The study covers is 262.5 km2, it is located in the northwestern region of Costa Rica and constitutes part of the Tempisque river basin. This region is one of the driest and fastest-growing areas in the country in terms of development. A conceptual hydrogeological model is proposed here as the basis to carry out specific studies as well as to plan the use of water resources in the territory.
The geology is composed mainly of Mio-Pliocene and Pleistocene rocks corresponding to the Bagaces and Liberia formations, as well as some Quaternary alluvial deposits. The Bagaces Formation is composed of a series of pyroclastic flows interbedded with a lava unit and sedimentary layers associated with the explosive events. Crystalline white tuffs correlated of Rio Liberia Unit of Liberia Formation are found overlying the previous formation. West of the study area, Tempisque River alluvium overlaying the Bagaces Formation.
Two aquifer systems are identified. The Bagaces aquifer is a complex and fractured system due to the heterogeneities of the reservoir. Some layers of tuffs and paleosols constitute horizons with lower hydraulic conductivities, while in general lavas present the best hydrogeological conditions. The aquifer varies from unconfined to confined, with the hydraulic conductivity is between 3.2x10-3 m/d and 31.22 m/d. The transmissivity varies from 2.43 m2/ d to 5520 m2/ d, with high to very high values in the northeastern sector, high values in the west }and intermediate values in the central-western area. The aquifer is classified as considerably heterogeneous.
On the other hand, Liberia is an unconfined aquifer, with transmissivity values between 3.56 m2/d and 54.6 m2/d. It is classified as homogeneous and the hydraulic conductivity varies from 0.0199 m/d to 0.2665 m/d.
The total average rainfall in the area is of 13.9 m3/s, of which 2.05 m3/s (14.73%) is intercepted by vegetation; the runoff volume is 0.35 m3/s (2.51%), mainly limited to the urban area; evapotranspiration is 7.98 m3/s (57%) and the potential recharge to aquifers is 3.46 m3/s, which constitutes only 24.8 % of the total rainfall.
Both aquifers are recharged by rainfall infiltration and in the case of the Bagaces aquifer, the recharge is also given by vertical percolation of the upper aquifer, while recharge from mountainous areas is not ruled out. A previous hydrogeochemical study indicates that the water recharged by both aquifers is of meteoric origin, with the water residence time being shorter in the Liberia aquifer than in Bagaces, based on tritium isotopes and CFCs, in both cases being less than 50 years old.