Microorganisms as contaminants and natural tracers: a 10-year research in some carbonate aquifers (southern Italy)

Limestone aquifers provide the main drinking water resources of southern Italy. Due to cattle grazing and/or manure spreading, these aquifers are often characterized by microbial contamination of groundwater. The aim of this paper is to summarize the results obtained during a 10-year research carried out in experimental field sites in southern Italy, analyzing (1) the influence of the topsoil of pyroclastic origin on the migration of microbial cells from the ground towards the groundwater, and then on the groundwater vulnerability, (2) the influence of the rainfall regime on the breakthrough at the springs, (3) the reliability of thermotolerant coliforms and fecal enterococci as bacterial indicators of microbial contamination, and (4) the effectiveness of microorganisms as natural tracers for some hydrogeological purposes. The results obtained showed that fecal enterococci are a more reliable indicator than thermotolerant coliforms for detecting contamination and that the entity and distribution over time of microbial contamination of fecal origin are influenced by several factors, such as precipitation regime, thermal regime and existence and thickness of the topsoil of pyroclastic origin. Moreover, the migration of a significant amount of bacterial cells through the topsoil and the underlying carbonate rocks allows the utilization of microorganisms as effective natural tracers, to be coupled with other classic tracers to study the recharge and the flow processes. In a broader perspective, these results can be used to optimize the investigations in other hydrogeological scenarios, with emphasis on those where different water types coexist and interact in same aquifer systems.