Mercury in California Rice Systems, 2014


Project Leader

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

The overall objective 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 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.

This is the second year of this research project. In 2014 the following objectives were addressed in two separate studies:

• 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 are higher in certain parts of the region than in others.

• Compare data from typical rice systems with those in the Sacramento–San Joaquin Delta.

Literature review continued

Analysis of data gathered from government reports in 2013 continued in 2014. This included measurements of methyl mercury concentration from sites along the Sacramento and Feather rivers, as well as two main agricultural drains (Sacramento Slough and Colusa Basin).

Analysis revealed a seasonal pattern, where methyl mercury concentrations were higher in winter and spring, and lower in summer and fall. (See graph on following page.) Agricultural drainage sites and locations downstream of the confluence of the Sacramento and Feather rivers had higher concentrations in the winter than in the summer. Upstream locations on the Sacramento and Feather rivers did not change much between seasons.

This study determined that loads from rice drainages are small compared to loads in the Sacramento an dFeather rivers.

Methyl mercury concentrations were higher in agricultural drainages than other locations in both seasons. Upstream Sacramento River and Feather River locations had the lowest concentrations, while intermediate concentrations were observed in the confluence.

To better understand how much methyl mercury is coming from the agricultural drains, loads were calculated. The load is the concentration times the volume of water over time. However, load calculations have been limited by data gaps. For instance, there was no flow data available for Sacramento Slough. Nonetheless, this study determined that loads from rice drainages are small compared to loads in the Sacramento and Feather rivers.

Richvale field study

During the early part of the growing season, water was applied to a 55-acre field near Richvale. Three pulses were released onto the field at separate times. A maintenance flood was then established with relatively constant inflows and outflows.

Methyl mercury concentrations in inlet waters were consistently low. Concentrations in outlet waters decreased through the season. The concentration patterns suggest that this decrease may be caused by particle settling or a transpiration-driven flux to the root zone.

The imported methyl mercury load was consistently higher than the exported load when loads were calculated. Thus, the data show that this field was clearly a sink for methyl mercury.

The outlets of four other fields in the area were sampled one day in August. This information, once analyzed, will help determine whether the Richvale field used in the study is representative of other fields in the area. Monitoring at this site will continue throughout the year.

This research also compared information from other studies with the data from the Richvale site. It showed variation in methyl mercury production and export from rice systems. In particular, methyl mercury exports from the Sacramento Valley were lower than those from rice fields in the Yolo Bypass. This variation highlights the need for additional study.