Freshwater, vital for life and ecosystems, accounts for only 2.5% of Earth’s water, and is primarily located in polar caps, underground reservoirs, and surface water. Its quality varies due to environmental interactions, especially in groundwater. Tenerife, located in the Canary Islands, Spain, relies mainly on underground aquifers and tunnels capturing 51.6 cubic hectometers annually. Ensuring safe drinking water is a global challenge due to health risks from poor water quality, including diseases and cancer. Fluoride, sodium, and potassium are essential for health, and are mainly derived from groundwater as fluoride ions (F⁻) and sodium and potassium cations (Na⁺, K⁺). However, excessive F⁻, Na⁺, and K⁺ in drinking water is harmful. The World Health Organization limits F⁻ to 1.5 mg/L, Na⁺ to 8.70 meq/L, and K⁺ to 0.31 meq/L. Geological, climatic, and human factors control the presence and transport of F⁻, Na⁺, and K⁺ in groundwater. Seismic events can impact water quality, with long-term effects linked to aquifer structure and transient effects from gas and fluid expansion during earthquakes. This study was motivated by a 3.8 mbLg earthquake in Tenerife in 2012, which allowed its impact on groundwater quality, specifically F⁻, Na⁺, and K⁻ concentrations, to be examined. Post-earthquake, F⁻ levels alarmingly increased to 8.367 meq/L, while Na⁺ and K⁺ showed no significant changes. This research quantifies the influence of earthquakes on increasing F⁻ levels and evaluates F⁻ reduction during low seismic activity, emphasizing the importance of water management on volcanic islands.

Understanding the intricate relationship between seismic events and groundwater chemistry is critical to effective water resource management and safeguarding public health. This study delves into the temporal evolution of magnesium (Mg²⁺) concentrations in groundwater along 60 tunnels in Tenerife, Canary Islands, over a decade from 2011 to 2021. By analysing Mg²⁺ concentrations together with precipitation patterns and seismic activity, this research reveals significant variations in groundwater chemistry after the earthquake of August 18, 2012, with a magnitude of 3.8 mbLg and intensity IV (EMS). Surprisingly, Mg²⁺ levels increased after the earthquake, exceeding World Health Organization (WHO) recommendations in certain tunnels, followed by a slow decline, preventing values from reaching pre-earthquake levels eight years later. Non-anthropogenic influences, particularly precipitation, were ruled out as important factors, underscoring the direct correlation between seismic events and changes in Mg²⁺ concentration. Using linear models, a gradual decrease in Mg²⁺ levels was observed over time, indicating possible implications for drinking water quality. This study highlights the importance of understanding the impact of seismic activity on groundwater chemistry, particularly in regions abundant in Mg²⁺-rich volcanic lithology. Emphasizes the need for proactive monitoring of groundwater quality in seismic zones and the development of adaptive management strategies to ensure the safety and sustainability of drinking water resources. By elucidating the dynamics of groundwater chemistry in response to seismic events, this research not only contributes to a broader understanding of non-anthropogenic influences on groundwater in deep aquifers, but also informs public health policies based in evidence in earthquake-prone regions.