Rapid Recent Recovery from Acidic Deposition in Central Ontario Lakes

Long-term dissolved organic carbon changes in Woronora drinking water system in Australia

Assessing historical records of DOC concentrations (DOC) in drinking water sources is important for water utilities to understand long-term planning for infrastructure needs. This study investigates 15-20 years of historical data of the Woronora water supply catchment in Australia inclusive of the water filtration plant (WFP), the lake from where the water was drawn for WFP supply, and the two primary river inputs. The DOC at each site ranged from 0.8 mg L-1 to 13.9 mg L-1, with the highest and lowest concentrations observed in Waratah Rivulet. The DOC in the lake and WFP significantly (p < 0.001) increased at annual change rates of 0.192 and 0.180 mg L-1 yr-1. However, Woronora River showed a ∼50% lower rate of DOC increase at 0.096 mg L-1 yr-1 (p < 0.001), while Waratah Rivulet showed no trend (p > 0.05). UV254 also showed increasing trends at Woronora River, Lake Woronora, and Woronora WFP, indicating an increase in aromatic DOC compounds in all three sites. Waratah Rivulet, however, transported more than 60% of the total DOC load into Lake Woronora due to high flow volumes (more than 65% of total annual system flow). Annual DOC load to the lake is positively correlated with annual rainfall (R2 > 0.92; p < 0.001). The higher percentage (>73%) of the samples had SUVA254 greater than 2 L mg -1 m-1 in all four sites indicating a dominance of hydrophobic DOC. The terrestrial plant-derived DOC has increased in Lake Woronora, predominantly influenced by historical rainfall magnitude. The results underscore the importance of considering the impact of increased DOC at the treatment plant intake for the planning and operation of the Woronora water supply system.

Critical loads for alkalization in terrestrial ecosystems

Critical loads are a risk assessment approach that has supported large decreases in atmospheric acidic deposition globally. In Canada, SOx emissions fell by approximately 70 % between 1990 and 2021, whereas total particulate matter (TPM) emissions increased by about 40 %, mostly after 2010. Base cations are a major component of TPM, and critical load models consider base cation deposition as beneficial to ecosystems insomuch as it reduces the risk of acidification. However, close to point sources, high levels of alkaline dust deposition have altered soil chemistry and caused an undesirable shift in ecosystem state; something that critical loads are designed to prevent. In this study, the simple mass balance model (SMB) was modified with the objective of preventing base cation accumulation in soil above an acceptable threshold. The concept was applied to a forested site close to large emission sources of sulphur, nitrogen, and base cations in the Oil Sands region of Alberta, Canada. At this site, base cation leaching measured at 25 cm was approximately three times higher than estimated background leaching and exceeded combined SO4 + NO3 leaching. The critical load for alkalization was exceeded under each scenario considered in this study, although the exceedance was marginal if all N in current deposition was assumed to leach from soil. While this framework can easily be applied to regional and national critical load efforts, the main uncertainties of the proposed approach include base cation deposition estimates, assumptions regarding the behavior of N in soil, the selection of an appropriate Alkle(crit) and the long-term immobilization of deposited base cations in soil.

Widespread chemical dilution of streams continues as long-term effects of acidic deposition slowly reverse

Studies of recovery from acidic deposition have focused on reversal of acidification and its associated effects, but as recovery proceeds slowly, chemical dilution of surface waters is emerging as a key factor in the recovery process that has significant chemical and biological implications. This investigation uses long-term chemical records from 130 streams in the Adirondack region of New York, USA, to evaluate the role of ongoing decreases in conductance, an index of dilution, in the recovery of these streams. Stream chemistry data spanning up to 40 years (1980s-2022) showed that acid-neutralizing capacity has increased in 92% of randomly selected streams, but that harmful levels of acidification still occur in 37% of these streams. Conductance and Ca2+ concentrations decreased in 79% of streams, and SO42- concentrations in streams continued to show strong decreases but remained several times higher than concentrations in precipitation. These changes were ongoing through 2022 even though acidic deposition levels were approaching those estimated for pre-industrialization. Further dilution is continuing through ongoing decreases in stream SO42-. Nevertheless, Ca2+ continued to be leached from soils by SO42-, organic acids and NO3-, limiting the replenishment of available soil Ca2+, a prerequisite to stem further dilution of stream water.

Acidity of Soil and Water Decreases in Acid-Sensitive Forests of Tropical China

Acid deposition in China has been declining since the 2000s. While this may help mitigate acidification in forest soils and water, little is known about the recovery of soils and water from previous severe acidification in tropical China. Here, we assessed the chemistry of mineral soils, water, and acid gases (SO₂ and NO_x) from three successional forest types in tropical China from 2000 to 2022. Our results showed that soil pH increased synchronously from 3.9 (2000-2015) to 4.2 (2016-2022) across all three forest types, with exchangeable acid initially decreasing and thereafter stabilizing. Surface and ground water pH also gradually increased throughout the monitoring period. Soil pH recovery was stronger in the primary than in the planted forest. However, soil pH recovery lagged behind the increase in rainfall pH by approximately a decade. The recovery of soil pH was likely related to the positive effects of the dissolution of Al/Fe-hydroxysulfate mineral and subsequent sulfur desorption on soil acid-neutralizing capacity, increased soil organic matter, and climate warming, but was likely moderated by increased exchangeable aluminum and potentially proton-producing hydroxysulfate mineral dissolution that caused the lagged soil pH recovery. Surface and ground water pH recovery was attributed to increased water acid-neutralizing capacity. Our study reports the potential for the recovery of acidified soil and water following decreased acid deposition and provides new insights into the functional recovery of acid-sensitive forests.

The Influence of Tree Species on the Recovery of Forest Soils from Acidification in Lower Saxony, Germany

Atmospheric acid deposition has increased sharply since the beginning of industrialization but has decreased considerably since the 1980s owing to clean-air policies. Soil acidification induced by an input of acidity has been demonstrated in numerous studies using repeated forest-soil inventories. So far, relatively few data have been sampled to analyze long-term soil trends and only a few studies show the recovery of forest soils from acidification, whereas the recovery of surface waters following declining acid deposition is a widespread phenomenon. To assess a possible recovery from acid deposition, soil resampling data from 21 forested permanent soil-monitoring sites in Lower Saxony (Germany) were evaluated. For most sites, at least three repetitions of inventories from a period of 30 to 50 years were available. Trend analyses of indicators for the acid-base status of unlimed forest soils using generalized additive mixed models (GAMM) show either a trend reversal or a stagnation of the acid-base status at a strong acidification level. The recovery, if indicated by an increase of soil pH and base saturation, of soils from plots with deciduous trees appears to have occurred faster than in coniferous forest stands. This observation may be attributed to a larger amount of temporarily stored sulfur in the soil because of the higher atmospheric input into coniferous forests. As indicators for the acid-base status still show considerable soil acidification, mitigation measures such as forest liming still appear to be necessary for accelerating the regeneration process.

Recovery Processes of Acidic Soils Experiencing Decreased Acidic Deposition

Reductions in acidic deposition rates through legislative actions in North America and Europe have stemmed further environmental degradation and shifted the focus to potential recovery [...]

Chemical recovery and browning of Nova Scotia surface waters in response to declining acid deposition

Declining emissions of sulfur and nitrogen have curtailed acid deposition across large areas of North America and Europe. This has allowed many lakes to recover from acidification, with decreases in sulfate, increases in pH, and increases in alkalinity. But reduced acid deposition has not always coincided with chemical lake recovery. Surface waters in Nova Scotia did not exhibit clear evidence of recovery as recently as 2007, due in part to increasing organic acidity and slow replenishment of base cations. In an updated assessment with data collected as recently as 2019, we analyze water chemistry representing 81 lakes and rivers and two precipitation monitoring stations over up to 41 years. We find that Nova Scotia surface waters are now exhibiting signs of chemical recovery. We estimated the linear decrease in precipitation sulfate and nitrate yield at up to 0.31 and 0.18 kg ha^-1 year^-2, respectively, and the linear increase in precipitation pH at up to 0.014 year^-1. Sulfate decreased in 60 of 62 lakes and 14 of 17 rivers (-0.0051 to -0.23 mg L^-1 year^-1), while pH increased in 55 of 64 lakes and 11 of 17 rivers (0.0015-0.072 year^-1). Apparent colour increased in 54 of 62 lakes and 13 of 17 rivers (0.0026-3.9 Pt-Co year^-1). We identified increasing aluminum trends in 46 of 61 lakes, and we show using size-exclusion chromatography that binding to organic and iron-based colloids may help to explain these trends. To the extent that increases in apparent colour are explained by chromophoric dissolved organic matter (DOM), they imply greater binding capacity for metals in surface waters, and greater capacity for DOM to stabilize metal (oxyhydr)oxide colloids.

Effects of climate and atmospheric deposition on a boreal lake chemistry: A synthesis of 36 years of monitoring data

Reduction in SO₄^2- and NO₃^- atmospheric deposition in the past decades has improved surface water quality in several catchments but recent studies suggest an increasing influence of climate and dissolved organic carbon (DOC). Here, we report on long-term trends in climate variables, strong acid anions and base cations concentrations in precipitation and at the lake outlet (stream) of a boreal catchment in Québec, Canada, and assess the combined effects of these trends on stream chemistry. Annual SO₄^2- and NO₃^- depositions respectively decreased by ~85% (from 23 to ~3 kg ha^-1) and ~70% (from 18 to ~5 kg ha^-1 yr^-1) from 1981 to 2016. As a response, stream SO₄^2- and Ca²⁺ concentrations decreased by 50% (from 3.9 to 1.9 mg L^-1) and ~35% (from 2.4 to 1.5 mg L^-1), respectively. Stream NO₃^- concentration decreased by ~89% (from 0.6 to 0.07 mg L^-1) mainly due to the decline in NO₃^- deposition and possibly to increased vegetation N uptake. Unexpectedly, stream alkalinity decreased, likely due to the decline in Ca²⁺ concentration and to an increase in DOC concentration. Variations in stream pH and Na⁺ concentrations were best explained by climatic changes than by changes in acid deposition, likely reflecting the effect of climate change on chemical weathering in the region. In addition, the average daily temperature between May and September had a strong influence on stream Ca²⁺ concentration in the last two decades (negative relationship), suggesting an increasing vegetation nutrient uptake caused by improved growth conditions. Overall, decreased acidic deposition resulted in a general recovery of surface water although the parallel increase in DOC concentration prevented from an increase in water alkalinity. Our data also indicate an increasing influence of climate on water chemistry at the study site, probably mediated by increasing weathering rate and vegetation nutrient uptake.