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The Contribution of Rice Agriculture to Methylmercury in Surface Waters: A Review of Data from the Sacramento Valley, California
Ist Teil von
Journal of environmental quality, 2017, Vol.46 (1), p.133-142
Ort / Verlag
United States: The American Society of Agronomy, Crop Science Society of America, and Soil Science Society of America, Inc
Erscheinungsjahr
2017
Quelle
Wiley Online Library Journals Frontfile Complete
Beschreibungen/Notizen
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.