Trends in Sulfate, Base Cations and H+ Concentrations in Bulk Precipitation and Throughfall at Integrated Monitoring Sites in Finland 1989–1995

Abstract

Temporal trends in sulfate, base cation (Ca2+ + Mg2+ + K+), and H+ ion concentrations in bulk precipitation and throughfall samples collected over a seven year period (1989–95) in four forested catchments in Finland are presented. The catchments are in remote locations and span the boreal zone (61–69 °N). The stands represent old, undisturbed forests, and are composed of varying proportions of Scots pine, Norway spruce and deciduous species (mainly Betula spp.). Monthly SO 4 2− -S and H+ ion concentrations in bulk precipitation averaged over the study period and catchments were: 18.7 µmol L−1 and 32.3 µmol L−1. The corresponding values for throughfall were: 37.4 µmol L−1 and 32.4 µmol L−1. Sulfate and H+ ion concentrations in bulk precipitation and throughfall both showed negative linear trends, which were significant (p > 0.05) for the three southernmost catchments. Concentrations and trend slope decreased northwards (e.g., bulk precipitation SO 4 2− -S slope estimates: −1.6 to −1.0 µcool L−1 yr−1). The decline was greater for throughfall than for bulk precipitation, indicating a proportionally greater reduction in dry deposition than wet. The sum of base cation concentrations averaged 12.1 µmol(+) L−1 in bulk precipitation and 83.1 µmol(+) L−1 in throughfall. There were no significant trends in the sum of base cations (p > 0.05). It is concluded that the reported reduction in S emissions over the study period has resulted in a significant reduction in the acidity and SO 4 2− concentration of bulk precipitation, and this reduction has has been reflected in throughfall concentrations. The greatest reduction has taken place in the southern part of the country.