Complex Aquifer Research Articles

The use of 3D geological models in the development of the conceptual groundwater model

The conceptual groundwater model is heavily dependent on the geological framework which is used to defi ne the aquifer being studied. In the past, two-dimensional datasets such as geological maps and cross-sections were used in coordination with site-specifi c point data to build a conceptual understanding at the site or catchment scale. This is then simplified and it is this simplifi ed version which is used to build the framework for the numerical groundwater fl ow model. Due to the way the geological framework model and the conceptual groundwater model were generated they could not be viewed together; this inevitably led to a signifi cant loss of information and understanding. With the current rapid developments in 3D modelling software and the increasing availability of digital geological data it is now possible to produce detailed 3D geological models of complex aquifer sequences. In this paper we will use two case studies (Chalk aquifer of the London Basin and the Jurassic limestone aquifer of the Cotswolds) to demonstrate that by developing a detailed 3D geological model signifi cant benefi ts are gained in the understanding and development of the conceptual groundwater model.

Trends and Transformation of Nutrients and Pesticides in a Coastal Plain Aquifer System, United States

Four local-scale sites in areas with similar corn (Zea mays L.) and soybean [Glycine max (L.) Merr.] agriculture were studied to determine the effects of different hydrogeologic settings of the Northern Atlantic Coastal Plain (NACP) on the transport of nutrients and pesticides in groundwater. Settings ranged from predominantly well-drained soils overlying thick, sandy surficial aquifers to predominantly poorly drained soils with complex aquifer stratigraphy and high organic matter content. Apparent age of groundwater, dissolved gases, N isotopes, major ions, selected pesticides and degradates, and geochemical environments in groundwater were studied. Agricultural chemicals were the source of most dissolved ions in groundwater. Specific conductance was strongly correlated with reconstructed nitrate (the sum of N in nitrate and N gas) (R(2) = 0.81, p < 0.0001), and is indicative of the relative degree of agricultural effects on groundwater. Trends in nitrate were primarily related to changes in manure and fertilizer use at the well-drained sites where aquifer conditions were consistently oxic. Nitrate was present in young groundwater but completely removed over time through denitrification at the poorly drained sites where there were variations in chemical input and in geochemical environment. Median concentrations of atrazine (6-chloro-N-ethyl-N'-(1-methylethyl)-1,3,5-triazine-2,4-diamine), metolachlor (2-chloro-N-(2-ethyl-6-methylphenyl)-N-(2-methoxy-1-methylethyl)acetamide), and some of their common degradates were higher at well-drained sites than at poorly drained sites, with concentrations of degradates generally higher than those of the parent compounds at all sites. An increase in the percentage of deethylatrazine to total atrazine over time at one well-drained site may be related to changes in manure application.

Comment on “Power law catchment‐scale recessions arising from heterogeneous linear small‐scale dynamics” by C. J. Harman, M. Sivapalan, and P. Kumar

Comment on “Power law catchment‐scale recessions arising from heterogeneous linear small‐scale dynamics” by C. J. Harman, M. Sivapalan, and P. Kumar

Conceptualization and classification of groundwater–surface water hydrodynamic interactions in karst watersheds: Case of the karst watershed of the Coulazou River (Southern France)

Over the last two decades, groundwater systems and surface water bodies are being more and more considered as a same entity which constitutes a single resource within the hydrologic system; understanding groundwater surface water (GW–SW) interactions is necessary to effectively manage the water resources. Concepts, methods of analysis and classifications of these hydrodynamic interactions have thus been developed, mostly in case of porous aquifer. Today, GW–SW interactions are still often poorly understood in many watersheds, particularly in the case of complex aquifers like karst aquifers. This study focuses on the influence of groundwater on the genesis and propagation of surface floods in the case of a highly karstified watershed. GW–SW hydrodynamic interactions are analysed (i) at a local scale focussing on hydrodynamic interactions between the karst aquifer in the vicinity of the river and the river, and (ii) at a larger scale focussing on hydrodynamic interactions between a karst spring and sinkholes in the riverbed. Hydrodynamic time series are described using the karst watershed of the Coulazou River (western Montpellier, France) as a case study. Results are used to adapt GW–SW classifications that are usually devoted to porous media. In case of the studied karst aquifer, it is shown that the initial groundwater level may be used to forecast the type of hydraulic connection between the river and the saturated zone during flood. However, initial water level in the karst aquifer does not give any information about the flow direction (gaining or losing stream) during a flood. The latter is found to be also controlled by the limited discharge capacity of karst conduits. A conceptual model of karst/river interactions during flood is given, with concepts that are derived from the analysis of the Coulazou watershed but that can be easily applied to most of the karst watersheds.

Limiting hydrochemical factors for sustainability of water resources: The Cisjordanian experience

In Cisjordan, surface- and groundwater flow are either towards the Jordan Valley–Dead Sea–Arava Valley (the Rift) or the Mediterranean Sea. Due to upstream exploitation by riparians to the Jordan River, the historical annual flow, which fluctuated between 250 and 1100 Mm 3, has declined to a mere 100–200 Mm 3. The remaining flow south of Lake Kinneret is highly polluted and heavily loaded with salts. Lake Kinneret (Sea of Galilee) is one of the major water resources in the area. Annually, between 200 and 700 Mm 3 reach the Lake as surface and groundwater flow. The relatively high salinity of the Lake is caused by thermomineral water discharging from springs and seepages located onshore and on the bottom of the Lake. The main factors causing deterioration of the groundwater quality in the Rift are of geogenic character. These are different types of brines, whose outflow and penetration into freshwater aquifers was triggered by overpumpage. Contemporary encroachment of seawater caused by intensive water exploitation in the Coastal Plain is manageable and reversible. However, due to lack of hydrogeological evidence, no such statement can be made about the circulation of seawater beneath the Coastal Plain and into the deep-seated Yarkon–Taninim aquifer or the upflow of brines in the Rift. The flow regimes of the different brine bodies could not be elucidated. Whether each such brine-body flows by its particular hydrological regime or whether the movement of the different bodies is intradependent or interdependent with the regional movement of fresh groundwater, remain open questions. Therefore, sustainable development of groundwater resources is clearly dependent on the elucidation of the relationship between changes in the pressure of the brine with depth and its relationship to the overlying freshwater. The average total annual recharge of all water sources in Cisjordan is 1820 Mm 3, which means that the total production of water must be managed within the limits of this annual volume. During drought years, total groundwater extraction exceeds the safe yield, causing drastic lowering of water levels and upflow of saline waters from greater depths. Because of the structural complexity of aquifers and hydrochemical variability of the numerous groundwater bodies, new hydrochemical methods have been developed for the identification of groundwater bodies and for the elucidation of their origins. These methods combine macrochemical, microchemical, and isotopic evidences. By combining distribution patterns of rare earths, yttrium and stable isotopes, a complete picture of catchment lithology and the altitude and latitude of precipitation could be obtained. The area west of the Jordan River is characterized by the occurrence of transboundary surface- and groundwater basins in which fresh and saline water and brines flow across political borders between Israel and the Palestinian Authority. It is very difficult to assess separately the annual safe yield of water resources for each of the two national entities. Neither country may dispose independently of its waters and is usually at the mercy of the other riparian. There is as yet no general multilateral international treaty in force allocating the water resources of international watercourses. At present there are two rules for the management of the waters of an international drainage basin—the rule of Equitable Distribution, and the obligation Not to Cause Significant Harm. The rules of equitable distribution have tended to focus on the issue of quantities of water rather than on quality of water, which is really relevant to the issue of equitable distribution. Future negotiations on the uses of the basin will need to deal with issues of characteristic salinities and geochemical features and on their impact on equitable sharing of water resources.

INTEGRAÇÃO DE MÉTODOS GEOFÍSICOS NUM ESTUDO HIDROGEOLÓGICO NA PLANÍCIE COSTEIRA NORDESTE DO ESTADO DO RIO DE JANEIRO - BRASIL

A geophysical pilot study has been undertaken in the Northern Coastal Plain of Rio de Janeiro State (Brazil), near Rio das Ostras Town, 200 km NE from Rio de Janeiro City. This region is characterized by both scarcity of drinking water and the existence of saline intrusions, which is caused mainly by the presence of a complex Quaternary geology and excessive aquifer exploitation. The research was performed by employing magnetic and electromagnetic methods (time - TEM and frequency - FEM domains), in order to evaluate the effectiveness of these methods under these particular geological conditions, and thus to allow the formulation of a more comprehensive hydrogeological research in the region in future. FEM profiles and one-dimensional (1D) interpretation of TEM data were capable of depicting different geological formations, to locate the main aquifer and to reveal the fresh-saline water contact. On the other hand, the magnetic method showed the presence of a NE-SW direction geological fault, which is located just in the geological contact between sand and clay, coincidentally with the fresh-saline water contact. Our results evince a promising future for using electromagnetic geophysical methods in this region in small scale surveys, as well as it confirmed that a precursory regional survey, chiefly using the magnetic method, is essential in the indication of most suitable places to locate productive fresh-water wells.

Evaluation of groundwater resources potential in the Ejisu-Juaben district of Ghana.

The increasing stress on freshwater resources due to ever-rising demands and profligate uses as well as growing population of Ejisu-Juaben District is an issue of great concern. The purpose of this study is to make a quantitative estimate of the available groundwater resources in the Ejisu-Juaben district for efficient utilization and management of groundwater resources. The methodology involved the collection and analysis of existing well data and chloride mass balance. The results indicate that the aquifers are composite and composed of weathered regolith of low permeability and high storage and overlying fissured bedrock of high permeability and low storage. Semi unconfined aquifers prevail in major portions, which constitute the principal source of groundwater. The depths of boreholes in the District range from 17 - 75 m with an average of 51 m. Generally Ejisu-Juaben district has low groundwater potential with mean yield, transmissivity and specific yield values of 2.8 m3/h, 12.5 m2/d and 0.95 m3/h/m respectively. Recharge in the district is estimated as 7 - 9% of the average annual rainfall of 1874 mm. The permanent water reserve of 326,064,375 m3 and recoverable water reserve of 130,425,750 m3 for the aquifer of the basement complex aquifer were estimated from the records of 97 boreholes.

Thermo-mineral waters from the Cerna Valley Basin (Romania)

In the south-west of the Southern Carpathians, upstream from the confluence of Cerna with Belareca, an aquifer complex has developed, strongly influenced by hydrogeothermal phenomena, acting within two major geological structures, the Cerna Syncline and the Cerna Graben. The complex consists mainly in Jurassic and Cretaceous carbonate rocks, as well as in the upper part of the Cerna Granite, highly fractured, tectonically sunken into the graben. As a result of the tectonic processes which occurred after the end of the Jurassic-Cretaceous sedimentation cycle, limestones may be encountered at 1100 m altitude in the Mehedinţi Mountains, at 150 ¬¬600 m in the Cerna Syncline or sunken into the Cerna Graben down to depths of 1075 m. In certain sections along Cerna, graben limestones outcrop in an intricate relationship with those of the slopes, facilitating the existence of very large scale uni- or bidirectional hydrodynamic links. The geothermal investigations have shown the existence of some areas with values of the geothermal gradient falling into the 110-200°C/km interval, and temperatures of 13.8-16°C at the depth of 30 m (VELICIU, 1978). The zone with the maximal flux intensity is situated between the Băile Herculane railway station and the Crucea Ghizelei Well, an area where 24 sources (10 wells and 14 springs) are known. The geothermal anomaly is also extended to the south (Topleţ), north (Mehadia) and north-east (Piatra Puşcată), a fact which is stressed by the existence of hypothermal springs with low mineralization. The physical-chemical parameters of the sources show a strong, north-south, variability. At the entire thermo-mineral reservoir scale, the temperature of the water sources, the total mineralization and the H2S quantity are increasing from the north to the south, and the pH and natural radioactivity are diminishing with the same trend.

Rain‐induced subsurface airflow and Lisse effect

Water‐level increase after rainfall is usually indicative of rainfall recharge to groundwater. This, however, may not be true if the Lisse effect occurs. This effect represents the water‐level increase in a well driven by airflow induced by an advancing wetting front during highly intensive rains. The rainwater, which may behave like a low‐permeability lid, seals the ground surface so that the air pressure beneath the wetting front is increased because of air compression due to downward movement of the wetting front. A rapid and substantial rise of the water level in the well screened below water table, which bears no relationship to groundwater recharge, can be induced when various factors such as soil properties and the rain‐runoff condition combine favorably. A transient, three‐dimensional and variably saturated flow model was employed to study the air and groundwater flows in the soil under rain conditions. The objectives of this paper are two‐fold: to evaluate the reliability of the theory of the Lisse effect presented by Weeks to predict its magnitude in modeled situations that mimic the physical complexity of real aquifers, and to conduct parametric studies on the sensitivity of the water‐level rise in the well to soil properties and the rain event. The simulation results reveal that the magnitude of the Lisse effect increases with the ponding depth. Soil permeability plays a key role in generating the Lisse effect. The water‐level rise in the well is delayed relative to the air‐pressure rise in the unsaturated zone when the soil permeability is low, and the maximum water‐level rise is less than the maximum air pressure induced by rain infiltration. The simulation also explores the sensitivity of the Lisse effect to the van Genuchten parameters and the water table depth.

Mixed, classical and hydrothermal karstification in a carbonate aquifer: Hydrogeological consequences. The case of the Saida aquifer system, Algeria

In the Saida area, Algeria, a regional aquifer has formed in Jurassic carbonate rocks. Recharged by direct infiltration and swallow holes on horsts, it discharges into the Saida graben. Geochemical and isotope contents show deep groundwater flow at temperatures of around 100 °C at depth and CO 2 of deep origin, developing a hydrothermal karst interconnected with the shallow, classical karst. This particular type of karst aquifer, mixing hydrothermal karst conduits in the phreatic zone and classical karst features in the infiltration zone occurs preferably in active tectonic regions. Such a situation is favourable to productive well boring, consequently to groundwater withdrawal and deterioration of water quality. Still badly known in general, this type of complex aquifer should be studied in depth for an efficient and proper exploitation and protection of the groundwater resource.

Geostatistical Analysis to Identify Hydrogeochemical Processes in Complex Aquifers: A Case Study (Aguadulce Unit, Almeria, SE Spain)

The Aguadulce aquifer unit in southeastern Spain is a complex hydrogeological system because of the varied lithology of the aquifer strata and the variability of the processes that can take place within the unit. Factorial analysis of the data allowed the number of variables to be reduced to 3 factors, which were found to be related to such physico-chemical processes as marine intrusion and leaching of saline deposits. Variographic analysis was applied to these factors, culminating in a study of spatial distribution using ordinary kriging. Mapping of the factors allowed rapid differentiation of some of the processes that affect the waters of the Gador carbonate aquifer within the Aguadulce unit, without the need to recur to purely hydrogeochemical techniques. The results indicate the existence of several factors related to salinity: marine intrusion, paleowaters, and/or leaching of marls and evaporitic deposits. The techniques employed are effective, and the results conform to those obtained using hydrogeochemical methods (vertical records of conductivity and temperature, ion ratios, and others). The findings of this study confirm that the application of such analytical methods can provide a useful assessment of factors affecting groundwater composition.

Chemical and isotopic ( δ18O‰, δ2H‰, δ13C‰, 222Rn) multi-tracing for groundwater conceptual model of carbonate aquifer (Gran Sasso INFN underground laboratory – central Italy)

Summary A hydrochemical and isotope study was conducted on the drainage waters of an underground laboratory, located inside the Gran Sasso massif (central Italy). The study was expected to improve the conceptual model of groundwater circulation at the base of an over 1000-thick unsaturated zone in the Gran Sasso partitioned karst aquifer. This lithostratigraphically and tectonically complex aquifer is typical of Africa–Europe thrust-and-fold collision belt in the Mediterranean area. In this case, investigations on water–rock interactions during recharge in complex aquifers, overlaid by a thick unsaturated zone, have been made thanks to the strategic location of the Gran Sasso underground laboratories, located in the core of a huge carbonate aquifer. Knowledge of the local basic hydrogeological setting was the starting point for a detailed hydrogeochemical and isotopic study, which was carried out at the aquifer scale and at the fine scale in the underground laboratories. The water–rock interaction processes were investigated both spatially and in temporal sequences, analysing recharge waters and groundwater in the underground laboratories by multitracing techniques, including major ions and δ18O‰, δ2H‰ and δ13C‰ stable isotopes. Use of 222Rn provides information on transit time in the aquifer. Processes proved to be typical of carbonate rocks, with clear influence of vertical movement of water on chemical–physical parameters through the unsaturated zone. Conversely, in the saturated zone, these processes proved to be dominantly affected by local geological–structural conditions. A conceptual model with dual flow velocity is proposed, directly related to the local geological-structural setting. 222Rn decay enables to calculate an effective velocity of around 10 m/day for the fracture network, through the sequence of less permeable dolomites and underlying limestone. Lag time between recharge and chemical changes in the saturated zone testifies to an effective velocity of about 35 m/day for fast flow through recent and active extensional faults.

Multi-disciplinary modeling, in stratigraphy and groundwater stratigraphy of the Jordan River basin

The study area is located along the Dead Sea Rift, the climate is considered arid in its southern margin near the Dead Sea, which is the lowest water reservoir found on the globe (412 m BSL), to semiarid in its northern part. During the last few decades, the water resources became depleted limiting the natural development of the agricultural settlements, which are the most common type of communities in the region. Previous studies suggested that a large amount of freshwater is lost as the result of salinization processes, which occur when fresh groundwater from the mountain aquifers, flow into the saline clastic Neogene aquifer complex. In order to comprehend this complex system, a detailed outlining of the regional hydrogeological system is essential. Since there are no boreholes, which penetrate the aquiferous rock sequences within the Rift, it was necessary to interpolate a large variety of data from several fields of geosciences. The methods applied included geological mapping, geophysical modeling based on interpretation of seismic profiles and geochemical modeling based on chemical and isotopic analysis of runoff, sediments and groundwater. The combined modeling based on results from the different types of analyses implied to several conclusions relevant to the regional water management policy: (1) groundwater becomes saline as it flows from the margins of the Rift to its center. Therefore, it is recommended to exploit it along the foothills of the rift escarpment. (2) Geophysical modeling indicated that the foothills and the Karstic mountain aquifer extend into the subsurface of the valley and can be farther exploited (up to 15 mm3 per annum) by relatively shallow wells. (3) Several mechanisms of groundwater salinization were deciphered: (a) the dense vertical faulting systems act as potential conduits for saline water, which flow-up from deep-seated sources and penetrate into the fresh aquifers. (b) Fresh groundwater in the clastic aquifer complex is rare, furthermore, two evaporates bodies were encountered (Auja and Zaharat el Qurein), also acting as sources for fresh water salinization. (c) Although the quantity of runoff recharge to the Jordan Valley aquifer complex is negligible, the increase in its salt-content (TDS) turns this negligible freshwater recharge to a significant contributor of salts.

Qualite des eaux souterraines et risque de pollution en milieu semi-aride. Cas de la cuvette de Cheria (NE Algerien)

Water shortage has become a key problem for all countries and particularly for those in development. In fact, the increase of the populations and the development of urbanism, industries and cultivated land lead to a degradation of the quality of the groundwater and a very significant reduction of the reserves that constitute at times the only source of water for human consumption. After the analisis of the situation of the land, the present work comes in an important moment to establish the characteristics of the complex aquifer of the plateau of Cheria in Algeria and the risks of contamination of the groundwater. This study, which relies in the recent causes of contamination takes into account several factors. We can mention the contamination by blackwaters (waste), the impact of livestock and the climatic conditions. The exploitation of the groundwater from carbonated aquifers, is today the only source that responds to the needs of water for the industrial, human, and agricultural comsumption in the region. In recent years there has been an increase in agricultural and agricultural-related activity. The first results show the existence of a lowering of the piezometric level, as well as important concentrations in nitrates due to anthropic activity which are leading to a deterioration of the quality of drinking water in Cheria.

European aquifer typology: a practical framework for an overview of major groundwater composition at European scale

Groundwater quality is determined by a variety of factors. Petrographic properties of the rocks in the vadose and in the groundwater saturated zones as well as the regional hydrological and hydrodynamic conditions are regarded as its major natural influencing factors together with groundwater's initial composition. For an assessment of major groundwater composition on a continental scale, the complexity of individual aquifers needs to be organized into simplified patterns. For this purpose, European scien- tists, among them representatives from several European national geological surveys, jointly defined a harmonized European aquifer typology, which includes the major classes of aquifers showing comparable petrographic properties. This process has led to the development of the European aquifer typology map presented in this paper. Similar groundwater compositions are expected for the various classes in comparable hydrodynamic and hydro- logic conditions. Application of the aquifer typology with regard to the intercomparison of hydrochemical data sets across Europe for several aquifer types clearly points out that typical groundwater compositions can be distinguished for the various aquifer types. In the future this typology may serve as a basis for referencing hydrogeochemical groundwater composition on a European scale.

Occurrence of minor toxic elements in volcanic-sedimentary aquifers: a case study in central Italy

The occurrence of minor toxic elements (arse- nic, vanadium and fluoride among others) in the volcanic aquifers of central Italy, with concentrations often above the national standards for drinking waters, has been recognized since the 1970s. However, these groundwaters are still often used for human consumption. With the aim of providing insight into the water-rock interaction processes, as well as into the evolution of arsenic and other elements, water-sampling campaigns where under- taken within a 100 km 2 area at the eastern margin of the Viterbo region, where volcanites overlie an alluvial aquifer complex. The samples were analysed and geo- chemical modelling was applied to characterize the thermodynamic state of the waters. The results indicate the existence of direct relations among arsenic, vanadium and fluoride concentrations and of their inverse relation- ship with calcium concentration. An evolution scheme of groundwater composition from the upstream volcanic aquifer to the downstream sedimentary aquifer is also discussed. Two hypotheses are proposed which justify the observed decrease of the cited minor element concen- trations, i.e. dilution processes and/or precipitation of fluorapatite, which can include in its structure small amounts of these elements. The precipitation hypothesis is supported by modelling results. Resume La presence de certains elements toxiques mine- urs (arsenic, vanadium et fluor entre autres) dans les aquiferes volcaniques d'Italie centrale, a des concentra- tions souvent superieures aux limites de potabilite nation- ales, est reconnue depuis les annees 70. Ces eaux restent cependant tres utilisees pour la consommation humaine. Afi nd 'apprendre les interactions eau/roche et l'evolution de l'arsenic et des autres elements, des campagnes de prelevement ont ete mises en œuvre dans un secteur couvrant 100 km 2 , sur la bordure orientale de la region de

Instream flows and the decline of riparian cottonwoods along the Yakima River, Washington, USA

AbstractUnder pre‐settlement conditions the Yakima River in Washington state, USA was characterized by multiple channels, complex aquifers and extensive riparian cottonwood forests. Subsequent implementation of headwater dams to supply irrigation water has altered river and floodplain processes critical to the cottonwoods and associated riparian vegetation. In this study, we analysed hydrology and floodplain forests and especially the dominant black cottonwoods (Populus trichocarpa) along sequential reaches of the Yakima River. Elevations were surveyed and vegetation inventoried along cross‐sectional belt transects, and cottonwood tree ring interpretations investigated historic associations between river hydrology and cottonwood establishment and growth. We analysed hydrographs relative to the apparent episodes of cottonwood recruitment and applied a quantitative model for seedling colonization that required: (1) floods, disturbance flows to produce barren nursery sites, and subsequent flows for seedling (2) establishment and (3) survival. In contrast to earlier conditions, flow patterns after the 1960s have generally been unfavourable for cottonwood recruitment although some cottonwood colonization has occurred in association with physical disturbance from gravel mining. With recent flow regimes, regulated flows along upper reaches maintain the river near bank‐full throughout the growing season, thus inundating suitable seedling recruitment sites. Downstream, irrigation withdrawals reduce the river stage, resulting in seedling establishment at low elevations that are lethally scoured by subsequent high flows. These regulated flow regimes have not hindered growth of established trees, but have reduced the recruitment of cottonwoods, and particularly disfavoured females, thus altering sex ratios and producing skewed cottonwood population age and gender structures. The cottonwood decline has also been associated with other changes in riparian plant community composition, including the encroachment of invasive weeds. Based on this ecohydrologic analysis we discuss flow adjustments that could rejuvenate cottonwood forests along the Yakima River. Copyright © 2007 John Wiley &amp; Sons, Ltd.

Determining The Salt Store and Mobility in the Regolith, Bland Sub-Catchment, Central Nsw

To manage salinity risk for a sub-catchment or a region, quantifying the salt store and degree of salt mobility are two fundamental issues to address. In a thick (e.g. 70 m) and complex aquifer system, where sand bodies are semi-confined to confined within spatially dominating mud and clay, quantifying the salt store and mobility based on information collected from sparsely and unevenly distributed water bores and exploration drill holes is a significant challenge. Airborne electromagnetic (AEM) data, using Fugro?s time-domain TEMPEST fixed wing system, were collected over ~ 1,500 km2 of the Bland sub-catchment in central NSW (Lawrie et. al., 2002). Conductivity responses depict the electrolyte concentrations (mainly sodium chloride) and water content of the regolith and basement rock (Lawrie et al., 2002). Statistical analyses of water content, salinity and electrical conductivity logs show that conductivity is a measure of salt load. Non-porous bedrock is resistive whereas porous and moist sediments with saline pore fluids are conductive. Amongst the sediments, less porous coarse textured palaeo-valley basal gravely sand is less conductive than highly porous clay-rich colluvium and lacustrine mud. Using conductive depth slices (CDI), the spatial salt load distribution was mapped and quantified using ArcGIS (Tan et al., 2005a).

Geostatistical Simulation and Numerical Upscaling, to Model Ground-Water Flow in a Sandy-Gravel, Braided River, Aquifer Analogue

Abstract An integrated approach combining sedimentological, geostatistical, and numerical analyses has been applied to model groundwater flow through a glaciofluvial, braided-stream, gravel–sand aquifer of Pleistocene age, exposed in a gravel pit in the eastern Piedmont (northern Italy). The aquifer under investigation consists of a stack of braided bar and minor channel fills and scour fills, where different gravel–sand bedforms migrated. It represents an analogue of buried sediments, which host some of the most important aquifer complexes and water resources of northern Italy. The work has been performed with three main steps: (1) drawing and discretization of the geological models of two orthogonal exposures and estimate of conductivity of every facies; (2) geostatistical simulation of the 3D facies heterogeneity, based on semivariogram analysis of the geological models; (3) application of a 3D numerical model of flow to upscale the conductivity field. Semivariogram analysis developed from facies indicator functions has shown that: (a) the variability in the data set in the horizontal direction is a representation of the oriented distribution of the sedimentary facies, corresponding to the average N–S elongation of sediment units, according to the paleoflow direction; (b) the vertical correlation length approximates the average thickness of the bedsets bounded by fourth-order erosional surfaces (unit bars, scour fill, and small channel fill associations) and may represent the thickness of an individual hydrostratigraphic unit; (c) the conductivity and facies distributions are more variable in the vertical direction than in the horizontal one, with a ratio between vertical and horizontal ranges of about 1:8. The 3D conductivity distribution within the considered deposits has been determined using a sequential indicator simulation, whereas transition-probability analysis has been used to confirm the correlation model obtained with semivariogram analysis and to check the result of the simulation. The equivalent hydraulic-conductivity tensor has been computed with finite-difference modeling of groundwater flow for cubic blocks of the aquifer analogue with side length of about 13 m. At this scale the medium is almost homogeneous, but anisotropic, with the lowest values of hydraulic conductivity in the vertical direction and the greatest values along the elongation of the sedimentary units, i.e., almost parallel to average paleoflow.

Hydraulic characteristics of a typical basement complex aquifer in Ajaokuta, southwestern Nigeria

Hydraulic characteristics of a typical basement complex aquifer in Ajaokuta, southwestern Nigeria