Seasonally recurring patterns of dominant Crenothrix spp. in a European alluvial drinking water well: Significance and potential indicator role.

Iron and manganese (Fe/Mn) often lead to aesthetic quality issues in water supply. Strong and problematic black-brown particle formation was persistently observed in an alluvial drinking water well, even though oxygen enrichment probes, intended for in situ i.e., subsurface iron/manganese removal, were installed. To investigate the cause of the problem, a comparative and multiparametric approach was undertaken at the problematic well, seven additional wells (with 0.3 to 70 km distance to the affected well) and all the adjacent surface waters. Via a time-series investigation of up to 2.5 years, microbiological analysis (high-throughput 16S rRNA gene amplicon sequencing, total cell count) and chemical analysis (high-resolution elemental analysis using inductively coupled mass spectrometry and others) of the water samples were performed. Results revealed previously unreported, extremely dynamic, and seasonally recurring patterns of genus Crenothrix (a sheathed, filamentous bacterial population) in water samples obtained from the particle-affected well. Crenothrix spp. dominated the microbial community in summer months (up to 82 % relative abundance), being virtually absent in winter. Explanatory models for the high dynamics and association with bio-geochemical processes were established. These included methane formation and manganese mobilization in relation to riverbank filtration in the summer months, as well as changing aerobic and anaerobic conditions in the aquifer. Dominance of Crenothrix spp. in the affected well, low abundance in weak particle-affected wells, and total absence in non-affected wells was observed. This led to the suggestion of Crenothrix spp. as a technical indicator for Fe/Mn treatment failure for alluvial groundwater (e.g., genetic marker quantification by q/dPCR), to be evaluated in future studies regarding their applicability across a broader geographic context. Despite being first described in association with drinking water deterioration 150 years ago, this is the first study reporting seasonally recurring dominant patterns of Crenothrix spp. in association with operational/aesthetic issues for drinking water production.