『Abstract
　This paper is the result of a study which was carried out in order to verify if the traditional methods to evaluate the intrinsic vulnerability or vulnerability related parameters, are able to clarify the problem of nitrate pollution in groundwater. In particular, the aim was to evaluate limitations and problems connected to aquifer vulnerability methods applied to nitrate contamination precision in groundwater. The investigation was carried out by comparing NO3^- concentrations, measured in March and November 2004 in the shallow aquifer, and the vulnerability classes, obtained by using GOD and TOT methods. Moreover, it deals with a comparison between NO3^- concentrations and single parameters (depth to water table, land use and nitrogen input). The study area is the plain sector of Piemonte (Northern Italy), where an unconfined aquifer nitrate contamination exists. In this area the anthropogenic presence is remarkable and the input of N-fertilizers and zootechnical effluents to the soil cause a growing amount of nitrates in groundwater. This approach, used in a large area (about 10,000 km²) and in several monitoring wells (about 500), allowed to compare the efficiency of different vulnerability methods and to verify the importance of every parameter on the nitrate concentrations in the aquifer. Furthermore it allowed to obtain interesting correlations in different hydrogeological situations. Correlations between depth to water table, land use and nitrogen input to the soil with nitrate concentrations in groundwater show unclear situations: in fact these comparisons describe the phenomenon trend and highlight the maximum nitrate concentrations for each circumstance but often show wide ranges of possible nitrate concentrations. The same situation could be observed by comparing vulnerability indexes and nitrate concentrations in groundwater . These results suggest that neither single parameters nor vulnerability methods (GOD and TOT) are able to describe individually the complex phenomena affecting nitrate concentrations in soil, subsoil and groundwater. In particular, the traditional methods for vulnerability analysis do not analyze physical processes in aquifers, such as denitrification and nitrate dilution. According to a recent study in the shallow unconfined aquifer of the Piemonte plain, dilution can be considered as the main cause for nitrate attenuation in groundwater.

Keywords: Nitrate; Shallow aquifer; Intrinsic vulnerability; Dilution; Denitrification; Depth to water table; Land use; Nitrogen input; Italy』

Introduction
Literature review
　Correlation between nitrate concentrations and several vulnerability related parameters
　Processes producing nitrate attenuation in groundwater
　　The denitrification process
　　The dilution process
Study area description
　Hydrogeological setting of the Piemonte plain
　Potentiometric surface reconstruction of the shallow unconfined aquifer
　Intrinsic vulnerability of the shallow unconfined aquifer
　Nitrate contamination in the shallow unconfined aquifer
Methodology of study
Comparison between nitrate concentrations and the parameters which affect the intrinsic vulnerability
Comparison between nitrate concentrations and factors affecting the pollution load
　Land use versus nitrate concentrations
　Nitrogen inputs to the soil versus nitrate concentrations
Comparison between nitrate concentrations and the shallow aquifer vulnerability
　GOD index versus nitrate concentrations
　TOT versus nitrate concentrations
Investigation about the role of nitrate attenuation processes in the study area
　The denitrification phenomenon in a sample area of the Piemonte plain
　Preliminary results about the dilution phenomenon in sample areas of the Piemonte plain
　　Evaluation of volumetric flow rate per unit perpendicular to the flow direction (qu) to assess the dilution process
　　Correlation between qu and nitrate concentration in output from the cell (Cu) in sample areas
Results
References