Rice field drainage water and methylmercury in the Sacramento Valley
In flooded soils, such as those in rice fields, inorganic mercury can be converted to methylmercury, which is far more toxic and bio-accumulative. In the Sacramento Valley, CA, mercury is a concern due to a history of mining, and rice is grown on 240,000 ha, but it is unclear how much methylmercury is exported from the rice-growing region.
In a recent article published in the Journal of Environmental Quality, researchers report on 10 years of historical methylmercury concentration data in the Sacramento River and major agricultural drainage canals.
Compared with irrigation source water that is diverted from the river, rice drainage water concentrations were significantly elevated during November–May, when fallow rice fields are flooded to decompose rice straw, but not during the rice-growing season (June–October). Methylmercury loads (concentration x flow) from agricultural drainage were lower than would be predicted based on studies of MeHg export from rice at the field scale.
These results suggest that rice may be a less important contributor of methylmercury to the Sacramento River than previously thought. Since methylmercury concentrations and loads in drainage water are higher in November–May, management efforts during this period will be best able to reduce methyl mercury export from rice fields.