Fluvial geochemistry in São Miguel Island (Azores, Portugal): source and fluxes of inorganic solutes in an active volcanic environment

Evaluating the Impact of Explosive Volcanic Eruptions on a Groundwater-Fed Water Supply System: An Exploratory Study in Ponta Delgada, São Miguel (Azores, Portugal)

Tephra fall is among the set of hazardous phenomena associated with volcanic activity that can impact water resources and services. The aim of this paper is to characterize the potential impacts of tephra fall on the groundwater-fed water supply system of Ponta Delgada (São Miguel, Azores) by comparing two scenarios of explosive eruptions. Vulnerability matrices were used to compute indexes, by multiplying the thickness of tephra fall deposits, corresponding to increasing hazard levels, by descriptors of the water supply system, representing the elements at risk. In a worst-case scenario, tephra covers a large area inland, severely constraining the abstraction of water to public supply, as 84.8% of the springs are affected. In 12 springs the expected hazard level is of perturbation (5157 m3/day; 12.1% of the daily abstraction) or damage (16,094 m3/day; 37.8%). The same trend is observed considering the storage capacity, as 75.4% of the reservoirs may be somehow affected. Moreover, 72.4% of the 68,392 inhabitants served by the water supply system will be in a zone where the damage level will be achieved. Results point out to the need of preparedness measures to mitigate consequences of a volcanic crisis over the water supply.

Evolution of the hydro-ecological environment and its natural and anthropogenic causes during 1985-2019 in the Nenjiang River basin

Lakes and vegetation are important components of the hydro-ecological environment and sensitive indicators of anthropogenic climate change and human activities. For the first time, the lake area, vegetation cover change (obtained from Landsat satellite images), terrestrial water storage (TWS; estimated from the gravity recovery and climate experiment), and hydrochemistry, stable isotopes of different waterbodies were combined to analyze the long-term evolution and controlling factors of lake area and vegetation cover in the Nenjiang River basin, China. Between 1985 and 1999, the lake area experienced natural shrinkage due to climatic and hydrological changes, whereas artificial shrinkage related to increased human activities dominated during 2000-2019. The normalized difference vegetation index (NDVI) increased during 1988-1999 and was affected by hydrometeorological factors in the same year and lake area in the previous year. The rate of increase in the NDVI accelerated during 2000-2012 owing to the impact of human activities which is reflected by GDP, and the related artificial shrinkage of the lake area. The main hydrochemical type of lake water changed from Ca-HCO₃ before 2000, which was similar to that of river water, to Na-HCO₃ after 2000, which was the main hydrochemical type of groundwater. In addition, most of the lake water and groundwater samples were rich in H₂SiO₃, and a strong correlation between the TWS and lake area for the period 2003-2019 indicated that lake water was strongly affected by groundwater, which was related to deep circulation. Therefore, the decreased river water recharge owing to human activities made groundwater contribute more to lake water. This study can substantially improve the decision-making support for environmental protection and water resource management in ecologically diverse areas, specifically in arid and semi-arid areas.

Spring geochemistry in an active volcanic environment (São Miguel, Azores): source and fluxes of inorganic solutes

Mineral waters were monthly sampled in selected springs from Furnas and Fogo trachytic central volcanoes (São Miguel, Azores, Portugal). Water temperatures between 15.1 °C and 90.2 °C, characterize poorly mineralized waters of Na-HCO₃ and Na-HCO₃-Cl types. According to the spring location, two watersheds were selected in each volcano to evaluate solute fluxes and chemical weathering rates (Fogo volcano: Ribeira Grande river - RRG; Furnas volcano: Ribeira Quente river - RRQ). Na, Mg, K and Ca fluxes in groundwater represented 43%, 60%, 46% and 57% of the total (subsurface plus surface fluxes) in RRG, and respectively 43%, 53%, 46% and 49% in RRQ. Average HCO₃ flux is ten times higher in RRQ (130.1 × 10(6) mol/yr) compared to RRG (13.8 × 10(6) mol/yr), reflecting the volcano degassing and the lower ratio between groundwater and river water fluxes. Based on these values, total CO₂-consumption by weathering ranged from 1.5 × 10(6)mol/km(2)/yr (RRG) to 4 × 10(6)mol/km(2)/yr (RRQ). TDS load varied between 3772 t/yr (RRG) and 15388 t/yr (RRQ), and the ratio between values in groundwater and in surface water is respectively equal to 0.72 and 2.04. The associated chemical weathering rates in groundwater were 206 t/km(2)/yr (RRG) and 399 t/km(2)/yr (RRQ). Coupled to river water, these values indicate that total chemical weathering rates are respectively equal to 493 t/km(2)/yr and 594 t/km(2)/yr. A similar approach developed for the entire archipelago showed that the chemical weathering rates due to groundwater are in the range from 33 to 321 t/km(2)/yr, being partially controlled by the age of the islands. Results of the present study point out to the need to consider groundwater solute fluxes due to weathering when establishing geochemical budgets.