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

Abstract

Abstract:Groundwater discharge is an important vector of chemical fluxes to the ocean environment, and as the concentration of nutrients is often higher in discharging groundwater, the deterioration of water quality in the receiving environment can be the result. The main objective of the present paper is to estimate the total NO 3 flux to coastal water bodies due to groundwater discharge in the volcanic Azores archipelago (Portugal). Therefore, 78 springs discharging from perched-water bodies have been monitored since 2003, corresponding to cold (mean = 14.9 • C) and low mineralized (47.2-583 µS/cm) groundwater from the sodium-bicarbonate to sodium-chloride water types. A set of 36 wells was also monitored, presenting groundwater with a higher mineralization. The nitrate content in springs range between 0.02 and 37.4 mg/L, and the most enriched samples are associated to the impact of agricultural activities. The total groundwater NO 3 flux to the ocean is estimated in the range of 5.23 × 10 3 to 190.6 × 10 3 mol/km 2 /a (∑ =~523 × 10 3 mol/km 2 /a), exceeding the total flux associated to surface runoff (∑ =~281 × 10 3 mol/km 2 /a). In the majority of the islands, the estimated fluxes are higher than runoff fluxes, with the exception of Pico (47.2%), Corvo (46%) and Faial (7.2%). The total N-NO 3 flux estimated in the Azores (~118.9 × 10 3 mol/km 2 /a) is in the lower range of estimates made in other volcanic islands.

Groundwater Governance in the Azores Archipelago (Portugal): Valuing and Protecting a Strategic Resource in Small Islands

“…In addition to salinization (mentioned in Section 3.1), other quality problems are observed, despite not being widespread, as nutrient and microorganism fecal pollution, mainly due to agriculture and waste water discharges [62][63][64], or local fluoride enrichments in São Miguel [65] and Terceira [66]. The latter is suggested to be closely associated to a volcanic activity source, which significantly alters the groundwater composition as shown by the numerous mineral waters spread along the archipelago [67,68].…”

“…The advantage of the scale factor toward more efficient public participation, common in other small islands where the identification of stakeholders is facilitated, has to be exploited, as well as the easier identification of societal values and local knowledge in small communities that suffer from a certain degree of isolation through centuries [64].…”