Hydrochemical and isotopic evaluation of groundwater and river water in the transboundary Silala River watershed

Abstract

This article reviews a hydro‐chemical and isotopic study aiming to provide information about groundwater‐surface water interactions in the Silala River, a transboundary basin shared between Chile and Bolivia. The chemical and isotopic data show: The springs located in the northern section of the lower course of the Silala River and the Cajones springs in Bolivia have similar characteristics and represent a shallow flow system feed by local recharge; In contrast, the springs located in the higher and southern sections of the lower course of the river have similar characteristics than the Orientales springs located in the higher part of river basin indicating these springs are associated with a high‐altitude recharge area. The river in the upper course in Chile has similar chemical and isotopic characteristics to the springs already mentioned, indicating most of the river flow comes from the high part of the basin in Bolivia. The groundwater shows different chemical characteristics, similar depleted isotopic values, and much lower 14C values (including Orientales groundwater) than the upper springs and river, providing the first evidence that both systems are not connected in the Chilean section of the river and the springs system is part of a perched aquifer confirmed by visual field observations. These data support a conceptual model which includes an interaction between a perched aquifer, probably in alluvial deposits on the flanks of the volcanoes, and the river as result of a complex system of fractures present in the near‐surface levels of Silala Ignimbrite. This study provides valuable information for modeling of hydrological and hydrogeological data in the study area.This article is categorized under: Science of Water > Hydrological Processes Science of Water > Water Quality

Introduction to the Silala River and its hydrology

“…Therefore, infiltration is a dominant process that promotes groundwater recharge (G omez et al, 2024;Y añez-Morroni et al, 2024aY añez-Morroni et al, , 2024b, and only a small part of the basin, mainly the saturated or near-saturated areas associated with spring-fed and riparian wetlands, contributes to overland flow. These findings are also supported by: (i) the great number of springs that feed the river along its course (Aravena et al, 2024;G omez et al, 2024;Su arez, Sandoval, et al, 2024); and (ii) the large baseflow index estimated F I G U R E 1 0 Location of the monitoring stations in the Chilean part of the Silala River basin. In addition to the stations that existed prior to 2017, surface and groundwater monitoring stations are shown.…”

“…The ravine of the Silala River crosses the international border at an elevation of $4277 m ASL (Figure 3, point C). In Chilean territory, the Silala River receives contributions of a series of perennial and/or ephemeral springs, as well as from groundwater (Aravena et al, 2024). For instance, the Quebrada Inacaliri (point A in Figure 3) is an ephemeral tributary of the Silala River, whereas the Quebrada Negra (point B in Figure 3) provides groundwater to the Silala.…”

“…According to Su arez, Sandoval, and Sarabia (2017), the SPW-DQN contributes with an additional flow of $90 L/s, which is discharged less than 50 m downstream of the confluence of the Silala River and the Quebrada Negra ravine. This additional flow comes from a deeper groundwater system (Aravena et al, 2024;G omez et al, 2024).…”

Understanding the Silala River—Scientific insights from the dispute over the status and use of the waters of the Silala (Chile v. Bolivia)

“…This work, together with the detailed geological mapping discussed above and hydrochemical investigations, discussed below, enabled an understanding of the major aquifers in Chile and their properties to be established, and was essential to the development of a hydrogeological conceptual model. These studies were supported by the "Hydrochemical and isotopic evaluation of groundwater and river water in the transboundary Silala River watershed" reviewed by Aravena et al (2024) in which several hydrochemical surveys are reported, providing data on major and minor ions and isotopes, including 14 C, 18 O, and Deuterium. The results contributed significantly to the understanding of the local and regional aquifer systems that operate in the Silala River Basin, and data from Bolivia proved useful in identifying the local and regional sources of groundwater recharge.…”

Hydrological modeling of the Silala River basin. 2. Validation of hydrological fluxes with contemporary data

“…Prior to this dispute, hydrological studies in the Silala River basin had focused on the characterization of hydrometeorological variables (precipitation, temperature, evaporation, and river flow) to define simple water balances for the basin (DICTUC, 2013;Muñoz et al, 2024). The legal dispute provided the opportunity to perform more detailed scientific investigations, aimed at quantifying the hydrological processes in the basin, for estimation of the water balance, and hence surface and groundwater flows across the international border, as well as groundwater recharge (Aravena et al, 2024;G omez et al, 2024;Su arez, Sandoval, et al, 2024;. A hydrological model was required to quantify these flows, and to estimate recharge to a groundwater model, developed by Chile to quantify the effects of small channels, constructed in the Bolivian headwater wetlands, on cross-border surface and groundwater flows.…”