Probabilistic approximation to risk assessment of basins by ecotoxicological evaluation

The present work shows a computer model for Risk Assessment of Basins by Ecotoxicological Evaluation (RABETOX), using a probabilistic approximation based on Monte Carlo analysis. This feature extends the forecasting capability of the model and provides the information needed to cover variability and uncertainty when supporting decision making. The model, based on system dynamics, has been developed on a spreadsheet (Microsoft Excel) under the following conditions: 1) The model comprises three subroutines: Volume Management, Hazard Management and Risk Forecasting. 2) The Volume Management block estimates flows for a variable number of segments settled on a constant time basis. Flow changes, inputs/outputs, and corrections based on measured data, are associated to the segments. Cumulative time an cumulative distance are also calculated. 3) The Hazard Management flow has as inputs, the effluent characteristics (Input position, flow, toxicity, half life and margin of safety) needed to calculate de hazard of each particular emission. The program calculates the initial hazard in the input points and their evolution downstream depending on the half life of each effluent. 4) The establishment of a general protocol for WEA, must deal with total hazard of effluents assessing not only toxicity but other parameters such persistence and bioaccumulation. These parameters have been introduced in the model through half live (persistence) and margin of safety (bioaccumulation). 5) The Risk Forecasting has been implemented by the Crystal Ball 2000 application (Decisioneering, Inc) based in Monte Carlo analysis. The risk probability distribution for the risk associated to each segment is presented. The RABETOX model is able to manage the whole risk of a complete basin, with different effluents inputs, informing about the risk characterization in every point of the basin and forecasting about the probability eds.