Methylmercury dynamics in Upper Sacramento Valley rice fields with low background soil mercury levels

Few studies have considered how methylmercury (MeHg, a toxic form of Hg produced in anaerobic soils) production in rice (Oryza sativa L.) fields can affect water quality, and little is known about MeHg dynamics in rice fields. Surface water MeHg and total Hg (THg) imports, exports, and storage were studied in two commercial rice fields in the Sacramento Valley, California, where soil THg was low (25 and 57 ng g⁻¹). The median concentration of MeHg in drainage water exiting the fields was 0.17 ng g⁻¹ (range: <0.007–2.1 ng g⁻¹). Compared with irrigation water, drainage water had similar MeHg concentrations, and lower THg concentrations during the growing season. Significantly elevated drainage water MeHg and THg concentrations were observed in the fallow season compared with the growing season. An analysis of surface water loads indicates that fields were net importers of both MeHg (76–110 ng m⁻²) and THg (1947–7224 ng m⁻²) during the growing season, and net exporters of MeHg (35–200 ng m⁻²) and THg (248–6496 ng m⁻²) during the fallow season. At harvest, 190 to 700 ng MeHg m⁻² and 1400 to 1700 ng THg m⁻² were removed from fields in rice grain. Rice straw, which contained 120 to 180 ng MeHg m⁻² and 7000–10,500 ng m⁻² THg was incorporated into the soil. These results indicate that efforts to reduce MeHg and THg exports in rice drainage water should focus on the fallow season. Substantial amounts of MeHg and THg were stored in plants, and these pools should be considered in future studies.