Influence of upwelling on the shallow water chemistry in a small wetland riparian zone (Basque Country)

Many hydrologic conceptual models in riparian areas assume that the alluvial deposits zone is hydraulically more active than the fractured bedrock below. Therefore, these models undervalue the possible contribution of deeper groundwater from the fractured bedrock system. A hydrochemical study, under various hydrological conditions, has been carried out in a small riparian zone of the Salburua wetland (Basque Country) in order to highlight the conceptual model. This wetland is included in a wide Quaternary aquifer, which has been declared a Nitrate Vulnerable Zone. The results of this study suggest that the fractured bedrock is at least as dynamic as the upper clayey deposits. The presence of more fractured zones, which act as hydraulic “windows”, allow the upwelling of deeper groundwater and, consequently, make the upper alluvial deposits and the fractured bedrock water systems to be cross-connected. Nevertheless, this upwelling is limited to some small areas in the riparian zone. As a result of this local interaction, several chemical reactions have been observed and the hydrochemical characteristics of shallow groundwater undergo seasonal variations. The study shows that a hydrologic conceptual model, which does not consider the hydraulic activity of the fractured bedrock, can be too simplistic. The presence of hydraulic windows could be considered throughout the entire Quaternary aquifer, knowledge of which could help the managers of the Vulnerable Zone and the wetland to take more effective measures for regulation and conservation.