engleski

The effect of increased sampling frequency on coastal bathing water quality assessment

Bathing water quality is crucial to prevent the health risks associated with bathing in coastal and inland bathing waters. As bathing water quality has been identified as one of the most important reasons for tourists' choice of destination, it is an important factor for island and coastal communities that depend on coastal tourism. Bathing water management in the European Union (EU) is regulated by Bathing Water Directive (BWD) 2006/7/ EC. The purpose of the BWD is to preserve, protect and improve the quality of the environment and to protect human health. The BWD sets the guidelines for monitoring, quality assessment, classification and quality status of bathing waters, as well as providing information to the public. Bathing water assessment is based on the levels of two fecal indicator bacteria (FIB), Escherichia coli and intestinal enterococci. According to the BWD, the final assessment is based on bathing water quality data sets compiled for this bathing season and the three previous bathing seasons. The bathing water quality datasets used for the final assessment always include at least 16 samples (based on an annual number of four samples) or 12 samples in the case of a bathing water located in a region with specific geographical restrictions. Since most water quality exceedances are single-day events, there is only a small chance (5%) of detection at such a low sampling frequency, even at the most frequently contaminated sites. This results in a significant misclassification of 15- 20% of bathing water sites. Therefore, estimating compliance based on such a small number of bathing water samples is unlikely to fulfill the main purpose of Bathing Water Directive, which is to protect public health, as too many poor quality beaches may be classified in the better category. Although the number of samples per bathing season is much higher than four in all Member States, the World Health Organization (WHO) in "WHO recommendations on scientific, analytical and epidemiological developments relevant to the parameters for bathing water quality in the Bathing Water Directive (2006/7/ EC)" (2018) recommends a further increase to 20 samples per season (80 data for the final assessment). This would certainly contribute to the protection of human health, but could also lead to an additional financial burden and technical difficulties for many Member States. Furthermore, it is questionable whether it is justified to increase the number of samples at sites classified as "excellent" over a longer period of time. In this study, we investigated the effect of sampling frequency on coastal water quality assessment. The study was conducted during the 2021 bathing season in 5 areas along the coast of Croatian Adriatic. A total of 27 bathing sites were included in the study, of which 14 bathing sites were rated as "excellent" over a longer period of time, and 13 bathing sites with variations in water quality. Samples were taken weekly at all bathing sites by official laboratories conducting official monitoring and by the Institute of Oceanography and Fisheries, which conducted sampling in Kaštela Bay. Both fecal indicator bacteria, E. coli and intestinal enterococci, were determined using the membrane filtration method, ISO 7899-2:2000 for intestinal enterococci and the temperature modified method ISO 9308-1:2014 for E. coli. All FIB counts were converted to log10 values and processed in accordance with Annex II of the BWD. Of the total 13 bathing sites with quality variation, seven had the same quality regardless of whether sampling was every two weeks or every week (three excellent, two good and two poor). At five bathing sites, water quality was poorer when sampling was weekly. At two sites the quality changed from excellent to good and at three from good to poor. All 14 excellent quality bathing sites had the same water quality regardless of sampling frequency (13 excellent and one poor). Based on the preliminary results, it appears that increasing the sampling frequency at bathing sites with variable water quality would improve the health protection of bathers. There does not appear to be any reason to increase the number of samples at sites that have been classified as "excellent" over a longer period of time.

Bathing Water Directive, sampling frequency, water quality, fecal indicator bacteria

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