Nitrate source identification in groundwater of multiple land-use areas by combining isotopes and multivariate statistical analysis: A case study of Asopos basin (Central Greece)
Graphical abstract
Introduction
Groundwater is one of the most important resources throughout the world, and is vital to development since around 2 billion people depend on groundwater for potable water (Morris et al., 2003). Over-exploitation of aquifers and human activities, e.g., agriculture, industries, environment has resulted in a decrease of groundwater tables, causing aquifers' depletion, and water quality degradation, respectively (Fetter, 2008, Matiatos and Alexopoulos, 2011).
Nitrate (NO3) is one of the most widely spread contaminants in groundwater resources primarily as a result of agricultural activities utilizing N-containing fertilizers, the disposal of sewage by centralized and individual systems, animal breeding operations and elevated atmospheric deposition (Bordeleau et al., 2008). Moreover, when the water infiltrates into the subsurface, biogeochemical processes (e.g., nitrification) can modify nitrogen species balance such that different forms of nitrogen (NO2, NH4, NH3) can be transformed into nitrate. In Greece, the application of fertilizers (e.g., NH4NO3, (NH4)2SO4) in cultivated areas (e.g., vineyard, cotton, tobacco, citrus), cattle breeding, industrial activities (e.g., cheese maker units, tanneries) and disposal of untreated domestic wastewater into rivers is considered the most important source of nitrate contamination to groundwater (EASAC–European Academies Science Advisory Council, 2010, Matiatos and Evelpidou, 2013, Matiatos and Papadopoulou, 2012).
Nitrate sources show different isotopic “fingerprints” of nitrogen (15N/14N) and oxygen (18O/16O) isotope ratios. Hence, the determination of δ15N–NO3 and δ18O–NO3 in water samples can provide meaningful insight for the identification of nitrate origins into water resources (Chang et al., 2002, Deutsch et al., 2006, Kaown et al., 2009, Kaushal et al., 2006, Mayer et al., 2002, Palau et al., 2010, Popescu et al., 2015, Urresti–Estala et al., 2015, Wankel et al., 2009, Wassenaar, 1995, Widory et al., 2013, Zhang et al., 2014) and the assessment of beneficial biogeochemical processes undergoing in the subsurface (Aravena and Robertson, 1998, Battaglin et al., 2001, Baulch et al., 2011, Böttcher et al., 1990, Clague et al., 2015, Fukada et al., 2003, Itoh et al., 2011, Lehmann et al., 2004, Zhi et al., 2015).
Nitrogen compounds exhibit a wide range of oxidation numbers, i.e. from + 5 (NO3−) to − 3 (NH4+), resulting in a wide range of isotopic compositions of different natural and anthropogenic nitrate sources. Hence, the δ15N–NO3 compositions of most nitrate sources fall between − 10‰ and + 25‰ (Kendall, 1998).
From a hydrochemical point of view, many different methodologies have been applied to study, evaluate and characterize the geochemistry of groundwater. These methods include multivariate statistical techniques like Principal Component Analysis (PCA) for unbiased methods for the analysis of water quality data (Liu et al., 2003, Matiatos et al., 2014, Shamsudduha et al., 2008, Wang et al., 2007, Xue et al., 2015).
Regarding nitrate source tracing, isotope mass-balance mixing models based on δ15N–NO3 and δ18O–NO3 approach have been successfully used to quantify NO3 sources (Deutsch et al., 2006, Phillips and Koch, 2002). Voss et al. (2006) used a mass-balance mixing model to quantify three major NO3 source contributions into 12 Baltic rivers, which were sewage, atmospheric deposition and pristine soils. Xue et al. (2012) implemented a Bayesian isotope mixing model to determine the proportional contribution of nitrate source to six different surface waters in Belgium affected by agriculture, greenhouses and households (Lee et al., 2008, Mayer et al., 2002, Pardo et al., 2004).
The objective of this study was to identify sources of nitrate in the groundwater of a heavily polluted area based on combining a Bayesian isotope mixing model to estimate the multiple NO3 source contributions in the area and a hydrochemical PCA analysis, with the aim to distinguish the factors controlling the nitrate content of the groundwater. The study aims to highlight that using isotopic ratios of nitrate coupled with conventional aquatic assessment approaches can lead to more reliable nitrate source identification in groundwater and help better inform remediation efforts.
Section snippets
Physiographic setting
The Asopos basin, covering approximately 680 km2, is located in Central Greece, and the Asopos River flows in a west to east direction (Fig. 1). The river crosses several towns, e.g., Oinofyta, Oropos, before discharging into the South Euboea Gulf. Mean annual precipitation reaches 400 mm in Oropos region, and exceeds 1000 mm at Parnitha Mt. which corresponds to the southern border of the study area (Dounas et al., 1978, Giannoulopoulos, 2008).
The Asopos basin is characterized by different
Sampling — analytical data
Hydrochemical data for groundwater were obtained from field campaigns carried out between November 2007 and February 2008 by the National Centre of Sustainable Development — Institute of Geology and Mineral Exploration (Giannoulopoulos, 2008) (Fig. 1). These campaigns provided abundant hydrochemical information and data, and helped in understanding the hydrochemical characteristics of the groundwater bodies in the area. These data were processed through multivariate statistical analysis and
Hydrochemical data
The nitrate concentrations determined in the groundwater samples of the study area during the sampling campaign of 2013 are summarized in Table 1. The nitrate values ranged between 3.2 and 124.3 mg/L, except sampling point A5 which reached 205.1 mg/L. Nitrate exhibited a high dispersion, implying a spatial variation, as indicated by the high standard deviation value, which reflected a different intensity of water degradation in terms of nitrate concentration. In natural systems, the background
Conclusions
Groundwater water quality characteristics in a nitrate contaminated area in Greece were examined with the aim to better identify sources and transformation processes of NO3− in groundwater, and the relationship with land-use. The comparison of data obtained in 2013 with historical geochemical data revealed that the nitrate contamination remains high over the years with an increasing time trend in the south-western part of the area. Based on the nitrogen isotope data, the industrial and urban
Acknowledgments
This work was financially supported by a STSM Grant (Reference Code: 12682) from the COST Action ES0806 SIBAE. I thank Prof. P. Boeckx at the University of Ghent (Belgium) and his Laboratory staff for training on the nitrate isotope measurements. Gratitude to Prof. P. Nicolopoulou-Stamati at the Medical School of the University of Athens (Greece) for support and to Dr. V. Paraskevopoulou, at the University of Athens for providing the nitrate measurements. Thanks to L. I. Wassenaar and the
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