Understanding Freshwater Quality Problems in a Changing World Proceedings of H04, IAHS-IAPSO-IASPEI Assembly, Gothenburg, Sweden, July 2013 (IAHS Publ. 361, 2013). Copyright  2013 IAHS Press 183 Assessment of groundwater quality contamination by nitrate leaching using multivariate statistics and Geographic Information Systems IOANNIS MATIATOS & NIKI EVELPIDOU Faculty of Geology and Geoenvironment, National and Kapodistrian University of Athens, 15784 Panepistimiopolis, Athens, Greece imatiat@gmail.com Abstract The present study examines nitrate contamination and groundwater quality in the Megara basin of Attica Prefecture (Greece). Hydrochemical data were assessed using descriptive and multivariate statistical analysis to: (1) classify the data into hydrochemically similar groups, and (2) to investigate geochemical and human-related factors responsible for the observed groundwater quality. Geographic Information Systems (GIS) were used to incorporate both thematic (land-use) data and groundwater chemistry to study the extent and variation of nitrate contamination and to establish spatial relationships with specific land-use types. The results indicate that more than 70% of the groundwater samples located around the national highway had nitrate concentrations that exceeded acceptable levels according to international legislation and guidelines (Directive 98/83/EC, EPA, WHO). The combined spatial analysis and statistical hydrochemical evaluation show that nitrate contamination in groundwater is closely associated with specific land-use classes and activities (e.g. agriculture, pasture, industries, urban effluents). Key words groundwater; nitrate leaching; multivariate statistical analysis; GIS analysis INTRODUCTION Nitrate (NO 3 ) occurs naturally as part of the nitrogen cycle but increasing concentrations in groundwater resources have resulted primarily from agricultural activities (Lord & Anthony, 2002; Rankinen et al., 2007). Other significant and widely spread anthropogenic non-point (diffuse) sources of groundwater contamination with nitrogen are the disposal of sewage by centralized and individual systems, leaking sewers, animal feeding operations and elevated atmospheric N deposition (Fraters et al., 1998; Salo & Turtola, 2006). In the Shandong province of the North China Plain, the nitrate concentration due to anthropogenic contamination has increased by 81.68% on average between 2002 and 2007, resulting in severe water quality degradation (Chen et al., 2010). The investigation of nitrate contamination in the water systems of the cities of Metro Manila, Bangkok and Jakarta showed that human waste via severe sewer leakage is the major source of nitrate leaching and groundwater nitrate enrichment (Umezawa et al., 2008). Given that consumption of water with high concentrations of nitrate can pose risks to human health, e.g. infant methemoglobinemia, human gastric cancer (Cuello, 1976; Fraser, 1979), organizations such as the World Health Organization (WHO) and governmental agencies, such as the US Environmental Protection Agency (EPA), have established quality standards for water resources and developed regulations and action guidelines for the use of water in the consumption sector (EU, 1998; WHO, 2004, EPA, 2009). In particular, WHO and European Union (EU) standards for drinking water quality are 50 mg/L NO 3 , whereas EPA set a maximum 10 mg/L NO 3 as an acceptable concentration level of nitrate for potable water. Multivariate Statistics and Geographic Information System (GIS) technology have been useful in determining nitrate contamination spatial variability as well as assessing the effects of land-use in water quality in terms of nitrate contamination (Babiker, et al., 2004; Liu et al., 2005). The objective of the present contribution is to determine the spatial distribution of nitrates together with other hydrochemical parameters in groundwaters of the Megara basin (Greece) via Cluster Analysis (CA) and to analyse its relationship with land use via GIS analysis, and hence to urge policy makers to develop strategies to protect groundwater resources.