In coastal regions, ecological quality and economic development are highly sensitive to the quality of seawater, which often depends on the discharge of anthropogenic pollution through marine outfalls. This is the case of Cullera Bay (Spanish Mediterranean coast), where bacterial pollution problems are frequent during the tourist season due to its physical configuration, prevailing meteorological conditions, wastewater discharge from a shallow marine outfall, and the input from the Júcar River. The analysis of the data obtained during five field campaigns done in Cullera Bay between July 2002 and April 2003 to characterize the distribution of bacteria in the seawater along the coast show that, although the quality of the water is good (according to the EU Bathing Water Quality Directive on E. coli), bacteria counts can become very high at the stations closest to Cullera Cape. These concentration peaks appear to be due to the sporadic filtration through the karstic headland of untreated wastewater from ill-maintained domestic sewage systems, but also to the barrier effect exerted by the Cape upon the hydrodynamic circulation induced by S-ESE winds, as shown by numerical simulations. Further modeling suggests that the water quality in the northern part of the bay could be improved by lengthening the marine outfall, thus reducing the trapping effect induced by Cullera Cape.