Microbiological condition of urban groundwater in the vicinity of leaky sewer systems

The possible deterioration of the microbiological quality of groundwater due to leaky sewer systems was investigated in a medium-sized city (Rastatt) in SW-Germany. Specially constructed focus observation wells drilled close to leaky sewers have been compared with the background observation wells in urban and rural areas. The sampling was performed in March (shallow depth of groundwater) and in July and October (deep groundwater table). It included the measurement of ammonium ions, temperature, electric conductivity, oxygen, and the depth of the groundwater table. Coliforms, Escherichia coli, and enterococci were enumerated as indicator organisms for wastewater contamination. In addition, the total number of colony forming units was determined. Bacterial counts of coliforms and E coli generally increased during the course of the investigation (March to October). This may have been caused either by a slightly increased groundwater temperature (higher activity rates) or by a higher proportion of sewage in the urban groundwater system as a result of the dry season (summer). The contamination with faecal indicator bacteria was greater adjacent to leakage locations. In addition, elevated concentrations of ammonium ions were observed at these areas. The results provide evidence that leaky sewer systems elevate faecal indicator concentrations in groundwater with associated potential health risk where such waters are used for potable water supply.