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Pesticides from agricultural practices are among the most pressing reasons why groundwater sources do not reach the good chemical status standards as required by the European Water Framework directive. Complementary to previous federal pesticide reports, we analysed groundwater-monitoring data from 13 German Länder assembled in a database consisting of 26.192 groundwater measuring sites sampled between 1973 and 2021 of in total 521 parent compounds and metabolites. This study focuses on agricultural plant protection products. The monitored substance spectrum and site density developed over time and differs between Länder. More than 95 % of all samples lie below the respective (multiple) limits of quantification (LOQ). We thus report the frequency of exceedance above concentration thresholds, which allows to compare measurements temporally and spatially. Pesticide detections were found in all aquifer types, land uses and well screen depths. Most detections of higher concentrations were found in agricultural areas, at shallow screen depth in porous aquifers. Karst aquifers showed also a higher percentage of samples in higher concentration classes. Metabolites with high mobility and persistence were found in higher concentration ranges. Herbicides and metabolites thereof dominate the top 20 of pesticides that most frequently exceed 0.1 μg L−1. The ranking for 2010–2019 includes both authorised and banned compounds and their occurrence is discussed in the context of their mobility, persistence and underlying monitoring density. Yearly exceedance frequencies above 0.05, 0.1 μg L−1 and higher thresholds of metazachlor and its esa-metabolite, and national sales data of the parent compound did not show a temporal correlation in subsequent years. This study stresses the need for the harmonisation of heterogeneous pesticide data. Further, a characterisation of the groundwater data used to analyse pesticide occurrence in selected concentration ranges for relevant site factors and compound properties and provides a pesticide ranking based on exceedance frequencies is provided. Plain 3D: simple with soil layers Attribution-ShareAlike 4.0 International (CC BY-SA 4.0). Tracey Saxby, Integration and Application Network (ian.umces.edu/media-library). field:harvesting Attribution-ShareAlike 4.0 International (CC BY-SA 4.0). Tracey Saxby, Integration and Application Network (ian.umces.edu/media-library). Urban: city 1 (Attribution-ShareAlike 4.0 International (CC BY-SA 4.0). Tracey Saxby, Integration and Application Network (ian.umces.edu/media-library). Quercus bicolor (Swamp White Oak) Attribution-ShareAlike 4.0 International (CC BY-SA 4.0). Annie Carew, Integration and Application Network (ian.umces.edu/media-library). Quercus palustris (Swamp Spanish Oak) Attribution-ShareAlike 4.0 International (CC BY-SA 4.0) Annie Carew, Integration and Application Network (ian.umces.edu/media-library). [Display omitted] • A nationwide data base of pesticides in groundwater in Germany has been harmonised. • Site characteristics and compound properties were compiled for further analysis. • Exceedance frequencies >0.05 and 0.1 μg L−1 reported as comparable measures. • Pesticide detections were found in all aquifer types, land uses and well screen depths. [ABSTRACT FROM AUTHOR]

Cyanobacterial harmful algal blooms (CHABs) have become a persistent seasonal problem in the upper San Francisco Estuary, California also known as the Sacramento-San Joaquin Delta (Delta). The Delta is comprised of a complex network of open water bodies, channels, and sloughs. The terminus of the Stockton Channel is an area identified as a CHAB "hotspot." As CHABs increase in severity, there is an urgent need to better understand CHAB drivers to identify and implement mitigation measures that can be used in an estuarine complex like the Delta. We investigated water quality conditions and nutrient dynamics in the Stockton Channel by measuring nutrients in the water column, sediments, and pore waters. In situ nutrient addition bioassay experiments were used to assess the effects of nutrient enrichment on total algal/cyanobacterial growth and pigment concentrations. In both June and September, relative to unamended controls, total chlorophyll and cyanobacterial pigment concentrations were unaffected by nutrient additions; hence, the study area showed signs of classical hypereutrophication, with ambient nitrogen and phosphorus present in excess of algal growth requirements. A cyanobacterial bloom, dominated by Microcystis spp. was present throughout the study area but was most severe and persistent at the shallowest site at the channel terminus. At this site, Microcystis spp. created water quality conditions that allowed for a prolonged bloom from June through September. While targeted nutrient reductions are recommended for long term mitigation, on a shorter timescale, our findings suggest that physical/mechanical controls are the more promising alternative approaches to reduce the severity of CHABs in the terminus of the Stockton Channel. • Examined nutrient dynamics and CHABs in the Sacramento-San Joaquin Delta. • The study area was hypereutrophic with nitrogen and phosphorus. • Nutrients alone do not explain why blooms were so severe. • Microcystis were most severe and persistent at shallowest site. • Physical/mechanical controls are promising options for CHAB mitigation. [ABSTRACT FROM AUTHOR]