Superficial Aquifer Research Articles

Groundwater salinization of the Sfax superficial aquifer, Tunisia

Groundwater salinization has become a crucial environmental problem worldwide and is considered the most widespread form of groundwater contamination. The origin of salinity in the coastal aquifer of the Sfax Basin, Tunisia was investigated by means of chemical analyses of groundwater samples from 65 wells. The groundwater samples present a clear gradation from calcium sulphate salinization to that of sodium chloride. The saturation indices for calcite and gypsum, and binary diagrams of different ions, together with multivariate analysis, indicate the existence of various salinization processes such as: dissolution of gypsum and calcite dispersed through the reservoir rock; ion exchange; intensive agricultural practices that produce effluents that infiltrate to the saturated zone; and sea-water intrusion, enhanced by excessive withdrawal of groundwater.

Hypoaigic influences on groundwater flux to a seasonally saline river

Hypoaigic zones are aquifer volumes close to and beneath the shores of saline surface water bodies, and are characterized by the presence of time-dependent natural convection and chemical stratification. When transient and cyclic processes are involved there is significant potential for complex flow and reaction in the near-shore aquifer, presenting a unique challenge to pollutant risk assessment methodologies. This work considers the nature of some hypoaigic processes generated by the seasonally saline Canning River of Western Australia near a site contaminated by petroleum hydrocarbons. A dissolved hydrocarbon plume migrates within the shallow superficial aquifer to the nearby bank of the Canning River. Beneath the river bank a zone of complex fluid mixing is established by seasonal and tidal influences. Understanding this complexity and the subsequent ramifications for local biogeochemical conditions is critical to inferring the potential for degradation of advecting contaminants. A range of modelling approaches throws light on the overall topographic controls of discharge to the river, on the saline convection processes operating under the river bank, on the potential for fluid mixing, and on the various important time scales in the system. Saline distributions simulated within the aquifer hypoaigic zone are in at least qualitative agreement with previous field measurements at the site and are strongly affected by seasonal influences. Groundwater seepage velocities at the shoreline are found to be positively correlated with river salinity. Calculations of fluid age distributions throughout the system show sensitivity to dispersivity values; however, maximum fluid ages under the river appear to be diffusion limited to a few decades. The saline convection cell in the aquifer defines a zone of strong dispersive dilution of aged (many decades) deep aquifer fluids with relatively young (several months) riverine fluids. Seasonal recharge and river salinity cycles induce regular perturbations to the convection cell, yielding intra-annual variations of 50% in seepage velocity and almost 30% in wedge penetration distance at the plume location.

Hydrochemical and isotopic behaviour of a Saharan phreatic aquifer suffering severe natural and anthropic constraints (case of Oued-Souf region, Algeria)

An approach combining the use of water dissolved chemical species and isotopic fingerprints has been used to understand the behavior of a phreatic aquifer and to determine the origin of its different water components. This aquifer is located in the large sedimentary basin of the Great Oriental Erg (Algeria) and overlies two deeper aquifers: the Complexe Terminal (CT) and the Continental Intercalaire (CI). Besides the deterioration of its groundwater quality, its water table has risen during the last 20 years. A water budget surplus between 950 and 2500 l s−1 was estimated. Down-gradient groundwater evolution (south-north) has shown that the mineralisation increases from 1.23 to 5.20 g l−1 due to evaporite minerals dissolution. Chemical and isotopic data demonstrated that in addition to rainfall there is a contribution from the CT and CI aquifers. The latter are tritium-free and less mineralized than the phreatic aquifer. Their radiocarbon contents are very low (<10 pmC, percent modern Carbon) (Pleistocene recharge) whereas quite the contrary is observed for the superficial aquifer which exhibits fairly high and variable C-14 activities (50–100 pmC), evidence of recent recharge. On the basis of tritium contents, two groundwater groups were identified for the phreatic aquifer.

Learning and Capacity Building for Irrigators in Western Australia's East Wanneroo Area: A Theoretical Framework for Educational Provision and a Sketch of the Socioecological Context

In Western Australia the East Wanneroo horticultural area is reliant on a superficial aquifer, the Gnangara Groundwater Mound, for irrigation. The area is affected by social and political change as the sprawling city of Perth expands, as well as by ecological changes resulting from a decline in groundwater levels. Horticulturalists face increasing competition for water allocation particularly with the drier conditions which are the consequence of two decades of reduced rainfall. This study provides a rationale for an educational intervention; explores the social, political, and ecological context of the East Wanneroo agricultural area; identifies gaps in current knowledge and understanding of the irrigation community and linkages between this community and their context which require further investigation and clarification; and outlines a theoretical framework which can be applied to develop a customized educational strategy aimed at improving irrigation practices and promoting sustainable communities and water usage.

Interconnectivity between the Superficial Aquifer and the Deep Confined Aquifers of the Gnangara Mound, Western Australia

Perth groundwater resources are obtained from three major aquifers that occur beneath the Perth metropolitan area: the Superficial aquifer, Leederville aquifer and Yarragadee aquifer. Each aquifer has a unique seasonal water level pattern controlled by soils, geomorphology and geology. Landuse is mainly responsible for variations in recharge; however, the hydraulic properties control aquifer response and water level pattern to a greater degree. Groundwater in the three aquifers is generally of very good quality except in localised areas. Salinity increases with depth and in direction of groundwater flow in the three aquifers. The best water quality is in the Superficial aquifer in the Wanneroo well field area. The geochemistry and stable isotope signatures from the three major aquifers revealed distinct water types that suggest very little hydraulic connection or mixing of waters between these aquifers at the present abstraction and recharge regimes. The results also show that the Leederville and Yarragadee aquifers were recharged during earlier cooler times while the Superficial aquifer is being recharged at present.

Soils Characteristics of The Bassendean and Spearwood Sands of the Gnangara Mound (Western Australia) and their Controls On Recharge, Water Level Patterns and Solutes of The Superficial Aquifer

The flow pathways of water in the soils of the Gnangara Mound are highly irregular and depend upon the moisture content, the repellency and preferential wettability potential of the soils. The occurrence of preferential flow is more evident in dry soils. As the soil wets during the rainy season, the water repellence and differential wettability decreases, the fingering and the preferential flow paths disappear. Most of the agricultural sites in the Spearwood Sands which showed more irregular flow than the Bassendean Sands are under continuous irrigation during cultivation season. As the repellency problems are chemically treated, it is therefore expected that the flow will be more uniform all the year round. Landuse is mainly responsible for variation in recharge rates; however, the hydraulic properties control aquifer response and water level pattern to a greater degree. Water levels in the mid 1970s were in a semi steady state. Since that time, a combination of increasing water use by pine plantations, heavy pumping from private boreholes in market gardens and private homes and intensive pumping from the Gnangara Mound for the metropolitan water supply have caused water levels to continually decline in the Superficial aquifer. Nitrate and phosphate concentrations in the regional Superficial aquifer are generally very low. None of the tested pesticides (atrazine, diazinon, dimethoate, endosulfan, fenamiphos, iprodione, malathion and chlorpyrifos) were detected in the groundwater samples collected from the monitoring bores.

Flow exchange between the deep and shallow groundwaters in the Sbe?tla synclinal basin (Tunisia): an isotopic approach

Stable (18O, 2 H) and radiogenic (3H, 14C) isotopes of water have been used to constrain the source, origin, age, migration pathway and mixing processes in the Sbeitla (Tunisia) system. The system is composed of an upper unconfined “Middle Miocene” aquifer with a variable thickness from 10–300 m, an intermediate confined/unconfined “Lower Miocene” aquifer about 100 m thick and a deeper confined “Lower Cretaceous” aquifer about 150 m thick separated by a thin clay layer. A total of 53 groundwater samples from the three aquifers and spring samples were collected during February and March 2000 and isotopically analysed using conventional methods. The stable isotopes composition of waters establishes that the deep groundwater (depleted as compared to present corresponding local rainfall) is ancient water recharged probably during the late Pleistocene and the early Holocene periods. The relatively recent water in the superficial aquifer is composed of mixed waters resulting presumably from upward leakage from the deeper groundwater. The radiogenic (3H, 14C) isotopes data confirm that the recent water, with a tritium content between 6.5 and 19 TU, represents post-nuclear recharge and the ancient groundwater with low carbon-14 contents between 7 and 26 pmC infiltrated between 8,000 to 20,000 years ago. When used in conjunction with the stable isotopes data, the mixing process can be clearly identified, especially in the Sbeitla sill area. Groundwater of the upper aquifer exhibits isotopic signatures of both the old and recent waters. By using isotopic mass balance, the computed contribution of the deep groundwater in recharging the upper aquifer is up to 94%.

Groundwater discharge from the superficial aquifer into Cockburn Sound Western Australia: estimation by inshore water balance

Submarine groundwater discharge (SGD) into Cockburn Sound Western Australia was quantified by applying a distributed groundwater flow model to estimate the inshore aquifer water balance. Spatially averaged SGD along the coast was estimated to be 2.5–4.8 ± 0.9 m3 day−1 m−1. The range in estimated average SGD reflected low and high estimates of average groundwater recharge, which ranged from 0.13 to 0.24 m year−1 (15–28% of average annual rainfall). The error ±0.9 m3 day−1 m−1 was calculated by assuming arbitrary ±20% errors in groundwater pumping and inflow across boundaries. SGD varied spatially along the coastal boundary due to variation in hydraulic connection between the coastal aquifers and ocean, and spatial variability in recharge, transmissivity and pumping. Under assumptions of low and high groundwater recharge, SGD along the coastline varied in the ranges 1.4–4.6 m3 day−1 m−1 and 2.4–7.9 m3 day−1 m−1, respectively.

Origine de la minéralisation et comportement hydrogéochimique d'une nappe phréatique soumise à des contraintes naturelles et anthropiques sévères : exemple de la nappe de Djebeniana (Tunisie)

The determination of the origin of the salinity in the superficial aquifer of Djebeniana (South-East of Tunisia), and the understanding of its hydrogeological and geochemical behaviours related to severe natural and anthropic constraints, were approached by the combined survey of some dissolved ions (especially the conservative elements: Br− and Cl−), and by oxygen-18, one of the stable isotopes of water molecules. These ‘tracers’ indicate that: (1) the present recharge during rainwater infiltration brings downward a high content of nitrates and other dissolved salts; (2) two other sources of dissolved salts in groundwater exist, favoured by the intensive exploitation of the phreatic aquifer. The first one is due to mineralised water uprising from a deep and confined aquifer. The sea intrusion is the second source of salinity. La détermination de l'origine de la salinité des eaux de la nappe superficielle de Djebeniana (Sud-Est de la Tunisie) et la compréhension de ses comportements hydrologique et géochimique, en réponse à des contraintes naturelle et anthropique sévères, ont été approchées par l'étude combinée des éléments chimiques (surtout les éléments «conservés » : Br− et Cl−) et par l'isotope 18 de l'oxygène de la molécule d'eau. Ces éléments montrent que, outre la recharge descendante par les eaux de pluie, qui ramènent aux eaux souterraines des teneurs élevées en nitrates, il existe deux autres apports d'eaux et de sels, favorisés par l'exploitation intensive de la nappe. Le premier apport correspond à une recharge ascendante à partir de la nappe profonde par des eaux relativement plus minéralisées. Le deuxième apport correspond à l'intrusion des eaux méditerranéennes.

Using chemical models for evaluating the geochemical confinement capacity of a geological barrier application to the Centre de Stockage de l'Aube (France)

The feasibility of using a chemical reaction-based approach for evaluating and modelling the role of adsorption reactions in determining the geochernical confinement capacity of natural geological barriers is being studied as part of an on-going R & D programme. The confined superficial aquifer underlying the Centre de Stockage de l'Aube facility, a geological barrier for this site, has been used as a case study with the following aims. First, development of a site characterisation protocol and demonstration of its use to determine the principal geochemical characteristics of aquifer materials using batch experiments and to represent the information obtained in terms of a chemical model. The experimental results obtained for Ni 2+ partitioning as a function of total Ni, pH, total Ca and total solid can be satisfactorily represented in terms of reactions with an ion exchange site and a single amphoteric surface hydroxyl site with ferrihydrite reaction constants. A second objective is the incorporation of the reactions in a coupled geochemistry/transport code, and to verify the applicability of the coupled code predictions for Ni 2+ mass transfer by comparison with the results obtained during column tracer experiments. The breakthrough curve and equilibrium solid phase Ni loading, predicted by a one-dimensional coupled model for a column tracer experiment, agree closely with observed data. Additional studies are underway to reduce model conditionality, to extend the adsorption model to other analogue cations and anions, to incorporate the effect of natural organic matter and to take into consideration precipitation/dissolution of amorphous Fe surface phases.

Groundwaters of Florence (Italy): Trace element distribution and vulnerability of the aquifers

Geochemical and hydrogeological research has been carried out on 109 wells in the alluvial plain of Florence, in order to evaluate conductivity and main chemistry of ground waters, the pattern of some possible pollutant chemical species (Fe, Mn, Cr, Cu, Pb, Zn, NO2, NO3), and the vulnerability of the aquifers. The plain is made up of Plio-Quaternary alluvial and lacustrine sediments for a maximum thickness of 600 m. Silts and clays, sometimes with lenses of sandy gravels, are dominant, while considerable deposits of sands, pebbles, and gravels occur along the course of the Arno river and its tributary streams, and represent the most important aquifer of the plain. The groundwaters analyzed belong to this aquifer or to the smaller ones, hosted in the gravel lenses. Most waters show conductivity values around 1000–1200 μS, and almost all of them have an alkaline-earth-bicarbonate chemical character; these features are consistent with the mainly calcareous lithology of the aquifers. In the western areas a higher salt content of the groundwaters is evident, probably related to the presence of industrial activities which use water desalinators. Heavy metal and NO2, NO3 analyses point out that no important pollution phenomena affect the groundwaters; all the mean values of the chemical considered species are below the maximum admissible concentration (MAC) fixed by the European Community for drinkable waters. Nevertheless, some anomalies of NO2, NO3, Fe, Mn, and Zn are present in the plain. Apart from Mn, which seems to be released by certain calcareous gravels, the other anomalies have a local influence, since they disappear even in the nearest wells. The most plausible causes can be recognized in losses of the sewage system (NO2=3–4 mg/t); use of nitrate compounds in agriculture (NO3=60–70 mg/l); oxidation of well pipes (Fe ≈ 20 mg/l; Zn ≈ 6 mg/l). As regards Cr, Cu, and Pb, all the observations are below the MAC; therefore, the median values of < 3, 3.9, and 1.1 μg/l, respectively, could be considered reference concentrations for groundwaters circulating in calcareous lithotypes, under undisturbed natural conditions. Finally, a map of vulnerability related to the most superficial and important aquifer has been elaborated on the basis of thickness and permeability of the covers. The map shows that the areas near the Arno river are highly vulnerable, for the minimum thickness (or lacking) of sediments covering the aquifer. On the other hand, in the case of pollution, several factors not considered in the map could significantly increase the self-purification capacity of the aquifer, such as the dilution of groundwaters caused by the feeding of the rivers, the bacteria oxidation of nitrogenous species, and the sorption capacity of clay minerals and organic matter with respect to trace metals.