Drinking water production using bank filtration is an important method for the sustainable usage of water resources, especially in urbanized areas. For example, bank filtration is used for 50% of the drinking water production of Berlin, and for the city of Düsseldorf it is even 100%.
Bank filtration can be threatened by polluted surface waters. From wastewater treatment plants and surface runoff of agricultural areas, pollutants including pathogens like viruses and bacteria can enter the surface waters.
The goal of this project is to develop a toolbox for waterworks managers for a better prediction of pathogen transport in bank filtration, taking into account the natural attenuation of the subsurface and naturally occurring variable hydraulic and geochemical boundary conditions (Figure 2).
While research in the past was mostly focused on pathogenic bacteria, knowledge about viruses in aquatic systems is still limited. From excretions of humans and animals they can get into the wastewater or directly into the environment. Results of previous studies for the natural attenuation of viruses in groundwater are ambiguous (Krauss & Griebler, 2011). Viruses can stay infectious in aquatic environments for more than a hundred days. Additionally, the infectious dose for viruses is generally lower than for pathogenic bacteria (Krauss & Griebler, 2011).
In the environment, different processes lead to the inactivation and elimination of viruses, for example UV-light or increased temperatures (Schijven et al., 1999). An important process for the retention of viruses in sediments is the sorption at charged surfaces (Figure 4). Various studies have shown that sorption processes can depend on water chemistry, water saturation, biological activity, sediment mineral composition or grain size distribution (Xagoraraki et al., 2014). Hydrological events can have a significant influence on the transport of viruses in groundwater, for example flooding or strong precipitation events can change the groundwater chemistry and/or lead to an increased entry of viruses into the groundwater.
Quantitatively, transport of virus can be described using the colloid-filtration-theory (CFT). But CFT has only been completely validated for sediments with homogeneous grains and without significant repulsion between colloids and grains, which is the typical case under natural conditions for pathogens (Hunt & Johnson, 2017; Tufenkji & Elimelech, 2004).
Analysis of samples from a 1-year monitoring at the waterworks Flehe in Düsseldorf showed, that the river Rhein contains significant concentrations of pathogen (indicators), for example up to 10000 MPN/100mL Coliforms and 100 PFU/100mL for Coliphages. But pathogen retention appears to be especially strong in the first part of the bank filtration.
Therefore, in the untreated water of the extraction wells concentrations are in most cases below the detection limit, for example not higher than 10 MPN/100 mL for Coliforms and viruses were never detected. Even though transport times from the river to the extraction wells for conservative species like chloride are comparably low with around 30 days.
Furthermore, a seasonal especially temperature-driven variability was found in the aquifer, which results in nitrate-reducing conditions in the late summer (Figure 5). Modelling of the hydrochemical data has shown, that hotspots for redox reactions (oxygen depletion are found in or close to the colmation layer at the interface between aquifer and river (Figure 5, depletion of oxygen after entry into the subsurface).
TU Berlin (Dep. Hydrogeology)
Funded by the Deutsche Bundesstiftung Umwelt (DBU)
Project period: 2017 - 2021