S.J. Langan

Riparian zone influence on stream water chemistry at different spatial scales: a GIS-based modelling approach, an example for the Dee, NE Scotland

A geographical information system (GIS-ARC/INFO) was used to collate existing spatial data sets on catchment characteristics to predict stream water quality using simple empirical models. The study, based on the river Dee catchment in NE Scotland, found that geological maps and associated geochemical information provided a suitable framework for predicting chemical parameters associated with acidification sensitivity (including alkalinity and base cation concentrations). In particular, it was found that in relatively undisturbed catchments, the parent material and geochemistry of the riparian zone, when combined with a simple hydrological flow path model, could be used to accurately predict stream water chemistry at a range of flows (Q95 to > Q5) and spatial scales (1-1000 km2). This probably reflects the importance of the riparian zone as an area where hydrological inputs to stream systems occur via flow paths in the soil and groundwater zones. Thus, evolution of drainage water chemistry appears to retain the geochemical characteristics of the riparian area as it enters the channel network. In more intensively managed catchments, riparian land use is a further influential factor, which can be incorporated into models to improve predictions for certain base cations. The utility in providing simple hydrochemical models, based on readily available data sets, to assist environmental managers in planning land use in catchment systems is discussed.

The sensitivity of surface waters of Great Britain to acidification predicted from catchment characteristics

Using a combination of soil, land use and geological information, a map of Great Britain has been derived which indicates the sensitivity of surface waters to acidification. For the geological information, a slightly modified version of an available map was used which indicated the sensitivity of groundwaters to acidification. For soils, 1-km databases of soil information for England and Wales and for Scotland were employed to map the soil sensitivity as determined by buffering capacity. The derived soils map was modified to take account of agricultural liming in arable and managed grassland areas using the ITE Land Classification. The final map of surface water sensitivity was obtained by using a geographic information system overlay procedure which enabled each combination of soil and geology sensitivity to be uniquely defined. The final sensitivity classification was based upon expert knowledge and the experience of a similar sensitivity mapping exercise for Wales.

The prediction and management of water quality in a relatively unpolluted major Scottish catchment: current issues and experimental approaches

Abstract The potential impacts of diffuse pollution from atmospheric deposition and land use on the water quantity and quality of the river Dee in N.E. Scotland are currently being assessed. The importance of headwater regions for supplying a large proportion of catchment runoff with water of a high quality is clearly demonstrated. However, the quality of this water is threatened by the impact of acid deposition in a number of sub-catchments. In some of the more agriculturally developed lowland sub-catchments, there are increasing levels of nitrogen runoff. The catchment attributes, together with hydrochemical data, are being considered in terms of an ongoing research programme established to predict the impact of future environmental and land-use change scenarios.

Factors regulating the spatial and temporal distribution of solute concentrations in a major river system in NE Scotland

Abstract The River Dee in NE Scotland, an oligotrophic soft water system, has a catchment area of approx. 2100 km2, its source in the Cairngorm mountains being approx. 140 km from its outlet to the North Sea at Aberdeen. A comprehensive sampling strategy and analytical programme, commensurate with the size and nature of the Dee system, have been established for major water quality determinands to identify the controls on, and origins of, dissolved species throughout the system at a range of catchment scales and over a range of flow regimes. Fifty-nine sites covering a range of catchment types and scales were therefore sampled bi-weekly for 1 year. At the basin scale, there is a general downstream increase in determinand concentrations. This produces strong linear relationships between many determinands which are unrelated in terms of a common terrestrial process or origin. At the sub-catchment scale, however, specific hydrochemical processes control streamwater chemistry. The Dee basin divides into two distinct geographic regions in terms of land use (upland and lowland) which produce clear differences in water chemistry. Individual sub-catchments can also be grouped in terms of temporal variations in streamwater chemistry. The strength of the relationship between weathering-derived ionic concentrations and flow in the upland sub-catchments has lead to the identification of specific concentration limits in sub-catchments which can be used as characteristics of soil water and groundwater end-members. This provides a basis for the prediction of upland weathering-derived component concentrations for each sub-catchment at a range of flows.

An empirical map of critical loads of acidity for soils in Great Britain.

The method used to produce a critical load map of acidity for soils in Great Britain is described. Critical loads were assigned to the dominant soil in each 1 km grid square of the UK national grid. Mineral soils were assigned a critical load based on mineralogy and chemistry, using approaches appropriate to UK conditions. Critical loads for peat soils are based primarily on a maximum acceptable reduction of peat pH, and results from laboratory equilibration studies. The map shows that soils with small critical loads (<0.5 kmol(c) ha(-1) year(-1)) i.e. highly sensitive to acidic deposition, dominate in the north and west of Britain; the south and east are dominated by soils with large critical loads, with small areas of more sensitive soils associated with sandy soil-forming materials. A modified critical load map illustrates the potential impact of agricultural liming on soil critical loads.

Considerations of uncertainty in setting critical loads of acidity of soils: the role of weathering rate determination

The concept of critical loads has been generally accepted throughout Europe, and increasingly in Asian countries and the rest of the world, as providing the data which forms the basis for international negotiations on abatement strategies for emissions of acidifying pollutants. Central to the determination of quantitative critical loads of acidity for forests (and other ecosystems) is the rate at which the minerals in the soil weather or dissolve. Seven methods for determining these rates on a regional basis for the production of critical load maps have been suggested by the official bodies which are responsible for co-ordinating the European critical load mapping efforts. These methods are largely correlations which require a knowledge of the soil parent material and/or the soil mineralogy. The purpose of this paper is to review these weathering rate calculation methods and to assess whether it is currently possible to calculate numerically accurate critical loads for the production of regional critical load maps. A consideration of the data used to generate these methods and comparisons of the weathering rates calculated using various methods leads to the conclusion that at present it is not. Further work is needed to develop and maintain the initial credibility of critical loads both scientifically and as an aid to policy decisions.

The influence of soil age on calculated mineral weathering rates

Abstract Mineral weathering rates for two chronosequences of soils have been calculated using an empirical method based on mineralogy, the depletion of elements relative to a conservative element and the computer model PROFILE. Weathering rates calculated by the empirical and depletion methods showed a decrease in rates with soil age whilst those calculated using the PROFILE model showed an increase with soil age. The counter intuitive PROFILE prediction is due to the use of surface area—normalised reaction rate coefficients which assume that: 1) mineral reactivity is constant with time and, 2) total mineral surface area is equivalent to reactive surface area. In Europe, mineral weathering rates in soils are an important input in determining levels of acid deposition above which ecosystem damage will occur (critical loads). As soils in Great Britain and much of NW Europe can range in age from 105 a, it is suggested that, until computer models can take account of soil age and the concomitant changes in mineral reactivity and surface area, modelled weathering rates will be subject to large uncertainties

A CRITICAL EVALUATION OF THE USE OF THE PROFILE MODEL IN CALCULATING MINERAL WEATHERING RATES

The PROFILE model is used extensively in the European Critical Loads programme as an aid to international negotiations on SO2 emission abatement. PROFILE calculates the rates of cation release by mineral weathering and it then uses these data to calculate soil solution and runoff chemistry. No independent assessment of the underlying assumptions and data in the model has been published and this paper reports such an assessment. The rate equations, which are the key to the PROFILE model require rate coefficients and constants. These have been derived from the literature but more work is required to produce a consistent set of constants. Manipulation of these rates to take into account the exposed reactive surface area of the minerals is fraught with problems. Calculation of exposed mineral surface area from soil textural data results in under-estimates and the requirement to determine the surface area fraction of the different minerals in the soil to be known is extremely difficult if not impossible. Further uncertainty is introduced by adjustment of the rates to take into account temperature differences and by the use of a default mineralogy which is compositionally unrealistic. Despite its flaws PROFILE usually predicts similar weathering rates to other methods of calculation. It is argued that the unrealistic constraints imposed by the use of the surface area equation may be responsible for limiting calculated weathering rate to a fixed range which coincides with characteristically determined values for weathering rates.

A sensitivity analysis of the PROFILE model in relation to the calculation of soil weathering rates

Abstract Recently there has been growing interest in, and use of, the PROFILE model, principally through the role of weathering rates in determining critical loads. In many cases the accuracy of the data used to run the model is not quoted. A simple sensitivity analysis of the model has been carried out by varying the input data one at a time in a systematic fashion. The variation in weathering rate that this generates for monomineralic soil profiles and for a soil profile from an acid sensitive forested catchment in Scotland has been recorded. Variations in weathering rates of over 100% can be generated using ranges of input parameters measured in field studies. The model broadly predicts the relative ease of weathering of the different minerals. Minerals which are particularly sensitive to input variations have been identified, e.g. Kfeldspar. Some input parameters exert a larger influence on the weathering rate as calculated by the model than others. The most sensitive input data are soil temperature, moisture content and exposed mineral surface area. The least sensitive input data are cation load and precipitation rate.

A long-term soil leaching column experiment investigating the effect of variable sulphate loads on soil solution and soil drainage chemistry

A series of leaching column experiments were set-up to investigate the effects of increasing and decreasing the sulphate load on the uppermost mineral horizon of an acidified podzolic soil from NE Scotland. The soils showed signs of recovery when the sulphate load was reduced and acidified further when the sulphate load was increased. For the soils in which simulated precipitation inputs were less acidic than the soil pH, cation adsorption was still occurring at the end of the experiment. Adsorption was probably limited by the rate of diffusion of ions from the bulk soil water to the surfaces of soil particles. The leachate and soil solution from the experiment in which the least acidic input was applied were more acidic than the equivalent solutions from the experiment with the intermediate pH input. This was due to the acidifying effect of the desorption of protons from soil particles. The leachate and soil solution chemistry and the weathering rate of the soil subject to the most acidic simulated precipitation were modelled using the PROFILE model. The predicted bulk weathering rate was over estimated by a factor of 1.2. The predicted weathering rates of individual ions varied from the measured values by factors of between 0.06 and 10. Both the measured and PROFILE predicted weathering rates fall in the critical load of acidity class suggested for the soil on the basis of soil parent material by the Skokloster classification. PROFILE failed to predict accurately the measured soil solution and leachate chemistry.