From low flows to ecosystem stress: Temporal patterns of ecological risk driven by emerging contaminants.

Ecological risk assessment (ERA) of river ecosystems receiving wastewater containing contaminants of emerging concern (CECs) is commonly based on the risk quotient of a CEC mixture (RQMIX), which is typically calculated under streamflow conditions at the time of sampling. However, this approach does not account for long-term streamflow variability and fails to quantify the proportion of time during which specific flow conditions occur and associated ecological risk over multi-year periods, thereby limiting its usefulness for optimising ecosystem protection. To address these limitations, an ERA based on RQMIX was conducted for a model mixture of CECs discharged into the Wisla River in Kraków, Poland, under representative streamflow conditions, including mean low, mean, median, and environmental flow, across two multi-year periods (1990-2010 and 2014-2023). The applied model incorporated simplifying assumptions, including temporal stability of CEC loads between the analysed periods. The model-based ERA indicated elevated RQMIX values, with substantial variation depending on streamflow conditions and the analysed time period. In 2014-2023, the model-estimated RQMIX reached 128.7 under median flow conditions and increased to 513.4 for mean low streamflow, representing a high potential risk. During 66% of this period of record, streamflow remained below the mean flow; the corresponding RQMIX exceeded 87.1. The RQMIX estimated under environmental flow condition was 482.6, with streamflow falling below this threshold during 7% of the 2014-2013 period. Cumulative ecological risk for the Wisla River ecosystem Kraków reach was assessed as high under environmental flow conditions during 2014-2023. The results indicate that ERA should be conducted using CEC concentrations and streamflow measured at the time of sampling and by incorporating median flow derived from multi-year records and environmental flow conditions in river reaches affected by wastewater discharge.