ARTICLE pubs.acs.org/est
Response of Surface Water Acidification in Upper Yangtze River to SO2 Emissions Abatement in China Lei Duan,†,* Xiaoxiao Ma,† Thorjørn Larssen,‡ Jan Mulder,§ and Jiming Hao† †
State Key Laboratory of Environmental Simulation and Pollution Control, School of Environment, Tsinghua University, Beijing 100084, China. ‡ Norwegian Institute for Water Research, Gaustadalleen 21, 0349 Oslo, Norway. § Department of Plant and Environmental Sciences, Norwegian University of Life Sciences, Box 5003, NO-1432 Ås, Norway.
bS Supporting Information ABSTRACT: Surface waters in Europe and North America are slowly recovering from acidification following major reductions in emissions of sulfur dioxide (SO2) since the 1980s. In contrast, regions affected by acid rain have been reported to be growing in China. Here we show that the rapid change in surface water chemistry in the 1990s in large areas in Southwestern China, specifically the tributary rivers of the Upper Yangtze River, caused by increasing SO2 emissions, has leveled off. During the 1990s the sulfate (SO42) concentrations in river water increased rapidly and, on average, doubled in only eight years. Simultaneously, calcium (Ca2þ) concentrations increased, while pH values decreased. In the following decade (2000s), SO2 emissions stabilized, causing a subsequent stop to the increasing SO42 concentrations and pH decline in river water. Although a rapid response to future reduction in SO2 emissions can be expected, a rapid increase of nitrogen (N) emissions could lead to increases in N leaching and delay recovery.
’ INTRODUCTION Many acidified surface waters in northwestern Europe and northeastern North America are slowly recovering following considerable emission abatement of sulfur dioxide (SO2) since the 1980s.14 In contrast, regions affected by acid rain continue to grow in China.5 Accompanying the rapid increase in emission of SO2, nitrogen oxides (NOx) and ammonia (NH3),69 high acid deposition rates occur in large areas in China, especially in the southwest and southeast.10,11 It is more than 30 years since the acid rain issue was first reported in the late 1970s, with the lowest annual average pH of rainwater in southwest China, especially near Chongqing and Guiyang.1215 In recent years, controls on SO2 emissions, carried out since 1998,10,16 have become quite successful. During the period of 20052010, the Chinese government set compulsory targets to reduce national energy intensity (i.e., energy consumption per unit GDP output) and SO2 emissions of 20% and 10%, respectively.9 According to the Report on the State of the Environment in China by the Ministry of Environmental Protection, national SO2 emissions decreased continuously from a maximum of 2.59 billion tons in 2006 to 2.17 billion tons in 2010, that is, a 14% of reduction had been achieved in relation to 2.55 billion tons in 2005, mainly through flue gas r 2011 American Chemical Society
desulfurization (FGD) installation in more than 70% of coal-fired power plants. However, the rapid increases in emissions of NOx and NH3, with the former mainly from coal combustion (without NOx emission control) and automobiles, and the latter mainly from agricultural sources, may counteract the positive effects of SO2 abatement on surface water acidification impacts.9 Emissions of SO2 in Sichuan and Chongqing, two provinces in southwest China (Figure 1), rapidly increased from 1985 to 1996. After this, SO2 emission stabilized and decreased slightly until 2002, following the pattern of coal use in the region. After 2002, SO2 emissions, relative to coal use, decreased substantially as a result of extensive measures for reducing emissions.16 In contrast, NOx and NH3 emissions kept increasing due to growing coal combustion, vehicle population, and N-fertilizer consumption. Here we present unique data on temporal trends in surface water chemistry in tributaries of the Upper Yangtze River in southwest China in response to the trends in atmospheric Received: November 16, 2010 Accepted: March 21, 2011 Revised: March 14, 2011 Published: March 30, 2011 3275
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’ MATERIALS AND METHODS The study region covers about 0.55 M km2 of the catchment of the Upper Yangtze River in Sichuan and Chongqing, Southwestern China. As the longest river in China and in terms of water discharge the fourth largest in the world, the Yangtze River flows from the Tangula Ranges on the Qinghai-Tibet Plateau, cuts through the Yun-Gui Plateau before entering the fertile Sichuan Basin and continues through the famous Three Gorges. There are about fifty tributaries of Yangtze River, including Yalongjiang, Minjiang, Tuojiang, Jialingjiang, and Wujiang in the upper part and Hanjiang, Xiangjiang, Yuanjiang, and Ganjiang in the middle and lower reaches. There are more than fifty stations for monitoring flow rate and water quality along the main stream and the tributaries under the authority of the Yangtze River Water Resource Commission. In the upper part of the Yangtze River we selected data for the period 19882007 from seventeen stations (Table 1 and Figure 2). Four of the stations are at the mainstream of the Yangtze River, whereas the remaining 14 are at the Minjiang, Tuojiang, Jialingjiang, and Wujiang respectively. These stations were chosen as they were not seriously polluted by wastewater; the criterion used was that the water quality could meet the National Environmental Quality Standards for Surface Water,
deposition. In particular, we assess changes in streamwater acidity to evaluate previous suggestions that surface waters in China are of high pH value and acid neutralizing capacity (ANC), and thus insensitive to acidification.2023
Figure 1. Coal and N-fertilizer consumption and anthropogenic SO2 and NOx emission in Sichuan and Chongqing province. Data from different sources: coal and fertilizer consumption from the China Statistical Yearbook;17 SO2 emission from China National Environmental Monitoring Center for recent years (shown as gray circle) and estimation based on coal consumption in earlier years (as white circle); NO2 emission from literatures 9,18,19 (as gray triangle), and estimation based on coal consumption (as white triangle).
Table 1. Seasonal Kendall Analysis of Water Dataa river
station
abbreviation
year
NO3
SO42
pH
Ca2þ
198897 982007 198897 982007 198897 982007 198897 Minjiang
Jialingjiang
Gaochang
GC
19882007
0.014
Pengshan
PS
19882006
0.014
Sanhuangmiao
SHM
19882006
0.040
Lijiawan
LJW
19882007
0.019
Fuluzhen
FLZ
19882006
0.020
42.4
46.9
41.0
72.6
68.9 0.033
51.4
74.0
6.71
2.78
5.30
7.44
8.61
18.50
0.51
Xindianzi
XDZ
19892007
0.030
Tingzikou
TZK
19892007
0.025
Wusheng
WS
19882007
0.015
Pingwu
PW
19892006
Fujiangqiao
FJQ
19892006
0.055 0.068
69.1
26.6
0.014 0.012
58.6
0.016
20.0
0.008
32.4
0.015
0.042
0.023
39.6
4.29
33.3
1.43
0.015 1.21
14.0
0.023
3.06
20.0
0.054
39.9
0.064
2.25
45.0
Shehong
SH
19892006
Luoduxi
LDX
198890,9407
Wujiang
Wulong
WL
19882007
Hanjiang
Baihe
BH
19882007
0.024
14.7
71.4
0.43
1.06
0.024
Xiantao
XT
19882007
0.025
18.6
131.0
0.47
1.67
0.026
Danjiangkou
DJK
19882007
Sigu
SG
198890,942007
Panzhihua Huidongqiao
PZH HDQ
19882007 0.063 198998,200607 0.030
Yangtze
54.8
198897
181.9
36.4
0.026
L.P.
0.017
4.64 117.7
42.0
3.00
174.0
53.9 25.3 0.063 0.040
40.0 34.3
42.0 38.5
0.065 0.031
3.61 3.00
Cuntan
CT
19882007
24.4
6.6
4.60
4.60
37.6
0.007
Yichang
YC
19882007
23.0
68.0
5.50
2.66
26.6
0.115
3.00
Hankou
HK
19882007
0.003
64.5
Datong
DT
19882007
0.015
42.0
2.82 1.38
Changes in concentration of SO42, NO3, and Ca2þ (in μeq 3 L1 3 yr1) and in pH (in yr1). Trend significant at p < 0.01. No data denotes trend not significant. No data for Ca2þ and L.P. (lime potential, pHþ0.5logCa) in 19982007 because the concentration of major ions except SO42 and NO3 was not measured since 2000 at any of the stations. a
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pH. The two sets of data were compared to assess regional water acidification, based on changes in acid neutralizing capacity (ANC), in these types of waters.
’ RESULTS AND DISCUSSION
Figure 2. Water pH of studied sites along Yangtze River. Underline: name of province; Square: station; Triangle: sampling site of small water; Circle: monitoring catchment. Numbers mean tributaries: 1, Yalongjiang; 2, Minjiang; 3, Tuojiang; 4, Jialingjiang; 5, Wujiang; 6, Hanjiang; 7, Xiangjiang; 8, Yuanjiang; 9, Ganjiang.
which combines chemical and biological parameters.24 In the middle and lower reaches, three stations at the mainstream and three at the Hanjiang were also selected for comparison. Tendency toward annual changes in the chemical composition were analyzed by a modified version of the seasonal Kendall test (SKT).25 Slopes associated with the SKT trends were estimated according to the method of Sen.26 For this study, flow-adjusted,27 bimonthly data of pH, SO42, NO3, and Ca2þ concentration were used for analysis. At stations where two or more samples were collected within two months, a mean value was used. Because the rapid increase of SO2 emission in this area was curbed around 1998 (Figure 1), as a result of emission abatement, the data were divided into two time periods (19881997 and 19982007). The national hydrological monitoring program focuses only on the larger rivers. In order to assess the trends in water chemistry for small streams, we selected the first order stream in the 16.2 ha forested TieShanPing catchment (Figure 2), which was part of the Integrated Monitoring Program on Acidification of Chinese Terrestrial Systems (IMPACTS).5 Data on atmospheric deposition and streamwater chemistry were available from 2001 to 2004 and from 2009 onward. Since there were not enough data to meet the requirement of the Seasonal Kendal Test, that is, a minimum of 10 years of data,25 the paired t test was used to show the significance of the difference between two years. In addition to regular monitoring, a regional survey program on surface water acidification were carried out in spring 1988, investigating small streams, ponds, and lakes in Southwestern China including some head waters.12 There were totally 37 studied sites in this region, most of which were in the countryside outside big cities such as Chengdu, Chongqing, Guangyuan, and Mianyang, whereas only a few were background sites (e.g., Emei Mountain). Nineteen of the survey waters (Supporting Information (SI) Table S2 and Figure 2) were resampled during spring 2009 (the remaining sampling sites were not included, due to the rapid development of cities and thus changes of land use). The pH value and electric conductivity (EC) were measured in the field, and the concentrations of major ions (Ca2þ, Mg2þ, Naþ, Kþ, NH4þ, SO42, NO3, Cl, and F) were measured in laboratory by IC (ion chromatography). In contrast, pH was measured in the laboratory, major cations by AAS (atomic absorption spectroscopy), and major anions by wet chemical analysis in the previous study. The analytical results by different methods could be comparable for ion concentrations but not for
Acidification of Yangtze River and Tributaries. The mean annual pH values and concentrations of the dominant ions from the 23 stations of the Yangtze River and the tributaries were shown in SI Table S1. The average pH values during 19982007 ranged from 7.40 to 8.15, with the lowest pH occurring at the HDQ station along the Minjiang River. Obviously, the acidity of the rivers is not high by North American or European standards. For example, water acidification has been most serious in Scandinavia, where waters (typically inland lakes and streams) with pH below 5.0 are common.28 On average during 19881997, bicarbonate (HCO3) was the dominant anion, accounting for about 72% of the anionic charge in river water. The concentrations of SO42 made up about 16% of the anionic charge. Ca2þ was the dominant cation, accounting for 54% of the cationic charge, whereas NH4þ contributed 36% of the cationic charge. Kþ and Naþ concentrations were generally low, making up only 4% of the cationic charge. Assessment of the temporal trend in ion concentrations of river water indicates a significant increase of SO42 concentration in the tributaries of Upper Yangtze River during both of the two periods (Table 1 and Figure 3). In 19881998, the concentration of SO42 showed a significant increase (p < 0.01) at 9 of the 13 stations, and no significant change (p g 0.01) at the others. The average increase of the SO42 concentration at these nine stations was 49.2 μeq 3 (L1 3 yr1, which implies that it took only about eight years to double the concentration to the current average level of 822 μeq/L (mean of the nine stations during 19982007 in SI Table S1). After 1998, the significant increase of SO42 concentration remained in only three of the stations, but continued at most of the stations in the middle and lower reaches of the Yangtze River. The SO42 trends were generally homogeneous within a region, suggesting that it is driven by changes in sulfur deposition. The trend of rapid increase of SO42 concentration in the 1990s in the upper reach of Yangtze River had leveled off in the 2000s, which coincided well with the trend in SO2 emission in the region (Figure 1). Continued increase in SO42 concentration in the middle and lower reaches may be due to the delayed SO2 emission reduction in the east of China.11 The concentration of NO3 increased significantly (p < 0.01) at seven of the 13 stations in the tributaries of Upper Yangtze River during 19881997 (Table 1 and Figure 3). The average increase of NO3 by 4.9 μeq 3 (L1 3 yr1 means that it took about 11 years to double its concentration to the current average level of 112 μeq/L (calculated on the bases of SI Table S1). The NO3 concentration continued increasing at most of the stations in 19982007, which coincided with the increasing atmospheric emission of NOx and NH3 (Figure 1). The ratio of NO3/ SO42 concentration (equivalence ratio, averaged to 0.11 at all stations) was much lower than that of N/S ratio in emission (equivalence ratio, about 0.47 in 2008), which indicates that the catchment was an important sink for N. Observation of the N balance in the small forested catchment TieShanPing in Chongqing indicated that denitrification might be the most important sink, whereas the net nitrogen uptake by the vegetation was relatively small.29,30 As a product of denitrification, nitrous oxide 3277
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Figure 3. Regional trends in solute concentrations in the catchment of the Upper Yangtze River. Histograms exhibit Sen slope of pH, and SO42, NO3, and Ca2þ concentrations for 19881997 (1990s) and 19982007 (2000s, no data for Ca). Slopes are in μeq 3 L1 3 yr1. The bar shows significant change (p < 0.01). T: tributaries in upper reaches; U: mainstream in upper reaches; ML: mainstream and tributaries in middle and lower reaches.
(N2O), a potent greenhouse gas31 was elevated at this site.29 Emission of N2O was reported to be increasing with enhanced nitrogen deposition32 and soil acidity.33 With the increasing concentration of the acid anions (SO42 and NO3) in 19881997, nine of the 23 stations had a decreasing trend of pH value (p < 0.01), 4 showed increasing trend (p < 0.01), and 10 had no significant trend (Figure 3). The stations with decreasing pH were all from the tributaries in the upper reaches, with only one exception in the lower reaches. This
indicates that surface water acidification was most prominent in the tributaries in the upper reaches of the Yangtze River. However, the trend of decreasing pH was curbed after 1998 at all eight stations with pH decreasing in 19881997, with the pH even recovering at some stations. This coincided with the trend in SO42, indicating the important role of SO42 as a driver for the decreasing pH values in the Upper Yangtze River catchment. It suggests that the surface water acidification is primarily caused by the rapid increase of SO2 emission, and that the slowdown in 3278
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Figure 4. Comparison of SO42, NO3, and Ca2þ concentrations and ANC of some small waters between 1988 and 2009.
SO2 emission after 1998 have limited further acidification of the surface waters. The significant increase in Ca2þ concentration at most stations was likely to be due to enhanced mobilization of Ca2þ in soils, caused by cation exchange and mineral weathering. The major soil type in the region is Purplish Soil (according to the Chinese soil taxonomy system, Eutric Regosol in FAO system), which has high base saturation and weathering rate.34 The larger increase in SO42 than Ca2þ concentration (32.6 μeq 3 (L1 3 yr1 on average) in tributaries of Upper Yangtze River (Figure 3) indicates that the input of sulfuric acid is only partly neutralized by Ca2þ mobilization in soils. The decrease of the socalled lime potential (pHþ0.5logCa)35 at most stations, may be interpreted as a measure of a decreasing trend in the base saturation of the soils. Acidification of Small Sized Waters. A comparison of concentrations in small waters in two regional surveys in this area, carried out in spring of 1988 and 2009, respectively, showed increased concentrations of SO42 (paired t test, p < 0.01) and NO3 (p < 0.05) in the past 20 years (Figure 4). As a result, ANC decreased (p < 0.01) while Ca2þconcentration did not show any trend. Generally, it showed therefore regional trend of water acidification in the past 20 years, as a result of increasing SO2 and NOx emission. The small, first order stream of the TieShanPing catchment in Chongqing did not show a significant increase in SO42 concentration from 2001 to 2004 to 2009 (Figure 5), which might coincide with the historical trend in SO2 emissions (Figure 1). The decreasing trend of ANC in 20012004 did not continue to 2009 (Figure 5), and neither did the pH and lime potential (pHþ0.5logCa). This may imply that soil acidification was
curbed, that is, no further decrease in soil base saturation (BS) in this catchment. However, concentration of NO3 increased from 2001 to 2004 to 2009, which was balanced by increased Ca2þ. It seems that recovery of streamwater from acidification caused by control of S deposition in recent years would be delayed in this catchment due to the increase of N deposition.
’ UNCERTAINTIES AND POLICY IMPLICATIONS The data from the national hydrological monitoring program used in this study were not originally intended for long-term trend assessment for water acidification, and great uncertainty might occur due to limited range of parameters monitored, insufficient period of records, and an absence of information on the analytical and quality control procedures used. However, there has been no other long-term monitoring of surface waters in southwest China and the data therefore provide valuable information on the environmental impacts in this region. Although the results appear to suggest clear regional acidification in the 1990s and a leveling off in recent years, the possibility remains that the observed pH changes are the result of causes other than change in atmospheric depositions. In this respect, the potential impact of land-use changes, agricultural activities, and water pollution should be considered. For at least a proportion of the rivers studied, it is believed that the increase in SO2 driven acid deposition, and subsequently surface water SO42 increase, has been the cause of pH decrease in the large tributary rivers of Upper Yangtze River. However, the decrease (maximum 0.7 units in 10 years, based on Table 1) was probably not dramatic in terms of biological effects, as the rivers have relatively high pH also after the decrease. Results indicate 3279
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Figure 5. Yearly median SO42, NO3, and Ca2þ concentrations and ANC of stream in TieShanPing, Chongqing. Range shows 2575th percentiles, and marked with different letters indicate significant difference (at the 0.01 level) according to paired t test.
that significant changes may occur even in large and well buffered river systems, which have not received much interest regarding acid rain impacts. The general trends of decreasing lime potential in rivers implied a decrease of base saturation, and thus acidification of soils. Earlier, soil acidification has been documented in China,5,36,37 both in forest areas with sensitive soils 9,38 and in agricultural soils due to excessive application of nitrogen fertilizer.36 Therefore, great attention should still be paid to the risk of acidification in China. Although current efforts to reduce SO2 emission since the end of 1990s has led to a halt of the pH decrease, the last two decades has seen a strong increase in the emission of reactive nitrogen (NOx and NH3), which has not been curbed. Till now, the growing N input by atmospheric deposition and fertilizer application has contributed less to the acidification of large rivers than S deposition. The NO3 leaching by runoff remains small, suggesting an effective retention of N in terrestrial ecosystems. However, increasing N deposition in small catchment such as TieShanPing, which already are saturated with N29 could lead to increasing leaching of N. Further studies on the effects of N deposition, from both NOx and NH3 emissions, and N cycling are needed to support the development of future N emission abatement strategies.
’ ASSOCIATED CONTENT
bS
Supporting Information. Two additional tables. This material is available free of charge via the Internet at http:// pubs.acs.org.
’ AUTHOR INFORMATION Corresponding Author
*Phone: (þ86-10)62771403; fax: (þ86-10)62773957; e-mail:
[email protected].
’ ACKNOWLEDGMENT We are grateful to the financial support of the State Key Laboratory of Environmental Simulation and Pollution Control (10Z02ESPCT) and the Norwegian Research Council (193725/ S30) to carry out this study. We would also like to thank Changjiang Water Resource Commission and Sichuan Water Environment Information Center for providing us monitoring data of Yangtze River, and three anonymous reviewers for their comments on the manuscript. ’ REFERENCES (1) Driscoll, C. T.; Lawrence, G. B.; Bulger, A. J.; Butler, T. J.; Cronan, C. S.; Eagar, C.; Lambert, K. F.; Likens, G. E.; Stoddard, J. L.; Weathers, K. C. Acidic deposition in the northeastern United States: sources and inputs, ecosystem effects, and management strategies. Bioscience 2001, 51, 180–198. (2) Evans, C. D.; Cullen, J. M.; Alewell, C.; Kopacek, J.; Marchetto, A.; Moldan, F.; Prechtel, A.; Rogora, M.; Vesely, J.; Wright, R. Recovery from acidification in European surface waters. Hydrol. Earth Syst. Sci. 2001, 5, 283–297. (3) Skjelkvale, B. L.; Wright, R. F.; Henriksen, A. Norwegian lakes show widespread recovery from acidification; results from national surveys of lakewater chemistry 19861997. Hydrol. Earth System Sci. 1998, 2, 375–577. 3280
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