Relevance of temporal and spatial variability for monitoring the microbiological water quality in an urban bathing area

The microbiological quality of bathing waters is a fundamental concern for the use of bathing areas. Although the present European Bathing Directive (2006/7/EC) broke new ground in what monitoring, harmonized methods and practices of analysis are concerned, several questions remain unanswered. This study investigated the variation of fecal indicator bacteria of an urban beach area at different spatial and temporal scales. Intensive hourly and spatial sampling was conducted during the most crowded months of the bathing season in order to identify major pollution sources and their influence on the bathing water quality. Temporal variations explained approximately 76% of the overall variability, emphasizing the importance of sample retrieval timing, but also regional events in obtaining reliable results. Although spatial variation explained only 24% of the total variability, it highlighted the fact that the physical and hydrodynamic characteristics of adjacent beach areas, as well as the magnitude of various local inputs also may decisively influence the overall water quality. As a result, small-scale variations of water quality should be identified and accounted when designing the bathing water profiles required by the Directive 2006/7/EC. In order to reduce the public health threats posed by pollution events, water monitoring programs should be planned to be beach specific, especially in areas where these events are more likely to occur.