Identification of Aquifer Systems in Weathered and Fractured Sandstone Based on 3D Geological Modeling in the Mesa de Los Santos (Santander, Colombia)

Mesa de Los Santos is an elevated plateau of the Eastern Cordillera of Colombia bordered by escarpments, where groundwater resources are limited to the local recharge. The geological unit with the greatest hydrogeological potential is Los Santos Formation (Lower Cretaceous), which presents three members (Lower, Medium and Upper). Based on stratigraphic information and hydrogeological information, three aquifer systems were characterized in the Upper Member: The Shallow Aquifer System (SAS), the Upper Aquifer 1 (UA1), and Upper Aquifer 2 (UA2). The SAS comprises discontinuous aquifers with groundwater flowing very close to the surface, circulating through weathered and fractured levels. UA1 and UA2 contain groundwater flowing through fractures. Groundwater in UA1 circulates through the top of the Upper Member, is underlain by a predominantly muddy base and exhibits an E-W and NE-SW flow consistent with the dip of the layers and the main directions of fractures. UA2 groundwater flows through the base of the Upper Member and is limited by the impermeable Middle Member. Stable water isotopes (δ18O, δ2H) data show three behaviors: (i) large temporal variability indicating a rapid flow through fractures in the three aquifers, and through primary porosity mainly due to weathering in the SAS; (ii) slower flows, with low temporal variability, showing well-mixed water of meteoric origin in the SAS, UA1, and UA2; (iii) groundwater with signs of evaporation indicating the connection between wetlands and the SAS in some cases.