139 6 Episodic Acidification 6.1 Background and Characteristics of Sensitive Systems The acid–base chemistry of surface waters typically exhibits substantial intra- and inter-annual variability. Seasonal variability in the concentration of key chemical parameters often varies by more than the amount of acidifica- tion that might occur in response to acidic deposition. Such variability makes quantification of acidification and recovery responses difficult, and also com- plicates attempts to evaluate sensitivity to acidification based solely on index chemistry. The latter term is applied to chemical characterization data that correspond with periods when the chemistry is expected to be relatively sta- ble. These are typically summer or fall for lakes and spring baseflow for streams. Lakes and streams exhibit short-term episodic decreases in ANC, and often also pH, usually in response to hydrological events, such as snow- melt or rainfall. Periods of episodic acidification may last for hours to weeks, and sometimes result in depletion of ANC to negative values with concurrent increases in potentially toxic inorganic Al in solution. Precipitation inputs to a watershed typically pass through the soil profile prior to reaching stream channels. The typical soil profile in acid-sensitive watersheds has lowest pH (approximately 4) in upper organic soil horizons, increasing down the profile to pH greater than 6 at depth (Norton et al., in press). Drainage water chemistry is generally somewhat reflective of condi- tions in the lower soil horizons and, therefore, generally has pH greater than 6. During high discharge snowmelt or rainfall events, however, flow routing favors water flowpaths through upper horizons. During such events, drain- age water chemistry, therefore, typically reflects the lower pH, higher organic content, and lower ANC of these upper soil horizons. This is one of the major reasons why many surface waters are lower in pH and ANC during hydro- logical episodes. Many of the same characteristics that predispose aquatic systems to chronic acidification from acidic deposition (discussed in Chapter 3) also pre- dispose aquatic systems to episodic acidification. Geology and soils charac- teristics are important in this regard. However, the single most important 1416/frame/C06 Page 139 Wednesday, February 9, 2000 2:13 PM © 2000 by CRC Press LLC 140 Aquatic Effects of Acidic Deposition factor governing the sensitivity of a given watershed to episodic acidification is hydrology. The pathways followed by snowmelt and stormflow water through the watershed and into streams or lakes largely determine the extent of acid neutralization provided by the soils and bedrock in that watershed. High-elevation watersheds with steep topography, extensive areas of exposed bedrock, deep snowpack accumulation, and shallow, base-poor soils are most sensitive. Such systems are common throughout the mountainous West and in portions of the Northeast and Appalachian Mountains. Episodes are generally accompanied by changes in at least two or more of the following chemical parameters: ANC, pH, base cations, SO 4 2- , NO 3 - , Al n + , organic acid anions, and DOC. These changes in chemistry can adversely impact biota, particularly when changes involve pH, Al i , and/or Ca 2+ (Baker et al., 1990c). Aquatic biota vary greatly in their sensitivity to episodic decreases in pH and increases in Al i in waters having low Ca 2+ concentration, and it is difficult to classify chemical episodes according to potential biologi- cal effects. Baker et al. (1990c) concluded, however, that episodes are most likely to impact biota if the episode occurs in waters with baseline (pre-epi- sode) pH above 5.5 and minimum pH during the episode of less than 5.0. In addition, for episodes that occur in systems that are chronically acidic or nearly so, the increase in acidity during the episode may be biologically sig- nificant, particularly when it is accompanied by increased concentrations of Al i (Baker et al., 1990c). Episodic acidification is nearly ubiquitous in drainage waters. Lakes and streams that have been studied throughout the U.S., Canada, and Europe nearly all experience loss of ANC during hydrologic events (Wigington et al., 1990). Chemical changes during episodes are controlled by a number of nat- ural processes, including dilution of base cation concentrations, nitrification, flushing of organic acids from terrestrial to aquatic systems, and the neutral salt effect.* Acidic deposition can also contribute to episodic acidification, particularly via enhanced NO 3 - leaching. Under some conditions, episodes can also be partially caused by increased SO 4 2- concentration, although S- driven episodes appear to be less common than N-driven episodes. There is also the likelihood that chronic acidification by acidic deposition can precon- dition a watershed, thereby increasing the severity of episodic acidification. Since preparation of the NAPAP 1990 Integrated Assessment, the EPA has completed the Episodic Response Project (ERP), an integrated evaluation of episodic acidification of surface waters during high-discharge periods (e.g., storms, snowmelt) in portions of the eastern U.S. (Wigington et al., 1993). This research provided important confirmatory evidence regarding the chemical and biological effects of episodic pH depressions in lakes and streams in parts of this country. The ERP clearly demonstrated that episodic processes are mostly natural, that SO 4 2- and, especially, NO 3 - attributable to * The neutral salt effect is a process whereby addition of a neutral salt (e.g., NaCl) to base-poor soils can cause acidification of drainage water that passes through that soil. The mechanism involves ion exchange between H + from the soil ion exchange complex and the neutral salt cation (e.g., Na + ) in solution. 1416/frame/C06 Page 140 Wednesday, February 9, 2000 2:13 PM © 2000 by CRC Press LLC Episodic Acidification 141 atmospheric deposition play important roles in the episodic acidification of some surface waters, and that the chemical response that has the greatest impact on biota is increased Al concentration. Similar findings have been reported elsewhere, especially in Europe, but the ERP helped to clarify the extent, causes, and magnitude of episodic acidification in portions of the U.S. Short-term pulses of increased NO 3 - concentration have been identified as the primary factor contributing to episodic depressions of pH and ANC dur- ing snowmelt in many acid-sensitive Adirondack lakes and streams (Driscoll and Schafran, 1984; Driscoll et al., 1987a,b; Stoddard, 1994). The magnitude of episodic acidification is strongly regulated by the base cation, and there- fore also ANC, concentration in lake waters. High-ANC Adirondack lakes experience episodes driven primarily by dilution of base cations during snowmelt, whereas low-ANC lakes often experience episodes driven by NO 3 - increases (Schaefer et al., 1990). The source of the N released during snow- melt in Adirondack watersheds includes nitrified snowpack N and also likely mineralized N from soil organic matter (Schaefer and Driscoll, 1993). Nitrogen has been experimentally added to a small pristine alpine catch- ment in Norway at deposition levels similar to those received by some Adirondack watersheds. Since 1993, 7 kg N/ha per year have been applied to the Sogndal minicatchment as part of the RAIN and NITREX projects to augment the ambient loading of 2 kg/ha per year (Wright and Tietema, 1995). Runoff contained high concentrations of NO 3 - only during events of high flow, however, during 9 years of treatment (Wright and Tietema, 1995). These findings suggest that during low-flow periods, the flow routing of drainage water and its contact with watershed soils and terrestrial biota allow for effi- cient utilization of essentially all of the incoming N. In contrast, during high- flow periods, a portion of the increased N is not utilized, mainly because drainage water containing relatively high concentrations of NO 3 - moves too quickly through the soil reservoir to allow efficient N utilization. The EPA's National Lake Survey (NLS), conducted in 1984 and 1985, pro- vided the most comprehensive database on the acid–base chemistry of lake waters in areas of the U.S. potentially susceptible to the effects of acidic dep- osition. This synoptic survey was conducted during the autumn index period, during which time lake-water chemistry typically exhibits low tem- poral and spatial variability. Although autumn is an ideal time for surveying lake-water chemistry in terms of minimizing variability, lake-water samples collected during autumn provide little relevant data on the dynamics or importance of N in most aquatic systems. Nitrate concentrations in lake water are elevated during the autumn season only in lakes having water- sheds that exhibit fairly advanced symptoms of N saturation (e.g., Figure 7.6; Stoddard, 1994). It is, therefore, not surprising that results of both the Eastern and Western Lake Surveys, both of which were conducted during the fall sea- son, suggested that NO 3 - is of only minor importance compared to SO 4 2- as an acid anion in lake waters in this country. For example, the median value of the ratio of lake water NO 3 - to (SO 4 2- + NO 3 - ) concentration in Florida, the upper Midwest, and the West were very low and varied from about 0.01 to 1416/frame/C06 Page 141 Wednesday, February 9, 2000 2:13 PM © 2000 by CRC Press LLC 142 Aquatic Effects of Acidic Deposition 0.06 (Landers et al., 1987; Stoddard, 1994). Survey data with which to evalu- ate the (largely episodic) effects that might be associated with N deposition were not collected in these surveys. Most research on episodic processes has been conducted on stream systems that are generally more susceptible to such effects than are lakes. Spatial vari- ability can be considerable in lakes, particularly during snowmelt episodes. Strong vertical and horizontal gradients in lake-water chemistry often pre- clude quantification of the magnitude of the effects in lake systems (Gubala et al., 1991). Because of the logistical difficulties and expense associated with sampling lake-water chemistry during episodic events in a manner sufficient to characterize these spatial gradients, few data are available for lakes in the areas of concern. A great deal of the research on episodic processes, both before and after 1990, has focused at least in part on the identification of source areas of storm flow within the watershed. To a large degree, the results of this research have been less than satisfying. It is clear that water flowpaths are enormously important in the regulation of drainage water chemistry. It is also clear that watershed hydrology is complex and varies with different flow regimes. This complicates the task of attributing chemistry to particular watershed soil horizons or other source areas. Efforts to “explain” stream-water chemistry on the basis of soil water chemistry at various points within the watershed have only been useful in the extent to which the results of such efforts have communicated to watershed scientists that we do not know everything. In fact, when it comes to drainage water flow routing in upland catchments, we seem to know little. It is very difficult to determine the immediate source of solutes in drainage water. It is intuitive that the soil water chemistry of the predominant soil/vegetation types within the watershed should correspond approxi- mately to the chemistry of drainage waters. In practice, however, this turns out not to be the case. Data from the Bear Brook watershed manipulation project in Maine provide a good example. Flow separation calculations using the ratio of 18 O to 16 O chemical isotopes suggested that the majority of the stream flow during high discharge events was derived from “old water” (Kendall et al., 1995), a finding common to most 18 O studies. However, none of the soil lysimeter sites showed soil water chemistry comparable to stream chemistry (Norton et al., 1999). The conclusion of Norton and co-workers was that storm water chemistry at Bear Brook is governed by a mixture of soil solution and deep groundwater, new water (precipitation and snowmelt), and micropore water that is not well-sampled by tension lysimeters. The observed constancy or only slight dilution of base cation concentrations dur- ing high discharge periods suggested water sources deep within the soil pro- file, whereas the observed episodic increases in NO 3 - and DOC concentrations suggested sources in shallow soil areas. Partly in response to these recognized uncertainties in the routing of drainage waters within the watersheds, a hydrologic analysis was conducted for the Bear Brook water- shed manipulation site by Chen and Beschta (in press). 1416/frame/C06 Page 142 Wednesday, February 9, 2000 2:13 PM © 2000 by CRC Press LLC Episodic Acidification 143 To simulate the dynamic hydrological processes at the Bear Brook water- shed manipulation site, Chen and Beschta (in press) used the Object Water- shed Link System (OWLS) model of Chen (1996), a physically-based, three- dimensional distributed watershed hydrologic model. The OWLS model attempts to represent the major hydrologic processes within the watershed and also allows dynamic three-dimensional visualization of flow separation processes and variable source areas. Results of the flow separations sug- gested that surface flow from riparian areas was the predominant component of the flood rising limb, whereas macropore flow from riparian areas domi- nated during the falling limb of the hydrograph. Downstream riparian areas appeared to be the major contributing areas for peak flow. Because the 18 O results suggested that most of the high-flow discharge was “old water,” it must be assumed that deep groundwater in the uplands re-emerges as near- surface flow in the lower riparian areas. More specific linkages between the simulated flow routing of drainage water and the observed soil water chem- istry may further refine our understanding of these complex interactions. 6.2 Causes Episodic acidification can be caused by several factors, including base cat- ion dilution and organic acid enrichment, both of which are natural compo- nents of the hydrological response. Other potentially important factors include S and N enrichment that can be either natural and/or result from acidic deposition. The relative importance of these various factors and the extent to which they contribute to episodic acidification vary by region and individual watershed. 6.2.1 Natural Processes The most important cause of episodic acidification of surface waters is base cation dilution. It is a completely natural process and typically accounts for a sizable fraction (often more than one-half) of the overall acidification response during snowmelt or rainfall events. Because hydrological episodes entail rapid water flow routing through upper soil horizons, base cations are contributed to drainage waters to a lesser extent than during periods of low flow. The additional large influx of water in the form of rain or meltwater, some of which makes only limited contact with watershed soils, further con- tributes to the observed dilution of base cation concentrations in stream water during high-flow events. The altered hydrological flow-routing during episodes that contributes to lower base cation concentrations in stream water also causes increased con- centrations of organic acid anions. This is because upper soil horizons tend to 1416/frame/C06 Page 143 Wednesday, February 9, 2000 2:13 PM © 2000 by CRC Press LLC 144 Aquatic Effects of Acidic Deposition be relatively rich in organic C. Some of the organic acidity of the upper soils is transported to streams during high-flow events. The fraction of the epi- sodic acidification that is caused by organic acid enrichment varies from watershed to watershed. In some cases, mainly in wetland-influenced high- DOC streams, the organic acid component of episodic acidification can dom- inate the episodic response. In other cases, episodic organic acidity is negligi- ble compared with other components of the episodic response. For the most part, base cation dilution and organic acid enrichment account for the ubiquitous nature of episodes. These processes operate with or with- out acidic deposition and can account for episodic loss of a few µ eq/L of ANC to losses of 50 µ eq/L or more during snowmelt or rainstorms. Episodic acidification owing to S or N enrichment can also be a natural pro- cess in some areas, but both are typically associated with anthropogenic effects of acidic deposition. In particular, N-driven episodic acidification is frequently associated with high levels of N deposition. 6.2.2 Anthropogenic Effects Nitrate in snowmelt runoff has been recognized for some time as an impor- tant component of biological damage resulting from atmospheric deposition (c.f., Wigington et al., 1990). Nitrate is the principal acid anion in snowmelt in many areas of northern Europe and the northeastern U.S. Selective elution of NO 3 - from the snowpack can result in early spring runoff having concentra- tions substantially greater than the average snowpack concentrations. The biological response to acidic runoff is similar, regardless of whether the pre- dominant acid anion is NO 3 - or SO 4 2- , assuming concentrations of other ions, including Al i , are the same. Nitrate concentrations in surface waters exhibit a strong seasonality; NO 3 - is typically elevated during late winter and spring, particularly during peri- ods of snowmelt, and reduced to low or nondetectable levels throughout summer and fall. This can be attributed to seasonal growth patterns of forest vegetation. Vegetation growth is reduced or stopped entirely during winter months, and microbial assimilation of N is also reduced during this season. Spring snowmelt can act to flush N into lakes and streams that was deposited in the snowpack from atmospheric deposition or N mineralized within the forest floor or soil during winter. Except in cases of excessive N saturation, the effects of N deposition on surface waters are expected to be primarily episodic in nature. Unfortu- nately, data required to make regional assessments of episodic effects are generally not available. Such data need to be collected on an intensive schedule and must include sample periods during late winter and early spring when snowmelt often causes the most severe N-driven episodes of surface water acidification. Sampling during this time of year is more diffi- cult and expensive than during the more common summer/fall sampling seasons. Sampling during snowmelt can be particularly difficult in the high 1416/frame/C06 Page 144 Wednesday, February 9, 2000 2:13 PM © 2000 by CRC Press LLC Episodic Acidification 145 mountains of the West, when study sites are often inaccessible, and when motorized transport (e.g., via snowmobile) is often not allowed owing to wilderness restrictions. Aluminum concentration in drainage water is also greatly affected by hydrological variations. For example, Wigington et al. (1993) sampled 4 Adirondack streams in New York, 3 of which contained maximum concentra- tions of Al i greater than or equal to 485 µ g/L and maximum weekly average concentrations of Al i greater than or equal to 264 µ g/L. These are high con- centrations of Al i by any standards, and would be toxic to many species of fish. Nevertheless, all of these streams had minimum weekly average concen- trations of 0 µ g/L and during 25% of the weeks during the course of the year, the average weekly Al i concentration was less than or equal to 94 µ g/L. In other words, an assessment of potential Al toxicity conducted during the times of year when Al concentrations were at their lowest (summer and fall) would likely conclude that Al was of minor or negligible importance in these streams. Such an assessment would of course be in sharp contrast to the extremely high (and toxic) levels of Al i achieved during winter and, espe- cially, spring. During periods of high discharge, especially during snowmelt, it has been frequently observed that increasing NO 3 - concentration contributes greatly to seasonal or episodic chemistry of streams, and to a lesser extent lakes. The observed NO 3 - pulse is often accompanied by a large increase in the concen- tration of Al i . This has led to speculation that NO 3 - may be a more effective mobilizer of Al i than SO 4 2- (Driscoll et al., 1991). However, the concentration of NO 3 - per se is not necessarily related to the concentration of Al i in surface waters. For example, Wigington et al. (1993) reported data from three streams in the Catskill Mountains, NY, that had very similar maximum NO 3 - concen- trations (129, 108, 106 µ eq/L) and maximum weekly average NO 3 - concentra- tions (68, 72, 67 µ eq/L). Despite these similarities in peak NO 3 - concentrations, however, peak Al i concentrations differed by a factor of 4 (159, 72, 505 µ g/L) and maximum weekly average Al i concentrations differed by a factor of 7 (92, 49, 380 µ g/L) in these 3 streams. Similarly, the 1 Adiron- dack stream (Biscuit Brook) sampled by Wigington et al. (1993) that had low concentrations of Al i (75th percentile of weekly average concentration equal to 20 µ g/L) had similar NO 3 - concentrations to the three Adirondack streams that exhibited much higher concentrations of Al i . The strongly elevated concentrations of Al i in drainage waters that are often observed during winter and spring in general, and snowmelt in particular, suggest that assessments of acidification effects based on fall index, or synop- tic survey, data are totally inadequate with respect to evaluating the dynamics and potential toxicity of Al. The general inadequacy of the hydrological com- ponent of most acidification assessments is widely acknowledged, and is per- haps most problematic when considering Al (Sullivan, 1994). In near coastal environments, the neutral salt effect can have a large influ- ence on episodic chemistry. For example, mechanisms of episodic acidifica- tion at Bear Brook watershed in Maine were at least partially controlled by 1416/frame/C06 Page 145 Wednesday, February 9, 2000 2:13 PM © 2000 by CRC Press LLC 146 Aquatic Effects of Acidic Deposition the volume and chemistry of the precipitation event, especially the contribu- tion of ions from seaspray (Kahl et al., 1992; Norton and Kahl, in press). Sim- ilar results were found at Lake Skjervatjern in western Norway (Gjessing, 1994). It was well-known previously that antecedent soil moisture is an important determinant of episodic chemical response. It now also seems clear that, for near-coastal watersheds, the ionic make-up of the rainfall event is also important. 6.3 Extent and Magnitude The ERP data for Adirondack streams (Figure 6.1) showed that as stream- water ANC decreased during hydrological episodes, the relative importance of NO 3 - vs. SO 4 2- acidity increased. In the acidic (ANC ≤ 0) samples from the ERP, NO 3 - concentrations were often of the same or similar magnitude as SO 4 2- concentrations. These are the chemical conditions (high concentrations of H + and Al i ) that are most toxic to fish. In samples that had positive ANC values, NO 3 - was of much less importance relative to SO 4 2- concentrations (Figure 6.1), but such conditions are not typically associated with adverse biological effects (Wigington et al., 1993). All four of the Adirondack ERP study sites were located in the southwest- ern highlands area of the Adirondack Park, with maximum elevations of 710 to 775 m. Buck Creek, Bald Mountain Brook, and Seventh Lake Inlet are typical first- or second-order Adirondack streams. The Buck Creek catch- ment is characterized by steep terrain with numerous rock ledges and thin soils. The Seventh Lake Inlet watershed is a combination of moderately sloping terrain with deeper soils and bedrock outcrops. Bald Mountain Brook contains a higher percentage of deeper soils (Wigington et al., 1993). Buck Creek and Seventh Lake Inlet showed similar relationships between the ratio of NO 3 - to (SO 4 2- + NO 3 - ) concentration and ANC, with increasing importance of NO 3 - as a strong acid anion at lower ANC values ( P < 0.0001, Figure 6.1). Both had ANC that generally remained below about 20 µ eq/L. The watershed with steepest slopes and shallowest soils (Buck Creek) had the lowest ANC. Bald Mountain Brook and Fly Pond Outlet showed much wider ranges of ANC values, and ANC was less related to the relative importance of NO 3 - as a strong acid anion in these streams. The stream gra- dient (9m/km) of Fly Pond outlet was the lowest of the 4 Adirondack ERP study sites. This stream had circumneutral water chemistry and constituted the reference stream for ERP biological studies in the Adirondacks (Wiging- ton et al., 1993). Episodic acidification of streams in Shenandoah and Great Smoky Moun- tains National Parks has been demonstrated in several recent studies (e.g., Hyer et al., 1995; Eshleman et al., 1995; Nodvin et al., 1995; Webb et al., 1995). Streams with chronic ANC less than about 25 µ eq/L, in particular, have been 1416/frame/C06 Page 146 Wednesday, February 9, 2000 2:13 PM © 2000 by CRC Press LLC Episodic Acidification 147 found to be subject to substantial ANC declines during precipitation events. Gypsy moth defoliation has also apparently contributed to episodic acidifi- cation at White Oak Run in Shenandoah National Park owing to increased NO 3 - leaching (Eshleman et al., 1995). Mean episodic ANC depressions increased by about a factor of two after the onset of defoliation. Webb et al. (1994) developed an approach to calibration of an extreme event (episodic acidification) model for VTSSS long-term monitoring streams in western Virginia that was based on the regression method described by Eshleman (1988). Median, spring quarter ANC concentrations from 1988 to 1993 were used to represent chronic ANC, from which episodic ANC was predicted. Regression results were very similar for the four lowest ANC watershed classes, and they were, therefore, combined to yield a single regression model to predict the minimum measured ANC from the chronic ANC. Extreme ANC values were about 20% lower than chronic values, based on the regression equation: FIGURE 6.1 Ratio of NO 3 - : (SO 4 2- + NO 3 - ) concentration vs. ANC in stream-water samples collected during hydrological episodes in the four streams included in the Adirondack region of the ERP. Site identifications: Buck Creek, • ; Bald Mountain Brook, º; Seventh Lake Inlet, ❏ ; Fly Pond Outlet, ∆ . (Source: Water, Air, Soil Pollut ., Vol. 95, 1997, p. 330, Increasing role of nitrogen in the acidification of surface waters in the Adirondack Mountains, New York, Sullivan, T.J., J.M. Eilers, B.J. Cosby, and K.B. Vaché, Figure 8, copyright 1997. Reprinted with kind permission from Kluwer Academic Publishers.) 1416/frame/C06 Page 147 Wednesday, February 9, 2000 2:13 PM © 2000 by CRC Press LLC 148 Aquatic Effects of Acidic Deposition ANC min = 0.79ANC chronic – 5.88 ( r 2 = 0.97; se of slope = 0.02, p ≤ 0.001)(6.1) Because the model was based on estimation of the minimum ANC mea- sured in the quarterly sampling program, it is likely that the true minimum ANC values were actually somewhat lower than 20% below the measured chronic ANC. Nevertheless, regression approaches for estimation of the minimum episodic ANC of surface waters, such as was employed by Webb et al. (1994) for western Virginia, provide a basis for predicting future epi- sodic acidification. A model such as MAGIC can be used effectively to derive estimates of future chronic ANC under various assumptions of atmospheric deposition. Episodic ANC can then be estimated by applying the regression approach. Simple mixing models have also been used successfully elsewhere to sim- ulate the magnitude of episodic acidification (c.f., Hendershot et al., 1992; Hooper and Christopherson, 1992; Schaefer and Driscoll, 1993). In the Sierra Episodes Study (Melack et al., 1998), minimum lake-water ANC was related to chronic lake-water ANC in a manner that was very similar to that found by Webb et al. (1994) for Virginia streams. For Sierra Nevada lakes, mini- mum ANC during snowmelt was about 12% lower than chronic ANC (Melack et al., 1998). Episodes of surface water acidification involving increases in NO 3 - con- centration have been reported for a few sites in the western U.S. (e.g., Lor- anger et al., 1986; Loranger and Brakke, 1988; Melack and Stoddard, 1991). Maximum NO 3 - concentrations in western lakes and streams reported dur- ing episodes tend to be low, however, typically less than 15 µ eq/L (Stod- dard, 1994). Although such episodic concentrations are quite low in comparison with many sites in the eastern U.S., increases in NO 3 - concentra- tion during episodes at Emerald Lake in the Sierra Nevada have apparently been sufficiently high, when coupled with natural base cation dilution, to drive lake-water ANC to zero on at least two occasions (Williams and Melack, 1991a,b; Stoddard, 1994). Episodic acidification is an important issue for surface waters throughout high-elevation areas of the West. A number of factors predispose western sys- tems to potential episodic effects. There is an abundance of dilute to ultradi- lute lakes (i.e., those having extremely low concentrations of dissolved solutes), exhibiting very low concentrations of base cations, and therefore ANC, throughout the year. Large snowpack accumulations occur at the high elevation sites, thus causing substantial episodic acidification via the natural process of base cation dilution. Many of the high-elevation drainage lakes exhibit short hydraulic retention times, thus enabling snowmelt to rapidly flush lake basins with highly dilute meltwater. The hydrology, physical char- acteristics (e.g., bedrock geology, lake morphometry), and climate through- out high elevation areas of the West provide justification for considering potential episodic acidification to be an important concern. In addition, the few studies that have been conducted to date confirm the general sensitivity of western lakes to episodic processes. 1416/frame/C06 Page 148 Wednesday, February 9, 2000 2:13 PM © 2000 by CRC Press LLC [...]... observed a 50% decline in the ANC of Bagley Lake to a low of 67 µeq/L following snowmelt Because the anion concentrations were so low in the snowpack, most of © 2000 by CRC Press LLC 14 16/ frame/C 06 Page 150 Wednesday, February 9, 2000 2:13 PM 150 Aquatic Effects of Acidic Deposition the ANC loss was attributed to dilution of base cations by meltwater The annual average ANC of Bagley Lake was 100 µeq/L From... issues of spatial variability Results of the FISH project will provide a wealth of biological response data, and will greatly improve our predictive capabilities in the area of biological effects of acidification The results of the episodic aspects of this project will be prepared for publication in the near future (Art Bulger, University of Virginia, personal communication.) The potential role of acid deposition. .. by CRC Press LLC 14 16/ frame/C 06 Page 152 Wednesday, February 9, 2000 2:13 PM 152 Aquatic Effects of Acidic Deposition in Shenandoah National Park, VA (Bulger et al., 1995) The aim is to assess potential impacts and to predict likely future effects based on current relationships between water chemistry and fish population responses The study design includes high-frequency measurement of stream discharge... Melack, 1991b) Concentrations of NO 3- in the Emerald Lake outlet increased from 2 to 3 µeq/L in the fall to 10 to 13 µeq/L during spring runoff The increase in NO 3- and SO4 2- (approximately 50%) was attributed to preferential elution from the snowpack and low retention rates in the watershed Reduction of NO 3- and SO4 2- within Emerald Lake was relatively small, and most of the acid anions passed through...14 16/ frame/C 06 Page 149 Wednesday, February 9, 2000 2:13 PM Episodic Acidification 149 Lakes and streams in the Sierra Nevada, Cascade, and Rocky Mountains are highly sensitive to potential acidic deposition effects because of the predominance of granitic bedrock, thin acidic soils, large amounts of precipitation, coniferous vegetation, and extremely... Notasha are likely far less sensitive to potential increases in acidic deposition than lakes with similar CB elsewhere in the West In contrast to lakes in the southern Cascades, lakes in the Sierra Nevada receive both greater acidic deposition and higher rates of runoff In the Emerald Lake watershed (elevation 2800 to 34 16 m), NO 3- increases of about 120% have been observed in the streams during snowmelt... Sierra Nevada lakes are not currently showing chronic biological effects of acidic deposition 6. 4 Biological Impacts The ERP (Wigington et al., 1993) included chemical and toxicological analyses of 13 streams in the northeastern U.S during multiple seasons In situ bioassays were conducted for brook trout, sculpins, and blacknose dace At all of the streams, some bioassays resulted in low mortality (less... substantially lower numbers and biomass of brook trout than were found in nonacidic streams Streams having acidic episodes showed significant mortality of fish Some brook trout avoided exposure to stressful chemical conditions during episodes by moving downstream or into areas with higher pH and lower Al This movement of brook trout only partially mitigated the adverse effects of episodic acidification, however,... degree in acid-sensitive areas of the West to permit any regional assessment of either episodic or chronic N-driven acidification In the Rocky Mountain region, Loch Vale has been the subject of intensive research on hydrochemical responses to snowmelt Loch Vale is located in Rocky Mountain National Park, CO, at an elevation of 3000 to 4000 m The watershed is comprised primarily (81%) of exposed bedrock... 30 µeq/L The shallow depth of the lake promoted rapid mixing and flushing (short residence time) Thus, most of the NO 3- in the snowpack passed through Bagley Lake (elevation 1800 m) during the period of most rapid snowmelt; by mid-June NO3concentrations in the lake had decreased from over 5 µeq/L to less than 2 µeq/L Loranger and Brakke (1988) concluded that NO 3- and SO4 2- concentrations in the snowpack . 142 Aquatic Effects of Acidic Deposition 0. 06 (Landers et al., 1987; Stoddard, 1994). Survey data with which to evalu- ate the (largely episodic) effects that might be associated with N deposition were. 2:13 PM © 2000 by CRC Press LLC 1 46 Aquatic Effects of Acidic Deposition the volume and chemistry of the precipitation event, especially the contribu- tion of ions from seaspray (Kahl et al.,. Mountains are highly sensitive to potential acidic deposition effects because of the pre- dominance of granitic bedrock, thin acidic soils, large amounts of precipita- tion, coniferous vegetation, and