin

The southwestern Chad basin is a semi-arid region with annual rainfall that is generally less than 500 mm and over 2,000 mm of evapotranspiration. Surface water in rivers is seasonal, and therefore groundwater is the perennial source of water supply for domestic and other purposes. Stable isotope has been measured for rainwater, surface water and groundwater samples in this region. The stable isotope data have been used to understand the inter-relationships between the rainwater, surface water, shallow and deep groundwater of this region. This is being used in a qualitative sense to demonstrate present day recharge to the groundwater. Stable isotope in rainwater for the region has an average value of –4‰ δ¹⁸O and –20‰ δ²H. Surface water samples from rivers and Lake Chad fall on the evaporation line of this average value. The Upper Zone aquifer water samples show stable isotope signal with a wide range of values indicating the complex character of the aquifer Zone with three distinguishable units. The wide range of values is attributable to waters from individual unit and/or mixture of waters of different units. The Middle and Lower aquifers Zones’ waters show similar stable isotopes values, probably indicating similarity in timing and/or mechanism of recharge. These are palaeowaters probably recharged under a climate that is different from today. The Upper Zone aquifer is presently being recharged as some of its waters show stable isotope compositions similar to those of average rainfall waters of the region.     

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 δ ²H?=?7.7δ ¹⁸O?+?14.4. The δ ¹⁸O–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.     

Uranium isotopes (234U/238U) in rivers of the Yukon Basin (Alaska and Canada) as an aid in identifying water sources, with implications for monitoring hydrologic change in arctic regions

The ability to detect hydrologic variation in large arctic river systems is of major importance in understanding and predicting effects of climate change in high-latitude environments. Monitoring uranium isotopes (²³⁴U and ²³⁸U) in river water of the Yukon River Basin of Alaska and northwestern Canada (2001?C2005) has enhanced the ability to identify water sources to rivers, as well as detect flow changes that have occurred over the 5-year study. Uranium isotopic data for the Yukon River and major tributaries (the Porcupine and Tanana rivers) identify several sources that contribute to river flow, including: deep groundwater, seasonally frozen river-valley alluvium groundwater, and high-elevation glacial melt water. The main-stem Yukon River exhibits patterns of uranium isotopic variation at several locations that reflect input from ice melt and shallow groundwater in the spring, as well as a multi-year pattern of increased variability in timing and relative amount of water supplied from higher elevations within the basin. Results of this study demonstrate both the utility of uranium isotopes in revealing sources of water in large river systems and of incorporating uranium isotope analysis in long-term monitoring of arctic river systems that attempt to assess the effects of climate change.     

Preliminary isotopic studies in the Bida basin,central Nigeria

The area of the Bida basin is lacking in essential data for effective groundwater management. Hydrological studies using environmental isotopes (²H,³H,¹³C,¹⁸O) were carried out in the basin during the 1987–1988 hydrological year on samples of rainfall, streams, springs, and groundwater. The results obtained results do not indicate the presence of paleowaters; rather all groundwaters in the basin are found to be of meteoric origin as bothD and ¹⁸O values of the samples, plotted in the conventional way, align with the meteoric water line (MWL). Moreover, these data show the deeper, and apparently older, groundwater to be more depleted in the heavy stable isotopes than the shallower, relatively younger groundwaters, indicating recharge under cooler climatic conditions for the former. Measured tritium values range from 0.7 to 23.8 TU and show a variation with both depth and lithology, while at the same time indicating that active recent recharge is taking place basin-wide. The few¹³C data available were used to illustrate the effect of pumping on the carbonate geochemistry of the groundwaters. Also, the stable isotope data from the basin were compared with similar data from other parts of the country to show the continental effect.     

Hydrological characteristics of karstic groundwater in the northeast Viet Nam as studied by isotopic techniques

The stable water isotopic composition (δ²H and δ¹⁸Ο), tritium (³H) activity, dissolved organic carbon, alkalinity, as well as the composition of carbon 13 (δ¹³C) in dissolved inorganic carbon (DIC) of 36 water samples taken from 16 resurgences in the northeast provinces of Viet Nam in the dry (Mar 2008) and rainy (June 2008) seasons were analyzed to elucidate hydrological characteristics of the karstic aquifers in the area. The stable water isotopic composition of the water samples collected clearly demonstrated that the karstic groundwater in the region was recharged from the local meteorological water. The tritium activity in the samples was found to be in between 3 and 4 TU, falling in the range of the ³H activity in the local precipitation and thus meaning that the traveling of recharge water to the resurgences was very short. Concentrated and diffuse allogenic recharges seem to be important sources of karstic groundwater in the study region. Water in the karstic aquifers could be classified into three types as: (a) water from karstic areas with dense vegetation cover that causes DIC be depleted in carbon 13 (¹³δ<?12‰ vs. Pee Dee Belemnite standard of Vienna, VPDB); (b) water from karstic areas with poor vegetation cover that originates DIC with carbon 13 composition ranging from ?11 to ?12‰; and (c) surface water from lakes, springs and rivers that has DIC with enriched carbon 13 (δ¹³C >?10‰). This implies that there are several sources of carbon dioxide contributing to the DIC in water of the karstic aquifers in the study region. Among other potential sources, the atmospheric CO₂, CO₂ from carbonate rock dissolution, biomineralization of soil organic matters and plant roots respiration seem to be important sources of the DIC in the waters of this region. The results show high vulnerability towards anthropogenic contaminants of karstic groundwater in the study region.     

An assessment of groundwater salinization in Haryana state in India using hydrochemical tools in association with GIS

Unplanned abstraction of groundwater due to various land use land cover activities and variations in monsoonal rainfall have greatly affected the availability and quality of groundwater resources in semi-arid regions of India. In the present study, a study of the hydrogeochemical characteristics of groundwater was undertaken in the Sonipat district of Haryana in India together with the use of stable isotope (δ¹⁸O and δD) measurements and GIS analysis. A total of 53 groundwater samples were collected from seven blocks of the district, and 14 water quality parameters and stable isotopes (δ¹⁸O and δD) were analysed to infer hydrogeochemical processes taking place in the area. The integration of hydrochemistry with GIS is very helpful to understand the factors governing in the area. The majority of the samples showed Na–Cl type of hydrochemical facies. The trilinear plot for major cations and anions in groundwater indicates dominance of sodium, calcium, chloride and bicarbonate ions. Nitrate plumes in the groundwater appear to be migrating in groundwater from the central and south-western parts of the area towards the urbanized areas. A total of 64% of the samples exceed the maximum permissible limit of 1.5 mg/L given by WHO for fluoride. Besides natural sources such as silicate and carbonate weathering, ion exchange, and reverse ion exchange, the leaching of surficial salts and untreated industrial wastes along with unregulated abstraction are contributing to poor groundwater quality in the study area. An assessment of saturation indices has shown that groundwater in the area is unsaturated with respect to anhydrite, halite and gypsum suggesting significant contribution of Ca²⁺, Mg²⁺ and other ions in the groundwater. A scatter plot of δ¹⁸O versus Cl also suggests mixing of saline water with fresh groundwater.     

基于氢氧稳定同位素的武汉北部新城地表水-地下水转换关系研究

【研究目的】揭示武汉北部新城地表水、地下水的氢氧稳定同位素特征及其相互作用。【研究方法】2019年，采集、测定了降水样7件、河水样6件、水库样14件、民井样98件、泉水样3件和钻孔样11件，并收集到武汉站1986—1998年的监测数据50件，以空间分析和流域分析为基础，氢氧稳定同位素分析为手段。【研究结果】（1）武汉降水氢氧同位素随季节变化，并表现出“降雨效应”明显、“温度效应”不明显的特点；（2）地表水在枯水期受到强烈的蒸散发，表现出一定的“地貌效应”与“干支流效应”的特征；（3）民井、泉和钻孔等地下水均源于大气降水，表现出“含水层埋深效应”与“山区平原效应”的特点；（4）枯水期，界河流域中界河获得了上游水库和地下水的补给，夏家寺水库流域中夏家寺水库得到了地下水补给。【结论】氢氧同位素能显著提高武汉北部新城地表水-地下水相互转换规律的认识。创新点：利用各类水体氢氧同位素组成及空间分布特征，揭示了武汉北部新城降水、地表水和地下水相互转换的规律     

Environmental isotopic study on the recharge and residence time of groundwater in the Heihe River Basin, northwestern China

The recharge and origin of groundwater and its residence time were studied using environmental isotopic measurements in samples from the Heihe River Basin, China. δ¹⁸O and δD values of both river water and groundwater were within the same ranges as those found in the alluvial fan zone, and lay slightly above the local meteoric water line (δD=6.87δ¹⁸O+3.54). This finding indicated that mountain rivers substantially and rapidly contribute to the water resources in the southern and northern sub-basins. δ¹⁸O and δD values of groundwater in the unconfined aquifers of these sub-basins were close to each other. There was evidence of enrichment of heavy isotopes in groundwater due to evaporation. The most pronounced increase in the δ¹⁸O value occurred in agricultural areas, reflecting the admixture of irrigation return flow. Tritium results in groundwater samples from the unconfined aquifers gave evidence for ongoing recharge, with mean residence times of: less than 36 years in the alluvial fan zone; about 12–16 years in agricultural areas; and about 26 years in the Ejina oasis. In contrast, groundwater in the confined aquifers had ¹⁴C ages between 0 and 10 ka BP.     

Geochemical and isotopic characterization of groundwater from shallow and deep limestone aquifers system of Aleppo basin (north Syria)

Stable isotopes (δ¹⁸O, δ²H and ¹³C) and radioactivity (³H, ¹⁴C) have been used in conjunction with chemical data to evaluate the processes generating the chemical composition, reconstruct the origin of the water and groundwater residence time. The Aleppo basin is comprised of two main limestone aquifers: the first one is unconfined of Paleogene age and the second is confined of Upper Cretaceous age. The chemical data indicate that the dissolution of minerals and evaporation are the main processes controlling groundwater mineralization. The groundwater from the two aquifers is characterized by distinctive stable isotope signatures. This difference in water isotopes is interpreted in terms of difference origin and recharge period. Fresh and brackish shallow groundwater were mostly recharged during the Holocene period. The presence of ³H in several groundwater samples of this aquifer gives evidence that groundwater recharge is going on. Brackish water of the deep confined aquifer has depleted stable isotope composition and very low ¹⁴C activity that indicates recharge during the late Pleistocene cold period.