Arsenic in Groundwater, Soil and Surface Water in Rice Growing Areas, 2013


Project Leader

Erin Thatcher, CH2M Hill

Lisa Porta,
CH2M Hill

Summer Bundy, CH2M Hill

The U.S. Food and Drug Administration (FDA) is assessing the human health risk of arsenic in rice and rice-based foods. From a study of more than 1,000 rice product samples, the agency concluded, “the amount of detectable arsenic is too low in the rice and rice product samples to cause any immediate short-term adverse health effects.” The FDA also is investigating the potential for long-term health effects from rice consumption.

To improve understanding of the role that arsenic plays in California rice production, the consulting firm CH2M HILL was contracted to summarize existing water quality information and current knowledge of arsenic occurrence in the rice-growing areas of the Sacramento Valley. CH2M HILL consolidated the available data and research results into a technical memorandum that evaluated arsenic presence in the soil, groundwater, and surface waters of the watershed and presented avenues of research to consider for future work.

Arsenic occurs naturally in the soils and groundwater of the Sacramento Valley, with higher concentrations in areas of volcanic or hydrothermal geologic influence. The most common sources of arsenic in soils are iron oxyhydroxide minerals in oxidizing (aerobic) environments and sulfide minerals in reducing (anaerobic) environments.

Surface irrigation water is not indicated as a primary source of arsenic to rice fields. CH2M HILL analyzed surface water sampling data contained in databases from the Central Valley Regional Water Quality Control Board, the California Department of Water Resources, and the U.S. Geological Survey (USGS). A total of 1,478 surface water samples were tested for arsenic. Samples included surface water, irrigation water, and drainage water.

In all of the sites with multiple samples, the most recent arsenic concentrations were lower than the maximum historical concentrations. None of the measurements within the last 30 years exceeded 6.5 micrograms/liter, well below the U.S. Environmental Protection Agency drinking water standard of 10 micrograms per liter.

Arsenic concentrations in shallow groundwater underlying rice fields previously were analyzed for the California Rice Commission in the Rice-specific Groundwater Assessment Report(GAR). This analysis relied on well-monitoring data from USGS in various locations throughout the rice-growing areas of the valley.

At 25 of 28 shallow groundwater wells located near rice fields, arsenic concentrations were less than 10 micrograms/liter. At one well in 1997, a detection of 15 micrograms/liter was observed. In 2006 the same well showed an arsenic concentration of 4.9 micrograms/liter. This well is located near the Sutter Buttes, a volcanic formation. Two other wells had maximum concentrations of 11 and 10.4 micrograms/liter, respectively. Analysis of five wells sampled six times showed relatively stable concentrations.

The areas with the highest detection of arsenic are located in deeper wells around the Sutter Buttes and in the Sutter Basin. Recent observations, analyses, and results suggest that arsenic is naturally occurring and that rice agriculture is not adding to background arsenic content.

No studies evaluating background levels of arsenic in the soils of Sacramento Valley rice areas were identified at the time the technical memorandum was produced. However, additional USGS soil arsenic data have since been identified and will be evaluated.


If additional information is needed to characterize arsenic uptake into rice, or to assess practices that may influence arsenic uptake, the priority should be on irrigation and pest management practices. Specific research areas to consider include:

Soil sampling—Soil sampling for arsenic in rice-growing areas should continue, focusing on areas with higher shallow groundwater arsenic concentrations. This work should be coordinated with the USGS, which has been conducting studies on soil arsenic levels within the Sacramento Valley’s rice-growing areas. Soil samples should be taken from areas with both high and typical arsenic concentrations in shallow groundwater. Ideally, these sites would be colocated with other arsenic studies.

Soil and pore water arsenic evaluations—No Sacramento Valley-specific literature on the partitioning of arsenic from soil and pore water into rice plants is currently available. This information would improve understanding of arsenic’s bioavailability to rice plants in California rice fields.

Regional analysis—Areas south of the Sutter Buttes show comparatively higher levels of arsenic in shallow groundwater. Some of these areas may rely on groundwater for supplemental irrigation water and would be likely candidates for field-level research.

Phosphate studies—Because rice plants uptake phosphate and arsenic similarly, the effect of increasing or decreasing phosphate levels on arsenic uptake should be determined. Additional literature review should be undertaken before advancing any field studies.

Trace arsenic in fertilizers—Trace concentrations of arsenic can be present in phosphate-based fertilizer and need to be documented. For cropping systems using these fertilizers, a study to quantify arsenic contamination in fertilizer may be warranted.

Field flooding duration—An area currently under study is examining reduced duration of flooded conditions in rice fields. Flooded conditions influence the mobility of arsenic, potentially increasing its bioavailability to rice. Additional study to understand annual variability, as well as impacts on agronomic performance, is warranted.