Study on maximal allowable discharge amount of toxicant based on ecological risk

Because of the absence of water quality standards, it is difficult to estimate maximal allowable discharge amount (MADA) into a river system for most toxicants. This paper introduces the procedure and methodology of ecological risk assessment to the research of MADA for toxicants. The Fuxi River in Ningbo City is taken as a case to study the MADA of toxicant acrylonitrile. The basic method is to calculate ecological risk under different discharge amount, and estimate MADA based on both risk values and water ecological standards. Two approaches are proposed in this paper: joint probability curve approach and total ecologic risk approach. MADA under different protection level and hazard probability is estimated through joint probability curve approach; MADA under different total ecological risk is estimated through total ecologic risk approach. The former can provide various standards for toxicants with complicated computation, and the latter has less reference value in determination of MADA due to the limited information though it is easy to compute.