Rice field surface microlayers: Collection, composition and pesticide enrichment

Abstract

The thin organic film (surface microlayer) found on natural waters can concentrate hydrophobic pollutants. Enrichment factors (EFs), the ratio of pollutant concentration in microlayer to that in subsurface water, previously have been measured in relatively thick (˜50–150 μm) aqueous sections of surface. A modified sampling method, using glass plates silanized with dichlorodimethylsilane, allowed the collection of samples with virtually no dilution by subsurface water. Surface microlayers from a flooded rice field, collected onto silanized plates and analyzed by GC-MS, showed 26 fatty acids and esters of 8 to 24 C atoms, saturated and unsaturated, and methyl esters of nonanoic, hexacosanoic, and octacosanoic acids as well as cholesterol. Microlayers were collected with both silanized and unsilanized glass plates after application of carbofuran (3.27 kg/ha) and a mixture of carbofuran (3.12 kg/ha) and thiobencarb (3.04 kg/ha) to 9.8-m2 field plots. Log EF was 0.14 to 1.43 for carbofuran and 0.04 to 2.30 for thiobencarb in samples collected with unsilanized plates; log EF from silanized plates was 3.88 to 7.55 and 4.04 to 5.74 for carbofuran and thiobencarb, respectively. Log EFs in samples collected from a commercial rice field with silanized plates were 3.88 and 4.18 for carbofuran and 4.77 and 5.15 for thiobencarb, greater than predicted from their Kow. Thiobencarb irradiated (285–340 nm) in isolated microlayers was photodegraded more rapidly than by itself. The only identified photoproduct was 4-chlorobenzyl mercaptan. These observations have significance for pesticide fate, analysis, and ecotoxicology.