Stormwater infiltration systems (SISs) are a source of pollution that may have adverse ecological and sanitary impacts. The incidence of a SIS on the coalescence of microbial communities from runoff waters and aboveground sediments with those of an aquifer was investigated. Aquifer waters showed lower coalescence with aboveground bacterial taxa than aquifer biofilms. These biofilms were colonized by bacterial hydrocarbon degraders and harboured undesirable human-opportunistic pathogens.