Single Aquifer System Research Articles

Influence of hydrostratigraphy on the distribution of groundwater arsenic in the transboundary Ganges River delta aquifer system, India and Bangladesh

Abstract The Ganges River delta complex contains a transboundary aquifer system shared between India and Bangladesh. Although it serves as the main freshwater source for the population inhabiting the delta, the aquifer system is severely contaminated with arsenic (As). This study aimed to determine the control of the delta hydrostratigraphy on the regional-scale depth distribution of As within the aquifer system. We developed the first high-resolution, regional-scale, transboundary hydrostratigraphic model of the Ganges River delta and analyzed the patterns of As distribution as a function of the hydrostratigraphy. Model results indicate that, despite the presence of a single aquifer system across the delta, the hydrostratigraphy is spatially variable and can be architecturally divided into three distinct aquifer subsystems from northwest to southeast: a single, thick continuous aquifer (type I); a vertically segregated, semi-confined aquifer subsystem (type II); and a multilayered, nearly confined aquifer subsystem (type III). Results indicate that the spatial distribution of As is characteristically different in each subsystem. In the type I aquifer, As concentrations tend to be homogeneous at all depths, while in type II and type III aquifers, As concentrations sharply decrease with depth. The intervening aquitards in the type II and type III aquifer subsystems appear to act as natural barriers to infiltration of surficial As or organic matter–rich water to the deeper aquifer zones. This delineation of the regional-scale hydrostratigraphic architecture and resulting understanding of its plausible controls on the depth-distribution of As within the delta should significantly aid in the systematic framing of sustainable management plans for the As-safe aquifers within the Ganges River delta aquifer system.

Contributions for the Understanding of the São Pedro do Sul (North of Portugal) Geohydraulic and Thermomineral System: Hydrochemistry and Stable Isotopes Studies

São Pedro do Sul thermomineral aquifer system is located in the North of Portugal. Hydrogeochemistry and isotopic studies were conducted in order to improve knowledge of this groundwater system, known since ancient Roman times for their therapeutic properties. One thermomineral spring (NT) and three boreholes (AC1, SDV1 and SDV2) have a mean temperature around 68 °C. Currently, these waters are mainly used in thermal spas and for geothermal energy supply. Major cations and anions were used to identify and characterize different water types and sub-systems. Stable isotopes composition, δ18O and δ2H, have been used to determine the origin and have been used as a first approach to estimate the mean preferential recharge altitude of thermomineral water. The results suggest a single aquifer system with a relative composition of cations and anions and similar mineralization values. δ18O and δ2H values indicate a meteoric origin with no significant evaporation before infiltration. Besides, the isotopic composition points to recharge areas located at altitudes of about 1000 m a.s.l. This suggests a recharge area in the Freita/Arada mountains located NW of the thermomineral poles.

Stochastic analysis of the groundwater velocity field and implications for contaminant transport within the Ga East and Adentan municipalities, Ghana.

The hydrogeological systems of the Ga East and Adentan Districts of the Greater Accra Region, Ghana were studied using a numerical model. Historical hydrogeological and groundwater monitoring data on twenty boreholes were used in conceptualising the hydrogeological system. The objective was to estimate the aquifer hydraulic conductivity and recharge, and also simulate the velocity field to assess the impact of contaminant transport within the study area. A steady-state model was developed depicting the groundwater flow conditions of the terrain. A single aquifer system (quartzite-schist formation) has been identified. The aquifer hydraulic conductivity estimates indicate that over 90% of the terrain is lower than 15.0 m day−1. The observed outliers are attributable to the fractured and jointed quartzites. The effective aquifer recharge through precipitation for this study ranges between 2.70×10−5m day−1 and 8.10×10−5 m day−1, representing 1.2%–3.6% of the average rainfall records per year in the area. Cases of local and intermediate flow systems, and potential recharge areas were identified. The estimated velocity field ranges from 0.002 to 11.2 m day−1, with over 90% of the area having velocities below 0.85m day−1. The spatial distribution of the velocity field ties in well with the hydraulic conductivity field observed for the terrain. The estimated velocity field makes contamination transport significant in the domain. For instance, contaminants leached into the aquifer zone through recharge by rainfall from the landfill site at Pantang will travel at approximately 0.85 m day−1 towards Oyarifa, Teiman and Ayimensa communities along an identified flowpath. Seven principal groundwater flowpaths have been identified using the particle tracking technique. The travel times along the flowpaths from recharge to discharge locations ranges from 7 years to 833 years. Stochastic simulations carried out on the calibrated model indicate that the model is unique with respect to the aquifer recharge, hydraulic conductivity and hydraulic head.

Darcy’s model with optimized permeability distribution for the simulation of Stokes flow and contaminant transport in karst aquifers

Simulation of fluid flow in karst aquifers is challenging because of the presence of porous and free-flow regions within a single aquifer system. One simple but less accurate approach to model such a system is to use Darcy’s model. This model has much lower computational overhead than many other more rigorous approaches. The results obtained from the Darcy model are accurate for the porous regions of the aquifer, but it produces inaccurate results inside the caves. An approach is proposed called the Darcy model with optimized permeability distribution (DMOPD) for modeling accurate pressure and velocity distributions within the aquifer while retaining almost the same computational cost as the Darcy model. This method comprises three main steps. First, at the first time-step, the pressure and velocity distribution of the entire aquifer is solved using the Brinkman model. Then each cave is divided into an odd number of zones, with the middle zone assigned a maximum permeability value. Subsequently, the permeability ratios of the other surrounding zones are estimated using a global optimization technique. The permeability ratio is the ratio of permeability in that zone to the maximum permeability within the cave. Finally, the Darcy model is run for the remaining time steps using the optimized values of permeability obtained in the second step. Example applications presented show that this method is able to approximate the Brinkman model very well and much faster when compared to the Brinkman model.

Regional-scale groundwater investigations for the Crossrail project

The Crossrail project includes a new underground railway through the heart of London. It will connect with 110 km of new or upgraded sections of surface rail to Reading, Shenfield and Abbey Wood. Ground investigations for the central tunnelled sections began during the 1990s and continued until 2011 according to a strategy that was based on a review of other major tunnelling projects and best practice. The level of detail in the investigations and subsequent groundwater monitoring varied according to local geological complexity and design requirements. A total of 1019 boreholes were drilled to a maximum depth of 114 m and contained a total of 1314 piezometers. Of these, 233 piezometers were installed during the 1990s and 1081 piezometers and several other monitoring types were installed in the following phase during the 2000s. The ground and groundwater investigation strategy included the following: (1) groundwater monitoring to accurately determine the multiple aquifer regional groundwater regime with varying underdrainage, in the west; (2) tidal monitoring in the east, where an unconfined single aquifer system predominated; (3) investigation of the impact of local features (e.g. fault zones and scour features); (4) determination of long-term groundwater trends. Crossrail data were supplemented with third-party data to provide more information on the far field and provide more definition on larger scale geological structures (e.g. faulting at Farringdon and Limmo). Installation trials were conducted on several types of installation, and their performance for monitoring multiple aquifers, in particular during permeability tests, is discussed. For large projects such as Crossrail, long-term performance of groundwater monitoring installations is critical for establishing a baseline in addition to obtaining information for design. Annual reviews of data backed up by remediation works allowed many installations to provide reliable groundwater data for up to a decade prior to the construction phase.

Variations in hydrostratigraphy and groundwater quality between major geomorphic units of the Western Ganges Delta plain, SW Bangladesh

Relationships among geomorphology, hydrostratigraphy, and groundwater quality with special emphasis on arsenic and salinity have been analyzed in the Bangladesh part of the Western Ganges Delta (WGD). On the basis of the presence of characteristic geomorphic features, the study area is divided into two geomorphic units: fluvial deltaic plain (FDP) and fluvio-tidal deltaic plain (FTDP). Lithostratigraphic sections demonstrate that FDP is composed predominately of sandy material whereas FTDP is characterized by alternation of sand and clay/silty clay material. Hydrostratigraphically, FDP is characterized as a single aquifer system, whereas FTDP is a complex multi-aquifer system. Spatial distributions of arsenic concentrations in groundwater reveal that elevated arsenic (>0.01 mg/l) occurs mostly in the FDP. Occurrences of high arsenic in deeper part of the aquifer system (>100 m) in the FDP, particularly in the south-western part, is probably due to the absence of any prominent impermeable layer between the shallow and deeper part of the aquifer system. Distributions of chloride concentrations show an increasing trend in groundwater salinity from north to south, i.e., from FDP to FTDP.

Hydraulic response of an unconfined-fractured two-aquifer system driven by dual tidal or stream fluctuations

Many islands consist of limestone sedimentary deposits that are better described as a two-aquifer system consisting of an unconfined aquifer (Quaternary sediments) above a fractured aquifer (fractured limestone) in which groundwater heads are closely regulated by tidal fluctuations on both sides of the islands (dual tides). Propagation of tidal signal, reflected in hydraulic head fluctuation in such a two-aquifer system is significantly different from that in a single aquifer system that is often assumed. The Laplace domain solution of the head fluctuation in such a two-aquifer system subjected to dual tides is obtained first and subsequently inverted to yield real-time solution. Unlike previous solutions, Fourier series with complex variables are avoided and Fourier sine transform is used instead. The solution takes into account an instantaneously drainable water table, hydraulic conductivity anisotropy and arbitrary time-dependent tidal fluctuations. The hydraulic connection of the underlying fractured aquifer and the overlying unconfined aquifer is explored in details. The presented solution can be used to evaluate the head fluctuations and the aquifer parameter estimation of the two-aquifer system underneath a strip-shape island subjected to dual tides. The results can be used to determine the optimum piezometer location to estimate hydraulic parameters of the two-aquifer system using groundwater head fluctuation data. Stream-aquifer interaction is similar to the tidal-aquifer interaction if the chemical difference of the salt water and fresh water is not a concern. The developed solution for tidal-aquifer interaction here can also be used to investigate the aquifer response to stream stage variations in river basin aquifers.

Geophysical mapping of Mount Bonnell fault of Balcones fault zone and its implications on Trinity-Edwards Aquifer interconnection, central Texas, USA

Geophysical surveys (resistivity, natural potential [self-potential], conductivity, magnetic, and ground penetrating radar) were conducted at three locations across the Mount Bonnell fault in the Balcones fault zone of central Texas. The normal fault has hundreds of meters of throw and is the primary boundary between two major aquifers in Texas, the Trinity and Edwards aquifers. In the near surface, the fault juxtaposes the Upper Glen Rose Formation on the Edwards Plateau, consisting of interbedded limestone and marly limestone, against the Edwards Group, which is mostly limestone, on the eastern down-thrown side (coastal plain). The Upper Glen Rose member is considered to be the Upper Trinity Aquifer and also a confining zone underlying the Edwards Aquifer. However, recent studies have documented a hydraulic connection between the Edwards and Upper Trinity aquifers. The uppermost portions of the Upper Trinity and the Edwards aquifers, in some places, operate as a single aquifer system, while the lowermost units of the Upper Glen Rose are confining layers between the Edwards Aquifer and the Middle Trinity Aquifer. Geophysical data, which include resistivity, natural potential, magnetic, conductivity, and ground penetrating radar, indicate not only the location of the fault but additional karstic features on the west side (Upper Glen Rose Fm.) and on the east side (Edwards Group) of the Mount Bonnell fault. Resistivity values of the Glen Rose and Edwards Group do not appear to have significant lateral variations across the fault. In other words, the Mount Bonnell fault does not appear to juxtapose different resistivity units of the Edwards Group and Upper Glen Rose member. Thus, the fault may not be a barrier for groundwater flow. With the abundant karstic features (caves, sinkholes, fractures, collapsed areas) on both sides of the fault, determined by the geophysical data, one can conclude that lateral groundwater flow (intra-aquifer) between the Edwards Aquifer and Upper Trinity is likely.

Delineation of Holocene–Pleistocene aquifer system in parts of Middle Ganga Plain, Bihar, Eastern India through DC resistivity survey

The study area forms a part of the Middle Ganga Plain (MGP) and experiences intensive groundwater draft due to domestic, irrigation and industrial purposes. Geoelectrical surveys were carried out in a geomorphic unit of MGP called South Ganga Plain, along the north–south traverse covering a total 50 km stretch. Interpreted results of the total of 17 vertical electrical soundings, carried out, provided information on aquifer and aquitard geometry and sediment nature in different aquifer systems. Bedrock topography is also demarcated along the north–south transect. The estimated dip of massive bedrock is less than 0.5° and dips toward north. The survey results show that a two-tier aquifer system exists in Newer alluvium parts of the study area and it is replaced by a single aquifer system at Older alluvium that occurs under thick clay/sandy clay bed in the southern part. An exponential decay of the aquifer potential is observed from north to south. Paleo channel Sone River is traced and it forms a potential aquifer.

Arsenic-safe alternate aquifers and their hydraulic characteristics in contaminated areas of Middle Ganga Plain, Eastern India

Arsenic groundwater contamination exceeding 0.05 mg/l affecting the Newer Alluvial tracts of Patna and Bhojpur, the two worst affected districts located in the Middle Ganga Plain in the Bihar state, has been studied The area is underlain by two-tier Quaternary aquifer systems within a depth of 300 m below ground level, separated by a 15-32-m-thick clay/sandy clay aquitard. The upper part (<50 m depth) of the shallow aquifer system is arsenic-contaminated. The deeper aquifer system (lying below 120-130 m depth) exhibits low arsenic load (max 0.0035 mg/l), having hydraulic conductivity between 64.88 and 82.04 m/day. Groundwater in the deeper aquifer occurs under semi-confined to confined condition due to poor hydraulic conductivity of the middle clay (4.7×10(-2)-7.2×10(-3) m/day). Hydraulic head of the deeper aquifer remains close to the surface than the shallow aquifer. The two aquifer systems in the Newer Alluvium are replaced by a thick single aquifer system in the adjoining Older Alluvium, within a depth of 330 m below ground. In the arsenic-contaminated area, deeper aquifer is protected by a middle clay, which may be developed for community drinking water supply by deep tube wells having a yield capacity of 150 m3/h.

Hydrogeologic and geomorphic settings of the Lower Subansiri Basin, Assam, India

Geomorphic settings of an area provide valuable supplementary information regarding groundwater recharge, their occurrence and distribution. The geomorphic settings of the Lower Subansiri Basin can broadly be represented by three distinct geomorphic units viz., structural hills, piedmont zone and alluvial plain. While the elevation, slope, lithology, drainage pattern and various relevant morphometric parameters vary from one geomorphic unit to another, the conditions of recharge and discharge, occurrence and distribution of groundwater also differ in different units. The structural hills occupying only 4.5% of the area along the north-western boundary represent a high run-off zone characterised by steep slope and fairly dense parallel to sub-parallel drainage. The piedmont zone, built up by the coalescence of alluvial fan deposits, represents 7.7% of the area occurring in a long and narrow NE-SW trending steeply sloping belt along the foothills of Arunachal Pradesh. Owing to high permeability, this zone hardly retains any water and hence forms a high recharge zone with relatively deeper groundwater level. The alluvial plain, covering 87.8% of the basin area and characterised by a gentle slope, serves both as recharge and discharge areas where groundwater occurs relatively close to the ground surface.&#x0D; Panel diagrams prepared for the Lower Subansiri Basin showing the thickness and extent of granular zones display that the unconsolidated alluvial sediments are primarily composed of sands of various grade and gravel with minor amounts of silt and clay. The sand-gravel isolith maps showing the cumulative thickness of the granular zones down to the depth of 40 m reveal that the subsurface formations in a major part of the alluvial plain of the Lower Subansiri Basin are entirely represented by granular zones. The granular zone in most parts of the area forms one single aquifer system where groundwater mostly occurs under unconfined to semi unconfined conditions. Due to the presence of thin clay and/or sandy clay lenses at shallow depths, however, the single aquifer system gets locally dissipated into multiple aquifer system where, barring the uppermost aquifer, groundwater mostly occur under semiconfined to confined conditions. The overall regional variation of depth to water level, from the piedmont zone in the north and north-west to the alluvial plain of the south and southeast, is controlled by the prevalent geomorphic settings of the area. The disposition of water table contours of the area indicates that the configuration of groundwater table closely controls to that of general topography of the area. The steeper hydraulic gradient observed to be present in the north and north-east indicates the possible control of the characteristically distinguished geomorphic setting, i.e., topography, relief and lithology.

Hydrogeological and hydrochemical characteristics of alluvial aquifers in the Brahmaputra Valley, Assam, India

The Brahmaputra Valley comprising a thick pile of unconsolidated alluvial deposits encompasses an area of about 63,450 km2 in the state of Assam, India. It is composed of unclassified Older and Newer alluvial deposits consisting of boulders, cobbles, pebbles, gravels as well as sand of various grades, silt, and clay. The area is occupied predominantly by Recent and Older floodplains represented by udifluvents (younger alluvial soils), and palenstalfs and haplaquents (older alluvial soils). The Piedmont zone fringing the northern foothill region of the valley is represented by usthorthents (Bhabar Soils) and haplaquolls (Terai Soils).&#x0D; Hydrogeologically, the northern and southern banks of the river Brahmaputra differ considerably. The northern bank is generally characterised by a single aquifer system of great thickness and varied composition down to a depth of about 150 m, where mostly an unconfined aquifer is found. Semiconfined aquifers are also seen at places where the sediment s are stratified. Shallow tube wells yield 30 to 60 ml/hr for a drawdown of 1- 2 m. The yield of deep tube wells is found to range from I00 to 250 ml/hr for a drawdown of 4- 10 m. The depth to water table in the piedmont zone varies from 5 to 40 m with an average fluctuation of 3-10 m. The average hydraulic conductivity of the aquifer system is found to range from 30 to 200 m/day.&#x0D; The southern bank is characterised by the presence of unconfined and semiconfined aquifers. Some perfectly confined aquifers as well as some perched aquifers are also encountered locally. Shallow tube wells can yield 25- 35 ml/hr for a drawdown of 2 m while deep tube wells are capable of yielding 100-250 ml/hr for a drawdown of 4-8 m. The water table rests within a depth of 2-6 m from the land surface. The average hydraulic conductivity varies from 30 to 200 m/day. The decadal variation of water table in the alluvial aquifer of the Brahmaputra Valley indicates a rising trend of water level. Dynamic groundwater potential of the shallow aquifer zone is within the range of 15,440 mcm.&#x0D; Hydrochemically, the groundwater of the Brahmaputra Valley is found to be suitable for domestic and irrigation purposes, except for higher concentration of iron. The pH value ranges from 6.8 to8.7, indicating neutral to slightly alkaline nature of groundwater. Low value of electrical conductivity and total dissolved solids implies fresh nature of groundwater. Piper trilinear plot reveals the preponderance of alkaline earths (Ca, Mg) and weak acids (HCO3 + CO3) over alkalis (Na, K) and strong acids (SO4, Cl) in the groundwater of the majority of the study area indicating Ca + Mg- HCO3 type.

Geoelectric investigations in Bakreswar geothermal area, West Bengal, India

A cluster of hot springs in Bakreswar geothermal area, which belongs to the Chotanagpur Gneissic Complex in the eastern part of Peninsular India, is characterised by varying temperature and similar chemical composition. Vertical electrical sounding (VES) investigations in and around Bakreswar reveal the presence of two to four prominent lithologic layers under prevailing hydrodynamic conditions. The intermediate weathered zone and the fractured rocks constitute a single aquifer system of varied hydraulic conductivity under water table condition. Lithology and groundwater conditions, as inferred from the VES, as well as hydrological studies, are in agreement with the nearby bore hole lithologs. Water table contours accompanied by VES findings of the region indicate that the occurrence and movement of groundwater take place mostly within the weathered and fractured rocks under unconfined condition. 1D interpretation of VES results reveals few promising groundwater potential zones in the eastern part of the region. Wenner resistivity profiling, coupled with VES and geological studies, indicates the presence of a nearly N–S striking buried fault providing passage for hot water to emerge in the form of springs.

Aquifer System and Salinity Hazards in Parts of Yamuna River Sub-Basin, India

The regional aquifer system, the hydrogeological condition and salinity hazards in the middle part of Yamuna river sub-basin were investigated. Locally there occurs two to three tier aquifer system, but on a regional scale a single aquifer system down to depth of 100 m can be visualized. The sediments forming the aquifers are medium to coarse grained sands and gravels with thin layers of clay beds. In general the groundwater flow is towards east and southeast with some variation. Groundwater troughs have been developed at many places due to excessive groundwater withdrawal. There is wide spatial variation in the salinity of groundwater as evident from wide range of electrical conductivity (E.C.) values (500-6700 µS/cm). Large scale practice of dumping solid industrial wastes in the open field, utilization of canal and river water containing effluents of industries for irrigational purpose may be responsible for some increase in the salinity of groundwater in the study area, where natural processes are mainly responsible for the high salinity of the groundwater.

Comparison of Marine-Bar with Valley-Fill Stratigraphic Traps, Western Nebraska

Marine-bar and valley-fill stratigraphic traps in the Cretaceous J sandstone in Cheyenne and Banner Counties, Nebraska, illustrate control of reservoir shape, size, and characteristics by depositional environment. Reservoirs deposited as shallow-marine bars are elliptical lenses 2-5 mi long, 0.5-1.5 mi wide, and less than 25 ft thick. Sandstone grades laterally into marine mudrock. There are two generations of bars in the area, closely spaced stratigraphically, but with different directions of elongation. These lenses now are tilted with a regional southwest dip. Entrapment is independent of structural closure. Most bar bodies are entirely oil filled. Reservoirs deposited as a valley fill are within a prism of sandstone more than 20 mi long, 2,000 ft wide, and 50-80 ft thick. The boundaries of this body are erosional. Oil is trapped only where the valley-fill trend crosses plunging anticlines. The valley fill interconnects all pools as a single aquifer system. Exploration and production efforts are guided by several considerations. Position of marine-bar reservoirs can be predicted by techniques which map gradients in sandstone-shale proportions, such as those based on mechanical logs. Bars in the study area are scattered and not in chains; orientation is varied. Structure is unimportant. In contrast, valley-fill reservoirs are separated by erosional boundaries from enclosing rocks; hence, they cannot be detected by examination of the enclosing facies. Where present, however, the valley fill has great continuity and persistence of trend. Structure is essential. Valley-fill reservoirs have water drive and high primary recovery, whereas marine-bar reservoirs have only solution-gas energy. Environmental interpretation of these reservoirs is based on fossils, sedimentary structures, textures, facies relations, and geometry. A single core commonly allows correct interpretation. Exploration and production programs are guided profitably by use of environmental concepts at an early stage.

Comparison of Marine-Bar with Valley-Fill Stratigraphic Traps, Western Nebraska: ABSTRACT

Marine-bar and valley-fill stratigraphic traps in the Cretaceous J sandstone in Cheyenne and Banner Counties, Nebraska, illustrate control of reservoir shape, size, and characteristics by depositional environment. Reservoirs deposited as shallow-marine bars are elliptical lenses 2-5 miles long, 0.5-1 mile wide, and less than 25 feet thick. Sandstone grades laterally into marine mudrock. There are two generations of bars in this area, closely spaced stratigraphically, but with different directions of elongation. These lenses presently are tilted with a regional southwest dip. Entrapment is independent of structural closure. Most bar bodies are entirely oil-filled. Reservoirs deposited as a valley-fill occur within a prism of sandstone more than 20 miles long, 2,000 feet wide, and 50-80 feet thick. The boundaries of this body are erosional. Oil is trapped only where the valley-fill trend crosses plunging anticlines. The valley-fill interconnects all pools as a single aquifer system. Exploration and production efforts are guided by the following. Position of marine-bar reservoirs can be predicted by techniques which map gradients in sandstone-shale proportions, such as those based on mechanical logs. Bars in this area are scattered and not in chains; orientation is varied. Structure is unimportant. In contrast, valley-fill reservoirs are separated by erosional boundaries from enclosing rocks; hence, they can not be detected by examination of the enclosing facies. Where located, however, the valley-fill has great continuity and persistence of trend. Structure is vital. Valley-fill reservoirs have water drives and high primary recoveries, whereas marine-bar reservoirs have only solution-gas energy. Environmental interpretation of these reservoirs is based on fossils, sedimentary structures, textures, facies relations, and geometry. A single core commonly allows correct interpretation. Exploration and production programs are guided profitably using environmental concepts at an early stage. End_of_Article - Last_Page 463------------