Dynamics of pathogens and fecal indicators during riverbank filtration in&#160;times of high and low river levels
Authors: Wang He , Knabe Dustin , Engelhardt Irina , Droste Björn , Rohns Hans-Peter , Stumpp Christine , Ho Johannes , Griebler Christian 
Published: March 2022

&lt;p&gt;Riverbank filtration is an established and quantitatively important approach to mine high-quality raw water for&amp;#160;&lt;br&gt;drinking water production. Bacterial fecal indicators are routinely used to monitor hygienic raw water quality,&amp;#160;&lt;br&gt;however, their applicability in viral contamination has been questioned repeatedly. Additionally, there are&amp;#160;&lt;br&gt;concerns that the increasing frequency and intensity of meteorological and hydrological events, i.e., heavy&amp;#160;&lt;br&gt;precipitation and droughts leading to high and low river levels, may impair riverbank filtration performance. In&amp;#160;&lt;br&gt;this study, we explored the removal of adenovirus compared with several commonly used bacterial and viral&amp;#160;&lt;br&gt;water quality indicators during different river levels. In a seasonal study, water from the Rhine River, a series of&amp;#160;&lt;br&gt;groundwater monitoring wells, and a production well were regularly collected and analyzed for adenovirus,&amp;#160;&lt;br&gt;coliphages, E. coli, C. perfringens, coliform bacteria, the total number of prokaryotic cells (TCC), and the number&amp;#160;&lt;br&gt;of virus-like particles (TVPC) using molecular and cultivation-based assays. Additionally, basic physico-chemical&amp;#160;&lt;br&gt;parameters, including temperature, pH, dissolved organic carbon, and nutrients, were measured. The highest&amp;#160;&lt;br&gt;log10 reduction during the &gt;72 m of riverbank filtration from the river channel to the production well was&amp;#160;&lt;br&gt;observed for coliforms (&gt;3.7 log10), followed by E. coli (&gt;3.4 log10), somatic coliphages (&gt;3.1 log10),&amp;#160;&lt;br&gt;C. perfringens (&gt;2.5 log10), and F+ coliphages (&gt;2.1 log10) at high river levels. Adenovirus decreased by 1.6&amp;#8211;3.1&amp;#160;&lt;br&gt;log units in the first monitoring well (&gt;32 m) and was not detected in further distant wells. The highest removal&amp;#160;&lt;br&gt;efficiency of adenovirus and most other viral and bacterial fecal indicators was achieved during high river levels,&amp;#160;&lt;br&gt;which were characterized by increased numbers of pathogens and indicators. During low river levels, coliforms&amp;#160;&lt;br&gt;and C. perfringens were occasionally present in raw water at the production well. Adenovirus, quantified via&amp;#160;&lt;br&gt;droplet digital PCR, correlated with E. coli, somatic coliphages, TCC, TVPC, pH, and DOC at high river levels. At&amp;#160;&lt;br&gt;low river levels, adenoviruses correlated with coliforms, TVPC, pH, and water travel time. We conclude that&amp;#160;&lt;br&gt;although standard fecal indicators are insufficient for assessing hygienic raw water quality, a combination of&amp;#160;&lt;br&gt;E. coli, coliforms and somatic coliphages can assess riverbank filtration performance in adenovirus removal.&amp;#160;&lt;br&gt;Furthermore, effects of extreme hydrological events should be studied on an event-to-event basis at high spatial&amp;#160;&lt;br&gt;and temporal resolutions. Finally, there is an urgent need for a lower limit of detection for pathogenic viruses in&amp;#160;&lt;br&gt;natural waters. Preconcentration of viral particles from larger water volumes (&gt;100 L) constitutes a promising&amp;#160;&lt;br&gt;strategy.&lt;/p&gt;