H2S was found to be Increasing in Gullfaks produced sea water. Wells supported by the same injector showed similar development indicating an association between the injector and the H2S. Statoil's biofilm model was used to history match this behaviour to set the input parameters and enable a prediction to be made for each well. Some wells will exceed the process H2S limits and solutions are discussed. Produced water re-injection was shown to be detrimental to souring reservoirs.