Sulfate deposition to surface waters Estimating critical loads for Norway and the eastern United States
Arne Henriksen Norwegian Institute for Warer Research 0314 Oslo 3, N o m y David E Brakke Western Washington University Bellinnham, Wash. 98225 Critical loads are defined here as the highest deposition of strong acid anions (principally sulfate and nitrate) in surface waters that will not cause harmful biological effects on populations, such as declines in or extinctionsof fish. Our analysis focuses on sulfate deposition 8
Enviran. Sci. Technol.,Val. 22. No. 1. 1988
because in glaciated regions sulfate is conservative in soils, whereas nitrate is biologically cycled. Sulfate also is the dominant anion in acidic deposition and in most acidic lakes. This analysis, which is derived from work sponsored by the Nordic Council ( l ) ,represents the first evaluation of certain data available from Norway and the eastern United States, with an emphasis on the data from Scandinavia. The concept of dose-response is widely used in connection with water pollution (2). Any lake system sub jected to an external dose (load) of pollutants will have an internal resistance (or buffer capacity) to the change. The
response of the lake system will depend on the relative magnitudes of the dose and the resistance parameters.
Dose-response models The dose-response concept is used in many mathematical and empirical water quality models. For example, empirical models of nutrient pollution of lakes (such as the Vollenweider model) and empirical ion-balance models of lake acidification (such as the Henriksen predictor nomograph) are both based on this principle (2, 3). There are, however, major differences between these two models, as illustrated in Figure I.
0013-938x18710922-W08$01.5010@ 1987 American Chemical Society
condition (bicarbonate or oligotrophic) on the graphs in each case.The acidification model estimates the pH of lakes as a function of sulfate (or H+) in precipitation. Lake water pH is determined by the concentrations of base cations and strong acid anions (mainly sulfate) in lakes. Although the model is simple, it is based on empirical water chemistry and its predictions must obey ion balance. Complete descriptions of the empirical model of acidificationhave been published (3,4), as has a critique of the empirical ion balance model and other more process-oriented models (5).
The Vollenweider model is devel-
ON. from log-log plots of lake charac-
tenstlcs, phosphoms loading, and mean depth (2). By contrast, the Henriksen predictor nomograph is built from simple h e a r relationships within the important constraint of the electroneutrality of the solution (3). Thus the predictor nomograph is based on thretical considerations. It does not suffer from the variability inherent in the Vollenweider model, and it incorporates bask elements of water chemistry. The slopes of the lines in the eutrophication model are given as phospborus concentration (areal loading divided by mean depth), and the slopes in the acidification model are in hydrogen ion concentration (pH). Both the phosphorus concentration and the pH can generally describe lake conditions in meaningful ways. The polluted condition (acidic or eutrophic) is placed above the unpolluted
Critical load The critical load in deposition is determined by the sensitivity of the aquatic resource, as illustrated in Figure 2. The systems with the highest sensitivity are protected only at the lowest levels of deposition. We define a target load as the load determined by political agreement. A government may set a target load that is more or less restrictive than a critical load for various reasons, including eccnomic considerations. A target may also be considered with respect to prw tection of a certain percentage of the total resource or the most sensitive resource in an area. The distribution of lake sensitivity varies between areas of concern, as does the distribution of key fish species; therefore, critical loads should be derived for eacb area of a p plication. Here we define the critical load for sulfur to be the values of deposition that will maintain pH levels above 5.3 in the most sensitive lakes in an area. Above a pH of 5.3 strong acids are not present, and aluminum concentrations are nor-
mally below levels toxic to fish. Biological effects to fish and other organisms can occur at pH values greater than 5.3. We have calculated loads relative to the likely presence or absence of fish. In this analysis the sulfur in deposition is in the form of sulfuric acid, although in some areas of the United States (e.g., the Midwest and portions of the Rocky Mountains) sulfate in precipitation is derived partly as neutral salt from dust contamination (6). We have attempted to relate the critical load to wet d e p i t i o n because of the problem associated with measuring the dry component. We recognize that the contribution of dry deposition is variable and is estimated to be high for some areas, such as southern Sweden (7). These assumptions are reasonable for most of Norway, where the contribution of dry deposition is a relatively small fraction of the total because of the high annual rainfall. Even in the areas of Norway receiving the highest amounts of total sulfate deposition, the dry deposition component is estimated to be not more than 50% of the wet deposition. The response of a lake will depend on its sensitivity, which can be related to the concentrations of base cations. For example, in most oligotrophic surface waters the sum of nonmarine calcium and magnesium is approximately equivalent to alkalinity; greater concentrations of base cations would lead to increased buffering capacity in a simple titration with acid. Consequently, as concentrations of calcium and magne sium increase, the critical load of sulfate calculated for an area would increase. The sensitivity of a group of lakes will also be influenced by other watershed factors, such as the hydrologic type of the lake and the hydrologic flow path (8-11). In acidified waters, bicarbonate has been replaced largely by sulfate. Because acidification appears to be reversible, the model can be used to estimate the recovery of alkalinity in lakes and increases in pH given reduced loads of sulfur (12).
Lake sensit*ity parameter Many scientists have attempted to define lake water sensitivity to acidification. Most of the classifications are based exclusively on lake water alminity. We feel this approach is misleadiig for areas experiencing acidic deposition. In such areas, the alkalinity distribution of a population of lakes will have changed over time. in response to the acidic components of deposition. For areas not receiving significant acidic inputs, lake water alkalinity may be a good estimator of sensitivity to deposition because the relationship beEnvirnn Sct Technol, Vol. 22,NO l , 1988 9
tween base cations and alkalinity has not changed. However, alkalinity is a poor predictor of the response of systems to decreased deposition for affected areas, such as the Adirondacks and southern Norway. We have therefore decided to use base cations as an estimate of sensitivity. Critical loads are based on empirical relationships between measured sulfate deposition and pH conditions in lakes that have very low concentrations of base cations (20-50 pequiv/L Ca Mg). Calculations of the response of acidic lakes to decreased levels of acids in deposition and the resulting effects on Ca or Ca+Mg levels in lakes are again determined considering the constraint of ion balance in the solutions. Lake sensitivity will be influenced by hydrology and in-lake processes; conservative solutions are given to loading estimates in the absence of regional estimates of in-lake processes that generate alkalinity.
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Loading parameter The loading parameter is the measured concentration of nonmarine sulfate (SO4*) in lake water. The concentration of SO4* in lake water can be converted to pH or SO4* in precipitation by means of empirical relationships. The acidification model can be used to estimate the response of lakes to changes in the loading parameter SO4* . The assumption is made that a change in sulfur deposition is reflected in a corresponding change in sulfate concentration in lake waters; sulfate is assumed to be in steady state and to be a mobile or biologically conservative anion (13). The concentrations of base cations (principally calcium and magnesium) are changed by increases in acid inputs. If a base cation increase factor (or F-factor) is used that assumes that 20 % of the changes in sulfate concentration are compensated by changes in base cation concentrations rather than by titration of alkalinity, then a conservative estimate of the number of lakes that will be restored can be predicted (13, 14).
This approach is justified based on the maximum estimate of base cation leaching in response to acidic deposition, which indicates that little if any increase in lake water Ca +Mg concentrations has resulted from weathering caused by acidic deposition (13). Thus the process of acidification involves the titration of alkalinity, whereas concentrations of base cations are relatively constant. A factor F, where F = A [Ca2+ + Mg2+]/A has been derived from a gradient of deposition and from acid lakes to range between 0.18 and 10 Environ. Sci. Technol., Vol. 22,No. 1 , 1988
0.24 (13). Based on the stated assumptions, lake alkalinities can be predicted for any change in sulfate load. The original alkalinity of the lakes can also be estimated. These calculations require that ion balance be maintained, as defined in the equation [Ca2+ Mgz+l* - [S042-]* [HC03-] [H'I - [Al"+] where [A"+]indicates nonmarine concentrations of aluminum. Thus the F-factor can be used to determine the change in HC03- concentration caused by an addition of sulfate to a known concentration of base cations.
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Alkalinity in Norwegian lakes Sets of data are currently available for the following Norwegian lakes: 7 15 lakes in southernmost Norway sampled in 1974 (Snekvik Lakes) (15); lakes included in 1974-81 regional lake surveys, mostly in southern Norway (some were sampled several times) (16); and 50 large lakes in southern Norway sampled twice between 1979 and 1981 (17). The lakes selected for this analysis will not provide a complete picture of the water quality of lakes in Norway. However, because of the large size of the sample and the fact that most of the lakes were originally selected from areas sensitive to acidification, the response of these lakes to changes in acid inputs will give acceptable figures for estimating a critical load for Norwegian lakes. Using the empirical model, we have estimated the cumulative distributions of lake alkalinity for 4 different levels of sulfate reductions in southern Norway: 30, 50, 80, and 100%. Because alkalinity measurements were not available for all sets of data, we estimated present and predicted levels of alkalinity from present concentrations of nonmarine S042-and Ca+Mg. The F-factor was used to take into account the possible change in base cation concentrations caused by acidification. Figure 3 shows the results of this analysis. Figure 3a uses a base cation change factor of 0.2; Figure 3b uses a factor of 0. Two base cation change factors were used because F approaches zero at low base cation concentrations. This is reflected by the fact that the estimates of lakes with alkalinities less than zero is 8 % with a factor of 0.2, whereas a factor of 0 gives 3 % . Therefore, this approach leads to conservative estimates of the original alkalinity in lakes. The base cation change factor, F, is probably not constant between areas. It will vary with base cation concentrations and with sulfur loads. At high base cation concentrations F is probably higher than 0.2, and at low concen-
trations the factor is probably lower. We have used a conservative average factor of 0.2 in our estimation of critical loads for Norway. Figure 3 estimates the percent of lakes with alkalinities less than any given value. The alkalinities -10, 0, 10, and 20 were chosen as levels of interest for further discussion. These alkalinities correspond to lake water pH values of 5.0, 5.3, 5.7, and 5.9, respectively (pC02 = 10-3.5atm). About half of the lakes included in this study have alkalinities less than or equal to zero (pH
