Decoding of groundwater recharge in deep aquifers of foreland Basins using stable isotopes (δ18O and δD) and anion-cation analysis: A case study in the southern Llanos Basin, Colombia

The topographic and structural configuration of the deformation front in foreland basins has implications in the process of hydrologic recharge of deep aquifer systems. Deep aquifer systems are favored when tectonic deformation has exhumed aquifer units to the surface, creating a connection between the surface and subsurface. The indirect water recharge processes established by this connection allow meteoric water to infiltrate and travel long distances through a basin. Therefore, establishing recharge zones and determining the influence of meteoric waters in deep aquifers represents a challenge to understand the dynamics and hydraulic potential of this type of system. This study presents new oxygen and hydrogen isotopic data from 162 samples of surface water and 109 samples from groundwater. Groundwater samples were taken from deep wells (700 m–4000 m deep) that penetrated the aquifer units that make up the Basal Flow Unit (BFU) present in the southern Llanos basin in Colombia. In addition, anion (−) and cation (+) analysis (HCO3−, CO3−2, SO4−2, Cl−, Na+, K+, Ca+2, Mg+2) was performed on 67 surface water samples and 93 groundwater samples from wells to evaluate their composition and existing relationship with the recharge areas. We used these isotope compositions (δ18O - δD) and hydrochemical data to identify the provenance and degree of influence of meteoric waters in the Basal Flow Unit. Thus, we evaluated the isotopic behavior of the Andean Foreland in Colombia. These data allowed us to propose the altitude ranges of the recharge zones for a deep regional aquifer (BFU) that extends along the southern Llanos basin in Colombia. The δ18O and δD values from surface water sources showed consistency with respect to the isotopic behavior reported in global datasets (GMWL), the Local Meteoric Water Line (LMWL) established for Colombia, and data from IAEA-GNIP stations examined in this study. The new δ18O and δD analysis, together with datasets from previous studies carried out in the area, made it possible to establish the Local Meteoric Water Line for the Llanos basin following the equation δD = 7.8 (±0.10) x δ18O + 11.66 (±0.75). δ18O and δD values of groundwater samples taken in the deep wells follow the proposed Llanos Local Meteoric Water Line. It supports a high influence of meteoric waters in the Basal Flow Unit, from areas close to the recharge zones up to more than 300 km away to the east of the basin and suggesting its origin from approximate altitude ranges that vary between 580 m and 2060 m, related to the La Macarena Range and the eastern flank of the Eastern Cordillera. The physicochemical data was used as an additional proxy and showed the lowest ion concentrations in proximity to the recharge zones, recognizing in general groundwater with low concentrations of Cl− (4.1–1278 mg/L) and Na (4.7–1749 mg/L) along the BFU present in the southern Llanos basin.