Pesticide concentrations in surface waters of the Sacramento Valley rice-growing regions, 2010

Surface water samples were collected from four stream/agricultural drain sites in the Sacramento Valley of California to measure pesticides commonly applied to rice. Samples were collected weekly from May through August 2010 to capture the rice pesticide application season. Water samples were filtered (0.7 ?m) and extracted via solid-phase extraction. Additionally, the filter paper was solvent extracted to measure suspended sediment-associated pesticides. Both fractions were analyzed via gas chromatography-mass spectrometry. A total of 89 pesticides and pesticide degradates were measured in the dissolved water phase, while 14 pyrethroid insecticides were measured in the suspended sediment-associated fraction (pyrethroids are hydrophobic and known to associate with particulates). For the dissolved phase, 18 compounds were detected. The most frequently detected compounds included those pesticides applied to rice: azoxystrobin (detection frequency 100%, maximum concentration 128,000 ng/L), clomazone (96%, 19,400 ng/L), 3,4-dichloroaniline (propanil degradate; 92%, 13,600 ng/L), thiobencarb (83%, 12,400 ng/L), and propanil (57%; 6,500 ng/L). Other frequently detected dissolved pesticides not applied to rice included boscalid (45%, 75.7 ng/L) and metolachlor (43%, 600 ng/L). Only two pyrethroids were detected in the suspended sediment fraction; this included lambda-cyhalothrin (17%, 14.8 ng/L), which was applied to rice, and bifenthrin, which is not specific to rice (6%, 26.3 ng/L).