Kaisa Heikkinen

Nutrient retention in the vegetation of an overland flow treatment system in northern Finland

Abstract The importance of vegetation for nutrient retention was estimated in an overland flow area (OFA) in northern Finland, where peat mining water is purified by conducting it across a virgin mire. In the OFA the phytomass of herbs, sedges, graminoids and below-ground parts of plants was greater, and that of shrubs and bryophytes smaller, than in a reference area (RA). Overland flow treatment favoured plant species, such as Menyanthes trifoliata and Carex lasiocarpa , and discriminated against Sphagnum spp. in particular. The differences in the nutrient contents of plants in the OFA resulting from nutrient enrichment by peat mining water were compared with the actual decrease in amounts of nutrients in the area. Total phytomass in the OFA increased by 20%, and its N concentration increased from 1.00 to 1.24% of dry weight, but the P concentration decreased from 0.26 to 0.22% of dry weight. These changes in total phytomass and nutrient concentrations resulted in a 40% increase in the N bound by plants in the area, but a 5% decrease in P. The increase in N bound by the total phytomass in the OFA during the 6 years of usage accounted for only about 4% of the observed total retention of inorganic N. In contrast, the vegetation in the area served as a source of P rather than a sink, with the decrease in P accounting for nearly 20% of the measured PO 4 -P retention. The average total decreases in both P and N in the OFA in 1987–1991 were 55%. Thus, the net retention of N and P from the peat mining water was mainly the result of other processes rather than retention in the plant cover.

Contribution of cation exchange property of overflow wetland peat to removal of NH4+ discharged from some Finnish peat mines

Abstract The importance of adsorption on peat for decreasing NH 4 + was estimated in four overland flow areas (OFA) in northern Finland, where peat mining water is purified by conducting it across a natural mire of a given size. The effective cation exchange capacity (ECEC) values of peat in the OFAs ranged from 21.4 to 92.7 meq per 100g, being generally highest in the surface 0–15 cm. It was estimated that about 4.6–5.8% of the peat cation exchan e sites could be occupied by NH 4 + , corresponding to a NH 4 + retention capacity of about 0.18 to 0.77 g kg −1 dry peat. Assuming that the retention was attributable to the 0–15 cm surface peat layer alone, the calculated NH 4 + retention potential of OFA peat should have lasted under 6 months with the loadings imposed on the OFAs and the decreases in NH 4 + amounts in peat mining water considered here. The actual duration of peat capacity to retain NH 4 + in the OFAs has, however, already been shown in this research to be longer, and apparently will be so also in future, probably as a result of two processes: nitrification and subsequent N loss through denitrification, and biological assimilation. Hence, cation exchange capacity of OFA was an important property that contributed to surface water protection against eutrophication by N.

Long-term purification efficiency of a wetland constructed to treat runoff from peat extraction

ABSTRACT Peat extraction increases the phosphorus, nitrogen, organic matter, suspended solids, and iron concentrations in runoff, resulting in negative effects on downstream water bodies. Wetlands are commonly used as natural cost-effective solutions to mitigate these negative effects. This study analyzed changes in the quality of runoff water from peat extraction areas and the long-term efficiency of constructed wetlands. The results indicate that the quality of runoff water changed after the initial drainage and during peat extraction. Nitrogen leached at high concentrations in the early stages of peat extraction following drainage, whereas the leaching of iron and phosphorus increased after peat extraction from deeper layers. Comparison of water quality and impurities retained immediately after treatment wetland construction and 14 years later showed that the treatment wetland remained functional, with good retention capacity, over a long period.