Envlron. Sci. Technol. 1988, 22, 185-190
of Lakes in the Eastern United States; EPA16OOf 4-861 007a; U.S.Environmental Protection Agency: Washington, DC, 1986; Vol. 1, 136 pp.
(15) Perdue, E. M. In Humic Substances in Soil, Sediment and
Hillman, D. C.; Fountain, J.; Simon, S. J.; Kramer, J. R.; Meier, E. P. Quality Assurance and Quality Control in the
pp 493-526. (16) Davis, J. C. Statistics and Data Analysis in Geology; Wiley: New York, 1973; pp 420ff. (17) Dempsey, D. A.; O'Melia, C. R. In Aquatic and Terrestrial Humic Materials; Christman,R. F., Gjessing, E. T., Eds.; Ann Arbor Science: Ann Arbor, MI, 1983; pp 239-274. (18) Oliver, B. G.; Thurman, E. M.; Malcolm, R. J. Geochim. Cosmochim. Acta 1983,47,2031-2035.
Water; Aiken, G. R., McKnight, D. M., Wershaw, R. L., MacCarthy, P., Eds.; Wiley-Interscience: New York, 1986;
EPA National Surface Water Survey. Application of Chemical Equilibria to the Evaluation of p H , DIC, ANC and BNC Data; LEMSCO Management Services: Las
Vegas, NV, 1986; 41 pp. Malcolm, R. L. In Humic Substances in Soil, Sediment and Water; Aiken, G. R., McKnight, D. M., Wershaw, R. L., MacCarthy, P., Eds.; Wiley-Interscience: New York, 1986; pp 181-210.
Received for review February 9,1987.Accepted August 17,1987.
Evaluation of the Role of Sea Salt Inputs in the Long-Term Acidification of Coastal New England Lakest Timothy J. Sullivan,*q~ Charles T. Drlscoll,gJoseph M. Eilers,s and Dlxon H. Landers''
Northrop Services, Inc., 200 S.W. 35th Street, Corvallis, Oregon 97333, Department of Civil Engineering, Syracuse University, Syracuse, New York 13244, and US. EPA Environmental Research Laboratory, 200 S.W. 35th Street, Corvallls, Oregon 97333
+
+ H 2 0 + A1(OH)3+ H+ + ClA1(OH)2++ H+ + A1(OH)2++ H 2 0
A1(OH)2+ C1-
I Input of neutral salt (NaC1) from sea spray, followed
by Na+-H+ exchange within the soil exchange complex, has been proposed as an important factor in surface water acidification of coastal areas. This hypothesis was tested on a regional basis by comparing the Na:C1 ratio of lake water with that of precipitation for the coastal lakes included in the U.S. Environmental Protection Agency Eastern Lake Survey-I in New England. The precipitation Na:C1 ratio closely approximated that of seawater at monitoring stations located within approximately 20 km of the coast. Few lakes in this coastal region exhibited a Na:C1 ratio less than that observed in precipitation. Those lakes that were acidic (ANC I 0) showed no tendency toward a lowered Na:Cl ratio. Sodium contribution from, rather than retention by, watershed soils was suggested by the data from these lakes. Although episodic acidification of runoff due to NaCl deposition may occur, there is little support for the neutral salt effect being an important long-term acidifying process in Northeastern lakes.
Introduction The neutral salt effect is a well-known soil phenomenon whereby the pH of an electrolytic solution is altered as it percolates through a soil column. The effect is due to ion exchange on soil surfaces and can lead to either a retention or a release of H+ depending on soil pH, bonding energy of the salt cation or anion, and the salt content of both the soil and added electrolytic solution (1, 3). This process is potentially relevant to surface water acidification, since a decrease in leachate pH may occur from very acidic soils. On neutral soils this effect is small due to increasing bonding energy of H+, with subsequent reduction in ion exchange (1). Wiklander (1)suggested that several reactions are important in acidification of leachate: R-H + Na+ + C1- + R-Na + H+ + C1(1) R-A1(OHI2 + Na+ + C1- + R-Na + A1(OH)2++ C1(2) 'This research is a contribution from the National Surface Water Survey of the EPA Aquatic Effects Research Program. t Northrop Services, Inc. 8 Syracuse University. 11 U.S.EPA Environmental Research Laboratory. 0013-936X/88/0922-0185$01.50/0
(3) (4)
where R indicates a negatively charged functional group within the soil exchange complex. Inputs of neutral salt result in acidification of leachate draining acidic soils (eq 1 and 3), but base saturation and soil pH are increased. Equation 4, which consumes H+, is more significant in circumneutral soils. Several researchers have speculated that the neutral salt effect could be an important factor contributing to the acidification of surface waters in parts of Scandanavia and North America ( 2 , 4 ) . Krug et al. ( 4 ) argued that many areas experiencing acidification, are coastal and that electrolyte concentrations in precipitation are more important than acidity (pH) in the acidification of surface waters. Further interest in this phenomenon has been stimulated by recent results in Acadia National Park, Maine, where Kahl et al. (5) have presented evidence for NaCl acidification of stream water in a coastal catchment on an event basis. Episodic acidification of stream water had previously been linked with NaCl inputs in the Breidvikdalen catchment of coastal western Norway (6). Particularly during sea salt enriched autumn rainstorms, water draining acidic soils within this catchment was acidified coincident with Na+ retention. Soil retention of Na+, however, was evidently a short-term phenomenon since there was close agreement between atmospheric input and drainage water output budgets of Na+ on an annual basis. Seip (7) examined these studies and others in the Norwegian SNSF Project. He concluded that, on the basis of input/output budgets, correlations between Na+ and C1- in runoff, and laboratory studies, Na+ is readily transported through soil compared to Ca2+ and Mg2+, and the acidification of drainage waters due to retention of Na+ from sea salt should be small in most cases. The relative importance of natural and anthropogenic causes of surface water acidification has been the focus of intense debate in recent years. A strong positive correlation has been established between precipitation chemistry and surface water chemistry in many areas, although it is difficult to prove a causal relationship (7). Contributions
0 1988 American Chemical Soclety
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to surface water acidification from natural sources, such as marine salt deposition, are therefore relevant. Issues of natural sources of acidification are particularly important in view of recent efforts throughout the northern hemisphere to curb emissions of acidifying sulfur compounds. In order to evaluate the possible regional importance of watershed Na+ retention leading to H+enrichment, it is first necessary to obtain an index of Na+ input. The Na:C1 ratio of precipitation provides such an index for coastal areas, where Na:C1 in precipitation very closely approximates that of seawater (0.86) (6,8,9). Variability in this relationship increases further inland. The predominant source of C1- in coastal precipitation is sea spray. A smalI amount may originate from anthropogenic emissions, but this quantity is apparently negligible compared to background coastal levels (10,11). Watershed sources of C1are also generally negligible (12). Thus, because the input ratios are reasonably well established, the Na:C1 ratio of surface waters provides a means to evaluate NaC1-induced acidification of coastal sites. An extensive data base has recently been developed by the US.Environmental Protection Agency in those regions of the eastern U S . which are expected to contain significant numbers of lakes with low acid-neutralizing capacity (ANC) (13). Phase I of the Eastern Lake Survey (ELS-I), conducted in autumn 1984, employed a statistical (probability) sampling design such that regional extrapolations with known uncertainty could be performed to estimate the number of acidic and low ANC lakes. The statistical sampling design and large data base provide a unique opportunity to evaluate the regional influence of NaCl on lake acidification, as well as other related issues. The survey determined that there were a number of acidic lakes (ANC 10 pequiv L-l) in coastal areas and that concentrations of sodium and chloride decreased rapidly with distance from the coast (14). These observations suggest the potential for the acidification of low-alkalinity lakes in coastal areas. The purpose of this paper is to estimate, with ELS-I data, the number of coastal lakes in the northeastern U.S.that have become acidic because of Na+ retention and to evaluate the role of NaCl in lake acidification processes. We examined NaCl acidification in the context of chronic effects on water chemistry using data from the autumn index sample. These data are not, however, suitable for investigating potential episodic effects of NaCl deposition from sea spray.
Methods Scope. Coastal precipitation chemistry data were made available through existing precipitation monitoring networks, NADP/NTN and W 3 S (15,16). Data were used from 21 monitoring stations located within 100 km of the east coast in order to establish Na:C1 wet input ratios as a function of distance from coast. Phase I of the Eastern Lake Survey (ELS-I) provided the lake chemistry data used in this study (13). In ELS-I, three regions were sampled which had been identified as having significant numbers of low ANC lakes by use of the map developed by Omernik and Powers: the Northeast (l), Upper Midwest (2), and Southeast (3) (17). Each region was further subdivided into two or more subregions on the basis of geographic homogeneity and physiography. Evaluation of potential sea salt acidification was necessarily restricted to coastal lakes where (1) NaCl deposition is significant and (2) the Na:C1 ratio in precipitation is reasonably constant (6, 8, 9). On the basis of our examination of coastal precipitation chemistry data (see, e.g., Figure 2), the pattern of concentrations of Na+ and Cl- in 186
Environ. Sci. Technol., Vol. 22, No. 2, 1988
*
(
A
50
50 -
200
00 bN
Figure 1. Map of ELS-I region 1 showing locations of the 144 lakes included in the analysis of potential neutral salt acidlflcation of lake water.
lakes (14), and these earlier studies, we limited this investigation to lakes within 20 km of the coast. In this coastal strip, Na:C1 precipitation ratios should approximate that of seawater (*analytical error) and watersheds experience the highest loading of sea salt. If evidence of surface water acidification by Na+ retention cannot be demonstrated for acid-sensitive coastal lakes, it is unlikely to be meaningful further inland as a regional phenomenon. Portions of two ELS-I regions coincided with the 20-km zone selected for investigation: coastal New England (in region 1) and Florida/southeast Georgia (in region 3). The Florida subregion (3B) contained the largest number of acidic lakes of any subregion. Lakes in this subregion, however, were generally farther from the coast; only 21 of the 150 lakes sampled were located within 20 km of the coast. Furthermore, lakes in subregion 3B exhibited a positive relationship between [Na+] and distance to seawater (13,18). This suggests a substantial Naf contribution from the watersheds, more specifically from shallow groundwater sources (18). Therefore, subregion 3B was excluded from our investigation, and further analyses were limited to coastal areas of region 1 (the Northeast) of the Eastern Lake Survey. Lake Selection. Subregions were stratified by alkalinity map classes according to a preliminary version of the regional surface water alkalinity map prepared by Omernik and Kinney (19). The designated alkalinity strata were 200 pequiv L-l. Approximately 50 lakes were randomly selected from each of the three map classes in each subregion and assigned a weighting factor on the basis of the frame population of each map class (universe of lakes considered for study in ELS-I, essentially including all lakes 1 4 ha, with specific conductance 11500 p S cm-l, and not locally influenced by intense anthropogenic activities). A total of 768 lakes were sampled in the Northeast, of which 144 were within 20 km of the coast (Figure 1). Sampling. Northeastern lakes were sampled by helicopter between 10 October and 18 November 1984 at a depth of 1.5 m in the deepest part of the lake. In most cases, sampling coincided with the period of complete mixing following fall turnover. Preliminary processing of
samples for water chemistry, including pH, was accomplished at a nearby mobile field laboratory. Subsequent analyses at an analytical laboratory included ANC (acidimetric titration with modified Gran analysis), C1- (ion chromatography), and Na+ (flame atomic adsorption spectroscopy). Details on sampling, analytical methodologies, statistical design, and QA/QC protocols were provided by Linthurst et al. (13), Best et al. (20), and Hillman et al. (21)and summarized in Landers et al. (22). Data Treatment. Precipitation chemistry data from NADP/NTN and MAP3S networks were analyzed by analysis of variance and linear regression of the Na:C1 ratio in precipitation as a function of distance from coast. Results of these analyses were used in delineating the coastal zone in which Na:C1 input ratios could be characterized and evaluating wet input ratios relative to lake ratios in the selected coastal zone. The lake water Na:C1 ratio was determined for all lakes in region 1 within 20 km of the coast. The relationship between potential Na+ wtention and lake acidity was evaluated by linear regression of both ANC and pH as a function of the ratio of Na+ to C1- in lake water. Analyses were performed for the following subgroups of ELS-I lakes in region 1: 0-5,0-10,6-10, and 11-20 km from the coast. A key assumption in this analysis was that chloride is conservative in the coastal watersheds under investigation. The assumption of conservative C1- behavior has been shown to be valid on the basis of soil column studies (23), sediment microcosm studies (24),and ion budgets (25-27). Bayley et al. (28) recently concluded that C1- retention is significant in some soils and wetlands in the Experimental Lakes Area (ELA) in Ontario. However, C1- deposition is very low a t ELA, yielding typical lake C1- concentrations 15 bequiv L-l, more than an order of magnitude less than that of the typical lakes included in this study. Retention of a small amount of C1- or minor analytical error would thus greatly influence ELA budget calculations. Implicit in this analysis were the additional assumptions that (1) the precipitation input Na:C1 ratio could be well established for coastal Northeast lakes and (2) Na+ retention in watershed soils (leading to H+ release and consequent leachate acidification) would be reflected in a decreased Na:C1 ratio of coastal lake water as compared to established precipitation input ratios. Road salt is a potential complicating factor in the interpretation of the sea salt effect, because lakes with high sodium and chloride were found a t considerable distance inland from the coast (13). A subset of coastal lakes was therefore selected which were determined to be without paved roads in their watersheds on the basis of examination of U.S.G.S. topographic maps (1:250 000). Analyses were repeated for this subset of lakes. We wed the following relationship to quantify the extent of acidification by retention of Na+ from sea salt inputs:
-
sea salt acidification = [C1-](0.86) - [Na+]
(5)
This expression compares the theoretical Na+ concentration calculated from measured C1- (assuming conservative behavior and the Na:C1 of seawater [0.86]) with measured Na+ values. The difference between theoretical Na+ (on the basis of measured C1-) and measured Na+ is an estimate of drainage water acidification due to Na+ retention, if the input ratio equals that of sea water. We compared sea salt acidification to ANC by linear regression analysis.
Results and Discussion Data from coastal precipitation stations indicated that the ionic composition of coastal precipitation closely ap-
I
1
I
I
I
I
I
/
t 1.4
h
.-0
1.3
tI
0
I
e e
I
1
e e
10 20 30 40 50 60 70 EO
90 100 110
Distance to Coast (km) Figure 2. Na:Ci ratio in precipitationas a function of distance to the seacoast. Data are from the 21 NADPINTN and MAPBS precipitatlon monitoring stations for the period 1979-1984. The seawater ratio Na:Ci = 0.86 is provided as a reference.
proximated that of seawater. Weighted annual means of the Na:C1 ratio of precipitation for all NADP/NTN and MAP3S monitoring stations within 100 km of the east coast are presented in Figure 2 for the period 1979-1984. The Na:C1 ratio of precipitation closely approximated that of seawater only for stations in close proximity to the coast. Further inland, the Na:C1 ratio in precipitation becomes more uncertain due to contributions of excess Na+ from the soil (29),enhanced inland transport of C1- relative to Na+ [possibly due to loss of Cl- from NaCl particles by volatilization (30)], and local anthropogenic sources of Cl(e.g., combustion of leaded gasoline). These factors become significant only as coastal NaCl loads diminish (31,II). Salt deposition as a function of coastal proximity is confounded by road salt in New England, but the minimum Na+ concentration in ELS-I lakes declined abruptly between 20 and 40 km from the coast, approaching a background concentration of 30 pequiv L-l (Figure 3; 13). This trend agrees with the observations of Ogden (8)that 80% of the marine aerosol in Nova Scotia was deposited within 20 km of the coast. Chloride patterns tend to be similar to those for Na+ (13),although background concentrations are not reached until 75-100 km inland (32). The data in Figure 2 include 8 precipitation monitoring stations located within 20 km of the coast with a total of 28 annual mean measurements during 1979-1984. Mean and standard deviatiofi for this subset were 0.83 and 0.04, respectively. Analysis of variance for this subset revealed a significant (P< 0.05) trend of decreasing Na:C1 with coastal distance (slope = -0.003, intercept = 0.85, R2 = 0.20). However, the slope of this relationship is relatively small, yielding minor variations in estimates for precipitation Na:C1 based on distance from the coast for stations within the 20-km coastal zone (95% CI = 0.75-0.86). Few lakes in coastal New England had Na:Cl ratios less than those measured by coastal precipitation monitoring stations (12% had Na:Cl < 0.86, Figures 4 and 5). Rather, most lakes contained proportionately higher sodium concentrations than were observed in precipitation, due to weathering reactions in the watersheds. For those lakes that did exhibit low Na:Cl ratios (although still within the Envlron. Sci. Technol., Vol. 22, No. 2, 1988
187
I .
I
t -
I
Seasalt Ratio 0
0
0
5.5
5.0 V
I
1
0
20
I
I
I
40 60 80 D I S T A N C E FROM COAST ( k m )
I
I
io0
420
Seasait Ratio
0
Q
0.6
I
1.0
0
model
8
ANC = a
+ b (Na:Cl)
300-
N
200-
e
100-
pH = a
+ b (Na:Cl)
Q z
Y
0-
0.8
1.0
1
I
f
1.4
I
1.6
1
1
1.8
I
I 2.0
1.2
1.4
1.6
1.8
2.0
Na:CI (ratio) Figure 4. Acid neutralizing capacity (ANC) (pequiv L-') as a function of the Na:Ci molar ratio in ELS-I region 1 lakes 0-10 km from the seacoast. One outlier was deleted at Na:Ci = 7. Five lakes wlth ANC > 600 pequiv L-' are not depicted.
range expected on the basis of wet input), there was no clear pattern of concomitant low ANC or pH (Figures 4, 5). Results of regression analyses are presented in Table I. ANC and pH as a function of the Na:C1 ratio for lakes 110 km from the coast are graphically depicted in Figures 4 and 5, respectively. Only one regression was significant at p < 0.05, that of pH = a + b(Na:Cl) for lakes within 10 km (Table I, Figure 5). Examination of Figure 5, however, reveals that the significance is derived from the right side of the graph (Na:Cl> Ll),suggesting excess Na+ inputs in many lakes with pH over 6.0. In the event of neutral salt acidification, we would observe a decreased Na:C1 (below the input ratio) for low-pH lakes. Such a relationship is not evident in Figure 5. Also, the pH as a 188 Environ. Sci. Technol., Vol. 22, No. 2, 1988
coastal distance, km 0-5 6-10 0-10 11-20 0-5 6-10 0-10 11-20
d.f.
F
P>F
60 33 94 51 60 33 94 51
0.159 2.151 1.284 3.052 1.453 0.939 4.338 0.252
0.6917 0.1523 0.2600 0.0868 0.2329 0.3399 0.0400" 0.6181
Significant at P I0.05.
e.
-1 00 0.6
1
1.2
Na:CI (ratio)
4001'
g3
I
Figure 5. pH as a function of the Na:CI molar ratio in ELS-I region 1 lakes 0-10 km from the seacoast. One outlier was deleted at Na:Ci = 7.
0
.-.--c
1 1 1
0.8
Table I. Results of Regression Analyses for Acidity as a Function of the Molar Na:CI Ratio for Coastal New England Lakes
500
'g n
4.5
I I
Figure 3. Lake water sodium concentration (pequiv L-') versus lake distance (km) from coast for region 1, Eastern Lake Survey-Phase I [from Linthurst et ai. (73)].
"oo6
1 \:
function of Na:Cl ratio was not significant for lakes in closer proximity to the coast ( 6 5 km), further suggesting watershed sources of Na+ due to weathering rather than retention. Results similar to those presented in Table I were obtained when analyses were restricted to drainage lakes only or to lakes without paved roads in their watersheds (presumably no road salt). Similar results were . also obtained when analyses were performed within individual alkalinity map strata. Those relationships that were significant derived their significance from high pH/ANC waters with Na:C1 > 1.1. The lack of a significant relationship between ANC and Na:C1 ratio does not by itself, however, demonstrate that Na+ retention has not occurred in the drainages of some lakes. Because lakes vary in their acid/base characteristics, a decrease in ANC due to Na+ retention would be independent of initial lake water ANC. But if Na+ retention was a significant regional process, causing lakes to become acidic (ANC 5 0 wequiv L-l), we should find a cluster of acidic lakes with Na:C1 ratios less than the observed input ratio. None of the acidic lakes in coastal New England showed an Na:C1 ratio less than the observed ratio in precipitation f one standard deviation (0.83 f 0.04) (Figures 4 and 5). An analagous situation exists between SO-: and ANC. Although lake Sod2concentration was not found to be negatively correlated with ANC for the
full data set of Northeastern lakes, almost all low ANC lakes in the Northeast had 502- as the major anion (13, 14). This is taken as strong circumstantial evidence for SOd2--inducedacidification. For the 18 lakes within 20 km of the coast that exhibited Na:C1 less than 0.86, the magnitude of sea salt acidification (eq 5) showed a mean f standard deviation of 28 f 34 pequiv L-l. Of these lakes potentially acidified by sea salt influence, the mean ANC was 240 f 246 pequiv L-l. Half of these lakes had ANC values greater than 150 pequiv L-l and therefore only nine of the low Na:C1 lakes could be significantly acidified by sea salt contribution. Linear regression analysis of the low Na:C1 lakes demonstrated a weak positive correlation between ANC and sea salt acidification [ANC = 131 pequiv L-l + 2.8(sea salt acidification); n = 18; R2 = 0.151, indicating that the extent of acidification by Na+ retention was generally greatest in high ANC lakes. In the low ANC lakes (150pequiv L-l), the potential effect was small in magnitude, but of some significance,showing a mean potential sea salt acidification of 25 pequiv L-l. These calculations should be viewed as approximations because small errors in the analytical determination of Na+ or C1- result in large uncertainty in estimating possible sea salt acidification. Furthermore, the mean observed Na:C1 ratio of coastal precipitation (0.83) was somewhat less than the marine ratio of 0.86 (Figure 2). Sodium ions are, in general, weakly bound in soils (33, 1) and have been reported to move readily through coastal catchments (25). It seems reasonable to assume that soils in coastal watersheds of the Northeast would be approximately at steady state with respect to long-term patterns of NaCl deposition. Long-term acidification due to cation exchange involving marine sources of Na+ therefore seems unlikely in the absence of major climatic changes affecting NaCl deposition. Similar conclusions were reached by Overrein et al. (25) in Norway. Seip (34) also emphasized that long-term accumulation of sodium will not occur in a stable terrestrial system. Mg:C1 ratios were also evaluated because Mg2+may have a higher retention on acidic soils than does Na+ (33),although the Mg2+concentration in seawater is only 23% of the Na+ concentration on an equivalent basis. We did not find evidence of Mg2+retention in watershed soils of drainage lakes. Three lakes within 20 km of the coast had a Mg2+deficit > 100 pequiv L-l, calculated as Mg2+deficit = [C1-](0.196) - [Mg2+] (6) where concentrations are in pequiv L-l. All three of these lakes were of the seepage (no outlet, no inlet) or spring (outlet, no inlet) type. Six coastal lakes had a Mg:Cl equivalent ratio less than that of seawater (0.196) and ANC I50 pequiv L-l. Only three of these were drainage lakes, and they all had a Mg2+deficit less than 24 pequiv L-l. Thus, we find no evidence supporting chronic Mg2+retention in Northeastern coastal drainage systems. We emphasize that samples were collected for this study during fall mixing and should be interpreted as an index of water chemistry on an annual basis only. These results, therefore, only apply for long-term evaluation of water chemistry. Episodic pH depressions caused by high inputs of sea salts can be a significant short-term acidifying phenomenon, as suggested by the research of Skartveit (6) and Kahl et al. (5) on stream systems. Sutcliffe et al. (35) demonstrated that sea salt deposition may show marked seasonal trends, and Skartveit (6) showed that as much as 30% of the annual wet C1- deposition can occur on just five to six episode days. Repeated neutral salt episodes would not, however, lead to long-term acidification. Once
a coastal terrestrial system has reached steady state with respect to long-term Na+ deposition, leachate would actually be less acidic than in an otherwise similar noncoastal site because the soil has become less acidic in response to higher Na+ saturation. Further research is needed to evaluate the potential effects of episodic sea salt inputs on low-alkalinity coastal surface waters. There is no evidence to suggest, however, that the group of low-alkalinity, low-pH lakes identified in coastal areas of region 1by the ELS-I fall survey (13) have become acidic as a result of the neutral salt phenomenon. Na+ deposition declines sharply inland of 20 km, as do lake water concentrations of Na+ and C1- (13,14). Therefore, there is little support for the neutral salt effect having caused a significant increase in the number of acidic Northeastern lakes.
Acknowledgments We gratefully acknowledge the assistance of M. T. Dana and J. Gibson in providing precipitation chemistry data. Valuable criticisms were received from D. F. Brakke, M. R. Church, E. Krug, R. A. Linthurst, S. Norton, and H. M. Seip. Registry No. NaCl, 7647-14-5; Na, 7440-23-5.
Literature Cited (1) Wiklander, L. Geoderma 1975, 14, 93. (2) Rosenqvist, I. Th. Sci. Total Environ. 1978, 10, 271. (3) Seip, H. M.; Tollan, A. Sci. Total Enuiron. 1978,10, 253. (4) Krug, E. C.; Isaacson, P. J.; Frink, C. R. J . Air Pollut. Control Assoc. 1985, 35, 109. (5) Kahl, J. S.; Anderson, J. L.; Norton, S. A. “Water Resource Baseline Data and Assessment of Impacts from Acidic Precipitation”. Acadia National Park, Maine; Technical Report 16; National Park Service, North Atlantic Region: Boston, MA, 1985; 123 pp. (6) Skartveit, A. Nord. Hydrol. 1981, 12, 65. (7) Seip, H. M. In Ecological Impact of Acid Precipitation; Drablos, D., A. Tollan, Eds.; SNSF Project; SNSF: Oslo, Norway, 1980. (8) Ogden, J. G. Water, Air, Soil Pollut. 1982, 17, 119. (9) Lebowitz, L. G.; de Pena, R. G. J.Geophys. Res., D: Atmos. 1985, 90, 8149. (10) Granat, L. Tellus 1972, 24, 550. (11) Kennedy, V. C.; Zellweger, G.; Avanzino, R. Water Resour. Res. 1979, 15, 687. (12) Gorham, E. Philos. Trans. R. SOC.London, B 1958,241,147. (13) Linthurst, R. A.; Landers, D. H.; Eilers, J. M.; Brakke, D. F.; Overton, W. S.; Meier, E. P.; Crowe, R. E. Characteristics of Lakes in the Eastern United States; Population Descriptions and Physico-Chemical Relationships. U.S. Environmental Protection Agency: Washington, DC, 1986; EPA-600J4-86-007A;Vol. 1. (14) Brakke, D. F.; Landers, D. H.; Eilers, J. M. Environ. Sci. Technol. 1988, 22, 155-163. (15) MAP3S Atmos. Enuiron. 1982, 16, 1603. (16) Watson, C. R.; Olsen, A. R. Acid Deposition System (ADS) for Statistical Reporting: System Design and User’s Code Manual; U.S. EnvironmentalProtection Agency: Washington, DC, 1974; EPA-60018-84-023. (17) Omernik, J. M.; Powers, C. F. Ann. Assoc. Am. Geogr. 1983, 73, 133. (18) Eilers, J. M.; Landers, D. H.; Brakke, D. F. Enuiron. Sci. Technol. 1988,22,172-177. (19) Omernik, J. M.; Kinney, A. J. Total Alkalinity of Surface Waters: A Map of the New England and New York Region; U.S.Environmental Protection Agency: Corvallis, OR, 1985; EPA-600lD-84-216, (20) Best, M. D.; Drouse, S. K.; Creelman, L. W.; Chaloud, D. J. National Surface Water Survey, Eastern Lake Survey-Phase I , Quality Assurance Report; US.Environmental Protection Agency: Las Vegas, NV, 1986;
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(21) Hillman, D. C. J.; Potter, J. F.; Simon, S. J. National
Surface Water Survey, Eastern Lake Survey-Phase I , Analytical Methods Manual; U.S.Environmental Pro-
tection Agency: Las Vegas, NV, 1986;EPA-60014-861009. Landers, D. H.; Overton, W. S.; Brakke, D. F.; Linthurst, R. A,; Eilen, J. M. Environ. Sci. TechnoZ. 1988,22,12&135. Mansell, R. S.; Elseftawy, A,; Hunt, P. G. Soil Sci. 1972, 15, 113. Baker, L. A.; Brezonik, P. L.; Edgerton, E. S.; Ogburn, R. W., I11 Water, Air, Soil Pollut. 1985, 25, 215. Overrein, L. N.; Seip, H. M.; Tollan, A. “Acid Precipitation-Effects on Forests and Fish”;’finalreport; SNSF Project; SNSF: Oslo, Norway, 1980. Christophersen, N.; Seip, H. M.; Wright, R. F. Water Resour. Res. 1982, 18, 977. Baker, L. A.; Pollman, C. D.; Brezonik, P. L. “Development of Interpretive Models for Lake Acidification in Florida”; Florida Acid Deposition Study, Phase IV; Environmental Engineering and Sciences, Inc.: Gainsville, FL, 1985;ESE 83-152-0602-0120.
(28) Bayley, S. E.; Schindler, D. W.; Turner, M. A. Presented at the American Society of Limnology and Oceanography Annual Meeting, Madison, WI, June 1987. (29) Junge, C. E.; Werby, R. T. J.Meterol. 1958, 15, 417. (30) Erickson, E. Tellus 1960, 12, 63. (31) Delmas, R.; Servant, J. J. Rech. Atmos. 1974, 8, 861. (32) Mairs, D. F. Water Resour. Res. 1967, 3, 1090. (33) Lag, J. Acta Agric. Scand. 1968, 18, 148. (34) Seip, H. M. Nature (London) 1986, 322, 118. (35) Sutcliffe, D. W.; Carrick, T. R.; Heron, J.; Rigg, E.; Talling, J. F.: Woof. C.: Lund. J. W. G. Freshwater Biol. 1982.12. , , 451. I
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Received for review February 9,1987. Accepted September 14, 1987. The research described i n this article has been funded as part of the National Acid Precipitation Assessment Program by the US.Environmental Protection Agency (EPA). I t has been subjected to the Agency’s peer and administrative review, and it has been approved for publication.
Reactions That Modify Chemistry In Lakes of the National Surface Water Survey Sljln Lee and Jerald L. Schnoor”
Department of Civil and Envlronrnental Engineering, University of Iowa, Iowa City, Iowa 52242 A simple mass balance equation was used to determine the important reactions that occur in selected lake watersheds of the Adirondack Park, the Southern Blue Ridge Province of the Appalachian Mountains, and a portion of northern Florida. The mass balance requires only three input parameters: precipitation ion concentration, lake ion concentration, and an evapoconcentration factor appliable to the watershed. It was determined that the most important alkalizing reaction in all three study areas was the production of base cations (Ca2+,Mg2+,and Na+) by ion exchange and chemical weathering, followed by sulfate adsorption in the Southern Blue Ridge watersheds and nitrate assimilation/reduction in the Adirondack Park. Dry deposition of sulfate was important to the sulfate balance in the Adirondack watersheds and was of the same order of magnitude as wet precipitation inputs. Chloride ion could not be used to provide an independent check on the mass balance methodology because unmeasured sources such as dry deposition and mineral dissolution appeared to be significant, especially in the Southern Blue Ridge Province and Florida watersheds. ~
Introduction The chemistry of lake water in dilute or headwater ecosystems is an admixture of precipitation solutes, dry deposition, and biogeochemical reaction products of the land and water (1). Precipitation water is modified as it passes through the canopy by leaching of vegetation and dissolution of dry deposited materials (2). Throughfall water is further affected by decomposition and mineralization products of leaf litter and organic soil (3). In the upper soil horizons, ion-exchange reactions, ammonia assimilation by vegetation, and microbially mediated nitrification reactions are likely to occur ( 4 ) . As the water percolates into the lower soil horizons, it becomes progressively more neutralized (less acidic) by sulfate exchange reactions and chemical weathering (dissolution) of minerals in the glacial till and bedrock (5-7). Lake chemistry represents a mixture of precipitation, runoff water, in190
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terflow, and groundwater chemistry. Within the lake, further reactions modify the chemistry, including nitrate and ammonium assimilation by plants and reactions at the sediment-water interface (e.g., cation exchange and sulfate reduction; ref 8-10). Lakes that have been acidified by acid deposition are those lacking carbonate minerals and characterized by sensitive hydrological settings (1). Schnoor and Stumm (I) presented various reactions that modify the H+balance in waters. Table I shows the stoichiometry of some of these processes. The overall change in alkalinity (or acid neutralizing capacity, ANC) of a lake can be calculated from the stoichiometry of the weathering reactioq, ion exchange, redox reactions, and biological production and respiration. In some instances, acidic deposition may be sufficient to disturb an existing H+balance and cause the acidification of a lake. A quantitative example is provided by the hydrogen ion budget in the Hubbard Brook Ecosystem by Driscoll and Likens (11). The purpose of this study was to assess (on a large-scale regional basis) which reactions are most important in controlling the acid-base chemistry of lakes, on the basis of a mass balance approach for major ions. There are two data sets that were used for the study. In 1984, the Eastern Lakes Survey of the National Surface Water Survey (NSWS) was begun by the U S . EPA (12,13) in order to assess the present acidity status of surface waters and to establish a base line for temporal variability associated with these surface waters. The second data base, of the Acid Deposition System (ADS) for Statistical Reporting, is a collection of precipitation chemistry data to monitor atmospheric deposition trends in the U.S.(14). The ADS data base was used to determine the chemistry of wet deposition to lakes of the NSWS in the eastern U.S. Lake chemistry was determined from the NSWS in order to estimate the net reaction or change of major cation and anion concentrations (Ca2+,Mg2+,K+, Na+, NH4+,Nos-, C1-, S042-). A simple mass balance equation was then used to calculate the magnitude of important processes that
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0 1988 American Chemical Soclety
