Organophosphorus Insecticides in Agricultural and Residential Runoff:  Field Observations and Implications for Total Maximum Daily Load Development

Development of total maximum daily loads (TMDLs) for nonpoint source pollutants requires mass flux estimates for targeted compounds from contributing sources. We measured organophosphorus insecticide concentrations in surface runoff from agricultural and residential land-use sites in a southern Californian watershed over the course of runoff-producing irrigation and rainfall events. Event mean concentrations (EMCs) for chlorpyrifos, diazinon, and malathion exhibited considerable variability among irrigation and storm runoff events at agricultural sites; residential storm runoff EMCs for these compounds were considerably less variable. Event loads and EMCs were higher for runoff events following reported insecticide applications. Organophosphorus insecticide EMCs were not consistently correlated with hydrologic characteristics of runoff events. Our results indicate that on an area basis, loads from residential land may exceed those from sites planted in row crops for a given rainfall depth, suggesting that residential land use warrants explicit consideration in TMDL development and implementation. No consistent first flush effect was discernible for organophosphorus insecticides in storm or irrigation runoff. A relative potency factor approach is introduced to permit evaluation of organophosphorus insecticides on a common toxicological basis and allow development of TMDLs and pollutant control strategies for these compounds as a class.