Areal identification of groundwater nitrate contamination sources in periurban areas

Background, Aims and Scope  The purpose of this study is to verify a method for groundwater contamination risk assessment in urban and periurban areas using Geographic Information Systems (GIS). The method is based on agricultural hazard evaluation. In urban and periurban areas, the problem of dealing with NO₃ concentrations mainly entails identifying nitrate sources from the excessive use of fertilizers, or leaks from the sewage network and old septic systems. Residential areas coexisting with farming, and high and increasing nitrate concentrations, were identified in a densely inhabited area (>2 million inhabitants in about 1,000 km²) with an alluvial aquifer, located in southern Italy. Methods  First of all, comparison between the Contamination Vulnerability Map and the distribution of NO₃ concentrations highlights anomalous areas with low or moderate vulnerability in spite of the diffuse nitrate contamination of the aquifer. Assuming the agricultural origin of the nitrates, the hazard is calculated using the ANHI (Agricultural Nitrate Hazard Index), a parametric index which assesses the potential hazard of nitrate contamination originating from agriculture on a regional scale (Padovani and Trevisan 2002). The ANHI integrates two categories of parameters: the hazard factors (HF), which represent all farming activities that cause, or might cause, an impact on soil quality in terms of nitrate (use of fertilizers, application of livestock and poultry manure, food industry wastewater and urban sludge), and the control factors (CF) which adapt the hazard factors to the characteristics of the site (geographical location, climatic conditions and agronomic practices). Organization, processing and mapping are performed using a Geographical Information System (GIS: ILWIS 3.3 and ARCGIS 9.1). Results  The ANHI Map is obtained by multiplying the hazard factors (HF) and the control factors (CF), and dividing the resulting values into 6 classes. Finally, the Potential risk Map (R) is obtained by coupling the potential hazard of nitrate pollution (ANHI) and the aquifer Contamination Vulnerability Map. Discussion  By using spatial statistics, the Potential Agricultural Nitrate Contamination Risk Map and the Nitrate content of the aquifer correlation can be correlated. Where areas with low correlation in the Land Use Map are in the ‘urban areas and artificial land’ class, the source of the groundwater nitrate is not necessarily related to intensive farming and livestock. Conclusions  The Potential Agricultural Nitrate Contamination Risk Map in urban and periurban environments indicates the relationship between the high nitrate values and land use, giving useful information about the source of nitrate in groundwater. Recommendations and Perspectives  Based on this study, we formulate a hypothesis on the source of groundwater nitrate which should be verified on several case studies using nitrogen isotope techniques.