Deep Groundwater Aquifers Research Articles

Hydrologic source identification using stable isotope ratios of bottled drinking water in Egypt

A convenient way to understand hydrologic source identification is to make use of a suitable tracer. Isotopes of oxygen and hydrogen provide that opportunity since the isotope ratios of the primary source, precipitation, can be modified by post precipitation processes. This approach has been tested by carrying out a comprehensive isotope ratios investigation of domestic and imported bottled water sourced and distributed across Egypt. Our collection of bottled water includes 33 samples distributed under 27 distinct brands. We compare the isotope ratios of the bottled water with the isotope ratios of the reported natural water source of this water. The bottled water isotope ratios preserve information about the water sources from which they were derived. The measured oxygen and hydrogen ratios of the studied bottled water samples range from −11.51‰ to 2.94‰, with an average value of −1.89‰ for δ18O and from −81.69‰ to 23.94‰, with an average value of −9.54‰ for δ2H. The observed variability of isotope ratios in the bottled water falls within the span of the natural groundwater resources in Egypt, such as shallow and deep groundwater aquifers as well as claimed non-Egyptian sources. In general, the bottled water samples in the data set can be divided into four distinct groups based on their isotope ratios. One group represents imported bottled water brands, and three groups include domestic brands. The first group represents the imported bottled water brands. Their measured isotopic compositions are similar to those of their claimed sources. The second group has the lowest δ18O and δ2H values and likely originated from the Nubian aquifer. The third and fourth groups appear to have been sourced from the Nile Valley and Delta aquifer. The third group isotopic content and d-excess values indicate a higher degree of mixing with evaporated return flow water. The results provided in this study indicate the diverse hydrogeology of the local bottled water brands. This promises extending the approach to several sites globally.

Improving deep groundwater aquifer characterization with deep learning inversion of audio-frequency magnetotelluric data

Deep groundwater is a crucial resource for drinking, industry, and ecosystems. However, its extensive subsurface distribution poses challenges for traditional hydrological sampling methods. Audio-frequency Magnetotelluric (AMT) is commonly used to image shallow subsurface resistivity distribution, but its application for quantifying deep groundwater aquifers is challenging. In this study, we propose a two-step strategy deep learning (DL) to estimate subsurface water information from AMT data. First, we employ an inversion DL network to directly predict subsurface resistivity distribution from AMT data. To assess the impact of data and prior information on DL inversions, we progressively include more input data during training, from apparent resistivity to apparent resistivity, phase, and Bostick transformed initial models. In the second step, we use another DL network to predict subsurface structure from AMT inversion results. We then estimate water content based on unit-specific petrophysical relationships. Evaluating our algorithm with synthetic cases, we find that including more information generally improves network performance, particularly when incorporating initial Bostick transformed models. Applying the algorithm to a field hydrogeological survey, we compare the inversion network trained with apparent resistivity, phase, and Bostick transformed initial models to traditional regularization inversions. The new approach shows better consistency with borehole data. Another network extracts structure information, enabling the estimation of subsurface water content and gaining valuable hydrogeological insights. To summarize, our study presents a novel DL-based approach to quantitatively delineate deep groundwater aquifers using AMT data. The proposed algorithm demonstrates promising performance in estimating subsurface hydrologic properties, providing valuable insights for hydrogeological investigations.

Examining the Relative Impact of Drivers on Energy Input for Municipal Water Supply in Africa

This study examines supply-side and demand-side drivers of municipal water supply and describes how they interact to impact energy input for municipal water supply in Africa. Several key compound indicators were parameterized to generate cluster centers using k-means cluster analysis for 52 countries in Africa to show the impact of water supply–demand drivers on municipal water supply and associated energy input. The cluster analysis produced impact scores with five cluster centers that grouped countries with similar key compound indicators and impact scores. Three countries (Gambia, Libya, & Mauritius) were classified as outliers. Libya presented a unique case with the highest impact score on energy input for raw water abstraction, associated with largescale pumping from deep groundwater aquifers. Multivariate analysis of the key indicators for 20 countries in sub-Saharan Africa that are either water-secure or water-stressed illustrate the relative impact of drivers on energy input for municipal water supply. The analytical framework developed presents an approach to assessing the impact of drivers on energy input for municipal water supply, and the findings could be used to support planning processes to build resilient drinking water infrastructure in developing countries with data challenges.

Geoelectric Sounding for Identification of Aquifer Layer in East Turatea Village South Sulawesi Province

East Turatea Village farmers still depend on rainwater for irrigating their rice fields. This study aims to identify groundwater aquifers in the village of East Turatea which can be used as water sources in irrigating rice fields. The method used is the correlation of well data and Vertical Electric Sounding (VES). Well data is result of water table mapping at three measuring stations. Measure the resistivity of rock using a resistivity meter with a length of 200 m with two Vertical Electric Sounding (VES) points. The measurement results are then processed using Progress software to produce a Vertical Electric Sounding (VES) with a resistivity variation to depth. The result this study found that East Turatea Village’s groundwater system in the form of shallow groundwater aquifer has a resistivity value of 0.84-5.5 Ohm.m at a depth of 6.04-31.88 m. Meanwhile, deep groundwater aquifers are at a depth of more than 53.8 m with a resistivity value of 1.94 - 2.75 Ohm.m. East Turatea Village groundwater aquifer is estimated to consist of permeable layer such as sand which is in a saturated zone.

Aspects of the Hydrogeology of southern Campeche and Quintana Roo, Mexico

This paper explores strong indirect evidence for existence of a previously unrecognized deep groundwater aquifer in southern Quintana Roo, adjacent parts of Campeche, and (probably) northern Belize. The region contains rocks of Cretaceous-to-Holocene age, including: 1) an up-thrust block of the late Cretaceous carbonate known in Belize as the Barton Creek Formation, which is the oldest formation exposed in the Mexican Yucatán Peninsula, 2) the Cretaceous/Paleogene Albion Formation consisting of weakly consolidated Chicxulub impact air-fall deposits, 3) the Paleocene-Eocene Icaiche Formation, containing a massive 25-35 m thick gypsum member that crops out over an estimated area of more than 10,000 km2 in the elevated interior region of the northern lowlands, and 4) younger rocks of relatively low permeability that flank the region on the east. Hydrogeology is dominated by groundwater and surface flow in and adjacent to the Rio Hondo Fault Zone (RHFZ) and by recharge in the elevated interior region. Groundwater in the elevated region has a high sulfate concentration and is approximately saturated with gypsum dissolved from the Icaiche Formation. High-sulfate groundwater and river water with a slightly lower gypsum saturation index than in the elevated region also occurs in the RHFZ, but no water of comparably high sulfate content is present elsewhere in the study area. This suggests that the elevated region is a recharge zone for high-sulfate groundwater carried eastward beneath a 50 km gap by a deep, previously unrecognized aquifer and then discharged into the RHFZ. Based on chemistry of chloride, sulfate and other ions it is proposed here that a deep aquifer comprising the strongly weathered upper surface of the Barton Creek Formation plus the overlying weakly consolidated Albion Formation connects the elevated recharge area with the RHFZ discharge area. If this composite permeable zone does extend westward beneath the elevated recharge zone, it is probably an excellent aquifer.

Subsurface structure investigation of the United Arab Emirates using gravity data

AbstractThe crustal structure beneath the United Arab Emirates (UAE) is still relatively unknown. Here, we use regional gravity data to constrain the subsurface density distribution and structure of the crust of the UAE by applying diverse gravity derivatives methods such as horizontal derivative (HDR), analytic signal (AS), and tilt angle (TA) to analyze the subsurface structure and perform three-dimensional (3D) gravity inversion for imaging crustal structure from the surface down to 35 km depth. The results are compared with known geological regional structures and the location of the petroleum fields.The Bouguer anomalies range from −100.8 to 113.5 mGal. The 3D gravity inversion results and the maximum Bouguer values coincide with the ophiolitic Hajar mountains in the east and the successive anticlines (uplifted basement rocks) and synclines in different parts of UAE, which could be promising sites for future mining and petroleum exploration. Also, the 3D density model results and the minimum Bouguer anomalies are located over the Aruma Basin, eastern UAE Platform, and Low Central UAE Platform, which can be the places for deep groundwater aquifers. These new results from HDR, AS, and TA successfully identify known geological structures, especially in the eastern part of UAE.

Groundwater recharge and water table response to changing conditions for aquifers at different physiography: The case of a semi-humid river catchment, northwestern highlands of Ethiopia

Groundwater recharge estimation, aquifer response to meteorological variables, and evapotranspiration calculations have been performed on a semi-humid catchment, in northwestern Ethiopian plateau. The Soil Moisture Balance (SMB), WetSpass water balance model, Water Table Fluctuation (WTF), and Chloride Mass Balance (CMB) methods are applied to estimate the groundwater recharge. Accordingly, 431 mm, 462 mm, and 477 mm recharge amounts are estimated as mean annual value, respectively, using SMB, WetSpass, and CMB methods. Based on the WTF method, the annual recharge rates of the volcanic aquifers range from 157 mm to 760 mm. The SMB and WetSpass methods are less effective for the flat physiographic area, where the recharge rate is storage controlled rather than precipitation amount. The calculated high recharge for maintain-front aquifers using WTF is attributed to extra rising due to lateral groundwater flow, which restricts the reliability of the method for such aquifer geometries. High groundwater level rising rate (121 mm/day) has been observed for the steeply sloping, low rates (11 mm/day) for the flat floodplain, and intermediate rate (52 mm/day) for the gently sloping volcanic aquifers. Similarly, receding rates of 3.18 mm/day were found for the steeply sloping, 0.40 mm/day for the floodplain, and 1.14 mm/day for the gentle sloping aquifers. The recession, in all of the topographies, is happening with second-order polynomial decay function. A strong connection between the shallow and deep groundwater aquifers is noted. Storage change in the relatively deeper volcanic aquifers is due to vertical groundwater flow from the overlying alluvial aquifer. This indicates that the recharge mechanism is local, and may be the reason for the low aquifer productivity of the Dangila wellfield. Diurnal water table fluctuation is detrended from the receding trend of the dry period, and evapotranspiration from the groundwater is estimated at 28% of total ET.

A Modified IHACRES Rainfall-Runoff Model for Predicting the Hydrologic Response of a River Basin Connected with a Deep Groundwater Aquifer

A flow regime is influenced by the degree of hydrologic connection between surface water and groundwater. As this connection becomes more transient and the basin’s runoff response more non-linear, such as for intermittent streams, the need for explicit representation of the groundwater component increases. The present study investigates the connection between Northern Etna groundwater system and the Alcantara river basin in Sicily (Italy). In particular, the upstream part of the basin, whose flow regime is essentially intermittent, is modeled through a modified version of the IHACRES rainfall-runoff model. The structure of the model includes a routing module formulated as a two-store model, with the upper store simulating the quick component of the runoff and recharging the lower store which, in turn, describes the slow component of the runoff and the groundwater extraction and losses. Both stores are conceptualized as simple linear reservoirs, with the lower one that maintains a continuous water balance account of groundwater storage volumes for the upstream basin area with respect to a control cross-section, assumed to be the stream gauging station. The model is calibrated at Moio Alcantara cross-section, where daily streamflow data are available. Model calibration and validation are carried out for the period 1980–1984 and 1986–1988, respectively. A first-order analysis is also performed to assess the sensitivity of model parameters. The adopted configuration is shown to improve model performance with respect to the original IHACRES model, with the proposed formulation able to better capture the interactions between the aquifer and the river.

Geological spatial plan toward groundwater resources in Kertek, Wonosobo Basin, Central Java, Indonesia

Human activity affects both natural resources and spatial land use, including its utilization as sand mining sites. Sand mines, as a pillar of building construction, have been over excavated in certain areas, which has impacted the environment. In this research, the purpose of this study was to determine a spatial allocation of the mining designation area that will not damage the groundwater flow. Therefore, it is imperative for understanding the depth of groundwater at the study site, understanding the direction of groundwater flow, and the impact of mining activities on groundwater based on the combination of geophysical and geological approaches to improve the current government policy. A geological-geophysics approach, vertical electrical sounding (VES), was followed at 12 different locations surrounding the Kertek District, which has a general geological formation of igneous volcanic rock sediment. The result of the geophysical measurements (possibilities) indicates the presence of shallow, medium, and deep groundwater aquifers, which tended to follow the slope direction to the south. This may be due to the unsaturated soil conditions as the geoelectrical measurements were taken at the beginning of the rainy season. Finally, the goals of this research were to integrate resources with spatial characteristics to allow proper resources management.

Stable isotope evidence for identifying the recharge mechanisms of precipitation, surface water, and groundwater in the Ebinur Lake basin

Arid areas cover more than one third of global land, and as such, water resources are vital for this fragile ecosystem. In order to reveal the recharge mechanisms among different water bodies in arid areas, precipitation, surface water, and groundwater were sampled in the Ebinur Lake basin, Xinjiang, China, and the isotopic values for hydrogen and oxygen were measured. The stable isotope values of precipitation showed significant seasonal variation, with minimum values in the winter, medium values in the spring and autumn, and maximum values in the summer. The slope and intercept of local meteoric water line were both lower than that of global meteoric water line, indicating subcloud evaporation effect. The vapor source of precipitation was dominated by the westerlies, but the regional re-evaporation vapor accounted for some proportions as well. In the Bortala River and Jinghe River, the stable isotopic values varied spatially, tending to be enriched with the river flow. The stable isotopic values for lake water were significantly higher than those of river water, which reflected a stronger evaporation and concentration effect of the lake water. The stable isotopic values of groundwater featured similar spatial variation compared to the river, and phreatic water evaporated to some extent. In the Bortala River, owing to its specific hydrogeological structure, the exchange rates between the groundwater and the river water were higher upstream than in the middle and lower reaches. In the Jinghe River, the deep groundwater aquifer received recharge from the shallow groundwater layers and from the river. At the edge of the Ebinur Lake, the interaction of groundwater and surface water was low and springs became the important recharge source for the lake. The results of this study provide insights into the determination of river hydrological processes and the management of water resources.

Dissolved CO2 Storage in Geological Formations with Low Pressure, Low Risk and Large Capacities

Geological CO2 storage is a mitigation technology to reduce CO2 emissions from fossil fuel combustion. However, major concerns are the pressure increase and saltwater displacement in the mainly targeted deep groundwater aquifers due to injection of supercritical CO2. The suggested solution is storage of CO2 exclusively in the dissolved state. In our exemplary regional case study of the North East German Basin based on a highly resolved temperature and pressure distribution model and a newly developed reactive transport coupling, we have quantified that 4.7 Gt of CO2 can be stored in solution compared to 1.5 Gt in the supercritical state.

Combining Resistivity and Aeromagnetic Geophysical Surveys for Groundwater Exploration in the Maghnia Plain of Algeria

The Maghnia plain in western Algeria is filled by Plio-Quaternary and Miocene sediments that rest unconformably on a basement of Jurassic rocks. Electrical sounding (VES), magnetic data, well information, and hydrogeological data have been used to explore for groundwater potential in the Maghnia plain. The interpretation of Schlumberger sounding data was first calibrated with the lithology of available nearby wells. Four geoelectrical layers were identified within the study area. They are a thin near surface topsoil layer with variable resistivities, a moderate resistive aquifer (15–30 ohm-m), a resistive aquifer (40–70 ohm-m), and a conductive clay layer (1–10 ohm-m). Near Sidi Mbarek, the geoelectric section is reduced to three layers: a topsoil layer, a conductive layer corresponding to the Miocene marls, and a deep resistive layer that correlates with the Oxfordian sandstones. The interpretation of VES data and the enhancement techniques of magnetic data enabled the identification of a number of unmapped faults that occur near recharge zones close to adjacent mountains. This study enabled us to study the extension of the known Plio-Quaternary aquifer of the Maghnia plain and to explore the possible existence of a second deep groundwater aquifer in Oxfordian sandstones.

Combined application of electrical resistivity and GIS for subsurface mapping and groundwater exploration at El-Themed, Southeast Sinai, Egypt

The study area is located at south central part of Sinai Peninsula at El-Thamed area. Geoelectrical resistivity method was used by measuring eighteen vertical electrical soundings using the Schlumberger configuration with total current electrode separation ranging between 3m and 4000m in order to investigate the deep groundwater aquifer and to delineate the subsurface structures of this area.The geoelectrical cross-sections show that the subsurface section consists of six geoelectrical units. The first unit is Quaternary deposits with thickness ranging from 0.6m to 30.9m and the resistivity values ranging from 177Ohm.m. to 39680Ohm.m. The second geoelectrical unit is composed of Egma Formation, with thickness ranging from 9.2m to 12.8m and the resistivity values ranging between 21Ohm.m. and 82Ohm.m. The third geo-electrical unit is composed of Paleocene Esna shale, also not detected at the study area with exception at profile 2, its thickness ranges from 15.6m to 166.5m and resistivity range from 8.9Ohm.m. to 20Ohm.m. This layer has not been detected at the extremely southern part of this profile, its depth ranges from 9.63m to 15.5m. The fourth geoelectrical unit is composed of Nubian Sandstone of Malha Formation (Lower Cretaceous) with depth ranging from 0.5m to 8.9m and its resistivity values ranging from 14Ohm.m. to 133Ohm.m. This layer represents the main aquifer in the study area with thickness ranging between 14.2m and 283m. The fifth geoelectrical unit is composed of claystone of Naqus Formation with depth ranging from 14.2 to 349m and its resistivity values ranging from 2.7Ohm.m. to 13Ohm.m., sometimes, it is extends to the maximum depth of penetration. The sixth geoelectrical unit is composed of Basement rocks. It is characterized by resistivity ranging from 970Ohm.m. to 25462Ohm.m. and depth ranging from 60.1m to 283m. This unit is detected at the eastern part of the study area.Structurally, the study area is affected by sets of faults trending mainly NE-SW (Aqaba Trend).

Salinity of deep groundwater in California: Water quantity, quality, and protection

Deep groundwater aquifers are poorly characterized but could yield important sources of water in California and elsewhere. Deep aquifers have been developed for oil and gas extraction, and this activity has created both valuable data and risks to groundwater quality. Assessing groundwater quantity and quality requires baseline data and a monitoring framework for evaluating impacts. We analyze 938 chemical, geological, and depth data points from 360 oil/gas fields across eight counties in California and depth data from 34,392 oil and gas wells. By expanding previous groundwater volume estimates from depths of 305 m to 3,000 m in California's Central Valley, an important agricultural region with growing groundwater demands, fresh [<3,000 ppm total dissolved solids (TDS)] groundwater volume is almost tripled to 2,700 km(3), most of it found shallower than 1,000 m. The 3,000-m depth zone also provides 3,900 km(3) of fresh and saline water, not previously estimated, that can be categorized as underground sources of drinking water (USDWs; <10,000 ppm TDS). Up to 19% and 35% of oil/gas activities have occurred directly in freshwater zones and USDWs, respectively, in the eight counties. Deeper activities, such as wastewater injection, may also pose a potential threat to groundwater, especially USDWs. Our findings indicate that California's Central Valley alone has close to three times the volume of fresh groundwater and four times the volume of USDWs than previous estimates suggest. Therefore, efforts to monitor and protect deeper, saline groundwater resources are needed in California and beyond.

Integrated geophysical interpretation for delineating the structural elements and groundwater aquifers at central part of Sinai Peninsula, Egypt

The study area is inhabited by Bedouins, suffering from scarcity of water necessary for domestic use and agricultural activities. The study area is located at central part of Sinai between Nakhl area and El Thamed area. Magnetic, gravity and geoelectric methods were used in this investigation to determine the groundwater aquifers and delineate the structural elements in the study area. Two hundreds and eighty eight magnetic and gravity stations were acquired by the EnviMag and Autograv CG3 instruments respectively. The magnetic data were processed by using Oasis Montaj. Reductions to the pole and 2D magnetic modeling were established to construct basement relief map. The depth to the basement rocks in the study area is ranging from 1200m to 7000m. The regional–residual separation and Euler deconvolution techniques were applied to the gravity data. Nine deep Vertical Electrical Sounding stations were measured to estimate the deep groundwater aquifer in the study area (Nubian Sandstone aquifer). The depth of upper surface of Nubian Sandstone aquifer is ranging between 975m and 1100m and affected by two major fault trends in the NE–SW and NW–SE directions.

HYDROGEOPHYSICS TECHNIQUES TO AQUIFER LOCATING AND MONITORING FOR INDUSTRIAL ZONE NORTH THANGLONG AND QUANGMINH, HANOI

HYDROGEOPHYSICS TECHNIQUES TO AQUIFER LOCATING AND MONITORING FOR INDUSTRIAL ZONE NORTH THANGLONG AND QUANGMINH, HANOI

Hydrological and hydrogeological characterization of groundwater and river water in the North Hanoi industrial area, Vietnam

Degradation of groundwater quality by human activities is a widespread environmental problem in Vietnam. Groundwater there is a major source of water for domestic and industrial purposes. This paper reviews, compiles, and comprehensively analyzes spatiotemporal variations of hydrological and hydrogeological characteristics of shallow and deep groundwater aquifers in northern Hanoi industrial zones and in nearby Red River water. Groundwater level, electrical conductivity, and water temperature were measured in six monitoring wells, complemented by anion, cation, and stable isotope analyses of ground and surface water. The results show that the groundwater in both shallow and deep aquifers was fresh, but mainly calcium-bicarbonate type contaminants and human activities affect groundwater and surface water composition. With the goal of devising sustainable water use regulations, more research must be directed toward long-term monitoring of groundwater and surface water quality, as well as toward detailed investigation of the hydraulic characteristics of local aquifers in the study area.

Feasibility Study for Geophysical Monitoring Renewable Gas Energy Compressed in Pore Storages

Most renewable energy sources are intermittent and need buffer storage (e.g., compressed air energy storage, CAES) to bridge the time-gap between power supply and demand peaks. Replacing pore brine with CAES causes changes in electro-elastic properties and density, and justifies applications of multi-geophysical approach. In this numerical study we apply techniques of the elastic full waveform inversion (FWI), electric resistivity tomography (ERT), transient electromagnetic induction (TEM) and gravity to detect and monitor CAES in deep reservoirs and possible leakages in shallow groundwater aquifers of North Germany. For different subsurface model scenarios of CAES reservoirs and leakages, synthetic data sets are generated and inverted using constraints on the initial model. Results reveal principally the capability of our applied approach to resolve the CAES plume in deep saline reservoirs and shallow groundwater aquifers. The ERT resolution for leakages is highly enhanced for the combined surface-borehole survey compared to the individual surface and borehole surveys. The applied gravity technique is highly sensitive to the mass deficit caused by CAES plume. The detect ability limit of the technique is determined by the least CAES volume causing an anomaly with amplitude just above the accuracy range of modern micro-gravimeters. The FWI technique can map the shallow CAES leakage by anomalies in the reconstructed ΔVp, ΔVs and Δdb tomograms within the background aquifer. However, these tomograms contain inversion artifacts and smearing effects related mainly to the dominance of the Rayleigh wave in the data. Obviously, applied multi-techniques complement and confirm each other. CAES plumes cause strong mass deficits and moderate resistivity highs and thus are more sensitive for gravity and FWI methods. Applying constrained inversion minimizes interpretation ambiguities and helps recovering almost realistic electro-elastic parameters that can be applied in adequate petrophysical equations to quantify CAES saturations.

Occurrence of pesticides in transboundary aquifers of North-eastern Greece

A five-year groundwater monitoring program undertaken in Evros (north-east Greece), showed a diversification in the levels of pesticide residues detected in adjacent transboundary aquifers. During the first two years 37 wells, including irrigation, drinking water and artesian wells were monitored while the next three years the survey was focused on the 11 most contaminated wells. The presence of pesticide residues was also monitored in the phreatic horizon (shallow groundwater) of four experimental boreholes drilled in the respective margins of four fields. Among the compounds found alachlor, metolachlor, atrazine, desethylatrazine (DEA), desisopropylatrazine (DIA) and caffeine were constantly detected. Pesticide concentrations were much lower (up to 1.54μg/L) in the water of the monitored drinking water wells (deep groundwater aquifers) compared to those found in the phreatic horizon (experimental boreholes) of the respective areas (up to 5.20μg/L). DEA to atrazine concentration ratios (DAR) determined for the phreatic horizon of the three boreholes and respective wells were lower than 1, indicating that preferential flow was the cause of the fast downward movement of atrazine to the phreatic horizon. In contrast the DAR for the fourth borehole and the adjacent well were greater than 1 indicating the absence of preferential flow of atrazine. Catabolic processes of the soil converted atrazine to DEA which is more mobile than atrazine itself through chromatographic (darcian) flow.This differential behavior of pesticides in adjacent aquifers (3km) was further investigated by determining the apparent age of water in the two wells. The apparent age of the water present in the first aquifer was 21.7years whereas the apparent age of that in the second aquifer was approximately 1.2years. The faster replenishing rate of the latter is an indication that this aquifer is very vulnerable to contamination with pollutants present in the infiltrated soil water.

Groundwater residence time and paleorecharge conditions in the deep confined aquifers of the coastal watershed, South-East Tanzania

Summary Analysis of radiocarbon and stable isotopes of C, H and O in groundwater samples from the coastal watershed of Southeastern Tanzania is used to decipher the age of groundwater and the climatic conditions at the time of recharge. Radiocarbon dating was corrected on the basis of stable carbon isotope ratios and hydrochemical data. Two main recharge periods are recognised: (1) a recent recharge period with residence times dated to younger than 1.3 ka BP, for the shallow aquifer and some deep groundwater aquifers; and (2) a paleorecharge occurring during the Late Pleistocene to Holocene transition (between 6 and 13 ka BP), for the lower confined aquifer. The climatic excursions (arid and humid phases) identified in this study match well with those identified from studies of paleo-climatic records in Tanzania and East Africa. The oxygen-18 and radiocarbon results show that there is a potential mixing of groundwater of varying ages along flow paths in the coastal aquifer system. This has also been shown by redox parameters in some of the deeper aquifer samples. The data also revealed that there is relatively simple pattern of lateral groundwater flow in the deeper confined aquifer. Isotopic evidence from this study and related studies in paleolimnology improves our knowledge of the past climates. The data suggests that the paleorecharge to the confined aquifer was from inland under wetter climatic conditions than present. Therefore, paleorecharge from the adjacent mountain regions to coastal area like Ifakara, Morogoro and Udzungwa mountains could be important parts of recharge area of the Kimbiji aquifer. However, the hinterland mountains of Dar es Salaam (mainly Pugu, Mkuranga, Ruvu and Kisarawe) could also play an important role in replenishing the Kimbiji coastal aquifers. In contrast, recharge in the shallow unconfined aquifer is characterized by local rainfall and river water infiltration. This study is the first of its kind, assessing fingerprinting of paleoclimates in Southeastern coastal Tanzania using the isotopic composition of deep groundwater aquifers. The freshwater resources in the Neogene aquifer contain paleo-groundwater aquifers that have been mixed with recently-recharged flow systems. Hence, the resources may be sustainable. This study also provides information that may be useful for designing optimum strategies for a sustainable management of groundwater resources in the coastal aquifer, and it testifies to the importance of groundwater as a paleoclimatic archive. Such information is also essential for consideration by modellers in their attempts to simulate the hydrogeological processes in confined aquifers over very long time spans.