Characterization of spatial heterogeneity in sedimentary aquifer systems at regional scale by using hydrogeological modeling: A case study of the Middle Magdalena Valley basin - Colombia.

The characterization of the inherent heterogeneity of aquifer systems on a regional scale represents one of the main challenges in the study of groundwater, especially when there are uncertainties associated with the scarcity of hydrogeological information (Maliva, 2016).

Based on the implementation of a numerical hydrogeological model in a regional scale, the objective of this study was to characterize the most recent geological formations which have the greatest hydrogeological potential in the Middle Magdalena Valley (VMM) in Colombia. The VMM is bounded by the Eastern Cordillera of the Colombian Andes and the Bucaramanga-Santa Marta fault to the east and the San Lucas Mountain range and the Central Cordillera to the west. This is considered one of the main hydrogeological basins in the country, in which a high potential of hydrocarbon production from the exploitation of unconventional deposits has also been identified (Sarmiento et al., 2015; Londoño, 2019).  For these reasons, the groundwater management, based on the characterization of this hydro system, has a national interest.

In this study the characterization of the heterogeneity was approached from the estimation of hydraulic parameters by solving the inverse problem by means of a hydrogeological model on a regional scale (Carrera et al., 2005; Zhou et al., 2014). For this, 153 pumping tests were carried out and interpreted, allowing to parameterize the model in seven iso-conductivity zones. Also 12,383 discrete static level data were measured and collected in 19 years -time window and they were used in the calibration process. Furthermore, three field campaigns for measuring in-situ parameters (electrical conductivity, pH, dissolved oxygen, and temperature) were performed they served to enhance the conceptual model.

Based on this, variations in hydraulic conductivities were identified on a regional scale for each iso-conductivity zone, fluctuating by four (4) orders of magnitude. The main flow direction was in the south-north, parallel to the Andes cordillera and therefore to the Magdalena River, and with some minor local flows perpendicular to them, producing important outflows from the system in permanent lentic water bodies in the north. The results of the research are encouraging, but at a regional scale they still do not allow to have a high resolution of the heterogeneity of the hydro system models for decision making, so it is suggested to implement stochastic models at a regional scale and to construct multi-purpose groundwater monitoring networks in this basin.

Acknowledgments
The researcher thanks the MEGIA Research Project, Contingent Recovery Contract FP44842-157-2018 funded by Minciencias and the National Hydrocarbons Agency.