Geochemical characterization of an urban lake in the centre of Rome (Lake Bullicante, Italy)

Urban lakes have become increasingly important in theplanning of urban ecology, green infrastructure and green areasin European cities. This paper describes the chemical, isotope andmicrobial features of Lake Bullicante, a small artificial lake locatedwithin the urban area of the city of Rome. It has an anthropogenicorigin due to excavation works that intercepted the underlyingaquifer, giving rise to a water body. The lake area is 7.000 m2, witha maximum depth of 7 m and located on the distal deposits of theAlban Hills Volcanic District in an area named "Acqua Bullicante"(i.e. Bubbling Water), where degassing phenomena were historicallyrecorded. The proximity of this volcanic district motivated thestudy on Lake Bullicante as a potential open-air laboratory totrace possible degassing phenomena in a highly urbanized area. Apreliminary geochemical and microbial sampling survey was carriedout in winter 2018. Samples were collected along a vertical profileof the lake from the surface to the maximum depth. Major, minor,trace elements, dissolved gases and stable isotopes (deltaD-H2O, delta18OH2O, delta13C-CO2) were analyzed, along with the analysis of 87Sr/86Srratio. The microbial community characteristics were analysed byepifluorescence microscopy (CARD-FISH) and flow cytometry. Thechemical composition and water isotopes suggest that lake waterhas a meteoric origin and is related to a Ca-HCO3 shallow aquiferhosted in volcanic rocks. This is confirmed by both the 87Sr/86Srratio of lake water, which falls in the range of values of Alban Hillsvolcanites, and the chemical-isotopic composition of neighboringwells. A relatively high concentration of dissolved CO2, its isotopicsignature (delta13C-CO2 20%o V-PDB), and the high content in organicmatter (DOC 10-30 mg/L) suggest for the lake an eutrophication statewith denitrification also occurring. Considering the relatively highconcentrations of dissolved CO2, an external input of carbon dioxidecannot be completely excluded and as a consequence, not even thehypothesis of mixing processes between biotic and inorganic CO2.This makes further investigations necessary especially during thesummer, when the lake is stratified. A summer survey could be alsouseful to better understand the microbial processes into the lake,its eutrophication evolution and health status, and to plan eventualproper remediation strategies, providing important tools to the localadministration and stakeholders to improve, protect and preservethis ecological niche.