Migration of arsenic in multi-aquifer system of southern Bengal Basin: analysis via numerical modeling

A heterogeneous anisotropic steady-state groundwater flow model for the multi-aquifer system of a part of southern Bengal Basin shows that human intervention has changed the natural groundwater flow system. At present, the shallow groundwater flow is restricted within the aquifer, with very short travel time of tens of years and vertical path length. The deep aquifer is fed by surface water or rainwater from distant locations with travel time of thousands of years and has no hydraulic connection with the arsenic-rich shallow aquifer. Numerical simulations indicate that the future pumping of deep groundwater is not likely to drive in arsenic from the shallow aquifer. Therefore, new wells may be installed in the deep aquifer. High pumping of shallow unpolluted aquifer consisting of brown sand will drive in groundwater containing organic matter from the post-Last Glacial Maximum aquifer-aquitard system. The organic matter drives reduction of manganese oxides at strip interfaces between palaeo-channel and palaeo-interfluve. After the completion of manganese reduction, FeOOH reduction may take place in the marginal palaeo-interfluvial aquifer and release sorbed arsenic. Arsenic then moves into the interior of palaeo-interfluvial aquifer polluting its fresh groundwater. Arsenic migration rates ranges between 0.21 and 6.3 and 1.3 × 10^?2 and 0.4 m/year in horizontal and vertical directions, respectively. Therefore, palaeo-interfluvial aquifer will remain arsenic-free for hundreds to thousands of years to supply safe drinking water.     

Geological and geophysical exploration of the groundwater aquifers of As Suqah area, Makkah district, Western Arabian Shield, Saudi Arabia

As Suqah area is a NW–SE trending wadi present in the west central part of the Arabian Shield. It comprises Precambrian–Cambrian basement rocks, Cretaceous–Tertiary sedimentary succession, Tertiary–Quaternary basaltic lava flows, and Quaternary–Recent alluvial deposits. The magnetic anomalies indicated the presence of many recent local buried faults. These affected the distribution of the clastic sedimentary succession and seem to have controlled the deep groundwater aquifers. Groundwater movement is towards the west and northwest, following in general the surface drainage system. Hydraulic gradient varies greatly from one point to another depending on the pumping rates and cross-sectional area of the aquifer in addition to its transmissivity. The detailed results of the resistivity and seismic measurements were integrated with those obtained from test holes drilled in the study area. Groundwater occurs mainly in two water-bearing horizons, the alluvial deposits and within the clastic sedimentary rocks of Haddat Ash Sham and Ash Shumaysi formations. The shallow zone is characterized with a saturated thickness of 3–20 m and water is found under confined to semi-confined conditions. Water levels were encountered at depths varying from 3 to 16 m in the alluvial wadi deposits and from 18 to 62 m in the sedimentary succession. The combinations of vertical electrical sounding, horizontal electrical profiling, and drilling led to the identification of groundwater resources in the study area. Resistivity soundings clearly identified the nature of the lithological depth and proved useful at identifying water-bearing zones. Significantly, the majority of the groundwater was found within the deep confined aquifer gravelly sandstone, rather than in the shallow unconfined aquifer.     

Hydrochemical evolution and arsenic release in shallow aquifer in the Titas Upazila,Eastern Bangladesh

Groundwater arsenic (As) concentrations above 10 μg/L (World Health Organization; WHO standard) are frequently found in the Titas Upazila in Bangladesh. This paper evaluates the groundwater chemistry and the mechanisms of As release acting in an underground aquifer in the middle-northeast part of the Titas Upazila in Bangladesh. Previous measurements and analyses of 43 groundwater samples from the region of interest (ROI) are used. Investigation is based on major ions and important trace elements, including total As and Fe in groundwater samples from shallow (8–36 m below ground level: mbgl) and deep (85–295 mbgl) tube wells in the aforementioned ROI. Principal hydrochemical facies are Ca–HCO₃, with circumneutral pH. The different redox-sensitive constituents (e.g., As, Fe, Mn, NH₄, and SO₄) indicate overlapping redox zones, leading to differences regarding the redox equilibrium. Multivariate statistical analysis (factor analysis) was applied to reduce 20 chemical variables to four factors but still explain 81% of the total variance. The component loadings give hints as to the natural processes in the shallow aquifers, in which organic matter is a key reactant. The observed chemistry of As, Fe, and Mn can be explained by simultaneous equilibrium between Fe-oxide and SO₄ reduction and an equilibrium of rhodochrosite precipitation/dissolution. A correlation test indicates the likeliness of As release by the reductive dissolution of Fe-oxides driven by the degradation of sediments organic matter. Other mechanisms could play a role in As release, albeit to a lesser extent. Reactive transport modeling using PHREEQC reproduced the observed chemistry evolution using simultaneous equilibrium between Fe-oxide and SO₄ reduction and the equilibrium of rhodochrosite dissolution/precipitation alongside organic matter oxidation.     

Delineation of groundwater prospective zones from a delta region of India,using geoelectrical and water quality approach

Geoelectrical survey was carried out in the western delta region of River Vasista Godavari, Andhra Pradesh, India, for delineation of groundwater prospective zones due to acute shortage of water supply for various purposes. Forty-six vertical electrical soundings (VES) were done, employing the Schlumberger configuration with a maximum AB/2 of 160 m. The interpreted results of VES show four to five layers with variable thicknesses, such as topsoil zone (1.5–3 m), clay zone (0.84–32 m), finer sand zone (2–72 m), medium to coarse sand zone (4 to 28.8 m) and clay zone (1.2–∞ m), indicating a multi-aquifer system. These results are corroborated with the known lithologs of the study area. Further, the resistivity is also compared with electrical conductivity (EC) of groundwater observed nearby shallow wells representing buried channel (BC), flood plain (FP) and coastal (C) zones, which indicate slightly brackish to brackish water (EC: 1470–6010 µS/cm), whereas the groundwater observed from deep wells shows the fresh (EC: 726–1380 µS/cm), fresh to brackish (EC: 1010 to 3250 µS/cm), and brackish water (EC: 3020 to 4170 µS/cm) located in BC, FP and C zones, respectively. This survey reveals the prospective aquifer zones with potable water at VES locations of 4–6, 8, 10, 11, 14, 16–28, 33–36, 39 and 42–44, where the resistivity values vary from 10 to 40 Ω m. The slightly brackish and brackish water zones are also observed from the resistivity of less than 10 Ω m at shallow depth in BC (VES-22, 37, 38 and 46), FP (VES-1, 2, 7, 29, 30 and 40) and C (VES-3, 4, 9, 12, 13, 15, 31, 32, 41 and 45) zones. As a result, the present investigation has delineated the freshwater zones at shallow (<?12 m) and also at deeper depths (30–45 m) as prospective areas, where BC zone occurs. Freshwater pockets also identified in FP (VES-8 and 39) and C (VES-11, 14 and 15) zones. Thus, this study helps to solve the drinking and irrigation water problems.     

Arsenic distribution along different hydrogeomorphic zones in parts of the Brahmaputra River Valley,Assam (India)

The spatial distribution of arsenic (As) concentrations along three classified hydrogeomorphological zones in the Brahmaputra River Valley in Assam (India) have been investigated: zone I, comprising the piedmont and alluvial fans; zone II, comprising the runoff areas; and zone III, comprising the discharge zones. Groundwater (150 samples) from shallow hand-pumped and public water supply wells (2–60 m in depth) was analysed for chemical composition to examine the geochemical processes controlling As mobilization. As concentrations up to 0.134 mg/L were recorded, with concentrations below the World Health Organization and the Bureau of Indian Standards drinking-water limits of 0.01 mg/L being found mainly in the proximal recharge areas. Eh and other redox indicators (i.e., dissolved oxygen, Fe, Mn and As) indicate that, except for samples taken in the recharge zone, groundwater is reducing and exhibits a systematic decrease in redox conditions along the runoff and discharge zones. Hydrogeochemical evaluation indicated that zone I, located along the proximal recharge areas, is characterized by low As concentration, while zones II and III are areas with high and moderate concentrations, respectively. Systematic changes in As concentrations along the three zones support the view that areas of active recharge with high hydraulic gradient are potential areas hosting low-As aquifers.     

Controls on groundwater flow in the Bengal Basin of India and Bangladesh: regional modeling analysis

Groundwater for domestic and irrigation purposes is produced primarily from shallow parts of the Bengal Basin aquifer system (India and Bangladesh), which contains high concentrations of dissolved arsenic (exceeding worldwide drinking water standards), though deeper groundwater is generally low in arsenic. An essential first step for determining sustainable management of the deep groundwater resource is identification of hydrogeologic controls on flow and quantification of basin-scale groundwater flow patterns. Results from groundwater modeling, in which the Bengal Basin aquifer system is represented as a single aquifer with higher horizontal than vertical hydraulic conductivity, indicate that this anisotropy is the primary hydrogeologic control on the natural flowpath lengths. Despite extremely low hydraulic gradients due to minimal topographic relief, anisotropy implies large-scale (tens to hundreds of kilometers) flow at depth. Other hydrogeologic factors, including lateral and vertical changes in hydraulic conductivity, have minor effects on overall flow patterns. However, because natural hydraulic gradients are low, the impact of pumping on groundwater flow is overwhelming; modeling indicates that pumping has substantially changed the shallow groundwater budget and flowpaths from predevelopment conditions.     

Temporal trends in concentrations of DBCP and nitrate in groundwater in the eastern San Joaquin Valley,California, USA

Temporal monitoring of the pesticide 1,2-dibromo-3-chloropropane (DBCP) and nitrate and indicators of mean groundwater age were used to evaluate the transport and fate of agricultural chemicals in groundwater and to predict the long-term effects in the regional aquifer system in the eastern San Joaquin Valley, California. Twenty monitoring wells were installed on a transect along an approximate groundwater flow path. Concentrations of DBCP and nitrate in the wells were compared to concentrations in regional areal monitoring networks. DBCP persists at concentrations above the US Environmental Protection Agency’s maximum contaminant level (MCL) at depths of nearly 40 m below the water table, more than 25 years after it was banned. Nitrate concentrations above the MCL reached depths of more than 20 m below the water table. Because of the intensive pumping and irrigation recharge, vertical flow paths are dominant. High concentrations (above MCLs) in the shallow part of the regional aquifer system will likely move deeper in the system, affecting both domestic and public-supply wells. The large fraction of old water (unaffected by agricultural chemicals) in deep monitoring wells suggests that it could take decades for concentrations to reach MCLs in deep, long-screened public-supply wells, however.     

Groundwater residence time in a dissected and weathered sandstone plateau: Kulnura–Mangrove Mountain aquifer,NSW, Australia

Groundwater residence time in the Kulnura–Mangrove Mountain aquifer was assessed during a multi-year sampling programme using general hydrogeochemistry and isotopic tracers (H₂O stable isotopes, δ¹³C_DIC, ³H, ¹⁴C and ⁸⁷Sr/⁸⁶Sr). The study included whole-rock analysis from samples recovered during well construction at four sites to better characterise water–rock interactions. Based on hydrogeochemistry, isotopic tracers and mineral phase distribution from whole-rock XRD analysis, two main groundwater zones were differentiated (shallow and deep). The shallow zone contains oxidising Na–Cl-type waters, low pH, low SC and containing ³H and ¹⁴C activities consistent with modern groundwater and bomb pulse signatures (up to 116.9 pMC). In this shallow zone, the original Hawkesbury Sandstone has been deeply weathered, enhancing its storage capacity down to ～50 m below ground surface in most areas and ～90 m in the Peats Ridge area. The deeper groundwater zone was also relatively oxidised with a tendency towards Ca–HCO₃-type waters, although with higher pH and SC, and no ³H and low ¹⁴C activities consistent with corrected residence times ranging from 11.8 to 0.9 ka BP. The original sandstone was found to be less weathered with depth, favouring the dissolution of dispersed carbonates and the transition from a semi-porous groundwater media flow in the shallow zone to fracture flow at depth, with both chemical and physical processes impacting on groundwater mean residence times.

Detailed temporal and spatial sampling of groundwater revealed important inter-annual variations driven by groundwater extraction showing a progressive influx of modern groundwater found at >100 m in the Peats Ridge area. The progressive modernisation has exposed deeper parts of the aquifer to increased NO₃^? concentrations and evaporated irrigation waters. The change in chemistry of the groundwater, particularly the lowering of groundwater pH, has accelerated the dissolution of mineral phases that would generally be inactive within this sandstone aquifer triggering the mobilisation of elements such as aluminium in the aqueous phase.     

Speciation and natural attenuation of arsenic and iron in a tidally influenced shallow aquifer

The mobility of subsurface arsenic is controlled by sorption, precipitation, and dissolution processes that are tied directly to coupled redox reactions with more abundant, but spatially and temporally variable, iron and sulfur species. Adjacent to the site of a former pesticide manufacturing facility near San Francisco Bay (California, USA), soil and groundwater arsenic concentrations are elevated in sediments near the prior source, but decrease to background levels downgradient where shallow groundwater mixes with infiltrating tidal waters at the plume periphery, which has not migrated appreciably in over two decades of monitoring. We used synchrotron X-ray absorption spectroscopy, together with supporting characterizations and sequential chemical extractions, to directly determine the oxidation state of arsenic and iron as a function of depth in sediments from cores recovered from the unsaturated and saturated zones of a shallow aquifer (to 3.5 m below the surface). Arsenic oxidation state and local bonding in sediments, as As-sulfide, As(III)-oxide, or As(V)-oxide, were related to lithologic redox horizons and depth to groundwater. Based on arsenic and iron speciation, three subsurface zones were identified: (i) a shallow reduced zone in which sulfide phases were found in either the arsenic spectra (realgar-like or orpiment-like local structure), the iron spectra (presence of pyrite), or both, with and without As(III) or As(V) coordinated by oxygen; (ii) a middle transitional zone with mixed arsenic oxidation states (As(III)–O and As(V)–O) but no evidence for sulfide phases in either the arsenic or iron spectra; and (iii) a lower oxidized zone in the saturated freshwater aquifer in which sediments contained only oxidized As(V) and Fe(III) in labile (non-detrital) phases. The zone of transition between the presence and absence of sulfide phases corresponded to the approximate seasonal fluctuation in water level associated with shallow groundwater in the sand-dominated, lower oxic zone. Total sediment arsenic concentrations showed a minimum in the transition zone and an increase in the oxic zone, particularly in core samples nearest the former source. Equilibrium and reaction progress modeling of aqueous-sediment reactions in response to decreasing oxidation potential were used to illustrate the dynamics of arsenic uptake and release in the shallow subsurface. Arsenic attenuation was controlled by two mechanisms, precipitation as sulfide phases under sulfate-reducing conditions in the unsaturated zone, and adsorption of oxidized arsenic to iron hydroxide phases under oxidizing conditions in saturated groundwaters. This study demonstrates that both realgar-type and orpiment-type phases can form in sulfate-reducing sediments at ambient temperatures, with realgar predicted as the thermodynamically stable phase in the presence of pyrite and As(III) under more reduced conditions than orpiment. Field and modeling results indicate that the potential for release of arsenite to solution is maximized in the transition between sulfate-reduced and iron-oxidized conditions when concentrations of labile iron are low relative to arsenic, pH-controlled arsenic sorption is the primary attenuation mechanism, and mixed Fe(II,III)-oxide phases do not form and generate new sorption sites.     

Geophysical characteristics of Wadi Hanifah water system, Riyadh, Saudi Arabia

Integrated geophysical techniques including resistivity image, vertical electrical sounding (VES), and seismic refraction have been conducted to investigate the Wadi Hanifah water system. The groundwater in Wadi Hanifah has problems caused by the high volumes of sewage water percolating into the ground. The combination of VES, resistivity image, and seismic refraction has made a valuable contribution to the identification of the interface between the contaminated and fresh water in Wadi Hanifah area. The contaminated groundwater has lower resistivity values than fresh groundwater due to the higher concentration of ions which reduces the resistivity. Resistivity image and sounding in this area clearly identified the nature of the lithological depth and proved useful at identifying water-bearing zones. Fresh groundwater was found in the study area at a depth of 100 m within the fractured limestone. Water-bearing zones occur in two aquifers, shallow contaminated water at 10 m depth in alluvial deposits and the deeper fresh water aquifer at a depth of about 100 m in fractured limestone. The interface between the contaminated water (sanitary water) and fresh water marked out horizontally at 100 m distance from the main channel and vertically at 20 m depth.     

Groundwater mixing in a sand-island freshwater lens: density-dependent flow and stratigraphic controls

This paper focuses on a small back-barrier sand-island on the southeast coast of Queensland. The freshwater lens in the study area exhibits anomalously high short-range salinity gradients at shallow depths, which cannot be explained using a standard seawater intrusion model. The island groundwater system consists of two aquifers: a semiconfined aquifer hosting saline to hypersaline groundwater and an overlying unconfined freshwater aquifer. The deeper aquifer is semiconfined within an incised paleovalley, and groundwater flow is restricted to an east – west direction. Tidal response observations show that the tidal signal propagates far more rapidly and is of much higher magnitude in the semiconfined aquifer than the unconfined aquifer. The tidal wave-pulse amplitude is also subject to greater attenuation in the unconfined aquifer. A conceptual hydrogeological model illustrates how upwelling of hypersaline groundwater, induced by density-dependent flow and tidal pumping, has contaminated the shallow groundwater resource. Salinisation at shallow depths is restricted to an area proximal to the paleovalley aquifer. The spatial distribution of lithological heterogeneity is an initial limiting control on the movement of the upwelling saline plume. The extent of shallow groundwater contamination is also limited by the presence of a baroclinic field, resulting from lateral variations in fluid density. Hydrochemical signatures have been used to support the model hypothesis and link the salinisation of fresh groundwater with the semiconfined aquifer as opposed to the surrounding estuarine surface water. The geometry and thickness of the freshwater lens are further controlled by the presence of the largely impermeable bedrock paleosurface between 9 and 12 m depth. The combination of hypersaline groundwater and hydraulically restrictive lithology at shallow depths has produced excessive thinning of the freshwater lens, demonstrating that the application of a model such as the Dupuit – Ghyben – Herzberg relationship would grossly overestimate the available groundwater resource.     

Shallow groundwater temperature in the Turin area (NW Italy): vertical distribution and anthropogenic effects

This study investigated the thermal regime of shallow groundwater in the Turin area (NW Italy), where the large energy demand has motivated a new interest for renewable sources, such as the use of ground-source heat pumps for domestic heating and cooling. The vertical variability of the groundwater temperature between the ground surface and 10–20 m was detected: deeper temperatures were higher than shallow temperatures in spring, while a decrease with depth occurred in autumn. These variations are connected with the heating and cooling cycles of the ground surface due to the seasonal temperature oscillation. Variations below the seasonal oscillation are likely to be connected with the presence of advective heat transport due to the groundwater flow, according to the hydraulic features of a shallow aquifer. Temperature values mostly ranged between 12 and 14 °C in rural areas, while the values were between 14 and 16 °C below the Turin city. This groundwater warming is attributed to a widespread urban heat island phenomenon linked to warmer land surface temperatures in Turin city. Sparse warm outliers are connected with point heat sources and site-specific conditions of land and subsurface use, which may cause the aquifer temperature to rise. A relatively stable temperature below the seasonal fluctuation zone combined with high productivity and legislated limits for deeper groundwater use represent favourable conditions for a large-scale diffusion of groundwater heat pumps within the shallow aquifer. Moreover, this heat surplus should be regarded as a resource for future geothermal installations.     

Groundwater condition based on geoelectrical technique and Dar Zarrouk parameters between El Alamein and El Dabaa—Egypt

El Alamein-El-Dabaa area lies in the western Mediterranean coastal zone of Egypt with about 50 km long. The aims of the present study are the shallow groundwater aquifer determination and calculate the electric parameters of the overburden to achieve the easiest way for detecting groundwater contamination and considered it during the planning of new development project(s). To attain this target, 44 vertical resistivity soundings using Schlumberger array of the maximum AB/2?=?1000 m in the form of four profiles were carried out. From the interpretation results, six geoelectrical layers have been established in the area, and iso-resistivity, depth to water, and isopach contour maps are presented. Four geoelectrical cross-sections (two geoelectrical cross-sections are parallel to the Mediterranean shoreline and the other two are normal to the Mediterranean shoreline) have been constructed. According to this work, the upper part of the Oolitic Limestone represents the shallow groundwater aquifer in this area and can be distinguished into two zones. The upper zone is brackish, whereas the lower one is saline. The geoelectrical succession reveals that the aquifer is free type. The depth to water ranges between 20 and 63 m; therefore, it is the choice as the best sites for groundwater exploitation. In the area under study, the depth to water and the thickness of the brackish increase towards the south side as well as the depth to the brackish water. The Dar-Zarrouk parameters clarified that there are some parts that may contaminate pathways and other parts are not.     

地下水流动对砷迁移的影响: 大同盆地试验场的观测与模拟

地下水流动特征对水文地球化学特征具有重要控制作用, 研究分析了大同盆地地下水流动特征对高砷水迁移的影响.以山阴县桑干河南岸地下水试验场(SYFS)的监测数据为基础, 建立了河岸带三维非稳定地下水流模型.结果表明, 灌溉在很大程度上影响着地下水位动态变化.灌溉活动减慢了地下水埋深和水平地下水流速, 加速了不同岩性地层之间的垂向水量交换.粉土(L1、L2、L3和L4)、粘土1(L5)和砂1(L6)之间始终存在由上至下的垂向水量交换, 粘土2(L7)、砂2(L8)、粘土3(L9)和砂3(L10)以水平水量交换为主.灌溉水和大气降水从地表向下垂直入渗至含水层的过程中, 推动了地表和包气带沉积物中的砷逐渐向下迁移; 到达含水层后, 水平交换量占主导, 地下水在水平方向上频繁的水量交换促使As在含水层中发生水平迁移.      

Spatial characteristics of water quality,stable isotopes and tritium associated with groundwater flow in the Hutuo River alluvial fan plain of the North China Plain

The groundwater flow system and the flow velocity in the alluvial fan plain of the Hutuo River, China, have been studied, with an emphasis on relating geochemical characteristics and isotopes factors. Seven stretches of one river, six springs and 31 wells, with depths ranging from 0 m (river waters) to 150 m, were surveyed. The groundwater has a vertical two-layer structure with a boundary at about 80–100 m depth, yielding an upper and a lower groundwater layer. The δ¹⁸O and δD values range from ?10.56 to ?7.05‰ and ?81.83 to ?59‰, respectively. The groundwater has been recharged by precipitation, and has not been subjected to significant evaporation during infiltration into the aquifer in the upper layer. Using a tritium model, the groundwater flow in the alluvial fan plain showed horizontal flow velocity to be greater than vertical velocity. Groundwater in the upper layer is characterized by Ca–HCO₃ type. From the spatial distribution characteristics of the stable isotope and chemical composition of the groundwater, agricultural irrigation was considered to have an influence on the aquifer by causing excessive groundwater abstraction and irrigation return.     

Geostatistics as a groundwater exploration planning tool: case of a brackish-saline aquifer

This paper discusses a geostatistical approach to model a groundwater aquifer in 3-D. The study aims at utilizing geostatistics as a tool for characterizing zones of better-water quality in a brackish-saline aquifer. In particular, the geostatistical model was constructed to characterize the aquifer’s salinity, represented by total dissolved solids (TDS), using logs of porosity and resistivity. Quality-checked estimated TDS vertical profiles were employed to construct and model horizontal and vertical semivariograms. Parameters of semivariogram models were used to develop both the kriging plan and the generated model. Results of this modeling process are shown in the form of horizontal salinity distribution maps. The aquifer was sliced into 20 layers, each 20 m thick, to represent its overall thickness. Salinity layers maps reflect vertical stratification of TDS concentrations in the aquifer and show that water quality deteriorates with depth and toward the northern part of the aquifer. Relatively better-quality water (TDS ≤10,000 mg/l) can be found at depths between 100 and 250 m below the aquifer’s top in both eastern and southeastern parts. Water in the same interval to the western and southwestern parts reflects the presence of higher TDS concentration. From a planning point of view, it is more feasible to target the eastern part of the aquifer for pumping and desalination purposes. In addition, the generated model could be utilized as an initial condition for flow simulation.     

Seasonal and spatial distribution of redox zones during lake bank filtration in Berlin,Germany

Redox processes during bank filtration were evaluated in Berlin, where bank filtered water is abstracted for drinking water production. The investigations included the mapping of the infiltration zone, a column study and hydrochemical analyses of the groundwater sampled between lake and production well. The organic carbon content increased and the permeability of the lake sediments decreased with distance from the shoreline. The most important changes with regard to the redox state of the infiltrate occurred within the first metre of flow. Infiltration was mostly anoxic, as oxygen was rapidly consumed within the organic rich sediments. The infiltration zone revealed a vertical redox stratification with hydrochemical conditions becoming more reducing with depth rather than with distance from the lake. The redox zones were found to be very narrow below the lake and wider towards the production wells, suggesting that other than differing flow paths, reaeration after infiltration may also occur and possible mechanisms are presented. Redox conditions were influenced by strong annual temperature variations of the surface water affecting the microbial activity. Aerobic infiltration only took place close to the shore in winter.     

A new model (DRASTIC-LU) for evaluating groundwater vulnerability in parts of central Ganga Plain,India

The study area is a part of central Ganga Plain which lies within the interfluve of Hindon and Yamuna rivers and covers an area of approximately 1,345 km². Hydrogeologically, Quaternary alluvium hosts the major aquifers. A fence diagram reveals the occurrence of a single aquifer to a depth of 126 m below ground level which is intercalated by sub-regional clay beds. The depth to water level ranges from 9.55 to 28.96 m below ground level. The general groundwater flow direction is northwest to southeast. Groundwater is the major source of water supply for agricultural, domestic, and industrial uses. The overuse of groundwater has resulted in the depletion of water and also quality deterioration in certain parts of the area. This has become the basis for the preparation of a groundwater vulnerability map in relation to contamination. The vulnerability of groundwater to contamination was assessed using the modified DRASTIC-LU model. The parameters like depth to water, net recharge, aquifer media, soil media, topography, impact of vadose zone, hydraulic conductivity of the aquifer, and land use pattern were considered for the preparation of a groundwater vulnerability map. The DRASTIC-LU index is computed as the sum of the products of weights and rating assigned to each of the inputs considered. The DRASTIC-LU index ranges from 158 to 190, and is classified into four categories, i.e., <160, 160–170, 170–180, and >180, corresponding to low, medium, high, and very high vulnerability zones, respectively. Using this classification, a groundwater vulnerability potential map was generated which shows that 2 % of the area falls in the low vulnerable zone, 38 % falls in the medium vulnerable zone, and 49 % of the area falls in the high vulnerable zone. About 11 % of the study area falls in the very high vulnerability zone. The groundwater vulnerability map can be used as an effective preliminary tool for the planning, policy, and operational levels of the decision-making process concerning groundwater management and protection.     

Groundwater occurrence and aquifer vulnerability assessment of Magodo Area,Lagos, Southwestern Nigeria

A combination of vertical electrical soundings (VES), 2D electrical resistivity imaging (ERI) surveys and borehole logs were conducted at Magodo, Government Reserve Area (GRA) Phase 1, Isheri, Southwestern Nigeria, with the aim of delineating the different aquifers present and assessing the groundwater safety in the area. The Schlumberger electrode array was adopted for the VES and dipole-dipole array was used for the 2D imaging. The maximum current electrode spread (AB) was 800 m and the 2D traverse range between 280 and 350 m in the east-west direction. The thickness of impermeable layer overlying the confined aquifer was used for the vulnerability ratings of the study area. Five lithological units were delineated: the topsoil, clayey sand, unconsolidated sand which is the first aquifer, a clay stratum and the sand layer that constitutes the confined aquifer horizon. The topsoil thickness varies from 0.6 to 2.6 m, while its resistivity values vary between 55.4 and 510.6 Ω/m. The clayey sand layers have resistivity values ranging from 104.2 to 143.9 Ω/m with thickness varying between 0.6 and 14.7 m. The resistivity values of the upper sandy layer range from 120.7 to 2195.2 Ω/m and thickness varies from 3.3 to 94.0 m. The resistivity of the clay layer varies from 11.3 to 96.1 Ω/m and the thickness ranges from 29.6 to 76.1 m. The resistivity value of the confined aquifer ranges between 223 and 1197.4 Ω/m. The longitudinal conductance (0.0017–0.02 mhos) assessment of the topsoil shows that the topsoil within the study area has poor overburden protective capacity, and the compacted impermeable clay layer shows that the underlying confined aquifer is well protected from contamination and can be utilized as a source of portable groundwater in the study area. This study therefore enabled the delineation of shallow aquifers, the variation of their thicknesses and presented a basis for safety assessment of groundwater potential zones in the study area.     

Groundwater flow dynamics in the complex aquifer system of Gidabo River Basin (Ethiopian Rift): a multi-proxy approach

Hydrochemical and isotope data in conjunction with hydraulic head and spring discharge observations were used to characterize the regional groundwater flow dynamics and the role of the tectonic setting in the Gidabo River Basin, Ethiopian Rift. Both groundwater levels and hydrochemical and isotopic data indicate groundwater flow from the major recharge area in the highland and escarpment into deep rift floor aquifers, suggesting a deep regional flow system can be distinguished from the shallow local aquifers. The δ¹⁸O and δ²H values of deep thermal (≥30 °C) groundwater are depleted relative to the shallow (<60 m below ground level) groundwater in the rift floor. Based on the δ¹⁸O values, the thermal groundwater is found to be recharged in the highland around 2,600 m a.s.l. and on average mixed with a proportion of 30 % shallow groundwater. While most groundwater samples display diluted solutions, δ¹³C data of dissolved inorganic carbon reveal that locally the thermal groundwater near fault zones is loaded with mantle CO₂, which enhances silicate weathering and leads to anomalously high total dissolved solids (2,000–2,320 mg/l) and fluoride concentrations (6–15 mg/l) exceeding the recommended guideline value. The faults are generally found to act as complex conduit leaky barrier systems favoring vertical mixing processes. Normal faults dipping to the west appear to facilitate movement of groundwater into deeper aquifers and towards the rift floor, whereas those dipping to the east tend to act as leaky barriers perpendicular to the fault but enable preferential flow parallel to the fault plane.