Formulation of water pollutant discharge limits for malathion based on nonsensitive aquatic organism protection

The "Integrated Wastewater Discharge Standard" was implemented for water pollutant discharge in China's pesticide industry, which has no control requirements for particular wastewater pollutants in the industry. In the standard, certain pollutants discharge limits are limited strictly or loosely, resulting in practical management implementation difficulties. In view of the highly selective targeting characteristics of organic pesticide active ingredients in fungicides, insecticides, and herbicides, a method for deriving discharge limits based on the water quality criteria for pesticides for the protection of nonsensitive species is established based on the idea of fully protecting aquatic organisms beyond sensitive objects. Through the use of malathion as an example, by screening its toxicity data in different species of aquatic organisms, the sequence point with the most significant change in the acute toxicity sensitivity of the species is taken as the variation point in the cumulative frequency of the sensitive and nonsensitive species to derive the water quality criteria, using the species sensitivity distribution method as the scientific basis for determining the discharge limits. After a comparative analysis of different simulation models, the sigmoid model, with the best fit, is selected to determine that the sensitive species hazard concentration (HC_s) of malathion to aquatic organisms in China is 46.4 µg/L, and the discharge limit derived from the HC_s based on the relationship between the environmental capacity and emissions is rounded to 250 µg/L. Studies showed that the relationship between the emissions limit derived from the water quality criteria for protecting nonsensitive species and malathion limit stipulated in the "Environmental Quality Standards for Surface Water" conforms to the corresponding relationship of the quality standard and discharge standard, which can be achieved by current pollution control technology, combined with water quality improvement. The discharge limit offers the advantages of technical accessibility and economic rationality.