This Test guideline describes studies on phototransformation in water to determine the potential effects of solar irradiation on chemicals in surface water, considering direct photolysis only.

It is designed as a tiered approach. The Tier 1 is based on a theoretical screen. The rate of decline of a test chemical in a direct photolysis study is generally assumed to follow pseudo first-order kinetics. If the maximum possible losses is estimated to be superior or equal to 50% of the initial concentration over a 30-day period, an experimental study is proceeded in Tier 2. The direct photolysis rate constants for test chemicals in the laboratory is determined using preferably a filtered xenon arc lamp capable of simulating natural sunlight in the 290 to 800 nm, or sunlight irradiation, and extrapolated to natural water. If estimated losses are superior or equal to 20%, the transformation pathway and the identities, concentrations, and rate of formation and decline of major transformation products are identified. An optional task is the additional determination of the quantum yield for various types of water bodies, seasons, and latitudes of interest.

The test chemical should be directly dissolved in the aqueous media saturated in air at a concentration which should not exceed half its solubility. For linear and non-linear regressions on the test chemical data in definitive or upper tier tests, the minimum number of samples collected should be 5 and 7 respectively. The exact number of samples and the timing of their collection is determined by a preliminary range-finding. Replicates (at least 2) of each experimental determination of kinetic parameters are recommended to determine variability and reduce uncertainty in their determination.