Exceptional floods and water quality

Degradation of surface and groundwater quality, caused by spreading of pollutants during exceptional flood events is getting serious concern. The ongoing changes in land use, is leading to a gradual increase of flood prone areas as well as flow volume and velocity. Inundated water breaks loose the surface deposits, soil minerals and transports them downstream into surface depressions, lakes and streams as NPS (Non Point Source) pollutants. Since exceptional flood are rare and unpredictable (in time and space) planning a program for collection of field data during and after exceptional flood is difficult, which makes the long-term impact on the groundwater quality difficult to evaluate. Problems related to surface or groundwater quality can be approached either by evaluating protective norms when the recipients receive emissions as close as possible to non point source; or more generally a risk oriented predictive approach. A properly planned protective approach in local flood prone areas appears to be sufficient to control contaminants transport into recipients during local floods but is ineffective for the exceptional flood events. The actual need for risk-oriented approach in water quality assessment for exceptional flood events is associated with estimation of contaminant sources, their geographical location and tendency to accommodate in runoff.