Disappearance rates of chlorothalonil (TCIN) in the aquatic environment

Chlorothalonil (TCIN) is a chlorinated isopthalonitrile fungicide with low water solubility. It is highly toxic to fish with 96h-LC50 values in the range 10 - 30 micro g/L, and is rapidly metabolised to glutathione conjugates. It is released into streams at low total concentrations after agricultural spraying operations. A series of experiments were performed using TCIN and 14C-TCIN in aqueous solutions from different sources, at different temperatures and with different stream substrates, in order or evaluate the nature and rates of TCIN disappearance and the appearance of polar derivatives of TCIN (DTCIN) in the aquatic environment. Results indicate that TCIN is readily biodegraded at low concentrations and cannot be regarded as a persistent pollutant. The log octanol-water partition coefficient was 4.38 (SD 0.12). Concentration decline was much greater at 15 deg C than 5 deg C, with a Q10 of 1.8. Analysis of suspended material contaminated with TCIN demonstrated significant binding to suspended material with an average log partition coefficient of 5.97, and with an average of 81% of total TCIN being bound to suspended matter. Periphyton (aufwuchs) on stones greatly increased the rate of decline in water column concentrations