Methylmercury output from the Svartberget Catchment in northern Sweden during spring flood

The problem of mercury (Hg) accumulation in the aquatic food chain is widespread in Sweden. The methylmercury (MeHg) in runoff from catchments may be an important component of the MeHg load in surface waters. The spring flood in northern Sweden constitutes a major portion of the annual catchment runoff. This brief, but large event, therefore, may be of significance for the annual output of MeHg from soils to surface waters in this region. Methylmercury, total mercury (Hg-tot) and other chemical parameters were measured in spring flood runoff during April and May 1993 from two tributaries as well as the headwater mire of the 50 ha Svartberget Catchment. Snow cores from April 1993 and April 1994 prior to the onset of spring snowmelt were also analyzed. Stable isotope techniques were used to estimate the proportion of snowmelt in runoff. During the spring flood, Hg-tot and TOC were diluted in output from the headwater mire compared to the concentrations observed prior to the flood. Over half of the runoff from the mire was snowmelt according to the isotope Hydrograph separation. In runoff from the two forested tributaries, however, TOC and Hg-tot concentrations increased. About a third of this runoff was snowmelt. MeHg concentrations in the spring flood declined at all locations to the lowest levels recorded during 1993 (<0.2 ng l^?1). The runoff concentrations of MeHg were less than the average snow core concentration of 0.3 4+/- 0.17 ng l^?1. The differences in MeHg dynamics in comparison to TOC or Hg-tot suggest that there are factors independent of the availability of Hg-tot or TOC, and even contemporary MeHg deposition (in this case the snowpack MeHg concentrations) which determine the concentrations and output of MeHg during spring flood.