Why should we care about Carrying Capacity? A novel screening tool for the health risk in recreational waters near estuary

The present work proposes a novel screening tool to improve the quality of recreational coastal water. Indeed, the recreational potential of beach resort depends on its health status, which in marine cities may be threatened by increasing stress produced by anthropogenic activity. In particular, we focus on the beach near an estuary, which may be affected by a considerable load of contaminants, especially when the urban sewage system is combined and designed to spill untreated wastewater directly in the coastal water. In a few words, when the Combined Sewer Overflows (CSOs) are activated, the bacterial concentration in the estuary increases, thereby resulting in a potential hazard for the swimmers’ health. In the present work, the bacterial transport is modelled through a physically-based stochastic framework, whereas the human health risk is evaluated by means of the Quantitative Microbial Risk Assessment (QMRA). As the human health risk is quantified, it is used to evaluate the Carrying Capacity indicator of the recreational coastal water. This indicator is defined as the number of swimmers that can be sustained by coastal water with an acceptable risk threshold. The results indicate that the Carrying Capacity increases by dilution processes and by reduction of the source concentration. This indicator may be viewed as a screening tool for policy-makers and other stakeholders. For instance, it can help to balance the resources needed to improve the sewage-system and the benefits coming from tourism and sustainable environmental policies, given that the beach quality, in turn, depends on the improvements in the sewage system.