BACTERIOLOGICAL INDICATORS OF FECAL CONTAMINATION - RESULT OF A LOADING EXPERIMENT WITH UNTREATED URBAN WASTE-WATER

Some observations were made on the behaviour of total coliforms, faeca l coliforms, enterococci, numbers of aerobic bacteria, salmonellas and sulphur-reducing clostridia as bacterial indicators of faecal contami nation 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 pa rameters as 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 a erobes which survived for much longer. Salmonellas were present at lev els too low to calculate the extent of faecal contamination and sulphu r-reducing clostridia were not detected. The results obtained show tha t irrigation with untreated wastewater offers a lower risk of microbio logical contamination of groundwater compared with the direct addition of waters decanted and/or previously filtered.