E. coli is a thermo-tolerant coliform bacterium from the family Enterobacteriaceae present in the faeces of mammals and birds. Most strains can also be found in the human gastrointestinal tract and represent an important part of our intestinal flora. E. coli is rarely found in water or soil without faecal contamination, making it suitable for detecting faecal contamination. To avoid infection in the marine environment or with seafood it is important to ensure that both meet the requirements - bathing water must meet minimum hygiene requirements while shell farms must not surpass the highest allowed number of E. coli in seashells. In this thesis we examined the factors affecting E. coli decay rate in seawater. Chick's law is used to calculate the changes in concentration of bacteria with time, where concentration is calculated using the exponential decay rate. We identified the equation that includes most of the impacts; however, due to the complexity of factors influencing the decay, they are limited to measurable physical factors. In Mancini's equation these are the intensity of solar radiation, water depth, temperature and salinity. With characteristic data for the Gulf of Trieste we first performed a sensitivity analysis, then calculated the decay rate also for the data obtained from the company Harpha Sea d.o.o. The equation is suitable for implementation in numeric models used at UL FGG. With the improved models it will be possible to evaluate the likelihood of pollution in bathing waters and
seashell farms in Slovenian waters.