Urban groundwater age modeling under unconfined condition – Impact of underground structures on groundwater age: Evidence of a piston effect

Abstract : In this paper, underground structures are shown to have a major influence on the groundwater mean age distribution described as a dispersive piston effect. Urban underground development does not occur with- out impacts on subsoil resources. In particular, groundwater resources can be vulnerable and generate disturbances when this space is exploited. Groundwater age spatial distribution data are fundamental for resource management as it can provide operational sustainability indicators. However, the application of groundwater age modeling is neglected regarding the potential effect of underground structures in urban areas. A three dimensional modeling approach was conducted to quantify the impact of two under- ground structures: (1) an impervious structure and (2) a draining structure. Both structures are shown to cause significant mixing processes occurring between shallow and deeper aquifers. The design technique used for draining structures is shown to have the greatest impact, generating a decrease in mean age of more than 80% under the structure. Groundwater age modeling is shown to be relevant for highlighting the role played by underground structures in advective–dispersive flows in urban areas.