Applied Geochemistry 22 (2007) 1209–1216

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AppliedGeochemistry

Speciation of aluminum in circumneutral Japanese stream waters

Masami Kanao Koshikawa a,*, Takejiro Takamatsu a, Seiichi Nohara a,Hideaki Shibata b, Xiaoniu Xu b,1, Muneoki Yoh c, Mirai Watanabe d,

Kenichi Satake e

a National Institute for Environmental Studies, Tsukuba 305-8506, Japanb Hokkaido University, Nayoro 096-0071, Japan

c Tokyo University of Agriculture and Technology, Tokyo 183-8509, Japand Chiba University, Matsudo 271-8510, Japan

e Rissho University, Kumagaya 360-0194, Japan

Available online 20 March 2007

Abstract

Speciation of Al, including inorganic monomeric Al (Ali, the sum of aquo, hydroxy and inorganically complexedforms), organic monomeric Al (Alo, the organically complexed form), and colloidal mineral Al (Alc, the fine particulateform that passes through a 0.4-lm pore size membrane filter), was investigated in stream waters (pH 6–8) of 4 watershedsin Japan. Total dissolved Al (Alt, the sum of Alc, Ali and Alo) ranged from 0.03 to 3.31 lM, and Alc was a minor com-ponent (<22% of Alt) in most of the streams. Ali was dominant (71% of Alt) in stream waters with low concentrations ofAlt (<0.25 lM), while the Alo fraction (37%) was almost as large as the Ali fraction (39%) in the highest class (Alt > 1 lM).In spite of the variation in the range of reactive Al (Alr, the sum of Ali and Alo) in the 4 watersheds (Miomote, 0.03–3.27 lM; Tsukuba, 0.06–0.71 lM; Dorokawa, 0.05–0.71 lM; Tama, 0.03–0.38 lM), the entire data set for Alr could beexpressed as a function of the ratio of dissolved organic C (DOC) and Ca: [Alr (lM)] = 0.13 [DOC/Ca (mol/mol)] + 0.11 (r = 0.86, P < 0.001). Alr increased in proportion to the DOC/Ca ratio in Japanese stream waters. Althoughacidic deposition in Japan has already resulted in elevated concentrations of NO�3 þ SO2�

4 in stream waters, a high level ofCa (instead of Al) is serving as a major counterion for NO�3 þ SO2�

4 . However, an additional loading of acidic depositionmay result in shortage of Ca and mobilization of Al as a counterion for NO�3 þ SO2�

4 , and continuous observation of thespeciation of Al in Japanese stream waters may reveal future change in the conditions for mobilization of Al from ‘‘low Caand high DOC’’ to ‘‘low Ca and high NO�3 þ SO2�

4 ’’.� 2007 Elsevier Ltd. All rights reserved.

0883-2927/$ - see front matter � 2007 Elsevier Ltd. All rights reserveddoi:10.1016/j.apgeochem.2007.03.013

* Corresponding author. Fax: +81 29 850 2576.E-mail address: mkanao@nies.go.jp (M.K. Koshikawa).

1 Present address: Anhui Agricultural University, Hefei 230036,PR China.

1. Introduction

Speciation of Al is important because inorganicmonomeric Al (the sum of aquo, hydroxy, andinorganically complexed forms) is more toxic thanorganically complexed Al. In areas where seriousacidification enhances the mobilization of Al,

.

1210 M.K. Koshikawa et al. / Applied Geochemistry 22 (2007) 1209–1216

speciation of Al has been intensively studied. Cro-nan and Schofield (1990) discussed importantinter-regional differences in speciation of Al in soilwaters and stream waters of forested watersheds ineastern North America and northern Europe. Pal-mer and Driscoll (2002) observed a significantdecline in Al in eastern North America in responseto decreasing acidic deposition and estimated thatinorganic monomeric Al in stream water will fallto below the toxic threshold for fish (3 lM) within�10 years if observed rates of decline aremaintained.

In Japan, soil solutions in some areas are alreadyacidified (pH 4–5), and speciation of Al in soil solu-tions (Sato and Wakamatsu, 2001; Umemura et al.,2003) is consistent with observations in northernAmerica and Europe that organically complexedAl is the dominant form of Al in soil solutions withlow concentrations of Al, while inorganic mono-meric Al dominates in soil solutions with high con-centrations of Al (Cronan and Schofield, 1990).Stream waters in Japan (pH 6–8) are not yet acidi-fied, and thus speciation of Al in stream watershas received little attention. In this study, speciationof Al in stream waters in Japan was examined toinvestigate a system under latent acidification and

Niigpref

Hokpref

Tokmet

Ibarpref

Fig. 1. Locations of the 4 watersheds studied. Streams selected have foand their catchment sizes are relatively small.

to determine the baselines for long-term study ofinorganic monomeric Al in Japanese stream waters.

2. Samples and analysis

Four watersheds in Japan were selected for thisstudy. The locations of the watersheds are shownin Fig. 1, and acid deposition levels for each arelisted in Table 1. The Miomote watershed (bedrock:granite) is in Niigata prefecture, where acidic depo-sition (mainly SO2�

4 Þ is believed to be transportedfrom China (Bellis et al., 2005). The Tsukuba (gran-ite) watershed is exposed to acidic deposition ofNO�3 derived mainly from local automobile traffic(Ohte et al., 2001), and the Tama watershed (shale,sandstone and limestone) is leeward of Tokyo and isthus exposed to heavy acidic deposition (Yoh et al.,2001). The Dorokawa watershed (andesite) isremote from any source of acidic deposition (Ogawaet al., 2006). Numbers of streams, times sampled,samples and sampling periods of the 4 watershedsare also listed in Table 1.

Water samples were filtered through a 0.4-lmpore size membrane filter and then analyzed byHPLC (cation-exchange chromatography with fluo-rescence detection of the Al–lumogallion complex;

ataecture

kaidoecture

yoropolitan prefecture

akiecture

Miomote watershed(105 streams, 0.01-20 km2)

Dorokawa watershed(41 streams,Catchment size: 0.01-38 km2)

Tama watershed(42 streams, 0.01-8 km2)

Tsukuba watershed(68 streams, 0.01-2 km2)

Nayoro city

Murakami city

Tsukuba city

Hachioji city

rested watersheds that receive no inflows from human activities,

Stream water samples

HPLC:

Lumogallion:

ICPMS:

MINEQL+:

AlHPLC = Alic + Alo

Alt = Alc + Alaq + Alic + Alo

Alic

Alr = Alaq + Alic + Alo

Ali = Alaq + Alic = Alr - Alo

Alc = Alt - Alr

Alo = AlHPLC - AlicCalculation:

Filtration (0.4-μm pore size membrane filter)

Fig. 2. Flowchart showing the procedure of Al speciation.

Table 1Observational setting, acid deposition levels, and sampling detail

Watershed

Dorokawa Miomote Tama Tsukuba

Acid deposition levela

Annual precipitation (mm) 1300 2100 1400 1300H+ deposition (mmol m�2 yr�1)a 10 54 51 20NO�3 deposition (mmol m�2 yr�1)a 17 32 34 49SO2�

4 deposition (mmol m�2 yr�1)a 19 54 32 26

Sampling

Number of streams 41 105 42 68Number of times sampled for each

stream1–32b 1–4 1 1–15c

Number of samples 234 224 42 142Dates September 2002–

January 2005May, July, October2002, May, August 2003

August 2001–November 2001

November 2001–November 2004

Monthly sampling was conducted at 6 streams (b) and 4 streams (c).a Acid deposition levels for each watershed were calculated using wet deposition data from the following sources: Dorokawa, Ogawa

et al. (2006); Miomote, Environmental Laboratories Association (2005); Tama, Tokyo Metropolitan Research Institute for EnvironmentalProtection (2004); Tsukuba, authors’ unpublished data.

M.K. Koshikawa et al. / Applied Geochemistry 22 (2007) 1209–1216 1211

Sutheimer and Cabaniss, 1995) for the sum(AlHPLC) of inorganically complexed Al (Alic) andorganically complexed Al (Alo), by the lumogallionmethod (Koshikawa et al., 2002) for reactive Al(Alr, the sum of aquo and hydroxy Al (Alaq), Alic,and Alo), and by inductively coupled plasma massspectrometry (ICPMS) for total dissolved Al (Alt)(Fig. 2). Alic concentrations were also calculated,using the MINEQL+ computer program (Schecherand McAvoy, 2003) with concentrations of F�

and SO2�4 (analyzed below) and equilibrium con-

stants (from the database within MINEQL+). Con-centrations of Alic were found to be negligible in allbut 4 of the samples (the low-pH samples from theDorokawa watershed). Concentrations of Alo werethen derived from AlHPLC–Alic. Therefore, concen-trations of inorganic monomeric Al (Ali, the sumof Alaq and Alic) are derived from Alr–Alo, and con-centrations of colloidal mineral Al (Alc), which is afine particulate form less than 0.4-lm i.d. and notdetected by the lumogallion method (Hydes andLiss, 1976), are derived from Alt–Alr. The detectionlimit for Al by HPLC, the lumogallion method, andICPMS was 0.01 lM. The relative standard devia-tions of analytical values (RSD) were 5% for Altand Alr and 10% for Alo. The RSDs for Ali (calcu-lated as 100 · dAli/Ali; dAli is the sum of dAlr anddAlo, which are the standard deviations for Alrand Alo, respectively) were 5–40%. Those for Alc(calculated as 100 · dAlc/Alc; dAlc = dAlt + dAlr)were 6–840%. Alc data with the RSDs above 100%

(165 samples) and Alc data below 0.01 lM (120samples) were treated as ‘‘not detected = zero’’ inthe calculation in Tables 2 and 3 and the correlationanalysis in Section 3.3.

Each filtered sample was also analyzed by ionchromatography (Dionex, DX-100) for F�, NO�3and SO2�

4 by inductively coupled plasma atomicemission spectroscopy (Nippon Jarrell–Ash, ICAP-750) for dissolved Na and Ca and by a total organicC meter (Shimadzu, TOC-5000) for dissolvedorganic C (DOC).

Table 2Mean concentration of each form of Al within each class of Alt

Class of Alt (lM) Population Mean concentration of each form of Al (lM)

Alt Alr Alc Ali Alo

0–0.25 331 0.15 0.13 (87%) 0.02 (13%) 0.11 (71%) 0.02 (16%)0.25–0.50 207 0.34 0.28 (83%) 0.06 (17%) 0.23 (68%) 0.05 (15%)0.50–0.75 70 0.60 0.48 (80%) 0.11 (18%) 0.35 (58%) 0.13 (22%)0.75–1.00 20 0.84 0.69 (82%) 0.15 (18%) 0.46 (55%) 0.23 (27%)Above 1 14 1.55 1.18 (76%) 0.34 (22%) 0.61 (39%) 0.57 (37%)

Notes: In each class, the lower limit is included while the higher limit is excluded. The proportion of each form of Al to Alt within eachclass is shown in parentheses.

Table 3Comparison of four watersheds

Watershed

Dorokawa (n = 234) Miomote (n = 224) Tama (n = 42) Tsukuba (n = 142)

Alt (lM) 0.29± 0.18b 0.37 ± 0.38a 0.28 ± 0.35ab 0.29 ± 0.16b

(0.05–1.23) (0.03–3.31) (0.03–1.70) (0.07–1.00)Alr (lM) 0.21 ± 0.12bc 0.34 ± 0.36a 0.15 ± 0.09c 0.24 ± 0.14b

(0.05–0.71) (0.03–3.27) (0.03–0.38) (0.06–0.71)Alc (lM) 0.08 ± 0.09b 0.03 ± 0.04c 0.13 ± 0.34a 0.05 ± 0.07c

(nd–0.68) (nd–0.25) (nd–1.52) (nd–0.44)Ali (lM) 0.18 ± 0.09b 0.23 ± 0.18a 0.14 ± 0.07b 0.21 ± 0.11a

(0.04–0.54) (0.03–1.49) (0.03–0.31) (0.06–0.60)Alo (lM) 0.04 ± 0.04b 0.10 ± 0.20a 0.01 ± 0.03b 0.03 ± 0.04b

(nd–0.24) (nd–2.05) (nd–0.15) (nd–0.18)F� (lM) 0.6 ± 0.3d 1.3 ± 1.0c 2.1 ± 1.1a 1.6 ± 0.7b

(nd–1.6) (nd–5.2) (0.6–4.2) (0.4–3.2)NO�3 ðlM) 11 ± 10c 15 ± 11c 69 ± 50b 115 ± 47a

(nd–63) (nd–60) (6–197) (37–270)SO2�

4 ðlM) 17 ± 6d 45 ± 25b 57 ± 32a 38 ± 24c

(7–37) (11–257) (10–137) (4–137)NO�3 þ SO2�

4 ðleq/L) 44 ± 20c 106 ± 55b 183 ± 101a 190 ± 76a

(16–106) (25–557) (27–438) (73–480)pH 7.1 ± 0.4b 7.3 ± 0.3a 7.3 ± 0.5a 7.1 ± 0.3b

(5.8–7.8) (6.5–7.9) (6.3–8.3) (6.5–7.8)Na (lM) 253 ± 46a 236 ± 89b 144 ± 51d 214 ± 76c

(151–407) (127–867) (43–237) (95–421)Ca (lM) 95 ± 25c 98 ± 63c 207 ± 132a 144 ± 87b

(30–147) (13–466) (41–828) (39–621)DOC (lM) 101 ± 52a 86 ± 79b 29 ± 16c 78 ± 35b

(38–307) (9–583) (9–89) (31–185)

Notes: Different letters indicate significant difference (P < 0.05) by Fisher’s PLSD test. SO2�4 was poorly correlated with Na (r = 0.07) and

not likely to be affected by sea salt. Ogawa et al. (2006) discuss NO�3 , SO2�4 , pH, and DOC in the Dorokawa watershed in detail.

n = number of samples; nd = not detected; data are presented as average ± standard deviation (minimum–maximum).

1212 M.K. Koshikawa et al. / Applied Geochemistry 22 (2007) 1209–1216

3. Results and discussion

3.1. Concentrations of Alt

Concentrations of Alt ranged from 0.03 to3.31 lM. In Table 2, the streams are divided into5 classes according to the concentrations of Alt,and populations are compared among classes. Thelargest population occurred in the class

Alt < 0.25 lM, and the next largest was found inthe class ranging from 0.25 to 0.50 lM. More than80% of the samples were below 0.50 lM, and only2% of the samples were above 1 lM.

3.2. Change in forms of Al with increasing Alt

Mean concentrations of each form of Al and theproportions of each form of Al to Alt are derived for

M.K. Koshikawa et al. / Applied Geochemistry 22 (2007) 1209–1216 1213

each class (Table 2). Concentrations of every formof Al increased as Alt increased. Alc was a minorfraction (below 22%) of Alt in all of the classes.Ali was the largest fraction (71%) of Alt in the low-est class (Alt < 0.25 lM), but the fraction of Alidecreased as Alt increased. On the other hand, Aloincreased as Alt increased, and the Alo fraction(37%) was almost as large as the Ali fraction(39%) in the highest class (Alt > 1 lM). AlthoughAli was the dominant fraction of Alt in all of theclasses, it appears that formation of Alo enhancedthe increase in Alt.

3.3. Comparison of four watersheds

The variation in concentration and speciation ofAl among streams was greater than variation withinany particular stream for different seasons orweather (details not shown). Therefore, the datacan be compared among the 4 watersheds, irrespec-tive of the different sampling times. The averages ofeach form of Al, together with F�;NO�3 , SO2�

4 , thesum of NO�3 and SO2�

4 , pH, Na, Ca and DOC in4 watersheds are listed in Table 3. The averageNO�3 was highest in Tsukuba, and the averageSO2�

4 was highest in Tama. From the amounts ofNO�3þ SO2�

4 , Tsukuba and Tama are the most pol-luted while Dorokawa is the least polluted. How-ever, pH values in Tama (7.3 ± 0.5) and Tsukuba(7.1 ± 0.3) were both neutral. In Miomote, the aver-age values of Alt, Alr, Ali, and Alo were the highestamong the 4 watersheds, and average values of Alrand Alo were significantly higher than in the otherwatersheds. On the other hand, the average Alcwas the lowest in Miomote but highest in Tama.

0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

Alr (

μM

)

Ca (μM)0 200 400 600 800 1000

MTT

Da

Fig. 3. Variation of Alr as a function of (

Each form of Al was poorly correlated withNO�3 þ SO2�

4 (r ffi �0.1 for Alt, Alr, Ali, and Alo,and r ffi 0.1 for Alc), and pH (r ffi �0.2 for Alt andAlc, r ffi �0.1 for Alr, Alo, and Ali). The influenceof more serious deposition of acidic pollutants inTama and Tsukuba appeared as higher NO�3þSO2�

4 but did not appear as either lower pH orhigher Al in stream waters.

3.4. Conditions for increase in Al

Alr varies as a function of Ca for all streams in 4watersheds (Fig. 3a). Alr was inversely related toCa, and the entire data set for the 4 watershedscould be expressed as Alr = 21(1/Ca) (r = 0.56,P < 0.001). Alr also varied as a function of DOCfor all streams in the 4 watersheds (Fig. 3b). Alrwas positively correlated with DOC, and the entiredata set for the 4 watersheds could be expressed asAlr = 0.0028 DOC + 0.02 (r = 0.67, P < 0.001).Alo was also positively correlated with DOC andcould be expressed as Alo = 0.0014 DOC � 0.06(r = 0.64, P < 0.001). These results suggest that thechanges in Alr concentrations reflect the concentra-tions of Ca and DOC in each stream rather than thedirect influence of acidic pollutants deposited intothe watersheds.

3.5. Relationship between Al, Ca and DOC

The relationship between Alr and DOC is differ-ent for Ca amounts less than or greater than110 lM (Fig. 4). The slope for the streams withCa < 110 lM (Alr = 0.0032 DOC + 0.01, r = 0.70,P < 0.001) was greater than that for the streams

DOC (μM)0 100 200 300 400 500 600

iomoteamasukuba

orokawa

b

a) Ca and (b) DOC in 4 watersheds.

0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

Al r(

μM)

0 5 10 15 20 25 30DOC/Ca (mol/mol)

Fig. 5. Relationship between Alr and DOC/Ca.

1214 M.K. Koshikawa et al. / Applied Geochemistry 22 (2007) 1209–1216

with Ca > 110 lM (Alr = 0.0007 DOC + 0.11,r = 0.36, P < 0.001). The ability of DOC to enhancedissolution of Alr seems to be greater when Ca islower. The correlation coefficient for the streamswith Ca < 110 lM was also greater than that forthe entire data set. The correlation coefficientbecame even greater when the ratio of DOC/Cawas substituted for DOC (Alr = 0.13 DOC/Ca + 0.11, r = 0.86, P < 0.001) (Fig. 5). Potentialprocesses behind this relationship are (1) lessexchangeable Ca in soil coinciding with moreexchangeable Al that is ready to be released fromsoil as Ali and/or Alo and (2) the higher ratio of(organic ligand concentration)/(competitive cationconcentration) in solution enhancing formation ofsoluble organic Al complexes.

3.6. Comparison with previous studies in northern

America and Europe

Concentrations of Alr and Ali were in the rangeof 0.03–3.27 and 0.03–1.49 lM, respectively (Table3), and variations of Alr and Ali were independentof pH in the neutral stream waters (pH 5.8–8.3) inthis study. These results are in accord with previousstudies in northern America and Europe (Figs. 6aand b), that is, concentrations of Alr and Ali remainlow (<0.1–5 lM) in neutral waters (pH > 5.5) andincrease with decrease in pH in acidic waters (5–10 lM at pH 5 to 40–50 lM at pH 4). This isexplained by the theoretical solubility of Al (OH)3

being consistently low in neutral waters and increas-ing exponentially with a decrease in pH (Figs. 6aand b).

0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

Alr (

μM)

DOC (μM)0 100 200

Ca < 110μM

Ca > 110μM

300 400 500 600

Fig. 4. Relationship between Alr and DOC.

Concentrations of Alo were correlated with DOCin this study. Similar relationships between Alo andDOC have been reported, and ratios of Alo/DOC invarious waters have been compared in previousstudies (Driscoll and Postek, 1996) (Fig. 6c). Theratio of Alo/DOC was low in neutral waters andincreased linearly with decrease in pH in previousstudies, although the ratios of Alo/DOC in somewetlands were lower than were expected on the basisof pH (Driscoll and Postek, 1996). The ratios ofAlo/DOC in this study were at about the levelexpected for pH 5.8–8.3, but the variations in Alo/DOC in this study were independent of pH andrather dependent on Ca, as shown in Section 3.5.

The fraction of Ali was dominant in neutralstream waters (pH 5.8–8.3) in this study (Table 2).In many studies in northern America and Europe,Alo is dominant at neutral pH, while the fractionof Ali increases as pH decreases and becomes dom-inant at pH < 5.5 (Campbell et al., 1983; LaZerte,1984; Driscoll and Postek, 1996). The exceptionsare: (1) when acidity is largely due to organic acid,Ali remains low and Alo is dominant at low pH(Driscoll et al., 1988; Helmer et al., 1990); and (2)when DOC is low, Alo remains low and Ali is dom-inant at neutral pH (Driscoll and Postek, 1996). Inthis study, Ca was higher (average, 114 ± 73 lM;range, 13–828 lM) and DOC was lower(86 ± 61 lM; 9–583 lM) (Table 3) than the levelsin northern America and Europe (Ca, 10–100 lM;DOC, 100–1000 lM; Cronan and Goldstein, 1989;Driscoll and Postek, 1996). Therefore, a possibleprocess to explain the low Alo at neutral pHobserved in this study is a high level of Ca actingas a competitive cation for complexation with alow level of DOC.

0

10

20

30

40

50

60

Alr

(μM

)

0

10

20

30

40

50

60

Ali(

μM)

0

0.005

0.010

0.015

0.020

Alo/

DO

C (

mol

/mol

)

3 4 5 6 7 8 9pH

Streams

Wetlands

Streams

Lakes

This study

Previous studies in northern America and Europe

Fig. 6. Variations of (a) Alr, (b) Ali, and (c) Alo/DOC as a function of pH, in this study and the previous studies in northern America andEurope. Data for the previous studies are from the summary of Driscoll and Postek (1996). In panels (a) and (b), the solid line representstheoretical solubility of amorphous Al(OH)3 (log[Al3+] = 9.66 � 3 pH, log[AlðOHÞþ2 � = �0.44 � pH, log[AlðOHÞ�4 � = �13.33 + pH), andthe dotted line represents theoretical solubility of gibbsite (log[Al3+] = 8.04 � 3 pH, log[AlðOHÞþ2 � = �2.06 � pH,log[AlðOHÞ�4 � = �14.95 + pH). The equations were calculated using log K values of �9.66 for Al3+ + 3H2O = 3H+ + amorphousAl(OH)3, �8.04 for Al3+ + 3H2O = 3H+ + gibbsite, �10.1 for Al3+ + 2H2O = AlðOHÞþ2 + 2H+, and �22.09 forAl3+ + 4H2O = AlðOHÞ�4 þ4H+ (Lindsay and Walthall, 1996; Nordstrom and May, 1996).

M.K. Koshikawa et al. / Applied Geochemistry 22 (2007) 1209–1216 1215

Cronan and Schofield (1990) evaluated the mobi-lization of Al in drainage waters in sites seriouslyimpacted by acidic deposition and found that twoconditions were necessary for the mobilization ofelevated concentrations of Al: low percent base cat-ion saturation in soil and elevated inputs of strong

acids. Under these conditions, concentrations ofAlr increased with increasing concentrations ofNO�3 þ SO2�

4 . Although the dominant factor affect-ing solution Al chemistry is pH, regional patterns ofvariation in the concentrations of Alr in naturalwaters are controlled in part by soil-exchange

1216 M.K. Koshikawa et al. / Applied Geochemistry 22 (2007) 1209–1216

chemistry and strong acid anion concentrations insolution (Cronan and Schofield, 1990). Exchangesites serve as the short-term source for rapid releaseof Al, but some forms of Al trihydroxide serve asthe longer-term resupply reservoir for exchangeableAl and as the key solid phase that limits Al3+ solu-bility (Cronan and Schofield, 1990). The results ofthe present study resemble their findings in thatthe authors observed low base cation concentrationsrepresented by Ca to be one of the conditions neces-sary for the mobilization of Al. In this study,though, it was observed that DOC, rather thanstrong acids, enhanced mobilization of Al.

Although acidic deposition in Japan has alreadyresulted in elevated concentrations of NO�3 þ SO2�

4

in stream waters, a high level of Ca (instead of Al)is serving as a major counterion for NO�3 þ SO2�

4

(for example, in this study, [NO�3 þ SO2�4 , l eq/

L] = 0.30 [Ca, leq/L] + 31, r = 0.59). However,an additional loading of acidic deposition mayresult in shortage of Ca and mobilization of Al asa counterion for NO�3 þ SO2�

4 , and continuousobservation of the speciation of Al in Japanesestream waters may reveal future change in the con-ditions for mobilization of Al from ‘‘low Ca andhigh DOC’’ to ‘‘low Ca and high NO�3 þ SO2�

4 ’’.

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