Can forest-soil liming mitigate acidification of surface waters in Sweden?

Acidification of surface waters and forest soils is severe in large parts of southern Sweden. The shallow groundwaters are also affected. Large scale liming of surface waters and streams is in operation, often combined with wetland liming to limit the effects of acid episodes, e.g. at snow melt. Acid episodes are perhaps the most severe problem in limed surface waters and in many as yet well buffered waters, because of temperature-layered acid inflow, often superficial. As a result of some investigations, a large scale forest liming programme covering 6.500–10.000 km² was recently suggested. The main objectives of this forest liming programme are to retard cation depletion and to prevent nutrient imbalance and forest decline in acidified areas. This paper deals with the effects of forest soil liming on streams and surface waters. The response of water chemistry is very dependent on hydrological and soil properties. Although pH itself may be little affected by liming, the acidity (or negative ANC) decreases, inorganic Al-species decrease and the Al/BC-ratio increases in the runoff water. Especially interesting is that this is also true during acid episodes. This means that toxicity for acid sensitive biota decreases. These results indicate that large scale liming may have beneficial effects on surface water chemistry. Furthermore, as surface waters are expected to respond to smaller decreases in acid deposition than do forests soils, forest soil liming may allow less frequent liming of lakes. Consequently, forest soil liming in combination with the anticipated emission reductions may have very beneficial results on surface waters in certain areas of Sweden.     

Operational liming of surface waters in Sweden

From 1977 to 1982, liming of acidified waters was performed for a trial period using governmental subsidies. Based on the experience gained, an operational liming program was started in 1982 by the National Environmental Protection Board of Sweden. All waters with pH of less than 6 and/or an alkalinity of less than 0.05 meq L^?1 have been eligible for subsidies. Individual water owners as well as organizations and federal agencies may apply for subsidies. Normally 85% of the costs are covered by government grants. Each county administration draws up a 5 yr liming plan and applies yearly for subsidies on a county basis. Since 1977, SEK (Swedish Kroner) 373 million have been spent on liming operations and SEK 33 million on administration and follow-up studies. A yearly budget of about SEK 110 to 130 million has been proposed for 1988 to 1991. The liming agent used has been powdered CaCO₃ with a particle size normally of 0 to 0.2 mm. At present, about 4,000 lakes have been treated. The results of the treatments are normally evaluated by analyzing water samples taken twice a year. These are analyzed for pH, alkalinity, conductivity, color, Ca and Mg. In addition, biological surveys are carried out in selected lakes and streams.     

Freshwater liming

Operational liming of surface waters is part of Sweden and Norway's strategy to counteract freshwater acidification caused by air pollutants. Smaller scale liming efforts are performed as research or experimental programs in other countries. Yearly, approx. 300,000 tons of fine-grained limestone (CaCO₃) is spread in lakes and streams and on wetlands to raise the pH in surface water at a cost of approximately 40–50 million $US. The chemical target is set by the biological goals and objectives. A total of over 11,000 lakes and streams are treated on a continuing basis. Dose calculations consider pH, inorganic monomeric Al, dissolved organic matter and the necessary buffering. Lake liming, limedosers at streams and terrestrial liming are used. A mix of different liming techniques is often preferred to get an optimal result. The vast majority of changes are desirable and expected Undesirable effects may appear and damaged wetlands are probably the most serious ones. Cost-benefit analysis show that liming may be profitable for the society. Recovery of the systems can take up to 10–20 years. Liming will in the long run restore the ecosystems but will not make them identical to what may be the original ones. In some cases, complementary measures, e.g. facilitation of recolonization, are necessary to enhance recovery. Reduced emissions of acidifying pollutants according to signed protocols will decrease the need for liming, but still liming is needed for several decades in large regions to preserve biodiversity.     

Liming and fertilization of acid forest soil: Short-term effects on runoff from small catchments

Increased atmospheric deposition of strong acids and deposition of potentially acidifying compounds (e.g. ammonium) has caused a decline in pH and exchangeable base cations in forest soils in Sweden. In recent years, attention has been paid to liming of forest soil as a method to counteract the effects of acid deposition. Experiments with liming, fertilization and woodash treatment of acid forest soils started in 1984. The aim of this study was to determine the effects of low doses of lime (500 to 1500 kg ha^–1) in combination with N fertilizers on tree growth, nutritional status of trees as well as soil, and runoff chemistry. This paper describes the short term effects of liming and fertilization on runoff from ten small catchments in two regions in south Sweden. The effects of liming were small in both areas. In the catchments fertilized with N (NH₄NO₃), a substantial leakage of various N species appeared in runoff after treatment. The increased N output was dominated by nitrate. The excess leakage of N during 2 yr after fertilization was 25 and 13% as an average of the applied N in the two study areas. The mobile nitrate increased the base cations output via runoff with 10 to 100% during 1 yr after N treatment. The runoff of Al increased with 60 to 1009, the first year in the fertilized catchments. Mobilization of cations was also influenced by ammonium, especially K that was exchanged by ammonium on the surface of the soil particles. The effects of woodash-treatment were small, however, sulfate in the ash leaked out following application and about 100% of the added sulfate was found in runoff during the first year.     

Effects of an acid precipitation event on the near-surface water chemistry of an oligotrophic lake

Reported here are the first data that examine direct chemical interactions between acid precipitation and near-surface lake waters. Temporal snapshots of the dissolved phase chemical dynamics at several depths in the surface 0.5 m water column of an oligotrophic low-alkalinity lake are presented for a storm event which occurred on August 17, 1983. During precipitating periods pH decreases of up to 0.35 pH units were observed in surface waters. The good agreement between the time-depth profiles of temperature, excess H⁺, and excess SO₄ ^2? strongly suggested that the major acidity component of the rain water (H₂SO₄) was primarily responsible for the decreased surface water pH. As a result of intrusion of cooler rain water into warmer surface waters, suspended particulate matter apparently became trapped within layers of cooler water and was subsequently removed from near-surface waters by the sinking of these layers. Significant solubilization of Zn occurred within these layers, presumably representing release from particulate matter subjected to lowered solution pH. In contrast to Zn, significant decreases occurred in the concentrations of dissolved Al and Fe that may have resulted principally from formation of solid phases.     

River Water Metal Speciation in a Mining Region – The Influence of Wetlands,Liming, Tributaries,and Groundwater

Changes in metal speciation occurring along the river Vormbäckenhave been investigated, and the potential for using such changes to reduce metal transport to areas further downstream has been evaluated.Vormbäcken is situated in a mining region in northern Sweden. Catchment area features likely to influence metal speciation include wetlands situated along the river, addition of treated (liming) effluent water from a mine area, and addition of other surface waters and groundwater. Surface water samples were collected from seven stations along the river on six occasions, representing different flow regimes. The total As, Ca, Cd, Cu, Fe, Pb, and Zn concentrations in the samples were partitioned into particulate (>0.4 μm and 0.2–0.4 μm, or only >0.2 μm) and dissolved (<0.2 μm, either associated with organiccarbon, or as free metal ions and inorganic complexes) fractions by means of filtration and an ion-exchange technique. The most important finding is that, with the exception of Ca, the fraction of particulate bound metals increased with increasing concentrations of particulate Fe. This Fe has its origin in surface waters and groundwater that join the river on its way through the catchment area. It is suggested that adsorption to, or co-precipitation with, such Fe-containing particles may have potential to be used as the initial step in a treatment method based on natural attenuation processes. Furthermore, the fraction of particulate bound metals decreased dramatically upon passing the lake Vormträsket, suggesting that some of these metals may be removed from the river system, at least temporally.     

Hydrochemistry of Waters of Volcanic Rocks: The Case of The Volcanosedimentary Rocks of Thrace, Greece

This work is referred to the characterization of the environmental hydrochemistry in the broader Sapes area – Thrace region, on the basis of physico-chemical properties of surface and groundwaters occurring in the volcanosedimentary formations of this area, where gold mining activities are planned to operate. Volcanic rocks are considerably altered where they are in contact with hydrothermal solutions. Aquifers are formed within these formations. Surface and ground waters are strongly metalliferous and their hydrochemical facies present similar but complex water types. Certain characteristic chemical types are the following: Ca-Mg-HCO₃-SO₄, Ca-Mg-SO₄-HCO₃. Ca-SO₄, Ca-Mg-SO₄. Ca-Na-Cl-HCO₃, Na-Cl. A small majority of the water samples present the following order of anion dominance HCO₃ ^? > SO₄ ^2? > Cl^?. Calcium is the dominant cation. Bicarbonates and sulfate ions are the dominant anions. The order of dominance for the heavy metals in surface and ground waters is as follows: Fe > Mn > Zn > Ni > Cu. The saturation index of waters regarding minerals is low. Computer simulation indicates that calcite and dolomite are common minerals in all water samples which are saturated in respect to quartz and argillaceous-siliceous minerals. The most pronounced property of waters is their acidic character. The high metal concentrations are related to water with low pH. Sulfide minerals control the low pH values of waters which is an important control factor for the evolution of the water chemical composition. The abundance of sulfates is attributed to the dissolution of the minerals pyrite (FeS₂) and alunite (KAl₃(SO₄)₂(OH)₆. The water–mineral interactions are responsible for the chemical composition of waters. Water quality problems can be successfully handled by the use factor analysis. 17 chemical parameters can be substituted by five factors which successfully represent the hydrochemical processes as well as their geographic distribution. Volcanic rocks in the study area have the potential to produce acid drainage.     

Contrasting the geochemistry of aluminum among peatlands

Aluminum concentrations were measured in surface waters, pore waters and surface peats of 15 wetlands in south-central Ontario. Wetlands were grouped floristically and chemically as mineralpoor, moderately-poor or mineral-rich fen. Mineral-poor fens were dominated bySphagnum, were low in alkalinity (0.31μeq L^?1) and pH (4.5–6.3). Moderately-poor fens had a mixture of vegetation (Sphagnum, sedges and grasses), mid-alkalinity (23–91μeq L^?1) and pH (5.8–6.4). Mineral-rich fens were dominated by sedges and grasses, had high alkalinity (104–181μeq L^?1) and circumneutral pH (6.2–6.3). Surface water Al concentrations were less in mineral-poor versus moderately-poor and mineral-rich fens (F=32.0; P<0.05). Pore water Al concentrations were lower in 4 of 5 mineral versus the mineral-rich fens (F=92.15; P<0.05). In all but two cases pore water Al (all species <0.2μm) were greater within the fen peats versus the overlying surface waters suggesting that peats could act as a source of Al to the overlying waters. In all wetlands, 70 and 30% of peat Al was recovered by a hydroxylamine hydrochloride/acetic extract (primarily inroganic) and an ammonium hydroxide extract (primarily organic), respectively. Differences in “extractable” Al recovered by the two reagents (i.e., inorganic+organic Al) among the 15 wetlands were independent of wetland type. Distribution coefficients, k _d , were different among the 3 types of wetlands (F=25.0; P<0.05) with theSphagnum dominated mineral-poor fens containing higher values versus the sedge and grass dominated mineral-rich fens. Lower surface and pore water concentrations of Al in mineralpoor versus mineral-rich fens may in part be a result of differences in the degree of minerotrophic influences between the two types of peatlands. As well, the greater binding capacity ofSphagnum peat as indicated by higher k _d 's in the mineral-poor fens, may have contributed to the observed lower pore water and surface water Al concentrations in mineral-poor versus mineral-rich fens. It has been postulated that anthropogenic acidification of peatlands will accelerate the transformation of a mineral-rich fen to that of a mineral-poor fen and ultimately to bog. Changes in Al geochemistry that may ensue as this transition occurs include decreases in pore and surface water Al concentrations with concurrent increases in peat bound Al.     

Liming and fertilization of acid forest soil: Short-term effects on runoff from small catchments

Increased atmospheric deposition of strong acids and deposition of potentially acidifying compounds (e.g. ammonium) has caused a decline in pH and exchangeable base cations in forest soils in Sweden. In recent years, attention has been paid to liming of forest soil as a method to counteract the effects of acid deposition. Experiments with liming, fertilization and woodash treatment of acid forest soils started in 1984. The aim of this study was to determine the effects of low doses of lime (500 to 1500 kg ha^?1) in combination with N fertilizers on tree growth, nutritional status of trees as well as soil, and runoff chemistry. This paper describes the short term effects of liming and fertilization on runoff from ten small catchments in two regions in south Sweden. The effects of liming were small in both areas. In the catchments fertilized with N (NH₄NO₃), a substantial leakage of various N species appeared in runoff after treatment. The increased N output was dominated by nitrate. The excess leakage of N during 2 yr after fertilization was 25 and 13% as an average of the applied N in the two study areas. The mobile nitrate increased the base cations output via runoff with 10 to 100% during 1 yr after N treatment. The runoff of Al increased with 60 to 100% the first year in the fertilized catchments. Mobilization of cations was also influenced by ammonium, especially K that was exchanged by ammonium on the surface of the soil particles. The effects of woodash-treatment were small, however, sulfate in the ash leaked out following application and about 100% of the added sulfate was found in runoff during the first year.     

Liming acid streams: Aluminium toxicity to fish in mixing zones

Liming to neutralize acidic surface waters involves a possible risk of toxicity to fish due to precipitation or changes in speciation of Al. We report the response of captive brown trout to the experimental liming of an acid stream rich in Al. Within 15 m of lime dosing 0.22 µm filterable Al fell from 580 to 230 µg L^?1, and to 120 jig L^?1, within 30 m, though total Al was unchanged. After 24 hr, fish mortality was 100% at untreated acidic sites, 80% up to 30 m downstream of liming, declining to zero within 100 m. Mortality was 70% at 15 m below the confluence of an acidic tributary with the limed stream, despite little change in pH or total Al concentration. Mortalities were significantly correlated with concentrations of Al and Fe in gill tissues, and with 0.22 µm filterable Al and Fe in the water, but not with particulate Al or Fe. AI(OH)₄ ^?, precipitating A1 or polymeric hydrolysis products are all possible causes of the observed toxicity. Iron may have also have contributed, but the stream concentrations of this metal were relatively low. The practical conclusion is that changes in Al chemistry, where waters of differing acidity mix, may be important in some circumstances where river systems are limed selectively.     

Effects of liming on soil magnesium on some soils in New Zealand

Abstract

Results from 2 pastoral field lime trials showed that liming reduced exchangeable Mg. This effect increased with increasing rate of lime and with time following lime application, and was greatest in the top 0–50 mm depth. Soil solutions, sampled 2 years after liming, showed that solution Mg increased in increasing rate of lime. This effect was greatest in the top 20 mm of soil.

Lime incubation studies indicated that Mg fixation did occur on some of the soil studied, at pH >6.2. However, this did not account for the size of the observed effects of liming on exchangeable Mg in the field or explain the observed effects of liming at pH <6.2.

It is suggested therefore, that the major mechanism by which liming reduces exchangeable Mg, on these soils, is through displacement of exchangeable Mg into solution by the added Ca in lime, and subsequent leaching.

Results from other field trials suggest that liming will decrease exchangeable Mg if the change in pH‐dependent CEC (?ECEC) per unit change in soil pH is <15 me 100 g‐¹.     

Water quality improvements in the Sudbury,Ontario, Canada area related to reduced smelter emissions

Data from water quality studies conducted in the Sudbury, Ontario, Canada area indicate that substantial decreases in the acidity of surface waters have accompanied reductions in SO₂ emissions from the Sudbury smelting industry since 1977. On average, acidic lakes in the Sudbury area showed a decrease in H⁺ of ～ 50% between 1974–76 and 1981–83, and the severity of springtime pH depressions in streams decreased. Although many Sudbury area surface waters remain highly acidic, general decreases in acidity appear to be continuing. The results demonstrate that reductions in emissions of acids and acid precursors result in concomitant improvements in water quality.     

Liming restores the benthic invertebrate community to “pristine” state

After liming of twelve acidified rivulets in central Sweden, the fauna increased its mean similarity to the fauna in unlimed non-acidified references. All species which were found after liming were also found in other waters north and south of the limed area. The species composition after liming should thus be considered as typical of the limed geographical area. Before liming, the fauna was characterized by the acid tolerant mayfly Leptophlebia spp. After liming the fauna was characterized by the acid sensitive mayfly genus Baetis, an important food organism for young brown trout. The restoration of the water quality by liming resulted in an apparently “pristine” benthic invertebrate community, enhancing the conditions for salmonid fish.     

The Swedish liming programme

To mitigate the acidification problem in surface waters the Swedish government is funding a liming programme. Limestone or dolomite powder has been applied to acidified waters since 1976 and on a large scale since 1982. In most projects, limestone is applied directly to the lake, but in several cases supplementary liming is carried out on wetlands and in streams using dosers or other techniques. At present 7,500 Swedish lakes and more than 11,000 kilometers of streams are limed repeatedly with a total of some 200,000 tonne of limestone every year. In 1994 about US$ 25 million was invested by the Swedish government in the liming programme. The biological objective of the liming operations is to detoxify the water so that the natural fauna and flora can survive or recolonize. The chemical aim is to raise the pH above 6.0 and the alkalinity above 0.1 meq/l, which gives an acceptable buffering capacity. In addition, dissolved metals will be deposited after liming, thus reducing their toxicity. Overdosing must be avoided, with natural softwater characteristics being the objective. The chemical and biological effects in water of the liming operations are encouraging. The Swedish liming programme has so far resulted in restoration in 80–90% of the limed surface waters. The fauna often shows an initial dominance by a few species but diversity increases with time, In general, flora and fauna in limed waters show a great resemblance to those in waters not acidified. An undesired effect of liming is significant changes in mosses and lichens after wetland liming.     

The effect of acidification,liming and reacidification on macrophyte development,water quality and sediment characteristics of soft-water lakes

Rapid expansion of Juncus bulbosus L. and the concomitant suppression of isoetid plant species has often been observed in acidifying soft water lakes in Western Europe. Experimental studies have shown that this mass development of J.bulbosus was caused by changes in the carbon and nitrogen budgets in these ecosystems. Acidification leads to temporarily strongly increased carbon dioxide (CO₂) levels in the slightly calcareous sediment and to accumulation of ammonium as a result of a reduced nitrification rate in acidifying waters. Many acidifying Scandinavian soft water lakes, however, have a well developed macrophyte vegetation. It is suggested that this is related with the non-calcareous sediments of these lakes. After liming, however, mass development of J. bulbosus and/or Sphagnum spec. has been observed in Swedish and S.W. Norwegian lakes. From field experiments it has become clear that part of the lime is deposited on the sediments leading to an increase of mineralisation rates, CO₂ production, sediment pore water levels of phosphate and ammonium and to a decrease of the nitrate concentrations in the sediment. These changes have been earlier observed in acidifying West European waters. Rooted species like J.bulbosus can only benefit from the higher nutrient levels in the sediment when the CO₂ level of the water layer is relatively high as this species is adapted to leaf carbon uptake. It is demonstrated that gradual reacidification by the acid water from the catchments and the increased flux of carbonic acid from the limed sediments to the overlying water leads to increased CO₂ levels in the water layer of the limed lakes already a few months after liming.     

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.     

Water Quality Dependent Recovery from Aluminum Stress in Atlantic Salmon Smolt

In Norway, a variable pH target (pH 6.2–6.4 during most of the year, but 6.4 during the smoltification period) is used to reduce the cost of liming salmon rivers. Here we test the adequacy of this liming strategy. Atlantic salmon presmolts exposed to sublethal acidic water (pH 5.9, <25 µg Ali·L^?1) for more than 3 months showed impaired sewater tolerance, elevated gill-Al concentrations, severe gill tissue changes, elevated blood plasma glucose concentrations, but no effect on blood plasma chloride. It is usually assumed that smolt will recover from prior aluminum (Al) exposure if water quality is restored. Recovery rate is here used as an indirect measure of water quality improvements achieved after treating acid water (pH 5.8, 85 µg Ali·L^?1) with lime to reach pH-target levels of 6.0 – 6.3. Fish were exposed in a channel-tank set-up for >210 hrs in water aged from 1 minute up to 2 hours after treatment (in a flow through system). More Al was eliminated from the gills when the fish were exposed to pH 6.3 than to pH 5.8 or 6.0, and when water was aged after pH increase. Recovery, defined as return of normal gill morphology, blood homeostasis and establishment of seawater tolerance was achieved within 210 hrs in channels treated with lime to pH 6.3, while a similar recovery was not obvious at lower pH values. Liming to pH 6.3 detoxified Al better than pH 6.1.     

Water chemistry in forested catchments after topsoil treatment with liming agents in South Sweden

Critical loads of sulphur and nitrogen are exceeded in South Sweden, and nutritional imbalances are expected to appear with time in forests. During 1984 paired catchments were established in a northwestern-southeastern gradient in South Sweden. The aim was to study long-term liming effects on throughfall, soil water, groundwater and runoff. Dolomitic limestone and wood ash were tested at one locality, Hagfors (59° N). Three adjacent catchments were used; one reference area, one treated with dolomitic lime (0.5 kg/m²) in 1985, and one with wood fly ash (0.22 kg/m²) in 1988. The lime and the fly ash was granulated and applied by a helicopter in the end of May. Measurements concerning chemistry of the precipitation, throughfall, soil water and runoff has been conducted since spring 1984. The results showed that top-soil spreading of liming agents, besides the desired effects on soil chemistry, after some years also affected the quality of the recipient water. In the dolomitic lime treated catchment the positive effects were most obvious, with raised pH-, Ca-, and Mg-values and lowered Al-, Fe- and Mn-values. A positive trend regarding lower nitrogen (NO₃ ^?) leaching could also be calculated. Wood ash in the used amount affected only slowly, but after six years the runoff water indicated increased pH-values as well as increased Ca- and K-values and Ca/Al-ratios. Dolomitic lime in the amounts of 0.5 kg/m² was concluded to be sufficient to achieve positive effects in catchments of the present type. Wood ash in the amount of 0.22 kg/m² although enough for recycling purposes, was not sufficient enough in increasing pH in runoff to prevent acid leaching from the forest soils.     

A summary of the impact of acid rain on atlantic salmon (Salmo salar) in Canada

Within the present North American range of Atlantic salmon, severe acid rain effects are limited to the Southern Upland area of Nova Scotia. In the Southern Upland, long range transport of H₂SO₄ has caused many rivers to decline in pH to the point where their Atlantic salmon stocks have been destroyed or much diminished. Chemical records show a declining pH trend in N.S. rivers since the early 1950s. Eighty % of the annual variation in H⁺ concentration can be accounted for by a multiple linear regression model on excess sulphate, total Al and organic anions. It is technically feasible to restore the acidified salmon habitat by the addition of limestone; the total cost of mounting a liming program to restore the lost habitat has been estimated at $4.75 × 10⁶ yr^?1. The pre-acidification Atlantic salmon production capacity of the Southern Upland was estimated from physical habitat surveys and tag return data to be about 45 000 fish yr^?1. Acidification has caused a 50% decline to the current production level of about 23 000 fish yr^?1. The costs of the liming program, when compared to the economic benefits of the anticipated salmon enhancement, are economically unjustifiable. The eradication of salmon from such large regions will hinder future programs to reestablish the species in their former range when pollution of the atmosphere is eventually brought under control. Present plans are for a small liming program to establish a series of refuges for the preservation of nuclei of native salmon stocks.     

Mercury in the Swedish environment — Recent research on causes,consequences and corrective methods

During the last decade a new pattern of Hg pollution has been discerned, mostly in Scandinavia and North America. Fish from low productive lakes, even in remote areas, have been found to have a high Hg content. This pollution problem cannot be connected to single Hg discharges but is due to more widespread air pollution and long-range transport of pollutants. A large number of waters are affected and the problem is of a regional character. The national limits for Hg in fish are exceeded in a large number of lakes. In Sweden alone, it has been estimated that the total number of lakes exceeding the blacklisting limit of 1 mg Hg kg-1 in 1-kg pike is about 10 000. The content of Hg in fish has markedly increased in a large part of Sweden, exceeding the estimate background level by about a factor of 2 to 6. Only in the northernmost part of the country is the content in fish close to natural values. There is, however, a large variation of Hg content in fish within the same region, which is basically due to natural conditions such as the geological and hydrological properties of the drainage area. Higher concentrations in fish are mostly found in smaller lakes and in waters with a higher content of humic matter. Since only a small percentage of the total flow of Hg through a lake basin is transferred into the biological system, the bioavailability and the accumulation pattern of Hg in the food web is of importance for the Hg concentrations in top predators like pike. Especially, the transfer of Hg to low trophic levels seems to be a very important factor in determining the concentration in the food web. The fluxes of biomass through the fish community appear to be dominated by fluxes in the pelagic food web. The Hg in the lake water is therefore probably more important as a secondary source of Hg in pike than is the sediment via the benthic food chain. Different remedy actions to reduce Hg in fish have been tested. Improvements have been obtained by measures designed to reduce the transport of Hg to the lakes from the catchment area, eg. wetland liming and drainage area liming, to reduce the Hg flow via the pelagic nutrient chains, eg. intensive fishing, and to reduce the biologically available proportion of the total lake dose of Hg, eg. lake liming with different types of lime and additions of selenium. The length of time necessary before the remedy gives result is a central question, due to the long half-time of Hg in pike. In general it has been possible to reduce the Hg content in perch by 20 to 30% two years after treatments like lake liming, wetland liming, drainage area liming and intensive fishing. Selenium treatment is also effective, but before this method can be recommended, dosing problems and questions concerning the effects of selenium on other species must be evaluated. Regardless how essential these kind of remedial measures may be in a short-term perspective, the only satisfactory long-term alternative is to minimize the Hg contamination in air, soil and water. Internationally, the major sources of Hg emissions to the atmosphere are chlor-alkali factories, waste incineration plants, coal and peat combustion units and metal smelter industries. In the combustion processes without flue gas cleaning systems, probably about 20 to 60% of the Hg is emitted in divalent forms. In Sweden, large amounts of Hg were emitted to the atmosphere during the 50s and 60s, mainly from chlor-alkali plants and from metal production. In those years, the discharges from point sources were about 20 to 30 t yr 1. Since the end of the 60s, the emission of Hg has been reduced dramatically due to better emission control legislation, improved technology, and reduction of polluting industrial production. At present, the annual emissions of Hg to air are about 3.5 t from point sources in Sweden. In air, more than 95% of Hg is present as the elemental Hg form, HgO⁰. The remaining non-elemental (oxidized) form is partly associated to particles with a high wash-out ratio, and therefore more easily deposited to soils and surface waters by precipitation. The total Hg concentration in air is normally in the range 1 to 4 ng m-3. In oceanic regions in the southern hemisphere, the concentration is generally about 1 ng m^?3, while the corresponding figure for the northern hemisphere is about 2 ng m-3. In remote continental regions, the concentrations are mainly about 2 to 4 ng m^?3. In precipitation, Hg concentrations are generally found in the range 1 to 100 ng L^?1. In the Nordic countries, yearly mean values in rural areas are about 20 to 40 ng L^?1 in the southern and central parts, and about 10 ng L^?1 in the northern part. Accordingly, wet deposition is about 20 (10 to 35) g km^?2 yr^?1 in southern Scandinavia and 5 (2 to 7) in the northern part. Calculations of Hg deposition based on forest moss mapping techniques give similar values. The general pattern of atmospheric deposition of Hg with decreasing values from the southwest part of the country towards the north, strongly suggests that the deposition over Sweden is dominated by sources in other European countries. This conclusion is supported by analyses of air parcel back trajectories and findings of significant covariations between Hg and other long range transported pollutants in the precipitation. Apart from the long range transport of anthropogenic Hg, the deposition over Sweden may also be affected by an oxidation of elemental Hg in the atmosphere. Atmospheric Hg deposited on podzolic soils, the most common type of forest soil in Sweden, is effectively bound in the humus-rich upper parts of the forest soil. In the Tiveden area in southern Sweden, about 75 to 80% of the yearly deposition is retained in the humus layer, chemically bound to S or Se atoms in the humic structure. The amount of Hg found in the B horizon of the soils is probably only slightly influenced by anthropogenic emissions. In the deeper layers of the soil, hardly any accumulation of Hg takes place. The dominating horizontal flow in the soils takes place in the uppermost soil layers (0 to 20 cm) during periods of high precipitation and high groun water level in the soils. The yearly transport of Hg within the soils has been calculated to be about 5 to 6 g km^?2. The specific transport of total Hg from the soil system to running waters and lakes in Sweden is about 1 to 6 g km^?2 yr¹. The transport of Hg is closely related to the transport of humic matter in the water. The main factors influencing the Hg content and the transport of Hg in run-off waters from soils are therefore the Hg content in soils, the transport of humic matter from the soils and the humus content of the water. Other factors, for example acidification of soils and waters, are of secondary importance. Large peatlands and major lake basins in the catchment area reduce the out-transport of Hg from such areas. About 25 to 75% of the total load of Hg of lakes in southern and central Sweden originates from run-off from the catchment area. In lakes where the total load is high, the transport from run-off is the dominating pathway. The total Hg concentrations in soil solution are usually in the range 1 to 50, in ground water 0.5 to 15 and in run-off and lake water 2 to 12 ng L^?1, respectively. The variation is largely due to differences in the humus content of the waters. In deep ground water with a low content of humic substances, the Hg concentration is usually below 1 ng L^?1. The present amount and concentrations of Hg in the mor layer of forest soils are affected by the total anthropogenic emissions of Hg to the atmosphere, mainly during this century. Especially in the southern part of Sweden and in the central part along the Bothnian coast, the concentrations in the mor layer are markedly high. In southern areas the anthropogenic part of the total Hg content is about 70 to 90%. Here, the increased content in these soils is mainly caused by long-range transport and emissions from other European countries, while high level areas in the central parts are markedly affected by local historical emissions, mainly from the chlor-alkali industry. When comparing the input/output fluxes to watersheds it is evident that the present atmospheric deposition is much higher than the output via run-off waters, on average about 3 to 10 times higher, with the highest ration in the southern parts of Sweden. Obviously, Hg is accumulating in forest soils in Sweden at the present atmospheric deposition rate and, accordingly, the concentrations in forest soils are still increasing despite the fact that the emissions of Hg have drastically been reduced in Sweden during the last decades. The increased content of Hg in forest soils may have an effect on the organisms and the biological processes in the soils. Hg is by far the most toxic metal to microorganisms. In some regions in Sweden, the content of Hg in soils is already today at a level that has been proposed as a critical concentration. To obtain a general decrease in the Hg content in fish and in forest soils, the atmospheric deposition of Hg has to be reduced. The critical atmospheric load of Hg can be defined as the load where the input to the forest soils is less than the output and, consequently, where the Hg content in the top soil layers and the transport of Hg to the surface waters start to decrease. A reduction by about 80% of the present atmospheric wet deposition has to be obtained to reach the critical load for Scandinavia.