The Río Grande watershed is located in the western of the Central Valley, Costa Rica, it covers an area of 634,4 km2, with a population of 150 000 inhabitants. The main economic activities in the area are coffee and sugarcane production as well as vegetables and ornamentals. The geology of the area is mainly composed by volcanic rocks with ages from Neogene to recent, but there are also some Pleistocene lacustrine deposits in Palmares and San Ramón, Alajuela. Regional basement outcrops of the Aguacate Group are located on the right bank of the river and include basaltic to andesitic lava flows interlayered with breccias, ignimbrites, and tuffs of Neogene age. Thick volcanic sequence related to the stratovolcanoes of the Central Volcanic Range overlays these rocks. On the left bank, the basement is represented by the Colima formation, composed by lava flows and breccias that are exposed in the east-striking Alajuela fault. Those rocks hoste fractured heterogeneous aquifers of low to medium potential.
Two groundwater flow systems were identified. The shallowest one is located above an elevation of 1200 meters above sea level, on the slopes of Poás and Barva volcanoes. Most of the sampled springs belong to this group. It is characterized by fast circulation through recent volcanic deposits. The intermediate flow converges toward the Colorado River. At the eastern side, groundwater flows to the southwest, at the western side, it flows to the southeast.
The chemical composition of groundwater reflects the geochemistry of the geological formations with small variations due to dilution and concentration processes. In general, the chemical composition of groundwater is a bicarbonate calcium type with some local variations. Stable isotope signature of the groundwater of the wells and springs evidence a direct recharge. It can be deduced that the aquifers present a short residence time, because lack isotopic variations between the two periods compared.
The stable isotope information shows the altitude effect in the study site, where the water recharged at higher elevation is more depleted in oxygen 18 and deuterium than waters recharged at intermediate and low elevations. From the excess of deuterium it can be deduced that La Laguna de Fraijanes, and the Chayotera and Prendas springs, located in the corridor between the Poás volcano and the Barva volcano, has a similar footprint of Atlantic rainfall. In the western side of the study area, in the town of San Ramón, a well without tritium was found, which indicates that it is the oldest water in the area (<60 years), which could correspond to a regional flow.
The regional conceptual model provides information to public institutions related to water resources, water operators and local governments to establish land planning programs and regional protection policies.