Capacité d'acceptation du milieu marin bactériologique de la rade de Brest

The study 'admission capacity of the marine environment" was conducted by IFREMER 1980 to 1983 in the Brest Roadstead at the request of the Ministry of Environment. This study was financed jointly by the Board of Health and IFREMER. The problem was the study of the fate of bacterial discharges in a sea subjected to tides. The selected approach consisted to simulate discharges from the principal pollution sources and to calculate the theoretical dilution of fecal bacteria in the Northern zone of the Roadstead. Spreading coefficients were determined by "on the site" measurements of parameters which may be considered as conservative in defined periods (e.g. ammonium in winter). Results show that in the conditions of the styay, the disappearance of fecal coliforms is very slow, the calculated T90 being of the order of 2 to 3 days. Various explanations may be proposed. The major one is due to the fact that most bacteria discharges by sewers are absorbed on or in the suspensions more or less rich in organic matter. In such conditions, bacteria which reach the sea are protected, they find on particles the conditions required for their persistence, and are able to settle. During agitation periods (plashmg, spring tide current. ..), they can be suspended agam, and thus cause a new contamination of coastal waters.