Groundwater composition in perched-water bodies in the north flank of Fogo volcano (São Miguel, Azores): main causes and comparison with river water chemistry

Abstract

A set of 46 springs located in the north flank of Fogo volcano (São Miguel, Azores) were sampled to characterize groundwater chemistry. The majority of the discharge from volcanic rocks with acidic character (95 %), at an elevation range of 200-400 m a.s.l., and the average discharge are usually not higher than 2 L/s. Nevertheless, the discharge in a few springs from trachytic lava flows can be higher (21.3 L/s). A group of 12 surface water samples were also collected twice along the Ribeira Grande River. Groundwater average temperature ranges between 19.6 and 20.7°C, and it is possible to depict two groups of discharges: thermal waters (16.7 %), with a temperature range from 27.8 to 71.2°C, and cold waters, with temperature under 20°C. In this latter group, temperature is inversely proportional to elevation. The average pH and conductivity range, respectively, between 5.7 and 7.8 and 291-425 lS/cm. Nevertheless, higher conductivity values were measured in boiling pools. Groundwater mainly belongs to the Na-HCO 3 to Na-Cl types. The major-ion relative composition of the cold springs shows a decreasing order for cations as Na [ Ca [ K [ Mg and for anions HCO 3 [ Cl [ SO 4 . In thermal discharges the relative composition is, respectively, Na [ Ca [ K [ Mg and SO 4 [ HCO 3 [ Cl. The average nitrate content ranges from 2.09 to 2.57 mg/L comparing all the sampling periods, and the cumulative probability plot shows that about 75 % of the samples present values lower than 1.98 mg/L. Results show clearly that the higher nitrate measurements are observed in river waters and springs sampled in areas where pasture lands are dominant.

Nitrates in Groundwater Discharges from the Azores Archipelago: Occurrence and Fluxes to Coastal Waters

“…Water temperature depicts an inverse relationship regarding spring elevation (r = −0.751), suggesting the influence of the colder air temperature observed at higher altitude in the islands ( Figure 4A). A similar pattern is observed in other volcanoes worldwide [53] as well as in detailed studies carried out in specific central volcanoes in the Azores [54]. The mean pH values range between 6.1 and 8 (mean = 7.1), showing that waters range from slightly acid to slightly basic.…”

“…Instead, a low correlation coefficient is computed for HCO 3 (r = 0.502; Figure 6G). Presented data points to the main drivers of the groundwater chemistry in the Azores already identified in previous studies [43,54,56], namely: (1) a marine source, through seawater spraying in springs and seawater intrusion in a few wells; and (2) identified in previous studies [43,54,56], namely: (1) a marine source, through seawater spraying in springs and seawater intrusion in a few wells; and (2) silicate dissolution. Table S1.…”

“…This low overall mineralization is common in springs located in perched-water aquifers in the archipelago [54,55]; an inverse relationship is presented by water conductivity and spring elevation (r = −0.566) which points out that discharges located at higher altitudes correspond to groundwater with a low residence time ( Figure 4B). …”

“…Previous research on river water chemistry in the archipelago has shown that the higher NO 3 contents are associated to areas where pasture lands are dominant [54], and despite the contribution of urban wastewater point sources, agriculture non-point sources are also critical for surface water pollution [42].…”

The Archaeology of a Landslide: Unravelling the Azores Earthquake Disaster of 1522 and its Consequences

“…pyroclastic flow, pumice fall), an active spring, and fault near its base. The southern flanks of the Fogo volcano have many springs fed by perched aquifers confined by impervious geological layers (such as ash fall; Cabral et al, 2015). These geological structures provide suitable conduits and storage for groundwater, which may have enhanced instability in the hillslope and transformed it into a low cohesion 'debris flow', as defined by Varnes (1978).…”