Understanding the 3D Structure of a Thermal Water Fissured Granite Aquifer by Use of Geophysical Studies

The thermal hot springs of Caldas de Monção, located in Northern Portugal, are supplied by a thermal water aquifer that must be preserved from pollution and anthropic effects. Enhancement of knowledge about the aquifer is therefore needed, so sustainable management and water quality preservation can be conducted, namely by replacing the springs by boreholes, which are less vulnerable to pollution. Several investigations have been conducted with this purpose, to review the available geological and hydrogeological information from the area. For this reason, several geophysical surveys were reprocessed and others were acquired and planned, so that a 3D conceptual groundwater flow model of could be drawn. Three seismic reflection profiles were acquired with the objective of locating deep faults capable of transporting groundwater to the surface and to complement information about shallow features available before the seismic survey. This information included interpretation of a dipole–dipole survey conducted in 2001, that reached an investigation depth of approximately 40 m, and two shallow boreholes drilled, after a deeper geoelectrical survey performed in the 1980s, to replace the springs. Besides the seismic reflection profiles, seven vertical electrical soundings were also acquired and refraction interpretation of the first reflection data to arrive was also undertaken to complement the information. This paper reports integrated interpretation of the geophysical and geological data to obtain a 3D conceptual model to achieve the above mentioned objectives. After interpretation and lateral correlation of the structures found in the geophysical profiles, the deep 3D conceptual model integrating the geological, hydrogeological, and geophysical information was constructed. This model was the basis for planning the location of an exploratory borehole that was drilled afterwards. From two initial locations appearing as the strongest candidates for a successful thermal water production well, one was chosen to drill a third borehole to supply the spa facilities. The third borehole results were very positive—250 m deep, temperature approximately 50°C, and water flow 12.5 l/s. This example shows the utility of using geophysical methods to increase knowledge about groundwater circulation and detect favourable areas for thermal water borehole drilling to replace the hot springs with the objective of protecting the aquifer from pollution.