Environ. Sci. Technol. 1089,23, 1258-1263
(20) Burdick, N. F.; Bidleman, T. F. Anal. Chem. 1981, 53, 1926-1929. (21) Foreman, W.T.;Bidleman, T. F. J. Chromatog. 1985,330, 203-216. (22) Ballschmitter, K.;Zell, M. Fresenius 2.Anal. Chem. 1980, 302,20-31.
(23) Mullin, M. D. PCB Workshop, Grosse Ile, MI, 1985. (24) Bidleman, T.F. Environ. Sci. Technol. 1988,22,361-367.
Received for review November 15,1988. Accepted April 25,1989. This work was supported by the Environmental Technology Division of the Westinghouse Electric Corp.
Biogeochemical Processes Affecting Arsenic Species Distribution in a Permanently Stratified Lake Patrick Seyier' and Jean-Marie Martin Institut de BiogQochlmie Marine, Unit6 Associ6e au CNRS 386, Ecoie Normale Superieure, 1 Rue Maurice Arnoux, 92 120 Montrouge, France
Arsenic species, iron, and manganese distributions were studied in the oxidizing and reducing waters of Lake Pavin, a small and well-stratified crater lake situated in the Maasif Central range (France). Arsenate and arsenite concentrations versus depth do not reflect the expected thermodynamic equilibria, indicating a slow and incomplete response to the redox conditions. The occurrence of arsenic in the anoxic zone results from transport on a particulate phase, due to adsorption onto iron and manganese oxides and probably to incorporation in phytoplanktonic organisms. Introduction
In recent years attention has been paid to those elements that exhibit multiple oxidation states in water. Arsenic (1, 2 ) , iodine and chromium (3), antimony (4, 5 ) , and selenium (6,7) are of particular interest to evironmentalists because certain of them may be more toxic than others. It has been shown that arsenic can be found in natural systems as arsenite [As(III)],arsenate [As(V)],and organic arsenic species (1,8-10). Thermodynamic calculations lead to the prediction that, at equilibrium, As(V), should be the only stable oxidation state in oxic water, whereas in anoxic systems As(II1) should be the stable dissolved form (11). Indeed, in waters containing dissolved oxygen, arsenate is the dominant species but arsenite is present in significant amounts (10% of total As). In anoxic basins or in the pore water of sediments, arsenite is found at concentrations exceeding those of arsenate, but arsenate is still present (3,12,13). Therefore the redox couple of arsenic does not appear to be in thermodynamic equilibrium, either in oxic or in anoxic systems. Several biotransformations, which could explain this disequilibrium, have been found to occur in laboratory cultures: redox transformation between As(II1) and As(V) by bacteria, fungi, and planktonic algae; biosynthesis of complex organoarsenic compounds by organisms and subsequent degradation giving stable methylated species (14-1 7). The aim of this paper is to determine which biological or chemical processes are controlling the redox state and the chemical cycle of this element across an oxic-anoxic interface. Due to its permanent anoxic deep layer, Lake Pavin, a deep crater lake in the volcanic range in the center of France (Figure 11, appeared to be most suitable for such a purpose. It is located at 45O55' N, 2'54' E, far from important industrial zones at an altitude of 1200 m. The main characteristics of the lake have been described by Martin (18). Owing to its restricted and forested watershed-a factor limiting mechanical erosion-and to the complete dominance of diatoms in the lake flora, the 1258
Environ.
Sci. Technol., Vol. 23, No. 10, 1989
bottom sediments are almost pure diatomaceous deposits with negligable detrital material. Chemical conditions within the water column vary from a well-oxygenated upper layer, through a transition zone between 50 and 65 m where oxygen concentrations decrease to zero and hydrogen sulfide appears, to a permanently anoxic bottom layer. According to limnological classification, the Pavin lake can be considered as strictly meromictic. As suggested by previous studies (19-22) the lake water budget must be balanced by an additional input from sublacustrine freshwater springs. Using mathematical modeling based on tritium data, Meybeck et al. (19) and Martin (18)have shown that such sublacustrian inputs would occur in the anoxic compartment. In addition to the determination of As(II1) and As(V) in the water column and in the surface layer of sediments, we determined the concentrations of dissolved oxygen, hydrogen sulfide, iron, and manganese and the pH. We measured all these parameters simultaneously and at close enough intervals near the oxic-anoxic interface to determine the detailed structure of the oxidized and reduced species profiles. Materials and M e t h o d s
Sampling. Samples were collected in December 1984. Owing to the small area and circular shape of Lake Pavin, sampling was performed at the middle and deepest part of the lake. In order to collect appropriate samples at the oxic-anoxic boundary, preliminary measurements of dissolved oxygen, temperature, and transmissivity were performed prior to sampling. Temperature and dissolved O2were determined with an Orbisphere Model 2609 apparatus, and transmissivity was obtained with a Montedoro-Whitney transmissometer. The water samples were collected in a precleaned Niskin bottle hung on a nylon hydrowire. To prevent oxidation of reduced species, water was immediately filtered on board through a syringe filtering assembly (Millipore, Swinnex) attached at the outlet of the Niskin bottle. Any contamination by air is thus avoided during the collection of the sample. The Nuclepore filters (0.4pm) were soaked before use in diluted hydrochloric acid. Two filtered subsamples were sealed in precleaned bottles; one for determination of arsenic species was stored in the dark at 4 OC,and another was acidified with 1%HNOB(Suprapur, Merck Inc.) to determine dissolved iron and manganese. All samples were analyzed for As less than 72 h after sampling. The effects of storage on the determination of As(II1) have been previously discussed (23). This study showed that no change in the As speciation was detectable for 240 h under
0013-936X/89/0923-1258$01.50/0
0 1989 American Chemical Society
Table I. Fe, Mn,As(III), As(V), and Total As in the Pavin Lake Waters and Neighboring Springs sample
Fe
Mn
As(II1)
Pretres spring surf 20 m 40 m 55 m 58 m 61 m 64 m 70 m 75 m 85 m 89 m 90 m
20.65 3.34 12.50 2.82 4.22 6.32 9600 27190 47600 46850 52850 54980 58600
1.47 5.59 6.75 3.21 130 400 1030 1040 1210 1350 1340 1350 1390
0.00 0.04 0.04 0.04 0.04 0.04 2.25 4.10 4.80 5.20 5.40 5.20 8.25
concn, pcg.L-' AsW) As(tot) 0.69 0.52 0.45 0.54 0.30 0.28 2.15 3.05 4.40 3.80 1.70 1.10 0.95 I ,
\ m
DIATOMS
\
I
IOXYGENATED)
As(III)/As(V)
5.5 7 8 7 12 12 51 57 52 58 76 82 90
0.08 0.08 0.07 0.13 0.14 1.05 1.34 1.09 1.4 3.2 4.7 8.68
,5
"' , F a
I( 7. SAT ,p
'p
a,*
rna,w
1,n
I
(ANOXIC/
I , 0
% As(III)/As(tot)
0.73 0.56 0.49 0.58 0.34 0.32 4.40 7.15 9.20 9.00 7.10 3.30 9.20
500
800
m
Figure 1. Schematic representation of Pavin crater lake. (PL V and PL VI are the location of the sedlment cores).
Figure 2. Stratification characterlstlcs of the Pavin lake. (Temperature, dissolved 02,pH, and transmissivity are measured in situ.)
experimental conditions [4 "C, pH 7, initial As(II1) concentration of 5 pgL-']. Sediments were collected during another survey with a modified version of Burke's corer (24): core PL V was collected below the oxic layer and core P L VI from the bottom of the anoxic layer. Analysis. Arsenic species were determined by the method of selective hydride generationlatomic absorption spectrometry (25,261. Arsenic(II1) and -(V) were selectively reduced with sodium borohydride to their corresponding hydrides. For the determination of As(II1) the samples were buffered at pH 4.8 by the addition of acetic acid-sodium acetate 1M buffer. The arsine produced was flushed from the solution by an argon stream and carried out into a quartz cell heated at lo00 "C and aligned in the beam of the atomic absorption spectrophotometer. Total dissolved arsenic was determined by the same procedure, but HC1 was added instead of the acetate buffer until the pH reached 1.0. Methylated As species were not reported in this study, as the analytical system was not optimized for their detection. Nevertheless, specific responses from organoarsenic species such as monomethylarsonic acid (MMAA) and dimethylarsinic acid (DMAA) were obtained at pH values less than 5 and they were consequently included in the total dissolved As determination (27). In view of the low temperature of the lake waters during the survey, it might be reasonably expected that organoatsenic species did not represent more than 10% of the total As concentration (28). Hydrogen sulfide was known to interfer in the determination of metalloids such As and Sb (5, 29, 30). To eliminate this problem without interfering with the As
species determination, the sample was degassed for 10 min rather than 2 min and the background corrector utilizing a deuterium lamp was systematically used (5). Hydride generation technique had been successfully applied to the marine anoxic waters at H2S levels comparable to those found in the Pavin lake waters (13). The apparatus used was a modified MHSlO (Perkin-Elmer) coupled with a Perkin-Elmer 30-30 atomic spectrophotometer equipped with an electrodeless discharge lamp. Iron and manganese were determined by atomic absorption spectrophotometry either with flame or flameless atomization. This last technique, called the "stabilized temperature platform furnace" method (31), was performed with a Perkin-Elmer HGA 500 graphite furnace. The detection limits for arsenic, manganese, and iron by these methods were 0.02,0.2,and 0.2 pgL-l, respectively. The precision was between 5 and lo%, depending on the element and its concentration. The hydrogen sulfide profile was computed from the data of F. Restituto (32). Particulate arsenic in sediments was determined according to a procedure similar to that used for dissolved arsenic but after hot chemical digestion in Teflon bombs. The digested sample was first oxidized with a mixture of HClO., and HNO,, dissolved with HF, evaporated to near dryness, and then diluted to 100 mL with HCl(O.6 M) (33). Replicate measurements of both samples and certified standards indicated that the standard deviation (lu)from the mean is below 10%.
Results The concentration of chemical species in the water column and in the sediments are given in Tables I and 11. Environ. Sci. Technol., Vol. 23, No. 10, 1989 1259
Figure 3. Vertical distribution of dissolved Fe, Mn, As(III), As(V), and total As concentrations in Pavin lake waters.
Table 11. As, Fe, and Mn Concentrations in the Pavin Lake Sediment Cores PL V and PL VI depth, cm
As, pgg-'
Fe, 9%
Mn, 70
Sediment Core PL V 0-2 2-4 4-6 6-8 10-12 12-14 14-16 20-22 30-32 40-42
5
1.08
8
0.88 0.88
20 10 8 5 10 5 4 13
0.92 0.93 0.77
0.017 0.017 0.017 0.015 0.013 0.012
0.84 0.74 0.78
0.013 0.011 0.012
Sediment Core PL VI 0-0.2 1-2 2-3 3-4 4-5 5-6 6-7 9-11 11-13 13-15
45 48 45 45 40 45 53 65 50 70
1.48 2.25 2.22 2.19 2.19 2.13 2.06 1.99 1.92 1.60
0.032 0.046 0.057 0.059 0.060 0.055 0.055 0.051 0.023 0.012
As illustrated in Figure 2, one may distinguish three stratified layers: Above the thermocline, located at a depth of 30 m, the upper layer of the mixolimnion is superoxygenated, with a pH value close to 7 and a transmissivity almost constant throughout the water column. In this layer chemical conditions are highly oxidizing. Below the thermocline, dissolved oxygen and pH decrease with depth and the temperature reaches a broad minimum (4"C). The bottom of this layer is characterized by the total depletion of dissolved oxygen and the occurrence of hydrogen sulfide simultaneously with a high turbidity near 61-m depth. The turbidity may result from accumulation of organic detritus, from a high bacterial population, or from iron and manganese oxyhydroxides at the top of high density water. The deeper layer, or monimolimnion, is totally anoxic and characterized by a thermal inversion of almost 1 "C. Its pH ranges from 6.0 to 5.75 at the bottom. The degradation of organic matter mainly associated with diatom tests is primarily responsible for the elevated hydrogen 1280
Envlron. Sci. Technol.. Vol. 23, No. 10, 1989
sulfide Concentration (0.6 mgL-') and the low redox potential occurring in this layer. The maintenance of the deep stratification results from the high total dissolved solids and high density of bottom water; spring and autumnal overturns reach depths of only 55-65 m. From mass balance considerations sublacustrine spring inputs to Lake Pavin have to be highly mineralized. The use of major and minor elements (Na, K, Ca, and Li) as geothermometers (34) indicates an original temperature of these sublacustrine inputs averaging 100 "C (39,consistent with the bottom temperature inversion. The chemical stratification is particularly well illustrated by the vertical distribution of total As, Mn, and Fe (Figure 3). The bottom concentrations are respectively 10, 200, and 5000 times higher than the corresponding top values. According to Davison (361,such convex-shaped profiles of iron and manganese indicate that almost total solubilization of particulate material occurs in the water column. In the upper layer the major elements @io2, Ca2+,Mg2+, Na+, K+, C1-, Sod2-, HNO,-) (23),as well as total arsenic, iron, and manganese concentrations, are relatively low and very similar to those of surficial inputs to the lake, as compared, for example, to Pretres spring composition (Table I). Arsenic(V)is the most abundant oxidation state of this element. However, the reduced form, As(III), is present at detectable concentrations throughout the water column and represents 5-10% of total dissolved concentrations. Below the thermocline the pentavalent As and total Fe concentrations decrease significantly, while the As(II1) concentration remains unchanged. The chemocline (50-65 m) is characterized by a drastic increase of Fe, Mn, and As. Manganese, the reduction of which occurs at pE values higher than those required for iron and arsenic, is reduced first, well before complete depletion of dissolved oxygen. The most telling feature in the redoxcline at 61-m depth is the rapid and correlative increase of Fe and As concentrations and a reversal of arsenic speciation. As(II1) concentrations increase from 0.04 c~g.L-labove the 02/H.&l interface to 2.25 pgL-' below, where this species becomes dominant. Surprisingly, arsenate also increases from 0.28 ~ g L -above l the interface to 2.12 pgL-I immediately below and remains high until 70-m depth. Below this depth and down to the water-sediment interface, total arsenic concentrations decrease gradually. In a sediment core collected above the redoxcline (PL V), As concentrations average 10 Ngg-l in the top 8 cm of sediment, whereas the
Table 111. Redox Half-Reactionsand Thermodynamic Equations for As, Os, and S Species for pE-pH range Occurring in the Pavin Lakea HzAsOd- + HASO,'-
+ H+
H3As02 + HzAsOC + H+
Arsenic Acid Dissociation pE = 6.96 Arsenious Acid Dissociation pE = 9.3
Redox Half-Reactions H3As02 + HzO ==H2As0c + 3H+ + 2epH + 2/3pE = 7.35 HSAsOl + HzO + HAsOlz- + 4H+ + 2epH + '/ZpE = 7.25 HzSo 4Hz0 * S042- + 10H+ + 8epH + 4/spE = 4.1 HS- + 4Hz0 S042- + 9H+ + 8epH + '/SpE = 3.8 HzSo H+ + HSpH = 6.9 2As + 3HzS0+ AszS3(s)+ 6H+ + 6epH + pE - '/ZpHzS = -2.5 2AsS(s) + HzSo As2S3(s)+ 2H+ + 2epH + pE - '/ZpHzS = -0.55 pH + '/ZpE + '/ZpAsS< - '/ZpHzS = 6.15 AsS(s) + HzSo+ Assz- + 2H+ + leAszS3(~)+ HzSO + 2AsSZ- + 2H+ pH + PA&- - '/zpHzS = 12.3
+
a
Gibbs free energy data are taken from ref 10 and 42.
concentrations in the deep core (PL VI) ranged from 40 to 70 ppg-'. Discussion Arsenic Speciation in the Upper Layer (0-50 m). Arsenic(II1) and -(V) are found in the upper layer of Lake Pavin. The thermodynamically unstable form, As(III), has been observed in oxic surface waters of lake and oceans. Generally, arsenate and phosphate are depleted in the same area (2,9,13). The concomitant depletion of As(V) and PO4* and the occurrence of As(II1) presumably results from the limited selectivity of phosphate uptake by phytoplankton. Direct evidence for reduction and methylation of arsenic species by algae has been reported in laboratory cultures (17) and ocean and river waters (1, 28, 37). Howard et al. (28)noted that the rate of bioreduction of arsenic is controlled by seasonal variations of phytoplankton populations, which are themselves dependent on surface water temperature, ambient light levels, and nutrient availability. In Lake Pavin, the concentration of the planktonic populations ranged from lo4 to lo6 ce1ls.L-l during winter (38).While the biological activity may have been insufficient to totally deplete As(V) in surface waters, it was apparently sufficient to reduce a measurable amount of this species. Arsenic concentrations averaged 9 ppg-' (dry weight) in the biogenic matter of the lake, which is mainly constituted of diatoms. The diatom species Melosira italica represents between 60 and 80% of total cell numbers. This relatively high value reflects the great ability of this algal species to take up arsenic. Below the thermocline the decrease of As(V) and Fe concentrations cannot be explained by biological uptake. This depletion is probably linked to iron oxyhydroxide formation. Adsorption and coprecipitation of arsenic with iron and manganese oxides have been shown in lacustrine or estuarine systems by various authors (39-41). Ferguson and Anderson (42)observed that arsenate was much more strongly adsorbed than arsenite onto iron and aluminium hydroxides. Adsorption of As(V) is at a maximum at pH 5.5, with a drastic decrease at both lower and higher pH values (43). This could explain the relative enrichment in arsenite just below the thermocline. Arsenic Speciation at the Redoxcline (50-65 m). The equilibrium speciation of arsenic at the redoxcline can be obtained by simple thermodynamic calculations. At pH 6, assuming that S042-/H2Sis the controlling couple of redox potential, the concentration of hydrogen sulfide below 61-m depth corresponds to pE N -2.8 (-0.165 V). By use of these values and equilibrium constants calculated from Gibbs free energy data (see Table 111),the theoretical
-
As(III)/As(V) ratio may be calculated to be 1O'O. The actual measured ratio is 1.05 (Table I). It is thus clear that arsenic is no more at equilibrium across the redoxcline than it is in surface water. If a strict thermodynamic interpretation fails to explain the occurrence of As(V), and even less its increase below the oxic-anoxic boundary, other mechanisms must be invoked. Since the exchanges by diffusion or advection between upper and deeper layers can only provide a negligible input of the arsenic(V) to the anoxic waters, a particular downward transport mechanism must be considered. This is evident by release from iron hydroxides. The highly correlated increases of pentavalent arsenic and dissolved iron concentrations between 60 and 70 m reflect the gradual solubilization of iron oxyhydroxides and the concomitant desorption of As(V). The marked decrease of pH at 60-m depth furthers the solubilization of colloidal hydroxides and the desorption of As(V). In addition to the desorption process, the release of biogenic arsenic from diatoms may also contribute to the occurrence of arsenic(V) in the deeper waters. The incorporation of As(V) into the biogenic particles in surface waters, its vertical transport, and its release to solution (as the result of particle degradation) can lead to the observed occurrence of As(V) in the anoxic layer. This mechanism could play an important role in Lake Pavin if the maximum turbidity evidenced by the nephelometric profile is interpreted as the accumulation of diatom tests. In situ measurements using sediment traps could confirm this hypothesis. Arsenic Speciation in the Anoxic Layer. In the deep waters the As(III)/As(total) ratio presents a steady increase from 52% at 70 m to 90% a t the water-sediment interface. To explain this fact a low reduction rate must be invoked. Several authors observed the occurrence of As(V) in anaerobic environments: e.g., in the deep waters of Saanich Inlet (12),the Baltic Sea (13),or in anoxic pore waters from lacustrine, estuarine, and oceanic sediments (3, 38, 44). In all cases, the observed disequilibrium speciation is attributed to a kinetic control of the arsenate reduction reaction. The kinetics of the arsenate-arsenite transformation in natural waters are not well understood, but are known to be chemically slow (12,14, 15). Reduction reactions in natural waters could be catalyzed by bacteria (15).Experiments measuring arsenate reduction rate carried out by Peterson and Carpenter (12)gave a value of -0.07 nmol-l-l-day-' without antibiotics. The conditions of the experiments [T= 6 OC, [H2S] = 0.8 mgL-', [As(V)] = 0.1 bpL-'] were close to the natural conditions occurring in the reducing water of Lake Pavin. Environ. Scl. Technol., Vol. 23, No. 10, 1989
1261
t
91
AsV,
21 ASIII
I
II/ MnS(s)
SEDIMENT
Figure 4. Conceptual madel of As behavior in the Pavin lake. (1) Uptake, transport, and release of As(V) by diatoms; (2) adsorption onto iron hydroxide and downward transport; (3) As(V) reduction in the anoxic zone: (4) adsorptlon-coprecipitation of As(II1) wtth Iron sulfide.
The presence of sulfide in deep water complicates the redox speciation of As. Sulfide influences the chemical forms and solubilities of iron, manganese, and arsenic. If we follow Peterson and Carpenter (12) and assume that the equilibrium between arsenite and thioarsenite is described by the equation H ~ A s O+~2HS-
+ H+ = A&- + 3Hz0
the As(III)/AsS2- ratio may be calculated. The HS- concentration is obtained from the H a concentration between 70- and 90-m depth mo1.L-'). The calculated ratio is -10l2, suggesting that thioarsenite should not be an important species in the anoxic layer. Similar ratios have been calculated in Saanich Inlet (12). In the same way, the solubility product of iron calculated for the ambient H2S concentration and pH is much lower than the measured dissolved concentrations, suggesting that Fe in the deep layer of Lake Pavin may be present in a colloidal form. Concerning the depletion of totalarsenic concentration just below the water-sediment interface, previous measurements (45) have shown a decrease of Fe(I1) concentrations between overlying waters (56 mgL-I) and sedimentary pore waters (42-44 mg.L-') in the first 5 cm. Coprecipitation of As with iron sulfide leading to the arsenopyrite formation (FeAsS) should be the predominant mechanism in the deeper layer of Lake Pavin. Similar mechanisms have been proposed for a Norwegian fjord where the major carrier phase for most metals was represented by pyrite framboids (46). Conceptual Model of the Arsenic Biogeochemical Cycle in Lake Pavin. The essential features of As biogeochemical behavior are illustrated in Figure 4. In summary, arsenic speciation and distribution in this lake appear to be dominated by redox processes complicated by physicochemical processes such as adsorption, coprecipitation, and desorption and biological phenomena such as uptake and regeneration. In a gross way the arsenate/ arsenite concentration ratio in the oxic and anoxic layers indicates a trend toward thermodynamic equilibrium; As(V) represents 88-95% of total dissolved arsenic in the oxic compartment, As(II1) is the dominant species in the 1262
Environ. Sci. Technol., Vol. 23, No. 10, 1989
anoxic zone (50-90%). A slow reduction rate from As(V) to As(II1) may be responsible for the incomplete response of the As redox couple to reducing conditions in the deep layer. Adsorption and coprecipitation of dissolved As by solid phases, and probably biological uptake and release, may control vertical transport of As in particulate form. To maintain a steady concentration of As in both layers, this model requires a sink for arsenic, iron, and manganese into the sediments below the anoxic layer. To verify the existence of this sink we can compare Fe, Mn, and As concentrations in the sediment core collected above the redoxcline (PL V) with those of the core (PL VI) collected below the redoxcline (Table 11). In PL V samples, As concentrations are close to those measured in particulate matter collected in lake surface waters (10 and 9 pgg-', respectively). By contrast, the PL VI sediment shows a drastic enrichment of As, Mn, and Fe. Arsenic concentrations ranged from 40 to 70 pgg-' and represent 4-7 times the concentrations determined in the PL V sample. In addition, arsenic and iron profiles in PL VI sediment are very similar to each other (correlation coefficient r = 0.85 for 13 pairs of data); this is consistent with the formation of arsenopyrite. The validity of this conceptual model is thus independently confirmed by the patterns observed for Fe, Mn, and As in the sediments of Lake Pavin. Acknowledgments
We thank F. M. M. Morel for the helpful comments and suggestions and P. Newton for editing improvements. Registry No. As,7440-38-2; Fe, 7439-89-6; Mn,7439-96-5; 02, 7782-44-7.
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Received for review November 19, 1987. Revised manuscript received November 22,1988. Accepted May 23,1989. This study has been supported by CNRS (A.T.P. Oceanographie Chimique, GRECO-ZCO and U.A. 386).
Use of Multivariate Analysis for Determining Sources of Solutes Found in Wet Atmospheric Deposition in the United States Rlchard P. Hooper' and Norman E. Peters US. Geological Survey, 64818 Peachtree Industrial Boulevard, Doraville, Georgia 30360
A principal-components analysis was performed on the major solutes in wet deposition collected from 194 stations in the United States and its territories. Approximately 90% of the components derived could be interpreted as falling into one of three categories-acid, salt, or an agricultural/soil association. The total mass, or the mass of any one solute, was apportioned among these components by multiple linear regression techniques. The use of multisolute components for determining trends or spatial distribution represents a substantial improvement over single-solute analysis in that these components are more directly related to the sources of the deposition. The geographic patterns displayed by the Components in this analysis indicate a far more important role for acid deposition in the Southeast and intermountain regions of the United States than would be indicated by maps of sulfate or nitrate deposition alone. In the Northeast and Midwest, the acid component is not declining a t most stations, as would be expected from trends in sulfate deposition, but is holding constant or increasing. This is due, in part, to a decline in the agriculture/soil factor throughout this region, which would help to neutralize the acidity. ~~
Introduction The National Atmospheric Deposition Program/National Trends Network (NADP/NTN) wet-deposition Not subject to
U.S.
monitoring network was established to determine the geographic patterns and temporal trends in atmospheric deposition, particularly acidic deposition (1). To date, analyses of data from this network have included the development of isopleth maps describing the geographic distribution of solutes and regional trends in deposition (2),an analysis of the spatial variability of the deposition (31, a comparison of deposition in the eastern and western United States (41, and the application of nonparametric statistical techniques to detect trends in deposition at single stations (5). All of these analyses considered individual solutes. However, the characteristics of individual solutes in defining source/receptor relations or determining the effect of control strategies can be misleading because the solutes may arise from more than one source (e.g., both fossil fuel combustion and sea salts can contribute to sulfate deposition). Rinella and Miller (6) did consider multiple ions in their analysis of the 1983 NADP/NTN data, but used them only for a qualitative comparison of different regions of the country. In this paper, we apply a standard multivariate technique, principal-components analysis (PCA), to perform a quantitative apportionment of the solutes found in wet deposition to their sources. PCA uses the covariation of solutes to determine new variables that are linear combinations of the solutes. These new variables are inter-
Copyright. Published 1989 by the American Chemical Society
Environ. Sci. Technol., Vol. 23,No. 10, 1989
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