115 5 Chemical Dose–Response Relationships and Critical Loads 5.1 Quantification of Chemical Dose–Response Relationships There has been a growing international recognition that air pollution effects, particularly from S and N, may in some cases necessitate emission controls to reduce or limit future increases in atmospheric deposition. Measures to reduce emissions must rely on known or estimated dose–response relation- ships that reflect the tolerance of natural ecosystems to various inputs of atmospheric pollutants. This need has stimulated interest in evaluating the efficacy of establishing one or more standards for acid deposition. The Clean Air Act Amendments of 1990 (CAAA) also included requirements to assess the effectiveness of the mandated emissions controls via periodic assess- ments, and to submit an EPA report on the feasibility of adopting one or more acid deposition standards to Congress. Diverse data are available from a variety of sources with which to quantify the watershed acidification response, as well as recovery from acidification. Such data shed light on the sensitivity of various kinds of watershed systems to changes in acidic deposition. Intercomparisons among the various studies that have been conducted are complicated by different relative watershed sensitivities, S deposition loading rates (and changes in those rates), the rela- tive importance of N leaching and N saturation, temporal considerations, and natural (especially climatic) variability. In addition, these quantitative data have been generated in vastly different ways, including monitoring, space- for-time substitution, whole-watershed or whole-lake acidification, whole- watershed acid exclusion, paleolimnology, and modeling. The only way in which different approaches can be compared on a quantitative basis is by nor- malizing surface water response as a fraction of the change in SO 4 2- concentra- tion (or SO 4 2- + NO 3 - concentration where NO 3 - is also important). The principal ions that change in direct response to changes in (SO 4 2- + NO 3 ) con- centration are ANC (which can be expressed as [HCO 3 - - H + ]), base cations (C B ), inorganic aluminum (Al i ), and organic acid anions (A - ). The proportional 1416/frame/C05 Page 115 Wednesday, February 9, 2000 2:09 PM © 2000 by CRC Press LLC 116 Aquatic Effects of Acidic Deposition changes in (HCO 3 - - H + ), Al i , C B , and A - concentrations should sum to 1.0 in order to satisfy the electroneutrality condition. For aquatic systems that are relatively insensitive to acidic deposition, ∆ C B approximates ∆ (SO 4 2- + NO 3 - ), and the F factor (Henriksen, 1982) approximately equals 1.0: ~ ~ 1.0 (5.1) where brackets indicate concentration in µ eq/L and changes in other constit- uents are insignificant. Where acidification occurs in response to acidic dep- osition, changes in (HCO 3 - - H + ) and/or Al i comprise an appreciable percentage of the overall surface water response and, therefore, the F factor is less than 1.0. The F factor is important in evaluating criteria for establishing acid deposition standards because it provides the quantitative linkage between inputs of acid anions (e.g., SO 4 2- , NO 3 - ) and effects on surface water chemistry. An important limitation of the F factor concept, however, is that the value of F is likely to change as the base cation pools in watershed soils become depleted by acid deposition inputs. Quantitative dose–response relationships for S have been determined, using a variety of approaches, in a number of regions in North America and Europe. Such studies have included, for example, measured changes in water chemistry during periods when S deposition changed appreciably, regional paleolimnological (e.g., diatom-inferred change in pH and ANC) investiga- tions, whole-catchment manipulation studies, and intensive process model- ing. Each type of study has provided quantitative estimates of dose–response that entail different sets of assumptions and limitations. Taken together, they provide a good indication of the range of quantitative acidification response. As a result of these recent studies, we are much better able to quantify acidi- fication and recovery relationships than we were in 1990. 5.1.1 Measured Changes in Acid–Base Chemistry Measured changes in surface water chemistry in areas that have experienced short-term (less than 20 years) changes in chemical constituents in response to changes in mineral acid inputs are available from a number of sources. Propor- tional changes in ANC, base cations, and Al i relative to changes in SO 4 2- or (SO 4 2- + NO 3 - ) concentrations were summarized by Sullivan and Eilers (1994) for lakes and streams in which such changes had been measured. They included lakes in the Sudbury region of Ontario, the Galloway lakes area of Scotland, a stream site at Hubbard Brook, NH, and catchment manipulation experiments in the RAIN project in Norway and Little Rock Lake in Wisconsin. Most of the observed changes were coincident with decreased acidic deposi- tion, and it is unclear to what extent acidification and recovery are symmetri- cal. F -factors in the range of 0.5 to 0.9 are apparently typical for lakes having low base cation concentrations, although lower values (0.35 to 0.39) were F C B []∆ SO 4 2- NO 3 - +[]∆ = 1416/frame/C05 Page 116 Wednesday, February 9, 2000 2:09 PM © 2000 by CRC Press LLC Chemical Dose–Response Relationships and Critical Loads 117 TABLE 5.1 Measured Short-Term Changes in Surface Water Chemistry Associated with Changes in Mineral Acid Anion Concentrations. (Units in µ eq/L.) Proportions are Expressed as Absolute Values. Site Location Period Type Initial pH ∆ S Ο 4 2− Ref. d Clearwater Lake Sudbury, Canada 1973–1977 to 1984 Recovery 4.2 -175 0.19 0.66 b 0.15 1 Swan Lake Sudbury, Canada 1977 to 1982 Recovery 4.0 -360 0.26 0.67 0.07 1 Baby Lake Sudbury, Canada 1968–1972 to 1983 Recovery 4.05 -750 0.12 —— 2 Whitepine Lake Sudbury, Canada 1980–1988 Recovery 5.4 -42 0.24 — 0.05 6 Laundrie Lake Sudbury, Canada 1974–1976 to 1979–1983 Recovery 4.7 -58 0.24 —— 3 Florence Lake Sudbury, Canada 1974–1976 to 1979–1983 Recovery 4.6 -42 0.22 —— 3 Average of 37 lakes having pH< 5.5 Sudbury, Canada 1974–1976 to 1979–1983 Recovery 4.7 -42 0.15 —— 3 Average of 105 trout lakes Sudbury, Canada 1980–1987 Recovery — -45 0.51 —— 6, 10 Average of 50 lakes Galloway, Scotland 1979–1988 Recovery 5.4 + - 0.71 -76 a 0.13 0.84 ~0.06 4 Little Rock Lake e Wisconsin 1983–1989 Acid addition 6.6 94 0.44 0.53 — 9 SOG2 catchment Sogndal, Norway 1984–1987 Acid addition 5.5 28 a 0.46 0.39 0.11 5 SOG4 catchment Sogndal, Norway 1984–1987 Acid addition 6.0 20 a 0.35 0.35 0.15 5 KIM catchment Risdalsheia, Norway 1984–1987 Acid exclusion 4.1 -139 a,c 0.09 0.55 0.05 5, 7 Bear Brook Maine 1987–1992 Acid addition 5.6 62 a 0.14 0.51 0.20 11 Hubbard Brook New Hampshire 1969–1979 Recovery 4.8 -30 a 0.15 0.91 — 8 a Also includes NO 3 - . b ∆ C B / ∆ SO 4 2- calculated by difference, assuming that the proportional changes in alkalinity, C B , and Al sum to 1.0. c Changes in the organic anion contribution to acidity were important at this site where DOC was very high (~ 1250 µ M). d 1—Dillon et al., 1986; 2—Hutchinson and Havas, 1986; 3—Keller et al., 1986; 4—Wright, 1988b; 5—Wright et al., 1988b; 6—Gunn and Keller, 1990; 7—Wright, 1989; 8—Sullivan, 1990; 9—Samp- son et al., 1994; 10—Gunn, personal communication; 11—Norton et al., 1993. e Little Rock Lake experiment involved manipulation of lake only. ∆ HCO 3 - -H + () ∆ SO 4 2- ∆C B * ∆SO 4 2- ∆Al ∆SO 4 2- 1416/frame/C05 Page 117 Wednesday, February 9, 2000 2:09 PM © 2000 by CRC Press LLC 118 Aquatic Effects of Acidic Deposition observed for the highly sensitive catchments at Sogndal, Norway that are char- acterized by thin soils and much exposed bedrock, as is common in many areas of southern Norway and the western U.S. The proportional change in ANC rel- ative to the change in (SO 4 2- + NO 3 - ) was variable, within the range of 0.1 to 0.5 (Table 5.1). The proportional change in Al was smaller, ranging up to 0.15. These measured values of acidification and deacidification change in ANC and Al are somewhat smaller than previously anticipated. Relatively early in the international efforts to quantify the acidification response, Henriksen (1982) proposed that F factors for softwater lakes would be in the range 0 to 0.4. More recent research (e.g., Table 5.1) has shown this earlier estimate to be too low in most cases. Based on measured values, only the most sensitive systems, for example at Sogndal, exhibit F factors below 0.4. TABLE 5.2 Inferred Long-Term Regional Changes in Surface Water Chemistry Associated with Estimated Changes in Mineral Acid Anion Concentrations, Using the Technique of Space-for-Time Substitution Region Reference Comments NE U.S. 0.13 0.54 0.07 Sullivan et al., 1990a Analysis restricted to lakes having current ANC ≤ 25 µ eq/L S. Norway 0.22 —— Brown and Sadler, 1981 Regional data set ( n = 471) S. Norway — 0.82 — Wright, 1988; Sullivan, 1990 Lakes located across depositional gradient from Bykle to Mandal TABLE 5.3 Diatom-Inferred Long-Term Changes in Lake-water ANC as a Fraction of Estimated Historic Changes in Lake-water SO 4 2- Concentration Region Number of Lakes References Comments Adirondacks, NY 48 0.11 Sullivan et al., 1990a Statistical sampling Adirondacks, NY 25 0.18 Sullivan et al., 1990a Acidic lakes only a Northern New England 12 0.30 Davis et al., 1994 Lakes were selected that were presumed to be acid-sensitive Florida (Lakes Barco, Suggs) 2 0.27 Sullivan and Eilers, 1994 Seepage lakes a The set of 25 acidic lakes was part of the regional data set of 48 lakes presumed to be acid-sensitive. ∆ANC ∆SO 4 2- ∆C B ∆SO 4 2- ∆Al ∆SO 4 2- ∆ANC ∆SO 4 2- 1416/frame/C05 Page 118 Wednesday, February 9, 2000 2:09 PM © 2000 by CRC Press LLC Chemical Dose–Response Relationships and Critical Loads 119 In addition to the measured acidification and recovery data presented in Table 5.1, there are several other sources of quantitative or semiquantitative data with which to evaluate the general applicability of the measured results that are available. These include the results of space-for-time substitution (Table 5.2), diatom-inferences of historical acidification (Table 5.3), and results of process-based model hindcasts or future forecasts (Table 5.4). Each of these methods has its own assumptions and limitations, and none are as robust as results of actual field measurements of response. Major advantages of these alternative sources of quantitative data, however, are that they primarily reflect acidification, rather than recovery, scenarios, and that they sometimes include longer periods of response than do the available direct measurements. 5.1.2 Space-for-Time Substitution Results of space-for-time substitution must be interpreted with caution. This approach is based on the assumption that changes in chemistry across space, for example, from low to high levels of acidic deposition, reflect changes that occurred over time as deposition increased from low to high. It is implicitly assumed that the waters included in the analysis were initially homogeneous in their chemistry, and also that potentially important factors other than dep- osition (e.g., soil characteristics, land use impacts) do not co-vary with depo- sition. Results should therefore be considered only semiquantitative. Nevertheless, available data using this method (Table 5.2) appear similar to results of measured values shown in Table 5.1. The spatial distributions of lake-water chemical variables across a longitu- dinal gradient in the upper Midwest for low-ANC groundwater recharge TABLE 5.4 Dynamic Model (MAGIC) Estimates of F -Factors for Hindcast or Future Forecast Projections of Acidification or Recovery Responses Number of Lakes or Streams F -Factor Region Type of Simulation Median 5th Percentile Reference Adirondacks Hindcast 33 0.56 0.25 Sullivan et al., 1996a Adirondacks 50-year forecast, 50% reduction in S deposition 33 0.73 0.39 Sullivan, unpublished Wilderness lakes, Western U.S. Forecasted 3-fold increase in S deposition 15 0.34 0.03 Eilers et al., 1991 Bear Brook, ME Response to experimental watershed acidification 1 0.85 — Norton et al., 1992 1416/frame/C05 Page 119 Wednesday, February 9, 2000 2:09 PM © 2000 by CRC Press LLC 120 Aquatic Effects of Acidic Deposition seepage lakes (Figure 4.5) provides a good example of the use of space-for- time substitution to evaluate acidification dose–response relationships. These distributions also constitute perhaps the best evidence available that many of the most sensitive lakes in the eastern portion of this region have acidified. In the absence of additional paleolimnological data for these sys- tems of most interest, however, it is difficult to substantiate in terms of mag- nitude much regional acidification in the upper Midwest. Nitrogen deposition does not appear to be an important issue for sensitive aquatic resources in the upper Midwest. This is likely attributable to the fact that snowmelt is less important to the acid–base chemistry of sensitive (i.e., seepage) lakes in this region, and hydrologic retention times are long. Sulfur deposition appears to be of greater importance, and potential chronic effects are of greater interest than episodic effects because of the nature of the hydrology of sensitive resources in the region. Based largely on the results of space-for-time substitution analyses, Sullivan and Eilers (1994) concluded that current deposition in the eastern portion of the region (approximately 5 kg S/ha per year) is a reasonable approximation of the deposition level required to protect the most sensitive aquatic receptors. Resources in the western portion of the region are less sensitive, however, and an appropriate standard for S deposition would be much higher. Because S deposition has been decreasing in recent years, it does not appear that acidic deposition is an important environmental concern in the upper Midwest at this time. An S deposition standard has been in effect in Minnesota since 1986. The Minnesota standard was based on the Acid Deposition Control Act, passed by the state legislature in 1982, which required the Minnesota Pollution Con- trol Agency (MPCA) to identify natural resources within the state that were threatened by acid deposition and to develop both an acid deposition stan- dard and an emissions control plan. Small, poorly-buffered lakes in northcen- tral and northeastern Minnesota were identified as the resources at greatest risk. Based on model simulations, MPCA selected a threshold pH for precip- itation of 4.7, below which damage to aquatic biota was thought to occur with prolonged exposure. This threshold pH was correlated with SO 4 2- deposition data, and a standard was determined that allowed no more than 11 kg/ha of wet SO 4 2- to be deposited during any 52-week period (3.7 kg S/ha per year) (MPCA, 1985). This standard is fairly stringent. In fact, 6 of 12 monitoring sites in Minnesota exceeded the standard in 1992 (Orr, 1993). There appears to be a limited scientific basis for such a standard for protection of aquatic resources in Minnesota. 5.1.3 Paleolimnological Inferences of Dose–Response Diatom-inferences of change in ANC from pre-industrial times to the present have been reported for a regional population of Adirondack lakes (Sullivan et al., 1990a), and for two lakes in Florida that have shown clear acidification in recent decades (Sweets, 1992). Proportional changes in diatom-inferred 1416/frame/C05 Page 120 Wednesday, February 9, 2000 2:09 PM © 2000 by CRC Press LLC Chemical Dose–Response Relationships and Critical Loads 121 ANC as a fraction of assumed increases in SO 4 2- concentration since pre- industrial times show estimates ranging from 0.1 to 0.3 (Table 5.3), in close agreement with measured values (Table 5.1). Diatom estimates of pH have been compared with measured pH values at numerous lake sites where changes in acid–base status have occurred. Such validations of the diatom approach have been performed for lakes that have been acidified and lakes that have recovered from acidification or have been limed in Canada (e.g., Dixit et al., 1987, 1991, 1992), Sweden (e.g., Renberg and Hultberg, 1992), and Scotland (e.g., Allot et al., 1992). Diatom-inferred pH histories generally agree reasonably well with the timing, trend, and magnitude of known acidification and deacidification periods. In several cases, however, the sedimentary reconstructions were slightly damped in comparison with measured values. That is, the diatom reconstructions did not fully reflect the magnitude of either the water pH decline or subsequent recovery. For example, Renberg and Hultberg (1992) compared diatom-inferred pH reconstructions with the known pH history for several decades at Lake Lyse- vatten in southwestern Sweden. The diatom-inferred pH history agreed well with both the acidification period of the 1960s and early 1970s and also the liming that occurred in 1974. The magnitude of pH change inferred from sed- imentary reconstructions was slightly smaller, however, than the measured changes in pH for both acidification and deacidification. Allot et al. (1992) found diatom reconstructions of pH recovery in the dea- cidifying Round Loch of Glenhead, Scotland to be somewhat smaller than the measured pH recovery since the late 1970s. pH reconstructions from the sed- iment cores showed an average recovery of 0.05 pH units. Measured increases in pH between 1978–1979 and 1988–1989 averaged 0.23 pH units. The authors attributed this difference to attenuation of the reconstructed pH record owing to sediment mixing processes. Dixit et al. (1992) analyzed sedimentary diatoms and chrysophytes from Baby Lake (Sudbury, Ontario) to assess trends in lake-water chemistry asso- ciated with the operation, and closure in 1972, of the Coniston Smelter. Extremely high S emissions caused the lake to acidify from pH approxi- mately equal to 6.5 in 1940 to a low of 4.2 in 1975. Following closure of the smelter, lake-water pH recovered to pre-industrial levels. The diatom- inferred acidification and subsequent recovery of the lake corresponded with the pattern of measured values. However, the diatom-inferred pH response was more compressed and did not fully express the amplitude of the pH decline or the extent of subsequent recovery. It is not known why diatom-inferences of pH change are often slightly attenuated relative to measured acidification or deacidification. Possible explanations include the preference of many diatom taxa for benthic habi- tats where pH changes may be buffered by chemical and biological pro- cesses. Alternatively, such an attenuation could be a result of sediment mixing processes. 1416/frame/C05 Page 121 Wednesday, February 9, 2000 2:09 PM © 2000 by CRC Press LLC 122 Aquatic Effects of Acidic Deposition Some upper Midwestern lakes have acidified since pre-industrial times. However, based on available paleolimnological data, there is little paleolim- nological evidence suggesting that widespread acidification has occurred in this region (Kingston et al., 1990; Cook et al., 1990). Land use changes and other human disturbances of upper Midwestern lakes and their watersheds have probably exerted more influence on the acid–base chemistry of lakes than has acidic deposition (Eilers et al., 1989a; Kingston et al., 1990; Sullivan, 1990). This is because acidic deposition has occurred at a much lower level in the upper Midwest than in most areas of the eastern U.S. The portion of the region most likely to have experienced acidification from acidic deposition is the Upper Peninsula of Michigan, where acidic seepage lakes are particularly numerous (Baker et al., 1990b), acidic deposition is highest for the region, and the [SO 4 2- ]/[C B ] ratio is commonly greater than 1.0 (Figure 4.5). The percent- age of acidic lakes in the eastern portion of the Upper Peninsula of Michigan (east of longitude 87 ° ) is 18 to 19% (Schnoor et al., 1986; Eilers et al., 1988b), which is comparable to heavily impacted areas of the Northeast. Diatom-inferred pH data are available for only two lakes in upper Michi- gan, McNearney and Andrus Lakes. McNearney Lake was naturally acidic prior to this century and is therefore atypical for the region. Andrus Lake is inferred to have experienced declines in pH and DOC since pre-industrial times that could be related to acidic deposition (Kingston et al., 1990). It is likely that other lakes in this subregion have also experienced recent acidifi- cation, although quantitative data are lacking regarding the amount of acid- ification that occurred in the past or the dose–response relationships of these systems. In addition to the scarcity of paleolimnological data within the por- tion of the upper Midwest most likely to have experienced widespread his- torical acidification, there is also a paucity of basic biogeochemical data on the response of the predominant lake type in this region to atmospheric inputs of S and N. Historical changes in Florida lake-water chemistry, as inferred from dia- toms, showed a distinct geographical pattern. All five of the paleolimnologi- cal study lakes in the Trail Ridge region showed some evidence of acidification, some strongly linked in timing to both the period of increasing acidic deposition and increased water consumption. Trail Ridge lakes showed diatom-inferred ∆ pH ranging from -0.2 (McCloud) to -0.9 (Suggs). No clear evidence of acidification was observed for lakes in the Ocala National Forest (three lakes) or the Panhandle (eight lakes), except Lake Five- O, where gross hydrological change was implicated. It is most likely that sev- eral factors have caused the recent acidification of lakes in the Trail Ridge area suggested by the diatom data. Acidic deposition is implicated, but changing lake stage and the linked phenomenon of evapoconcentration may also be important (Sweets et al., 1990). Diatom-inferred historical changes in pH for all lakes in the Florida Pan- handle, except Lake Five-O, were less than -0.10 units. These results appear surprising insofar as the Panhandle seepage lakes are the most dilute lakes in Florida, and have been believed to receive minimal hydrologic in-seepage 1416/frame/C05 Page 122 Wednesday, February 9, 2000 2:09 PM © 2000 by CRC Press LLC Chemical Dose–Response Relationships and Critical Loads 123 (ca. 1 to 3% of total hydrologic budget; cf. Baker et al., 1988b). Groundwater monitoring data collected adjacent to Lake Five-O suggested, however, that groundwater may contribute one-third to one-half of the overall hydrologic budget of this lake (Pollman et al., 1991). Calibrated inflows based on Cl - balances for Panhandle lakes also suggested substantial groundwater inflows, ranging from 10 (Moore Lake) to 29% (Lofton Ponds) (Pollman and Sweets, 1990). Superimposed on the complex heterogeneity of Florida lakes is a high inci- dence of anthropogenic disturbance. Of the 159 total lakes sampled by ELS-I in Florida, all but 37 were judged by Baker et al. (1988b) to have substantial shoreline or watershed disturbances, mostly related to agriculture. Besides the increased atmospheric deposition in Florida in the 1950s, other changes have also occurred. The human population has increased markedly, as has freshwater withdrawal from the Floridan aquifer (Aucott, 1988). As a result, the potentiometric head has declined substantially in the Trail Ridge area (Healy, 1975; Aucott, 1988; Pollman and Canfield, 1991). The effects of water withdrawal on the acid–base status of lakes is not well understood. For undeveloped lakes in the northcentral peninsula, lake-water chemis- try is consistent with an hypothesis of acidification by acidic deposition (Hendry and Brezonik, 1984; Eilers et al., 1988c; Baker et al., 1986, 1988b). Evaporative concentration of modest amounts of acidic deposition, and in- lake retention of SO 4 2- and NO 3 - appear to be important processes. However, Eilers et al. (1988c) concluded it is unlikely that the mechanisms of acidifi- cation of clearwater lakes in Florida and the linkages to atmospheric depo- sition will be satisfactorily understood until the hydrologic pathways are better known. Slight differences in groundwater inputs can have a major influence on base cation supply and lake-water chemistry in these precipi- tation-dominated seepage systems. Based on limited paleolimnological data, it appears that recent acidification of lakes in Florida may have been restricted to the Trail Ridge district. Furthermore, it is unclear to what extent recent acidification of lakes in the Trail Ridge district may be attrib- utable to acidic deposition, as compared to other anthropogenic activities, especially groundwater withdrawal. 5.1.4 Model Estimates of Dose–Response Dynamic model estimates of F -factors for watersheds in the northeastern U.S., using the MAGIC model, show reasonably close agreement with mea- sured F -factors for acid-sensitive systems (Tables 5.1 and 5.4). Model-gen- erated median values of the F -factor ranges from 0.56 to 0.85, and values of the 5th percentile of Adirondack lake projections (0.25 to 0.39) were reason- ably comparable to the measured values at the highly sensitive Sogndal site (0.35 to 0.39). MAGIC forecasts for western lakes, however, yielded esti- mated F -factors that were substantially lower (median 0.34, 5th percentile 0.03; Table 5.4). It is not clear how representative these forecasts might be 1416/frame/C05 Page 123 Wednesday, February 9, 2000 2:09 PM © 2000 by CRC Press LLC 124 Aquatic Effects of Acidic Deposition for western lakes, in general, or how accurate the estimates are for the mod- eled lakes. Nevertheless, these comparative data suggest that western sys- tems are as sensitive, or perhaps more sensitive, than any of the watersheds for which acidification and/or recovery responses have been more rigor- ously quantified. 5.2 Critical Loads 5.2.1 Background It has been well documented that acidic deposition has caused environmen- tal degradation of surface waters, soils, and forests in certain areas. Such deg- radation has been more widespread in Europe than in North America, owing partly to the fact that many regions of Europe have received much higher deposition of S and N for a longer period of time than have comparable North American ecosystems. Recent emissions control efforts have focused on attempts to reduce deposition sufficiently to permit ecosystem recovery, if not to pre-acidification levels, at least to ecologically acceptable levels. The key questions facing scientists and policy-makers, therefore, have to do with the degree in space and time to which S and N emissions will need to be reduced in order to allow ecosystem recovery to proceed (Jenkins et al., 1998). Public policy measures to reduce emissions must be based upon quantified dose–response relationships that reflect the tolerance of natural ecosystems to various inputs of atmospheric pollutants. This need has given rise to the concepts of critical levels of pollutants and critical loads of deposition (e.g. Bull, 1991, 1992), as well as interest in establishing one or more standards for acid deposition. A critical load can be defined as “a quantitative estimate of an exposure to one or more pollutants below which significant harmful effects on specified sensitive elements of the environment do not occur according to present knowledge” (e.g., Nilsson, 1986; Gundersen, 1992). Such an approach to establishing a standard is intuitively satisfying. However, the assignment of a standard or critical load of S or N for any particular region may be difficult to defend scientifically. A variety of natural processes and anthropogenic activities affect the acid–base chemistry of lakes and streams, in addition to atmospheric deposition of S and N. The loadings of N or S that may be required to protect the most sensitive elements of an ecosystem may be unrealistically low in terms of economic or other considerations, and may be difficult to quantify. The basic concept of critical load is relatively simple, as the threshold con- centration of pollutants at which harmful effects on sensitive receptors begin to occur. Implementation of the concept is, however, not at all simple or straightforward. Practical definitions for particular receptors (soils, fresh 1416/frame/C05 Page 124 Wednesday, February 9, 2000 2:09 PM © 2000 by CRC Press LLC [...]... selection of appropriate acid deposition standards involves consideration of a matrix of factors, as outlined in Table 5. 5 5. 2.4 Establishment of Standards for Sulfur and Nitrogen Sulfur deposition is a potential concern in all of the acid-sensitive regions of the U.S Some degree of chronic acidification attributable to S deposition has occurred in the Adirondacks, northern New England, mid-Appalachian... Region- or subregion-specific standards Chronic or episodic acidification ANC or pH ANC . results of space-for-time substitution (Table 5. 2), diatom-inferences of historical acidification (Table 5. 3), and results of process-based model hindcasts or future forecasts (Table 5. 4). Each of. and 5. 4). Model-gen- erated median values of the F -factor ranges from 0 .56 to 0. 85, and values of the 5th percentile of Adirondack lake projections (0. 25 to 0.39) were reason- ably comparable. 1416/frame/C 05 Page 1 15 Wednesday, February 9, 2000 2:09 PM © 2000 by CRC Press LLC 116 Aquatic Effects of Acidic Deposition changes in (HCO 3 - - H + ), Al i , C B , and A -