Low Acid Deposition Research Articles

Disentangling the responses of dissolved organic carbon and nitrogen concentrations to overlapping drivers in a northeastern United States forested watershed

The concurrent reduction in acid deposition and increase in precipitation impact stream solute dynamics in complex ways that make predictions of future water quality difficult. To understand how changes in acid deposition and precipitation have influenced dissolved organic carbon (DOC) and nitrogen (N) loading to streams, we investigated trends from 1991 to 2018 in stream concentrations (DOC, ~3,800 measurements), dissolved organic nitrogen (DON, ~1,160 measurements), and dissolved inorganic N (DIN, ~2,130 measurements) in a forested watershed in Vermont, USA. Our analysis included concentration-discharge (C-Q) relationships and Seasonal Mann-Kendall tests on long-term, flow-adjusted concentrations. To understand whether hydrologic flushing and changes in acid deposition influenced long-term patterns by liberating DOC and dissolved N from watershed soils, we measured their concentrations in the leachate of 108 topsoil cores of 5 cm diameter that we flushed with solutions simulating high and low acid deposition during four different seasons. Our results indicate that DOC and DON often co-varied in both the long-term stream dataset and the soil core experiment. Additionally, leachate from winter soil cores produced especially high concentrations of all three solutes. This seasonal signal was consistent with C-Q relation showing that organic materials (e.g., DOC and DON), which accumulate during winter, are flushed into streams during spring snowmelt. Acid deposition had opposite effects on DOC and DON compared to DIN in the soil core experiment. Low acid deposition solutions, which mimic present day precipitation, produced the highest DOC and DON leachate concentrations. Conversely, high acid deposition solutions generally produced the highest DIN leachate concentrations. These results are consistent with the increasing trend in stream DOC concentrations and generally decreasing trend in stream DIN we observed in the long-term data. These results suggest that the impact of acid deposition on the liberation of soil carbon (C) and N differed for DOC and DON vs. DIN, and these impacts were reflected in long-term stream chemistry patterns. As watersheds continue to recover from acid deposition, stream C:N ratios will likely continue to increase, with important consequences for stream metabolism and biogeochemical processes.

Is surface water acidification a serious regional issue in China?

Acidic deposition used to cause severe surface water acidification in Europe and the North America, but no damage of surface water acidification was reported under continuous heavy acid deposition in China. We present the first detailed study on regional surface water acidification in southern and northeastern China based on a survey of 255 forested headwater streams, which were investigated during 2010-2015 to explore the status and mechanism of surface water acidification. South China has a subtropical climate with high acid deposition, whereas northeast China is located in a cold temperature zone with relatively low acid deposition. High pH and acid neutralizing capacity (ANC) were observed for almost all of the streams, indicating that surface water acidification may not be a serious regional issue in China both at present and in the future. In northeast China, where soil types are similar in low weathering rate to those in Northern Europe and North America, the ANC of surface waters was lower than that in South China, indicating a higher acid-sensitivity of the surface water. In comparison, the high pH and ANC of streams in southwest China resulted from high soil weathering and atmospheric calcium (Ca) deposition, despite elevated atmospheric acid inputs due to the high sulfur (S) and nitrogen (N) deposition. Comparison of stream nitrate (NO3-) and sulfate (SO42-) concentrations with modeled N and S deposition showed significant N and possible S sinks in the catchments in south China, probably due to denitrification of NO3- in groundwater discharge zone and SO42- adsorption by acid soils respectively, which may also buffer the acidifying effect of S and N depositions. It seems that considerable NO3- leaching to stream waters does not occur in China unless N deposition is higher than 1.8keq·ha-1·yr-1 (2.50g N·m-2·yr-1) (or greater values in certain regions).

Dissolved Al reduces Mg uptake in Norway spruce forest: Results from a long-term field manipulation experiment in Norway

Dissolved aluminium (Al) in soils, mobilized by acid deposition, is considered a threat to forest health through hampering root growth and nutrient uptake. Since the end of the 1980s dissolved Al in forest soil water plays a key role in the assessment of critical loads of acid deposition. So far, most evidence for toxicity of dissolved Al in forest soil water is based on nutrient solution studies and pot experiments. Here, we present results from one of the few in situ ecosystem-scale forest manipulation experiments to study the effect of Al on mature forest trees. A plotwise addition of dilute AlCl 3 was conducted during seven years in an even-aged spruce forest ( Picea abies) in an area in Norway with low acid deposition. Soil solution concentrations of Al were increased to potentially toxic levels (up to 500 μmol L −1) and base cation (Ca + Mg + K) to inorganic Al ratios in the soil solution in the root zone were mostly below 1 in the Al-addition treatments. In the control treatment (only water addition) Al concentrations did not exceed 15 μmol L −1 and base cation to inorganic Al ratios were above 1. The toxic effects of Al on fine root growth and plant growth found in hydroponic studies and pot trials are not confirmed by this field manipulation. However, magnesium (Mg) contents in needles decreased significantly and persistently in plots with elevated Al concentrations, whereas the needle Ca content did not respond. The depletion of the Mg content in needles is suggested to be due to antagonistic effects of high Al concentrations at the root surface, consistent with observed reductions in Mg to Al ratio of inner bark. This study clearly supports a role for Al in critical load functions for forests as dissolved Al causes a decrease in uptake of Mg. However, other signs of reduced forest vitality were not observed. Soil base cation status may need to be included in risk evaluations of forest health under acid deposition.

The present is the key to the past, but what does the future hold for the recovery of surface waters from acidification?

Analogue matching was used to identify close modern water quality analogues for a set of 59 acid-sensitive lakes in the Galloway region of south-west Scotland. Modern analogues were identified that closely matched the pre-disturbance conditions of these lakes using simulated water quality parameters from the MAGIC (Model of Acidification of Groundwater in Catchments) model for key years from 1860 to 2100. The lakes were matched with hydrochemical samples from a large spatial data set in the UK. For the majority of the 59 lakes, several close modern analogues were identified from the training set for specified years. The close modern analogues for the reference year (1860) were predominantly located in north-west Scotland, an area of low acid deposition and high-status water quality. A clear recovery in the regional surface water acid neutralising capacity (ANC) was simulated by MAGIC in 2015 compared to the situation in 1970 at the height of acid emissions. Predicted trends in surface water chemistry from present day to 2015 indicate some improvement in water quality with c. 23 ± 0.97% recovery towards pre-acidification (1860) ANC for the region. Output from the MAGIC model was used with the analogue technique to investigate the combined influence of future changes in deposition and climate on biogeochemical processes and water quality at the Round Loch of Glenhead (RLGH). Our results demonstrate that pre-acidification restoration targets will not be achieved by simply reducing acid deposition, and climate change will further confound the beneficial effects of deposition reductions. Results for 2015 and beyond show that modern analogues for these periods were predominantly concentrated in North Wales, with some in north-west Scotland, Galloway and the Lake District. Evidence from model simulations and modern analogues indicate that more stringent measures to further reduce acid deposition and combat climate change in the future are necessary if the majority of lakes in the Galloway region are to be restored to their pre-acidification target chemistry. The identified analogues for selected periods may be used to study wider ecological conditions to better define reference conditions and future recovery trajectories. That modern analogues were identified for the simulated chemistry in 2100 at RLGH suggests that unprecedented chemical conditions are unlikely to be observed as a result of future climate change.

Does Acidification Policy Follow Research in Northern Sweden? The Case of Natural Acidity During the 1990’s

The situation in northern Sweden did not figure prominently in the intense period of research during the 1980's that laid the basis for many acidification-related policies now in effect in Europe and Sweden. Northern Sweden has not only relatively low acid deposition levels and significant sources of natural acidity, but also intense episodes of pH decline during spring flood that are a major focus of liming activity. Controversy over that liming and natural acidity has led to scientific advances. These include discovery of a correlation between sulfur in snow and the anthropogenic contribution to the subsequent spring flood ANC decline, but also that natural organic acidity is responsible for most of the spring pH decline in the region. This paper compares the developments in liming policy with the scientific developments of relevance to the region during the last decade. Considerable discrepancies are noted which create opportunities for revising remediation policies to better reflect the state of knowledge in 2000.

Quantifying sources of acid neutralisation capacity depression during spring flood episodes in Northern Sweden

Quantitatively separating the anthropogenic and natural contributions to acid neutralisation capacity (ANC) depression during spring flood is complicated by the many factors which contribute to this depression. Besides anthropogenic sulphate and nitrate, there are natural influences which include dilution, organic acids, sea salts, as well as strong mineral acid anions from bedrock and/or sediment. This paper presents a model for separating and quantifying the effects of dilution, organic acids and strong mineral acids on the ANC for five streams in Nothern Sweden during the spring floods of 1994 and 1995. The basis of the model is that the baseflow prior to and following the event is essentially unaffected by acid deposition and hence can act as the chemical baseline for the episode. The relative contribution of different factors to the spring flood ANC depression are calculated with reference to the dilution of a conservative hydrological tracer. The most important contributor to the ANC decline was found to be dilution of baseflow and organic acids in this remote region of Europe with relatively low acid deposition levels.

Soil and stream water chemistry in a pristine and boggy site in mid-Norway

Stream- and soilwater at the 18.7 ha pristineIngabekken catchment, on gneiss bedrock at Hoylandet,have been studied for three years, including intensiveepisode studies in spring and autumn. The site mainlyconsists of blanket bogs which are typical for thesemarine west coast climates. Water drains through theblanket peats by means of two major flowpaths. Eachflowpath contributes to the stream with a distinctchemical fingerprint rendered by the soil/soil waterinteractions along the flowpath, i.e. they may beregarded as end-members. The soil water from the upperpeat layers is the end-member representative ofstormflow discharge whereas baseflow originates mainlyfrom seepage of the other end-member, which is themineral soil water from beneath the peat. ThepHBaCl2 of the soils that control the runoffchemistry during highflow conditions was as low as2.6, allowing for a substantial pH drop in streamwaterin the case of a seasalt episode. pH in the streamvaried from more than 7 at baseflow to 5 or slightlybelow at stormflow. The lowest pH (4.8) was observedduring early snowmelt due to release of meltwaterhighly enriched in seasalts. The fraction ofexchangeable aluminum (AlS) was much higher in thesurface layers of the lower reaches of the catchmentthan close to the water divide. This suggests atransport of Al, much like podzolisation, thoughdownslope by a lateral flowpath. A Principal ComponentAnalysis on the stream water chemical data shows theimportance of water flowpaths in addition to dilutionor ionic strength and antecedent conditions as afactor in determining the water quality. On the planeof the two major principal components the base cations(Ca2+, Mg2+, Na+, K+) werenegatively related to [H+], and the total organiccarbon (TOC) was negatively related to strong acidanions (Cl-, SO42-, NO3-).These relationships between the parameter loadingsalong the two main principal components remainedindifferent to the effects of both dilution andflowpaths.Under the present conditions of low acid deposition,this sensitive system is effectively buffered by itsweak acids and all released Al is complexed by naturalorganic acids. Similar boggy areas located in regionswith heavy anthropogenic acid deposition may not beable to neutralize the mineral acids. A shallow waterflowpath and a high H+ saturation of the ionexchanger in the soils controlling the highflowchemistry may lead to discharge episodes where strongmineral acids are allowed to pass through the systemreleasing elevated levels of toxic aluminum in thestream.

Matching diatom assemblages in lake sediment cores and modern surface sediment samples: the implications for lake conservation and restoration with special reference to acidified systems

Restoration goals for damaged freshwater habitats can bedefinedaccording to ecological as well as to chemical criteria. Fordisturbed lakes, the sediment microfossil record can be usedtoselect potential modern analogue sites as possible restorationtarget ecosystems.Fossil diatom assemblages in two acidified lakes (Round LochofGlenhead and Loch Dee) in Galloway, Scotland, were comparedfloristically with modern surface sediment samples from ca. 200lakes in Britain, Ireland, Sweden and Norway using numericaltechniques. Mean squared Chi-squared dissimilarity (SCD)valuesbased on between sample Chi-square distance measures were usedtocompare samples.‘Space-for-time substitution’ using diatom assemblage matchingtechniques identified several modern analogue sites withHebrideanLoch Teanga and Irish Lough Claggan possessing modern diatomflorasmost similar to those which existed at the Round Loch ofGlenheadand Loch Dee before acidification.From the point of view of atmospheric pollution, the mostcloselymatching modern analogue sites were not necessarily in themostpristine regions. Some analogues occurred in UK regions ofmoderateor low acid deposition and modern diatom assemblages inatmospherically cleaner mid Norway were generally less similarfloristically.It is argued that identification of modern analogue sitesraisesthe possibility of using time-space substitution of closelymatchedmodern and fossil samples to infer whole lake ecosystems.Diatoms are however poor indicators of some water chemistryvariables and the two closest matched modern analogue siteshavetoo high calcium concentrations making faunistic comparisonsquestionable.Identification of good modern analogue lakes can be improvedbyusing selection criteria, other than diatoms, to pre-selectsites.Screening inappropriate sites according to water chemistry andbasin features combined with a larger biological database ofmodernand fossil samples offers a promising way of refining theselectionprocesses.Despite necessary refinements, modern analogue matching canpotentially identify whole lake ecosystems that can serve asbiological target communities for currently disturbed sites.Beingbased on biological rather than chemical criteria, thisapproachdoes not rely on species-water chemistry transfer functions.It istherefore directly relevant to lake conservation andrestorationobjectives and offers an alternative method for reconstructing lakepalaeo-environments.

Interactions between anthropogenic sulphate and marine salts in the Bs horizons of acidic soils in Scotland

In laboratory adsorption experiments, the comparison of podzol Bs horizons from coastal and inland moderately-impacted catchments with those from a severely-acidified inland region has demonstrated the effect of marine inputs on SO 4 2- -retention. Moderate sea-salt inputs and low acid deposition leads to the retention of most of the SO 4 2- and the release of soluble Mg 2+ ; increasing the marine salt loading causes the development of a selectivity towards retention of acidic SO 4 2- and the retention of Mg 2+ . In the highly-impacted soil, the marine input caused a decrease in SO 4 2- retention in open moorland soils. The opposite occurred under forest, due to the ion-exchange of marine Mg 2+ for soil Al 3+ , increasing soil acidity towards the pH 0 (Gillman and Uehara, 1980), which is depressed below that of its moorland equivalent

The causes of lake acidification, with special reference to the role of acid deposition

Acid surface waters (pH < 5.5) occur throughout western and northern Europe. The claim that many of these waters have been acidified in recent decades and that the acidification results from acid deposition has been well-substantiated by palaeolimnological studies. At almost all sites acidification post-dates 1800 A. D.; it is accompanied by increases in the concentration of trace metals and carbonaceous particles and the spatial pattern of acidified lakes coincides with areas of high acid deposition (greater than 0.5 g sulphur m -2 a -1 ). Very sensitive sites (Ca 2 + < 50 ueq 1 -1 ) in areas of low acid deposition are not acidified. Palaeolimnological tests to evaluate the contribution of other factors suggest that leaching and paludification processes are important on a post-glacial time scale but imperceptible over the last 200 years, and that alterations to catchment burning and grazing regimes over this time scale have little or no effect. Only the afforestation of sensitive catchments in areas of high sulphur deposition appears to be significant, an effect attributed to the enhanced sulphur-scavenging efficiency of the forest canopy rather than to the direct effect of forest growth.

Palaeolimnological changes related to acid deposition and land-use in the catchments of two Norwegian soft-water lakes

This is a palaeolimnological study of two Norwegian soft-water lakes, one receiving high, the other low, deposition of sulphur and nitrogen compounds. At the site with low acid deposition inferred pH has oscillated between 5.6 and 5.9 and there is little evidence of atmospheric contamination. At the site with high acid deposition, many centuries of stability are followed by a rapid acidification from pH around 5 in 1900 to the present (1986) level of pH 4.4. In this lake, the sedimentary record indicates a close connection between acid deposition and recent lake acidification.

Restoring acidified streams in upland Wales: A modelling comparison of the chemical and biological effects of liming and reduced sulphate deposition

Increasing emphasis is being placed on the restoration of surface waters which have been affected by acidification. Amongst the possible strategies are management of the causes, by reducing acidic deposition, and management of the symptoms, by treating affected areas with basic material such as limestone. In few cases have there been comparisons of the likely effect of these two strategies on surface water chemistry and ecology, although there is widespread belief that the two are similar in outcome. At present, only a modelling approach permits such a comparison. This paper describes chemical and biological responses of three Welsh streams whose catchments were limed experimentally in 1987-1988 as part of the Llyn Brianne project. Actual changes are compared with simulated changes which occur following reduced acid deposition according to the hydrochemical model, MAGIC (Model of Acidification of Groundwaters in Catchments). The results indicate that liming and 90% reduction in sulphate deposition reduce concentrations of toxic aluminium to similar levels. However, calcium concentrations and pH were increased by liming to values which were high by comparison with conditions simulated at low acid deposition, either in the past or future. Trout density increased in two of the streams following liming to levels similar to those simulated under low acid deposition. By contrast, the aquatic invertebrate fauna changed after liming so that streams acquired species typical of higher calcium concentrations than those simulated under low acid deposition. Species characteristic of 'soft water' communities were apparently lost, although more data are required to separate treatment effects from random change in the longer term. The 'soft water' community also declined in the model as a result of acidification, indicating that both liming and acid deposition resulted in a different faunal community from that prior to acidification. The results support those who conclude that liming is suitable for the restoration or protection of a fishery, but indicate that there may be other ramifications, for example to conservation, which must be considered when liming is implemented. However, the simulation of biological conditions under low acid deposition involves extrapolation from the initial data base. Further data are now required to assess empirically the likely biological character of British streams which have low base cation concentrations unaffected by acid deposition.

Effects of liming on fisheries

Liming measures for treating acidified surface waters have been carried out in Sweden for a trial period with the aid of Government grants. The aim was to study whether lime treatment was a possible method of protecting waters of special value for fisheries, nature conservation or recreational uses. In all waters where liming resulted in a sufficient and durable pH increase, the fish started to reproduce again, even if the populations were composed of very few and old individuals at the time of treatment. In waters with weak recruitment one or more rich year classes appeared after liming. Restocking of extinct populations of Arctic char ( Salvelinus alpinus )and brown trout ( Salmo trutta ) after lime treatment resulted in self-reproducing populations. In populations with intact recruitment, individual growth rates were generally not affected but in weak populations with fast individual growth rates, growth rates decreased after liming. Liming of acidic inlet waters that have high concentrations of aluminium led to heavy mortality in fish farms as did acid spring runoff flowing into a lime treated lake. Rainbow trout introduced into lakes shortly after liming also experienced high mortality. Lime treatm ent can never be a definite cure for acidified waters, but may protect fish populations in lakes with long turnover times in areas with relatively low acid deposition. Periods of high runoff in streams, rivers and lakes with short turnover times were difficult to treat with lime. In areas with a high acid load and acidified watersheds, liming of lakes and running waters will not prevent acid groundwater with elevated concentrations of toxic metals from entering surface waters.