The Contribution of Rice Agriculture to Methylmercury in Surface Waters: A Review of Data from the Sacramento Valley, California

Abstract

Methylmercury (MeHg) is a bioaccumulative pollutant produced in and exported from flooded soils, including those used for rice (Oriza sativa L.) production. Using unfiltered aqueous MeHg data from MeHg monitoring programs in the Sacramento River watershed from 1996 to 2007, we assessed the MeHg contribution from rice systems to the Sacramento River. Using a mixed‐effects regression analysis, we compared MeHg concentrations in agricultural drainage water from rice‐dominated regions (AgDrain) to MeHg concentrations in the Sacramento and Feather Rivers, both upstream and downstream of AgDrain inputs. We also calculated MeHg loads from AgDrains and the Sacramento and Feather Rivers. Seasonally, MeHg concentrations were higher during November through May than during June through October, but the differences varied by location. Relative to upstream, November through May AgDrain least‐squares mean MeHg concentration (0.18 ng L−1, range 0.15–0.23 ng L−1) was 2.3‐fold higher, while June through October AgDrain mean concentration (0.097 ng L−1, range 0.6–1.6 ng L−1) was not significantly different from upstream. June through October AgDrain MeHg loads contributed 10.7 to 14.8% of the total Sacramento River MeHg load. Missing flow data prevented calculation of the percent contribution of AgDrains in November through May. At sites where calculation was possible, November through May loads made up 70 to 90% of the total annual load. Elevated flow and MeHg concentration in November through May both contribute to the majority of the AgDrain MeHg load occurring during this period. Methylmercury reduction efforts should target elevated November through May MeHg concentrations in AgDrains. However, our findings suggest that the contribution and environmental impact of rice is an order of magnitude lower than previous studies in the California Yolo Bypass. Core Ideas We studied the impact of rice production on MeHg at the watershed scale. MeHg concentration was elevated in agricultural drainage water during November through May. Watershed‐scale MeHg loads were lower than expected based on field studies.

Methylmercury Dynamics in Upper Sacramento Valley Rice Fields with Low Background Soil Mercury Levels

“…1). Butte and Yolo are located within the area studied by Tanner et al (2017) who reported similar irrigation and drainage water MeHg concentrations and seasonal patterns.…”

“…Finally, decomposing rice straw may promote Hg(II)‐methylation by providing labile organic carbon, if the Hg(II)‐methylating microbes are limited by carbon (Marvin‐DiPasquale et al, 2014; Windham‐Myers et al, 2014b). However, the relationship between increased MeHg production and bioaccumulation or export is complex (Eagles‐Smith et al, 2014; Zhu et al, 2015; Tanner et al, 2017). …”

“…More research is needed to understand fluctuations of MeHg concentrations in rice drainage water during the fallow season before management recommendations are identified. Furthermore, previous studies of MeHg export from rice systems included a flooded fallow season for straw decomposition (Eagles‐Smith et al, 2014; Windham‐Myers et al, 2014b; Tanner et al, 2017). Cropping systems with different annual cycles of dry and wet periods (such as the absence of a flooded fallow season, or production of multiple crops per year), likely have different seasonal patterns of MeHg export.…”

“…Downstream in the Sacramento‐San Joaquin Delta (hereafter referred to as the “Delta”), MeHg concentrations are elevated and negative impacts of Hg on wildlife have been documented (Ackerman et al, 2014). Both field‐scale and watershed‐scale studies report rice field drainage waters have elevated MeHg concentrations and/or loads during the fallow season (Alpers et al, 2014; Bachand et al, 2014; Eagles‐Smith et al, 2014; Tanner et al, 2017), when fields are flooded to decompose rice straw. However, the field‐scale studies (Alpers et al, 2014; Eagles‐Smith et al, 2014) report concentrations that are considerably higher than observed at the watershed scale (Tanner et al, 2017).…”

“…Both field‐scale and watershed‐scale studies report rice field drainage waters have elevated MeHg concentrations and/or loads during the fallow season (Alpers et al, 2014; Bachand et al, 2014; Eagles‐Smith et al, 2014; Tanner et al, 2017), when fields are flooded to decompose rice straw. However, the field‐scale studies (Alpers et al, 2014; Eagles‐Smith et al, 2014) report concentrations that are considerably higher than observed at the watershed scale (Tanner et al, 2017). This may be because field‐scale studies were conducted in areas known to receive Hg‐laden sediment from mining regions during flood events (Singer et al, 2013), whereas sediment transport from mining areas into the main Sacramento Valley rice‐growing region is limited by dams (Slotton et al, 1995; Supplemental Fig.…”