Long-term effects of stump harvest on mercury and general chemistry in discharge water

This synoptic study evaluates water chemistry in stump-harvested and conventionally logged reference areas, located in South-Central Sweden, 20-30 years after harvest and reforestation. Electrical conductivity, pH, alkalinity (HCO3-), nitrate nitrogen (NO3-) and total organic carbon (TOC) were measured in discharging groundwaters and in low-order streams and ditches. The groundwaters were further analysed for total mercury (THg) and methyl mercury (MeHg). Additionally, solid phase Hgtot and MeHg were measured in stream mudbottoms. The mean MeHg concentration and mean MeHg/TOC ratio were significantly (p <.05) higher in discharging groundwater from stump-harvested compared to reference areas. In contrast, the concentration of THg did not differ. Likewise, there were no significant treatment differences for electrical conductivity, pH, NO3- or TOC. In the stream-/ditchwaters also no significant differences were found, but electrical conductivity (p = 0.07) and pH (p =.08) tended to be lower in stump-harvested areas. Hg-tot and MeHg concentrations in stream-/ditch sediments did not differ between treatments. These results suggest that weakly enhanced MeHg concentrations may be present in the riparian groundwater zone 20-30 years after stump harvest.