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A multidisciplinary approach including environmental isotope (δ2H, δ18O and 3H), hydrochemistry and electrical resistivity tools were used to revive a part of the drought prone areas of Vidarbha region, Maharashtra, India. In this study, groundwater recharge processes, monsoon characteristics, climate variables during evaporation in the region were evaluated using environmental isotope techniques. Additionally, electrical resistivity surveys were also conducted to understand the aquifer geometry and weak zones (fractures, lineament) for the sustainable groundwater development. Significant seasonal variation observed in stable isotopic composition, deuterium excess and tritium in precipitation helped to delineate the sources of vapour generating the rains during both the southwest and northeast monsoons. A negative correlation between stable isotopic content and amount of precipitation is observed. Two sources of groundwater recharge, namely, natural recharge from precipitation and contribution from artificial recharge structures (detention tanks) were deduced. Hydrochemical data also corroborates the above findings. Environmental isotope data show that the groundwater is predominantly recharged from the southwest monsoon, and the contribution from the northeast monsoon is insignificant. Tritium data indicate that the groundwater is modern and its residence time is of the order of a few years. Modelling of evaporative isotopic enrichment helped in estimating the climatic variables such as temperature and humidity during evaporation. Also, the recharge from the detention tanks was quantified based on isotope balance model. Aquifer geometry and groundwater potential zones were delineated in a part of the study area using electrical resistivity method. An integrated approach of multidisciplinary techniques is found to be effective in assessing the recharge processes and groundwater potentiality in drought prone areas.

Hydrogeochemical and multi-isotopic (18O, 2H, 3H, 13C, and 14C) approaches were applied accompanied with hydrogeological investigations to determine the origin and mechanism of groundwater salinization in Kashan Plain Aquifer (KPA). Evolution of hydrogeochemical facies in KPA is Ca-HCO3 (19.04%), Mix Ca-Cl, and Ca-Cl (9.52%), Mix Na-Cl, and Na-Cl (72.26%) based on the HFE-M. The isotopic composition of δ2H and δ18O varies from −8.38 to 6.48‰ with average of −4.49‰ for δ18O and −57.09 to 22.68‰ with average of −40.28‰ for δ2H. The fresh and saline groundwater samples were plotted near the GMWL, with a minor shift towards more enriched values, indicating that the present meteoric water is the main origin of groundwater in the study area. Intrusion from salt lake and saltpan was rejected by ionic ratios and stable isotopes approaches. Further, geophysical investigations ruled out contributions from the Nasr Abad salt dome, as it is buried at about 480 m depth underneath the KPA. In addition, the radioisotopes results (3H, and 14C) showed that the KPA groundwaters comprise of modern meteoric waters mixed with deep paleo-brine waters (10000 to 21000 yr BP). Further, the Na/Cl, I/Cl, Cl/Br, and Li/Cl/Br/Cl ratios and 2H and 18O data showed that, most probable source of salinity for this deep paleo- brine water is halite dissolution in Miocene Marl and URFs formation in the bedrock of KPA. The mixing ratio calculated by chloride concentration showed about 60% mixing between deep brine and groundwater samples. Consequently, the source of salinity in KPA groundwater is dissolution of halite and gypsum in deep paleo-brine which is likely upconing and mixing with fresh groundwaters during heavy pumping.

Summary Weekly variation of δ 18 O was measured over 2 years in precipitation and river water in four relatively large catchments in north east Scotland. The River Dee (1712 km 2 ) is predominantly upland, with impermeable geology and hydrologically responsive soils. The headwaters of the River North Esk (732 km 2 ) are similar, but the lower third of the catchment is underlain by a major sandstone aquifer and is lowland (i.e. 2 ) is upland, but the remainder is lowland with freely draining soils recharging significant groundwater reservoirs in superficial drifts. The River Ythan catchment (662 km 2 ) is entirely lowland and similar to the lower Don. The hydrological responsiveness of the catchments was directly related to their upland area, with the Dee and the North Esk generating the highest specific discharges during high flow events. Conversely, the Don and Ythan had more subdued hydrological regimes, but higher specific discharge under baseflows. Despite broadly similar δ 18 O variation in precipitation inputs, the variability of stream waters was increasingly damped in the order Ythan > Don > North Esk > Dee. Convolution integral models were used to estimate Mean Transit Times (MTTs) for the four catchments from the isotope data. These were reasonably well-constrained at around 2 years for the Dee and 3 years for the North Esk. Estimates for the Don and Ythan were poorly constrained and therefore highly uncertain, but are both likely to exceed 10 years. MTTs in upland catchments in the Scottish Highlands are relatively short (ca. 2 months–4 years) and have been shown to be strongly correlated with soil hydrology, topographic indices and precipitation intensity. However, these relationships change in lowland areas as catchments become less surface water dominated and greater groundwater storage and deeper mixing processes result in much higher MTTs. Nevertheless, a close correlation between soil cover and MTT remains.

Summary The stable isotopic composition of hydrogen and oxygen of precipitation from Debrecen, Eastern Hungary was analysed in event-based samples collected from the beginning of 2001 to the end of 2009. During the monitoring period, the δ 18 O values varied between −22.3‰ and 6.64‰ and the δ 2 H values between −176.8‰ and 10.7‰. The LMWL for the monthly based data is close to the GMWL, but shows the effect of secondary evaporation of falling raindrops with lower intercept and slope. LMWL of each year shows highly different parameters due to differences in precipitation amount and summer temperatures, especially in the extreme years of 2002 and 2003. On the basis of our data, deuterium excess is considered to be the best parameter to reveal the extremities of dry and warm periods. Deuterium excess also proved to be a useful tool to show the different formation histories of certain precipitation events. Good correlation of δ 18 O with temperature was obtained for the samples. The slope of the δ 18 O–T functions for the whole sampling period was 0.32 ± 0.03‰/°C for the monthly samples, however, a slope of 0.37 ± 0.03‰/°C was obtained if monthly mean temperatures were replaced with the monthly mean temperatures of the rainy days. Considering the temperature dependency of the δ 18 O values in the past, it can be concluded that Δ δ 18 O/ΔT relationship using monthly mean temperatures of the rainy days might be a better approach than monthly mean temperatures.

Summary A synthesis of isotope hydrological (2H, 3H, 18O, 13C, 14C, 3He, 4He) and hydrogeochemical tools was used to analyze flow regimes and their time scales, to determine recharge conditions, and to describe the regional-scale hydrogeochemical evolution of groundwater from the Triassic Ntane Sandstone Aquifer between Serowe and Orapa in the Kalahari of Botswana. This location is taken as an example of a complex hydrodynamic setting relating to semi-arid climatic conditions, low recharge amounts, thick unsaturated zones (up to 100 m) and long groundwater residence time scales. Capabilities and limits of the individual methods are discussed, and relations to solute transport are demonstrated for nitrate. Chemical classification allows the distinction of six different groundwater types. This characterization is reflected in the isotopic composition and in part can be attributed to different age ranges (up to greater than 20 ka). We show that the regional-scale groundwater flow regime in the aquifer is only slow, and that groundwater is recharged in Serowe and Orapa. However in both investigated regions, mixing between over- and underlying aquifers is evident which limits a clear derivation of one regional flow regime. The challenges imposed on the identification of the regional groundwater flow regime by resulting mixing processes and for groundwater dating are discussed and evaluated.

Groundwaters from the phreatic aquifer within and surrounding of the Cerro Prieto geothermal field were analyzed geochemically and isotopically in order to establish a hydrodynamic model of the study zone, which is located in the Mexicali Valley between 655,000–685,000 m E–W and 3,605,000–3,576,000 m N–S relative to UTM coordinates. Based on their chemical composition three types of water were recognized: chloride, sulfate and bicarbonate. However four groups of water were identified on a statistical multi-variable method of cluster analysis (A–D). The average temperature is 25 °C; with a few exceptions in the south where temperature can be as high as 47 °C. Stable isotope ratios for some waters plot close to the world meteoric line, corresponding to the original unaltered waters of the zone. The hydrogeochemistry varies in relation to three principal processes: evaporation, infiltration of water used in agriculture and rock interaction by reaction with evaporitic deposits. Major quartz, calcite and plagioclase and minor smectite, kaolinite, halite, sylvite and gypsum were identified by X-ray diffraction in lacustrine sediments of the central part of the zone. Chemical modeling indicates saturation with respect to calcite and undersaturation with respect to gypsum. By incorporating chemical and isotope data into geological and isopotential well information, a hydrodynamic model has been postulated. In this hydrodynamic model the water (A) enters the study zone from the east and it is originally of the old Colorado River water. The water samples on which the model is based were draw from agricultural wells that intersected two aquifers, a shallow and a deep one, representing the recharge to the zone. The salinity of the deep aquifer water (B) is lower than that of the shallow aquifer water (C) and so is the stable isotope ratio. The difference is though to be due to dissolution of evaporates, evaporation and possible infiltration of spent agriculture water. Both waters then pass through lacustrine sediments and gain in salinity and become isotopically heavier mainly by evaporation in a stagnate flow. They eventually emerge as a saline water (D) in the central part of the study zone. This saline water is the one that mixes with thermal fluid discharges from the geothermal reservoir in the south.

Between 1937 and 1995 a complex of more than 100 interdunal wetlands disappeared from Great Sand Dunes National Monument, Colorado. We investigated three hypotheses that could explain wetland disappearance: (1) dune movement during a severe drought in the 1950s buried the wetlands, (2) agriculture related ground water pumping lowered the regional water table, and (3) changes in local hydrologic processes led to wetland loss. We used regional stream flow records, ground water level measurements, natural stable isotope analyses, soil stratigraphy, buried seed banks, and ground water modeling to address these hypotheses. Hydrologic data and stable isotope analyses illustrated the interaction between Sand Creek, a nearby stream, and the unconfined aquifer in the area where wetlands occurred. When the intermittent Sand Creek flows, seepage through its bed creates a large ground water mound under the creek. The seasonal development and dispersion of this mound propagates pressure waves through the aquifer that influence ground water levels up to 2 km from Sand Creek. Our data suggest the primary factors contributing to wetland disappearance were recent climatic fluctuations and incision of the Sand Creek channel. Below average stream flow between 1950 and 1980 reduced the duration of Sand Creek flow across the dune complex, minimizing ground water mound development. Consequently, the water table in the unconfined aquifer dropped ∼1.0 m and interdunal wetlands dried up. Twentieth Century incision of Sand Creek's channel reduced ground water mound height ∼2.5 m, decreasing seasonal water table fluctuations at interdunal wetlands and contributing to the overall water table decline. Long-term wet and dry cycles affect the water table elevation more than channel incision, leading us to conclude that many interdunal wetlands are ephemeral features. Wetland area is maximized during consecutive years of above average Sand Creek discharge and minimized as the water table drops during dry periods.

Stable isotopes of precipitation, ground water and surface water measured on the windward side of East Maui from 0 to 3055 m altitude were used to determine recharge sources for stream flow and ground water. Correct interpretation of the hydrology using rainfall 18 O gradients with altitude required consideration of the influence of fog, as fog samples had isotopic signatures enriched by as much as 3‰ in 18 O and 21‰ in D compared to volume-weighted average precipitation at the same altitude. The isotopic analyses suggested that fog drip was a major component of stream flow and shallow ground water at higher altitudes in the watershed. Oxygen-18/altitude gradients in rainfall were comparable for similar microclimates on Maui (this study) and Hawaii Island (1990-95 study), however, East Maui 18 O values for rain in tradewind and high-altitude microclimates were enriched compared to those from Hawaii Island. Isotopes were used to interpret regional hydrology in this volcanic island aquifer system. In part of the study area, stable isotopes indicate discharge of ground water recharged at least 1000 m above the sample site. This deep-flowpath ground water was found in springs from sea level up to 240 m altitude, indicating saturation to altitudes much higher than a typical freshwater lens. These findings help in predicting the effects of ground water development on stream flow in the area.

Sampling of springs, for which the mean elevation of recharge could be estimated, provided an estimate of −0.44‰ 100 m−1 for the groundwater altitude effect on the δ18O value in Eastern Macedonia and −0.21‰ in Thrace. A slight difference in both the slope and the intercept of the lines expressing the regression of elevation against δ18O values in the continental part of Thrace and the island of Thassos, is also observed. As case studies, the isotopic composition of surface water and groundwater was used to investigate the mechanism of recharge of groundwater in the areas of Lekani Mountains and Thassos Island. As a result, the contribution of water leaking from the river Nestos to the recharge of the springs of Paradissos, near to the river bed, Kastro Filippon and Aghios Athanassios, at a distance of over 25 km from the river bed, as well as of the aquifers tapped by boreholes in the plain between Kastro Filippon and Aghios Athanassios, was indicated. The estimate is 54 ± 12% for the contribution of the river water to the spring yield, in the case of Aghios Athanassios, and 43 ± 10% in the case of the springs of Paradissos. As another result, the hydraulic connection between the marbles of Profitis Ilias and the marbles and dolomites of Potamia series, in the case of Thassos Island, was confirmed. Finally, the extent of the carbonate rocks and the amount of precipitation effective in recharge to the main springs, as well as to the aquifers tapped by boreholes in the different plains of the areas under investigation were estimated.

The more than 1550 km2 (600 mi2) Hat Creek Basin in northeastern California is host to several first magnitude cold springs that emanate from Quaternary basaltic rocks with individual discharge rates ranging from 1.7 to 8.5 m3 s−1 (60–300 ft3 s−1). Stable isotope (δ18O, δD, δ13C) and 14C measurements of surface and groundwater samples were used to identify recharge areas, and to evaluate aquifer residence times and flow paths. Recharge locations were constrained from the regional decrement in meteoric water δ18O values as a function of elevation, determined to be −0.23‰ per 100 m for small springs and creek waters collected along the western Cascade slope of this region. In general, the large-volume springs are lower in (δ18O than surrounding meteoric waters, and are inferred to originate in high-elevation, high-precipitation regions up to 50 km away from their discharge points. Large spring 14C abundances range from 99 to 41 % modern carbon (pmc), and most show evidence of interaction with three distinct carbon isotope reservoirs. These reservoirs are tentatively identified as (1) soil CO2 gas equilibrated under open system conditions with groundwater in the recharge zone [δ13CDIC ≈ −18‰, 14C > 100 pmc], (2) dissolved carbon equilibrated with atmospheric CO2 gas [δ13CDIC ≈ +1‰, 14C > 100 pmc], and (3) dissolved carbon derived from volcanic CO2 gas emissions [δ13CDIC≈0‰, 14C=0 pmc]. Many regional waters show a decrease in 14C abundance with increasing δ13C values, a pattern indicative of interaction with dead carbon originating from volcanic CO2 gas. Several lines of evidence suggest that actual groundwater residence times are too short (⩽ 200 years) to apply radiocarbon dating corrections. In particular, water temperatures measured at springs show that deep groundwater circulation does not occur, which implies an insufficient aquifer volume to account for both the high discharge rates and long residence times suggested by 14C apparent ages. The large springs also exhibit rapid decreases in flow during periods of drought that suggests a high level of aquifer interconnectivity to the recharge area. The estimated amount of volcanic CO2 dissolved in surface and groundwater originating from the Lassen highlands is consistent with the conversion of approximately 10% of the geothermal CO2 flux into dissolved inorganic carbon.

With the aim of studying the hydraulic behaviour and isotopic effects in refuse dumps, measurements of the 18O and 2H content of rainwater infiltrating into large cylindrical refuse lysimeters (1.5–4 m height with diameter of about 5 m), installed by the Technical University of Brunswick, Germany, were performed. The 11 lysimeters were filled with household refuse and sewage sludge under different conditions (aerobic/anaerobic, compaction, layering). The long-term hydraulic parameters of flow through the lysimeters (transit time, apparent dispersivity, volume of total water) were estimated with a specially adapted black-box model, which used isotopic content curves of rainwater and infiltrated water sampled at different depths in the lysimeters. The transit times of rainwater through the lysimeters (3–6 years) and the mean flow velocities deduced from them (0.4–1 m year−1), correspond to values in the unsaturated zone of sandy aquifers. The dispersivities are in the range of 0.1–1 cm. A special attempt was made to follow individual heavy rain events and estimate the travel times and portions of rainwater percolating swiftly through the lysimeter. It was found that up to 40% of those precipitation events drains off directly, i.e. within a few weeks. However, as it remains only a short time in the lysimeter, this water should not be greatly affected by the lysimeter filling. In addition, by evaluating the δ2H-δ18O relationship of the water samples from the different lysimeter sampling points, it was possible to differentiate, on a preliminary basis, isotopic effects caused by evaporation and biochemical reactions.

The isotopic composition of surface water and groundwater was used to investigate the mechanism of recharge of groundwater in the Axios River plain in northern Greece. Sampling of springs, for which the mean elevation of recharge could be estimated, in the Paikon Mountains provided an estimate of −0.28‰ per 100 m for the altitude effect. The δ18O estimated for the recharge of the Neogene formation between the river valley and the Paikon Mountains agreed with many of the sampled locations. However, in the southwestern part of the area there is an indication of a contribution from the limestone formation of the Paikon Mountains. The stable isotopic composition of shallow groundwater close to the river indicates contributions from the river. Deeper groundwater farther away from the river also contains a proportion of infiltrated river water. Some of the boreholes close to the river have chlorinities ranging up to about 6000 mg 1−1. The stable isotopic and chemical data indicate that the salinity is due to varying proportions of sea water, infiltrated river water and local recharge. The isotopic composition of Aravissos Spring water suggests that the spring is recharged by the upper series of the semi-metamorphic Paikon zone (dolomitic limestones, spilites-keratophyres and Gropi series; the mean altitude of recharge computed for the area so defined is 867 m and its potential annual yield, i.e. the amount of annual precipitation effective in recharge in this area, is 43.6 ± 10 × 106 m3). This suggests that the actual annually exploited quantity of the spring water for the water supply of Thessaloniki (about 44 × 106 m3) is at the limit of the corresponding potential yield.

The water resources in northern Chile are extremely scarce and development is limited by this. This paper discusses the isotope hydrology of groundwaters in the Pampa del Tamarugal — an area of extreme aridity (rainfall < 1 mm/yr.) — whose groundwater resources are mined for the town of Iquique, as well as for industrial and agricultural purposes. The aim of the project was to obtain information on modern recharge and to delineate, if possible recharge environments. To obtain the necessary background information a precipitation survey in the high Andes, as well as spring- and surface-water studies were carried out. The results show that a well-defined meteoric water line exists where σ2H = (7.8 σ 18O + 10.3)%, and altitude effects depend on air-mass movements and cannot be defined without a broader regional and detailed sampling programme. However, it is still possible to assign maximum altitudes of recharge to springs in the Andes and at the eastern border of the Pampa del Tamarugal. Comparison of these data with groundwater compositions show, that these groundwaters originate from infiltrating surface water rather than directly infiltrated precipitation. A dependence of individual groundwater systems on specific quebradas (river valleys) is recognized. However, low 14C activities indicate that most of the waters pumped today are fossil and at least several hundreds if not thousands of years old. Some minor subsurface recharge does occur at the foot of the Andes, especially at Pica where high-altitude waters discharge, but even there groundwater appears to be a diminishing resource.

The environmental isotope ( 18 O, D = 2 H ) composition of the groundwaters of central Mexico divides these into three distinctive groups: (1) groundwaters whose isotope composition fits the contemporary meteoric-water line with d = 10‰ ; these are from shallow aquifers, mainly in volcanic and alluvial rocks; (2) waters whose isotope data plot to the right of the meteoric-water line, describing a new meteoric-water line of d = 5–7‰ ; this group is made up of waters from the confined limestone aquifers which are of regional dimensions; (3) groundwaters with marked oxygen excess which emerge in regions of recent/subrecent volcanic activity and are geothermal in nature. The second group is considered to be of greater age, the isotopic composition being that of paleowaters. The third group's isotope composition reflects water-rock interactions and isotope fractionation which accompanies steam separation. In spite of some local peculiarities, it is shown that the water sources of central Mexico can be fitted to a general hydrogeological model, which is presented.

The town of Alice Springs is situated within the arid zone of central Australia and derives its water supply from production bores in Mereenie Sandstone. Twenty-nine samples of this water and other surrounding water have been analysed for their chemical, radiocarbon, stable carbon, deuterium and 18O composition to investigate recharge to the aquifer system. The measured 14C contents range from 5.1 to 63.4% modern. There is no correlation between borehole depth and percentage modern carbon; however, the samples fall into several groups when the percentage modern carbon is plotted against the reciprocal of the total carbonate content. Within each group, the δ D and δ 18O values are constant and isotopically much lighter than the average annual rainfall value. It is believed that each group represents a single recharge event, and that recharge to the aquifer system occurred at intervals of a few millennia, when climatic conditions were different from those of today.

Variations in the isotopic composition of water were used to define the mechanism of recharge of the Assopos riverplain and the Kalamos Attikis coastal brackish karst springs. The plain is mainly recharged by local rain water, while the karst springs are fed by the Parnis mountain system, the mean recharge altitude being estimated in the order of 870 m. The apparent velocity and the underground pathway of the water feeding the springs were deduced and the most convenient sites for the drilling of production wells for recovery of the spring water before salinization were sited. As a result, 73,000 m3 of water are now pumped per day and used as additional potable water for the Athens area, comprizing about one-sixth of the mean daily consumption.

Although the isotope content of precipitation in the semi-arid regions of southern Africa is extremely variable, groundwater samples from the same district are found to have a remarkably constant isotopic composition. The oxygen-18 content of the underground water, in general, varies by about 0.5‰ in a given area. The differences that occur between different regions are sufficiently large to allow the groundwater of an area to be characterized by means of its oxygen-18 content. In order to localize the infiltration area of an aquifer, radiocarbon dating of the water is used. It appears that the groundwater contains, in general, less of the heavy isotopes than does the precipitation in the recharge area. This indicates that infiltration only takes place during periods of heavy rainfall. Examples are given where the isotope content of the groundwater is used to distinguish between different aquifers in the same region.

Deuterium and oxygen-18 data have been used to identify the origin of recharge to a coastal complex of springs and also aquifers developed for supplying water to Aghios Nikolaos, Crete. Tritium data have been used to estimate the age of groundwater at different sampling locations. The results of this study indicate that the Almyros Aghios Nikolaos springs are recharged mainly from the part of the Gavrovo-Tripolitza limestones forming the mountains of Selena, Katharo Tsivi and Lassithiotika, while the part of the same limestones forming the mountainous area of Mavro Dassos provides recharge to the northern part of the Aghios Nikolaos plateau. The total surfaces of these areas, their corresponding potential annual recharges, as well as the flow velocity of the water recharging the springs of Almyros Aghios Nikolaos during the last few kilometers of its way are estimated.

Deuterium, oxygen-18, tritium and chloride concentrations were used in three salinized experimental catchments to gain insight into the mechanism of solute concentration and flow processes in the saturated and unsaturated zones. The three experimental catchments were studied because of their location in different rainfall regions, their status with respect to clearing of native vegetation and with respect to secondary salinization. Two uncleared catchments have average annual rainfalls of approximately 820 and 1220 mm, respectively. The third cleared catchment has an annual rainfall of 650–750 mm. This catchment was in an advanced state of secondary salinization and displayed large areas of saline groundwater discharge with halite encrustation at the ground surface. The stable isotope compositions of the solution phase in solute bulge profiles in the unsaturated zone showed a close agreement with the amount-weighted mean isotopic composition of rainfall and only surficial evidence of isotopic enrichment due to evaporation. Evaporation from the soil surface plays a minor role as a mechanism of solute concentration in the unsaturated zone. The dominant process of solute concentration in the unsaturated zone was ion exclusion during uptake of water by tree roots which was evidently a solute but not isotope fractionating process. Tritium analyses of unsaturated zone water and grondwater indicated movement of recent recharge to 7–10 m depth at the low rainfall site but over the full depth of the 15 m unsaturated zone at the higher rainfall site. The variability in δ18O and δ2H values of groundwaters was used in association with chloride concentrations to provide information on mixing characteristics of groundwaters within the catchments.