Impediments to recovery from acid deposition: Fid-Bau Portal

Impediments to recovery from acid deposition

Watmough, Shaun A. / Eimers, Catherine / Baker, Scott

Abstract In response to large reductions in sulphur (S) emissions over the past 30 years, sulphate (SO4 ²⁻) concentrations in precipitation at Plastic Lake in south-central Ontario, Canada, have declined by more than 70%. More recent decreases in NOx emissions have similarly led to a reduction in nitrate deposition (NO3 ⁻) and consequently the pH of bulk precipitation has increased by approximately 0.8 pH units since 1980. Despite the large decrease in acidic deposition, chemical recovery of the streams, as measured by an increase in pH and decrease in aluminum (Al), has been much less than expected, primarily due to losses of base cations from the shallow, base-poor soils. While nitrogen (N) is almost totally retained within the terrestrial catchment, S export continues to exceed inputs measured in bulk deposition and during the early part of the record approximately 70% of the anions in streams were buffered by calcium (Ca²⁺) and magnesium (Mg²⁺) compared with only 60% in 2011/12. In the wetland-draining stream (PC1), peak depressions in stream pH and peaks in SO4 ²⁻ and Al concentration in the fall are significantly positively correlated with annual drought days defined as the number of days when stream flow ceases. Even though reductions in SO2 and NOx emissions in Canada have resulted in large improvements in precipitation chemistry, the combined influence of soil acidification and climate-mediated biogeochemical processes occurring in wetlands cause acidification-related issues to persist. Forecasting the longer-term response of soils and surface waters in light of these observations is required to fully assess the need for further emission reductions.

Highlights • Precipitation and stream chemistry measured at Plastic Lake for 30 years. • Reduction in SO2 and NO2 emissions reflected in lower SO4 ²⁻ and NO3 ⁻ in precipitation. • Number of drought days correlated with peak fall SO4 ²⁻ in the wetland-draining stream. • Fall peaks in stream Al and depressions in pH in the 2000s are as high as the 1980s. • Soil acidification and excess S export delay recovery from acid deposition.