Bruce Linquist, UCCE Rice Specialist, Dept. of Plant Sciences, UC Davis

The overall 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 guiding this research include:

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 alternating wet/dry (AWD) irrigation as a potential mercury management practice. A new study commenced in the fall of 2017 to compare mercury dynamics in six representative Sacramento Valley commercial rice fields receiving two types of irrigation water, fresh and recycled.

Background on mercury

While rice systems may be a source of mercury, loads from most 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 Preserve, 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 Preserve. Mercury concentrations in drainage water from Sacramento Valley rice fields are lower than drainage water concentrations in the other two areas.

Research to date also has shown 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.

Previous research also has shown that alternate wetting and drying water management affects methyl mercury in rice systems. This practice reduces mercury concentrations in water, soil, and rice grains and could be a mitigation practice to reduce mercury if needed.

Methyl mercury quantification

An important goal for 2017 and 2018 research was to quantify methyl mercury in filtered and particulate water samples going into and out of rice fields.

Current regulatory focus on methyl mercury is all based on unfiltered water samples.  However, much of the methyl mercury is likely to be bound in suspended sediment. This methyl mercury may be less bioavailable than that which is dissolved in water. From an ecosystem standpoint, dissolved methyl mercury is a better indicator of ecosystem health.

With that in mind, this newer area of research began evaluating in-field methyl mercury concentrations from filtered samples, while also quantifying the filtered methyl mercury concentrations in water leaving rice fields. Three pairs of commercial fields located in Butte, Colusa, and Yolo counties were selected to determine whether higher levels of methyl mercury in irrigation water result in higher concentrations in field flood water and tailwater. These fields are representative of major rice growing areas, had rice grown on them in the previous season, and kept straw on the flooded field during the winter. The inlets and outlets were set up with monitoring equipment to measure water flow rate throughout the year. The equipment is unobtrusive, so growers were able to manage their fields as usual.

One field from each pair received fresh irrigation water, while the other received recycled irrigation water. Samples were taken from the fields over the course of a year during both winter and the growing season. Grain from each field was sampled and analyzed for mercury and methyl mercury concentrations. Not all the data has been analyzed yet, but there are some preliminary results.

There was no significant difference in filtered inlet water methyl mercury concentrations between fresh water and recycled water fields. Across the whole year, filtered inlet water methyl mercury concentrations were lowest in winter. Methyl mercury inlet water concentrations started low in the spring, increased in early summer, and then declined near the time when fields were to be drained.

Outlet water methyl mercury concentrations were highest in the winter and lowest in late summer. There was little difference between the fresh and recycled water fields during this time. Concentrations were somewhat higher during summer maintenance drains. During this time, recycled fields had significantly higher methyl mercury concentrations in outlet water than the fresh water fields.

Field water methyl mercury concentrations were generally highest in the fall flood and lowest during preharvest drain. There was little difference between recycled water and fresh water fields during all sampling periods except during the winter drain and spring flood events. During those times, recycled water fields had significantly higher field water methyl mercury concentrations than did the fresh fields.

All six fields used in this research were net methyl mercury sources in the fallow season and net sinks in the growing season. Across all fields, filtered outlet water concentrations were about three to eight times higher in the fallow season than in the growing season.

Furthermore, because there is significant accumulation of rainfall in the field and relatively low rates of evapotranspiration in the winter, there are significantly larger volumes of water exported as drainage water during the fallow period (40% to 60% of applied irrigation water), causing the field to act as a net exporter of methyl mercury. In the growing season, outlet concentrations of methyl mercury are relatively low and only 13% to 25% of irrigation water applied was exported as drainage water, causing the fields to act as net sinks of mercury.

On an annual basis, fields receiving fresh irrigation water were net sinks of methyl mercury, while fields receiving recycled water were net sources of methyl mercury. The magnitude of import and export varied greatly regardless of irrigation type.

Public engagement and publications

In addition to these research activities, scientists working on this project are involved with various stakeholder groups that are concerned with methyl mercury in the Sacramento–San Joaquin Delta. In conjunction with the California Rice Commission, project scientists have met several times with growers to discuss findings and to gather thoughts on mitigation options. At this point, however, there is no foreseen need for mitigation given the relatively low levels of methyl mercury.

Publication of research results is crucial, as mercury in rice systems is likely to become more of a regulatory issue. Peer reviewed papers are critical in guiding scientifically based regulation.

Several papers have been published on this work, including The Contribution of Rice Agriculture to Methylmercury in Surface Waters: A Review of Data from the Sacramento Valley, California in the Journal of Environmental Quality; Alternate Wetting and Drying Decreases Methyl Mercury in Flooded Rice (Oryza sativa) Systems in the Soil Science Society of America Journal; and Methyl Mercury Dynamics in Upper Sacramento Valley Rice Fields with Low Background Soil Mercury Levels in the Journal of Environmental Quality.