Groundwater is an important technical difficulty regarding the feasibility of open pit mines. An example, shown here, is a complex limestone formation quarry in North Spain. Numerical groundwater simulations help to quantify the problem. Here we present the development and results of a numerical model for this aquifer, using both FEFLOW (finite elements) and MODFLOW (finite differences). The results match reasonably well the measured flows and have allowed simulation of practical scenarios of inmediate interest. Furthermore, the model gives aditional information regarding the location of a groundwater divide and propose a permeability zonation, dependent of the degree of fracturing. This zonation is able to explain the spatially localized groundwater measured flows and is consistent with other observations.