| Document 6/29/2005 | North Ostrobothnia Regional Environment Centre |
Changes caused by peat mining to the quality of runoff
Changes in the quantity of runoff from peatlands have a bigger impact on the loading of water systems than changes in substance concentrations. This is why the environmental impacts of peat mining on watercourses cannot be reliably estimated solely on the basis of pollutant concentration data. Changes to the quality of runoff flowing from a drained peatland are dependent on factors such as peatland type, peat quality, peat layer thickness and drainage depth.
The quality of runoff from peat mining areas and from natural peatlands
| Substance | Peat mining area | Natural peatland |
| Suspended solids (mg/l) | 1,5–2 400 | 1,2–10,4 |
| CODMn (mg/l) | 5,4–136,0 | 15,3–86,0 |
| Total N (mg/l) | 0,5–5,6 | 0,3–0,9 |
| NO3-N (mg/l) | 0–1,0 | 0,001–0,1 |
| NH4-N (mg/l) | 0,04–4,2 | 0,02–0,1 |
| Total P (µg/l) | 18–230 | 16–90 |
| PO4-P (µg/l) | 2–196 | 7–63 |
| Total Fe (mg/l) | 0,8–20,4 | 1,4–4,5 |
The concentrations of different substances, such as suspended solids (SS), organic matter (CODMn, TOC, DOC), nitrogen, phosphorus and iron, are often higher in the runoff flowing from peat mining areas than in the runoff flowing from natural peatlands.
Waters flowing from peat mining areas have high SS concentrations especially during floods and periods of heavy rain. This is because the heavy water flow carries peat particles away from ditches and from the surface of peat mining fields. SS transport from peat mining areas can be reduced with different kinds of water pollution control structures, such as field ditch systems, overland flow areas and sedimentation basins. The SS concentrations in runoff waters are also affected by different mining operations. The concentrations increase especially during the drainage phase, which is why overland flow areas and sedimentation basins are nowadays constructed and taken into use before the peat mining area is ditched.
As SS concentrations increase, so do concentrations of organic matter, nitrogen, phosphorus and iron. Concentrations of humic substances, inorganic nitrogen, inorganic phosphorus and humus-bound soluble iron increase because of increased peat decomposition. When peat is dried, its oxygen content increases. This benefits the bacteria that decompose peat. As a result of increased peat decomposition, larger quantities of humic substances, nitrogen and phosphorus are released into the runoff water. Especially concentrations of ammonium nitrogen increase. As a consequence of bacterial activity, ammonium nitrogen is transformed partly into nitrate nitrogen, and as a result, also concentrations of nitrate nitrogen increase. Concentrations of humus-bound iron increase primarily because peatland drainage causes waters to flow through deeper and more iron-rich layers of the peatland and through the mineral soils under the peatland. The transport of organic matter, nitrogen, phosphorus and humus-bound soluble iron from peat mining areas can be effectively reduced with wetlands constructed on peatlands (so called overland flow areas). Nowadays this method is widely used in Finnish peat mining areas.