Potential effects of forest fire and storm flow on total mercury and methylmercury in sediments of an arid-lands reservoir

A study was conducted from July 1995 to June 1996 to examine the spatial and temporal changes of mercury concentrations in sediments of an arid-lands reservoir. Prior to the first sample collection in July, a forest fire burned 2930 ha of mixed conifer and ponderosa pine in the watershed of Caballo Reservoir in south-central New Mexico. The fire was eventually extinguished by summer rains and storm runoff resulting in the mobilization and transport of charred vegetative material into an intermittent tributary (Palomas Creek) that drains the watershed into Caballo Reservoir. Concentrations of total mercury (THg), monomethlymercury (MMHg), and total organic carbon (TOC) in surficial sediments revealed fire, followed by storm runoff, enhanced the transport of mercury and organic matter to the reservoir. Concentrations of THg in sediments increased from 7.5 etag/g in July to 46.1 etag/g by November 1995 at one site (Palomas) nearest the outflow of Palomas Creek. No other spatial or temporal trends were observed for THg at other sites throughout the remainder of the study. Concentrations of MMHg in sediments at the Palomas site increased from 0.428 etag/g in July to 12.46 etag/g by October 1995 compared to concentrations in sediments at the remaining sites which ranged from 0.11 to 1.50 etag/g throughout the study. The ratio of MMHg to THg (a gross index of methylation activity) was greatest in sediments from the Palomas site (5.4-33.8%) compared to the remaining sites (0.01-3.60%). The ratio was mirrored by elevated TOC in sediments at the Palomas site (2.5-11.8%) that remained elevated throughout the study. Fire and subsequent late-summer rains may have had a twofold effect on mercury concentrations in Caballo Reservoir. The storm-driven runoff following the forest fire carried mercury complexed to organic matter which resulted in elevated levels of mercury as well as providing a carbon source for microbial methylation processes in sediment.