Major Aquifer Systems Research Articles

Modeling of aquifer geo-hydraulic characteristics with geo-electrical methods at a major coastal aquifer system in Uyo, southern Nigeria

Abstract Characterization of aquifer geo-hydraulic properties is essential for groundwater management and conservation. This work demonstrates the use of electrical resistivity surveying data in the characterization of important geo-hydraulic properties within a major coastal aquifer in southern Nigeria. Geo-electrical data acquired along profile lines with the use of Schlumberger array configuration were used to determine the aquifer geo-hydraulic parameters such as aquifer resistivity ρb, water resistivity ρw, porosity Φ, formation factor F, hydraulic conductivity K, transmissivity T, permeability Kp, and tortuosity τ. The resistivity inversion models delineated the occurrence of four lithological layers, with resistivity of the geo-layers ranging from 2.4 to 4,393 Ω m. Contour maps were created to show the distribution of the various geo-electrical and geo-hydraulic parameters over the study area. Regression curves showing the relationship between the geo-hydraulic parameters were generated and used to generate empirical relationships between the parameters. The empirical formulas developed for these geo-hydraulic parameters will aid in future work seeking to characterize aquifer zones and model aquifer geo-hydraulic units. In all, electrical resistivity surveying was found to be a noninvasive, cost-effective, and efficient method of characterizing aquifer geo-hydraulic properties via nonpoint geo-electrical data collected over the earth's surface.

Understanding global groundwater-climate interactions

Global warming is emerging as an important predictor of water availability and future water supplies across the world through inducing the frequency and severity in hydrological extremes. These extremes (e.g., drought) have potential impacts on groundwater, environmental flows, as well as increase social inequalities (limited access to water by the poor), among a range of other issues. Understanding the influence of global climate on groundwater systems is thus critical to help reshape global water markets through policies underpinned by the knowledge of climatic processes driving the water cycle and freshwater supply. The main aim of this study is to improve understanding of the influence of climate variability on global groundwater using statistical methods (e.g., multi-linear regression and wavelet analyses). The response of groundwater to climate variability are assessed and the feasibility of identifying climatic hotspots of groundwater-climate interactions are explored (2003−2017). Generally, climate variability plays a major role in the distribution of groundwater recharge, evidenced in the groundwater-rainfall relationship (r ranging from 0.6 to 0.8 with lags of 1–5 months) in several regions (Amazon and Congo basins, West Africa, and south Asia). Some of the areas where no relationship exists coincide with major regional aquifer systems (e.g., Nubian sand stone in north Africa) in arid domains with fossil groundwater. Our results also show that groundwater fluxes across the world are driven by global climate teleconnections. Notable among these climate teleconnections are PDO, ENSO, CAR, and Nino 4 with PDO showing the strongest relationship (r= 0.80) with groundwater in some hotspots (e.g. in South America). The explicit role of the Pacific ocean in regulating groundwater fluxes provides an opportunity to improve the prediction of climate change impact on global freshwater systems. As opposed to remarkably large productive hydrological systems (Amazon and Congo basins), in typically arid domains, groundwater could be restricted during prolonged drought, constraining the persistence of surface water in the maintenance of a healthy surface-groundwater interactions.

The integration of geochemical and isotopic approaches for thermo-mineral water characterization: the case of Tebessa (North Eastern Algeria)

This research aims to assess the hydrogeochemical evolution and the assessment for drinking and irrigation use of the spring water from the alluvial aquifer and major karst aquifer systems in Tebessa (the northeastern part of Algeria). For achieving this goal, 25 groundwater samples from several springs, including thermo-mineral springs, were examined and subjected to multivariate statistical analysis (principal component analysis), isotopic approaches, and geochemical modelling. However, it was revealed that the hot waters interact at depth with Triassic evaporates located in the hydrothermal conduit (fault), giving rise to the Na+-Cl- water type. Furthermore, the freshwater characterized the karst aquifer marked by the Ca2+ HCO3 - water type with low salinity concentrations. On the other hand, the majority of cations and anions and electrical conductivity, which characterize the chemical composition of the overall water springs, were below the limits allowed for drinking water according to the standards. In terms of hydrochemical facies, it was discovered that throughout the study area, two chemical facies were predominant (Ca2+- HCO3 - and Na+- Cl-). Water-rock interaction, characterized by the dissolution of carbonates and silicates, plays a primordial role in the chemical composition of the groundwater. Stable isotopic analyses of the δ18O and δ2H compositions of the waters suggest that the cold waters of the study area are of meteoric origin. Anyway, it was concluded that the meteoric recharge was precipitation, which recharged from a higher altitude (600–1700 m) and infiltrated through deep faults and fractures in the carbonate formations of the Tebessa Mount.

Estimating Aquifer System Storage Loss With Water Levels, Pumping and InSAR Data in the Parowan Valley, Utah

AbstractIn the Parowan Valley of Utah, groundwater levels have declined by as much as 30 m over the past 50 years with accompanying subsidence rates of up to 5 cm/year. Traditional methods to estimate groundwater storage change use a combination of groundwater level and storativity estimates, but there is often considerable uncertainty in these. In this study, we demonstrate a new method that relies on a combination of geodetic data from InSAR, as well as groundwater level and pumping data, to estimate both the total groundwater storage loss and the percentages of storage loss in fine‐ and coarse‐grained layers within an aquifer system. We find that when aggregated over all of Parowan Valley, fine‐ and coarse‐grained layers account for roughly equal portions of the total groundwater storage loss. However, in confined aquifers, fine‐grained layers account for most of the storage loss. This has important implications on the source of groundwater in depleting aquifer systems, as many models do not account for fine‐grained layers as a source of water. We find that in the Parowan Valley, the aquifer depletion is roughly 12.5% of the volume of pumped groundwater, meaning that the remainder of pumped groundwater is sourced from net inflow. This study presents the first method that combines geodetic and in situ groundwater data to provide estimates of groundwater storage change that account for both coarse‐ and fine‐grained intervals, which are typically present in significant amounts in the major unconsolidated aquifer systems of the world.

Evaluation of the groundwater quality index (GWQI) and the human health risk (HHR) on fluoride concentration in Namakkal district, South India

This research aims to determine the health consequences of fluoride contamination of groundwater in the Namakkal region in south India using the groundwater quality index (GWQI). Study area latitude and longitude: 11° 00' and 11° 30' in the north, and 77° 45' and 78° 15' in the east. Statewide, it is among the largest districts in the state. The study region occupies an area of 3406.37 km2. The geology of the studied area is mainly based on the Archaean crystalline and metamorphic complex. The district's major aquifer systems are composed of crystalline rocks that are weathered and fractured and of colluvial deposits. Alluvium and colluvium are examples of porous formations in the cross-section. Only the main river channels have alluvial deposits. The phreatic properties of groundwater Depending on the topography, these aquifers may reach 5 m saturation thickness. Groundwater samples were obtained from 58 bore well sites across the study area during the North-East Monsoon (NEM) of 2015. pH concentrations in suitable drinking water regions during the seasons assist in limiting the availability of groundwater for drinking purposes. TDS are an important factor in determining water suitability for various purposes. The groundwater sample in the study area shows cation domination in ascending order of Na+ > Ca2+ > Mg2+ > K+ due to the dissolution of aquifer minerals in rainfall in the study area. In the NEM seasons, rock dominance and anthropogenic contributions to higher Na+ > Mg2+ > Ca2+ > K+ values. Fluoride concentration differentiates into three groups such as < 0.5 indicates low risk, 0.5 to 1.5 indicates moderate risk, and > 1.5 means high risk. More than 2 fluoride implies very high risk, whereas fluoride in the range of 1.5 to 2.24 suggests a very high risk. The appraisal of non-carcinogenic risk was done to stress the health issues that succeed due to the intake and dermal contact of drinking water in the Namakkal district. The percentage of risk HQ >1 shows that 48 men groundwater samples, followed by 46 groundwater samples women and 30 groundwater samples children, constitute possible health hazards. Overall, health risk estimation results showed that all the groundwater samples have surpassed the permissible limit of HQ <1 for children.

Isotopic composition of groundwater resources in arid environments

The availability and quality of limited groundwater resources in Egypt are negatively affected by both natural and anthropogenic forces. It is crucial to understand the recharge sources and mechanisms of country-scale aquifers in order to develop sustainable management scenarios for Egypt's groundwater resources. In this study, we compiled all historical hydrogen (δ2H) and oxygen (δ18O) stable isotopic compositions of 1618 ground- and surface-water samples collected for seven major aquifer and reservoir systems in Egypt. These data were then hosted in a public-domain web map application (https://bit.ly/3n9MtrM) that enables users to access, navigate, display, and analyze the isotopic composition of any water sample in Egypt. The developed application and customized web tools were then used to provide improved understanding of the country-wide water resources’ isotopic composition and to investigate the recharge sources and mechanisms of, and provide sustainable management scenarios for, the seven major Egyptian aquifer systems. Results indicate that (1) the alluvial and coastal aquifers are recharged from modern rainfall, floodwaters, and upward leakage from deep aquifers; (2) the Nile Valley and Delta, Moghra, and carbonate aquifers are receiving recharge from the Nile River, modern rainfall, irrigation canals and drains, and upward leakage from deep aquifers; (3) the Nubian aquifer receives modern recharge in areas of higher rainfall or close to surface water reservoirs; and (4) the fractured basement aquifers are recharged from modern rainfall and floodwaters. The developed cost-effective web application will guide locations and techniques for future field data collection; reduce time, efforts, and resources required for field activities; enable improved understanding the country-wide stable isotopic compositions of water resources; enhance understanding of nationwide recharge mechanisms; and enable the scientific community to address country-scale science questions in a unique way.

Hydrogeophysical Evaluation of Parts of River Mamu Sub-Basin, Southeastern Nigeria

Hydrogeophysical evaluation of parts of River Mamu Sub-Basin, Southeastern Nigeria was carried out using the Vertical Electrical Sounding technique (VES). Schlumberger array was used to acquire thirty (30) VES stations and were interpreted using software 1D-Interpex. The predominate VES curve types within the area are typically K-curves which is about 47%, whereas, the remaining 53% belongs to the other five curve types namely; KH, HK, H, Q, and KHK-curves. The calculated Da-Zarrouk parameters outcomes ranged from low to high within the study area: longitudinal conductance (0.0028-0.0571 mhom); transverse resistance (6630-10290 ohm-m2); aquifer resistivity (455-4900 ohm-m), aquifer thickness (4.70-38.00 m); depth to the aquifer (33-172 m); conductivity (0.0002-0.0022 mho. Two aquifer parameters were computed within the area namely; hydraulic conductivity (6.137-0.792 m/day) and transmissivity (1.007-20.597 m2/day). Two major aquifer systems were obtained specifically; a shallow aquifer belonging to Nanka Sands with depth (33-62m) and a far aquifer belonging to Ebenebe Sandstone with depth (69-172m). The groundwater flow paths within the study area trend predominantly in the NW-SE direction. The findings revealed that the study area possesses high groundwater potentials in some areas, hence, it is more economical to drill a borehole at the southwestern part of the study area than the southeastern part.

Groundwater Storage Changes in the Major North African Transboundary Aquifer Systems during the GRACE Era (2003–2016)

Groundwater is an essential component of the terrestrial water cycle and a key resource for supplying water to billions of people and for sustaining domestic and economic (agricultural and industrial) activities, especially in arid and semi-arid areas. The goal of this study is to analyze the recent groundwater changes which occurred in the major North African transboundary aquifers in the beginning of the 21st century. Groundwater storage anomalies were obtained by removing soil moisture in the root zone (and surface water in the case of the Nubian Sandstone Aquifer System) from the terrestrial water storage anomalies estimated using the Gravity Recovery and Climate Experiment (GRACE) over the 2003–2016 time period. Spatio-temporal changes in groundwater storage contrast significantly among the different transboundary aquifers. Low changes (lower than 10 km3) were observed in the Tindouf Aquifer System but they were found to be highly correlated (R = 0.74) to atmospheric fluxes (precipitation minus evapotranspiration, P − ET) at annual scale. The GRACE data revealed huge water loss in the North Western Sahara and the Nubian Sandstone Aquifer Systems, above 30 km3 and around 50 km3, respectively. In the former case, the aquifer depletion can be attributed to both climate (R = 0.67 against P − ET) and water abstraction, and only to water abstraction in the latter case. The increase in water abstraction results from an increase in irrigated areas and population growth. For these two aquifers, a deceleration in the water loss observed after 2013 is likely to be attributed either to an increase in rainfall favoring rain-fed agriculture or to measures taken to reduce the over-exploitation of the groundwater resources.

Multi-tracer approach for assessing complex aquifer systems under arid climate: Case study of the River Tata catchment in the Moroccan Anti-Atlas Mountains

Within the arid Tata River catchment of the Moroccan Anti-Atlas Mountains groundwater is the only source for drinking and irrigation water supply. Nevertheless, the hydrogeological and hydrogeochemical setting of the area is understudied. In order to develop sustainable groundwater management strategies a profound understanding of the region's groundwater system is essential. In terms of comprehensive field surveys, aquifer structure and properties as well as water composition were evaluated. The Lower Cambrian Limestone and Dolomite Series as well as the Quaternary sediments were identified to be the major aquifer systems. Isotopic signatures of groundwater indicate direct groundwater recharge trough fractures and a major flow direction similar to the drainage system of the Tata River. Hydrochemical composition mainly reflects the mineralogy. The primary processes are calcite and bassanite dissolution as well as dedolomitization and feldspar weathering. Water of irrigation return flow drains downstream and is pumped repeatedly, with each cycle contributing to contamination and water loss through evaporation, while the water is used for irrigation and daily needs. In turn, groundwater quality decreases along its flow path from N to S. The hydrogeological conceptual model, visualizing all results, can be used by stakeholders to integrate groundwater related aspects into site-specific agricultural practices as well as to set up numerical flow and transport models.

Aquifer characterization and groundwater flow dynamics in the upper left bank of Jemma River catchment, Blue Nile Basin, Ethiopia

The complex nature of the Ethiopian aquifer requires detail, characterization, and analysis. Consequently, characterization of groundwater flow and major aquifer systems was conducted in the upper left bank of the Jemma River catchment in the Blue Nile Basin. The research area covered 1255 km2. The main objective of the research was to characterize the aquifer systems and groundwater flow dynamics of the area. The software applied during the research work were aquifer test, ArcMap 10.2, global mapper 17, AutoCAD, Exelis ENVI, geomatics, and rockworks. Pumping test data and spring discharge together with qualitative field description were used to classify the aquifer system. Accordingly, the different aquifer classes are fissured aquifer developed on basalts on the plateau (moderate to low groundwater potential), mixed aquifer (high permeability and very high productivity), fissured aquifer developed on basalts in the valleys (high permeability and low to moderate storage), porous and fissured sandstone aquifer in deep valleys, and localized aquifer with intergranular porosity and permeability (high permeability and high productivity). From the analysis of water level measurements of boreholes, hand-dug wells, and springs, the groundwater flow system is controlled by surface morphology and lineaments. Therefore, the groundwater flows from SE and S-SSE corner towards the N and NW corner of the catchment. Understanding groundwater dynamics and characterizing the aquifer in the catchment are very crucial which in turn help to manage the available water resource for domestic, industrial, and agricultural purpose. Besides, the research will be used to manage aquifers from depletion.

Learning and flexibility for water supply infrastructure planning under groundwater resource uncertainty

Water supply infrastructure planning in groundwater-dependent regions is often challenged by uncertainty in future groundwater resource availability. Many major aquifer systems face long-term water table decline due to unsustainable withdrawals. However, many regions, especially those in the developing world, have a scarcity of groundwater data. This creates large uncertainties in groundwater resource predictions and decisions about whether to develop alternative supply sources. Developing infrastructure too soon can lead to unnecessary and expensive irreversible investments, but waiting too long can threaten water supply reliability. This study develops an adaptive infrastructure planning framework that applies Bayesian learning on groundwater observations to assess opportunities to learn about groundwater availability in the future and adapt infrastructure plans. This approach allows planners in data scarce regions to assess under what conditions a flexible infrastructure planning approach, in which initial plans are made but infrastructure development is deferred, can mitigate the risk of overbuilding infrastructure while maintaining water supply reliability in the face of uncertainty. This framework connects engineering options analysis from infrastructure planning to groundwater resources modeling. We demonstrate a proof-of-concept on a desalination planning case for the city of Riyadh, Saudi Arabia, where poor characterization of a fossil aquifer creates uncertainty in how long current groundwater resources can reliably supply demand. We find that a flexible planning approach reduces the risk of over-building infrastructure compared to a traditional static planning approach by 40% with minimal reliability risk (<1%). This striking result may be explained by the slow-evolving nature of groundwater decline, which provides time for planners to react, in contrast to more sudden risks such as flooding where tradeoffs between cost and reliability risk are heightened. This Bayesian approach shows promise for many civil infrastructure domains by providing a method to quantify learning in environmental modeling and assess the effectiveness of adaptive planning.

Using hydrochemical and isotopic data to determine sources of recharge and groundwater evolution in arid region from Eastern Desert, Egypt

In arid areas, identifying the recharge sources and hydrogeochemical evolution of groundwater is essential for effective water resource management. The hydrogeological, hydrogeochemical, and isotopic methods are used to understand the origin of water and associated groundwater recharge processes and conceptualize the groundwater flow within the major aquifer systems in the Idfu-Esna Area. In the study area, two main aquifers are studied: the unconfined Quaternary aquifer and the confined Nubian Sandstone aquifer. The groundwater from the two aquifers is characterized by distinct stable isotope signatures. This difference in isotopic compositions is interpreted in terms of difference in origin and recharge mechanisms. The recharge mechanisms are dominated by direct infiltration of precipitation, lateral flow from the surface water and leakage from deep aquifer system through the main faults. The hydrochemical results indicate that the dissolution of carbonate and silicate minerals, leaching of soils and/or ion exchange are the main processes controlling groundwater mineralization. Established conceptual model has clarified the groundwater hydrodynamics within the studied aquifer systems. Based on this model, groundwater mixing can occur between the different aquifers.

Global Land Surface Modeling Including Lateral Groundwater Flow

AbstractThe groundwater system is an essential part of Earth's systems. However, most current land surface models (LSMs) for climate modeling do not explicitly account for the lateral groundwater flow process. In this study, schemes describing LSM‐lateral groundwater flow module coupling, model resolution conversion, and parallel simulation were designed and implemented to incorporate a lateral groundwater flow module into the Community Land Model 4.5. The depth to less permeable bedrock also was included in the large‐scale groundwater flow modeling. Model validation was performed using multiple observations from the 20‐year continuous groundwater table depth measurements at 67 stations in 10 countries, 1.6 million worldwide time‐averaged groundwater table depth measurements, previous knowledge about the locations of major aquifer systems, and inversed terrestrial water storage anomalies derived from satellite data. The simulated results show that the groundwater table pattern is a combined reflection of climatic and topographic factors across a range of spatial scales. Lateral groundwater flow significantly modified the equilibrium water table patterns in North Africa, the Arabian Peninsula, central Asia, and southern Australia, deepening the water tables by more than 6 m. The trend of deepening groundwater tables observed between 1970 and 2010, which was found to be 0.025 to 0.125 m/decade, was exacerbated by the lateral flow; however, the seasonal variability of the groundwater table depth was reduced by the buffering effect of the lateral flow.

Characterization of general and singular features of major aquifer systems in the Saguenay-Lac-Saint-Jean region

The hydrogeology of the municipalized territory (13,210 km2) of the Saguenay-Lac-Saint-Jean (SLSJ) region has been studied as part of the groundwater knowledge acquisition program entitled Programme d’acquisition de connaissances sur les eaux souterraines (PACES) launched in 2008 by the Government of Quebec, Canada. This study involved the collaboration of numerous contributors (municipal policymakers, government agencies, watershed organizations and universities) to meet the program’s multiple and wide-ranging requirements. The key deliverables included a numerical geodatabase, 38 regional-scale maps and a scientific report, all elaborated after 4 years of data gathering and compilation, fieldwork and information synthesis. In addition, numerous collateral research projects were undertaken by undergraduate and graduate students. The results of the SLSJ-PACES project provided new insights into regional groundwater resources and led to a generalized conceptual model of regional hydrostratigraphic features and groundwater quality. This paper summarizes the particular aspects of the major aquifers in the SLSJ region as uncovered by PACES, and presents the emerging challenges for updating and improving the region’s hydrogeological knowledge and ensuring the sustainable management of regional groundwater resources.

Social tipping points in global groundwater management.

Groundwater is critical to global food security, environmental flows, and millions of rural livelihoods in the face of climate change 1 . Although a third of Earth's largest groundwater basins are being depleted by irrigated agriculture 2 , little is known about the conditions that lead resource users to comply with conservation policies. Here we developed an agent-based model 3,4 of irrigated agriculture rooted in principles of cooperation 5,6 and collective action 7 and grounded on the World Values Survey Wave 6 (n = 90,350). Simulations of three major aquifer systems facing unsustainable demands reveal tipping points where social norms towards groundwater conservation shift abruptly with small changes in cultural values and monitoring and enforcement provisions. These tipping points are amplified by group size and best invoked by engaging a minority of rule followers. Overall, we present a powerful tool for evaluating the contingency of regulatory compliance upon cultural, socioeconomic, institutional and physical conditions, and its susceptibility to change beyond thresholds. Managing these thresholds may help to avoid unsustainable groundwater development, reduce enforcement costs, better account for cultural diversity in transboundary aquifer management and increase community resilience to changes in regional climate. Although we focus on groundwater, our methods and findings apply broadly to other resource management issues.

A Market-Based Solution to Arkansas's Groundwater Crisis

In the southern United States, prolonged over-extraction of groundwater has resulted in a lowering of the water table in major aquifer systems that are relied on as sources for irrigation water and, in some cases, municipal water. Among the areas most affected by over-extraction is the Grand Prairie region in eastern Arkansas. Using the Grand Prairie as a case study, this paper examines historical and current environmental policy in Arkansas and offers a new solution to the persistent problem of over-extraction. The solution: to create a groundwater market – a cap and trade system aimed to decrease reliance on groundwater and promote sustainability.

Assessment of the Efficacy of Home Remedial Methods to Improve Drinking Water Quality in Two Major Aquifer Systems in Jaffna Peninsula, Sri Lanka.

Chunnakam and Vadamaradchi are two major aquifer systems in Jaffna Peninsula, Sri Lanka. This study was performed to compare water quality in the domestic wells in these aquifers and to assess the efficacy of household water treatments for treating contaminated water. Replicate well water samples were collected from each aquifer and pH, dissolved oxygen (DO), conductivity, total dissolved solids (TDS), salinity, temperature, total solids (TS), total hardness (TH), chemical oxygen demand (COD), oil and grease (OG), nitrate N (N), and total phosphate (TP) were measured. The sampled water from the domestic wells was filtered through commercial mineral filter and Moringa oleifera leaf powder and boiled at 100°C for 10 minutes and the TH, OG, N, and TP were measured. Both OG and N in Chunnakam were significantly higher and the DO were significantly lower than those of Vadamaradchi. TH, N, and OG of some wells exceeded the drinking water quality standards established by Sri Lanka Standards Institution. Moringa oleifera leaf powder filtration reduced N significantly and filtering through commercial mineral filter reduced OG, TH, and N significantly. Boiling at 100°C could remove TH significantly but may cause significant increase in N which might result in health impacts.

Nitrate distribution and potential attenuation mechanisms of a municipal water supply bedrock aquifer

The Silurian bedrock aquifer constitutes a major aquifer system for groundwater supply across the Ontario province in Canada. The application of natural and industrial fertilizers near urban centers has led to groundwater NO3−-N concentrations that sometimes have exceeded the drinking water limit, posing a threat to the usage of groundwater for the human consumption. Therefore, there is a growing interest and concern about how nitrate is being leached, transported and potentially attenuated in bedrock aquifers. This study assesses the local distribution of groundwater NO3− in the up-gradient area of two historically impacted municipal wells, called Carter Wells, in the City of Guelph, Canada, in order to evaluate the potential nitrate attenuation mechanisms, using both groundwater geochemical and isotopic analysis (3H, δ15N-NO3, δ18O-NO3, δ18O-SO4, δ34S-SO4) and a detailed vertical hydrogeological and geochemical bedrock characterization. The results indicate that probably the main source of nitrate to the Carter Wells is the up-gradient Arkell Research Station (ARS), an agricultural research facility where manure has been historically applied. The overburden and bedrock groundwater with high NO3 concentrations at the ARS exhibits a manure-related δ15N and δ18O signature, isotopically similar to the high nitrate in the down-gradient groundwater from domestic wells and from the Carter Wells. The nitrate spatial distribution appears to be influenced and controlled by the geology, in which more permeable rock is found in the Guelph Formation which in turn is related to most of the high NO3− groundwater. The presence of an underlying low permeability Eramosa Formation favors the development of oxygen-depleted conditions, a key factor for the occurrence of denitrification. Groundwater with low NO3−-N concentrations associated with more oxygen-limited conditions and coincident with high SO42− concentrations are related to more enriched δ15N and δ18O values in NO3− and to more depleted δ34S and δ18O values in SO42−, suggesting that denitrification coupled with pyrite oxidation is taking place. The presence of macro crystalized and disseminated pyrite especially in the Eramosa Formation, can support the occurrence of this attenuation process. Moreover, based on tritium analysis, some denitrification can occur in shallow bedrock and within relatively short residence times, associated with less permeable conditions in depth which facilitates oxygen consumption through sulfide oxidation. The role of denitrification mediated by organic carbon cannot be discarded at the study site. This study suggests that the geological configuration and particularly the presence of low permeability Eramosa Formation can play an important role on nitrate natural attenuation, which may serve as a decision factor on defining the bedrock water supply system for both domestic and municipal purposes.

Water Resources Evaluation in Ghadamis Basin, Libya

Ghadames basin is one of the important groundwater basins in Libya. This basin covers an area of about 215,000 km2 in northwestern Libya. This groundwater basin consists of two major aquifer systems. The upper aquifer system mostly formed of carbonate rocks (Mizdah and Nalut formations). The lower aquifer system formed mostly of sandstone known as Kiklah formation. The two aquifer systems are separated by semi-confining layers known as Yefren marl. The lower aquifer system is considered as the main aquifer for water supply. The water resource (mostly from the lower aquifer system) in these systems is used for domestic, irrigation and fodder drinking. The domestic use of water is to supply water to the cities and towns in western part of the country and in the northern flanks of the basin, and plans to transport water to the coastal cities in northwestern part of the country. The existing total amount of extracted water is 132 Mm3/year, and about 20% of it used for domestic uses. The planned water extraction is 223.8 Mm3/year about 55% of it will be used for domestic uses. The hydrogeological situation for this basin has been studied in this study using data from previous hydrogeological exploration programs, evaluation of the data from the existing extraction and observation water wells, pumping tests data used to evaluate aquifers hydraulic parameters, and construction of hydrogeological cross-sections to know the aquifer systems geometry. Hydogeological computer model was constructed to evaluate the aquifer hydraulic parameters using the historical water extraction, and water levels and drawdowns. The model is then used to predict the future drawdown which has been found to be in the range 55 to 60 m after 50 years of extraction.

Hydrogeochemical and isotope evidence of groundwater evolution in El Guettar Oasis area, Southwest Tunisia

In southern Tunisia, the major aquifer system is characterized by the superposition of three principal levels: the upper unconfined Quaternary alluvial aquifer; the intermediate semi-confined/confined complex terminal (Segui, Beglia and Zebbag Formations) and the Continental intercalaire (Bou Dinar and Sidi Aich Formations) which constitutes the deeper confined aquifer. The hydrodynamic functioning of this aquifer system is largely influenced by tectonics (Gafsa and Sehib faults), lithologic variation and recharge conditions. The tectonics has contributed to the discontinuity of the aquifer levels by creating some barriers, which play the role of hydraulic sills. A study of the potentiometric maps shows four principal flow directions. These flow directions converge at the sill zones (Chott El Guettar and Chott Djerid depressions). The total mineralization evolution shows an increase from the periphery of the basin to the discharge area (1.75 ≤ TDS ≤ 14.4 g/l). Dissolution of halite, gypsum and/or anhydrite-bearing rocks is the principal source of the salinity of the groundwater. The isotopic signatures permit to classify the studied groundwaters into two different groups: Non-evaporated groundwaters that are characterized by depleted δ18O and δ2H contents highlight the importance of modern recharge at higher altitude. Evaporated groundwaters with enriched contents reflect the significance infiltration of return flow irrigation waters and the influence of the water dam. Tritium data in the studied groundwaters lend support to the existence of pre-1950 and post-1960 recharge. According to radiocarbon residence time estimates, the deep groundwater is approximately 40 kyears old, and was recharged during a period when the climate was wetter and colder. Based on radiocarbon content, the shallow groundwater is older than 0.5 kyear, and represents modern waters mixed with a limited quality of palaeo-recharge. Moreover, the insulation of certain compartments of the reservoir and inter-communications between the aquifer levels seem to play a principal role in the degradation of quality of groundwater.