Comparison of different critical load approaches for assessing streamwater acid-sensitivity to broadleaf woodland expansion.

Due to its potential adverse effects on freshwater acidification, risk assessments of the impacts of forest expansion on surface waters are required. The critical load methodology is the standard way of assessing these risks and the two most widely used models are the Steady-State Water Chemistry (SSWC) and First-order Acidity Balance (FAB) models. In the UK the recommended risk assessment procedure for assessing the impact of forest expansion on freshwater acidification uses the SSWC model, whilst the FAB model is used for guiding emission policy. This study compared the two models for assessing the sensitivity of streamwater to acidification in 14 catchments with different proportions of broadleaf woodland cover in acid-sensitive areas in the UK. Both models predicted the exceedance of streamwater critical loads in the same catchments, but the magnitudes of exceedance varied due to the different treatment of nitrogen processes. The FAB model failed to account for high nitrogen leaching to streamwater, attributed to nitrogen deposition and/or fixation of nitrogen by alder trees in some study catchments, while both models underestimated the influence of high seasalt deposition. Critical load exceedance in most catchments was not sensitive to the use of different acid neutralising capacity thresholds or runoff estimates, probably due to the large difference between critical load values and acidic deposition loadings. However, the assessments were more sensitive to differences in calculation procedure in catchments where nitrogen deposition was similar to the availability of base cations from weathering and/or where critical load exceedance values were <1keqH(+)ha(-1)yr(-1). Critical load exceedance values from both models agreed with assessments of acid-sensitivity based on indicator macroinvertebrates sampled from the study catchments. Thus the methodology currently used in the UK appears to be robust for assessing the risk of broadleaf woodland expansion on surface water acidification and ecological status.

[1]  Mark A. Sutton,et al.  A multi-layer model to describe the atmospheric transport and deposition of ammonia in Great Britain , 1998 .

[2]  C. Curtis,et al.  Critical loads of sulphur and nitrogen for freshwaters in Great Britain and assessment of deposition reduction requirements with the First-order Acidity Balance (FAB) model , 2000 .

[3]  A. Wade,et al.  Riparian zone influence on stream water chemistry at different spatial scales: a GIS-based modelling approach, an example for the Dee, NE Scotland. , 2001, The Science of the total environment.

[4]  R. Flower,et al.  Biological responses to the chemical recovery of acidified fresh waters in the UK. , 2005, Environmental pollution.

[5]  M. Cresser.  Why Critical Loads of Acidity and N for Soils Should be Based on Pollutant Effective Concentrations Rather Than Deposition Fluxes , 2007 .

[6]  Thorjørn Larssen,et al.  Afforestation, seasalt episodes and acidification--a paired catchment study in western Norway. , 2006, Environmental pollution.

[7]  J. B. Ellis,et al.  Diffuse pollution impacts: the environmental and economic impacts of diffuse pollution in the UK , 2000 .

[8]  C. Curtis,et al.  Status of UK critical loads and exceedances part 1 - critical loads and critical load maps. Update to January 1998 Report , 2001 .

[9]  M. Cresser,et al.  An assessment of the possible impact of expansion of native woodland cover on the chemistry of Scottish freshwaters , 1995 .

[10]  M. Posch,et al.  The UN/ECE International Programme on Mapping Critical Loads and Levels , 2001 .

[11]  Maximilian Posch,et al.  Steady-State Models for Calculating Critical Loads of Acidity for Surface Waters , 2001 .

[12]  Maximilian Posch,et al.  ENVIRONMENTAL AUDITING: Exceedance of Critical Loads for Lakes in Finland, Norway, and Sweden: Reduction Requirements for Acidifying Nitrogen and Sulfur Deposition , 1997, Environmental management.

[13]  Rodney J. Keenan,et al.  Planted forests and water in perspective , 2007 .

[14]  S. Ormerod,et al.  Relationships between the physicochemistry and macroinvertebrates of British upland streams: the development of modelling and indicator systems for predicting fauna and detecting acidity , 1990 .

[15]  C. Curtis,et al.  Nitrate leaching as a confounding factor in chemical recovery from acidification in UK upland waters. , 2005, Environmental pollution.

[16]  R. Harriman,et al.  The relationship between salmonid fish densities and critical ANC at exceeded and non-exceeded stream sites in Scotland , 1995 .

[17]  J. Nilsson,et al.  Critical Loads for Sulphur and Nitrogen , 1988 .

[18]  K. Heal,et al.  Effects of broadleaf woodland cover on streamwater chemistry and risk assessments of streamwater acidification in acid-sensitive catchments in the UK. , 2008, Environmental pollution.

[19]  B. Reynolds.  Continuous cover forestry: possible implications for surface water acidification in the UK uplands , 2004 .

[20]  C. Neal.  Alkalinity measurements within natural waters: towards a standardised approach. , 2001, The Science of the total environment.

[21]  R. Wright,et al.  The NITREX project: an introduction , 1995 .

[22]  Comparison of Critical Load Exceedance and Its Uncertainty Based on National and Site-specific Data , 2007 .

[23]  D. Monteith,et al.  Trends in surface water chemistry of acidified UK freshwaters, 1988-2002. , 2005, Environmental pollution.