Changes in acidification of lochs in Galloway, southwestern Scotland, 1979–1988: The MAGIC model used to evaluate the role of afforestation, calculate critical loads, and predict fish status

Abstract

Decades of acid deposition in the Galloway area, southwestern Scotland, have resulted in acidification of surface waters and damage to fish. In the period since 1980, however, acidic deposition has decreased substantially. A survey of 50 lochs conducted in 1979 and repeated in 1988 reveals major changes in water chemistry over this 9 year period. Together these two data sets separated in time by 9 years and covering a period of relatively large and rapid change in acid deposition offer a valuable basis for the evaluation of acidification models. Concentrations of SO 4 in the lochs were on the average 42% lower in 1988 relative to 1979. The decline is readily explained by the large and rapid decline in sulphate concentrations in precipitation in the area. Concentrations of non-marine base cations decreased from 155 to 90 μequiv1 −1 and acid neutralising capacity (ANC) increased. The change in ANC was due mostly to decreased concentrations of Al. pH levels showed no systematic change from 1979 to 1988. The regional changes in water chemistry over the period 1979–1988 are corroborated by regular measurements at several of these lochs over this 9 year period. MAGIC (Model for Acidification of Groundwater In Catchments) successfully reproduces the major changes in water chemistry observed over the period 1979–1988. Both calibration to the 1979 data with prediction of 1988, and calibration to the 1988 with reconstruction of 1979 give close fits to the observations. The model provides a means by which the future impact of acidic deposition and afforestation can be evaluated. A fish response function coupled to MAGIC provides the basis for evaluation of past and future fish status in the region. MAGIC predictions under different acid deposition and forestry scenarios indicate that if acidic deposition is held constant at 1988 levels, afforestation causes further acidification of the lochs. Acidic deposition emerges as the major cause of soil and water acidification in the Galloway region, although forestry practices can exacerbate the effects.