Elsa Cristina Ramalho

A geophysical and hydrogeological study of aquifers contamination by a landfill

Abstract Landfills are the classical solution for waste disposal. During the last years there has been a growing concern about the effect of landfills in public health, because leaching water can contaminate nearby aquifers. Hydrogeological studies are very important to investigate and remedy these contamination problems but an integrated use of both geophysical and hydrological methods can be even more effective. In this paper, the groundwater contamination by a landfill, located on very porous and permeable formations, is assessed by both geophysical and hydrogeological techniques. Bearing in mind the geological and hydrogeological characteristics of the area, electrical methods were chosen to carry out the geophysical survey. At first, electromagnetic methods were used and a ground conductivity survey was carried out to determine the contamination plume. Then several resistivity soundings were proposed and two resistivity pseudo-sections obtained so that a more detailed study of the contamination was accomplished. From the geophysical data interpretation, strategically located boreholes were drilled, water samples obtained and chemical analysis carried out. Finally, the overall results are integrated and discussed as well as the merits and disadvantages of the techniques that have been employed.

Using resistivity logs to estimate hydraulic conductivity of a Nubian sandstone aquifer in southern Egypt

The present study was carried out in the Khor-El-Ramlah area, in a Nubian sandstone aquifer. It is located in the western bank of Nasser Lake, southern Egypt, in a tectonically active area, which is dissected by many regional faults extending towards the lake. For this reason, the Nubian sandstone aquifer in this area is expected to be recharged by a large amount of seepage water from the lake. One important parameter used to quantify seepage is hydraulic conductivity. Since in this area there is no available information about hydraulic conductivity via pumping tests or laboratory examina- tion of core samples, an attempt is made in this study to estimate hydraulic conductivity using available resistivity logs by applying the Kozeny-Carman model. The clay contamination effect on porosity estimation has been removed via application of the Waxman-Smits model. The hydraulic conductivity is estimated to be 9.5 ± 2.5 m/day for the Nubian sandstone aquifer in this area. The present result is in agreement with the estimated and measured hydraulic conductivity values for the same aquifer in nearby areas and could be used to estimate seepage in this zone. The application of geophysical well logging often provides a cost-effective and efficient alternative to estimate aquifer parameters. out. Therefore, no information about hydraulic conductivity or seepage water quantity is known for that area. In particular, the seepage quantity is expected to be higher than in other parts around the lake due to the local dense fault system. The main objective of the present study is to estimate the hydraulic conductivity of the Nubian sandstone aquifer for this area, from the measured resistivity logs of well RML, in order to make it available for any future estimation of seepage.

Contributions of hydrogeophysics to the hydrogeological conceptual model of the Albufeira-Ribeira de Quarteira coastal aquifer in Algarve, Portugal

Geoelectrical and electromagnetic (time and frequency domains) hydrogeophysical methods were applied and jointly interpreted together with auxiliary information such as regional piezometric maps, borehole lithological logs and offshore data. The objective was to retrieve the structure and geometry of the Albufeira-Ribeira de Quarteira coastal aquifer system (Algarve, Portugal) and to upgrade the current hydrogeological conceptual model. The results allowed for the detection of the freshwater–saltwater interface along the coastline and identification of the water-bearing layers and aquitards and their hydraulic relationships. An explanation for the location of the inter- and subtidal fresh groundwater discharge is also presented and a new modeling unit is proposed for groundwater flow modeling. Limitations of the used hydrogeophysical methods are indicated and recommendations are made for follow-up studies.RésuméLes méthodes hydrogéophysiques de type géoélectrique et électromagnétique (domaines temporel et spatial) ont été appliquées et conjointement interprétées en utilisant comme information auxiliaire des surfaces de piézométrie régionale, des logs lithologiques de forage et des données de la plate-forme littorale. L’objectif était d’obtenir la structure et la géométrie du système aquifère côtier Albufeira-Ribeira de Quarteira (Algarve, Portugal) et de réviser l’actuel modèle conceptuel hydrogéologique. Les résultats ont permis de détecter l’interface eau douce-eau salée le long du littoral, de différencier niveaux aquifères et aquitards et d’établir leurs relations hydrauliques. Une explication de la localisation des décharges d’eau souterraine douce dans la zone inter- et subtidale est avancée et une nouvelle unité de modélisation de l’écoulement d’eau souterraine est proposée. Les limitations des méthodes hydrogéophysiques utilisées sont indiquées et des recommandations sont faites pour les études à venir.ResumenSe aplicaron métodos hidrogeofísicos geoeléctricos y electromagnéticos (en los dominios del tiempo y frecuencia) y se interpretaron en forma conjunta con información auxiliar, tales como mapas piezométricos regionales, registros litológicos de pozos y datos offshore. El objetivo fue recuperar la estructura y geometría del sistema acuífero costero Albufeira-Ribeira de Quarteira (Algarve, Portugal) y actualizar el actual modelo hidrogeológico conceptual. Los resultados permitieron la detección de una interfaz agua dulce-agua salada a lo largo de la línea costera y la identificación de las capas, acuíferos y acuitardos y sus relaciones hidráulicas. Se presenta también una explicación para la ubicación de la descarga de agua dulce subterránea inter y submareal y se propone una nueva unidad de modelado para el modelación de flujo de agua subterránea. Se indican las limitaciones de los métodos hidrogeofísicos utilizados y se hacen recomendaciones para los estudios de seguimiento.摘要应用了地电和电磁(时间和频率范围)水文地球物理方法并跟根据诸如区域测压图、钻孔岩性记录和近海资料等辅助信息对此进行了解译。目标就是获取(葡萄牙阿尔加维)Albufeira-Ribeira de Quarteira沿海含水层的结构和几何形状,对目前的水文地质概念模型进行升级。依赖获得的结果能检测到沿海岸线的淡水-咸水界面,识别出含水层和隔水层及其水力关系。还论述了交互的和潮下的地下淡水补给位置,为地下水流模拟提出了新的模拟单元。指出了所用水文地球物理方法的局限性,为后续研究提出了建议。ResumoOs métodos hidrogeofísicos geoelétricos e eletromagnéticos (domínios de tempo e de frequência) foram aplicados e conjuntamente interpretados utilizando como informação auxiliar mapas piezométricos regionais, logs litológicos de sondagens e dados da plataforma costeira. O objectivo foi obter a estrutura e a geometria do sistema aquífero costeiro Albufeira-Ribeira de Quarteira (Algarve, Portugal) e actualizar o modelo hidrogeológico conceptual. Os resultados permitiram detectar a interface água doce-água salina ao longo da cost, identificar as camadas aquíferas e aquitardas e definir as suas relações hidráulicas. Uma explicação para a localização das descargas de água doce subterrânea inter- e subtidais é avançada e uma nova unidade de modelação do fluxo de água subterrânea é proposta. As limitações dos métodos hidrogeofísicos utilizados foram indicadas e recomendações foram feitas para futuros estudos.

Assessment of the leachate movement in a sealed landfill using geophysical methods

This paper describes a case study concerning the use of integrated geophysical methods applied to environmental assessment. The study is focused on an old municipal solid waste sealed landfill site, located in Gaeiras, Central Portugal. The problem is related with leachate overproduction in this domestic and industrial waste landfill that became an environmental problem with urgent assessment, so that a solution could be planned. Due to the lack of accurate information regarding the shape, history and development of the landfill, the use of a set of classical geophysical methods was the option, since they are non-invasive and non-destructive. The available area was small, almost restricted to the landfill area. To conduct this assessment, electromagnetic RF-EM and Geonics EM34, spontaneous potential (SP), vertical electrical soundings (VES) and magnetic prospecting surveys were planned to understand the various problems that could be related with the leachate overproduction. The joint use of these classical geophysical methods was targeted to investigate bedrock depth and structure (RF-EM, EM34 and VES), waste and leachate characteristics (EM34 and magnetics) and groundwater flow (SP) in the landfill. Geophysical results were correlated with hydrogeological information, integrated and interpreted, using geographic information system tools. The results obtained were important to understand the geological mechanisms that are responsible for leachate overproduction and to suggest remedial measures.

An approach to estimate porosity and groundwater salinity by combined application of GPR and VES: a case study in the Nubian sandstone aquifer

Ground-penetrating radar signal propagation is strongly controlled by water content. Therefore, it can be employed to address numerous hydrogeological problems, ranging from geological structures to petrophysical properties. In the present study, ground-penetrating radar and vertical electric sounding measured data are analysed and combined to answer the most hydrogeologically simple and geophysically complex question of groundwater salinity. Geophysical logging, in particular resistivity logs were used to correct the static shift’s usual effect on vertical electric soundings (VES), as well as to correlate with surface data. The dielectric constant of the water bearing horizon was estimated through ground-penetrating radar data interpretation, which in turn plays an important role in the estimation of the volumetric water content. On the other hand, vertical electrical sounding was used to estimate water saturation. Combining the results obtained from both methods, the groundwater salinity of a shallow aquifer, mostly clay-free, can be estimated. The method is applied to a data set measured over a sand aquifer in the Aswan area, Egypt. The results obtained indicated that the present technique is efficient, since the estimated groundwater salinity using this technique is about 1152 mg/l, whereas the groundwater salinity estimated from water sample analysis is about 950 mg/l.

Environmental assessment of a uranium experimental rock blasting in Portugal, using geophysical and hydrogeological methods

The Nisa uranium deposit, located in Central Portugal, has been known since the late 1960s. Some areas were explored at that time. Today, a few open pits and dumps remain in place and are a concern to local authorities. To assess the geoenvironmental problems caused by the main mining exploration composed of an experimental rock blasting, 3D electrical conductivity and resistivity models were made to develop a hydrogeological model to understand the possibility of contaminants transportation, such as uranium, from the dumps towards a dam located nearby. These 3D models were the support to show alteration layer thickness variations and fault zones at depths controlling groundwater circulation. Spectrometric surveys were also carried out and correlated with geology and geoelectrical structure. All this information was used in the construction of the 3D steady state hydrogeological model of the experimental rock blasting of Nisa. In this model, groundwater flow and the contaminant pathways were simulated. Some areas have very high radioactive values resulting from the geological formation characteristics and old dumps. However, results of the environmental assessment using geophysical and hydrogeological methods point to a critical situation restricted only to the area of the experimental rock blasting of the Nisa uranium deposit and its dumps.

Hydrogeophysical Survey in a Hydrocarbon Contaminated Site

In 2002, an accidental release of unlead gasoline from a ruptured underground storage tank of a gas station has contaminated soils and groundwater. Although remediation actions have taken place, groundwater is still contaminated. During the last year, hydrogeological and geophysical investigations have been carried out in the area between the gas station and the local river, for site characterisation. The hydrogeological survey aimed at defining the groundwater flow and eventual tectonic control. The geophysical survey consisted of EM31 and 34, GPR, Resistivity and IP, so that a more complete diagnosis of the situation could be conducted.

Input of geophysics to understand hydrogeology towards the assessment of geoenvironmental conditions in Beira city, Mozambique

As the fourth largest city in Mozambique, and located right beside the Indian Ocean, Beira faces sanitary and environmental problems that are a strong concern to the local and national authorities. One of the major problems that Beira city has to deal with is the existence of undrinkable groundwater in many dug-wells that supply a large amount of the city population. The cooperation project among Direcção Nacional de Geologia of Mozambique, the Laboratório Nacional de Energia e Geologia of Portugal and the Instituto de Apoio ao Desenvolvimento Português that took place from 2007 to 2011 has as one of its main goals the production of the Geological and Geoenvironmental Maps of Beira at scale 1:50,000, intending to create a tool to support the implementation of governmental policies regarding sustainable groundwater and natural resources exploitations, land management and environmental protection. These goals were achieved through the identification, characterization and mapping of the different geological units and the evaluation of the city environmental problems regarding water and soil contamination in urban areas, involving the use of several methodologies that were subsequently integrated and jointly interpreted. These comprised remote sensing techniques, hydrogeology analysis and evaluation and the use of geochemistry and electrical geophysical methods. This work emphasizes the input of geophysical and hydrogeological methods to assess the geoenvironmental conditions of Beira, namely those related with the prediction of occurrence of saline and brackish water in the shallow dug-wells that supply a large amount of Beira populations, especially in the crowded suburbs. Several geophysical surveys using electrical and electromagnetic methods were conducted and interpreted regarding the evaluation of the several hypotheses that were stood for this feature. Results are related with hydrogeological conditions and organic and inorganic hydrochemical analysis. Considering these facts, this study was an important input for mapping areas with saline and brackish water imprisoned in the subsurface geological formations, using a wider set of methodologies that were integrated in these results.

Geophysical Studies to Detect a Favourable Location for a Thermal Water Well

Several geophysical surveys were carried out in order to determine the best location for a new production well for the spa of Caldas de Moncao, in order to increase production of thermal waters due to an increasing demand of the facilities. After a dipole-dipole survey conducted in 2001 that reached a depth of investigation of about 40 m and 2 shallow wells, three seismic reflection profiles were acquired to locate deep rooted faults capable of groundwater transport to the surface. Vertical electrical soundings and the refraction interpretation of the first arrivals of the reflection data were also undertaken to complement the information. This paper presents the integrated interpretation of geophysical and geological data in order to achieve the above mentioned goal. After the interpretation and lateral correlation of the structures found in the geophysical profiles, two locations appear as the strongest candidates for a successful thermal water production well. This example shows the utility of geophysical methods in detecting favorable areas for water borehole drilling, reducing economical costs considerably.