Article pubs.acs.org/est
Six-Decade Change in Water Chemistry of Large Freshwater Lake Taihu, China Yu Tao, Zhang Yuan, Wu Fengchang, and Meng Wei* State Key Laboratory of Environment Criteria and Risk Assessment, Chinese Research Academy of Environmental Sciences, Beijing 100012, China S Supporting Information *
ABSTRACT: Taihu lake has become a hot spot internationally due to its algae bloom. However, its natural water chemistry (major ions) received little attention though it is equally important for drinking water and aquatic ecology. Using historical data (1950s−2012) we explored the drastic change of Taihu water chemistry over the past six decades and the driving factors. Results show that major ions increased around 2−7-fold and TDS increased nearly 3-fold during the last 60 years. The dominant cation has shifted from Ca2+ to Na+, and the current Cl− is dominant over HCO3−, the predominant anion before the 2000s. Analyses show that population increase and human activities were the major driving factors responsible for the drastic change. Whereas the mechanism of increase was different for ions, i.e., Na+ and Cl− increase was directly related to the population increase and sewage discharge in the basin; SO42‑ was related to atmospheric deposition derived from increasing coal consumption and SO2 emissions; hardness (Ca and Mg) increase was closely linked to the acidic precipitation. No increase trend of HCO3− was attributable to frequent outbreaks of algae bloom which consumed HCO3−. Estimation indicated that sewage discharge in the basin contributed 23% to the lake in terms of Cl−, exceeding the contribution from rock weathering. Current water chemistry of Taihu lake has become “anthropogenic dominance” from its original rock dominance.
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INTRODUCTION It is widely recognized that anthropogenic activities have caused global environment change,1,2 which in turn has brought great impact on human life and social development due to the decreased availability of fresh water resources3 and degraded water quality.4 Freshwater lakes play an important role in the socioeconomic development and in maintaining a healthy ecosystem as they provide 46% of the global renewable freshwater and habitat for aquatic organisms.5 However, freshwater lakes have been undergoing more intensive disturbance by human activities since freshwater lake watersheds are suitable environments for development and food production, thus attracting more and more human settlement.6 Furthermore, lakes are more sensitive to human disturbance than rivers due to their longer residence time. Unfortunately, while there have been many studies focusing on degradation of freshwater lakes, long-term alteration of natural water chemistry (major ions) has been much less studied although major ions are vital factors for drinking water and lacustrine systems, and the few ones seemed to focus on the Great Lakes in North America thanks to the long-term surveillance program there.7,8 Gibbs first reported the mechanisms controlling world surface water chemistry including lakes,9 whereas it was pointed out that Gibbs’ model could be misleading in the interpretation of the water chemistry of some African dilute lakes10 and some European upland lakes.11 However, these inapplicabilities were © 2013 American Chemical Society
caused by variations of the local natural conditions such as hydrologic flowpaths, local soil geology, and catchment processes. There has been no discussion reported on the human-caused water chemistry change of lakes. Furthermore, in the international literature, there have been no reports on the long-term major ion change of lakes from China, the largest developing country in the world that has experienced drastic environmental changes resulting from human activities as well as from global change,3 mostly due to data unavailable. Taihu lake is the third largest freshwater lake in China and an important drinking water source for surrounding cities such as Wuxi and Suzhou.12 It provides other significant services, including water supply for industrial and agricultural uses, flood mitigation, and fish production.13 Because of its important ecological functions, Taihu lake has unique biodiversity and important habitats for some valuable species. The natural resources afforded by Taihu lake, and its advantageous location in the Yangtze Delta, one of the most economically developed regions in China throughout the history, have caused rapid population growth around the lake.14 Received: Revised: Accepted: Published: 9093
April 8, 2013 July 17, 2013 July 22, 2013 July 22, 2013 dx.doi.org/10.1021/es401517h | Environ. Sci. Technol. 2013, 47, 9093−9101
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Figure 1. Location of the Taihu basin in China and sampling sites in Taihu lake (solid triangle, square, circle, and star in the lake are sampling sites of different sources; double circles around the lake are major cities with population over one million; the solid circle in Xishan Island is also a rainfall/ evaporation gauging station).
of 15−17 °C. The long-term average annual precipitation is 1177 mm, with up to 60% of that occurring in May to September, and the long-term average evaporation is 821.7 mm.20 The matrix in this region is dominated by carbonates and clastic rocks.21 The lake’s shallow average depth ensures that it remains polymictic with no seasonal stratification.22 The turnover time of the lake water was nearly 20 days in Song Dynasty (A.D. 960−1279), but it is now 309 days.23 The drainage basin involves two major provinces (Jiangsu and Zhejiang) and Shanghai Municipality (Anhui province was not considered due to its quite minor area in the basin). Shanghai is located in the downstream of Taihu lake (Figure 1). The population density in the Taihu basin is 1500 population/km2, over 10 times higher than the national average. Data Sources and Manipulation. Historical major ion (K+, Na+, Ca2+, Mg2+, Cl−, SO42‑, HCO3−) data and other parameter data (hardness, pH, alkalinity, and precipitation/ evaporation) were collected from various sources. Data of the 1950s−1970s were from the “Hydrological Yearbook of the Yangtze River Basin, China”(HY), data of the 1980s−1990s were from the UN “Global Environment Monitoring System” Water Programme(GEMS/Water), data of the 2000s were from the “National Ecosystem Research Network of China”(CNERN).24 Present water chemistry data (2011− 2012) were obtained from our analyses of the field samples in this study. Lake water samples at a depth of around 0.5 m were taken as the water column of Taihu lake is well mixed without stratification due to its shallow depth and wind disturbance. The concentration of total dissolved salts (TDS) is the sum of individual ions. The sampling sites and data sources are shown in Figure 1. Detailed information about the data set is listed in Table S1. To explore the impact of wastewater discharge on the
While the area of the Taihu drainage basin accounts for 0.38% of the national land area, the population and gross domestic product (GDP) in this region accounted for 3% and 12% of the total national population and GDP, respectively, and its average GDP per capita was 3.5 times higher than the national average in 2010. With the rapid economic development in this area, drastic change of water quality has taken place over the past decades.13,15 A typical example was the outbreak of algae bloom in 2007 which led to one million local people being short of drinking water for a week.4 This event made Taihu lake a hot topic both domestically and internationally.16 However, existing studies mostly have focused on its nutrients or eutrophication,13,17 pollution of organic pollutants,18 and metals.19 There has been no report on the water chemistry (major ions) change of Taihu lake, especially at the decadal time scale. In this work, we used six decades of monitored major ion data to (1) show drastic water chemistry change in Taihu lake from the 1950s to the present; (2) explore the driving causes responsible for the change; (3) estimate the contribution of domestic wastewater discharge to the current water chemistry of the lake. This information is expected to contribute to a new case study from a different perspective on lake evolution and to water quality management under the circumstances of strong human pressures.
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MATERIALS AND METHODS Site Delineation. Taihu lake is located in the downstream of the Yangtze River (Yangtze Delta), in the southeast of China. It covers a water area of 2338 km2, and its average water depth is 1.89 m and water volume 44.3 × 108 m3. In this region, the river network is densely distributed. The regional climate is subtropical monsoon, with the long-term average temperature 9094
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Figure 2. Long-term trend (1950s−2010s) of major ions (in mmol/L) and TDS (in mg/L) in Taihu lake. (The box represents 25th and 75th percentiles of all data; the whiskers represent fifth and 95th percentiles; the small square represents mean; the line in box represents median; values above or below whiskers are outliers; n is the data number in the statistical calculation.)
water chemistry of the lake, we also sampled and analyzed wastewater from 12 sewage treatment plants (STPs) with the help of the local Environment Supervision Center. These STPs
are mostly located in the north and northwest of the lake, i.e., in the upstream of the lake (The plant names and their location were not presented in this paper due to the confidential 9095
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dominant anion was HCO3− before the 2000s (including in the 2000s), but presently (average of 2011−2012), Cl− concentration (2.19 mmol/L ± 0.65) is dominant over HCO3− (1.41 mmol/L ± 0.38). The current water chemistry has become a nontypical freshwater type, and this water chemistry is not consistent with the natural lithology of the basin.21 Long-Term Variations of Monthly Patterns. Monthly statistics of the water chemistry data for each decade from the 1950s−2000s indicated that, prior to the 2000s, the monthly variation of individual ions had no regularity throughout the year (Figure S1). While in the decade following 2000, it had a clear pattern with many ions presenting higher concentration in dry seasons and lower concentration in wet seasons (May to September, Figure 3). This monthly pattern has the same
purpose.). The treated wastewater from STPs is mostly discharged to Taihu lake via channels, ditches, or rivers. Socioeconomic data, such as population, GDP, energy consumption, etc., were collected from the Statistical Yearbook of the provinces and municipality in the basin. Wastewater discharge and sulfur dioxide (SO2) emission data were collected from either local Statistical Yearbook or the National Environment Statistical Yearbook. The provincial statistical data were first converted into drainage basin data by multiplying the ratio of the provincial area covered by the Taihu basin to the entire provincial area calculated with GIS tool (Arcview 3.3), and then the individual provincial numbers were added to obtain the socio-economic data for the entire Taihu drainage basin. In the calculation of population, GDP, and wastewater discharge, etc., in the basin, the two most influential provinces upstream of Taihu lake (Zhejiang and Jiangsu, Figure 1) were included, 15 whereas Shanghai was not included as it is in the downstream of the lake and has no direct impact. However, for the calculation of SO2 gas emitted in the basin, Shanghai was included as its emissions are likely to have impacts on the basin through atmospheric deposition. Frequency and acidity data of precipitation were collected from Shanghai Statistical Yearbook. Domestic sewage contribution in terms of Cl− was calculated by following expression [(Cdis × Vdis × 80%)/(C lake × Vlake)] × t /365
(a)
−
where Cdis is the mean Cl concentration of sewage discharge (145.25 mg/L); Vdis is the sewage discharge amount in 2011 in the basin (7.989 × 108 m3); 80% is the delivery ratio of discharged sewage to the lake;25 Clake is current mean Cl− concentration of Taihu lake (77.6 mg/L); Vlake is the average annual water storage in the lake (44.3 × 108 m3); t is the residence time of the lake water (309 days). All data calculations and statistical analyses were performed using Excel and SPSS 19. Distribution tests were performed before doing comparison or selecting appropriate statistical parameters. The result was expressed as mean ± one standard deviation. A significance level of 0.05 was used for statistical testing.
Figure 3. Monthly variation of major ions during the 2000s: point source characteristic.
characteristics as point source discharges, that is, solutes have higher concentration during low flow seasons and lower concentration during high flow seasons. Monthly variations also showed the great change in ion proportion over the past 60 years, that is, SO42‑ > Cl− prior to the 2010s, but in 2011−2012, Cl− > SO42‑ (in meq/L). Furthermore, in some special dry months, for example, June of 2011, when the lake had the lowest water level in this month in the past ten years due to consecutive drought,26 either Cl− (2.61 mmol/L ± 0.74) or Na+ (2.81 mmol/L ± 0.69) concentration was higher than HCO3− (1.02 mmol/L ± 0.05) (p < 0.05). While in August of the same year, a high flood month, our sampling data showed significantly lower concentrations of Cl−, Na+, and SO42‑ than in the June (p < 0.05), also indicating the point source characteristic.
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RESULTS Long-Term Trend of Major Ions and TDS. Using historical water chemistry data for Taihu lake, we found that many major ions and TDS have drastically changed in the past six decades (Figure 2). Figure 2 shows that, except for HCO3−, all major ions and TDS had a significant increasing trend (p < 0.05). The ion concentration increased from 2.35-fold (Mg2+) to 6.76-fold (Cl−), and TDS increased by a factor of 2.85 compared with that of 60 years ago. The increase rate of the ions was different; it is 0.002 mmol·L−1 y−1 for K+, 0.029 mmol·L−1 y−1 for Na+, 0.012 mmol·L−1 y−1 for Ca2+, 0.004 mmol·L−1 y−1 for Mg2+, 0.030 mmol·L−1 y−1 for Cl−, 0.012 mmol·L−1 y−1 for SO42‑, and 3.88 mg·L−1 y−1 for TDS. In addition, Na+, Cl−, and SO42‑ have seen an abrupt increase since the 2000s; Ca2+ and Mg2+ increased gradually throughout the 1950s to present day. Compared with 60 years ago, the water chemistry type of Taihu lake also radically altered. The dominant cation changed from Ca2+ before the 1990s to present Na+, and the dominant anion was characterized by HCO3− > (Cl−+ SO42‑) and SO42‑ > Cl− before the 1990s, but current Cl− + SO42‑ was much higher than HCO3− in 2011−2012 and Cl− > SO42‑. Remarkably, the
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DISCUSSION Gibbs summarized three major determinants for the water chemistry of surface waters, that is, rainfall dominance, rock dominance, and evaporation-crystallization dominance.9 For most of the world’s major rivers, rock weathering processes dominate the dissolved and suspended loads,27 and it is also the case for Chinese major rivers including the Yangtze river,28 in which Taihu lake is located in the downstream. For Taihu lake, the weight ratio of Na/(Na+Ca) or Cl/Cl+HCO3) versus TDS of the 1950s−2010s fell into neither the ascending limb (evaporation-crystallization process dominance over rock 9096
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Figure 4. Correlation between Cl− (a−c) and SO42‑(d−f) concentration in the lake water and socioeconomic variables in the Taihu basin. (All based on best fit regression. The could-be level off in c is most likely caused by component change in GDP, i.e, GDP proportion of the secondary industry, with more Cl discharge, kept decreasing and GDP from the tertiary industry, with less or no Cl discharge, kept increasing in the past decades, especially since China called for “green GDP” policy.)
natural water chemistry of Taihu lake. In the downstream of the Yangtze basin and especially around the Taihu basin, the most widely distributed rocks are carbonates (limestone and dolomite) and clastics.21,28 This determines that the major
weathering) nor descending limb (precipitation dominance over weathering) of the Gibbs boomerang-shaped plot9 (Figure S2). It fell into the mid part of the boomerang-shaped plot, indicating that the rock weathering process is the basis of the 9097
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2011 was around 1357 times higher than that in 1950 (including Shanghai). In addition, ferric chloride used for phosphorus removal from municipal wastewater also contributes Cl increase in the surface water.7 This is particularly true for Taihu lake since the later 1990s as a large amount of STPs were built and operated after then in the region (as we were told by the staff of the Environment Supervision Center of South Jiangsu Province when we investigated there). This can be further evidenced by comparing Cl− concentration of the wastewater from the inlet (before treatment) and outlet (after treatment) of STPs, that is, Cl− concentration from effluents (4.09 mmol/L ± 1.79) was significantly higher than that from influents (3.53 mmol/L ± 1.81) (p < 0.05, n = 12). Effluents from urban discharges are typical point source pollution for surface waters, with the characteristic of higher concentration in dry seasons and lower concentration in wet seasons. This is consistent with the monthly variation of many major ions in Taihu lake in the 2000s (Figure 3), further evidencing human impact on the ion change especially since the early 2000s. Besides, potassium chloride (KCl) used in agricultural fertilizers and lost with nonpoint source runoff 32 also was a contributor to the Cl increase in the lake water, although the amount of contribution cannot be estimated with the current data availability. Although road salts used to deice have been reported to contribute Cl in surface waters,7 it is not the case for the Taihu region due to its climate. The atmospheric deposition also contributes Cl− for some surface waters, but it is negligible in the Taihu basin.21 Additional sources of SO42‑ are quite different than Cl− and Na+ though its increase was also significantly correlated with growth in population (r = 0.885, p < 0.05) and GDP (r = 0.90, p < 0.05) in the basin. As shown in Figure 4d−f, long-term variation of SO42‑ concentration in Taihu lake was significantly correlated to the SO2 emission, coal consumption, and energy consumption in the basin (p < 0.05). Particularly, coal and energy consumption better explained the SO42‑ increase (r2 = 0.89 and 0.96, respectively, Figure 4e and f) in Taihu lake than simply SO2 emission (r2 = 0.38, Figure 4d), indicating the major source (energy use) of the air pollutants in this region. It is believed that emissions associated with energy use are the primary source resulting in atmospheric composition including sulfate.33 As the largest developing country, China has been using coal as the major energy source, which constitutes about 75% of all energy sources. Therefore, one of the most concerned atmospheric pollutants in China is sulfur dioxide (SO2).34 Ever-increasing coal consumption caused atmosphere SO2 to increase in the past 50 years. As a result, sulfate accounted for nearly 50% of the total ions in the rain in the Taihu region.35 Although the control policy of SO2 emission was implemented from the late 1990s and SO2 decreased by 15% by the early 2000s,36 it still remains high in many Chinese cities. For example, the average atmospheric SO2 concentration in Wuxi City in the Taihu basin (Figure 1) was 59 μg/m3 in 2011,37 nearly 3 times higher than the WHO standard.34 Nevertheless, unlike Cl− with the ever-increasing trend throughout the 1950s−2010s, the lower concentration of SO42‑ in the 2010s than in the 2000s (p < 0.05) (Figure 2) in Taihu water is likely a response of the SO2 emission control mentioned above. The increased atmospheric sulfate has caused widespread acidic precipitation, characterized by increased frequency and intensity of the acid rain in the Taihu basin (Figure 5a) as well
water chemistry in such an area is typically characterized by the Ca-HCO3 type,21,29 that is, Ca2+ is the dominant cation and HCO3− is the dominant anion. The lake water chemistry of the 1950s−1990s was in agreement with this type (Figure S1). However, the current water chemistry (average of 2011− 2012), characterized by high Na+ over Ca2+ since the 2000s and high Cl− or sometimes even SO42‑ over HCO3− (Figures 2 and 3), is inconsistent with the above theory. Although this water chemistry does exist in the upstream of the Yangtze river due to evaporites and strong evaporation there,28,29 the Taihu basin is excluded from these natural conditions. This inconsistency indicates that a great amount of additional Na+, Cl−, and SO42‑ inputs to Taihu lake has altered its natural water chemistry. The triangle plot used by Hu et al. to indicate the dominant factors of the major ions28 will greatly bias the result (see the abstract art) if used for the dominance analysis of the current Taihu water chemistry. We believe that it is the intensive anthropogenic activities in the basin that have caused these ions as well as TDS to be increased and altered the natural water chemistry of Taihu lake. However, the mechanism of the increase is different between Na+ or Cl− and SO42‑. In natural waters, Na+, Cl−, and SO42‑, which are mainly derived from the weathering process of evaporites, are more sensitive to human disturbances than other ions.21 The lower concentration of Na+ relative to Ca2+ and lower concentration of Cl− or SO42‑ relative to HCO3− before the 1990s (Figure 2 and 3) represented the typical freshwater chemistry of Taihu lake and were consistent with the regional geography.30 The increased concentration of these ions is closely linked to the rapid development of the regional population and economy, especially since the 1990s, when China began to accelerate its economy policy. The two major anions Cl− and SO42‑ with higher increase rate (>1 mg·L−1 y−1) over the past 60 years were used to exemplify the correlation between their long-term change in the lake and the socioeconomic development in the basin (Figure 4). Statistics of the historical data showed that increasing Cl− concentration had a significant correlation with population, domestic sewage, and GDP (p < 0.05) in the Taihu basin (Figure 4a−c), indicating that the natural balance of the chemical species in the lake has been broken by anthropogenic additions derived from the domestic and industrial activities, and these socioeconomic variables explained over 80% of the variance of Cl− increase based on best fit regression (p < 0.05), with the domestic sewage having the highest explanation (Figure 4b). This indicates that domestic sewage is a direct dominant for Cl− increase. Na+ had a similar correlation with the above parameters. Human wastes contain salt (NaCl) and constitute an important source of Na+ and Cl− in domestic sewage. It was reported that Cl contribution from human wastewater could be calculated as the product of population and a per capita human generation rate of 18 kg/capita/year.7 However, for the Chinese people, this value could be higher as their daily consumption of salt (12−15 g or even more) is much higher than the recommended amount by WHO31 due to food habits. With the current population in the basin increasing by 3-fold compared with that in 1950, a huge amount of additional salt was excreted and contributed to the increase of Cl and Na in the lake. Another important input of Cl− is from industrial discharges, which is indicated by the significant correlation between Cl− concentration and GDP in the basin (Figure 4c); and statistical calculation indicated that GDP in the basin in 9098
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Figure 5. Long-term variation of acidic rain in the Taihu basin (a) and its relationship with SO42‑ concentration (r = −0.869, p < 0.01) in the lake (b).
as in the middle and downstream of the Yangtze River.36,38 This process, additionally, led to increased SO42‑ in the Taihu water (Figure 5b). Figure 5b shows the significant negative correlation (r = −0.869, p < 0.01) between the pH value of the rainfall in the basin and SO42‑ concentration in Taihu lake, indicating that acidic precipitation in this region played an important role in the SO42‑ increase. This mechanism was also reported for some rivers in the Yangtze basin.39 Although some major ions also increased in these rivers in a similar way as Taihu water, the extent is much less compared with Taihu lake. In addition, as the Yangtze river naturally does not discharge into Taihu except when artificial water transfer to alleviate pollution in the lake,18 it is safely concluded that the upstream change in the Yangtze river had little impact on Taihu and it is drivers within the Taihu basin that caused its alteration. Different from Na+, Cl−, and SO42‑, increased Ca2+ and Mg2+ in Taihu lake were not directly from human discharges but closely linked to the acidic atmospheric deposition. The hardness to alkalinity ratio (equivalent concentration) can be used to indicate whether the natural water is affected by acidification from human sources.40 The ratio ≤1 implies that the dissolution of carbonates is only caused by carbonic acid, which is an equilibrium result of carbon dioxide in the air dissolved in the rain; while the ratio >1, it indicates that the dissolution of carbonates is also driven by anthropogenic acid inputs40 such as sulfuric and nitric acids. The long-term variation of hardness to alkalinity ratio (1950s−2010s) of Taihu water clearly shows that before the 1970s there was no human addition of acid contributing to carbonate dissolution, but, subsequently, human inputs played a significant role (ratio >1, Figure S3a) in the leaching process of the carbonates that are widely distributed around the Taihu basin.21 As acidic rain in most of the areas of China is mainly the sulfuric acid type,34 this process can be expressed by the following formulations:
pH value of Taihu lake water had no significant decrease during the past decades (Figure S3b) under the circumstance that acidic rain became increasingly serious (Figure 5a) in this region, that is, the carbonates around the lake basin have neutralized acidic rain and therefore buffered the pH value of the lake water. The above formulations also show bicarbonate (HCO3−) produced in the process; however, unlike other ions, HCO3− had no increase trend (Figure 2), and alkalinity did not increase along with hardness in the past decades (Figure S3a). This inconsistency is attributable to increasingly frequent outbreaks of algae bloom13 that consumed HCO3− in photosynthesis.42 With the increase of net primary production, an increased proportion of inorganic carbon is needed in maintaining the phytoplankton biomass in the lake,43 and this response has offset HCO3− increase. This mechanism can also largely explain why the pH value has not dropped during the period of acidification (Figure 5a). In actuality, the pH value increases with the increasing photosynthetic metabolism of phytoplanktons because hydroxyl ions are produced in the biochemical process.44 This is in good agreement with our findings (Figure S3b) that pH had the highest value during the 2000s when algae blooms were the most intensive and 2007 saw the peak bloom.4,13,45 However, the acidification in this region (Figure 5a) has neutralized alkalinization in the lake, resulting in no significant trend of pH during past decades (Figure S3b, p = 0.652). Although the increased major ions had different mechanisms, they were all attributed to human activities in this basin. This homogeneity can be further illustrated by the change of correlations between ions from the 1950s−2010s, i.e., before the 1980s, except for Cl−Na, there was no significant correlation between them; while since the 1990s, they all became significantly correlated (Table S2), reflecting that human impact became increasingly significant on the major ions. One of the important factors influencing the water chemistry of surface waters is the rainfall and evaporation relationship.9 However, the rainfall in Taihu area had no significant change, while the evaporation has decreased somewhat over the past decades (Figure S4), indicating that evaporation-crystallization is not a controlling influence over the change in water chemistry of the lake.
2CaCO3 + H 2SO4 =2Ca 2 + + 2HCO3− + SO4 2 − , 2MgCO3 + H 2SO4 =2Mg 2 + + 2HCO3− + SO4 2 −
Together with other anthropogenic acidification of the topsoil in this region,41 the leached Ca2+ and Mg2+ resulted in a very large increase of these two cations in Taihu lake (Figure 2). Consequently, the current hardness is 2.84 times higher than 60 years ago. These formulations can also partially explain why the 9099
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reviewers for their careful work and insightful comments to improve an earlier version of the manuscript.
Turnover period of lake water is another influencing factor for water chemistry.46 For the purpose of reclamation of farmlands and flood prevention, many gates and dykes were constructed around the lake since 1949,47 which led to a major decrease of the water surface area of Taihu lake and thus significantly delayed the turnover of lake water. The prolonged residence time also facilitated the increase of ions and TDS in the lake. Although Taihu is still a freshwater lake in terms of TDS (
