35 3 Chronic Acidification Chronic acidification of surface waters refers to loss of ANC or reduction in pH on a chronic, or annual-average, basis. Chronic acidification is often evaluated by studying changes in surface water chemistry during periods when that chemistry is expected to be relatively stable. These are generally summer or fall for lakes and spring baseflow (in the absence of storms) for streams. Attempts to measure chronic acidification focus to some extent on a moving target. Lake-water chemistry tends to be relatively stable during summer and fall, compared to other times of the year, as does spring base- flow chemistry in streams. There is still, however, often significant variabil- ity in that chemistry. Water chemistry exhibits changes on both intra and interannual time scales in response to a host of environmental factors. Key in this regard are short-term and long-term climatic fluctuations that gov- ern the amount and timing of precipitation inputs, snowmelt, vegetative growth, depth to groundwater tables, and evapoconcentration of solutes. Many years of data, therefore, are required to establish the existence of trends in surface water chemistry, much less assign causality to changes that are found to occur. There have been many advancements in the scientific understanding of chronic surface water acidification since 1990. Several studies that had been initiated during the original NAPAP research effort were completed post- 1990 and research results from those programs continue to be published. A major research effort was conducted in Europe regarding the dynamics of N- driven acidification and related processes in both terrestrial and aquatic eco- systems. New predictive models have been developed and some previously existing models have been extensively tested and improved. Finally, the availability of increasing volumes of data from long-term monitoring pro- grams and experimental manipulation studies have provided considerable insights regarding quantitative dose–response relationships, as well as data that provide the foundation for the establishment of standards for the protec- tion of acid-sensitive aquatic resources. 1416/frame/C03 Page 35 Wednesday, February 9, 2000 1:59 PM © 2000 by CRC Press LLC 36 Aquatic Effects of Acidic Deposition 3.1 Characteristics of Sensitive Systems Broad areas in the U.S. that contain large populations of low-ANC lakes and streams include portions of the Northeast (particularly Maine and the Adirondack Mountains), the mid-Appalachian Mountains, northern Florida, the Upper Midwest, and the western U.S. (Figure 3.1). The Adirondack and Mid-Appalachian Mountains include many acidified surface waters that have been impacted by acidic deposition. Portions of northern Florida and to a lesser extent the Upper Midwest also contain appreciable numbers of acidic lakes and streams, although the role of acidic deposition in these areas is less clear. The western U.S. contains many of the surface waters most susceptible to potential acidification effects, but the levels of acidic deposition in the West are generally low and acidic surface waters are rare. It was recognized relatively early in acidification research that most of the major concentrations of low ANC surface waters were probably located in areas underlain by bedrock resistant to weathering. Subsequent compilations of available water chemistry data (e.g., Omernik and Powers, 1982; Eilers and Selle, 1991) refined and expanded this image of sensitive areas in North FIGURE 3.1 Major areas of North America containing low-ANC surface waters as defined by Charles (1991). 1416/frame/C03 Page 36 Wednesday, February 9, 2000 1:59 PM © 2000 by CRC Press LLC Chronic Acidification 37 America. The extensive research programs conducted in Europe, Canada, and through NAPAP provided additional insight into factors contributing to the sensitivity of surface waters to acidic deposition by revealing the impor- tance of soil composition and hydrologic flowpath, in addition to geology, in delineating sensitive regions. The geologic composition of a region plays a dominant role in influencing the chemistry and, therefore, sensitivity of surface waters to the effects of acidic deposition. Bedrock geology formed the basis for a national map of surface water sensitivity (Norton et al., 1982) and has been used in numer- ous acidification studies of more limited extent (e.g., Bricker and Rice, 1989; Dise, 1984; Gibson et al., 1983). Analysis of bedrock composition continues to be an important element for assessing sensitivity of surface waters in mountainous regions (e.g., Stauffer, 1990; Stauffer and Whittchen, 1991; Vertucci and Eilers, 1993). The presence of large populations of acidic and low-ANC lakes and streams in regions such as Florida that are underlain by calcareous bedrock illustrate that if the surface waters are isolated from highly weatherable bed- rock minerals, acid–base status is not controlled by bedrock geology (Sulli- van and Eilers, 1994). Many Karst lakes in northern Florida are situated in highly weathered marine sands that are capable of providing comparatively little neutralization of acidic inputs. For lakes located above calcareous bed- rock in areas with minimal hydrologic connection with the Floridan aquifer, the surface waters can be acidic despite groundwaters saturated in carbonate minerals. Conversely, where calcareous soils have been deposited over resis- tant bedrock such as granite, lakes and streams draining such soils are pre- dominantly alkaline. Thus, both soil and bedrock composition may exert strong influence on surface water acid-base chemistry and, therefore, are important factors to be considered in defining acid-sensitive regions. The third principal factor now recognized as critical in contributing to the sensitivity of aquatic resources is watershed hydrology. The movement of water through the soils, into a lake or stream, and the interchange between drainage water and the soils and sediments regulate the type and degree of watershed response to acidic inputs. Lakes in the same physiographic setting can have radically different sensitivities to acidic deposition depending on the relative contributions of near-surface drainage water and deeper, more highly buffered groundwater (Eilers et al., 1983; Chen et al., 1984; Driscoll et al., 1991). The movement of water through natural conduits in peat can circumvent hydrologic routing through wetlands (Gjessing, 1992). Even acidic deposition that does not pass through the watershed, but instead falls as precipitation directly on the lake surface, may eventually be neutralized by in-lake reduction processes that are controlled in part by hydraulic residence time (Baker and Brezonik, 1988). Natural hydrologic events also radically alter sensitivity to acidification by bypassing normal neutralization processes during snowmelt or changing flowpaths during extended droughts (Webster et al., 1990). The importance of hydrologic factors in influencing the acid–base chemistry of sur- face waters across the U.S. was reinforced by Newell (1993), who identified 1416/frame/C03 Page 37 Wednesday, February 9, 2000 1:59 PM © 2000 by CRC Press LLC 38 Aquatic Effects of Acidic Deposition hydrology as a key component associated with changes in the acid-base chem- istry of lakes included in EPA's Long Term Monitoring Program. 3.2 Causes of Acidification 3.2.1 Sulfur Several watershed processes control the extent of ANC generation and its contribution from soils to drainage waters as acidified water moves through undisturbed terrestrial systems. These are the major processes that regulate the extent to which drainage waters will be acidified in response to ambient levels of acidic deposition. Of particular importance is the concentration of acid anions in solution. Naturally occurring organic acid anions, produced in upper soil horizons, normally precipitate out of solution as drainage water percolates through lower mineral soil horizons. Soil acidification processes reach an equilibrium with acid neutralization processes (e.g., weathering) at some depth in the mineral soil (Turner et al., 1990). Drainage waters below this depth generally have high ANC. The addition of strong acid anions from atmospheric deposition allows the natural soil acidification and cation leach- ing processes to occur at greater depths in the soil profile, thereby allowing water rich in mobile anions such as SO 4 2- and NO 3 - to emerge from mineral soil horizons into drainage waters. If these anions are charge-balanced by H + and/or Al n + cations, the water will have low pH and could be toxic to aquatic biota. Thus, the mobility of anions within the terrestrial system is a major fac- tor controlling the extent of surface water acidification. The scientific community has continued to make significant progress since 1990 in refining understanding of acidification processes and quantifying dose–response relationships. In particular, knowledge has been gained regarding the role of natural organic acidity, the depletion of base cation reserves from soils, interactions between acidic deposition and land use, and N dynamics in forested and alpine ecosystems. Each of these topics, in which significant recent advancements have been made, is discussed in the sections that follow. An expanded discussion of N dynamics is also provided in Chap- ter 7. It is now clear that the flux of SO 4 2- through watersheds is only one part of a complex set of watershed interactions that govern the response of both aquatic and terrestrial ecosystems to acidic deposition. 3.2.2 Organic Acidity Organic acids commonly exert a large influence on surface water acid–base chemistry, particularly in dilute waters having moderate to high dissolved 1416/frame/C03 Page 38 Wednesday, February 9, 2000 1:59 PM © 2000 by CRC Press LLC Chronic Acidification 39 organic carbon (DOC) concentrations. Some lakes and streams are naturally acidic as a consequence of organic acids in solution. The presence of organic acids also provides buffering to minimize pH change in response to changes in the amount of mineral (e.g., SO 4 2- , NO 3 - ) acid anions contributed to solu- tion by atmospheric deposition. The fact that there are many lakes and streams throughout the U.S. that are chronically acidic (ANC less than or equal to zero) primarily owing to the presence of organic acids is well known. NAPAP (1991) concluded that about one-fourth of all acidic lakes and streams surveyed in the National Surface Water Survey (NSWS, Linthurst et al., 1986; Kaufmann et al., 1988) were acidic largely as a consequence of organic acids. A more intensive survey of 1400 lakes in the Adirondacks by the Adirondack Lake Survey Corporation (ALSC; Kretser et al., 1989) that included lakes much smaller than those sur- veyed by NSWS, found a higher percentage of organically acidic lakes. Baker et al. (1990b) concluded that 38% of the lakes surveyed by ALSC had pH less than 5 owing to the presence of organic acids and that organic acids depress the pH of Adirondack lakes by 0.5 to 2.5 pH units in the ANC range of 0 to 50 µ eq/L. However, the importance of organic acids in comparison with other sources of acidity has remained a subject of debate. In addition, the role of organic acids in the process of changing the acid–base character of surface waters (acidification or alkalization) is still poorly known. Organic acids in fresh water originate from the degradation of biomass in the upland catchment, wetlands, near-stream riparian zones, water column, and stream and lake sediments (Hemond, 1994). The watersheds of surface waters that have high concentrations of organic matter (DOC greater than about 400 µ M) often contain wetlands and/or extensive organic-rich riparian areas (Hemond, 1990). Specification of the acid–base character of water high in DOC is somewhat uncertain. Attempts have been made to describe the acid–base behavior of organic acids using a single H + dissociation constant (pK a ), despite the fact that organic acids in natural waters are made up of a complex mixture of acidic functional groups. It has also been assumed in the past that organic acids are essentially weak acids, whereas a portion (perhaps one-third) of the acidity is actually quite strong, with some ionization occurring at pH values well below 4.0 (Hemond, 1994; Driscoll et al., 1994). A number of modeling approaches have been used to estimate the acidity of organic acids in fresh waters, often as simple organic acid analogs having different pK a values (Oliver et al., 1983; Perdue et al., 1984; Driscoll et al., 1994). In lakes sampled by the ALSC, estimated values of organic acid anion concentration per mol DOC (RCOO - /DOC), often called the organic acid charge density, were consistent with patterns anticipated from the presence of both strong and weak organic acid functional groups (Driscoll et al., 1994; Figure 3.2). Even at pH values below 4.5, the charge density of ALSC lakes was in the range of 0.03 to 0.05, corresponding to about 25 to 30% of values found at circumneutral pH (Driscoll et al., 1994). Thus, some of the functional groups associated with naturally occurring organic acids are 1416/frame/C03 Page 39 Wednesday, February 9, 2000 1:59 PM © 2000 by CRC Press LLC 40 Aquatic Effects of Acidic Deposition strongly acidic, and do not dissociate unless pH is below 4.0. Values of charge density in the ALSC lakes increased with increasing pH between pH values of 5.0 to 7.0 owing to the presence of weakly acidic functional groups. Thus, organic acids in surface waters include a mixture of func- tional groups having both strong and weak acid character. This concept was not well understood prior to 1990. FIGURE 3.2 Mean organic anion concentration (A) estimated from anion deficit, and (B) charge density expressed as A n - /DOC at 0.1 pH unit intervals, as a function of pH for the reduced ALSC data set included in the analyses of Driscoll et al. (C.T. Driscoll, M.D. Lehtinen, and T.J. Sullivan, 1994, Modeling the acid-base chemistry of organic solutes in Adirondack, NY, lakes, Water Resour. Res. , Vol. 30, p. 301, Figure 1; copyright by the American Geophysical Union. With permission.) B A 1416/frame/C03 Page 40 Wednesday, February 9, 2000 1:59 PM © 2000 by CRC Press LLC Chronic Acidification 41 The ALSC data were fitted by Driscoll et al. (1994) to a triprotic organic acid analog representation that provided a good fit to the data ( r 2 = 0.92), with pK a values of 2.62, 5.66, and 5.94 to represent a range of strong to weak acid char- acter. Inclusion of organic acidity from this analog in model calculations FIGURE 3.3 MAGIC model hindcast estimates of pre-industrial pH versus diatom-inferred pH for 33 sta- tistically selected Adirondack lakes. (A) Without including organic acid representation in the MAGIC simulations, and (B) including a triprotic organic acid analog model in the MAGIC simulations. (Source: Water Air Soil Pollut ., Vol. 91, 1996, p. 301, Influence of organic acids on model projections of lake acidification, Sullivan, T.J., B.J. Cosby, C.T. Driscoll, D.F. Charles, and H.F. Hemond, Figure 1, copyright 1996. Reprinted with kind permission from Kluwer Academic Publishers.) A B 1416/frame/C03 Page 41 Wednesday, February 9, 2000 1:59 PM © 2000 by CRC Press LLC 42 Aquatic Effects of Acidic Deposition resulted in good agreement between measured and predicted values of lake- water pH and ANC in this large database (Driscoll et al., 1994). The importance of naturally occurring organic acids as agents of surface water acidification has recently been substantially reinforced by several modeling studies (e.g., Sullivan et al., 1994, 1996). These have shown that inclusion of organic acids in the MAGIC model have an appreciable effect on model predictions of surface water pH, even in waters where DOC con- centrations are not particularly high. Concern was raised subsequent to NAPAP’s Integrated Assessment (NAPAP, 1991) regarding potential bias from the failure to include organic acids in the MAGIC model formulations used in the IA. MAGIC hindcasts of pre-industrial lake-water pH of Adirondack lakes showed poor agreement with diatom inferences of pre- industrial pH. Revised MAGIC simulations, therefore, were constructed that included the organic acid analog model developed by Driscoll et al. (1994). The revised MAGIC hindcasts of pre-industrial lake-water pH that included an organic acid representation showed considerably closer agree- ment with diatom inferences (Figure 3.3). The mean difference between MAGIC and diatom estimates of pre-industrial pH was reduced from 0.6 pH units to 0.2 pH units when organic acids were included in the model, and the agreement for individual lakes improved by up to a full pH unit (Sullivan et al., 1996). Inclusion of organic acids in the MAGIC simulations for watershed manip- ulation data sets at Lake Skjervatjern (Norway), Bear Brook (Maine), and Ris- dalsheia (Norway) also had dramatic effects on model simulations of pH. In all cases, MAGIC simulated considerably higher pH values when organic acids were omitted from the model. Even at Bear Brook, where annual aver- age DOC concentrations are very low (less than 250 µ M C), incorporation of organic acids into the model reduced simulated pH by 0.1 to 0.3 pH units for the years of study. At Lake Skjervatjern and Risdalsheia, where organic acids provide substantial pH buffering, omission of the organic acid analog repre- sentation from MAGIC resulted in consistent overprediction of pH by about 0.2 to 0.5 pH units (Sullivan et al., 1994; Figure 3.4). Rosenqvist (1978) and Krug et al. (1985) hypothesized that a significant component of the mobile acid anions contributed from atmospheric deposi- tion (e.g., SO 4 2- , NO 3 - ) merely replace organic anions that were previously present in solution. Under this anion substitution hypothesis, the net result of acidic deposition is not so much an increase in cations (including poten- tially toxic H + and Al n + ) as much as an exchange of SO 4 2- and NO 3 - anions for organic anions, with little or no change in ANC and pH. Data are scarce with which to directly evaluate the hypothesis that acidic deposition causes decreased organic acidity, but a variety of indirect evidence was summarized in the review of Marmorek et al. (1988). They concluded that there were a number of inconsistencies in the available data, but most data suggested that organic acids have been lost from lake water as a conse- quence of acidic deposition. Hypothesized mechanisms included: 1416/frame/C03 Page 42 Wednesday, February 9, 2000 1:59 PM © 2000 by CRC Press LLC Chronic Acidification 43 FIGURE 3.4 MAGIC simulated pH with and without inclusion of the triprotic organic acid analog, and observed pH, in the treatment and control lake/stream at (A) Skjervatjern, (B) Risdalsheia, and (C) Bear Brook. A B C 1416/frame/C03 Page 43 Wednesday, February 9, 2000 1:59 PM © 2000 by CRC Press LLC 44 Aquatic Effects of Acidic Deposition 1. Decreased mobilization of organic materials from soils and wet- lands because of increased H + concentration. 2. Reduced microbial decomposition of organic materials in soils. 3. Changes in dissociation and/or physical structure of humics. 4. Increased loss from solution to sediments through chelation with metals (e.g., Al, Fe) mobilized by increased H + , and subsequent precipitation of the metal–organic complex. Of the preceding mechanisms, complexation of organic acids by metals (Almer et al., 1974; Lind and Hem, 1975; Dickson, 1978; Cronan and Aiken, 1985) and pH dependent changes in dissociation of organic acids (Oliver et al., 1983; Wright et al., 1988b) appeared most likely to be significant. Quanti- tative estimates of change in DOC were not possible, but based on the avail- able data, Marmorek et al. (1988) concluded that potential DOC losses of up to 250 µ M C were not unreasonable. Subsequent research has suggested, however, that decreases in DOC concentrations in surface water in response to acidic deposition have probably been less than 250 µ M C (Wright et al., 1988b; Kingston and Birks, 1990; Cumming et al., 1992). Furthermore, Krug and co-workers contended that interactions between acidic deposition and organic matter can either increase or decrease DOC, depending on the nature of the organic matter interacting with the acid (e.g., Krug et al., 1985; Krug, 1991a,b). Kingston and Birks (1990) presented diatom-based paleolimnological reconstructions of DOC for lakes studied in the Paleoecological Investigation of Recent Lakewater Acidification (PIRLA-I) project. The DOC optima and tolerances of diatom taxa in four regions (Adirondack Mountains, northern New England, northern Great Lakes states, and northern Florida) were esti- mated using maximum likelihood and weighted averaging regression. The cumulative fit per taxon as a fraction of the taxon's total variance revealed that few taxa were consistent in terms of their explanation of the DOC gradi- ent from region to region. DOC explained a small, but significant, amount of taxon variance in lakes in the Adirondack Mountains, northern Florida, and the northern Great Lakes States, but the signal was much weaker in northern New England. Calculated species optima were not consistent among regions and the best indicators of DOC in the PIRLA data sets were not always in good agreement with those found in Norway and Canada (e.g., Davis et al., 1985; Anderson et al., 1986; Taylor et al., 1988). The authors, therefore, cau- tioned that taxa that are good indicators for one region may not be good indi- cators of DOC in other regions. Example reconstructions were provided for Big Moose Lake in the Adirondack Mountains, NY and Brown Lake in north- ern Wisconsin. The magnitudes of inferred DOC changes were small relative to the mean squared error of the predictive relation in each region (98 and 80 µ M, respectively), but in each case DOC was inferred to have declined coin- cident with lake-water pH. For the recently acidified PIRLA-I lakes in gen- eral, inferred declines in DOC were coincident with recent acidification. 1416/frame/C03 Page 44 Wednesday, February 9, 2000 1:59 PM © 2000 by CRC Press LLC [...]... (Mast et al., 1995) 3. 3 Effects of Acidification Acidification from acidic deposition has a number of important chemical and biological effects The most noteworthy relate to changes in the acid–base status of surface and soil water, sometimes resulting in short-term or long-term toxicity of the water to aquatic or terrestrial biota Acidic deposition generally increases the concentration of acid anions in... episodic pulses of NO 3- acidity which could be very important biologically The Uinta Mountains of Utah and the Bighorn Mountains of central Wyoming had the greatest percentages of high NO 3- lakes in the West, irrespective of lake-water ANC, with 19% of the lakes included within the Western Lakes Survey having NO3 more than 10 µeq/L This is a high percentage of lakes with measurable NO 3- for fall samples... the result of decreased base saturation of soils caused by acidic deposition (Lawrence et al., in press) Lawrence et al (1995) proposed that the dissolution of Al in the mineral soil by mineral acid anions supplied by acidic deposition (SO4 2- , NO 3 - ) can decrease the availability of Ca2+ in the overlying forest floor This conclusion was based on the results of a survey in 1992 and 19 93 of soils in... PM 58 3. 2.6 Aquatic Effects of Acidic Deposition Climate Climate can have a large influence on acid sensitivity and the effects of elevated S or N deposition in several ways Drought can alter hydrologic flowpaths and change the relative contribution of near-surface runoff vs deeper baseflow Because these source areas typically generate different levels of ANC, such changes in hydrologic input can profoundly... levels of deposition cause elevated levels of NO 3- in drainage waters (Aber et al., 1989, 1998; Stoddard, 1994) This enhanced leaching of NO 3- causes depletion of * The term nitrogen-saturated has been defined in a variety of ways, all reflecting a condition whereby the input of nitrogen (e.g., as nitrate, ammonium) to the ecosystem exceeds the requirements of terrestrial biota and a substantial fraction of. .. relative importance of land use activities in exacerbating or ameliorating acidic deposition effects The importance of acidic deposition as an agent of acidification does not preclude the fact that land use and landscape changes may also be important and, in some cases, more important than acidic deposition (Sullivan et al., 1996b) It is now clear that acidic deposition causes acidification of some sensitive... high-elevation catchments of the Colorado Front Range at N deposition levels considered quite low by European standards Total N deposition is 4 to 7 kg N/ha per year in this region, about double that in most other mountainous areas of the West and approaching the deposition levels found © 2000 by CRC Press LLC 1416/frame/C 03 Page 50 Wednesday, February 9, 2000 1:59 PM 50 Aquatic Effects of Acidic Deposition. .. development of this understanding has evolved slowly During most of the 1980s, the generally accepted paradigm of watershed response to acidic deposition was somewhat analogous to a large-scale titration of ANC (Henriksen, 1980) It was widely believed that atmospheric input of acidic anions (mainly SO4 2-) resulted in movement of those anions through soils into drainage waters with near stoichiometric loss of. .. regarding the role of NO 3- in acidification of surface waters, particularly during hydrologic episodes, the role of NO 3- in the long-term acidification process, the contribution of NH4+ from agricultural sources to surface water acidification, and the potential for anthropogenic N deposition to stimulate eutrophication of freshwaters and estuaries © 2000 by CRC Press LLC 1416/frame/C 03 Page 48 Wednesday,... concomitant increase in labile Al and decrease in (Ca2+ + Mg2+) (SFT, 1988) Analysis of selected lakes and streams with longer-term records also showed increases in NO 3- concentrations, providing additional evidence for an increasing trend in NO 3- Although SO42remained the dominant anion in most systems, the ratio of NO 3- / (NO 3- + SO4 2-) reached 0.54 on an equivalent basis in some lakes and rivers in southwestern . establishment of standards for the protec- tion of acid-sensitive aquatic resources. 1416/frame/C 03 Page 35 Wednesday, February 9, 2000 1:59 PM © 2000 by CRC Press LLC 36 Aquatic Effects of Acidic Deposition . the role of NO 3 - in acidification of sur- face waters, particularly during hydrologic episodes, the role of NO 3 - in the long-term acidification process, the contribution of NH 4 + . 1:59 PM © 2000 by CRC Press LLC 38 Aquatic Effects of Acidic Deposition hydrology as a key component associated with changes in the acid-base chem- istry of lakes included in EPA's Long