As part of the actions of the Integrated Sanitation Plan for the Matanza Riachuelo basin, a first version of the numerical model of groundwater flow was developed. A series of three articles presents the work undertaken; in this article the conceptual and numerical models are presented. The numerical model successfully reproduces the observed levels; in general, simulated hydraulic gradients match measured ones. Calibrated/calculated hydraulic conductivities are within the same order of magnitude as those measured, except at some areas that are seemingly greater probably due to a poor definition of the local geology. The mass balance is sounded and flow rates calculated in streams are consistent with observed measurements. The article ends with the presentation of the conceptual and numerical model uncertainties.