Pollution natural attenuation in karst groundwater environments

In karst areas, the contaminants with anthropogenic origin reach ultimately the subsurface, with detrimental effects on the fragile subsurface ecosystems and the downstream springs. We sampled during four seasons water from upstream and downstream locations in five hydrokarst systems (HKS), and the samples were subjected to chemical and microbiological analysis. The concentration of organic matter, considered here as pollution, decreased towards the downstream springs. The water becomes richer in oxygen underground, increasing the oxidation-reduction potential, and therefore the chances for pollution biodegradation. The microbial community composition was complex and varied largely between upstream and downstream locations. A large fraction of microorganisms remains underground, and can contribute here to pollutant biodegradation. This suggests that the HKS can act as partial filters against potentially hazardous substances and microorganisms. Water quality, including the presence of pathogens in the downstream springs, must be further monitored to ensure whether the water is appropriate for consumption. In parallel, the human populations located upstream should be advised on the risks resulting from their daily activities, improper stocking of the various wastes and dumping of their refuses in the surface streams.