Abstract:
the proximity to emission sources and seasonal dynamics of emission and mobilization. While spatial-temporal
information on water pollution in Europe is increasing, there are substantial knowledge gaps on seasonal
pollution dynamics in tropical countries. Thus, we took Lake Victoria South Basin in western Kenya as a case
study to identify spatial and seasonal hot spots of contamination, quantified toxic risks to different groups of
organisms, and identified seasonal risk drivers. For this purpose, we analyzed grab water samples from five rivers
with agricultural and wastewater treatment plants in their catchment in four different seasons. We used liquid
chromatography coupled to high resolution mass spectrometry (LC-HRMS) with a target list of 785 organic
micropollutants. A total of 307 compounds were detected with concentrations ranging from 0.3 ng/L to 6.6 μg/L.
Using a Toxic Unit (TU) approach based on mixture toxicity to standard test organisms, crustaceans were
identified as the most affected group followed by algae and fish. For crustaceans, chronic risk thresholds were
exceeded in 96 % of all the samples, while 56 % of all samples are expected to be acutely toxic, with the highest risk in February during the dry season. High toxic unit values for algae and fish were recorded in July dry season
and May wet season. Diazinon, imidacloprid, clothianidin and pirimiphos-methyl were the major drivers for
crustacean toxicity while triclosan and different herbicide mixtures drive risks to algae in dry and wet seasons,
respectively. A total of 18 chemicals were found to exceed acute and chronic environmental risk thresholds. With this study, strong spatial-temporal patterns of pollution, risks and risk drivers could be confirmed informing
prioritization of monitoring and abatement to enhance water quality and reduce toxic risks.
