Dissolved Organic Carbon Concentration, Soil Type, and Forestry Activities Influence the Continued Acidification of Sensitive Streams in Galloway, Southwest Scotland

ABSTRACT

The decrease in acidic air pollution, particularly SO ₂ , between the 1990s and 2020s has led to recovery from acidification of many surface waters globally. However, recovery has been relatively slow in some areas. This is particularly relevant in acid‐sensitive forested catchments with peaty soils and vulnerable fish populations, such as those in Galloway, SW Scotland. We report the results of a 1‐year study in an acid‐sensitive catchment in Galloway investigating the trends and drivers of pH and Dissolved Organic Carbon (DOC) concentration. Stream water samples were collected from 13 locations in a catchment at seven times throughout a year and the impacts of soil type, forestry operations, and weather conditions on DOC, pH, and other water quality parameters were investigated. pH in 31.9% of samples was below 5.6, an ecological threshold for salmonid mortality, with 9 out of 13 sites having pH < 5.6 at least once during the sample period (median pH of all samples at all sites = 5.91, SD = 0.55, min = 4.49, max = 6.71). Sub‐catchments with peaty soil types had higher DOC concentration and lower pH, with a strongly significant negative correlation between DOC and pH (Spearman's ρ = −0.530, p  < 0.001). In addition, there was an indication that clear‐felling forestry activities during the sampling period increased nitrate, particulate organic carbon and DOC concentrations. The results highlight the continued acidification of surface waters draining organic soils, despite predicted recovery due to the decrease in acidic sulfate deposition. Environmental and anthropogenic land‐use factors combine to influence water quality in forested catchments, significantly degrading ecological condition.