Mercury in California Rice Systems, 2013

 

Project Leader

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

The overall objective of this new 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 the soil, but mining (gold and mercury) operations in the mountain ranges around the Sacramento Valley may also contribute to soil mercury levels. Under flooded conditions mercury methalates and forms methyl mercury, which is more bioavailable.

The first step in this project was to assemble a research team to study the issue: representatives from the California Rice Commission, the U.S. Geological Survey, and UC Davis.

The team identified four government reports on methyl mercury in Sacramento Valley water bodies that covered eight years over a 10-year period from 1996 to 2006. Also, some unpublished data from the Central Valley Regional Water Quality Control Board was collected in 2011.

Analysis of this data focused on sites surrounding rice-growing areas. Sites on the main rivers above rice areas were compared to identify any variation of methyl mercury concentration in irrigation source waters. There was more variation in methyl mercury concentrations among sites on the Sacramento River than between the Sacramento River and sites on the Feather River and its tributary, the Yuba River. The lowest concentrations were seen on the Sacramento River above Bend Bridge. The Yuba River at Marysville, Feather River at Nicolaus, and the Sacramento River at Hamilton City all showed similar concentrations. The Sacramento River at Colusa had the highest methyl mercury concentrations of sites on main rivers above agricultural drainages.

Program Sampling period Number of sites Number of samples
National Water Quality Assessment (NAWQA)
February 1996 -
June 1998
5
23-29
Sacramento River Watershed Program (SRWP)
July 2000 -
April 2003
10
14-18
CALFED Bay-Delta Program
March 2003 -
June 2006
16
23-31
SRWP Proposition 50 funding
April 2006 -
August 2007
12
17-18
Central Valley regional Water Quality control Board
August and September 2011
18
2

All of the data were analyzed for any long-term changes in methyl mercury concentrations in the waterways of the region. Levels have remained consistent throughout. It is important to note that there were substantial changes in rice straw management during this period. Field burning was phased out and winter flooding increased. This does not appear to have affected methyl mercury concentrations in rice drainage waters.

Methyl mercury concentrations were plotted together using month and day of sample collection to determine whether there is a seasonal pattern in rivers and agricultural drains. A clear period of consistently low concentration values exists from June to October. From November through May the majority of measurements are also low, but there are spikes of much higher concentrations. This pattern is seen in agricultural drainages and mainstream sites. Averaged across sites, the winter season concentrations are about double that observed during the growing season.

This analysis has shown that methyl mercury concentrations in rice drainages tend to be elevated in the winter. Without stream flow data, however, it is difficult to determine whether this results in an increased export of methyl mercury to the Sacramento River. Existing data are not sufficient to determine the effect of rice systems in winter, so field studies will be needed.