Time-of-day dependent UV filter concentrations and derived risk estimates at a recreational site

The objective of this study was to present a time-resolved investigation to evaluate temporal patterns and relative variation in ultraviolet (UV) filter concentrations in surface waters at a recreationally used site, Warnemünde Beach, at the German coast. Two 28-hours sampling campaigns were conducted in August 2023 and August 2024, during which surface water samples were collected approximately every two hours. The samples were then analysed to quantify the concentrations of 14 organic UV filters. Octocrylene (OC), avobenzone (BMDBM) and phenylbenzimidazolsulfonic acid (PBSA) were the most prominent UV filters detected at Warnemünde Beach. The highest concentrations of OC and BMDBM were detected at midday, coinciding with peak recreational hours. In contrast, PBSA showed its highest concentration during the evening hours, indicating its persistence in water after the period of maximum recreational activity. Concentrations of UV filters varied depending on the time of sampling, influenced by direct input from recreational activities as well as changing environmental and hydrodynamic conditions. The risk quotients (RQs) calculated from both sampling campaigns show that the RQs for UV filters for August 2024 were significantly higher than those for August 2023. During the period of highest recreational activity, the cumulative RQs from both campaigns exceeded the threshold value of 1, with the highest risks attributed to OC and BMDBM. This highlights that short-term variation significantly impacted the potential risk estimates from UV filters in coastal regions. However, the results are inherently site-specific and represent a preliminary screening-level risk assessment of UV filters. Future research should include multi-site analysis and additional long-term risk assessments to support comprehensive environmental risk evaluation.