INFLUENCE OF THE LOW VELOCITY AREAS ON THE CONTAMINANTS PERSISTENCE: A FIELD STUDY (MILAN, ITALY)

The city of Milan, and generally the Functional Urban Area, is the location of many contaminated sites. The urban soils and aquifer are interested both by local pollution hotspots and by multiple-sources diffusive contamination where identification is necessary for optimal environmental management. Moreover some inner areas in the city of Milan show a persistence presence of contamination due to chlorinated solvents and chrome. The quality is significantly influenced by many parameters such as the piezometric fluctuations and the change of the groundwater direction flow. Recently, Alberti et al. (2014), have underlined the possibility that in the some contaminated areas a presence of a high concentration can be due to a lower velocity respect to the proximity zones. This situation prevents a total renewal of the groundwater quality. The studied domain of this work, where a huge number of contaminated sites are investigated, is located on the north-eastern portion of the Milan Province. In this region, hydro-chemical concentrations data and piezometric levels constitute a suitable population in order to implement a numerical model in transient state. The results will be a useful toolbox for a comparison between the transient velocity and the change pollution concentration during time; moreover, the individuation of hotspots and diffusive regions will be evaluated and investigated. Therefore this paper deals with a Nord-Eastern portion of the Milan Province, where a huge number of hydrochemical and piezometric data (spatial and in time domain) are available in order to implement a numerical transient model. The analysis allows with more detail to compare the velocity variation in time in different areas with the pollutants concentration, in order to verify the hypothesis in the previous work. The possible contaminants sources will be considered active during time with a transient concentration which will be compared with a piezometric map calibrated in transient state. The numerical model now allows to adequately evaluate the 3D local piezometric gradients, due to the significant changes in piezometric head observed in recent years and the model results show the low velocity areas which are in high correlation with the higher concentrations of contaminants.