T. E. H. Allott

The impact of nitrogen deposition on upland surface waters in Great Britain: A regional assessment of nitrate leaching

A national dataset of water chemistry collected for critical loads mapping is used to make a regional assessment of surface water nitrate concentrations in Great Britain. The primary data are dominated by high concentrations in lowland regions Where N inputs are dominated by non-atmospheric sources. Land cover data are used to screen out sites with potential catchment sources of N, allowing the evaluation of nitrate leaching due to atmospheric deposition alone. In the screened dataset several upland regions show elevated nitrate concentrations, notably Wales, the Pennines, Cumbria, Galloway and the Cairngorms, and there is a clear relationship between surface water nitrate and total N deposition.

The Prediction of Nitrate Leaching with the First-Order Acidity Balance (FAB) Model for Upland Catchment in Great Britain

The relative contribution of N deposition to the acidification of freshwaters in Great Britain has increased over the last few years as S deposition has fallen in line with reduced emissions. In certain high deposition areas of Great Britain, NO 3 − -based acidity can equal or exceed the contribution of SO 4 2− -based acidity in some upland waters. Here we apply the first-order acidity balance model (FAB) to predict the maximum N leaching from 13 study catchments at future steady state. Using mean water chemistry and catchment soils data, along with long-term default values for N sink processes, we predict NO3-N leaching at much higher rates than currently are being measured in surface waters, with a mean increase of 10.5 kg ha−1 yr−1. As a result, mean acid neutralizing capacity would decline to less than 0 meq L−1 at 4 sites. While there are uncertainties associated with model parameterization relating to the short-term storage of N within catchment soils and vegetation, model outputs do indicate much greater leaching of N at some time in the future as steady-state is achieved.

Predicting Freshwater Critical Loads of Acidification at the Catchment Scale: An Empirical Model

Current applications of the critical loads concept are geared primarily toward targeting emission control strategies at a national and international level. Maps of critical loads for freshwaters have been produced in grid form based on water samples of representative sites within each grid square. However, the water chemistry data required to calculate freshwater critical loads are not always readily available at a national level and maps are therefore limited to catchments where such data exist. This paper describes the development of an approach that uses nationally available secondary data to predict freshwater critical loads for catchments lacking the appropriate water chemistry information. An empirical statistical model is calibrated using data from 78 catchments throughout Scotland. Water chemistry for each catchment has been determined. Each catchment is characterized according to a number of attributes. Redundancy analysis of these data shows clear relationships between catchment attributes and the critical load derived from the water chemistry. The key variables that explain most of the variation in critical load relate to soil, geology and land use within the catchment. Using these variables as predictors in a regression analysis, the critical load can be predicted across a broad gradient of sensitivity (R2adj=0.81). The predictive power of the model was maintained when different combinations of explanatory variables were used. This accords the approach a degree of flexibility in that model parameterization can be geared toward availability of secondary data. There are limitations with the model as presently calibrated. However, the approach offers considerable scope for environmental managers to undertake national inventories of catchment sensitivity and specific assessments of individual catchments.

Predicting freshwater critical loads from national data on geology, soils and land use

Using information on geology, soils and land use, a map has been generated for Great Britain which indicates five classes of sensitivity of surface waters to acidification. This map has been used for designing sampling strategies for mapping critical loads of acidity for freshwaters. This paper evaluates the freshwater sensitivity map using a data set of water chemistry collected as part of the UK critical loads programme. Discriminant analysis was used to predict five critical load classes from information on geology and soil sensitivity for freshwater sites. This showed geology and soil information can correctly predict approximately 50% of all critical loads classes. In addition, 77% of sites fall within one critical loads class of that predicted. Predictions may be improved by including other variables eg altitude and geographical location. Differences between lake, stream and reservoir sites are also examined. Ranges of critical loads values were determined for each of the five classes of surface water sensitivity. While a trend in critical load values was evident between classes, there was significant overlap. A simplified sensitivity map with only three classes related more closely to critical loads values. The paper demonstrates the usefulness of the surface water sensitivity map for assessing acidification at a national scale, but highlights the difficulties of predicting critical loads for individual sensitive catchments using national data.

Validation of the UK critical loads for freshwaters: Site selection and sensitivity

Critical loads maps for UK freshwaters have been produced on a 10×10 km grid square basis, and used to map critical load exceedances under various deposition scenarios. A single lake or stream site was selected to represent the most sensitive water body in each grid square using predefined criteria. In the UK a major programme of data screening and validation has been undertaken in order to address issues of accuracy and validity. A major part of this validation exercise, the within-square variability study, is designed to test the extent to which the site chosen for mapping represents the most sensitive water body within each grid square or mapping unit. Sampling of all lake sites in thirty-two randomly chosen 10 × 10 km grid squares has shown that in two thirds of cases, the selection exercise has identified a site in the lowest critical load class within a square. However, up to a third of all sites selected to represent grid squares could be replaced by more sensitive sites with a critical load smaller by at least one Skokloster class. The mean overestimate of “diatom model” critical loads for sulphur in the within-square variability study is 0.188 keq ha−1 yr−1. This means that current critical load maps show overestimates for some grid squares. In order to determine where the most sensitive site has not been identified, further work on catchment scale classification of freshwater sensitivity is being carried out.

A palaeolimnological assessment of the impact of acid deposition on surface waters in North-West Scotland, a region of high sea-salt inputs

Recent critical loads assessments suggest that sensitive surface waters in the north-west of Scotland have acidified, whereas earlier surveys indicate little chemical or biological evidence of acidification. It has been suggested that regionally high sea-salt inputs are affecting either critical loads calculations or the susceptibility of surface waters to acidification. We use palaeolimnological techniques to test the hypothesis that the critical load exceedances in north-west Scotland are real. Pre-industrial and present day loch-water pH are inferred from diatom assemblages in sediment cores from 21 lochs in order to estimate recent pH change. The results indicate consistent post-1800 declines in loch-water pH, although the magnitude of this decline is small (<0.4 pH unit) and in most cases within the error of the technique. It is concluded that although slight acidification might have taken place, this has not been of sufficient magnitude to significantly effect most biological communities (e.g. higher plants, invertebrates and fish).

Assessing the Impacts of International Emissions Reduction Scenarios on the Acidification of Freshwaters in Great Britain with the First-Order Acidity Balance (FAB) Model and the Hull Acid Rain Model (HARM)

Critical loads models for acidity underpin international negotiations for the reduction of acid deposition through emissions controls. In Great Britain and Scandinavia, critical loads for freshwater ecosystems are calculated with the First-order Acidity Balance (FAB) model, which can provide a catchment based estimate of deposition reduction requirements of sulphur and nitrogen species in order to protect any aquatic target organism for which a critical chemical threshold is defined. The FAB model is applied to a national freshwaters database for Great Britain using three deposition scenarios generated with the Hull Acid Rain Model (HARM). Critical load exceedance and changes in three important chemical indicators (non-marine sulphate, nitrate and acid neutralising capacity) are assessed for 1990 baseline deposition levels, planned emissions reductions under existing international commitments (REF scenario), and a potential stringent emission reduction scenario under a multi-pollutant, multi-effect strategy (E10 scenario). Model outputs indicate that the number of sampled sites exceeding their critical load would be reduced by 60% and 73% respectively under the two future deposition scenarios. There is a clear need for a strategy to reduce both S and N deposition from 1990 levels if British freshwaters in sensitive areas are to be protected.