Sensitivity of soils and waters to acidification, Finnish Lapland

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Applied GtoeMmis/ry, Suppl. Issue No.2. pp. 49--50. 1993

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Sensitivity of soils and waters to acidification, Finnish Lapland ANNE-MAT KAHKONEN

Geological Survey of Finland, Regional Office of Northern Finland, P.O. Box 77, SF-96101 Rovaniemi, Finland

and Otrrr

MAHONEN

Water and Environmental District Office of Lapland, P.O. Box 8060, SF-96101 Rovaniemi, Finland Abstract-This paper reports the preliminary results of an investigation into the effects of the geological characteristics of catchment areas on the sensitivity to acidification of small lakes in Finnish Lapland. The sensitivity of mineral soil to acidification becomes more apparent due to special geological features and the relatively harsh climate. The sensitivity to acidification of lakes strongly depends on the geochemical properties of soil in the catchment areas. The buffering capacity of the drainage and the headwater lakes exhibit a positive correlation with the sum of the base cation equivalents of mineral soil. According to the preliminary results the buffering capacity of closed lakes seem to have no correlation with the base cation contents ofthe catchment areas. The study was carried out as a co-operative effort of the Geological Survey of Finland and the Water and Environmental District Office of Lapland.

The lakes were divided into the following groups (KAMAIu,

INTRODUCTION

1984): 1. The drainage lakes (inflow and outflow exist); 2. The headwater lakes (no inflow, outflow exists); 3. The closed (or seepage) lakes (inflow exists, no outflow or no inflow, no outflow). The sum of the soluble base cation equivalents (Ca 2++Mg2+ +K+meq/kg) in mineral soil was used for determining the sensitivity of the catchment areas to acidification. The water quality variables of 409 lakes are linked with the AI, Ca, Fe, K, Mg and P concentrations. The content of organic anion, an indication of natural acidity, was estimated using the method of OLIVER et al. (1983).

THE SENSITIVITY of waters to acidification varies as a function of the amount, quality and availability of soluble elements in the soil in the catchment area. The geochemistry of soil varies extremely by area. Only a small proportion of the acidifying deposition passes directly into the waters. Acid rain changes the quality of runoff through weathering, leaching and ion exchange reactions. In fact, the quality of the runoff better reflects the combined influence of physical, chemical and biological factors than does the nature of precipitation. The anions of the strong acids, S04 and N03 originate chiefly in deposition, whereas the organic anion indicative of natural acidity comes from the soil. Sulphate is considered to be a mobile anion, which means that the S04 deposited on the catchment area is leached out of the soils and simultaneously transports equivalent amounts of acidity (H+, Al3+) and/or base cations to the receiving surface water (SEIP, 1980). Many humic lakes are acid due to organic acids, even in areas where acidifying deposition is small.

RESULTS

The alkalinity of the drainage and the headwater lakes exhibits a clear correlation to the equivalent sum of the base cations of the mineral soil in the catchment areas. The drainage lakes have the shortest detention period and mainly the largest catchment area, and the geochemistry of the soil has a great effect on the characteristics of the runoff. The runoff of the headwater lakes depends only on the geochemistry of the soil of their own catchment area. The closed lakes have the longest detention period and part of the runoff can consist of groundwater or perched water. No connection can be demonstrated between the alkalinity of the closed lakes and the base cation concentrations of catchment areas (Fig. 1) (KONnO and KAHKONEN, 1991). Acidified (alkalinity <0.0 meq/l) or acidifying (alkalinity 0.0-0.05 meq/l) drainage and headwater lakes are located in catchment areas with few soluble base cations (median Ca + Mg + K = 406 meqlkg). They are most common in parts of northeastern, northwestern and central Lapland. At present 6% of

MATERIALS AND METHODS The sensitivity and risk to acidification of mineral soils in the catchment areas was determined using the till analysis results of the Geological Survey of Finland (1 sample/ 4 km 2) . Collection and analysis ofthe lake water samples in Lapland were carried out by the Water and Environmental District Office of Lapland. Samples were taken during the fall overturn. The hierarchic position of the lakes was determined by using maps at scales 1:20,000 and 1:50,000.

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the lakes have no HC03 buffering capacity and 20% of the lakes have little HC03 buffering capacity (KINNUNEN, 1990). Lakes well buffered against acidification (alkalinity >0.15 meq/l) are located in areas with an abundance of soluble base cations (median Ca + Mg + K = 732 meq/kg), most commonly in western Lapland and parts of central and eastern Lapland. Both acidifying S deposition and the S04 concentration of lakes are highest in northeastern Lapland. Sulphate has the greatest concentration of all anions in 9% of the investigated lakes in northeastern and central Lapland. Bicarbonate has the highest concentration of all anions in 66% of the investigated lakes, mainly in the greenstone belt of central Lapland and in parts of eastern Lapland. The organic anion is the dominant anion in 25% of the investigated lakes. High organic anion concentrations are characteristic of the lakes in peaty catchment areas located in southern Lapland. DISCUSSION

The preliminary investigations of the Geological Survey of Finland show that the changes in soil

chemistry due to acidification are associated with areas which are sensitive to acidification on the basis of the low content of base cations (KoNTIo and KAHKONEN, 1991). The lakes in these areas are the most sensitive to acidification. When the connections between the geochemistry of the soil and the lake water are studied in more detail and when the results cover the whole Finnish Lapland it should be possible to determine and to model both the regional sensitivity and risk to acidification of soil and water. These results can be used among other methods in estimation of the critical loads in Finnish Lapland. Acknowledgements-The authors acknowledge the help of Harri Kinnunen in data treatment, and Kari Kinnunen and Matti Kontio for helpful discussionsfor this study. Editorial handling: Brian Hitchon.

REFERENCES KAMARI J. (1984) Suomen karujen pienvesistojen happamoitumisherkkyys. Vesihallitus, Tiedotus No. 239. KINNUNEN K. (1990) Acidification of waters in Finnish Lapland. Effects of Air Pollutants and Acidification in Combination with Climatic Factors on Forests, Soils and Waters in Northern Fennoscandia (eds K. KINNUNEN and M. VARMOLA), Nordic Council of Ministers, Nord 1990, Vol. 20, pp. 72-78. KONnO M. and KAHKONEN A.-M. (1991) Pohjois-Suomen jarvien alkaliniteetin ja valuma-alueiden geokemiallisten ominaisuuksien valiset yhteydet. In Geokemian Paivat Oulussa (eds R. SALMINEN and S. Roos) , Vuorimiesyhdistys-Bergsmannaforeningen r. y., Ser. B No. 50, pp. 12-18. OLIVER B. G., THRUMAN E. M. and MALCOLM R. L. (1983) The contribution of humic substances to the acidity of colored natural waters. Geochim. cosmochim. Acta 47, 2031-2035. SEIP H. M. (1980)Acidification of freshwaters, sources and mechanism. Ecological impact of Acid Precipitation (eds D. DRABLOS and TOLLAN A.), pp. 358-366. SNSFProject, NISK, 1432As.