The Hidden Information in Observations of Groundwater Well Records in a Complex Multilayer Aquifer

The Los Arenales region in the Duero river basin (Spain) is an extensive area which today is formed by three adjoining groundwater bodies. The traditional conceptual model for this area assumes three different hydrostratigraphic units: Quaternary (unconfined aquifer), and Upper (unconfined and semi/confined aquifers) and Lower Miocene (deep semi/confined aquifer) formations. The Medina del Campo groundwater body (GWB) occupies the central part of the Los Arenales region. Within this GWB, the La Moraña region is perhaps the most important groundwater irrigated area. An experimental project by the Castile-Leon regional government for the period 1999–2013 recorded data from a set of eight groundwater monitoring wells. In this paper, we analyze the records of groundwater levels and Electrical Conductivity (EC) at the monthly scale and compare the records between the observed wells. Our interpretation highlights a transition from the Upper Miocene leaky aquifer (in the east) to the Lower Miocene semi/confined aquifer (in the west). If this hypothesis is confirmed with further research, it should be checked at the entire GWB.