Micropollutants in Large Lakes: From Potential Pollution to Risk Assessments

This book is divided into six additional chapters: Chapter 2 deals with the agricultural source of micropollutants; Chapter 3 with the urban source of micropollutants; Chapter 4 with lake circulation; Chapter 5 with the influence of a wastewater treatment plant on a bay; Chapter 6 with the input of sediments and their transport in a bay; and Chapter 7 with the risk assessment of the mixture of the compounds present in lake water. A last chapter (8) synthesizes the main findings and presents perspectives for future studies. More specifically: Chapter 2 is dedicated to the micropollutants that enter lakes through agriculture. These are mainly pesticides. After a review of the different types of compounds, the authors will put forth the main pathways these compounds follow to enter lakes. The case of glyphosate, an herbicide used worldwide, will be discussed. Chapter 3 illustrates the urban sources of pollutants with pharmaceuticals and biocides. Two models will be covered that allow estimating the load and dynamics of these chemicals that may exit from an urban catchment and therefore reach lakes. Special attention will be paid to the “end of pipe” removal of these compounds at a WWTP. Chapter 4 is dedicated to lake circulation. The aim of this chapter is to present an overall description of lake hydrodynamics, which is driven by three factors: wind, temperature and Coriolis forces. The main inflow to Lake Geneva is provided by the Rhône River. In addition, the topography surrounding the lake varies rapidly, for example, with mountains in the eastern part. As a consequence, the lake’s currents can vary markedly. To achieve our objective, we will utilise hydrodynamic modelling. A model will be proposed that permits describing the behavior of the top layer of the lake based on wind direction. The importance of stable hydrogen and oxygen isotopes for characterising the sources of water and mixing processes in the lake will be described and illustrated with the example of the Rhône river interflow to it. The measured variations of the isotopic compositions within the lake can serve as verification of the lake hydrodynamic models. However, the emphasis of the chapter will be on explaining the major current features and how they relate to wind forcing, which is one of the main drivers. Chapter 5 and 6 are devoted to the Vidy Bay in the Lausanne area of Lake Geneva. This bay receives the treated effluents of the largest wastewater treatment plant of the lake catchment. The latter represents therefore one of the major point sources of contaminants for the lake. These two chapters will focus on the various processes that control the transfer of contaminants (associated with particles or in a dissolved state) discharged into the bay and transported to the lake’s main water body. More specifically, Chapter 5 describes the WWTP plume dynamics. In relation to hydrodynamics (Chapter 4), specifically the behaviour of the plume formed through the treatment plant effluent release, field measurements are utilised to attempt to determine the plume behavior under various lake conditions. Furthermore, it investigates the degradation of organic pollutants. Photodegradation and other degradation processes are studied with theoretical, experimental and modelling approaches. Chapter 6 focuses on sediment path tracing. The pollutants associated with particles (trace metals, hydrophobic components, etc.) settle to the lakebed, but can be remobilized and transported to deeper parts of the lake. Bottom currents, and other physical forces, can cause resuspension and advection. Tracers used to identify this advection comprise natural and anthropogenic radioisotopes along with mercury as a major toxic trace element. Chapter 7 concentrates on one major issue of micropollutants in aquatic systems, like lakes, i.e., the risk of the mixture. The evaluation of the risk of mixture of micropollutants is not trivial and we will first present critically the models that allow doing so. We will also demonstrate their validity as predictive tools with the example of herbicide mixtures in Lake Geneva. Finally, we will map the risk of micropollutants around the lake. Finally in Chapter 8, we synthesize the main findings of all chapters and discuss recommendations for the management of mid-sized lakes regarding micropollution.