Groundwater discharges were sampled in selected springs from São Miguel (Furnas and Fogo trachytic central volcanoes) and Santa Maria islands (Azores, Portugal), in order to characterize natural background levels (NBLs) and proceed to the determination of threshold values (TVs). Besides being a key issue in order to fully assess the anthropogenic pressures, NBLs are also instrumental to derive TVs, therefore complying with requirements from the European Union Groundwater Directive. The composition of groundwater corresponds mainly to low mineralized Na-HCO3 to Na-Cl water types, the latter dominant in Santa Maria island, with a decreasing order of Na>Ca>Mg>K and Cl>HCO3>SO4>NO3 for cations and anion respectively. The majority of the samples are slightly acid to slightly alkaline (pH range of 5.45-7.43), and the electrical conductivity range between 180 and 1458 μS/cm. Groundwater composition is controlled by two major drivers, addition of sea salts and dissolution of silicate minerals. Results shown that TVs established along the present study are in general in the lower rank when compared to the range of values proposed by the several EU member states, with the main exception of NO3, reflecting the impact of agriculture activities over water quality in the Azores, and lower than the national ones. The comparison between the estimated NBL and TV with values derived with another dataset from the Azores, usually higher, depicts the effect of a larger and diverse number of groundwater sources over calculations. On the other hand, all samples which show a contribution from volcanic/hydrothermal systems were excluded from the dataset, which explains why the derived NBLs and TVs are lower comparing to other active volcanic areas, which is also a conservative approach on a subject that has regulatory implications.
Keywords: Azores; Groundwater chemistry; Natural background levels; Threshold values; Volcanic aquifers.
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