Hydrogeological, hydrochemical and isotopic study of the Chibunga river basin (Ecuador)

This study provides a description of the hydrogeologic, hydrogeochemical and isotopic setting of the Chibunga river basin in the Province of Chimborazo (Ecuador). The basin has an area of approximately 514 km2 and 44 km in length. Its main river, the Chibunga River, originates on the slopes of the Chimborazo Volcano. The study area is located at an altitude comprised between 6268 aslm of Chimborazo volcano and 2598 aslm. This area has suffered for years of social conflicts for the access to water, due to an inadequate water resources management system.  Moreover, there is a deficiency of information about the water quality and groundwater recharge. The aims of this study was to investigate hydrogeochemical features of the Chibunga water resources and to improve information about hydrogeological structure, groundwater recharge processes and water quality.

From a hydrogeological point of view, a multilayer aquifer system is present, consisting in volcanoclastic deposits, and alternating pyroclastic and lava layers. An unconfined shallow aquifer is located in the most superficial part, feeding the plain springs. More in depth, confined and semiconfined aquifers are hosted in the more permeable layers, and are used for drinking water purposes. The plain is bordered by volcanic formations, mainly of andesitic rocks, characterized by a low permeability by fracturing. According to the literature, the aquifer system is mainly recharged by melting glaciers from the Chimborazo volcano. However, glaciers have been affected by a generalize retreat in the last decades that influences the water availability.

In this view, sampling campaigns were carried out to improve the hydrogeochemical characterization of precipitation, surface water and groundwater of the area.

The analyses of major elements highlight that, although Ca-HCO3 hydrochemical facies is the most common, a wide variability can be found in the groundwater of the area. On the whole, excluding the wells nearby urban settlements, chemical analyses show good quality of the water for drinking, irrigation, and agricultural use. Isotopic results, represented in δ18O – δ2H plot, are close to the Global Meteoric Water Line with a d-excess on average 16.3. Water stable isotopes reveal the contribution of precipitation to springs (δ18O ranging between -15.9 and -14.5; and; δ2H ranging between -110.5 and -99.2) at different altitude, while, near-stream groundwater has a similar isotopic signature to that of the river water (δ18O ranging between -14.4 and -13.7; and; δ2H ranging between -96.5 and -92.7) and reveal the interaction between surface and groundwater system.