Long-term dynamics of pesticide metabolites in soil and aquifers
Authors:   , Hintze Simone 
Published: January 2024

Groundwater is one of the most important resources for drinking water, and has to be protected from the input of persistent substances. Nevertheless, pesticides and especially their degradation products (metabolites) are frequently detected in groundwater. Metabolite concentrations often exceed those of their parent pesticides. In order to reduce the metabolite contamination in groundwater, some pesticides have been banned (e.g. atrazine). However, the corresponding metabolites often show a high persistence in aquifers, even decades after the introduction of these measures. The main aim of this dissertation was to study the factors and mechanisms that control the long-term dynamics of metabolites in aquifers and pumping wells after the application stop of pesticides. The pesticide chloridazon (CLZ) and its frequently detected metabolites, namely desphenyl-chloridazon (DPC) and methyl-desphenyl-chloridazon (MDPC), were used as an example. The following three research questions were defined: 1) How do surface water – groundwater interactions influence the spatial distribution of metabolites in aquifers and in particular, can surface water – groundwater interactions act as an additional source of metabolites in aquifers? What are the implications for the concentration level and long-term dynamics of metabolites in pumping wells? 2) Is the high persistence of metabolites in aquifers mainly related to a high residence time in the aquifer or can the soil/unsaturated zone be a long-term source for metabolites as well? 3) Which compound- and site-specific factors can influence the long-term dynamics of metabolites in pumping wells after an application stop of pesticides and, therefore, have to be considered for the knowledge transfer between different compounds and field sites? The first research question was addressed by a field-based study in a small alluvial aquifer, which interacts with surface water bodies originating from catchments exhibiting contrasting land uses. The characterization of surface water – groundwater interactions with various environmental tracers (electrical conductivity, stable water isotopes, wastewater tracers, major ions) has shown that surface water bodies from mountainous watersheds can dilute the metabolite concentrations in aquifers, whereas small surface water bodies from lowland watersheds with intensive agriculture can act as an additional source of metabolites in aquifers. Our focus was on the latter scenario, as it has been so far received little attention. An endmember mixing analysis using hydro-chemical data revealed that about 20 % of the DPC mass of the pumping well enters the aquifer via surface water – groundwater interactions. This mass would on its own cause concentrations above 0.1 μg L-1 in the pumping well. Our investigations highlighted that the infiltration of surface water bodies from lowland watersheds with intensive agriculture into aquifers can lead to an important input of metabolites from outside of the exploited aquifer and, thereby, to a prolongation of travel times until the metabolites reach the pumping well. This can affect the long-term dynamics of metabolites in pumping wells after an application stop of the parent pesticides. Metabolites from further away can still reach the pumping well, even after the soil/unsaturated zone above the exploited aquifer no longer being a source of metabolites. The role of the soil and unsaturated zone as a long-term metabolite source (research question 2) was investigated by combining soil analysis, groundwater analysis and numerical modelling. Soil samples were taken from a small agricultural area (0.7 km2), where the last CLZ application was 5 to 10 years ago. Groundwater was sampled upgradient (multi-level piezometers) and downgradient (pumping well) of this agricultural area. In soil, the DPC and MDPC concentrations were 10 times (DPC: 0.22 – 7.4 μg kg-1) and 6 times (MDPC: 0.12 – 3.1 μg kg-1) higher compared to CLZ (< 0.050 – 1.0 μg kg-1). Their concentration correlated with the organic carbon content, but not with the time since the last CLZ application. Despite a small fraction of metabolites dissolved in pore water (median: 7.9 % for DPC; 5.1 % for MDPC), they reached a median pore water concentration at a depth of 75 – 100 cm (2.1 μg L-1 for DPC; 1.0 μg L-1 for MDPC) which was high enough to cause concentrations above 0.1 μg L-1 in the aquifer. The CLZ metabolite concentrations in groundwater increased by a factor of about 3 between monitoring wells upgradient of the agricultural area and the downgradient pumping well. The absolute increase amounted to 0.49 μg L-1 for DPC and 0.12 μg L-1 for MDPC. This highlights that the key factor for the high metabolite persistence in this aquifer after the application stop is their retention in soil and unsaturated zone (median thickness: 2 m) and not the retention of CLZ and its ongoing degradation. Based on model simulations, it is expected that even more than one decade after the last CLZ application, the metabolite input from soil and unsaturated zone can cause DPC concentrations in the aquifer of our study site that exceed values of 0.1 μg L-1. The influence of various compound- and site-specific factors on the long-term dynamics of metabolites in pumping wells (research question 3) was studied by using simple analytical models and applying them to different metabolites and field sites. The time scale necessary to decrease the metabolite concentration in pumping wells after an application stop of pesticides to a certain threshold value is controlled by the metabolite concentration at the point in time when the pesticide application was stopped (steady state concentration) and by the rate at which the metabolite concentration decreases. The latter is mostly governed by the mean residence time of metabolites in soil, unsaturated zone and the aquifer. Our focus was on the soil and unsaturated zone. In soil/unsaturated zone, we found a higher mean residence time for metabolites with a higher sorption coefficient (e.g. DPC, MDPC, deethylatrazine (DEA)) and at field sites with a higher organic carbon content and/or a thick soil/unsaturated zone. findings illustrated that the mean residence time in soil and unsaturated zone is mostly governed by the loss via leaching, whereas degradation might only play a role in the uppermost soil layer. In soil, the loss via leaching is mostly controlled by the sorption related factors (foc content, sorption coefficient, bulk density), whereas in the unsaturated zone, the hydrology related factor (volumetric water content) and therefore, the retained amount of water becomes more important. A comparison of the mean residence time in the soil/unsaturated zone and in the aquifer for different metabolites and field sites highlighted the importance of considering not only the mean residence time in aquifers, but also the mean residence time in soil/unsaturated zone. This is especially essential for estimating the metabolite long-term dynamics in pumping wells for metabolites with a high sorption coefficient and at field sites with a thick and/or organic carbon rich soil/unsaturated zone. Considering the mean residence time in the entire system, the steady state concentration in the pumping well and the corresponding compound- and site-specific factors allow a simplified comparison of the long-term dynamics in pumping wells between different compounds and field sites. The findings of this dissertation illustrated that it is crucial to consider the entire system comprising aquifer, soil and unsaturated zone as well as possible surface water – groundwater interactions in order to estimate the long-term dynamics of metabolites in pumping wells after an application stop of pesticides. Considering only the mean residence time of metabolites in the aquifer can lead to an underestimation of the required time scale that is necessary to decrease the metabolite concentration in the pumping well below a certain threshold value. Based on these findings, practical recommendations for the estimation of the long-term dynamics of metabolites in pumping wells can be given. In addition, these new insights can help to improve the effectiveness of measures aimed at reducing the metabolite contamination in aquifers and pumping wells.