Groundwater recharge in arid areas induced by tropical cyclones: lessons learned from Gonu 2007 in Sultanate of Oman

Younger groundwater found in some Omani aquifers is a result of recent recharge from cyclonic and storm events [Weyhenmeyer et al. (Science 287:842–845, 2000); Young et al. (J Appl Geophys 57:43–61, 2004)]. The analysis of the meteorological data in Oman indicates an anomalous rainfall on a decadal interval whereas cyclones frequency is expected to increase due to global climatic changes. The cyclone Gonu has severely struck the eastern Omani coasts in 2007 resulting in devastating floods. Huge volume of water (3,672 mm³) spread over the coastal plain calling for an assessment of potential groundwater recharge subsequent to this event. The present study evaluates groundwater recharge with respect to Gonu 2007 to assess the potential of recharge induced by such cyclones in the arid zones. The hydrographs of several piezometers sited along the coastal plain in Muscat Province have been studied and variation in water table rise has been analyzed. Significant water table rise is indicated for areas with geological and structural settings favoring rapid infiltration of water yielding considerable groundwater mound, whereas piezometers located in less favorable zones show minimum rise of water table. However, soon after the floods the aquifer hydrodynamics has readjusted to attain equilibrium and the groundwater mound dissipated. The cumulative rise of the water table on an areal extent does not exceed a few centimeters indicating lesser volume of recharge. Comparatively, recharge from frequent precipitation along favorable zones produces more significant recharge compared with cyclonic events where surface water residence time is shorter to allow for efficient infiltration.     

Oxygen and hydrogen isotopes for the characteristics of groundwater recharge: a case study from the Chih-Pen Creek basin,Taiwan

Assessing the seasonal variation of groundwater recharge is important for effective management of groundwater resources. Stable isotopes of oxygen and hydrogen were used to estimate the sources of groundwater and seasonal contributions of precipitation to groundwater recharge in Chih-Pen Creek basin of eastern Taiwan. Based on the isotopes of precipitation (n = 177), two different local meteoric water regression lines (LMWL) can be obtained for the different seasons: δD = 8.06¹⁸O + 10.08 for wet season precipitation (May through October) and δD = 8.65δ¹⁸O + 17.09 for dry season precipitation (November through April). The slope and intercept of regression line for wet season precipitation are virtually identical to the global meteoric water line (GMWL) of Craig (1961). In contrast to during dry season precipitation due to evaporation effect the intercept of 17.09 is much higher than of the GMWL of 10. The results show the stable isotopes compositions of precipitation decrease with increasing rainfall amount and air temperature, due to the amount effect of precipitation is pronounced. The amount effect is clearly but do not show the temperature effect from January to December 2007. Using a mass-balance equation, a comparison of deuterium excess or d values of precipitation and groundwater indicates the groundwater consist of 76% wet season precipitation and 24% dry season precipitation, representing a distinct seasonal variation of groundwater recharge in study area. About 79% of the groundwater is recharged from the river water of the mountain watershed and 21% is from the rain that falls on the basin.     

Groundwater recharge and flow in the Lower Cretaceous Nubian Sandstone aquifer in the Sinai Peninsula, using isotopic techniques and hydrochemistry

The present work was conducted in the Sinai Peninsula (1) to identify the recharge and flow characteristics and to evaluate the continuity of the Lower Cretaceous Nubian Sandstone aquifer; and (2) to provide information for the aquifer's rational appraisal. Isotopic and hydrochemical compositions combined with the geological and hydrogeological settings were used for this purpose. A considerable depletion in isotopic content (oxygen-18 and deuterium) and low d-excess values exist in the studied groundwater, reflecting the contribution of old meteoric water that recharged the aquifer in pluvial times. Modern recharge also occurs from precipitation that falls on the aquifer outcrops. The wide scatter of the data points around the two meteoric lines, the global meteoric water line (GMWL) and Mediterranean meteoric water line (MMWL), in the δ¹⁸O–δD diagram indicates considerable variation in recharge conditions (amount, altitude, temperature, air masses, distances from catchment, overland flow, etc.). The isotopic composition in the El-Bruk area is minimum (¹⁸O=–9.53‰), very close to the average value of the Western Desert Nubian Sandstone (¹⁸O=–10‰), where the local structural and lithologic conditions retard groundwater flow and the main bulk of water becomes noncyclic. The continuity of the aquifer in northern and central Sinai is evidenced by the isotopic similarity between samples taken from above and below the central Sinai Ragabet El-Naam fault, the distribution of potentiometric head, and hydrogeological cross sections. The combination of isotopic composition in terms of ¹⁸O and chemical composition in terms of TDS and salt contents is the basis for separating the studied groundwater into groups that reflect the recharge sources and isotopic and chemical modifications during flow. Electronic Publication     

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.

     

Groundwater recharge and salinization in the arid coastal plain aquifer of the Wadi Watir delta,Sinai, Egypt

The Quaternary coastal plain aquifer down gradient of the Wadi Watir catchment is the main source of potable groundwater in the arid region of south Sinai, Egypt. The scarcity of rainfall over the last decade, combined with high groundwater pumping rates, have resulted in water-quality degradation in the main well field and in wells along the coast. Understanding the sources of groundwater salinization and amount of average annual recharge is critical for developing sustainable groundwater management strategies for the long-term prevention of groundwater quality deterioration. A combination of geochemistry, conservative ions (Cl and Br), and isotopic tracers (^87/86Sr, δ⁸¹Br, δ³⁷Cl), in conjunction with groundwater modeling, is an effective method to assess and manage groundwater resources in the Wadi Watir delta aquifers. High groundwater salinity, including high Cl and Br concentrations, is recorded inland in the deep drilled wells located in the main well field and in wells along the coast. The range of Cl/Br ratios for shallow and deep groundwaters in the delta (∼50–97) fall between the end member values of the recharge water that comes from the up gradient watershed, and evaporated seawater of marine origin, which is significantly different than the ratio in modern seawater (228). The ^87/86Sr and δ⁸¹Br isotopic values were higher in the recharge water (0.70,723 < ^87/86Sr < 0.70,894, +0.94 < δ⁸¹Br < +1.28‰), and lower in the deep groundwater (0.70,698 < ^87/86Sr < 0.70,705, +0.22‰ < δ⁸¹Br < +0.41‰). The δ³⁷Cl isotopic values were lower in the recharge water (−0.48 < δ³⁷Cl < −0.06‰) and higher in the deep groundwater (−0.01 < δ³⁷Cl < +0.22‰). The isotopic values of strontium, chloride, and bromide in groundwater from the Wadi Watir delta aquifers indicate that the main groundwater recharge source comes from the up gradient catchment along the main stream channel entering the delta. The solute-weighted mass balance mixing models show that groundwater in the main well field contains 4–10% deep saline groundwater, and groundwater in some wells along the coast contain 2–6% seawater and 18–29% deep saline groundwater.A three-dimensional, variable-density, flow-and-transport SEAWAT model was developed using groundwater isotopes (⁸⁷Sr/⁸⁶Sr, δ³⁷Cl and δ⁸¹Br) and calibrated using historical records of groundwater level and salinity. δ¹⁸O was used to normalize the evaporative effect on shallow groundwater salinity for model calibration. The model shows how groundwater salinity and hydrologic data can be used in SEAWAT to understand recharge mechanisms, estimate groundwater recharge rates, and simulate the upwelling of deep saline groundwater and seawater intrusion. The model indicates that most of the groundwater recharge occurs near the outlet of the main channel. Average annual recharge to delta alluvial aquifers for 1982 to 2009 is estimated to be 2.16 × 10⁶ m³/yr. The main factors that control groundwater salinity are overpumping and recharge availability.     

Isotope hydrology of deep groundwater in Syria: renewable and non-renewable groundwater and paleoclimate impact

The Regional Deep Cretaceous Aquifer (RDCA) is the principal groundwater resource in Syria. Isotope and hydrochemical data have been used to evaluate the geographic zones in terms of renewable and non-renewable groundwater and the inter-relation between current and past recharge. The chemical and isotopic character of groundwater together with radiometric ¹⁴C data reflect the existence of three different groundwater groups: (1) renewable groundwater, in RDCA outcropping areas, in western Syria along the Coastal and Anti-Lebanon mountains. The mean δ¹⁸O value (?7.2 ‰) is similar to modern precipitation with higher ¹⁴C values (up to 60–80 pmc), implying younger groundwater (recent recharge); (2) semi-renewable groundwater, which is located in the unconfined section of the RDCA and parallel to the first zone. The mean δ¹⁸O value (?7.0 ‰) is also similar to modern precipitation with a ¹⁴C range of 15–45 pmc; (3) non-renewable groundwater found in most of the Syrian interior, where the RDCA becomes confined. A considerable depletion in δ¹⁸O (?8.0 ‰) relative to the modern rainfall and low values of ¹⁴C (<15 pmc) suggest that the large masses of deep groundwater are non-renewable and related to an older recharge period. The wide scatter of all data points around the two meteoric lines in the δ¹⁸O-δ²H diagram indicates considerable variation in recharge conditions. There is limited renewable groundwater in the mountain area, and most of the stored deep groundwater in the RDCA is non-renewable, with corrected ¹⁴C ages varying between 10 and 35 Kyr BP.     

Evaluating the impact of artificial groundwater recharge structures using geo-spatial techniques in the hard-rock terrain of Rajasthan,India

Groundwater levels in hard-rock areas in India have shown very large declines in the recent past. The situation is becoming more critical due to a paucity of rainfall, limited surface water resources and an increasing pattern of groundwater extraction in these areas. Consequently, the Ground Water Department with the aid of World Bank has implemented the water structuring programme to mitigate groundwater scarcity and to develop a viable solution for sustainable development in the region. The present study has been undertaken to assess the impact of artificial groundwater recharge structures in the hard-rock area of Rajasthan, India. In this study groundwater level data (pre-monsoon and post-monsoon) of 85 dug-wells are used, spread over an area of 413.59 km². The weathered and fractured gneissic basement rocks act as major aquifer in the area. Spatial maps for pre- and post-monsoon groundwater levels were prepared using the kriging interpolation technique with best fitted semi-variogram models (Spherical, Exponential and Gaussian). The groundwater recharge is calculated spatially using the water level fluctuation method. The entire study period (2004–2011) is divided into pre- (2004–2008) and post-intervention (2009–2011) periods. Based on the identical nature of total monsoon rainfall, two combinations of average (2007 and 2009) and more than average (2006 and 2010) rainfall years are selected from the pre- and post-intervention periods for further comparisons. All of the water harvesting structures are grouped into the following categories: as anicuts (masonry overflow structure); percolation tanks; subsurface barriers; and renovation of earthen ponds/nadis. A buffer of 100 m around the intervention site is taken for assessing the influence of these structures on groundwater recharge. The relationship between the monsoon rainfall and groundwater recharge is fitted by power and exponential functions for the periods of 2004–2008 and 2008–2011 with R ² values of 0.95 and 0.98, respectively. The average groundwater recharge is found to be 18% of total monsoon rainfall prior to intervention and it became 28% during the post-intervention period. About 70.9% (293.43 km²) of the area during average rainfall and more than 95% (396.26 km²) of the area during above-average rainfalls show an increase in groundwater recharge after construction of water harvesting structures. The groundwater recharge pattern indicates a positive impact within the vicinity of intervention sites during both average and above-average rainfall. The anicuts are found to be the most effective recharge structures during periods of above-average rainfall, while subsurface barriers are responded well during average rainfall periods. In the hard-rock terrain, water harvesting structures produce significant increases in groundwater recharge. The geo-spatial techniques that are used are effective for evaluating the response of different artificial groundwater recharge techniques.     

Application of the water balance model J2000 to estimate groundwater recharge in a semi-arid environment: a case study in the Zarqa River catchment, NW-Jordan

Pollution and overexploitation of scarce groundwater resources is a serious problem in the Zarqa River catchment, Jordan. To estimate this resource’s potential, the amount and spatial distribution of groundwater recharge was calculated by applying the hydrological model J2000. The simulation period is composed of daily values gathered over a 30-year period (July 1977 to June 2007). The figure finally obtained for estimated groundwater recharge of the Zarqa River catchment is 105 × 10⁶ m³ per year (21 mm a^?1). This is 19 % higher than the value previously assumed to be correct by most Jordanian authorities. The average ratio of precipitation to groundwater recharge is 9.5 %. To directly validate modelled groundwater recharge, two independent methods were applied in spring catchments: (1) alteration of stable isotope signatures (δ¹⁸O, δ²H) between precipitation and groundwater and (2) the chloride mass balance method. Recharge rates determined by isotopic investigations are 25 % higher, and recharge rates determined by chloride mass balance are 9 % higher than the modelled results for the corresponding headwater catchments. This suggests a reasonably modelled safe yield estimation of groundwater resources.     

Impact of recharge variations on water quality as indicated by excess air in groundwater of the Kalahari, Botswana

Groundwater is an important and often exclusive water resource in arid and semi-arid regions. The aim of the present paper was to gain insight into the processes and conditions that control the deterioration of groundwater quality in the semi-arid Kalahari of Botswana. Measurements of ³He, ⁴He, ²⁰Ne, ²²Ne, and of ¹⁴C of dissolved inorganic carbon (DIC) were combined with existing isotopic and hydrochemical data to investigate groundwater from the Ntane Sandstone Aquifer, which is affected by high nitrate concentrations of non-anthropogenic origin. All groundwater samples revealed neon concentrations in excess to air-saturated water, which we attributed to the addition of excess air during recharge. Neon concentrations ranged from values close to air saturation for ¹⁴C DIC rich samples (up to 80.5%MC) up to values of 90% in excess to air-saturated water for lower ¹⁴C DIC contents (2.6-61.3%MC). A strong linear correlation of excess Ne with nitrate concentrations suggests an intimate connection between groundwater quality and the processes and conditions during groundwater recharge. Low groundwater recharge rates under present-day semi-arid conditions are associated with low amounts of excess Ne and elevated nitrate concentrations. In contrast to this, higher excess Ne values in groundwater of lower ¹⁴C DIC and nitrate contents indicate that the high quality groundwater end-member presumably is related to higher groundwater table fluctuations during wetter climatic conditions in the past. We attribute the decline in groundwater quality with respect to nitrate to a decreasing rate and temporal variability of groundwater recharge, and to concurrent changes in biogeochemical activities following a transition to a drier climate during the Holocene. Under such conditions, a much stronger decrease in groundwater recharge compared to the release of nitrate from soil organic matter may result in elevated nitrate concentrations in the vadose zone and groundwater. This implies a strong impact of climate change on the transport of solutes like nitrate through the vadose zone which needs to be considered in predictions of future groundwater quality.     

Assessment of recharge to groundwater systems in the arid southwestern part of Northern Territory, Australia, using chlorine-36

 The sustainability of community water supplies drawn from shallow aquifers in the arid southwest of the Northern Territory has been evaluated using the radioactive isotope chlorine-36 (³⁶Cl). These aquifers include fractured sandstones of the Ngalia Basin, fractured metamorphic rocks and Cainozoic sands and gravels. ³⁶Cl/Cl ratios for these shallow, regional groundwaters exhibit a bimodal distribution with peaks at 205 (±7) and 170 (±7)×10^–15. The higher ratio probably represents modern (Holocene) recharge, diluted with windblown salts from local playa lakes, and occurs mostly around the margin of the basin. The lower ratio corresponds to a ³⁶Cl "age", or mean residence time, of 80–100 ka, implying that the last major recharge occurred during the last interglacial interval (Oxygen Isotope Stage 5). These values are mainly observed in the interior of the Ngalia Basin. Lower values of the ³⁶Cl/Cl ratio measured near playa lakes are affected by addition of chloride from remobilised salts. Finite carbon-14 (¹⁴C) data for the groundwaters are at variance with the ³⁶Cl results, but a depth profile suggests low recharge, allowing diffusion of recent atmospheric carbon to the water table. The ³⁶Cl results have important implications for groundwater management in this region, with substantial recharge only occurring during favourable, wet, interglacial climatic regimes; most community water supplies are dependent on these "old" waters. Received, September 1997 · Revised, August 1998, March 1999 · Accepted, March 1999     

Application of the chlorine-36 method for the delineation of groundwater infiltration of large river systems: example of the Danube River in western Hungary (Szigetköz area)

The gravel aquifer within the Szigetköz Plain in northern Hungary is mainly fed by the infiltration of the Danube River. This infiltration process can be identified using the tritium/helium method to a distance of about 30 km away from the infiltration area of the Danube. In this study, chlorine-36 analyses are used as additional method. This natural radioisotope was also produced by nuclear bomb tests in the atmosphere. It is an ideal, stable constituent for this particular study due to its very long half-life (300,000 years), and consequently acts as an independent check of the established model of aquifer recharge. The chlorine-36 data of the analysed selected groundwater samples of this area clearly show the effect of the atmospheric nuclear bomb tests, with enhanced ³⁶Cl/Cl ratios of up to a factor 10 higher than the unaffected groundwater of the pre-bomb period within the study area. Finally, the observed ³⁶Cl values were introduced into a transport model with dispersion/advection-type flow of groundwater to deduce the transport parameters.     

Delineating volcanic aquifer recharge areas using geochemical and isotopic tools

Relative recharge areas are evaluated using geochemical and isotopic tools, and inverse modeling. Geochemistry and water quality in springs discharging from a volcanic aquifer system in Guatemala are related to relative recharge area elevations and land use. Plagioclase feldspar and olivine react with volcanically derived CO₂ to produce Ca-montmorillonite, chalcedony and goethite in the groundwater. Alkalinity, Mg, Ca, Na, and SiO_2(aq) are produced, along with minor increases in Cl and SO₄ concentrations. Variations in groundwater δD and δ¹⁸O values are attributed to recharge elevation and used in concert with geochemical evolution to distinguish local, intermediate, and regional flow systems. Springs with geochemically inferred short flow paths provided useful proxies to estimate an isotopic gradient for precipitation (??.67 δ¹⁸O/100?m). No correlation between spring discharge and relative flow-path length or interpreted recharge elevation was observed. The conceptual model was consistent with evidence of anthropogenic impacts (sewage and manure) in springs recharged in the lower watershed where livestock and humans reside. Spring sampling is a low-budget approach that can be used to develop a useful conceptual model of the relative scale of groundwater flow (and appropriate watershed protection areas), particularly in volcanic terrain where wells and boreholes are scarce.     

Karst groundwater management by defining protection zones based on regional geological structures and groundwater flow fields

In a semiarid region, the karst aquifer generally forms a large groundwater reservoir that can play an important role in regional water supply. But because of the specific physical properties of karst aquifers, they are vulnerable to pollution and anthropogenic impacts. Karst groundwater management strategies are vital. As representative of karst springs in a semiarid area, Niangziguan Springs is located in the east of Shanxi Province, China with an annual average rate of discharge of 10.34 m³/s (1956–2003) (Y. Liang, unpublished data). The Niangziguan Spring Basin covers an area of 7,394 km² with an annual average precipitation of 535 mm (1958–2003) (Hao et al. in Carsologica Sinica 23(1):43–47, 2004). Over the past three decades, accelerated groundwater exploitation has caused water-table decline in the aquifer, reduction of the spring discharge, and deterioration of water quality. In this study, three protection zones were defined to ensure the quality and capacity of this resource. The confluence of the 11 spring systems and the discharge areas were defined as I protection zone, the recharge basin was II protection zone, and the slack water area where there is little surface recharge was the III protection zone. Management strategies for each zone were suggested and evaluated to provide a scientific foundation for sustainable utilization.     

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.     

Spatial and temporal constraints on regional-scale groundwater flow in the Pampa del Tamarugal Basin,Atacama Desert,Chile

Aquifers within the Pampa del Tamarugal Basin (Atacama Desert, northern Chile) are the sole source of water for the coastal city of Iquique and the economically important mining industry. Despite this, the regional groundwater system remains poorly understood. Although it is widely accepted that aquifer recharge originates as precipitation in the Altiplano and Andean Cordillera to the east, there remains debate on whether recharge is driven primarily by near-surface groundwater flow in response to periodic flood events or by basal groundwater flux through deep-seated basin fractures. In addressing this debate, the present study quantifies spatial and temporal variability in regional-scale groundwater flow paths at 20.5°S latitude by combining a two-dimensional model of groundwater and heat flow with field observations and δ¹⁸O isotope values in surface water and groundwater. Results suggest that both previously proposed aquifer recharge mechanisms are likely influencing aquifers within the Pampa del Tamarugal Basin; however, each mechanism is operating on different spatial and temporal scales. Storm-driven flood events in the Altiplano readily transmit groundwater to the eastern Pampa del Tamarugal Basin through near-surface groundwater flow on short time scales, e.g., 10⁰–10¹ years, but these effects are likely isolated to aquifers in the eastern third of the basin. In addition, this study illustrates a physical mechanism for groundwater originating in the eastern highlands to recharge aquifers and salars in the western Pampa del Tamarugal Basin over timescales of 10⁴–10⁵ years.     

Hydrological and reactive processes during rapid recharge to fracture zones: The Äspö large scale redox experiment

The hydrochemical response of fracture zones to enhanced recharge into the upper bedrock environment has been studied during a 3 a project at the Äspö Hard Rock Laboratory (HRL) in Southeastern Sweden. Hydrochemical data obtained during the experiment provides a basis for development of a model for the impact of accelerated recharge on groundwater composition and reactive processes during repository construction and operation. Tunnel construction at the HRL resulted in a 50-fold increase in recharge rates, and a 30-fold decrease in groundwater residence times in the fracture zone studied. Up to 80% dilution of the native groundwater created the greatest impact on groundwater composition. In addition, comparison of mass balances for solutes with known conservative behaviour, and reactive solutes, indicates a significant source of HCO⁻₃, SO²⁻₄ and Na⁺ ions and a significant sink for Ca²⁺ ions within the fracture zone. These trends are explained by ion-exchange processes and microbial degradation of organic C transported from the soil with recharge. The increased microbial activity helps maintain anoxic conditions within the fracture zone. The enhanced recharge favours the performance of the geological barrier since anoxic conditions help to protect against corrosion of engineered barriers, and because long-lived isotopes of Np, Tc and U are less soluble under reducing conditions. A secondary impact is the strong dilution which affects trace element speciation, and also the stability and possible transport of colloids, through ion strength effects. Results from this experiment are primarily significant for national radioactive waste disposal programs that consider potential repository sites in granite geology, and for other programs considering disposal in fractured rock.     

Geochemistry analysis and evolution of a bolson aquifer,basin and range province in the southwestern united states

This paper expands significantly on the major-ion geochemical characterization, evolution, and differentiation of groundwater in the Presidio-Redford Bolson (PRB) Aquifer of Texas as presented in Chowdhury et al. (2008). For 19 groundwater samples from the PRB Aquifer, the author calculated major cation–anion balance errors, equilibrium carbon dioxide partial pressure values and saturation indices for selected minerals. Comparison of major-ion analyses for groundwater from basin margin wells with those for basin center wells is documented and illustrated with ion-concentration maps and Piper and Stiff diagrams and reveals significant increases in concentrations of chloride, sulfate and sodium coupled with notable decrease of calcium in bolson-center well samples. These geochemical changes suggest dissolution of aquifer minerals and cation exchange as groundwater migrates downgradient to the bolson center. The US Geological Survey (USGS) computer code, NETPATH, was used to interpret probable net geochemical mass-balance reactions that potentially have occurred within the PRB Aquifer along groundwater flowpaths from bolson margin to bolson center. For all four upgradient–downgradient well pairs studied, at least three NETPATH models contain cation exchange values; calcium is being exchanged for sodium. The Rio Grande Alluvium Aquifer and Rio Grande River are notably minor sources of recharge to the PRB Aquifer, based on Chowdhury et al. (2008) and geochemical evaluations of this study.     

Characterisation of recharge processes and groundwater flow mechanisms in weathered-fractured granites of Hyderabad (India) using isotopes

In order to address the problem of realistic assessment of groundwater potential and its sustainability, it is vital to study the recharge processes and mechanism of groundwater flow in fractured hard rocks, where inhomogeneties and discontinuities have a dominant role to play. Wide variations in chloride, δ¹⁸O and ¹⁴C concentrations of the studied groundwaters observed in space and time could only reflect the heterogeneous hydrogeological setting in the fractured granites of Hyderabad (India). This paper, based on the observed isotopic and environmental chloride variations of the groundwater system, puts forth two broad types of groundwaters involving various recharge processes and flow mechanisms in the studied granitic hard rock aquifers. Relatively high ¹⁴C ages (1300 to ～6000 yr B.P.), δ¹⁸O content (?3.2 to ?1.5‰) and chloride concentration (<100 mg/l) are the signatures that identified one broad set of groundwaters resulting from recharge through weathered zone and subsequent movement through extensive sheet joints. The second set of groundwaters possessed an age range Modern to ～1000 yr B.P., chloride in the range 100 to ～350 mg/l and δ¹⁸O from ?3.2 to +1.7‰. The δ¹⁸O enrichment and chloride concentration, further helped in the segregation of the second set of groundwaters into three sub-sets characterized by different recharge processes and sources. Based on these processes and mechanisms, a conceptual hydrogeologic model has evolved suggesting that the fracture network is connected either to a distant recharge source or to a surface reservoir (evaporating water bodies) apart from overlying weathered zone, explaining various resultant groundwaters having varying ¹⁴C ages, chloride and δ¹⁸O concentrations. The surface reservoir contribution to groundwater is evaluated to be significant (40 to 70%) in one subset of groundwaters. The conceptual hydrogeologic model, thus evolved, can aid in understanding the mechanism of groundwater flow as well as migration of contaminants to deep groundwater in other fractured granitic areas.     

Estimation of groundwater recharge in a major sand and gravel aquifer in Ireland using multiple approaches

Groundwater recharge was investigated in the most extensive sand and gravel aquifer (area of approximately 200 km²) in the Republic of Ireland as part of a wider study seeking to derive recharge estimates using aquifer vulnerability mapping. The proportion of effective rainfall (total rainfall minus actual evapotranspiration) that leads to recharge is known as the recharge coefficient. The recharge investigation involved a variety of approaches, including soil moisture budgeting, well hydrograph analysis, numerical modelling and a catchment water balance. The adoption of multiple techniques provided insights on recharge and also on aquifer properties. Comparison of two soil moisture budgeting approaches (FAO Penman-Monteith with Penman-Grindley) showed how variations in the effective rainfall values from these methods influence groundwater levels simulated in a numerical groundwater model. The catchment water balance estimated the recharge coefficient to be between 81 and 85%, which is considered a reasonable range for this aquifer, where overland flow is rarely observed. The well hydrograph analysis, using a previous estimate of specific yield (0.13), gave recharge coefficients in the range of 40–80%, considered low for this aquifer: a revised specific yield of 0.19 resulted in a more reasonable range of recharge coefficients of between 70 and 100%.     

Isotope geochemistry and modelling of the multi-aquifer system in the eastern part of Lithuania

A steady-state groundwater flow model of three Quaternary intertill aquifers in the eastern part of Lithuania has been compiled. The distinction of separate modelled layers is based on hydraulic and isotope-hydrochemistry data criteria. ³H data were used to estimate the corrected groundwater age and were coupled with a groundwater-flow-dynamics model of the Quaternary aquifer system along a cross-section flow pathway from the Baltic Upland recharge area in eastern Lithuania towards the discharge area in the lowlands near the city of Kaunas in central Lithuania. The bicarbonate content in groundwater (214–462 mg/l) increases downgradient towards the lowland area. The other major constituents and total dissolved solids (TDS) have a trend analogous to the bicarbonate. The ¹⁴C activity of dissolved inorganic carbon (DIC) in the groundwater ranges from 41.4 to 85.7 pMC. With aquifer-system depth, active precipitation of aqueous solution takes place by dissolving minerals of calcite and dolomite and leakage of “old” groundwater from lower aquifers; the process is also traced by lower ¹⁴C and ³H activities and by more positive δ¹⁸O values in lowland areas.