Steady-State Water Chemistry Model Research Articles

The sensitivity of Afromontane tarns in the Maloti-Drakensberg region of South Africa and Lesotho to acidic deposition

Despite their remoteness from sources of atmospheric pollutant emissions, the Afromontane tarns in the Maloti-Drakensberg region are perfect candidates to study the negative effects of acidifying atmospheric pollution, because mountain lakes are widely recognised as sentinel ecosystems, unimpacted by direct human disturbance within their catchments. Thirty-four tarns were sampled in the Maloti-Drakensberg region and most were found to be extremely sensitive to acidic deposition, as indicated by their low acid neutralising capacity. There are very few studies of freshwater critical loads for any region within South Africa. The steady-state water chemistry model (SSWC) was adapted and used to determine critical loads, whereas exceedance was estimated relative to modelled regional deposition data, in order to understand the risk of harmful effects to aquatic ecosystems. Seventy-six percent of sampled sites across the Maloti-Drakensberg would exceed critical loads even at the lowest modelled deposition levels, but there are no current measured deposition data for the region. The sensitivity of the Maloti-Drakensberg tarns needs to be considered in future policy formulation regarding acceptable levels of acidifying atmospheric pollution from South Africa’s energy sector and indicates the need for assessing aquatic ecosystem impacts in other regions of South Africa.

Critical loads of acidity for 90,000 lakes in northern Saskatchewan: A novel approach for mapping regional sensitivity to acidic deposition

Atmospheric emissions of sulphur dioxide (SO2) from large point sources are the primary concern for acidic deposition in western Canada, particularly in the Athabasca Oil Sands Region (AOSR) where prevailing winds may potentially carry SO2 over acid-sensitive lakes in northern Saskatchewan. A novel catchment-scale regression kriging approach was used to assess regional sensitivity and critical loads of acidity for the total lake population of northern Saskatchewan (89,947 lakes). Lake catchments were delineated using Thiessen polygons, and surface water chemistry was predicted for sensitivity indicators (calcium, pH, alkalinity, and acid neutralizing capacity). Critical loads were calculated with the steady state water chemistry model using regression-kriged base cations, sulphate, and dissolved organic carbon concentrations modelled from surface water observations (n > 800) and digital landscape-scale characteristics, e.g., climate, soil, vegetation, landcover, and geology maps. A large region (>13,726 km2) of two or more indicators of acid sensitivity (pH < 6 and acid neutralizing capacity, alkalinity, calcium < 50 μeq L−1) and low critical loads < 5 meq m−2 yr−1 were predicted on the Athabasca Basin. Exceedance of critical loads under 2006 modelled total sulphate deposition was predicted for 12% of the lakes (covering an area of 3742 km2), primarily located on the Athabasca Basin, within 100 km of the AOSR. There have been conflicting scientific reports of impacts from atmospheric emissions from the AOSR; the results of this study suggest that catchments in the Athabasca Basin within 100 km of the AOSR have received acidic deposition in excess of their critical loads and many of them may be at risk of ecosystem damage owing to their sensitivity.

Critical Loads of Acid Deposition for Wilderness Lakes in the Sierra Nevada (California) Estimated by the Steady-State Water Chemistry Model

Major ion chemistry (2000–2009) from 208 lakes (342 sample dates and 600 samples) in class I and II wilderness areas of the Sierra Nevada was used in the Steady-State Water Chemistry (SSWC) model to estimate critical loads for acid deposition and investigate the current vulnerability of high elevation lakes to acid deposition. The majority of the lakes were dilute (mean specific conductance = 8.0 μS cm−1) and characterized by low acid neutralizing capacity (ANC; mean = 56.8 μeq L−1). Two variants of the SSWC model were employed: (1) one model used the F-factor and (2) the alternate model used empirical estimates of atmospheric deposition and mineral weathering rates. A comparison between the results from both model variants resulted in a nearly 1:1 slope and an R 2 value of 0.98, suggesting that the deposition and mineral weathering rates used were appropriate. Using an ANClimit of 10 μeq L−1, both models predicted a median critical load value of 149 eq ha−1 year−1 of H+ for granitic catchments. Median exceedances for the empirical approach and F-factor approach were −81 and −77 eq ha−1 year−1, respectively. Based on the F-factor and empirical models, 36 (17 %) and 34 (16 %) lakes exceeded their critical loads for acid deposition. Our analyses suggest that high elevation lakes in the Sierra Nevada have not fully recovered from the effects of acid deposition despite substantial improvement in air quality since the 1970s.

Site-specific estimates of water yield applied in regional acid sensitivity surveys across western Canada

Runoff or water yield is an important input to the Steady-State Water Chemistry (SSWC) model for estimating critical loads of acidity. Herein, we present site-specific water yield estimates for a large number of lakes (779) across three provinces of western Canada (Manitoba, Saskatchewan, and British Columbia) using an isotope mass balance (IMB) approach. We explore the impact of applying site-specific hydrology as compared to use of regional runoff estimates derived from gridded datasets in assessing critical loads of acidity to these lakes. In general, the average water yield derived from IMB is similar to the long-term average runoff; however, IMB results suggest a much larger range in hydrological settings of the lakes, attributed to spatial heterogeneity in watershed characteristics and landcover. The comparison of critical loads estimates from the two methods suggests that use of average regional runoff data in the SSWC model may overestimate critical loads for the majority of lakes due to systematic skewness in the actual runoff distributions. Implications for use of site-specific hydrology in regional critical loads assessments across western Canada are discussed.

Recently surveyed lakes in northern Manitoba and Saskatchewan, Canada: characteristics and critical loads of acidity

Based on minimal information, lakes in the western Canadian provinces of Manitoba (MB) and Saskatchewan (SK) have long been considered unaffected by acid rain. However, emissions of acidifying pollutants from MB smelters and oil sand processing in Alberta (AB) may pose a developing threat. Surveys of 347 lakes located on geologically sensitive terrain in northern MB and SK were conducted to assess their acidification sensitivity and status. The survey domain (~193,000 km2) contained 81,494 lakes ≥1 ha in area. Small lakes dominated the inventory in terms of numbers, and large lakes dominated in terms of area. Survey lakes were selected using a stratified-random sampling design in 10 sampling blocks within the overall survey domain. Few lakes had pH &lt;6, and only three (all in SK) were acidic, i.e., Gran Alkalinity (Alk) &lt;0 μeq L–1. A broad range in lake sensitivity was apparent, and very sensitive lakes (low specific conductance, base cations and Alk) were present in all sampling blocks. Dissolved organic carbon (DOC) was an important constituent of many lakes. Critical loads (CL) of acidity calculated using the Steady-State Water Chemistry model (SSWC) revealed extremely low 5th percentile values for every block (range 1.9 to 52.7 eq ha–1 y–1). Block CL exceedances calculated using estimated S and N deposition for 2002 ranged from 54.5 to 909 eq ha–1 y–1. The largest exceedances were for sampling blocks located near smelter sources or downwind of the oil sands. Lake chemistry revealed by our surveys was compared to others conducted both nearby and outside Canada.

Inter-annual variations in water yield to lakes in northeastern Alberta: implications for estimating critical loads of acidity

Stable isotopes of water were applied to estimate water yield to fifty lakes in northeastern Alberta as part of an acid sensitivity study underway since 2002 in the Athabasca Oil Sands Region (AOSR). Herein, we apply site-specific water yields for each lake to calculate critical loads of acidity using water chemistry data and a steady-state water chemistry model. The main goal of this research was to improve site-specific critical load estimates and to understand the sensitivity to hydrologic variability across a Boreal Plains region under significant oil sands development pressure. Overall, catchment water yields were found to vary significantly over the seven year monitoring period, with distinct variations among lakes and between different regions, overprinted by inter-annual climate-driven shifts. Analysis of critical load estimates based on site-specific water yields suggests that caution must be applied to establish hydrologic conditions and define extremes at specific sites in order to protect more sensitive ecosystems. In general, lakes with low (high) water yield tended to be more (less) acid sensitive but were typically less (more) affected by interannual hydrological variations. While it has been customary to use long-term water yields to define a static critical load for lakes, we find that spatial and temporal variability in water yield may limit effectiveness of this type of assessment in areas of the Boreal Plain characterized by heterogeneous runoff and without a long-term lake-gauging network. Implications for predicting acidification risk are discussed for the AOSR.

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 < 1 keq H + 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.

Testing the steady-state water chemistry model predictions of pre-industrial lake pH with paleolimnological data from northern Sweden

Criteria are needed for distinguishing naturally acid water from that acidified by air pollution, especially in the organic-rich waters of northern Sweden. The Steady-State Water Chemistry Model (SSWC) was augmented to include organic acidity so that it could predict pre-industrial pH in organic-rich waters. The resulting model predictions of pre-industrial ANC and pH were then tested against diatom predictions of pre-industrial pH and alkalinity in 58 lakes from N. Sweden (after alkalinity was converted to ANC using the CBALK method). The SSWC Model's predictions of pre-industrial lake pH in N. Sweden did not correspond well with the diatom predictions, even when accounting for the uncertainty in the diatom model. This was due to the SSWC's sensitivity to short-term fluctuations in contemporary water chemistry. Thus the SSWC Model is not suitable for judging the acidification of individual lakes in areas such as northern Sweden where the degree of chronic acidification is small, or without a good average value of contemporary water chemistry. These results should be considered when assessing the accuracy of critical loads calculated using SSWC.

Estimating calcium weathering rates and future lake calcium concentrations in the Muskoka–Haliburton region of Ontario

Calcium (Ca) concentrations in surface waters on the Precambrian Shield are determined primarily by the Ca weathering rate in soil, which requires extensive soil data that generally do not exist. From a water chemistry database comprising approximately 550 lakes in south-central Ontario, Canada, 130 lakes were selected with low Ca concentrations (Ca &lt; 75 µmol·L–1). Calcium weathering is primarily dominated by silicate minerals such as plagioclase and hornblende, allowing the use of Ca–sodium (Na) ratios in lake water to estimate Ca weathering rates. Soil profile data at seven sites indicated that the Ca–Na ratio from mineral weathering is 0.86; correspondingly, Ca weathering rates in lakes ranged from 0.04 to 0.24 kmol·ha–1·year–1 (median of 0.09 kmol·ha–1·year–1). This compares with a range of 0.06–0.24 kmol·ha–1·year–1 (median of 0.14 kmol·ha–1·year–1) obtained using the steady-state water chemistry model. Using these methods to bound potential weathering rates, Ca concentrations in individual lakes at steady state are predicted to decline by 10%–40% compared with current values. Dynamic soil hydrochemical model predictions indicate that Ca concentrations in lakes will be considerably lower than these steady-state predictions within decades if timber harvesting occurs in the drainage basins.

Recovery from acidification in Nova Scotia: temporal trends and critical loads for 20 headwater lakes

The chemical response of 20 headwater lakes in Nova Scotia to reduced acid deposition was investigated using trend analysis, and the need for further reductions was assessed using two steady-state, critical load models. Significant decreases were observed in the concentration of nonmarine sulphate (SO42–) and hydrogen (H+) at four wet deposition monitoring stations across Atlantic Canada since 1984. Dominant trends in surface water were decreasing SO42–concentrations, with little improvement in alkalinity and H+. Based on the Steady State Water Chemistry (SSWC) and First-order Acidity Balance (FAB) models, and using a critical chemical limit for acid-neutralizing capacity of 20 µmolc·L–1, critical load is exceeded at 9 and 13 of the 20 study lakes, respectively. Application of the SSWC model suggests that sulphur (S) deposition must be reduced by 37.3 mmolc·m–2·year–1from 1997 levels to prevent critical load exceedance at 95% of the study lakes. Using the FAB model, the minimum reductions in nitrogen and S deposition necessary to protect 95% of the study lakes are 32.7 and 42.1 mmolc·m–2·year–1, respectively. Additional reductions beyond those proposed for 2030 are required to minimize critical load exceedance and promote recovery in alkalinity and pH of surface waters at the study catchments.

Effect of Declining Lake Base Cation Concentration on Freshwater Critical Load Calculations

Steady-state critical load models have been extensively used as the scientific underpinning for air pollution control policies in Europe and are currently being applied to other parts of the world. An important assumption of steady-state models is that critical load estimates do not change through time (or time scale of interest). The most commonly used model for estimating freshwater critical loads is the steady-state water chemistry (SSWC) model. In this study we examined changes in SSWC critical load estimates for 29 lakes in south-central Ontario using data collected 13 years apart (1985-1998), during which time bulk sulfate (SO4(2-)) deposition decreased by 35%. In lakes with the lowest base cation concentrations (<120 microequiv L(-1) Ca2+), the decrease in SO4(2-) concentration was accompanied by an approximately equivalent decrease in base cation concentration, resulting in only a minimal increase in Acid Neutralizing Capacity (ANC) during the 13-year period (median increase 1.6 microequiv L(-1)), and the median critical load for acidity (CL(A)) estimated by the SSWC model decreased by 14.6%. These changes may have been brought about by declining base cation concentrations owing to continued soil acidification in the region. In contrast, in lakes with higher base cation concentrations (>150 microequiv L(-1) Ca2+), the relative decline in base cation concentration was not as great, resulting in a larger increase in ANC (median increase 13.1 microequiv L(-1)) and an increase in the estimated CL(A) (median 5.1%). Lakes with moderate base cation concentrations (120-150 microequiv L(-1) Ca2+) exhibited an intermediate response; the median ANC increased by 8.8 microequiv L(-1) and the estimated CL-(A) decreased by 2.2%. In central Ontario, SSWC critical load estimates based on data taken only 13 years apart change quite dramatically due to changing lake base cation concentrations, and the response appears to depend on the base status of the lakes. The changing values obtained from the SSWC model have important consequences for policy decisions regarding acceptable levels of acid deposition. The application of dynamic models that take into account changes in lake/soil chemistry appears more appropriate for estimating acceptable levels of acid deposition in the region.

Critical loads of acidity for Irish lakes

Critical loads of acidity have been estimated for Irish lakes (277) using survey data from the Republic of Ireland and Northern Ireland, and compatible inputs and methods across both regions. This is the first trans-region application of the Steady-State Water Chemistry model for the island of Ireland. Results indicate that a small percentage of lakes (10.8%, 30 lakes) have low critical loads of acidity (<50.0 mmolc m–2 yr–1). Based on bulk sulphate deposition, exceedance of critical load was observed in 6.9% of the sampled lakes (19 lakes). Inferences relating to the whole lake population cannot be made, as statistical lake surveys were not carried out. However, as the surveys were weighted towards acid-sensitive lakes, the current study suggests that Irish lakes are not under a significant threat from anthropogenic acidification. The acidifying impact of nitrogen was incorporated into the exceedance calcula tion by superimposing the present leaching of nitrogen onto sulphate deposition. This increased exceedance by 40% (i.e., from 11 to 19 lakes). Despite the potential uncertainties associated with using default values and relationships in the Steady-State Water Chemistry model, uncertainty analysis indicated that the critical loads were relatively stable. The critical acid neutralising capacity limit (specified water quality criterion) had the greatest effect on the critical load. In the current study, a catchment dependent limit was employed (ranged: 0–50 μmolc L–1; mean: 35 μmolc L–1) rather than a single fixed value.

The impact of acid deposition and forest harvesting on lakes and their forested catchments in south central Ontario: a critical loads approach

Abstract. The impact of acid deposition and tree harvesting on three lakes and their representative sub-catchments in the Muskoka-Haliburton region of south-central Ontario was assessed using a critical loads approach. As nitrogen dynamics in forest soils are complex and poorly understood, for simplicity and to allow comparison among lakes and their catchments, CLs (A) for both lakes and forest soils were calculated assuming that nitrate leaching from catchments will not change over time (i.e. a best case scenario). In addition, because soils in the region are shallow, base cation weathering rates for the representative sub-catchments were calculated for the entire soil profile and these estimates were also used to calculate critical loads for the lakes. These results were compared with critical loads obtained by the Steady State Water Chemistry (SSWC) model. Using the SSWC model, critical loads for lakes were between 7 and 19 meq m-2yr-1 higher than those obtained from soil measurements. Lakes and forests are much more sensitive to acid deposition if forests are harvested, but two acid-sensitive lakes had much lower critical loads than their respective forested sub-catchments implying that acceptable acid deposition levels should be dictated by the most acid-sensitive lakes in the region. Under conditions that assume harvesting, the CL (A) is exceeded at two of the three lakes and five of the six sub-catchments assessed in this study. However, sulphate export from catchments greatly exceeds input in bulk deposition and, to prevent lakes from falling below the critical chemical limit, sulphate inputs to lakes must be reduced by between 37% and 92% if forests are harvested. Similarly, sulphate leaching from forested catchments that are harvested must be reduced by between 16 and 79% to prevent the ANC of water draining the rooting zone from falling below 0 μeq l-1. These calculations assume that extremely low calcium leaching losses (9–27 μeq l-1) from forest soils can be maintained without any decrease in forest productivity. Calcium concentrations in the three lakes have decreased by between ∼10 and 25% over the past 20 years and calculations assume that calcium concentrations in lakes can fall to around 30% of their current values without any harmful effects on biota. Both these assumptions require urgent investigation. Keywords: acid deposition, calcium, critical loads, forests, harvesting, lakes

Present and potential nitrogen outputs from Norwegian soft water lakes – an assessment made by applying the steady-state First-order Acidity Balance (FAB) model

Abstract. The steady-state First-order Acidity Balance (FAB) model for calculating critical loads of sulphur (S) and nitrogen (N) is applied to 609 Norwegian soft-water lakes to assess the future nitrate (NO3‾) leaching potential under present (1992-96) S and N deposition. The lakes were separated into five groups receiving increasing levels of N deposition (&lt;25, 25-49, 50-74, 75-99 and 100-125 meq m-2yr-1). Using long-term sustainable N sink rates presently recommended for FAB model applications, N immobilisation, net N uptake in forests, denitrification and in-lake N retention were estimated for each group of lakes. Altogether, the long-term N sinks constituted 9.9 ± 3.2 to 40.5 ± 11.4 meq m-2yr-1 in the lowest and highest N deposition categories, respectively. At most sites, the current N deposition exceeds the amount of N retained by long-term sustainable N sinks plus the NO3‾ loss via the lake outlets. This excess N, which is currently retained within the catchments may, according to the FAB model, leach as acidifying NO3‾ in the future. If these predictions are fulfilled, NO3‾ leaching at sites in the various N deposition categories will increase dramatically from present (1995) mean levels of 1-20 meq m-2yr-1, to mean levels of 7-70 meq m-2yr-1 at future steady state. To illustrate the significance of such an increase in NO3‾ leaching, the mean Acid Neutralising Capacity (ANC) at sites in the highest N deposition category may decrease from -18 ± 15 μeq L-1 at present, to -40 ± 20 μeq L-1. Under present S and N deposition levels, the FAB model predicts that 46% of the Norwegian lakes may experience exceedances of critical loads for acidifying deposition. In comparison, the Steady-State Water Chemistry model (SSWC), which considers only the present N leaching level, estimates critical load exceedances in 37% of the lakes under the same deposition level. Thus far, there are great uncertainties regarding both the time scales and the extent of future N leaching, and it is largely unknown whether the FAB model predictions will ever be fulfilled. Hence, long-term monitoring and further studies on N immobilisation processes under varying N deposition levels and ecosystem types seem necessary to make better predictions of future NO3‾ leaching. Keywords: Lakes, hydrochemistry, nitrogen, nitrate, sinks, leaching, acidification, critical loads, FAB model

Application of Static Critical Load Models for Acidity to High Mountain Lakes in Europe

Critical load models for acidityprovide a measure of the sensitivity of surfacewaters to acid deposition, and can be used todetermine critical load exceedance and potentiallong-term harmful effects. Three static models,the Steady-State Water Chemistry model, diatommodel and First-order Acidity Balance model, arehere applied to 11 high mountain lakes in Norway,Scotland, the Alps, the Pyrenees and the Tatras.Between five and seven of the lakes show criticalload exceedance, depending on the model used.Nitrogen as well as sulphur deposition isimportant in causing exceedance. Since soil andvegetation cover are generally sparse, geologyand lake retention time appear to be key factorsin the determination of critical load. Retentionof nitrogen is observed, but it is unclearwhether this occurs within the lake or theterrestrial part of the catchment.

The link between the exceedance of acidity critical loads for freshwaters, current chemical status and biological damage: a re-interpretation

Critical loads have for several years been employed bypolicymakers to aid in the development of strategies for aciddeposition abatement. They provide an effects-based approachwhereby an acid deposition flux greater than the critical load(known as critical load exceedance) implies that long-termharmful effects on a selected target organism will occur.Implicit in this approach are two assumptions: first, theexceedance of a critical load will harm the target organism,and second, the severity of biological impact is related to themagnitude of exceedance. However, static models give noindication of when the predicted damage might occur. One suchmodel, the Steady-State Water Chemistry (SSWC) model, employs aseries of empirical relationships to derive the pre-industrial,baseline leaching rate of base cations from measured waterchemistry using the so-called `F-factor'. The SSWC model setsthe critical load relative to pre-industrial base cationleaching (a permanent buffer of acid deposition) and a selectedacid neutralizing capacity (ANC) value which corresponds with aknown likelihood of damage to a biological target organism.Here we interpret the meaning of critical load exceedance as aprediction of steady-state ANC, and explore the relationshipbetween exceedance of the critical load and current chemistry. We demonstrate that a critical loadexceedance with the SSWC model does not necessarily indicatethat the critical chemical threshold (zero ANC) has alreadybeen crossed, and there may be no correlation betweenexceedance and biological status. A reformulation of the SSWCmodel is proposed which provides a direct link between currentdeposition and current chemical conditions, and is thereforemore likely to indicate current biological damage. Thereformulation illustrates the discrepancy between currentchemical status and that predicted by the SSWC model atsteady-state, which is a function of the `F-factor'.

How are Results from Critical Load Calculations Reflected in Lake Water Chemistry

The Steady State Water Chemistry Model (SSWCM) is a common method for determinations of critical loadof acid and subsequently of critical loadexceedance for lakes. One way to verify the modeloutput, is to compare with chemical indicatorssuch as pH-value, alkalinity or ANC. When themedian chemical status (as ANC) is used 65% ofthe lakes responded according to the exceedancevalue. For these the calculated exceedanceresulted in violation of the critical chemicalvalue while non-exceedance gave no violation.Since biota react on extreme conditions a morecorrect evaluation should be based on minimumvalues for the chemical indicator. This raises thefraction of lakes responding to 78%. Non-exceedance is seldom found inlakes with acid conditions. The evaluationindicates that the calculation of critical load ofacidity by means of SSWCM is very reliable.

Freshwater Critical Loads in Wales

The Steady State Water Chemistry (SSWC) modeland the Diatom model have been used to calculate criticalloads for acidity using annual mean chemistry for 102 acidupland streams in Wales sampled as part of the Welsh AcidWaters Survey (WAWS) in 1995. Diatom critical loads werelower than SSWC values reflecting the higher effective[ANC]limit of the Diatom model compared to the[ANC]limit of zero used in the SSWC model. The WAWSstream sites were all located within 41 10 × 10 km squares andeach square was assigned the lowest critical load value fromamongst the sites located within it. Comparison with valuesassigned under the UK national critical loads mappingprogramme (UKCLMAP) showed that WAWS critical loads were lowerthan UKCLMAP values in approximately 40% of squares.Differences in critical load class were variable, but exceeded2 keq ha-1 yr-1 in up to a maximum of seven squares.It cannot be assumed, therefore, that reducing acid depositionto the currently mapped UKCLMAP critical load will protect allstreams occurring within a given 10 × 10 km grid square inWales. The limited number of sample sites means that even inthose squares where all WAWS sites will be protected, theremay be other, more acid sensitive freshwaters with lowercritical loads. This has important implications for theinterpretation and use of critical loads data for regional andlocal environmental planning.

Spatial and temporal variations in critical loads for rivers in N.E. Scotland: a validation of approaches

Previously, critical load maps for UK surface freshwaters have been created by applying the steady state water chemistry model to a spot sample of water from a lake, from each 10 km square. If no lake is present, as was the case for many squares in the north east of Scotland, a sample from a river thought likely to be most sensitive to acidification was used. This study evaluates the effectiveness of this approach for protecting all the waters of the complex river network of a major catchment in N.E. Scotland, under high and low flow regimes. Samples were collected and analysed over a period of one year, at intervals of two weeks, from 59 sub-catchments of the River Dee catchment. Critical load values were calculated for every sample, and also using mean annual concentrations of Na +, Ca 2+, Mg 2+, K + and Cl −. In addition, to investigate discharge effects, they also were calculated for each site for base and high flows, using stream water chemistry corresponding to the three lowest and the three highest discharge levels respectively. Only two rivers were insufficiently protected under base flow conditions, whereas 20 apparently were not sufficiently protected under high flow conditions. It is concluded that the system used in the UK is generally very effective for protection of rivers, but only under low flow conditions. More work is needed to assess the biological impacts of episodic critical load exceedance in upland rivers. A more recently suggested approach for classifying UK river water sensitivity to acidification from the Na + dominance of the solute base cations in water samples is also shown to be effective for this river system.