Surface and groundwater in volcanic islands: water from Azores islands

Abstract

Small volcanic islands face unique challenges in the assessment and management of their hydrological systems and water resources, which distinguish them from other regions around the world. This chapter summarizes the latest knowledge of the water resources in the Azores Archipelago. Although precipitation is abundant, the Azores islands have poorly developed watersheds and few perennial streams. Most water is contained in volcanic lakes whose geomorphological characteristics are being controlled by volcano-tectonic activity. Some lakes interact with volcanic fluids. Several lakes are subject to anthropogenic pollution, mainly related to agricultural production and/or open air grazing. Lakes on the Azores are under the influence of various chemical processes: (1) sea-salt spraying effect, (2) water-rock interaction due to hydrolysis, and (3) contamination with volcanic fluids. Groundwater is a vital resource in Azores islands for human consumption, agriculture and industry. The occurrence, movement, and storage of groundwater show that heterogeneity and anisotropy directly depend on volcanic eruptions and secondary processes, such as fracturing and weathering. The total volume of groundwater resources in the Azores equals 1.6 × 109 m3/year. Groundwater occurs in two main aquifer systems: perched and basal aquifer systems. The spatial distribution of springs is very heterogeneous, with densities varying between 0.01 to 0.72 springs/km2 and median discharge values varying between 1.73 and 36.29 m3/day. 160 wells were known, located at distances between 300 and 5825 m from the coastline and had capture depths from 25 to 284 m. The specific flow rates (q) range from 1.4 × 10−2 to 266.7 L/sm, with a median value equal to 22 L/sm, and transmissivity varies between 1.43 and 34809 m2/day, with a median of 3162 m2/day. Groundwater chemical composition facies varies from sodium chloride to sodium bicarbonate waters and show low mineralization. Volcanic degasification and hydrothermal steam rising from deeper geothermal systems contribute to the contamination of shallower aquifers resulting in the occurrence of a large number of mineral and thermal waters. With the exception of those sites, associated with active faults and volcanoes, the groundwater quality in some of the Azores islands is mainly deteriorated by salt water intrusion in the coastal aquifers, inland agricultural activities, and the lack of sanitation system.