Current-use and legacy contaminants evidence dissolved organic matter transfer and dynamics across a fractured-rock groundwater recharge area

Abstract

AbstractAnthropogenic activities cause the release of vast amounts of contaminants into the environment which eventually reach even groundwater resources. With usually sparse regulatory monitoring of limited priority compounds, the large spectrum of contaminants, and the intricacies of intra- and inter-annual contaminant dynamics, such as the emergence and mobilisation of contaminants, are easily overlooked. Utilizing an 6-year record of untargeted LC-MS assessment of dissolved organic, we report the detection and tracing of selected environmental chemicals in the Hainich Critical Zone Exploratory (central Germany), representing a model groundwater flow system under different land use. The insect repellent DEET (N,N-diethyl-m-toluamide) and the coniferous resin acid 7-ODAA (7-oxodehydroabietic acids) show phases of seasonal dynamics in line with their expected periods of release. The legacy herbicides simazine, the triazine transformation product hydroxypropazine, and the flame retardant/plasticiser TPP (triphenyl phosphate) occurred episodically at various locations in the fractured sedimentary bedrock. Within the period of monitoring, extreme weather events (i.e., the severe 2018 drought) and extreme subsurface responses (i.e., 5-year groundwater highstand 2018) likely contributed to long-term organic matter dynamics, potentially causing re-emergence of legacy agrochemicals. This investigation points to the persistence and mobilisation of anthropogenic contaminants, and highlights the importance of long-term combined untargeted and targeted analysis of the groundwater dissolved organic matter for understanding subsurface ecosystems processes. The results add a note of caution for regulatory monitoring since also legacy contaminant levels may considerably vary over time.Highlights6 years of monitoring with non-target LC-MS screenings; 4-weekly samplings5 current-use and legacy compounds that evidence dissolved organic matter transferCompounds comprise regularly targeted and non-monitored features in groundwaterPronounced phases of non-detects help to further investigate surface-subsurface couplingThis illustrates the need for spatiotemporally highly resolved non-target monitoring