Combined Use of Electrical Resistivity Tomography and Hydrochemical Data to Assess Anthropogenic Impacts on Water Quality of a Karstic Region: A Case Study from Querença-Silves, South Portugal

Abstract

The presence of highly vulnerable karstic systems in areas of intense human activities often results in the degradation of existing surface- and groundwater quality status. Water quality protection and improvement, as required by the Water Framework Directive (WFD), depends on correct identification and prioritisation of the most relevant pollution sources within complex multi-stressor conditions. This paper presents a methodology that combines the use of hydrogeology, water quality and quantity data, and electrical resistivity tomography methods to access different anthropogenic impacts upon the water cycle, focusing on the water quality of a section of the Portuguese karstic Querenca-Silves aquifer. During this study an interpretation of the possible interconnections between pollutant sources, their pathways and local surface-groundwater connections was made, based on data obtained from field campaigns. As a result of this study, the areas with more interconnection between surface- and groundwater were identified, namely the influent sites from the local stream into the aquifer. The impact of the diffuse pollution sources was verified at the monitoring points, especially those located downstream of large farming plots. Pollution in this karst aquifer results from seepage through agricultural areas and infiltration at the influent points of the stream. This aspect of stream influence upon the aquifer means that pollution sources located upstream of the area of the aquifer (e.g., a WWTP—Wastewater Treatment Plant) can also contribute to the aquifer pollution.