Document Type

Master's Thesis

Degree Name

Master of Science (M.S.)


Moss Landing Marine Laboratories


The San Francisco Bay-Delta Complex is contaminated with mercury, and many fish tissue concentrations exceed US Food and Drug Administration (FDA) limits for human consumption. Much of the mercury is historic and can be traced to contaminated sediments from hydraulic mining. Today, contamination continues from two major sources: mercury mines in the coast range and gold mines in the Sierra foothills. Mercury from both watershed sources is methylated in receiving sediments within the Delta. Little is known about the relative bioavailability and chemical reactivity of this mercury once incorporated into Delta sediments. To prioritize mitigation options, this study assessed methylation efficiency (ratio of methylmercury:total mercury) at three Delta locations using laboratory and field experiments with mixed and transplanted sediment. Methylation efficiency was found to be greatest for Sierra sediment and lowest for coast range sediment. Methylation efficiency of ionic mercury was spatially and temporally variable, though during the summer was greater than for other forms of Hg (including controls). Methylmercury production was proportional to the total mercury (THg) concentration in sediments, yet efficiency of this transformation depended on receiving and source sediment. Overall, field results using in situ sediment transplant experiments substantiate laboratory findings. Together, these results indicate that reductions in THg are an effective strategy for the reduction of methylmercury in the San Francisco Bay-Delta Complex and should reduce biota methylmercury exposure. Due to source strength and reactivity, this study suggests that elemental mercury from abandoned gold mines in the Sierras should be the highest priority for mitigation.


Thesis (M.S.) Division of Science and Environmental Policy. Moss Landing Marine Laboratories