Finnish lake survey: the role of organic and anthropogenic acidity

A lake survey consisting of 987 randomly selected lakes was conducted in Finland in autumn 1987. The survey covered the whole country, and the water quality of the lakes can be considered as representative of the approximately 56 000 lakes larger than 0.01 km² in Finland. The median TOC concentration is 12 mg L^-1 and the median pH 6.3. The proportion of lakes with TOC concentrations > 5 mg L^-1 in the whole country is 91 %. Organic anion is the main anion in the full data set (median 89 μeq L^-1). The high organic matter concentrations in Finnish lakes are associated with catchment areas containing large proportions of peatlands and acid organic soils under coniferous forest. The survey demonstrated that organic matter strongly affects the acidity of lakes in Finland. The decreasing effect of organic matter on the pH values was demonstrated by both regression analysis and ion balances. At current deposition levels of ^*SO₄ the pH of humic lakes in Finland is determined to a greater extent by high TOC concentrations than by ^*SO₄ in most areas. In lakes with pH values lower than 5.5 the average organic anion contribution is 56 % and non-marine sulfate contribution 39 %. However, in the southern parts of the country, where the acidic deposition is highest, the minerogenic acidity commonly exceeds the catchment derived organic acidity.     

Regional monitoring of lake acidification in Finland

The primary objective of this monitoring is to detect long-term Long-Range Transboundary Air Pollution (LRTAP) induced changes in the water quality of small lakes, throughout Finland, with low conductivity. The monitored lakes (n=171, sampled every autumn since 1990 and in 1987) have a smaller watershed (usually headwater or seepage lakes), a larger lake/catchment ratio, and lower base cation concentrations, alkalinity and pH than Finnish lakes on average. The monitoring network provides background data for air pollution dose/response studies, critical load calculations and for the modelling of acidification scenarios. The declining sulphate deposition seems to be reflected in small headwater lakes all over southern and central Finland as a lowering of the sulphate concentration in the waters. Nitrate concentrations in these lakes have been typically low in the autumn. The base cation concentration is not generally declining, as it is in deposition in many areas. The sulphate concentration in lakes has declined more than base cations. Hydrologically, the recent years have been quite variable because of varying annual precipitation. The variation in alkalinity and pH in typical Finnish lakes is dependent on the content of humic material derived from catchments. The monitoring period is too short to reveal consistent trends in major ion chemistry. However, signs of improvement in recent years can be seen; in comparing 1993 to 1987, years with similar organic acidity and base cations, it seems that the sulphate decline in lakes monitored is compensated by a significant rise in both alkalinity and pH.     

Watershed vs in-lake alkalinity generation: A comparison of rates using input-output studies

As a means of assessing the relative contributions of watershed (terrestrial) and in-lake processes to overall lake/watershed alkalinity budgets, alkalinity production rates for watersheds and low alkalinity lakes were compiled from the literature and compared. Based on net alkalinity production data, derived using wet or bulk deposition data, mean and median alkalinity production for 20 watersheds in North America and Europe were 89 and 69 meq m^?2 yr^?1 (range 20 to 235 meq m^?2 yr^?1). For a subset of 10 watersheds with dry deposition data, terrestrial alkalinity production neutralized an additional 35 meq m^?2 yr^?1 of acidic deposition. For 11 lakes, mean and median in-lake alkalinity generation were 99 and 88 meq m^?2 yr^?1 (range 22 to 240 meq m^?2 yr^?1). Analysis of data indicates that for the low alkalinity systems described here, areal alkalinity production rates for watersheds and lakes are approximately equal. This relationship suggests that watershed area to lake area ratio can be used as a convenient estimator of the relative importance of watershed and in-lake sources of alkalinity for drainage lake systems. For precipitation-dominated seepage lakes and other systems where hydrology limits soil-water contact, hydrologic flow paths and residence times can be of overriding importance in determining alkalinity sources. For regions dominated by drainage lakes with high watershed area to lake area ratios (such as the Northeastern U.S.), however, alkalinity budgets are dominated by watershed processes. Omission of in-lake alkalinity consideration for most lakes in such regions would have little impact on computed alkalinity budgets or on predicted response to changes in acidic deposition loadings.     

Calculating critical loads for acidity with the simple mass balance method

The critical load for acid deposition to forest soils, groundwater and surface water can be calculated using a simple mass balance model, taking into account all sources of acidity and alkalinity in the system. The model is very simple to apply, and the major difficulty lies primarily in the estimation of reasonable values for the required input data such as the weathering rate. Methods for its estimation are indicated here. Examples are given for the application of the simple mass balance method (SMB) to Dutch and Swedish forest soils, lakes and shallow groundwater. This work outlines and explains the actual application of the consept as it is being carried out regionally for all European nations.     

Gårdsjön Project: lake acidification,chemistry in catchment runoff,lake liming and microcatchment manipulations

The Gårdsjön Project is an ecosystem approach in studying acid deposition and its effects on terrestrial parts and lakes within a catchment. The study is an attempt to quantify the processes causing the acidification as well as an analyses of the chemical and biological conditions before liming of the lake and manipulations in microcatchments. This paper gives a brief overview of some studies already performed and recently started experiments within the project, such as lake liming, lime application on land, forest fertilization, clearcutting and applications of acid and neutral sulphate on land.     

SOURCES OF TRACE METALS IN STREAMS AND HEADWATER LAKES IN FINLAND

Distributions of Mn, Zn, Cu, Ni, Cr, Pb, As, and Cd in Finnish surface waters were studied by comparing two data sets: samples from 154 headwater lakes collected by the Water and Environment Administration in 1992 and samples from 1165 headwater streams collected during the environmental geochemical mapping program of the Geological Survey of Finland in 1990. It was expected that headwater lakes with catchments smaller than 1 km² and high lake percentage (ratio of lake area to catchment size) would be more influenced by atmospheric trace metal deposition than the streams, with average catchment size of 30 km². The lakes with highest arsenic concentrations lie in an area with greenstones and arsenic-rich black schists. The same lakes have high copper concentrations, which evidently are derived from the Cu-rich greenstones of the catchment. The high copper concentrations of streams and lakes in the industrialized region of the southwest coast are due to several anthropogenic sources. The highest concentrations of chromium occur in brown stream and lake waters rich in humic matter, while manganese and zinc concentrations, which are controlled by acidity, tend to be elevated in low-pH waters. The high nickel concentrations in lakes in southwestern Finland probably are due to anthropogenic input, while Ni anomalies in stream and lake water in eastern Finland are correlated with high Ni contents of glacial till. The lead concentrations in lakes are mainly of airborne anthropogenic origin. The pattern of atmospheric deposition is reflected in the concentrations of Cd, As, Cu, Zn, and Ni in headwater lakes, but land-use, the natural distribution of metals in the overburden, water acidity, and the amount of humic substances influence the distribution of trace metals in both lakes and streams. Thus the trace metal distribution in headwater lakes cannot be used alone to estimate the contribution of anthropogenic atmospheric deposition to metal anomalies in Finnish surface waters.     

Modeling the transport of acidity in soil profiles with front — A dynamic transport model

A dynamic transport model, FRONT, that describes the downwards transport of acidity in podzolized forest soils is presented. In this model the downward transport of acidity with the soil solution is counteracted by a production of alkalinity through the weathering of primary minerals and delayed by the adsorption of sulfate and hydrogen ions on iron- and aluminium oxides. The heart of the model is a massbalance equation that describes the transport of bulk acidity/alkalinity. The FRONT model was tested on 23 deep soil profiles situated along three transects in west-to-east direction across Sweden. Using a deposition scenario starting at 1910 the model was able to account for the large regional differences in the present depth of the acid front. Assuming a linearly decreasing deposition until 30% of present deposition is reached in 2010 the model was used to simulate a scenario for profiles in different parts of Sweden. The scenarios indicated that the upper parts of soil profiles that are severely acidified today will recover and assume a new steady-state in 2030. However, for soil profiles that have large stores of adsorbed sulfate in the B horizon the simulations indicate that one can expect an increased acidity in the deep soil layers several decades after the deposition has ceased due to downward transport of acidity.     

The significance of in-lake production of alkalinity

Alkalinity production in terrestrial and aquatic ecosystems of Canada, the U.S.A., Norway and Sweden is calculated from either strong acid titrations or budgets for base cations and strong acid anions, using mass-balance budgets. Where alkalinity budgets for lakes and their catchments are calculated in acid-vulnerable geological settings, in-lake processes often contribute more to lake alkalinity than yield from terrestrial catchments. Nitrate and sulfate removal, and Ca exchange with sediments are the predominant alkalinity generating mechanisms in lakes. Nitrate and sulfte removal rates increase as the concentrations of NO^? ₃ and SO₄ ^2? in lake water increase, so that in-lake acid neutralizing capacity increases as acid deposition increases. Both processes occur in sediments overlain by oxic waters, at rates which seem to be controlled primarily by diffusion.     

Neutralization of atmospheric acid inputs in small spring catchments in the Frankenwald mountains,Germany

Investigations on soil and freshwater acidification are usually focused on well-aerated systems. This study deals with the role of reductive processes for the neutralization of acid soil solution within helocrene springs. Two toposequences consisting each of three profiles (forest soil, margin of fen, fen) were established to study the chemistry of the solid phase (soil pH, CEC, pedogenic Fe- and Al-oxides) and the soil solution in two small spring catchments on three dates during 1991 and 1992. Despite high acid inputs and acidified forest soils the pH of the spring outflow is near neutral, and the soil solid phases of the spring fens are not acidified. The results support the following hypothesis: Aluminum with its corresponding anion sulfate is leached with the soil solution into the water-saturated fens. Dissimilatory iron and sulfate reduction take place within the fen and generate alkalinity. Reduced iron either reacts with sulfide to form pyrite or migrates within the fen profile and precipitates in the uppermost, oxic horizons, consuming part of the generated alkalinity. Due to the higher pH values in the fens the incoming aluminum precipitates releasing acidity. The alkalinity generated exceeds the amount of acidity released by oxidation and precipitation of iron and the precipitation of aluminum. A balance of alkalinity consuming and alkalinity generating processes based on solid phases showed that iron and sulfate reduction can account for at least 67% of the neutralization of acidity entering the fen of one of the catchments. Due to shorter water retention times and higher discharge these processes are of minor importance in the other catchment.     

Soil geochemistry in relation to water chemistry and sensitivity to acid deposition in Finnish Lapland

Ecosystems in Finnish Lapland are threatened by heavy metal pollution and acid deposition derived from emissions of Cu-Ni smelters in Kola Peninsula and to varying extent to pollution from southern Fennoscandian and Central European sources. Extensive chemical analyses of small lake waters collected in Finnish Lapland have demonstrated that a significant number of lakes are acidified (ANC < 0μcq/l) or their buffering capacity is critical (ANC = 0–50μe/l). The relative abundance of mafic, ultramafic and carbonate rock components in the catchment were the chief factors controlling ANC and the main base cation (Ca²⁺, Mg²⁺) concentrations of lake waters. Both humic and clearwater lakes with low buffering capacity (ANC < 50μeq/l) were mainly located in the catchment areas identified as sensitive to acidification on the basis of low content of base cations (Ca²⁺ +Mg²⁺ +K⁺ < 500 meq/kg) in till. The ratio of the catchment area to the lake area was distinctly smaller for acidic lakes than for the well-buffered lakes, indicating the importance of catchment processes in determining the ANC and main base cations. The high sulphur concentrations (median 60μeq/1) of acidic lakes in northeastern Lapland, near the Finnish-Norwegian border, were strongly correlated with the highest deposition of sulphur derived from smelters of Kola Peninsula. The anomalously high concentrations of sulphur of well-buffered lakes in the western part of Lapland were due to sulphide minerals of soil and bedrock. The acidity of humic lakes in southern Lapland was in large part due to the organic acidity derived from peatlands.     

Comparison of paleolimnological with magic model reconstructions of water acidification

In sensitive areas receiving acidic deposition, paleolimnolgical data indicate changes in lake pH over 1 to 3 decades during the past century. Estimates of deposition of SOx and NOx over this same period suggest that deposition rates changed (1) earlier and (2) more slowly than did changes in lake chemistry. Clearly chemical and biological processes in the terrestrial catchment damp, delay, and moderate the response of surface water pH to deposition of acidifying compounds. This response is controlled by key terrestrial processes that include chemical weathering, sulfate adsorption, cation exchange, dissolution and precipitation of Al compounds, and dissolution and dissociation of inorganic C. MAGIC (Model of Acidifcation of Groundwater In Catchments) provides a tool by which these processes can be simultaneously and quantitatively linked to examine the impact of acid deposition on surface water chemistry. We have applied MAGIC to 4 lakes from which paleolimnological reconstructions are available — Big Moose Lake in the Adirondacks, Loch Grannoch in Scotland, Lake Gårdsjøn in Sweden, and Lake Hovvatn in Norway. The results indicate that the processes linked in MAGIC can account for temporal trends in pH and alkalinity such as those obtained from paleolimnological data.     

Influence of atmospheric deposition on lake mass balances in the Turkey Lakes Watershed,Central Ontario

Ion mass budgets were measured for 2 water yr (June–May, 1981–83) for a high and a ).ow elevation lake and their associated catchments. The lakes are located in the Turkey Lakes Watershed (TLW) in central Ontario, Canada, which is an undeveloped basin located on the Canadian Shield, 50 km north of Sault Ste. Marie. The ionic budgets of the lakes show that atmospheric deposition directly to the lakes' surfaces is the principal input pathway for H⁺ and NH₄, whereas basic cations, SO₄, NO₃, and probably alkalinity are supplied primarily by inflow from the surrounding terrestrial basin and/or upstream lake. The lakes strongly retain H⁺ (i.e. output ? input), weakly retain the N species, and are in balance (i.e. output = input) for other ions except Ca and alkalinity which show an excess output compared to measured + estimated inputs. We hypothesize that an input of groundwater and/or seepage accounts for most of the Ca and alkalinity imbalance although the existence of within-lake alkalinity generation is probable also.     

Headwater lake chemistry during the spring freshet in North-Central Ontario

From data on 30 headwater lakes in north-central Ontario we found that, during the spring snow melt of 1981, all lakes underwent serious declines in alkalinity. Generally, SO₄ ^2?, alkalinity, Ca⁺ and Mg²⁺ concentrations were reduced by runoff and rain then recovered to intermediate levels after the major inputs declined. As expected, a range in responses was evident with lower alkalinity systems showing the greatest changes. The observed changes, however, were consistent with acid loading having depleted alkalinity. In calculating an input-output budget for each lake, we found that changes in Cl^?, Na⁺, and K⁺ were consistent with atmospheric inputs being the major source as the difference between the expected input and the actual contribution from rain and snow had a mean near zero. There appears to be a significant, ? 45%, watershed source of sulphate that we hypothesize is from dry deposition occurring prior to snowfall and is eluted with the melting process. With refinements to a mass balance approach explaining the watershed source of SO₄ ^2? and Al, we feel it is possible to predict springtime lake changes given a few chemical and simple morphometric variables.     

The Quantification of Sulfate Reduction in Sulfate-Rich Freshwater Lakes - A Means for Predicting the Eutrophication Process of Acidic Mining Lakes?

A comparison of neutral freshwater lakes and acidic coal mining lakes with respect to both, in-lake alkalinity generation and P mobilization, has been made to predict the extent of the possibility of P remobilisation in acidic mining lakes creating eutrophication. It is hypothesized that the maturing process of an acid mining lake is comparable to the recent history of the increasing productivity observed in SO42--rich freshwater lakes. This hypothesis is based on the observation that (1) with rising pH over time the atomic S:Fe ratio in the acidic waters is increasing because only a fraction of the SO42--S but nearly all Fe is usually buried in the sediment; (2) the potential of their sediments to immobilize P is at present linked to its continuous accumulation at the sediment together with the stock of its binding partners, mainly Fe(III) compounds; (3) the input of organic matter stimulating the SO42- reduction and the formation of insoluble complexes of sulfide with ferrous Fe will enhance not only the generation of alkalinity, but will also increase the mobility and release of P.     

Fluxes,pools, and turnover of mercury in Swedish forest lakes

In boreal forest lakes, high Hg concentrations in fish are common, even in remote areas. In this paper, the effects of atmospheric Hg pollution in Sweden are synthesized and related to a concept based on the strong interaction of Hg with biogenic matter (Hg/B). Based on this concept, a compartment model is developed to predict concentrations, pool sizes, flux rates and turnover times of Hg along the biogeochemical cycle, including atmosphere, forest soils, surface runoff, lake waters, and aquatic biota. The aim is to provide a conceptual framework, both for a comprehensive mechanistic model, and for predictions from readily available information, such as regional data on acid deposition, air temperature and surface runoff, and local data on the trophic status of lakes with respect to humus and nutrient concentrations. The model is in good agreement with observations from recent Swedish field studies in all compartments. It suggests a strong influence of climate on the susceptibility of soil and lake ecosystems in the boreal region to Hg contamination. The high Hg concentrations in fish from forest lakes can be largely attributed to the low productivity of both terrestrial and aquatic biota. The impact of historical point sources of Hg is considered, as well as the slow turnover of Hg in forest soils, both resulting in elevated fish Hg levels in humic lakes for centuries following atmospheric deposition.     

Role of precipitation chemistry versus other watershed properties in Wisconsin lake acidification

Data for over 100 watershed properties, including aspects of topography, hydrology, geology, soils, vegetation, lake morphometry and input precipitation chemistry, have been developed since 1980 for 316 watersheds in northern Wisconsin. The hypothesis being evaluated for this lake population is that the observed water chemistry, can be accounted for as a function of antecedent water and chemical inputs, after considering exchange processes in the lake and watershed and the lake/groundwater interactions. The variables found by regression analysis to explain observed variability in color, sulfate, and acid neutralizing capacity (ANC) levels in Wisconsin lakes are: for color, vegetative characteristics, mean depth, and water renewal times; for sulfate, precipitation concentration of sulfur, evaporative concentration, and lake water renewal time; ANC appears to be controlled by the size of the watershed, lake depth or water renewal time, and the intensity of anthropogenic inputs and cultural developments in the watershed. These results differ from previous studies in Wisconsin and nearby areas of Michigan and Minnesota by indicating that in some lakes acidity may not be in equilibrium with current precipitation chemistry.     

The coupling of mercury and organic matter in the biogeochemical cycle — towards a mechanistic model for the boreal forest zone

In boreal forest lakes, high Hg concentrations in fish are common, even in remote areas. In this paper, the effects of atmospheric Hg pollution in Sweden are synthesized and related to a concept based on the strong interaction of Hg with biogenic matter (Hg/B). Based on this concept, a compartment model is developed to predict concentrations, pool sizes, flux rates and turnover times of Hg along the biogeochemical cycle, including atmosphere, forest soils, surface runoff, lake waters, and aquatic biota. The aim is to provide a conceptual framework, both for a comprehensive mechanistic model, and for predictions from readily available information, such as regional data on acid deposition, air temperature and surface runoff, and local data on the trophic status of lakes with respect to humus and nutrient concentrations. The model is in good agreement with observations from recent Swedish field studies in all compartments. It suggests a strong influence of climate on the susceptibility of soil and lake ecosystems in the boreal region to Hg contamination. The high Hg concentrations in fish from forest lakes can be largely attributed to the low productivity of both terrestrial and aquatic biota. The impact of historical point sources of Hg is considered, as well as the slow turnover of Hg in forest soils, both resulting in elevated fish Hg levels in humic lakes for centuries following atmospheric deposition.     

Siltation and radiocesium pollution of small lakes in different catchment types far from the Fukushima Daiichi nuclear power plant accident site

The Fukushima Daiichi nuclear power plant accident caused radioactive pollution in northeastern Honshu Island, Japan. This study examined the influence of snowmelt and rainfall on soil erosion processes and siltation of small lakes in Miyagi Prefecture (150 km northwest of the power plant). Two sets of slopes and lakes, respectively in pasture and forest catchments, were examined. Snowpack thickness, soil infiltration, surface runoff volume, soil and sediment physicochemical properties, Cs concentration of precipitation, meltwater, and rainwater, and lake siltation rates were determined. The total radioactive Cs content in precipitation was 0.7–7.4 BqˑL⁻¹ and was below the Japanese standard (10 BqˑL⁻¹). Total radioactive Cs was at the allowable level in water flowing down the pasture catchment slope (0.1–9.2 BqˑL⁻¹) during snowmelt and rainfall, as well as in pasture (0.9–8.8 BqˑL⁻¹) and forest (0.7–5.2 BqˑL⁻¹) catchment lake water. There was no soil erosion (surface runoff) in the forest catchment. Soil losses in the pasture catchment were 23 due to rainfall and 9 kg ha⁻¹ yr⁻¹ following spring snowmelt. After snowmelt, a 0.5 and 0.2 mm thick layer of silt was deposited in pasture and forest catchment lakes, respectively, and 1.4 and 0.6 mm were deposited during the rainfall period. Average siltation rates were 1.9 and 0.8 mmˑyr⁻¹ for pasture and forest catchment lakes, respectively. The upper layer of lake bottom sediments is represented mainly by silt fractions (2–50 μm), with high organic matter (4.0–5.7%) and radiocesium (1100–1600 kgˑha⁻¹) contents.     

Metal fluxes to Swedis forest lakes

Data on atmospheric deposition, transport via run-off water and contents in soil were used to estimate the fluxes of Zn, Cd, Cu, Pb, and Hg to Swedish forest lakes. The calculations refer to a hypothetical lake with a surface area of about 9% of the total catchment area. There are clear differences in the pathways and transport mechanisms for the different elements. The dominating pathway for the input of Zn and Cd to lakes is via run-off from the drainage area. About 60 to 95% of the total load comes from run-off, highest in the acidified areas. The acidification status of the soil is by far the most important factor regulating the mobility of these elements. The amounts of Hg and Pb deposited on the drainage area are mainly accumulating in the surficial soil layers. The transport of these elements is primarily associated with humic substances. About 30 to 50% of the total load of Pb and 25 to 75% of the total load of Hg to lakes originates from run-off. The transport mechanisms for Cu are similar to those of Pb and Hg, but due to the lower anthropogenic airborne load, the direct deposition is of less importance.     

Influence of Physical and Chemical Characteristics on Mercury in Aquatic Sediments

Given the variation observed in mercury in fish from natural lakes, it is difficult to determine what represents a background mercury level. Mercury in aquatic sediments is a potential source of this trace metal to biota, notably fish. Site specific factors, such as acidity and dissolved organic carbon have been shown to affect the mobilization of mercury and methylation of mercury. Methyl mercury is the most toxic form of this metal and the form most readily accumulated by biota. Thirty-four headwater lakes, selected for a range in pH, were sampled for sediment mercury levels as part of an investigation of the impacts of acid rain on insular Newfoundland lakes. Selected physical and chemical data were also collected on all of the study sites. Acidity was not found to be significantly related to sediment mercury concentrations despite the wide range in pH. Pearson correlation analysis indicated that sediment mercury level was positively correlated with WA:LA (watershed to lake area ratio). WA:LA was also correlated with Secchi depth and colour. Linear regression was used to estimate the parameters of a model relating sediment mercury to WA:LA. Watershed area to lake area ratio was more important than site specific factors in governing the concentration of sediment mercury in lakes without industrial input.