Dynamics of hydrological process in karst systems - stable isotope and geochemical approach

Mid to high-altitude karst terrains in NW Romania are the sole sources of water for the numerous rural communities that dot the area. Due to the combination between the 1) specific hydrologic functioning of karst systems, 2) contamination from agricultural activities and 3) lack of waste water treatment plants, the water used for household purposes is subject to potentially high levels of pollution, with adverse effects on health and general well-being. Water flow through fractured systems is generally thought to be less effective in removing contaminants than flow through porous media, but a limited set of studies have shown that despite the lack of mechanical and chemical cleaning of water, microbial activity could lead to attenuation of pollution. To test this hypothesis, we have devised a two stage-approach in which we first targeted the hydrological functioning of karst systems and than analyzed the potential for microbial-mediated degradation of contaminants. We have investigated five hydrokarst systems (HKS, defined as systems with a point input given by either a ponor or cave and an outlet through springs or caves) in the Apuseni Mountains (Romania, East-Central Europe), by collecting monthly samples of water for three years and analyzing their physical, chemical and microbial characteristics. We present here the results of the stable isotope, physical and chemical analyses of water and discuss the sources of water feeding the HKSs and the transfer times and nature of flow, in order to make inferences on the connectivity between the different components of the HKSs. The stable isotope data indicates a rapid transfer of water through the conduits, with low underground retention times. Chemical and stable isotope data reveals a high degree of mixing between waters that enters the karst systems through pint inputs with diffuse infiltration. The different systems behave differently in terms of interaction with the host rock, in three (of the five) cases limestone being dissolved while in two cases no apparent dissolution takes place. On this hydrologic background, microbial analysis indicates an overall downstream decrease of microorganism diversity, potentially suggesting a natural attenuation of pollution.

 

Acknowledgments. This work was supported by a grant of Ministry of Research and Innovation, CNCS - UEFISCDI, project number PN-III-P4-ID-PCCF-2016-0016 (DARKFOOD), within PNCDI III. The research leading to these results has received funding from the EEA Grants 2014-2021, under Project contract no. 4/2019 (GROUNDWATERISK).