Mercury cycling in agricultural and managed wetlands, Yolo Bypass, California: Spatial and seasonal variations in water quality Charles N. Alpers a, ⁎, Jacob A. Fleck a,1 , Mark Marvin-DiPasquale b,2 , Craig A. Stricker c,3 , Mark Stephenson d,4 , Howard E. Taylor e,5 a U.S. Geological Survey, Placer Hall, 6000 J St., Sacramento, CA 95819, United States b U.S. Geological Survey, 345 Middlefield Road, Mailstop 480, Menlo Park, CA 94025, United States c U.S. Geological Survey, Fort Collins Science Center, Building 21, Mailstop 963, Denver, CO 80225, United States d Moss Landing Marine Laboratory, Moss Landing, CA 95039, United States e U.S. Geological Survey, 3215 Marine St., Suite E-127, Boulder, CO 80303, United States HIGHLIGHTS • Surface water Hg chemistry was studied in agricultural and non-agricultural wetlands. • MeHg concentrations were highest during wild-rice harvest and winter flooding. • MeHg concentrations correlated with Mn, Fe, DOC, and δ 34 S of dissolved sulfate. • Sulfate-bearing fertilizer amendments had no effect on Hg(II)-methylation. abstract article info Article history: Received 5 August 2012 Received in revised form 25 October 2013 Accepted 27 October 2013 Available online 14 December 2013 Keywords: Methylmercury Sulfate-reduction Iron-reduction Manganese Rice agriculture Wild rice The seasonal and spatial variability of water quality, including mercury species, was evaluated in agricultural and managed, non-agricultural wetlands in the Yolo Bypass Wildlife Area, an area managed for multiple beneficial uses including bird habitat and rice farming. The study was conducted during an 11-month period (June 2007 to April 2008) that included a summer growing season and flooded conditions during winter. Methylmercury (MeHg) concentrations in surface water varied over a wide range (0.1 to 37 ng L -1 unfiltered; 0.04 to 7.3 ng L -1 filtered). Maximum MeHg values are among the highest ever recorded in wetlands. Highest MeHg concentrations in unfiltered surface water were observed in drainage from wild rice fields during harvest (September 2007), and in white rice fields with decomposing rice straw during regional flooding (February 2008). The ratio of MeHg to total mercury (MeHg/THg) increased about 20-fold in both unfiltered and filtered water during the growing season (June to August 2007) in the white and wild rice fields, and about 5-fold in fal- low fields (July to August 2007), while there was little to no change in MeHg/THg in the permanent wetland. Sulfate-bearing fertilizer had no effect on Hg(II) methylation, as sulfate-reducing bacteria were not sulfate- limited in these agricultural wetlands. Concentrations of MeHg in filtered and unfiltered water correlated with filtered Fe, filtered Mn, DOC, and two indicators of sulfate reduction: the SO 4 2- /Cl - ratio, and δ 34 S in aqueous sul- fate. These relationships suggest that microbial reduction of SO 4 2- , Fe(III), and possibly Mn(IV) may contribute to net Hg(II)-methylation in this setting. Published by Elsevier B.V. 1. Introduction Freshwater wetlands are important sites for inorganic mercury (Hg(II)) methylation and monomethylmercury (MeHg) bioaccumula- tion (Fitzgerald and Lamborg, 2007). For example, in the experimental lake area of Ontario, Canada, it was shown that watersheds with wet- lands contributed far more MeHg than watersheds with lakes (stratified and non-stratified) and riparian habitats (St. Louis et al., 1996). Similar results have been found in other areas where seasonally flooded Science of the Total Environment 484 (2014) 276–287 ⁎ Corresponding author. Tel.: +1 916 278 3134; fax: +1 916 278 3013. E-mail addresses: cnalpers@usgs.gov (C.N. Alpers), jafleck@usgs.gov (J.A. Fleck), mmarvin@usgs.gov (M. Marvin-DiPasquale), cstricker@usgs.gov (C.A. Stricker), mstephenson@mlml.calstate.edu (M. Stephenson), hetaylor@usgs.gov (H.E. Taylor). 1 Tel.: +1 916 278 3063; fax: +1 916 278 3079. 2 Tel.: +1 650 329 4442; fax: +1 650 329 4463. 3 Tel.: +1 303 236 7908; fax: +1 303 236 4930. 4 Tel.: +1 831 771 4170; fax: +1 831 633 0128. 5 Tel.: +1 303 541 3007; fax: +1 303 541 3084. 0048-9697/$ – see front matter. Published by Elsevier B.V. http://dx.doi.org/10.1016/j.scitotenv.2013.10.096 Contents lists available at ScienceDirect Science of the Total Environment journal homepage: www.elsevier.com/locate/scitotenv