Optimizing short time-step monitoring and management strategies using environmental tracers at flood-affected bank filtration sites.

Bank filtration is a popular pre-treatment method to produce drinking water as it benefits from the natural capacity of the sediments to attenuate contaminants. Under flood conditions, bank filtration systems are known to be vulnerable to contamination, partly because flow patterns may evolve at short timescales and result in a rapid evolution of the origin and travel times of surface water in the aquifer. However, high frequency monitoring for water quality is not common practice yet, and water quality management decisions for the operation of bank filtration systems are typically based on weekly to monthly assays. The aim of this study is to illustrate how monitoring strategies of environmental tracers at flood-affected sites can be optimized and to demonstrate how tracer-based evidence can help to define adequate pumping strategies. Data acquisition spanned two intense flood events at a two-lake bank filtration site. Based on bacteriological indicators, the bank filtration system was shown to be resilient to the yearly recurring flood events but more vulnerable to contamination during the intense flood events. The origin of the bank filtrate gradually evolved from a mixture between the two lakes towards a contribution of floodwater and one lake only. Automatized measurements of temperature and electrical conductivity at observation wells allowed to detect changes in the groundwater flow patterns at a daily timescale, while the regulatory monthly monitoring for indicator bacteria did not fully capture the potential short timescale variability of the water quality. The recovery to pre-flood conditions was shown to be accelerated for the wells operating at high rates (i.e., ≥1000 m

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Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.