Abstract

OCHOA-GONZALEZ, Gil Humberto; CARREON-FREYRE, Dora; CERCA, Mariano  and  LOPEZ-MARTINEZ, Margarita. Assessment of groundwater flow in volcanic faulted areas. A study case in Queretaro, Mexico. Geofís. Intl [online]. 2015, vol.54, n.3, pp.199-220. ISSN 2954-436X.  https://doi.org/10.1016/j.gi.2015.04.016.

This work integrates local stratigraphy and faults to constraint a numerical model of groundwater flow in the North area of the Valley of Queretaro aquifer. Basic geological information was established from the reinterpretation of well logs, 40Ar*/39Ar dating of rocks, and field mapping. In particular, dating allowed to obtain the timing of emplacement of a magmatic dyke inferred to be emplaced along the El Nabo fault. The increase of flow rate and temperature in the El Nabo well suggest that the fault and dike transport a regional flow. A numerical model was implemented to evaluate the contributions of faulting and volcanic structures in the overall groundwater flow. The model, accomplished with the Visual-Modflow software and Modflow 2000 code, was partially constrained by the results of a pumping test in the El Nabo well, the deepest well (Lat. 20°42'14"N, Long. 100°28'45"W, ca. 1000 m depth) located in the Queretaro area. The interpretation of the model results and field observations suggest that faults collect the regional flow, compartmentalize the reservoir, and are responsible of the dramatic increase of flow rate from 6 to 47 l/s when drilling the El Nabo well below a depth of 850 m. The study presented here allowed the assessment of normal faults and intrusives determining groundwater flow and might be relevant to the knowledge of flow dynamics in nearby volcanic valleys. The analysis of the pumping test suggest the presence of two different groundwater flow systems: (1) a flow related with the limestone and shale unit and (2) a regional linear flow through faults. The implemented numerical model, based on geological information, allowed a better understanding of complex aquifer systems that cannot be evaluated by analytical methods. Moreover it can be a useful application for a better interpretation of pumping tests and to add complementary data of other pumping tests and/or hydrogeochemical analysis.

Keywords : Groundwater flow; faulted aquifers; numerical modeling; volcanic structures; dykes; Mexicos.