Long term surface water simulations with STEPS-1-2-3-4

The new model STEP-1-2-3-4, an extension of the official FOCUS tool STEPS-1-2, is presented that can be used throughout the whole tiered approach for estimating PEC in surface water beginning a step 1 and finishing at level 4. In contrast to SWASH/TOXSWA, STEPS-1-2-3-4 is able to calculate the formation of metabolites in surface water and sediment and runs significantly faster. Results of test simulations demonstrate that STEPS-1-2-3-4 is able to adequately assess the fate of pesticide using the European FOCUS scenarios on all established tiers. Simulations considering the effect of additional buffer zones on level step 4 resulted in concentrations about ten times lower than the respective step 3 concentrations. First experiences with long term simulations over 20 years performed with STEPS-1-2-3-4 demonstrate that the official PEC surface water concentrations calculated based on the FOCUS scenarios and procedures are very much dependent on the selected simulation year and the year selected by FOCUS does not necessarily represent the desired 50th weather situation.