Mahjoub Himi

Geochemical processes controlling groundwater quality under semi arid environment: A case study in central Morocco

Bahira plain is an important area for Morocco due to its agriculture and mining activities. Situated in a sub-arid to arid climate, this plain hosts an aquifer system that represents sequences of carbonates, phosphates, evaporates and alluvial deposits. Groundwater flows from Ganntour plateau (recharge area) to the basin-fill deposits and Zima Lake and Sed Elmejnoun where water evaporates. The objective of this study was to characterize the chemical properties of the groundwater and to assess the processes controlling the groundwaters chemistry. We can divide water samples into three hydrochemical water groups: recharge waters (Ca/Mg-HCO3), transition zone waters (Ca-HCO3-SO4/Cl) and discharge waters (Na-Cl/SO4). Accordingly, compositions of waters are determined by the availability of easily soluble minerals like calcite (Ca-HCO3 dominant), halite (Na-Cl dominant) and gypsum (Ca-SO4 dominant). Cl/Br ratios show that Cl concentration increases from dissolution of natural halite. When groundwater is affected by extreme evaporation Cl/Br ratios may increase up to 1900. High fluoride concentrations are associated with low Ca2+ concentrations (<100mg/L). That means when recharge waters enter the aquifer, it starts dissolving fluorite since the Ca2+ concentration is low. Once groundwater becomes saturated with Ca2+, the immobilization of fluoride is occurring by precipitation of fluoride-rich minerals like fluoro-apatite. According to the environmental isotope (18O and 2H) analyses, they are three potential processes affecting groundwater: 1. Evaporation as verified by low slope value, 2. Water-rock interaction, 3. admixture of waters showed different stable isotope compositions and salinities.

Integrated Geophysical Surveys in the Tarragona Cathedral

An integrated geophysical survey has been conducted at the Tarragona’s Cathedral (Catalonia, NE Spain) with the aim to detect the existence of archaeological remains of the Roman’s temple devoted to August. Many hypotheses have been proposed about its possible location, the last ones regarding the inner part of the Cathedral, which is one of the most famous temple of Spain (12th century) evolving from Romanesque to Gothic styles. A project including electrical resistivity tomography (ERT), ground probing radar (GPR) and frequency domain electromagnetics (FDEM) has been planned over one year and conducted during a week of intensive field survey. From all the methods applied, both ERT and GPR have provided some detailed information on subsoil structures. The ERT method has been applied with different techniques and arrays, ranging from standard Wenner-Schlumberger 2D sections to full 3D electrical imaging using the MYG array. Electrical resistivity data have been recorded extensively and therefore, 70.000 apparent resistivity data were available to obtain a full 3D image after an integrated inversion. In conclusion, some significant buried structures have been revealed providing conclusive information for archaeologists.

An evaluation of aquifer vulnerability in two nitrate sensitive areas of Catalonia (NE Spain) based on electrical resistivity methods

The protection of groundwater has become one of the most important European environmental policies as evidenced by the Orders relating to the protection of water from contamination, Directive 2000/60/CE of the European Parliament and the European Union Council, and more concretely Directive 2006/118/118/CE, related to the protection of groundwater from pollution and degradation. Traditional methods for assessing vulnerability include soil surveys, drilling and analysis of lithology logs from wells with the objective of characterising the thickness, hydraulic properties and lateral extend of the protective layers. However, such studies can be labour-intensive and expensive. In addition, the parameters measured may have high spatial variability, which makes accurate characterization over large areas difficult. Fortunately, a numerical index of protection can be assigned from the longitudinal electrical conductance parameter derived from electrical resistivity surveys (VES, ERT or any other electrical or EM method). This can be more accurate and reliable than any other vulnerability index derived only from visual inspection or interpolated from sparse borehole data.

Archaeological Excavation Confirms the Geophysical Anomalies Recorded at the Cathedral of Tarragona - A Comparative Study

An integrated geophysical survey was conducted in September 2007 at the Cathedral of Tarragona to search for archaeological remains of the Roman temple dedicated to the Emperor Augustus. Many hypotheses about its location have been put forward, the most recent ones suggesting it could be inside the present cathedral. A project including Electrical resistivity tomography (ERT) and Ground probing radar (GPR) was planned for a year and conducted during a week of intensive field survey. Both ERT and GPR provided detailed information about subsoil structures. Different ERT techniques and arrays were used, ranging from standard Wenner-Schlumberger 2D sections to full 3D electrical imaging using the MYG array. Electrical resistivity data were recorded extensively, making available many thousands of apparent resistivity points to obtain a complete 3D image after full inversion. The geophysical results were clear enough to persuade the archaeologists to excavate the area. The excavation confirmed the geophysical interpretation. In conclusion, the significant buried structures revealed by geophysical methods under the cathedral were confirmed by recent archaeological digging as the basement of the impressive Roman Temple that headed the Provincial Forum of Tarraco, seat of the Concilium of Hispania Citerior Province

Structure of the Pliocene Camp dels Ninots maar-diatreme(Catalan Volcanic Zone, NE Spain)

Maar volcanoes expose shallower or deeper levels of their internal structure as a function of the degree of erosion. In El Camp dels Ninots maar-diatreme (Catalan Volcanic Zone, Spain), the tephra ring has been largely eroded, and the remaining volcanic deposits infilling the diatreme are hidden under a lacustrine sedimentary infill of the crater. The volcano shows hardly any exposure, so its study needs the application of direct (e.g., boreholes) and indirect (shallow geophysics) subsurface exploration techniques. Additionally, this maar-diatreme was built astride two different substrates (i.e., mixed setting) as a result of its location in a normal fault separating Neogene sediments from Paleozoic granites. In order to characterize the internal structure and post-eruption stratigraphy of the maar-diatreme, we did geological studies (mapping, continuous core logging, and description of the tephra ring outcrops) and near-surface geophysics, including nine transects of electric resistivity tomography and a gravity survey. Results show that the deeper part of the diatreme is excavated into granites and is relatively steep and symmetrical. The uppermost diatreme is asymmetrical because of mechanical contrast between granites and Pliocene sands. The maar crater contained a lake permanently isolated from the surrounding relief and was deep enough to host anoxic bottom waters while its margins had shallower waters. These lake conditions preserved the remarkable Pliocene fossil record found in the lacustrine sediments.

Assessing the Vulnerability of Groundwater Pollution at Sensitive Areas by Geophysical Methods

The protection of groundwater has became one of the foremost environmental European policies according the Orders on the Protection of Water Against Contamination Directive 2000/60/CE of the European Parliament and the European Union Council, and more concretely with the Directive 2006/118/118/CE, related to groundwater protection against the pollution and degradation. Traditional methods for assessing vulnerability include soil surveys, drilling and analyses of logs from wells with the objective to characterize the thickness, hydraulic properties and lateral extent of the protective layers. Disadvantages of such investigations are that they can be labour-intensive and expensive. In addition, the measured parameters may have high spatial variability making accurate characterization over large areas difficult. As a result, a numerical index of protection can be assessed from the longitudinal electrical conductance parameter derived from electrical resistivity surveys (VES, ERT or any other electrical or EM method), that can be more accurate and reliable than any other vulnerability index assumed only by visual inspection or interpolated from sparse borehole data.

Hydrogeophysical Characterisation of Artificial Recharge Basins on the Llobregat River Barcelona, Spain

Over the past ten years there has been growing interest in the use of the subsurface for water storage using shallow ponds, either natural or excavated, for the infiltration of water into the subsurface and then uses wells to recover the water. The design and operation of these subsurface systems require an understanding of the hydrogeologic structure and properties that control both the movement and storage of water. In our study, we have performed a detailed hydrogeophysical characterization about the subsurface of the pond from electrical resistivity tomography.

Integrated Geophysical Exploration for Geothermal Resources in Caldes de Montbui (NE Spain)

Six geothermal structures were identified on the Moesian Platform territory based on the analysis of the hydrogeological, geological and geophysical data. Four of them are developed on the Wallachian part of the Moesian Platform, known as the warmer segment. Three of these structures are overlapped with areas characterized by high value of the geothermal gradients and heat flow. The temperature values of 80oC and 120 oC were measured at depths of 2000-3000 m and 2500-3500 m, respectively. Different sources were proposed for the presence of these high values of temperature and heat flow. The layers with thermal waters were intercepted by the wells drilled for oil exploration. The water temperature values vary from 50 to 100oC. Overlapping the geothermal structures on the latest tectonic image proposed for the Moesian Platform, we notice a correlation between the high temperatures and systems of deep faults with east-west and north-south orientations. These two system of faults are developed at west of the Intramoesian Fault, known as a major crustal faults which affect the basement and sedimentary cover of the Moesian Platform.