Natural background groundwater composition in the Azores archipelago (Portugal): A hydrogeochemical study and threshold value determination

doi:10.1016/j.scitotenv.2015.03.057

Science of The Total Environment

Volume 520, 1 July 2015, Pages 127-135

https://doi.org/10.1016/j.scitotenv.2015.03.057 Get rights and content

Highlights

•
Groundwater NBL and TV are derived for the first time in the Azores (Portugal).
•
Groundwater corresponds mainly to low mineralized Na-HCO3 to Na-Cl water types.
•
Addition of sea salts and dissolution of silicate minerals influences chemistry.
•
Two dataset were compared using a diverse number of groundwater sources.
•
NBL and TV should be derived with samples with similar hydrogeological setting.

Abstract

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.

Introduction

The determination of the pristine composition of groundwater is a key issue in order to fully assess any modifications that may occur during flow in the subsoil, namely the effects of an anthropogenic pressure, therefore being an important issue for groundwater management. This baseline chemistry may be defined as the composition resulting from natural sources (geological, biological, atmospheric), and whenever for regulation purposes the basal concentration of a given element, species or chemical substance dissolved in groundwater is derived from the conditions observed at the beginning of monitoring, thus possibly including an anthropogenic component signature, may be defined as background composition (Edmunds et al., 2003). The usefulness of the concept of background concentration of a particular element, species or substance has been recognized for long by earth scientists, as a premise for the derivation of threshold values or allowing detection of anomalous results when exceedance occurs (Panno et al., 2006).

In European Union (EU) the so-called Groundwater Directive (GD; Directive no. 2006/118/CE), that came to force on 12th December 2006, among other requirements implies the determination of threshold values for all the pollutants that may imply that groundwater is not able to achieve a good status. As a result numerous case studies have been developed in diverse hydrogeological environments (Hinsby et al., 2008, Marandi and Karro, 2008, Wendland et al., 2008, Coetsiers et al., 2009, Radu et al., 2010, Ducci and Sellerino, 2012) and the outcome and pitfalls of the process of definition of groundwater threshold values among EU Member States were summarized by Scheidleder (2012).

The Azores Archipelago is made of nine volcanic islands located in the North Atlantic between latitudes 37° and 40°N and longitudes 25° and 31°W. The islands are spread along a NW–SE trending strip of about 600 km, and are geographically associated in three groups: western Group, which includes the islands of Flores and Corvo; central Group, that includes the islands of Graciosa, São Jorge, Terceira, Faial and Pico; and eastern Group, made by the islands of São Miguel and Santa Maria.

Being an autonomous region of the Portuguese republic, the Azores have specific authority to define its own policies for water resource management and protection. Regarding compliance toward both the EU Water Framework Directive (WFD) and the GD, the Azores is one of the Portuguese river basin districts, in which a total of 54 groundwater bodies were delimited.

The overall groundwater quality in the Azores is good, despite problems associated with agricultural diffuse pollution (Cruz et al., 2013), faecal pollution caused by a lack of appropriate sewage drainage and treatment systems (Cruz et al., 2009), and groundwater salinization through mixture with seawater (Cruz et al., 2011). The former two impact groundwater composition mainly in perched-groundwater bodies, being the pollution risk characterized by Cruz et al. (2015), while the latter occurs in coastal basal aquifers and is responsible by the poor chemical status in three groundwater bodies in Graciosa and Pico islands. Diffuse pollution is also responsible by the nutrient enrichment in surface water bodies leading to lake eutrophication (Santos et al., 2005, Ribeiro et al., 2008, Martins et al., 2008, Cruz et al., 2010a).

The objectives of the present study are (1) to characterize groundwater composition in springs from São Miguel (Fogo and Sete Cidades volcanoes) and Santa Maria islands, (2) to present the source of dissolved solutes in these waters, and (3) estimate the natural background composition and threshold values of selected major, minor and trace elements in the groundwater. The present paper depicts the first estimates of the background and threshold values for solutes in groundwater from the Azores archipelago, and results are of significance for water management in the archipelago and contribute to the compliance regarding EU WFD and GD. Additionally, data on natural background composition and threshold values in volcanic islands are scarce in the international literature, despite the fact that aquifers made of volcanic deposits are vital for water supply in numerous islands and continental areas where these materials are dominant.

São Miguel is located between 37°55′N to 37°04′N of latitude and 25°52′W to 25°08′W of longitude and is the major island of the archipelago, presenting an area of 744 km² and 138,638 inhabitants (year 2013). This island has a length of 66 km and a maximum width of 16 km. On the other hand, Santa Maria is the westernmost island of the Azores and one of the smallest islands of the archipelago (97 km²), being located between 37°01′N and 36°55′N of latitude and 25°00′W to 25°11′W of longitude and presenting 5663 (year 2013) inhabitants.

Section snippets

Geology

Volcanic activity that originated the islands ranges from more effusive to highly explosive eruptions, resulting in diverse types of volcanic deposits and landforms, and since the beginning of the Azores archipelago settlement in the 15th century about 30 eruptions occurred.

The geology of São Miguel is dominated by three active central volcanoes (Sete Cidades, Fogo and Furnas), associated with highly explosive eruptions of magmas of trachytic nature, which explain the large Quaternary calderas

Groundwater sampling and analysis

A set of 19 springs were selected across São Miguel and Santa Maria islands in order to collect periodically groundwater samples for analysis. A set of eight springs was selected in Sete Cidades Volcano, four in Fogo volcano, both in São Miguel islands, and seven springs are spread in Santa Maria island (Fig. 1). Selection was made considering the higher discharge-springs and usefulness to public supply, that restrained the search in the more important aquifers used for drinking water supply,

Data presentation

The major-ion composition data, as well as results for master variables (temperature, pH, electrical conductivity) and SiO₂ content, is shown in Table 1 (electronic supplementary material — esm). In Table 2 (esm) results for selected minor and trace elements are also shown.

Spring water is cold, as temperature range between 13.1 °C (spring LIM.03) and 18.1 °C (LIM.15), being respectively 32% and 26% of the samples in the range of 15.4–16.6 °C and 13.0–14.2° (Fig. AA). Mean pH values reveals that

Conclusions

Groundwater natural background levels (NBLs) and threshold values (TVs) are derived for the first time in the Azores (Portugal), allowing to comply with requirements from EU Groundwater Directive. The study was based on a set of 19 springs selected across São Miguel and Santa Maria islands and followed a previous step in order to exclude samples on basis of balance error, high-NO₃ content and hydrothermal influence over water chemistry. Results were compared to the ones calculated with a larger

Acknowledgements

Authors are grateful to the Azores Directorate-Regional for Environment for the funding of the present project and for the permission to handle the data for this paper.

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Natural background groundwater composition in the Azores archipelago (Portugal): A hydrogeochemical study and threshold value determination

Highlights

Abstract

Introduction

Section snippets

Geology

Groundwater sampling and analysis

Data presentation

Conclusions

Acknowledgements

J. Volcanol. Geotherm. Res.

Sci. Total Environ.

Appl. Geochem.

J. Volcanol. Geotherm. Res.

J. Volcanol. Geotherm. Res.

J. Volcanol. Geotherm. Res.

Sci. Total Environ.

J. Volcanol. Geotherm. Res.

Sci. Total Environ.

Appl. Geochem.

J. Volcanol. Geotherm. Res.

Environ. Sci. Pol.

Chemosphere

Desalination

Plano de Gestão da Região Hidrográfica dos Açores - RH9. Versão para consulta pública

Standard Methods for the Examination of Water and Wastewater

Geochemistry, Groundwater and Pollution

Does groundwater protection in Europe require new EU-wide environmental quality standards?

Front. Chem.

A quantitative study of five thousand years of volcanism on São Miguel, Azores

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Environ. Earth Sci.

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Guidance on Groundwater Status and Trend Assessment

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Estudo hidrogeológico da Ilha do Pico (Açores – Portugal)

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