Influence of microbial activity on carbonate dominated mineral precipitates in tunnel drainage systems

Carbonate scaling in motor-and railway tunnel drainages causes seriousproblems due to narrowed flow cross sections and clogging. Their removal generates high costs and traffic disturbances, arguing for an enhancedunderstanding of installation-specificchemical and mineralogical compositions, microstructures, and environmental controlson theirformationand material characteristics. From asubstantialset of case studiesconductedin Austria, distinct types of scale materialscan be distinguished ranging from unconsolidated softand poroussediments to compactincrustationswith distinct microstructure, mineralogy and variable porosity. In contrast to existingconcepts, nearly all of the investigated scale types indicate a significant impact of microbial communitieson scaledeposition. Even in site-specific aqueous solutions, several types of bacteria could be detected. Documentedchemoautotroph bacteria in these tunnel drainage solutions comprise ofGallionella sp., Leptothrix sp.,Thiothrix,methylococcales, as wellas rare photosynthetic algae. Metabolisms rangefrom Fe(II) to Fe (III) oxidation, methanotrophy, sulfur oxidizing and sulphat reduzingto photosynthesis. Thus,next toinorganically driven mineral precipitation of,e.g. calcite, aragonite, brucite, ferrihydriteand silica, distinct biomineralization products such asFe/Mn–(hydr)oxides and(Mg-) calcite have to be considered. In the latter casesextracellularpolymeric substances seem to play a vitalrole for the consolidation and stiffness of the resultingcomplexand spatiotemporally evolvedscale deposits. An advanced process understandingofthe individual evolution, composition, and character of scaling in drainage systemswould allow fortailored monitoring procedures, countermeasuresand cleaning strategies in order to tackle unwanted scaling in tunnels.