Using hydrodynamic and hydraulic modelling to study microbiological water quality issues at a\xa0backwater area of the Danube to support decision-making

Julia Derx,Regina Sommer,Julia Vierheilig,Christina Frick,Alfred Paul Blaschke,Alexander K T Kirschner,Jürgen Komma,Philipp Reiner,Rita Linke,Domenico Savio,Andreas H Farnleitner

doi:10.1007/s00506-021-00797-7

Abstract

The alluvial backwater areas of the Danube are valuable ecological habitats containing important drinking water resources. Due to the river regulation and the construction of power plants, the river water levels and natural dynamics of the backwater areas continuously decline, threatening their typical characteristics. The aim of this study was to evaluate how an increased connectivity of the backwater branch located in a nature-protected riverine floodplain (enabled by diverting river water into the backwater system via a weir) affects the microbiological quality of groundwater resources. The defined quality criterion was that the diversion measures must not lead to an increased detection frequency of faecal indicators in groundwater. The microbiological water quality of the Danube, its backwater branch and the groundwater was analysed from 2010 to 2013. E. coli was selected as bacterial indicator for recent faecal pollution. C. perfringens (spores) was analysed as indicator for persistent faecal pollution and potentially occurring pathogenic protozoa. We simulated the microbial transport from the Danube and the backwater river into groundwater using a 3‑D unsaturated-saturated groundwater model coupled with 2‑D hydrodynamic flow simulations. Scenarios for no diversion measures were compared with scenarios for an additional discharge of 3, 20 and 80 m3/s from the Danube River into the backwater branch. While the additional discharge of 20 and 80 m3/s of Danube water into the floodplain strongly improved the ecological status according to ecological habitat models, the hydraulic transport simulations showed that this would result in a deterioration of the microbiological quality of groundwater resources. The presented approach shows how hydraulic transport modelling and microbiological analyses can be combined to support decision-making.

Highlights

The wetlands of the Danube play an important role for recreation and drinking water supply
While the additional discharge of 20 and 80 m3/s of Danube water into the floodplain strongly improved the ecological status according to ecological habitat models, the hydraulic transport simulations showed that this would result in a deterioration of the microbiological quality of groundwater resources
A low level of faecal pollution was found at sampling sites LSW 5, 6 and 9, with 90th percentile values ranging from 30 to 130 colony forming units (CFU)/100 mL for E. coli, and from 20 to 200 CFU/100 mL for C. perfringens, respectively (Fig. 3)

Summary

Introduction

The wetlands of the Danube play an important role for recreation and drinking water supply. One possible measure is to divert river water into the backwater branches or to improve the connectivity of the side branches Such measures could support the development of a more dynamic alluvial backwater area with a larger biodiversity and help preserving the high value of the natural areas in the long term. Such measures, must be in line with the Water Rights Act (1959) and must not lead to a restriction of drinking water production. According to the Austrian Drinking Water Ordinance (2001) and the Water Rights Act (1959), faecal pollution is not permitted in drinking water and protected groundwater resources The compliance with this requirement is assumed fulfilled when standard faecal indicators are not detectable in 100 ml sample volume

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