"The main aim of this study was to analyze the influence. of rainwater pH on retention of Escherichia coli cells and Bacillus subtilis spores in a pyroclastic soil. Nine column experiments were performed in intact soil blocks. The experiments were carried out. using antibiotic resistant strains, to be distinguished from the natural background of soil. Column tests were performed at three different. solution pH (1.5, 5.6, 8.0). The pH range was chosen taking also into consideration pH values measured in rainwater samples collected at the site were soil blocks were carved. Some physical and hydraulic. properties of soil were analyzed by means of standard laboratory tests. Despite the various experimental conditions, comparable results. were obtained in all tests. It was observed that retention of both E. coli cells and B. subtilis spores is independent from rainwater pH. This phenomenon is due to a high buffering capacity of the studied soil that inhibits pH-dependent processes that influence microbial transport. The findings presented here have applications within the. risk assessment for microbial pollution of groundwater and have the advantage of showing that variations in rainwater pH over time do not cause significant modifications in self-purification processes. within the analyzed soil."