Dynamics of pathogens and fecal indicators during riverbank filtration in times of high and low river levels

Abstract

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