Geophysical techniques applied to shallow aquifer characterization

12/04/2017

The general objective of this doctoral project was the use of geophysical techniques in groundwater research, more specifically in shallow-aquifer research, and particularly in groundwater-dependent ecosystems (GDE) research. The EU Water Framework Directive (WFD) establishes that all groundwater masses related to GDE have to be identified and characterized. The geophysical techniques used in this work fulfil this requirement perfectly. The project developed through (1) an extensive bibliographical survey on the use of ground penetrating radar (GPR) in groundwater research, especially in GDE research; and (2) geophysical and hydrogeological field work in plot-scale study sites associated to GDE located in Portugal and Spain. The geophysical techniques used on field were GPR, electrical resistivity tomography (ERT), and multi-channel analysis of surface waves (MASW). Scientific outputs to this project are presented in this thesis as three manuscripts prepared for publication. The first has been published as a SCI paper, the second is submitted to a SCI scientific journal and the third is being prepared to be submitted in coming months. The first manuscript reviews GPR applications in GDE research. The large dataset compiled from the literature was completed with data from the study sites. This work showed that the most covered hydrogeological contexts are alluvial, colluvial and glacial, and that the most common application is water table depth determination. Oppositely, weathered-marl and crystalline-rock environments as well as the delineation of salinity interfaces in coastal and inland areas are less studied. The second manuscript assesses the current status ofattenuation expressions in shallow groundwater research, for which a well-agreed expression doesn’t exist in GPR literature. The manuscript compiles attenuation expressions found in literature and analyses the performance of the attenuation values compiled according to geological environments, antenna frequencies, and penetration depths, contributing to the design of proper GPR surveys in shallow groundwater research. The third manuscript combines electrical and seismic geophysical prospecting to deduce geometry and groundwater dynamics in a coastal alluvial aquifer determining the hydrological functioning of the Santo André lagoon, a GDE, located in SW Portugal. This research covers different sections along the main groundwater flow path system of the alluvial aquifer, where geophysical electrical surveys provide ambiguous information because of difficult distinction between high-conductivity pore fluids and conductive geological structures such as clay-rich and salty sediments. ERT, GPR, and MASW were applied quarterly over one year. The combination of techniques permitted to characterize faults that control the E-to-W longitudinal increase of the thickness. Once the aquifer geometry was solved and the highconductivity lithology was discarded, the time-lapse ERT results were used to evaluate seasonal resistivity alterations related to stream-aquifer interactions, changes in salinity consequence of aquifer recharge from atmospheric sources, transient salinity due to old marine brackish water presence in low-recharge periods, direct groundwater evaporation when water table is shallow, and agricultural fertilizers presence in the cropping season. A similar combination of techniques was implemented in the other study sites, despite results have been omitted to reduce the length of this PhD thesis.

Authors in the community:

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  M

  Maria Catarina Custódio da Paz

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Supervisors of this institution:

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  L

  Luís Filipe Tavares Ribeiro

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  F

  Francisco Javier Alcalá Garcia

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101352034

Doutoramento em Georrecursos

earth-and-related-environmental-sciences - Earth and related environmental sciences

eng - English

Instituto Superior Técnico

Financing entity

Fundação para a Ciência e a Tecnologia