Natural acidity or anthropogenic acidification in the spring flood of northern Sweden?

doi:10.1016/S0048-9697(99)00259-4

Science of The Total Environment

Volume 234, Issues 1–3, 30 August 1999, Pages 63-73

https://doi.org/10.1016/S0048-9697(99)00259-4 Get rights and content

Abstract

Episodic pH decline associated with spring flood runoff in a typical headwater stream in the boreal forest of northern Sweden is driven almost exclusively by natural processes. Despite a drastic decline in acid neutralisation capacity (ANC), a 75-fold increase in hydrogen ion concentration and an associated increase in inorganic monomeric aluminium fractions, anthropogenic deposition (the strong acids SO₄²⁻ and NO₃⁻ associated with anthropogenic deposition) made only a minor contribution (5–8%) to the ANC and pH decline during the spring flood of 1997. Instead the ANC decline from 60 μeq. l⁻¹ during winter baseflow to −26 μeq. l⁻¹ at peak flow, as well as the associated pH decline from approximately 6.4–4.6 was, according to an episodic acidification model, driven almost exclusively by organic acids originating from the soil and dilution by low ionic strength snowmelt water. The natural component of ANC and pH decline during spring flood, such as that reported here, has important implications for the aquatic ecology of the boreal zone in general, and for the Swedish liming program in particular, since government subsidies are used to keep pH above 6.0 throughout spring flood, in the belief that this is the natural pH level prior to the appearance of acid rain in Europe.

Introduction

Episodic pH decline during spring flood is an ubiquitous phenomena that has been reported from several regions of the world (i.e. Tranter et al., 1987, Wigington et al., 1992, Wigington et al., 1996). In many regions the large and rapid pH decline in surface waters during spring flood, which is a particularly sensitive time for the early life stages of many aquatic species, has in acidified regions of the world been mainly ascribed to anthropogenic SO₄²⁻ and NO₃⁻ deposition in combination with dilution of the buffering capacity (Molot et al., 1989, Davies et al., 1992, Eshleman et al., 1995). The anthropogenic contribution to this acid shock is intensified by elevated concentrations of acidity eluted preferentially from the snow pack at the onset of snowmelt (Johannessen and Henriksen, 1978). There is, however, growing evidence that the concentration of organic acids may rise naturally during snowmelt (Kahl et al., 1991, Campbell et al., 1992, Jansson and Ivarsson, 1994, Kortelainen and Saukkonen, 1995, Laudon and Bishop, 1999).

Using an ANC dilution model outlined in Laudon and Bishop (1999) in combination with a organic dissociation model (Köhler, unpublished data) we quantified the effects of sulphate, chloride, organic acids and dilution in driving a large ANC decline and a 75-fold increase in hydrogen ion activity during the peak of spring flood in a forested boreal catchment from northern Sweden. This quantification is an important tool in understanding the role of anthropogenic acidification and for improving the liming strategy during the spring flood in northern Sweden.

Section snippets

Field site

Västrabäcken is a forested 8.6 ha, headwater catchment, typical of the boreal region in northern Sweden. The catchment is located at the Svartberget Research Station in the county of Västerbotten (64°14′ N, 10°46′ E) approximately 50 km inland from the Baltic Sea coast. The drainage area is mainly covered by pine (Pinus sylvestris) in the upper, drier areas (20% of the catchment) and Norway spruce (Picea abies) in the lower reaches (75% of the catchment). A locally derived glacial till with an

Methods

Water sampling and discharge measurements were carried out at a thin plate 90° V-notch weir at the outlet of the catchment. The sampling program was based on weekly samples of baseflow prior to the onset of the spring flood, and then daily sampling until the water flow had receded to levels close to those of baseflow. Water samples for chemical analysis were collected in acid-washed 250-ml polyethylene bottles. The samples were stored cold and in the dark until analysed.

Nine 1.5-m² acid washed

Results

The spring flood of the Västrabäcken catchment commenced on the 1 May 1997 and reached maximum discharge 2 weeks later. During baseflow, prior to the spring flood, the pH of the stream varied between 6.2 and 6.5 for over 1 month. At the onset of the rising hydrograph, pH declined rapidly to a minimum of 4.6 at peak flow (Fig. 1a). The decline in pH was accompanied by a decline in ANC (Fig. 1b), coinciding with a decrease in base cations (BC) (Fig. 1c), SO₄²⁻ (Fig. 1d) while Cl⁻ stayed at

Discussion

Paleolimnological reconstructions of pre-industrial lake acidity in 120 geographically representative head water lakes from northern Sweden showed an average decline of only 0.05 pH units in the region during the industrial era (Korsman, 1993, Korsman, in press). The paleolimnic record, however, is only believed to indicate the yearly mean pH value of the surface water, and does not provide specific insight into the anthropogenic influence on spring flood episodes in head water streams, which

Conclusion

The spring flood ANC and pH decline at Västrabäcken demonstrates that a large pH decline and Al_i, increase can be a natural property of the boreal surface water ecosystems. The natural acidity of surface waters in northern Sweden is a situation which the aquatic life has adapted to over thousands of years. Headwater streams in this region also represent important habitats for fish reproduction. Liming to raise the pH of such naturally acid streams is, therefore, a threat to the natural

Acknowledgements

We thank the Swedish Environmental Protection Agency, NFR and Lilli and Oscar Lamms minnesfond for their financial support. The doctoral position for Stephan Köhler was financed by the Centre of Environmental Research in Umeå. Kevin Bishop also received research support from Umeå University, Faculty of Natural Sciences.

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Natural acidity or anthropogenic acidification in the spring flood of northern Sweden?

Abstract

Introduction

Section snippets

Field site

Methods

Results

Discussion

Conclusion

Acknowledgements

J Hydrol

Environ Pollut

J Hydrol

J Hydrol

Environ Pollut

J Hydrol

Geochim Cosmochim Acta

Environ Pollut

Episodic acidification of small streams in the northeastern United States: effects on fish populations

Ecol App

Geochemistry of Quebec north shore salmon rivers during snowmelt — organic-acid pulse and aluminum mobilization

Can J Fish Aquat Sci

Linking hydrological, geochemical and soil chemical processes on the catchment scale: an interplay between modeling and field work

Water Resour Res

The determination of quickly reacting aluminium in natural waters by kinetic discrimination in a flow system

Int J Environ Anal Chem

The role of organic acids in the acid–base status of surface waters at Bickford Watershed, Massachusetts

Water Resour Res

A 2-component mixing model for predicting regional episodic acidification of surface waters during spring snowmelt periods

Water Resour Res

Determination of the equivalence point in potentiometric titrations

Analyst London