Tools for quantification of the duration and impact of pesticides in groundwater resources

The widespread occurrences of pesticides and their metabolites in Danish groundwater resources pose a challenge to water utilities and regional authorities. In recent years, several “new” pesticide metabolites have been discovered in Danish groundwater with an alarming detection frequency. These metabolites can typically be classified as persistent and mobile organic compounds (PMOCs), a group which also encompasses other micropollutants such as short-chained per- and polyfluorinated alkyl substances (PFAS), pharmaceuticals and biocide metabolites.The fungicide metabolite N,N-dimethylsulfamide (DMS) was the most frequently detected pesticide compound in Danish drinking water wells in 2021 and findings are also many across the EU. The ubiquity of DMS in groundwater has created a need for mapping the sources and origin of contamination to determine whether remedial measures can address the sources. Furthermore, tools for estimating the duration of DMS in groundwater systems are required by water utilities to support their management of well fields and planning of long-term investments.The aim of this PhD thesis was to improve the understanding of the sources of DMS in groundwater systems and to provide tools for estimation of its duration at drinking water well fields. Knowing possible origins of DMS is the foundation for tracking its sources in groundwater systems.Three parent compounds for DMS were identified. Tolylfluanid and dichlofluanid were formerly in use as both pesticides (horticultural crops, such as strawberries and pome fruits) and biocides (outdoor paint/wood protection), while cyazofamid is currently in use for potato cultivation. Due to these diverse applications, sources of DMS can be found in both rural and urban areas. Both types of sources can adversely affect downgradient drinking water well fields and cause concentrations above the groundwater quality criterion (0.1 μg/L).Several field investigation methods were tested in the context of tracking DMS sources or estimating its duration at well fields in agricultural areas. Review of site history, sales statistics, aerial photographs, and screening of topsoil was key in identifying possible sources and building source functions for modelling, while porewater sampling from clayey till proved useful in estimating source strength. Based on depth-discrete groundwater samples obtained from several investigation transects, contaminant mass discharge (CMD in g/yr) was estimated, and vertical concentration profiles were applied for tracking diffuse- and point sources of DMS.At catchment scale, several concurrent water samples from a gaining stream obtained in combination with flow measurements helped identify stretches of the stream which were subject to DMS discharge. This could be a tool to narrow down the locations of contaminant sources within a catchment.To estimate the duration of DMS contamination at a drinking water well field, several methods were applied in combination: CMD estimations based on comprehensive field data, numerical modelling, and groundwater dating. Simulated CMD was compared with estimations based on field observations, and simulated travel time was supported by retention time from dating of groundwater samples. On this basis, a prognosis for DMS load at the well field was developed: the DMS load was apparently still increasing, and a peak could be expected around year 2040. The well field may be affected by DMS until the end of the 21st century.The predicted duration in this groundwater system was surprisingly long, considering that DMS is highly mobile, and that the parent compounds have probably not been in use at the main source area for about 25 years. Simulations indicated that part of the explanation could be prolonged leaching caused by fracture-matrix interactions governed by diffusion during vertical transport in clayey till. This mechanism affects even highly mobile compounds, such as DMS, which is practically not retained by sorption.In conclusion, this PhD project has elucidated the origin and sources of the fungicide metabolite DMS and a comprehensive set of methods and tools was presented for estimation of its duration in groundwater systems