The present paper utilizes a multi-step modeling approach and in situ observations to (a) assess the washout (removal) of N compounds from contaminated floodplain water bodies (floodplain lakes and quarries whose origin is related to the large-scale mining of nonmetallic building materials in the floodplain zone) during flood events of various frequencies and magnitudes, (b) determine the rate of dispersal of the contaminant plume along the river, and (c) assess the magnitude of contamination that reaches the intake for the domestic water supply.