Bacteriological indicators of faecal contamination: result of a loading experiment with untreated urban wastewater

Some observations were made on the behaviour of total coliforms, faecal coliforms, enterococci, numbers of aerobic bacteria, salmonellas and sulphur-reducing clostridia as bacterial indicators of faecal contamination of groundwater. A controlled irrigation experiment was carried out with untreated residual water in the alluvial aquifer of the Vega of Granada (Spain). The results obtained confirm the value of these parameters are useful indicators of very recent faecal contamination; and changes were detected as the level of the freatic layer increased and the chemical composition of the groundwater changed. These groups of micro-organisms persisted for about 200 h, with the exception of the aerobes which survived for much longer. Salmonellas were present at levels too low to calculate the extent of faecal contamination and sulphur-reducing clostridia were not detected. The results obtained show that irrigation with untreated wastewater offers a lower risk of microbiological contamination of groundwater compared with the direct addition of waters decanted and/or previously filtered.