Fate of malathion andO,O,S-trimethyl phosphorothioate by-product in Hawaiian soil and water

Abstract

The fate of malathion (I) and a formulation byproduct,O,O,S-trimethyl phosphorothioate (II), were determined in soil and water from an agricultural area on Kauai, Hawaii. Rapid degradation of I was observed in laboratory soil microcosms (t1/2 = 8.2 h) and in the field (t1/2 = 2 h) and dimethyl phosphorodithioic acid and diethyl fumarate were identified as degradation products. Degradation of II was relatively slow in laboratory soil microcosms (t1/2 = 6.4 d) and the degradation rate of both I and II decreased significantly after γ irradiation. Degradation of I in river, ground and seawater (avg. t1/2 = 4.7 d) was controlled by an elimination reaction; photolysis and biodegradation played minor roles. Dissipation of II in river water was primarily by biodegradation. Strong sorption of I to Lihue silty clay soil (Koc = 405) was determined while II was weakly bound (Koc = 9.8). Rapid degradation and strong sorption of I suggest a low potential for groundwater contamination. Although II degrades slowly and is weakly bound to Lihue soil, the extremely low concentrations found in the dilute formulations applied in fruit-fly control should not present a groundwater problem.