Long-term volcanic unrest in Tenerife (Canary Islands): Anomalies in dissolved gases.

Tenerife, the largest and most populated island of the Canary Islands, hosts a complex volcanic system characterized by rift-related activity, long-lived magma reservoirs, and an active hydrothermal system. Although historically characterised by relatively sporadic eruptive activity, the island’s volcanic system remains active, as evidenced by historical eruptions, such as those of Siete Fuentes (1704) and Chinyero (1909), as well as the 2004-2005 unrest. After more than a decade of relative quiescence, the Instituto Geográfico Nacional (IGN) began detecting seismic and geochemical anomalies in 2016 and a continuous slow deformation in 2023, which has persisted to the present.

Since 2015, numerous groundwater samples have been taken by the IGN Volcano Monitoring Group in distinct locations on the island, in order to determine total dissolved gas composition. Sampling points were chosen from among the existing hundreds of water mining galleries based on its proximity to Las Cañadas caldera, the presence of volcanic dissolved gas in the water and its physico-chemical properties such as pH, electric conductivity and temperature. Since 2022, additional sampling points were incorporated, resulting in eight-sampling point surveillance network of dissolved gases: QT61, QT62, QT63, QT65, QT66, QT72, QT73 and QT77, sampled approximately every three months. Groundwater samples were taken using the method described by Capasso and Inguaggiato (1998) and analyses were carried out at Istituto Nazionale di Geofisica e Volcanologia (INGV) laboratories in Palermo (Italy) following the methodology described by Paonita et al. (2012). 

In this work, we present CO₂ and H₂ dissolved concentrations in the above-mentioned sampling sites, pointing out the apparent relationship between some of the observed gas changes and the evolution of seismicity and ground deformation recorded in Tenerife during the last ten years.

Regarding dissolved CO₂, a wide range of concentrations have been measured in the different sampling points. The location with the maximum dissolved CO₂ concentration detected was QT77 with a value of 61.67 %. Analysis of the temporal evolution highlights two key observations. On the one hand, an increase in the CO₂ concentration base level in QT61, QT62, QT63 has been detected since mid 2024. On the other hand, a pronounced peak in one of the sampling sites (QT61) was recorded in September 2024, reaching 49.37% compared with the baseline mean of 24.06%.

In contrast the temporal evolution of dissolved H₂ concentration is distinct from that observed for CO2. Time series are characterized by an almost absence of dissolved H₂ (values below the detection limit) sporadically interrupted by significant peaks reaching variable concentrations over time. This behaviour may be associated with micro fracturing induced by increased stress in the island due to seismicity and/or ground deformation. There is a particularly interesting period covering a span of six months from December 2022 to June 2023 when almost simultaneous H₂ peaks in QT61, QT62, QT72 and QT77 occurred.