Forestry practices increase mercury and methyl mercury output from boreal forest catchments.

We observed significant increases in the runoff output of total mercury (TotHg) and methyl mercury (MeHg) from a small spruce forest catchment (0.071 km2) after clear-cutting and soil treatment. Here we show that forest regeneration practices may act as an important additional source of TotHg and MeHg to forest lakes. TotHg and MeHg in runoff from two small forested catchments were monitored during the period 1994 to 2001. In the autumn of 1997, one of the catchments was clear-cut Soil preparation (mounding) was carried out in the autumn of 1998 and replanting in the summer of 1999. During the 3 years after the silvicultural treatment, medians of monthly flow-weighted TotHg and MeHg concentrations (12.02 ng L(-1) and 0.35 ng L(-1), respectively) and output loads (0.80-0.97 g km(-2) a(-1) and 0.011-0.036 g km(-2) a(-1), respectively) increased significantly compared to the 3 years calibration period (8.13 ng L(-1) and 0.15 ng 1(-1); 2.0-5.3 g km(-2) a(-1) and 0.11-0.16 g km(-2) a(-1), respectively). These results indicate that clear-cutting and/or soil treatment significantly increases the mobility of TotHg and MeHg accumulated in forest soil and may thus be an important factor for the total input of Hg to boreal freshwater ecosystems.

Effectiveness of constructed overland flow areas in decreasing diffuse pollution from forest drainages.

Forestry is the largest scale human impact affecting catchments in Finland and a prominent source of diffuse pollution in many water courses. Among the forestry activities, draining of wetlands had the most pronounced impacts on sediment, nutrient, and metal loading in the past. At present, renovation of old ditches and fertilization of peatlands constitute the major risk of forestry-induced diffuse pollution. Contemporary forestry aims at decreasing this risk with various riparian buffer strip designs. Among such designs, creation of overland flow areas by plugging the outlet ditches is increasingly used. Our objectives were to evaluate the potential of constructed overland flow areas to function as riparian buffers and estimate the quality and quantity of diffuse pollution from old versus recent forest drainages. We studied retention and release of pollutants from 20 constructed, 2- to 10-m-wide overland flow areas receiving drainage water from forested peatlands. Drainage waters were sampled above and below the plugged ditches three times per year from 1998 to 1999. Chemical oxygen demand and nutrient and metal loads and concentrations varied strongly between seasons, years, and drainage areas. Areas subjected to recent ditch renovations and fertilizations had clearly elevated seasonal loads and concentrations of total phosphorus (TP), PO4, Fe, and Al in comparison to old treatment areas. Especially TP loads were high above the national average values measured for forestry-induced diffuse pollution. In general, water quality above and below the buffer strips did not differ significantly. Our results indicate that plugged outlet ditches and associated narrow overland flow areas do not function as proper buffers in peatland areas. We suggest that wider buffers with extensive overland flow areas are needed in order to control diffuse pollution from forested and drained peatlands.