The hydrological and hydrochemical conditions of groundwater-related wetlands may vary greatly both spatially and temporally depending on multiple variables: the size, location and lithology of the recharge area; the zone of the wetland where groundwater discharge takes place; the magnitude of this discharge and its chemical composition; the regional and local distribution of the hydraulic permeability; the topographic features around the wetland; the contribution of other water sources; the variability of all these conditions; etc. In the framework of a project of the MESCyT to know the functioning of the Ozama Wetlands National Park (OWNP, with about 48 km2), hydrodynamic, hydrochemical and isotopic studies are underway since 2017 to understand, among other things, the relationship between the wetlands and groundwater. In this paper we present some results of the piezometric, hydrochemical and isotopic studies of river water and groundwater in the basin (about 2700 km2) of the OWNP. The main results indicate that regional piezometric levels and main groundwater flow directions are controlled by the Ozama River, which is the main course and drainage axis of the basin. The Ozama River contributes and exports water to/from the OWNP. To the west of the OWNP, groundwater discharges to the wetlands, while to the east are the wetlands that recharge the aquifer. This situation seems to be influenced by the intense exploitation of groundwater that occurs east of the OWNP, mainly for domestic supply. The chemical composition of surface water is mostly HCO3-Ca type, while groundwater samples have two main facies, HCO3-Ca and Cl-Na, with a certain proportion of mixed samples. The HCO3-Ca facies is the lithological signature of the carbonates, which are the most abundant minerals in the basin. The Cl-Na facies is due to the mixture with seawater in the aquifer, since a good part of the sector to the east of the OWNP is affected by marine intrusion. Quite a few water samples of rivers and wells are isotopically enriched by evaporation. Groundwater samples that are mixed with seawater are also enriched for this reason. The isotopic composition of surface waters taken at the source of rivers has allowed us to identify the possible altitudinal isotopic gradient and also to deduce that there are two groups of wells that tap groundwater recharged at two different altitude ranges. This information can be useful for the management of the aquifers. For logistical reasons, the study of the wetlands themselves has begun recently.