Hydrochemistry and isotopic characterization of water resources in Santa Rosa mining area, Ecuador.

Water has memory: Uncovering its story in Santa Rosa, Ecuador

The hidden origin of the water we use

Water is an essential resource for life, yet we rarely think about its origin or the processes that shape its quality. In the Santa Rosa canton, located in El Oro Province, the Santa Rosa River is the main source of water supply for 85% of the population, making it a strategic resource for the region’s social and economic development. However, this area is also characterized by intense mining activity, along with agriculture and urban growth, raising concerns about the condition of its water resources.

To better understand this situation, researchers from the Escuela Superior Politécnica del Litoral (ESPOL) conducted a study in the Santa Rosa River Basin using hydrochemical and isotopic tools—scientific techniques that make it possible to identify the origin of water, the processes controlling its chemical composition, and potential sources of contamination.

Between 2022 and 2024, five sampling campaigns were carried out during both dry and rainy seasons. Water samples were collected from the Santa Rosa River, its main tributary streams, and La Tembladera lagoon, as well as from groundwater extracted from wells used for commercial activities.

The results showed that both surface water and groundwater are primarily of meteoric origin, meaning they originate from precipitation. However, as water moves through the environment, its chemical composition changes due to interactions with rocks and surrounding environmental conditions.

How the environment shapes water quality

The study also revealed important differences between surface water and groundwater, reflecting distinct hydrogeochemical processes.

Surface water was found to be mainly of the calcium-sulfate and calcium-bicarbonate type, indicating a strong influence of water–rock interactions within the basin. In contrast, groundwater was primarily classified as sodium-chloride type, a composition typically associated with longer residence times and greater interaction with soluble minerals. Additionally, surface water chemistry is largely controlled by water–rock interaction processes, whereas groundwater is influenced by the dissolution of evaporitic minerals, explaining its higher concentrations of dissolved salts.

The chemical composition of surface water was observed to change progressively along the Santa Rosa River, reflecting the combined influence of natural factors and human activities throughout the river. In particular, streams located near mining areas showed low pH values and elevated sulfate concentrations, suggesting acidification processes linked to the oxidation of sulfide minerals.

Isotopic analyses provided further insight into the origin of these compounds. The results indicated that, in these streams, sulfate is mainly derived from the oxidation of sulfide minerals such as pyrite, a process commonly associated with mining environments. This type of isotopic signature serves as key evidence to distinguish between natural sources and those influenced by mining activities.

On the other hand, groundwater used in commercial wells exhibited a different composition, characterized by higher concentrations of dissolved salts. This occurs because water remains underground for longer periods, allowing the progressive dissolution of minerals and enrichment in salts. Isotopic analyses also indicated that groundwater is relatively old, meaning it may have infiltrated decades ago. Its slow renewal rate makes it a particularly vulnerable resource in the event of contamination.

Science to protect water and the future

Understanding the origin and evolution of water is essential for its protection and sustainable management. This study represents the first comprehensive hydrochemical and isotopic characterization of the Santa Rosa River basin, providing key scientific insights into the processes that control water quality in an active mining area.

The results show that water chemistry is mainly controlled by natural processes such as water–rock interaction, but also reveal the localized influence of human activities, particularly in areas close to mining operations and urban zones.

The application of isotopic techniques made it possible to identify the origin of sulfate and distinguish between natural sources and those associated with the oxidation of sulfide minerals, providing a valuable tool for environmental monitoring.

In addition, the identification of relatively old groundwater highlights the importance of protecting this resource, as its capacity to recover from potential impacts is limited.

These findings provide a solid scientific foundation for developing sustainable water management strategies, enabling authorities and communities to make informed decisions to protect water resources.

This study demonstrates that water is not only a vital resource, but also a natural archive that stores information about its history, origin, and the processes it has undergone.

Thanks to science, it is now possible to read that history and use this knowledge to protect one of the most important resources for life and sustainable development.