Soil and surface water acidification – KSLAT nr 18-2003

– in theory and practice

Workshop November 11–12, 2002

One of the scientific theories within the field of limnology states that water quality mirrors the soil status in the drainage basin, if the water is not affected by direct discharge of any substance. According to this soil and surface water linkage theory, generally accepted by the scientific society, soils acidified by acid precipitation would cause more acidic surface waters. The reverse would be true if acid deposition decreases and the surface waters exhibit recovery.

Since the middle of the 1980’s, the deposition of acidifying compounds has decreased substantially and it is today at the same level as in the early 1950’s in Central Sweden. The decreased deposition of sulphur can be observed in many streams and lakes in the forested landscape, demonstrated by reduced sulfate concentrations, increased buffer capacity (ANC and alkalinity) and increased pH (SEPA 2000). In Norway, reduced  oncentrations
of inorganic aluminium are documented as well (Skjelkvåle et al. 1996). Unfortunately, long-term  time series on inorganic aluminium are not available from Sweden.

According to the theory, the acid status of the soils should also have improved parallel to the surface water recovery. However, recently published (SEPA 2000) soil data and model simulations indicated a continuous soil acidification during the 1990’s down to the B-horizon  (increased exchangeable Al) and that the acid soil status should not be expected to improve (base saturation and ANC) during the forthcoming decades in the most acid sensitive areas of  Southwest Sweden, respectively. In the perspective of one decade, the results from the soil and surface water assessments were obviously in conflict with the theory of surface waters acting as a mirror of the soil status. Questions arose whether the theory is too simple, if our conclusions have been based on different definitions of acidification/recovery of soil and water or if data and/or mathematical models are non-representative and not comparable with each other.

This scientific workshop was initiated in order to answer those questions and to define important issues for future research related to acidity transfer from soils into surface waters in forested, Swedish catchments dominated by podzolic soils.