Mercury in California Rice Systems, 2016

 

Project Leader

Bruce Linquist, UCCE rice specialist, Dept. of Plant Sciences, UC Davis

The goal of this project is to determine whether methyl mercury discharged from California rice systems poses a health risk to humans, fish, or wildlife and, if so, how this risk can be cost-effectively minimized.

Mercury is naturally present in some soils of the Sacramento Valley. Under flooded conditions in California rice fields, mercury forms methyl mercury and becomes more bioavailable. Objectives in this research project include:

  • Identify the annual cycle of methyl mercury concentration and loads in the major rivers of the Sacramento Valley.
  • Determine whether methyl mercury production and discharge from rice systems is higher in certain parts of the region.
  • Compare data from typical rice systems with those in the Sacramento–San Joaquin Delta.
  • Identify viable management practices to cost-effectively minimize methyl mercury production where risks occur.
  • Identify underlying causes where methyl mercury poses a risk.

Since 2013, when this project began, three studies have been completed. These include using historical data to assess methyl mercury export from rice producing areas; determining mercury budgets at the field scale; and using Alternate Wetting and Drying (AWD) irrigation as a potential mercury management practice.

  • Mercury assessment

    Mercury concentration data in irrigation sources (Sacramento and Feather rivers) and agricultural drains (Colusa Basin, Sacramento Slough) collected between 1996 and 2007 were used to assess mercury contributions from rice field discharges in the Sacramento Valley.

    While Sacramento Valley rice systems may be a source of mercury, loads from rice fields are small—much less than what is found in rice grown in the Sacramento–San Joaquin Delta. Peak periods of mercury concentration in drainage water exiting rice fields occur primarily during the fallow season and to a lesser extent during the early growing season. This is a consistent pattern in studies conducted in the Sacramento Valley, Cosumnes River area, and the Yolo Bypass.

    Sacramento Valley rice fields have lower levels of mercury in soils and irrigation water than rice fields in the Yolo Bypass and Cosumnes River area. Mercury concentrations in drainage water from Sacramento Valley rice fields are lower than drainage water concentrations in the other two areas.

    Research also showed that rice grain concentrations of mercury are very low—among the lowest reported in the literature and well below levels of concern for human health.

    Growing season rice drainage water mercury concentrations are lower than in the fallow season. During the growing season, most rice fields become mercury sinks. During the fallow season, most rice fields are mercury sources. That is, they export more mercury than they import through irrigation water.

    AWD irrigation

    A controlled, replicated experiment was conducted to determine how AWD affects methyl mercury in rice systems. This practice was found to reduce mercury concentrations in water, soil, and rice grains and could be a mitigation practice to reduce mercury if needed.

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