The purpose of this test is to measure the time course of biodegradation of a test substance at low concentration in aerobic natural water and to quantify the observations in the form of kinetic rate expressions. The test is performed in batch by ‚"pelagic test", or ‚"suspended sediment test‚" to simulate a water body with suspended solids or re-suspended sediment. The test flasks are incubated in darkness at an environmental temperature under aerobic conditions and agitation during 60 days normally. At least two different concentrations of test substance (non-volatile or slightly volatile organic) should be used. The maximum test concentration should be less than 100¬µg/L (biodegradation following first order kinetics) and the lowest test concentration preferably in the range of ‚Äπ1-10¬µg/L. Two subsamples should be withdrawn from each test flask at each sampling time. Degradation is followed at appropriate time intervals, by measuring either the residual 14C or the residual concentration of test substance. The total mineralisation and the primary biodegradation are determined by a different 14C labelling part of the molecule. Periodic measurements of pH and oxygen concentration in the test system must be conducted.