Just out, news that rice is a major pathway for methylmercury exposure in a mining community in Inland China. This is big news because fish consumption has been the focus of exposure assesments for mercury exposure. Mercury, when methylated (hence methylmercury) can be devastating to developing fetuses, causing severe brain damage. For a detailed history of a methylmercury incident look up “Minimata, Japan“. Keep in mind this was an unusual tragedy and human exposures are normally much lower than those that occurred there.
Would have to search to see if anyone has done similar work to test produce or other agricultural projects near US coal plants, but it would be interesting to know (hopefully soon). Mercury uptake by plants apparently varies by species, with most of it remaining in roots, and most mercury detected in/on plant leaves may be from simple settling of dust. To the right is a USGS map of wet mercury deposition in the (on the) US. Above is the link for further information. Health effects are minimal for unmethylated mercury, with methylation being performed by anaerobic bacteria in oxygen poor environments . . . which you would see in swamp bottoms, ocean floor etc. . . and would probably not (to the best of my knowledge) be occurring in common agricultural conditions. Rice fields, being flooded and marsh-like, may provide good conditions for methylated microorganisms.
Further note, China does not export much rice and mercury in rice may be only a local problem.
Zhang H, Feng X, Larssen T, Qiu G, Vogt RD, 2010 In Inland China, Rice, rather than Fish is the Major Pathway for Methylmercury Exposure. Environ Health Perspect doi:10.1289/ehp.1001915