Identification of the natural background levels in the Phlaegrean fields groundwater body (Southern Italy)

https://doi.org/10.1016/j.gexplo.2019.02.007Get rights and content

Highlights

  • In the examined coastal volcanic GWB, GW facies is influenced by several processes.

  • It depends on seawater intrusion, geothermal field and regional groundwater flow.

  • As probability map permits the identification of geochemical anomalies

  • As contamination is of geogenic origin.

  • In the same groundwater body, different NBL can be distinguished.

Abstract

The paper deals with the identification of Natural Background Levels (NBLs) in a groundwater body located in a volcanic area. NBLs were assessed through spatial analysis adopting both the probability plot and the pre-selection methods. They were performed on 33 sampling points, focusing on F, Fe, Mn and As ions. The approach is discussed adopting the groundwater body of Phlaegrean Fields as relevant case study. The area is an active volcanic caldera, hosting a geothermal system superimposed on the regional groundwater flow and characterized by hydrogeochemical anomalies. The results, especially referred to As, suggest a local geogenic contamination or concentrated volcano-related phenomena, as fluids upwelling along faults, in the most active portion of the caldera. The application of the probability maps generated by the indicator kriging, resulted effective to distinguish within the same groundwater body areas characterized by peculiar geochemical anomalies.

Introduction

The quality of groundwater resources is constantly threatened by the introduction of pollutants originated from urban, industrial and agricultural activities, which can be transported from soils to groundwater bodies and to their discharge areas (i.e. springs, rivers and wells). The result is the deterioration of the groundwater quality, making it unsafe for both humans and natural biological communities. Therefore, the European Commission provided several regulatory tools to protect the quality of the groundwater bodies (GWB) and to achieve the sustainability of the territory. The first EU regulation on groundwater is the Directive 80/68/EEC on the protection of groundwater against pollution by some dangerous substances, which was followed by the Water Framework Directive (WFD 2000/60/EC), the Groundwater Directive or ‘daughter directive’ (GWD 2006/118/EC) and finally by the Directive on the protection of groundwater against pollution and deterioration (2014/80/EU). The latter provides criteria for establishing National Guidelines for the determination of background levels and threshold values for all pollutants and indicators of pollution.

The “background level” corresponds to the concentration of a substance or the value of an indicator in an undisturbed groundwater body, considering the absence or only minor anthropogenic alterations (art 2.5 of the GWD 2006/118/EC). Several natural factors contribute to the definition of the Natural Background Level (NBL): climate, rainfall composition and frequency, water-gas-rock interactions and residence time. European groundwater quality standards rely on the concept of threshold value (TV). In some cases, NBLs can exceed the law limits or reference values (REF) for certain substances due to peculiar geological conditions, i.e. tectonic structures, upwelling of hydrothermal fluids, interactions with other groundwater bodies. In these cases the TVs are evaluated on the basis of the NBLs. The determination of NBL is based on the characterization of the GWB, on the results of groundwater monitoring and on the identification of the natural population within a broad chemical analysis dataset. The methods may include either a geochemical approach, in which the dataset is composed exclusively of “undisturbed” waters, not belonging to aquifers on which anthropogenic pressure exists, or data collected in an area in which such kind of pressure did not exist or was very mild (Limbrick, 2003; Chery, 2006; Muller et al., 2006; Griffioen et al., 2008). Whether the aquifer is subjected to not negligible anthropogenic pressures, or the data set has been collected in such a way that do not permit to exclude the presence of potentially contaminated samples, the statistical methods are certainly the most suitable for the purpose. These approaches aim to distinguish the natural population from the anthropogenic one by assuming a normal or log-normal distribution, (Matschullat et al., 2000; Gałuszka, 2007; Nakić et al., 2007, Nakić et al., 2010; Urresti-Estala et al., 2013) such as the component separation method (Wendland et al., 2005; Kunkel et al., 2007; Molinari et al., 2012, Molinari et al., 2014; Rotiroti et al., 2015). Conversely, some other statistical methods do not assume a typical sample distribution, such as the probability plot method (Sinclair, 1974; Edmunds et al., 2003; Lee and Helsel, 2005; Panno et al., 2006; Walter, 2008; Zabala et al., 2016) or the method of pre-selection (Muller et al., 2006; Hinsby et al., 2008; Wendland et al., 2008; Coetsiers et al., 2009; Molinari et al., 2012; Ducci et al., 2016).

As highlighted above, some peculiar hydrogeological features, i.e. reducing environments (Carraro et al., 2013), upwelling of deep groundwaters (Desiderio et al., 2010), water–rock/sediment interactions in volcanic environments (Rango et al., 2010), geothermal processes, can affect elements concentrations with remarkable differences in groundwater chemistry within a GWB. The determination of NBLs allows to distinct between anthropogenic pollution and natural origin of the contamination.

In this paper, the possibility of combining the NBLs with the statistical analysis, by means of probability maps, for the individuation and characterization of different hydrogeological features in a GWB is analysed. The study is carried out for a volcanic aquifer system, where geothermal activity can affect groundwater quality. It is based on the hydrogeological and geochemical characterization and the assessment of NBLs for F, Fe, Mn, As ions. The latter were identified using spatial analysis and statistical testing, through the Indicator kriging (IK - Journel, 1983), widely adopted for both soil (Cattle et al., 2002; Smith et al., 1993) and water quality studies (Liu et al., 2004; Stigter et al., 2005; Lee et al., 2007; Delbari et al., 2016), which permits to generate probability maps. They express for a given ion the probability that a threshold value could be exceeded in a certain area. These maps are a valuable tool as they permits: i) to verify the applicability of the REF or NBLs values, ii) to distinguish areas in which the contamination is geogenic or anthropogenic, iii) to identify sectors, where differentiated NBL should be applied when hydrogeochemical anomalies exist tool (Ducci et al., 2016; Dalla Libera et al., 2017).

Section snippets

Study area

The study area is represented by the Phlaegrean Fields (Southern Italy), it includes part of the city of Napoli, the town of Pozzuoli and numerous densely inhabited villages. The area shown in Fig. 1 is about 200 km2 wide and it is an active volcanic field formed after two relevant eruptions: the Campanian Ignimbrite (CI; ∼39 ky; De Vivo et al., 2001; Fedele et al., 2008) and the Neapolitan Yellow Tuff (NYT; ∼15 ky; Deino et al., 2004). Those two large-scale volcanic events identified the

Data

The hydrochemical dataset adopted for the study is composed of 27 sampling points coming from wells collected in previous studies of the regional monitoring network, with at least a single annual sampling (in Winter season) in the period 2003–2004 (AA.VV, 2004; Adamo et al., 2007), and 6 springs reported in Aiuppa et al. (2003).

The monitoring wells have a depth between 2 and 220 m and they always intercept the main aquifer. In the sector outside the CI caldera (Fig. 1), i.e. in the

Results

In Fig. 4, the Schoeller-Berkaloff diagram of the groundwater samples coming from both wells and springs has identified two main groundwater facies: the type “a” enriched in alkaline, is characterized by high salinity and constitutes the typical Phlaegrean groundwater (Corniello, 2009), the facies “b” is an evolution of the type “a” influenced by the seawater, which surrounds the GWB to W and S, and it is mainly enriched with chlorides. This phenomenon is also confirmed by the results of the

Discussion

At small scale, the lithological and hydrogeological setting (e.g. the uniform piezometric gradient) shows the GWB of Phlaegrean Fields as a single porous aquifer. However, at a more detailed scale the presence of an active volcanic caldera, whose boundary can be identified in the NYT caldera, creates differences within the groundwater flow. Hydrogeochemical pattern of the area is characterized by a relevant groundwater mixing, mainly consisting in geothermal fluids upwelling and seawater

Conclusions

In this paper, the hydrogeological and hydrogeochemical characterization of the Phlaegrean Fields GWB, focused on the ions of F, Fe, Mn and As, was carried out. Two different facies of groundwater were identified: one influenced by mixing with seawater (Facies “b”) and another influenced by geothermal fluids, but also other processes due to water–rock/sediment interactions cannot be neglected (Facies “a”). The assessment of the Natural Background Levels for the groundwater body was carried out

Acknowledgments

The authors are grateful to the editor and to the anonymous reviewers for their time and the useful suggestions provided for enhancing the overall quality of the paper.

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