There aretoo few reported toxicity tests on whole sediments containing ammonia to estimate an appropriate ‘safe’ pore water concentration based on chronic effects data. However, given the ability of benthic organisms to regulate their exposure to porewater, the acute LC50 data are considered appropriate for guideline development.A value of 4.5 mg total NH3–N/L was calculated from the PC95 value based on acute benthic LC50s divided by 5 to convert to a NOEC, however, only 9 acute benthic data were available and mainly for amphipods, so the value is questionable. This value is not that different from the upper 80th percentile value of 3.9 which was therefore adopted as a conservative guideline.