Spatial and temporal biotransformation of micropollutants in a lake ecosystem

European lakes are subject to the input of a large range of micropollutants, ranging from pesticides to pharmaceuticals, artificial sweeteners and personal care products. These micropollutants, often measured in the ng/L to µg/L range, can exert toxicological effects on the aquatic communities residing in the lake, but can also threaten the quality of drinking water as they might not be completely removed during the drinking water treatment process. Scientists have measured these micropollutants sources and concentrations extensively in the past decades, and started elucidating their dissipation pathways, such as flushing, sorption and phototransformation. However, there still exists a knowledge gap regarding the dissipation of these micropollutants through biotransformation processes. This project aimed at evaluating the extent of biotransformation of a large range of micropollutants in a lake ecosystem, and at different seasons and water depths. The influence of environmental parameters (temperature, pH, dissolved oxygen…) relative to microbial density, composition and diversity was evaluated using data coming from a lake monitoring station and nutrient concentration measurements, and DNA sequencing, respectively. Statistical approaches were used to identify the most important factors driving the transformation of micropollutants at different periods of the summer.
This study is important to understand the persistence of micropollutants in surface water, identify the drivers of biotransformation, and foster improvements in risk assessment of typical contaminants. In addition, the biotransformation rate constants for pharmaceuticals obtained in laboratory experiments were used in a lake mass balance model to verify if inclusion of biotransformation as an elimination process would improve prediction of micropollutant concentrations in the studied lake.

The beginning of the project involved literature research, selecting the studied micropollutants, ordering material necessary for the experiments, preparing solutions and standards, getting acquainted with the laboratory equipment (especially the analytical instruments) and testing the experimental approach. The original proposal involved the use of dialysis membranes to conduct the experiments directly in the lake, and these membranes needed to be tested. Several preliminary experiments with pure water and lake water led to the conclusions that the chemicals were leaching out of the dialysis membranes and that dialysis membranes could therefore not be used to assess the biotransformation in situ. An environmental chamber was then prepared to be able to run the experiments in close-to-field conditions in the laboratory.
In April 2018, the sampling period started with the first lake sampling of the project. Water was collected for chemical analysis, biotransformation experiments and microbial community analysis. Biotransformation experiments were conducted over the following three weeks, with measurements of micropollutant concentrations every 1-5 days. These samplings, biotransformation experiments and measurements of micropollutant concentrations and microbial community were repeated 6 times between April 2018 and October 2018. The following months were used for data analysis and interpretation. From October 2018 to April 2019, the modelling of the lake mass balance was done within the frame of a master student project. The results of that project demonstrated that the biotransformation rate constants measured in our environmental chamber did help to explain the changes in micropollutant concentrations in the lake, but were not high enough to completely explain the decrease during the summer months. This suggested that some other processes were not adequately captured in our experimental setup, e.g. the importance of daily temperature variation or transformation in the sediments of the lake. More research involving in situ studies or more complex laboratory setup are thus required to determine biotransformation rate constants that are fully representative of the extent of biotransformation in actual lake systems.

In the spring of 2019, DNA and RNA were extracted from the water samples and 16S and 18S rRNA amplicons were sequenced. The following months were used for data analysis (from the sequencing work) and preparation of the main publication. The results showed that only a fraction of the micropollutants studied would undergo biotransformation to a measurable extent (12 compounds out of 40 studied), and that there were large variations in micropollutant biotransformation over the summer, with higher transformation rate constants measured when the biomass of microorganism and/or its diversity were highest in the lake.

The results of the project were presented at the TransCon conference in Ascona (Switzerland) at the end of April 2019, at SETAC Europe in Helsinki (Finland) at the end of May 2019, and in several group seminars at Eawag. The results of the modelling project were presented at the Office of Waste, Water, Energy and Air of the canton of Zürich (German: AWEL).

This project is the first to assess the biotransformation rate constants of a large range of micropollutants simultaneously, under environmentally relevant conditions, during six different periods of the year. The summer of 2018 saw the formation of a large blooms of phytoplankton, and the sampling of lake water and subsequent biotransformation experiments helped assess the variations in micropollutant biotransformation before, during and after a bloom of phytoplankton. The combination of lake environmental parameters, biotransformation rate constants and sequencing amplicons generated a large dataset that could be further analysed by ecologists and environmental chemists. This data set will be available for further analysis from the internal repository of Eawag.

The project will lead a to a high quality publication in a high-ranking journal and is expected to attract attention from the specialised scientific community. It also helped strengthen the collaboration between the Aquatic Ecology department and the Environmental Chemistry department of Eawag, as well as with the Genetic Diversity Centre of ETH Zürich. Finally, it also promoted our research in the cantonal authority for Waste, Water, Energy and Air (AWEL) and the sharing of data between our two institutions.

Periodic Reporting for period 1 - LakeMP (Spatial and temporal biotransformation of micropollutants in a lake ecosystem)

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