Clay Lenses Research Articles

Modelling seawater intrusion in the Burdekin Delta Irrigation Area, North Queensland, Australia

The Burdekin Delta is a major irrigation area situated in the dry tropics of North Queensland. It is unique in that (i) it overlies shallow groundwater systems that serve as a major water supply for the irrigation of sugarcane, and (ii) it is adjacent to the world heritage listed Great Barrier Reef. Water management practices include large recharge pits and surface spreading of water to assist with replenishment of the groundwater. This has been useful in maintaining groundwater levels to help control seawater intrusion. This technique, however, can be costly and ineffective in unconfined aquifer systems, which are subjected to large amounts of groundwater pumping for irrigation. There are more than 1800 production bores currently used for irrigation in the Burdekin Delta and the large volumes of water extracted have at times lowered the regional water tables and made it difficult to control seawater intrusion. In this paper we describe the use of a variable density flow and solute transport model, SUTRA, to define the current and potential extent of seawater intrusion in the Burdekin Delta under various pumping and recharge conditions. A 2D vertical cross-section model, which accounts for groundwater pumping and recharge, was developed for the area. The Burdekin Delta aquifer consists mainly of sand and clay lenses with granitic bedrock. The model domain uses vertical cross-sections along the direction of groundwater flow. The initial conditions used in the model are based on land use prior to agricultural development when the seawater wedge was in its assumed natural state. Results of this study demonstrate the effects of variations in pumping and net recharge rates on the dynamics of seawater intrusion. Simulations have been carried out for a range of recharge, pumping rates and hydraulic conductivity values. Modelling results show that seawater intrusion is far more sensitive to pumping rates and recharge than to aquifer properties such as hydraulic conductivity. Analysis also shows that the effect of tidal fluctuations on groundwater levels is limited to areas very close to the coast. Tidal influences on saltwater intrusion therefore can be neglected when compared with the effects due to groundwater pumping. The impacts of various management options on groundwater quality are also discussed.

Uncertainty Analysis of Mound Monitoring for Recharged Water from Surface Spreading Basins

Uncertainty Analysis of Mound Monitoring for Recharged Water from Surface Spreading BasinsThis study has conceptually reviewed issues related to implementation of a groundwater mound monitoring well (GMMW) for monitoring recharged water from a surface spreading basin with emphasis on uncertain hydrogeological conditions. For this, we selected a recharge site in the City of Mesa, Arizona, that is characterized with near surface clay lenses of low permeability. A geostatistical...Author(s)Heekyung ParkDong-Hoon ChaPeter FoxSourceProceedings of the Water Environment FederationSubjectSession 58: Reclaimed Water: Groundwater and Surface Water IssuesDocument typeConference PaperPublisherWater Environment FederationPrint publication date Jan, 2007ISSN1938-6478SICI1938-6478(20070101)2007:14L.4382;1-DOI10.2175/193864707787974067Volume / Issue2007 / 14Content sourceWEFTECFirst / last page(s)4382 - 4401Copyright2007Word count160

Hydrogeology and hydrochemistry of the Parmelia aquifer, northern Perth Basin, Western Australia

This study aims to characterise the hydrogeology and hydrochemistry of the Parmelia aquifer and to understand controls on recent water-level changes as these are needed to underpin a quantitative analysis of recharge. The Parmelia aquifer, a layered sequence of sand, silt and discontinuous lenses of clay, receives diffuse rainfall recharge on its outcrop and groundwater recharge occurs across the Dandaragan Plateau at different rates. Water levels have risen steadily over the last three decades between 10 and 55 cm/y in response to the replacement of native vegetation with pasture and annual crops. The mean aquifer properties from sediment analyses indicate a very wide range of porosity (8.9 – 49.5 %) with an arithmetic mean of 26% and consequently a very broad range of specific yield (0.0004 – 0.4) with an arithmetic mean of 0.14. Groundwater in the Parmelia aquifer has an underlying meteoric origin with compositional changes due to reactions with silicate minerals and leaching of chloride that has concentrated in the soil by evapotranspiration. The hydrochemistry sampled at different depths and locations in the aquifer indicates that the groundwater is not well mixed, and variations arise due to relatively recent recharge that has undergone evaporation in some areas.

Impact of change in vegetation cover on groundwater recharge to a phreatic aquifer in Western Australia: assessment of several recharge estimation techniques

Groundwater levels in a deep phreatic aquifer in the northern Perth Basin, Western Australia have risen between 10 and 55 cm/y over the last three decades due to the replacement of deep-rooted native vegetation with pasture and annual crops. This has sparked debate over increasing groundwater allocation limits to meet growing demand. Estimates of recent and historical rates of groundwater recharge are needed to manage the resource effectively. The Parmelia Formation, composed predominantly of sandstone, supplies domestic, stock and production bores for rural towns. Groundwater recharge was estimated using several recharge estimation techniques including the water-table fluctuation method and tracer techniques; groundwater chloride, chlorofluorocarbons (CFCs) and C-14. Vertical fluxes of soilwater under pasture and native vegetation were determined using chloride as a tracer. Apparent groundwater ages determined using CFCs and carbon-14 are generally younger than 40 years in piezometers that are screened within a few metres of the water-table from recharge under pasture, while groundwater is older than several thousand years in bores that are screened well below the water-table, and mixed waters occur at discharge areas. Recharge-rate estimates obtained under native vegetation are mainly <12 mm/y, whereas after replacement with pasture and annual crops, recharge rates are generally 20 – 50 mm/y. The presence of thick clay lenses in some areas may be responsible for observed delays of approximately one decade in the water-table response to land-use change. The study highlighted the fact that several recharge estimation techniques are required for better understanding of recharge processes and evaluation of recharge.

Uncertainty Analysis of Mound Monitoring for Recharged Water from Surface Spreading Basins

This study has conceptually reviewed issues related to implementation of a groundwater mound monitoring well (GMMW) for monitoring recharged water from a surface spreading basin with emphasis on uncertain hydrogeological conditions. For this, we selected a recharge site in the City of Mesa, Ariz., that is characterized with near-surface clay lenses of low permeability. A geostatistical simulation technique was used for generating hydrogeological fields under the recharge basin, using soil boring logs and historical hydrological data. More than 50 hydrogeological fields were generated and used for modeling. Five scenarios were formulated with varying parameter values and different initial and boundary conditions, and each scenario was evaluated with the 50 hydrogeological fields generated. Results of this study indicate that travel times to the mound may vary by over one order of magnitude and the use of a GMMW will only be practical for regulatory compliance in a homogeneous system.

Mathematical Modeling of Chemotactic Bacterial Transport through a Two-Dimensional Heterogeneous Porous Medium

ABSTRACT The impact of bacterial chemotaxis on in situ ground-water bioremediation remains an unanswered question. Although bacteria respond to chemical gradients in aqueous environments and under no-flow conditions, it is unclear whether they can also respond in porous media with advective flow to improve overall contaminant degradation. The effect of chemotaxis is most profound in regions with sharp chemical gradients, most notably around residual nonaqueous phase liquid (NAPL) ganglia and surrounding clay lenses or aquitards with trapped contamination. The purpose of this study is to simulate bacterial transport through a two-dimensional subsurface environment, containing one region of low permeability with trapped contaminant surrounded above and below by two regions of higher permeability. Using mathematical predictions of the effect of pore size on measured bacterial transport parameters, the authors observe a 50% decrease in both motility and chemotaxis in the finer-grained, low-permeability porous medium. The authors simulate how chemotaxis affects bacterial migration to the contaminated region under various flow and initial conditions. Results indicate that bacteria traveling through a high-permeability region with advective flow can successfully migrate toward and accumulate around a contaminant diffusing from a lower permeability region.

Enhancement of Remediation of NAPL Contaminated Fractured Permeable Formations—Modeling Study

This study has been originated and motivated by a series of discussions, concerning the containment and use of polluted groundwater of a comparatively wide part of the Coastal Plain Aquifer CPA in Israel that has been polluted by kerosene light nonaqueous phase liquid LNAPL. A variety of types of information have indicated that hydraulic barriers should be employed. However, such an operation should be subject to optimize the aquifer remediation, which is also obtained due to the hydraulic barrier operation. The particular part of the CPA is comprised of fractured permeable namely sandstone interbedded with sandy clay lenses. Therefore, in this study a simplified conceptual model is applied to represent the formation and implement the pump-and-treat remediation procedure, whose major objective is cost effective containment of the polluted area. Three physical measures, aimed at the remediation process enhancement, have been analyzed: 1 changing the pumping-injection discharge, 2 use of surfactant additives or other types of solubilizing agents, and 3 use of controlled means to increase the aperture size and density of fracture segments. Possibly, an appropriate combination of such means is most feasible and should be determined. However, the present study evaluates the separate possible effects of each one of such measures on major parameters of the remediation process time and volume of water that should be treated. It is shown that a particular set of parameters can be applied to evaluate the optimal design and adequate combination of such physical measures aimed at remediation enhancement.

Electrical Methods for Detection and Discrimination of Saline Groundwater in Clay-Rich Sediments in Northern Victoria

Conductivity surveys (Induced Polarization and Transient Electromagnetics) have been conducted on the southern bank of the Goulburn River, near Murchison, Victoria, to discriminate between clay zones and saline discharge streams. The combined surveys provide a consistent dataset and multiple conductive zones are delineated within the regolith. The ambiguity between salinity and clay content variations is resolved by means of the induced polarization parameters. In particular, normalised chargeability values are shown to highlight the location of clay lenses containing saline pore water.

Equivalent images derived from very-low-frequency (VLF) profile data

We describe the implementation of a new fast imaging technique for filtering very-low-frequency (VLF) data measured on profiles into corresponding equivalent current systems in the earth. Single-frequency VLF data using magnetic measurements alone are often used to delineate lateral changes in electrical conductivity, e.g., fracture zones in crystalline terrains or changes in lithology in the sedimentary cover. Here, an attempt is made to add depth information to the conductivity distribution by realizing that the single-frequency VLF profile data contain information about (1) the background medium through their decay away from the conductors, (2) the position, and (3) the depth of the dominating conductors through the relative contribution of in-phase and quadrature components to the VLF anomaly in addition to the rate of change of the anomaly close to the conductors. Synthetic data from a model containing a shallow and a deeper conductor are filtered to show that the estimated current distributions coincide well with the horizontal position of the conductors, but even they provide some smeared images of the depth distribution of the conductors. A comparison with models obtained from regularized inversion of the same data shows good correspondence. The VLF field data from an area with clay lenses overlying wet sand and crystalline basement are filtered into current distributions that grossly mimic the electrical conductivity distribution of the clay lenses as obtained from radiomagnetotelluric measurements along the same profile.

Comparison between VLF and RMT methods: A combined tool for mapping conductivity changes in the sedimentary cover

We compare the resolution power of single frequency very low frequency (VLF) electromagnetic data (real and imaginary parts of the tipper) and multi-frequency RadioMagnetoTelluric (RMT) data in delineating conductive structures typical for the sedimentary cover over crystalline basement in Scandinavia. Using VLF field data from five parallel profiles reveals that the estimated models have responses that fit the observed data well, and the models show an overall agreement with more detailed models derived from broadband RMT data. It is suggested that VLF data be used as a fast mapping tool to fill in the gaps between profiles along which more detailed RMT measurements are made. A generic model with conductive clay lenses and sandy formations over crystalline basement is used to generate synthetic data for the two cases. Using regularized inversion the corresponding estimated models clearly shows the strength and the weakness of both methods. Being inductive methods, they both have difficulties in clearly resolving the depth transition from conductive to resistive units. Especially the single frequency VLF data can be interpreted with very smooth models at depth. However, both methods resolve very well the lateral boundaries of the clay lenses and the RMT data also constrain the thickness of the clays quite well compared with the VLF models, which are less distinct at depth. Single frequency scalar VLF data emphasize those conductive structures that have dominant strikes in the direction of the transmitter. Multi-frequency VLF (tensor VLF) measurements provide the tipper vector which depends upon the underlying conductivity structure only. Real conductivity structures have significant 3D components which can be delineated easily by tensor VLF measurements. We propose that new VLF instrumentation be developed with this in mind.

Deformation of the northeastern Venezuelan Andes. Relationships with the Caribbean overthrusts

The Merida Andes (Venezuela) formed in the middle Miocene due to oblique convergence between the South American plate and the Maracaibo block [Audemard et al., 2002] (figs. 1A and 1B). The study area corresponds to the so-called "Barbacoas platform" [Renz, 1960], which constitutes the northeastern termination of the belt, NE of Valera (fig. 2). It is located in the northeastern part of Trujillo block [Hervouet et al., 2001], considered as an independent block separated from the main Maracaibo block along the Valera fault. According to Stephan [1982], the N170°E-trending Caribbean compression developed in this area from late Cretaceous to Eocene. It was followed by a N105°E-trending compression older than middle Miocene, and finally by the NW-SE Andean compressional stage that lasted till now in most of the chain. However, east of El Empedrado fault, a NNE-SSW compression presently occurs that is oblique to the classical Andean stage. The tectonic evolution of the Andean stage is not well understood. The Merida Andes are mainly composed of Precambrian and Paleozoic rocks. The northern part of the belt only comprises a complete and continuous Jurassic to Paleogene cover. This lithologic pattern is probably a consequence of the tectonic escape of the Maracaibo block, and more particularly of the smaller Trujillo block. The Merida and Carribean belts being close to each other, the influence of the Andean deformation on the Caribbean allochthonous must be taken into account. In order to make a structural analysis at regional scale, we privileged the use of remote sensing data (Landsat, Spot and Radar images) and aerial photographs. This was complemented by structural data obtained in the field, allowing the study of geometric and chronological relationships between the tectonic structures.

Till genesis and glacier motion inferred from sedimentological evidence associated with the surge-type glacier, Brúarjökull, Iceland

AbstractA study employing macro- and micro-sedimentological techniques was conducted at three sites with recently deglaciated sediments in the proglacial area of Brúarjökull, a surge-type outlet glacier of the Vatnajökull ice cap, Iceland. Tills at these sites were likely deposited and deformed during the 1963/64 surge. At the height of the last surge, these sediments were beneath 90-120 m of ice, and associated basal shear stresses would have been 24-32 kPa. Tills associated with the surge at these sites formed by a combination of subglacial sediment deformation and lodgement and are thus regarded as ‘hybrid tills’. The tills show evidence of both ductile and brittle deformation. Discontinuous clay lenses within the tills, indicating local ice-bed decoupling and sliding, imply that subglacial water pressures were spatially and temporally variable during the surge. The thickness of the subglacial deforming-till layer was 50-90 cm.

Groundwater exploration using combined controlled-source and radiomagnetotelluric techniques

Radiomagnetotelluric (RMT) (14–250 kHz) combined with controlled-source magnetotelluric (CSMT) (1–12 kHz) measurements were applied to the exploration of groundwater located in sandy formations at depths as great as 20 m below thick clay lenses. A combination of approximately 30 radio frequencies and controlled-source frequencies is essential for penetrating the thick clay layers. The electromagnetic transfer functions of impedance tensor and tipper vectors point toward a structure that is largely two-dimensional, although clear three-dimensional effects can be observed where the sandy formation is close to the surface. The determinant of the impedance tensor was chosen for inversion using two-dimensional models. The final two-dimensional model fits the data to within twice the estimated standard errors, which is considered quite satisfactory, given that typical errors are on the level of 1% on the impedance elements. Comparison with bore-hole results and shallow-reflection seismic sections show that the information delivered by the electromagnetic data largely agrees with the former and provides useful information for interpreting the latter by identifying lithological boundaries between the clay and sand and between the sand and crystalline basement.

A Fossil Byblidaceae Seed from Eocene South Australia

A single mummified angiosperm seed is described from a middle Eocene clay lens deposit at the Monier East Yatala Sand Pit, Golden Grove, South Australia. The seed is small (0.7 mm long and 0.45 mm wide), elliptical, black, and shows complex raised reticulate honeycomb sculpturing with deeply excavated cell floors and verrucate sculpturing on the anticlinal ridges. The fossil was compared against extant species of Byblis and the Droseraceae, especially the Drosera indica L. complex, common annual carnivorous plants that grow in seasonally damp environments in northern Australia and that have similarly small sculptured seeds. The combination of deep reticulately honeycombed cells and the verrucate anticlinal walls places the seed close to extant taxa in the Byblis liniflora Salisb. complex. However, in the absence of a larger sample and/or of definitive features to assign the fossil unequivocally to an extant species, as well as nomenclatural restrictions preventing the typification of a fossil by an illust...

Multiphase flow and transport through fractured heterogeneous porous media

The migration of Dense, Non-Aqueous Phase Liquid (DNAPL) and dissolved phase contamination through a fractured heterogeneous porous medium has been investigated through the use of a multiphase compositional model. The sensitivity of the timescales of migration and the distribution of contaminant in the subsurface to the mean permeability, the variance of the permeability, and the degree of fracturing of the domain were examined. It was found that increasing the mean permeability of the domain allowed the DNAPL to penetrate deeper into the subsurface, while decreasing the mean permeability caused the DNAPL to pool at shallower depths. The presence of fractures within the system was found to control the infiltration only in the most fractured domain. Moment analysis of the nonwetting phase showed that large-scale movement had ceased after approximately 9 years (maximum duration of the source-on condition was approximately 4.5 years). This tended to be due to a redistribution of the DNAPL towards a residual configuration, as was evidenced by the gradual trending of average nonwetting phase saturations within the domain to a static value. The dissolved phase plume was found to migrate at essentially the same rate as the nonwetting phase, due to the reduced relative permeability of lenses containing DNAPL, and due to diffusive losses of mass to the matrix of fractured clay and silty-clay lenses. Some exceptions to this were found when the DNAPL could not overcome the displacement pressure of a lens, and could not by-pass the lens due to the lack of available driving force after the source had been shut off.

Using amplitude variation with offset and normalized residual polarization analysis of ground penetrating radar data to differentiate an NAPL release from stratigraphic changes

Amplitude variation with offset analysis of ground penetrating radar data (AVO/GPR) may improve the differentiation of nonaqueous phase liquid (NAPL) from stratigraphic changes. Previous controlled experiments have shown that common offset (CO) GPR methods can detect the presence of NAPL in soil by examining amplitude and travel time (velocity) anomalies. Unfortunately, stratigraphic changes such as the presence of a silt or clay lens or perched water table may produce similar amplitude and velocity anomalies. Therefore, it is difficult to delineate NAPL in a terrain with unknown stratigraphy exclusively using CO data collection methods. Forward models based on the Fresnel equations predict that amplitude responses exist at various incidence angles that will allow for differentiating NAPL from hydrogeologic changes. Models generated as part of this study indicate that analyzing the difference in amplitude responses from linearly polarized electric field vertically oriented ( EV) to the horizontally oriented ( EH) signals at various incidence angles improves target discrimination. A case history is presented demonstrating that collecting common-midpoint (CMP) GPR data using EH and EV polarized signals at anomalous CO amplitude responses and analyzing the data using AVO and normalized residual polarization (NRP) methods may improve the detection and differentiation of NAPL from stratigraphic changes in the subsurface. These results are corroborated using a capacitively coupled resistivity instrument and subsequent intrusive sampling.

Variations in the composition and adsorption behavior of dissolved organic matter at a small, forested watershed

This study investigated the properties and sorption by goethite of bulk (unfractionated) dissolved organic matter (DOM) from surface and shallow groundwaters at McDonalds Branch, a small freshwater fen in the New Jersey Pine Barrens (USA). Water samples were collected in the spring and fall seasons from two surface-water sampling sites, an upstream potential recharge area and a downstream discharge area, as well as from a set of in-stream nested wells in the upstream potential recharge area. Changes in DOM concentration, molecular weight distribution, and molar absorptivity at 280 nm were measured. Surface and shallow (1.6 m below land surface) groundwater samples collected in spring 1997 in the potential recharge zone (actual recharge impeded by an extensive clay lens) were found to be very similar in terms of DOM concentrations and physicochemical properties and is believe to originate from a common source. Samples taken in fall 1997 yielded no surface water because of drought conditions, and the shallow groundwater DOM collected from the recharge well contained significantly less and chemically altered DOM. This change in chemical properties is believed to be caused in part by fractionation resulting from sorption to mineral phases. Batch isotherm experiments show that sorption by goethite of the DOM from both spring surface and shallow groundwaters in the potential recharge area were similar, whereas the fall groundwater possessed a much lower affinity for the sorbent. This study demonstrated that shallow groundwaters collected under different climatic and hydrologic conditions (spring, high flow versus fall, drought conditions) resulted in different physicochemical properties and adsorption affinities.

Functional equivalency of natural and excavated coastal plain ponds

A major obstacle to assessing functional equivalency of restored or created wetlands is the time needed to develop the functions of natural wetlands. We compared hydrologic, water-quality, and vegetation-composition functions of nine natural coastal plain ponds in the New Jersey Pinelands with those of four small, well-established excavated basins that are at least 50 years old. Our study revealed that well-established (> 50 yr old) excavated ponds achieved a moderate degree of functional equivalency with Pinelands wetlands, representing a range of coastal plain pond characteristics. Based on water-depth-fluctuation patterns and the similarity of most hydrologic indices, including high-water pond area, mean water depth, area of exposed substrate (drawdown), and the presence of a clay lens, the excavated ponds seemed to achieve hydrologic equivalency with the natural reference wetlands. However, steeper bank slopes found at most of the excavated ponds affected nearshore water depths and resulted in the absence of plant zonation that characterizes coastal plain ponds. The water-quality function, represented by pH, specific conductance, and total organic carbon, differed between pond types. The pH and specific conductance of the excavated ponds were higher and total organic carbon concentrations were lower compared with the natural ponds. We attributed these differences to landscape setting, reflected by adjacent vegetation and contrasting plant zonation. Elevated specific conductance values in the natural ponds were likely due to the higher hydrogen ion concentrations. Reduced light transmission due to higher organic carbon concentrations in the natural ponds may have greater ecological importance. However, differences in water-quality functions between the pond types may make excavated ponds more prone to changes in pH if constructed within landscapes with extensive developed or agricultural lands. The excavated ponds met or exceeded most vegetation-composition reference criteria associated with the natural wetlands. Total and herbaceous species richness were greater in the excavated ponds. Most importantly, the excavated ponds supported a native Pinelands species composition, thus preserving regional biodiversity. Because all ponds were acidic and displayed pH values within the range associated with native Pinelands plants, differences in pH may not have been the cause of the greater species richness. Although overall species composition differed between the two pond types, the flora of the created wetlands was similar to that of coastal plain ponds found in other regions and other areas of the Pinelands. The major difference in vegetation composition between ponds was both the lack of distinct vegetation zonation due to steeper slopes and lower patch-type diversity in the excavated ponds. These structural differences can be overcome by constructing ponds with slopes that are comparable to natural ponds. Because the transitional-upland location of the excavated ponds is a more likely location for a mitigation wetland, the effect of landscape setting on water quality may not be as easily remedied as the lack of nearshore slopes.

Conodonts and brachiopods from the Volkhov Stage (Lower Ordovician) microbial mud mound at Putilovo Quarry, north-western Russia

Microbially mediated clay mud mounds are widely developed in the Lower Ordovician succession east of St. Petersburg (Russia) and are associated with a diverse and abundant fauna of brachiopods, ostracodes, echinoderms, bryozoans and conodonts. The lithology of one such mud mound in Putilovo Quarry has previously been studied, but the faunas associated with the mounds have not been investigated to date. Clay lenses in the Putilovo mud mound yield conodont assemblages belonging to the Baltoniodus triangularis and lowermost part of the Paroistodus originalis zones and these stratigraphical intervals are much thicker in the mud mound than in the coeval Lower Ordovician succession lateral to the mound. The compositions of the conodont and brachiopod assemblages are generally the same in the mud mound as in contemporaneous beds. The occurrence of relatively fewer conodont elements in the mud mound than in the surrounding successions probably indicates the higher rate of accumulation of the mud mound clays. Juvenile brachiopods are more numerous in the clays of the mud mound than outside the build-up, supporting the hypothesis that the mounds included ecologically stressed environments.

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.