Analysis of black water aggregation in Taihu Lake

Gui-hua LU*, Qian MA, Jian-hua ZHANG

1. National Engineering Research Center of Water Resources Efficient Utilization and Engineering Safety, Hohai University, Nanjing 210098, P. R. China

2. Hydrology and Water Resources Investigation Bureau of Jiangsu Province, Nanjing 210029, P. R. China

3. Water Resources Department of Jiangsu Province, Nanjing 210029, P. R. China

Analysis of black water aggregation in Taihu Lake

Gui-hua LU*1, Qian MA2, Jian-hua ZHANG3

1. National Engineering Research Center of Water Resources Efficient Utilization and Engineering Safety, Hohai University, Nanjing 210098, P. R. China

2. Hydrology and Water Resources Investigation Bureau of Jiangsu Province, Nanjing 210029, P. R. China

3. Water Resources Department of Jiangsu Province, Nanjing 210029, P. R. China

Black water aggregation (BWA) in Taihu Lake is a disaster for the lake environment. It is a phenomenon resulting from water environmental deterioration and eutrophication caused by accumulation of pollutants in the lake, according to research on the water quality, pollutants of BWA, and occurrence mechanisms of BWA. Dead algae are the material base of BWA, the polluted sediment is an important factor for the formation of BWA, and hydrological and meteorological conditions such as sun light, air temperature, wind speed, and water flow are the other factors that may lead to the formation of BWA. Thioether substances such as dimethyl trisulfide are the representative pollutants of BWA. Parameters such as chlorophyll-a, DO, pH, and water temperature are sensitive indicators of BWA. Measures such as algae collection, ecological dredging, pollution control, and water diversion from the Yangtze River to the lake, are effective, and strengthening aeration is an emergency measure to control BWA.

black water aggregation; water quality indicator; control measure; Taihu Lake

1 Introduction

Being one of the five largest freshwater lakes in China, Taihu Lake is of vital importance in regional flood control and water supply. Accounting for about 0.4% of the land area and 3.5% of the population in China, the Taihu Lake Basin creates more than 11% of the national gross domestic product (GDP) and 20% of the national fiscal revenue. With the most centralized urban areas, a highly developed economy, and the greatest social productivity, the basin plays a large role in the socio-economic development as one of the core areas in China.

Black water aggregation (BWA) appeared at the Nanquan water source in Gonghu Bay in the northern part of Taihu Lake at the end of May in 2007. The drinking water supply for millions of people in Wuxi City near the lake had to be cut off because the water was so polluted and cannot be purified to the required standard, causing a water supply crisis that shocked the world. BWA appeared again in the coastal areas of Zhoutie Town and Dapu Town in Yixing City in Taihu Lake at the end of May in 2008, with a maximum area of up to 17 km2. It was scattered over the area and took a week to disappear gradually with the wind waves.Several small BWAs were also found in the western and northern parts of Taihu Lake in 2009 and 2010. The occurrence of BWA resulted in rapid deterioration of water quality and severe destruction of the ecosystem, and posed a significant challenge to the water supply for urban and rural residents. Since research on BWA is still in its preliminary stage currently, there exist various views concerning the formation of BWA. Some have said that BWA is the result of a blue-green algal bloom; some have insisted that it is caused by exotic pollution sources, and others think it is brought in through water transfer. For large shallow lakes like Taihu Lake, in order to reduce the occurrence of BWA, protect the lake ecosystem, and secure the water supply, it is of great importance to analyze the formation mechanism of BWA, to explore sensitive indicators of its occurrence, and to study prevention and control measures based on tracking and monitoring data.

Analysis of tracking and monitoring data from recent years indicates that there are two possibilities regarding BWA’s formation. One is that BWA results from the biochemical reaction of accumulated and dead algae after algal blooms in eutrophic lakes with organics from sediment under appropriate meteorological and hydrological conditions. The other is that BWA is directly caused by the contaminated sediment under certain meteorological and hydrological conditions (Lu and Ma 2009).

2 Occurrence of BWA in Taihu Lake since 1990s

BWA has been observed many times in western Taihu Lake, Meiliang Bay, and Gonghu Bay since the 1990s. According to the records of the Wuxi Water Resources Department, BWA took place after algal blooms in Meiliang Bay during the periods of July 1 to 25, 1990, July, 1994, July 5 to 8, 1995, August 1 to 10, 1998, and August, 2003. At the end of May, 2007, BWA occurred following a large-scale algal bloom in the Nanquan water source and caused a drinking water supply crisis in Wuxi City. From May 26 to June 9, 2008, BWA, with a maximum area of 17 km2, occurred in the coastal area of Taihu Lake in Yixing City. In 2009, BWA occurred 11 times within the area of 22 km2along the western and northern parts of the lake: once in May, in June, and in September, and five and three times respectively in July and August. In 2010, BWA was again found four times in the western and northern areas of the lake, the area of BWA on July 23 reaching nearly 6 km2in the western part of the lake. Fig. 1 is the location map of the occurrence of BWA in Taihu Lake from 2008 to 2010.

According to the study of Kong et al. (2007), there are four key stages in the occurrence of BWA, which have been confirmed by simulation experiments, reviews of the formation process, and field investigations. Based on observation, tracking, and monitoring of the phenomenon of BWA in Taihu Lake, we note details of the four key stages: (1) Stage of substance accumulation or the early stage: the algal bloom provides a material base for BWA, and the combination of accumulated algae and organics in the sediment is likely to be the substance factors of BWA formation. (2) Stage of aerobic decomposition: the accumulation ofalgae is closely related to the process of aerobic decomposition. With the decreasing of oxygen, the hypoxic process begins to appear. On the other hand, the amount of oxygen in water has a close relationship with the flow pattern and weather conditions. (3) Stage of anaerobic decomposition: with the depletion of oxygen in the water, the decomposition of organics transforms into anaerobic decomposition by utilizing small-molecule organic matter as electron acceptors with microbial action. (4) Stage of formation of BWA: with the decomposition of large amounts of anaerobic organics, malodorous black substances emerged. The combination of ferromanganese materials and reduced sulfur compounds in the sediment floats upward to water under the effects of volatile organic compounds, resulting in BWA, which moves along with the water flow and wind, leading to ecological disasters.

Fig. 1 Location of BWA occurrence in Taihu Lake from 2008 to 2010

From the four key stages in the occurrence of BWA in Taihu Lake, it can be seen that the chemical and physical characteristics and the hydrological and meteorological factors affect the formation of BWA in each stage. Therefore, they should be given more attention in the following study.

3 Basic characteristics of BWA

3.1 Appearance of BWA

BWA occurring in the coastal area of Taihu Lake near Yixing City from May 26 to June 10, 2008 is taken as an example. The BWA took place in the water area near Zhoutie Town and Dapukou Town in Yixing City, and is named BWA526. The area of BWA526 changed with a daily average area of 7.5 km2. Bubbles appeared sometimes, and the water was turbid and black, showing clear boundaries with other water in the lake. There were smells of sewage and hydrogen sulfide, and some dead fish, mainly lake loaches, were found in water surface.

Table 1 shows the monitoring results of BWA526. Compared with the pollutants in thewater without BWA, the dissolved oxygen (DO) concentration of BWA526 was 6.7 mg/L lower, and the concentrations of permanganate index (CODMn), chemical oxygen demand (COD), total phosphorus (TP), ammonia nitrogen (NH3-N), and total nitrogen (TN) were 4.1 mg/L, 29.0 mg/L, 0.327 mg/L, 4.54 mg/L, and 2.03 mg/L higher, respectively. The lowest concentration of DO was almost 0 mg/L, and the maximum CODMn, COD, TP, NH3-N, and TN were, respectively, 2.13, 2.19, 3.79, 4.72, and 1.64 times of the background value.

Table 1 Monitoring results of BWA526 in Taihu Lake

3.2 Monitoring results of BWA

Table 2 provides the monitoring data and the statistics of major water quality indicators of BWA in Taihu Lake in recent years, which shows the following results:

(1) The DO concentration has been terribly low, close to zero, while, the concentrations of CODMn, TP, NH3-N, and TN have been high. Compared with Grade Ⅲ inEnvironmental Quality Standards for Surface Waterin China, the concentration was three to four times higher for CODMn, about 100 times higher for TP, and about ten times higher for NH3-N and TN.

(2) The concentrations of CODMn, TP, NH3-N, and TN of BWA were quite high on May 11 and July 20, 2009. The concentrations of major water quality indicators of BWA on May 12 and July 21 in 2009 were close to those in 2007 and 2008.

(3) Comparing the water quality data of BWA occurring on May 26, 2008 with those on June 2, 2007, when BWA occurred 1.5 km east of the Nanquan water source, the results were roughly similar to each other. The DO concentration in the former was slightly higher, the CODMnconcentration was roughly equal, the TP and NH3-N concentrations were respectively 0.25 mg/L and 0.65 mg/L higher, and the TN concentration was 5.8 mg/L lower. Dimethyl trisulfide (DMTS), geosmin, and other materials were found in both water samples of BWA. Therefore, the composition of BWA occurring near Yixing City in 2008 was similar to that near the Nanquan water source of Wuxi City in the lake, and the causes were considered basically the same.

3.3 Main pollutants of BWA

According to the monitoring data, the odor of BWA is significantly different from the smell caused by algal bloom. Smell from an algal bloom is mainly produced by secondary pollutants like 2-Methylisoborneol or geosmin, directly generated by the metabolism of algae, while the odor of BWA is very distinct. Yang et al. (2008) pointed out that the main cause ofthe odor of BWA in Taihu Lake near the Nanquan water source of Wuxi City in 2007 was sulfide material, particularly DMTS, produced by the decay of organisms in the anaerobic environment. Those materials can often be detected in contaminated river water. The concentration of DMTS in BWA was up to 0.011 4 mg/L, far more than 2-Methylisoborneol and geosmin. However, DMTS cannot be produced by the metabolism of algae and can only be generated in the process of protein decomposition of organisms under anoxic conditions (Yu et al. 2007).

Table 2 Results of main water quality indicators of BWA in Taihu Lake in recent years

3.4 Indicators of BWA

The sensitive indicators and representative pollutants of the key stages in the occurrence of BWA can be obtained through simulation experiments and statistical analyses. In the stage of substance accumulation, we need to focus on the meteorological and hydrodynamic factors resulting in the agglomeration of algal blooms. These factors are water temperature, dissolved oxygen, oxidation-reduction potential, pH, and the indicators of aerobe, as well as the contents of chlorophyll-a and xanthophyll, which are closely related to the stage of aerobicdecomposition. In the stage of anaerobic decomposition, the indicators are water temperature, DO, oxidation-reduction potential, pH, indicators of anaerobic microorganisms, typical smelly substances, NH3-N, hydrogen sulfide, and manganese sulfide. The variations in DO, oxidation-reduction potential, pH, and NH3-N need to be given close attention in the stage of BWA formation.

Although the detected odor materials like sulfide can be used significantly as indicators of the occurrence of BWA, the collection and analysis of those materials are not practical, usually taking long time. Based on the simulation of BWA and statistical analysis, the following parameters can be selected as indicators of BWA: chlorophyll-a, DO, pH, and water temperature. In general, anoxic or anaerobic conditions can form easily under conditions of water temperature above 20℃, chlorophyll-a up to 2 000 μg/L, and pH in water close to or less than 7.0. The creation of a great amount of odor materials like sulfide is indicated when the DO concentration drops below 2.0 mg/L.

4 Causes of formation of BWA

It was believed that the cause of BWA was the lake eutrophication caused by increasing pollutant loads in recent years (Zhu 2008). Based on the tracking and monitoring of BWA, analysis of water quality, algal blooms, sediment distribution, and meteorological and hydrological data over a long period, a large-scale blue algal bloom and accumulation of contaminated sediment are mainly blamed for the occurrence of BWA, and the sun light, air temperature (generally higher than 25℃), wind speed (3 m/s to 5 m/s), water flow, and other meteorological and hydrological conditions are the trigger factors of BWA.

4.1 Water quality and algal bloom in Taihu Lake

4.1.1 Significant deterioration of water quality

According to analyses of the monitoring data of water quality in Taihu Lake, the water quality was Grade Ⅰ to Grade Ⅱ in the 1960s, Grade Ⅱ in the 1970s, Grade Ⅱ to Grade Ⅲ on average in the early 1980s, Grade Ⅲ and partly Grade Ⅳ or Ⅴ in the late 1980s, and GradeⅣon average in the mid-1990s with Grade Ⅴ water in one third of the lake area. Water quality in the lake has declined by one grade level every ten years on average, and the deterioration rate has been exacerbated noticeably in the past ten years (Xu and Qin 2005; Zheng et al. 2001).

Since 2004, the water quality in Taihu Lake has been Grade Ⅴ or worse than Grade Ⅴ in the whole lake except the eastern part of the lake where it has been Grade Ⅳ, and the main pollution parameters have been TP and TN. From 2004 to 2009, the average concentration throughout the lake varied from 0.071 mg/L (Grade Ⅲ) to 0.104 mg/L (Grade Ⅳ) for TP and from 2.25 mg/L (inferior Grade Ⅴ) to 3.38 mg/L (inferior Grade Ⅴ) for TN. Values varied from 0.080 mg/L (Grade Ⅲ) to 0.165 mg/L (Grade Ⅳ) for TP and from 3.01 mg/L (inferior Grade Ⅴ) to 6.57 mg/L (inferior Grade Ⅴ) for TN in Meiliang Bay, and varied from0.092 mg/L (Grade )Ⅲ to 0.158 mg/L (Grade ) Ⅳfor TP and 2.99 mg/L (inferior Grade )Ⅴ to 4.57 mg/L (inferior Grade )Ⅴ for TN in Gonghu Bay.

4.1.2 Pollutant loads into lake

According to 2009 monitoring data from the Taihu Basin Authority of the Ministry of Water Resources of China, the loads of TN, TP, and CODMninto Taihu Lake were, respectively, 4.4 × 104t, 1.9 × 104t, and 5.6 × 104t, which were 4.7, 3.8, and 1.7 times, respectively, of the permitted loads in theComprehensive Program of Water Environmental Rehabilitation in the Taihu Lake Basinratified by the State Council of China in 2008. However, pollutant loads of TN, TP, and CODMnflowing out of Taihu Lake were, respectively, 1.4 × 104t, 455 t, and 3.4 × 104t. This indicates that the pollutant loads discharged into the lake were far greater than those flowing out of the lake. It is estimated that there were 1.5 × 103t of TP, 3.0 × 104t of TN, and 2 × 104t of CODMnretained in Taihu Lake in 2009 (Ma et al. 2009). From 1998 to 2009, the average pollutant loads flowing into the lake were 5.6 × 104t for CODMn, 1.7 × 104t for TP, and 4.0 × 104t for TN, values far beyond the environmental carrying capacity of Taihu Lake, 34 369 t for CODMn, 514 t for TP, and 8 510 t for TN.

4.1.3 Internal pollutants

Over a long period of time, a large amount of external sources of pollutants has been brought in by river water running into the lake, particularly the areas with massive breed aquatics in the east of the lake, and the accumulation of pollutants in the lake has been increasing (Wang et al. 2009). According to theSediment Dredging Planning Program in Taihu Lakeissued by the Taihu Basin Authority, the deposition area of sediment in the lake in 2005 was 1 547 km2, accounting for 66% of the whole lake area. Accumulation of pollutants in sediments in Zhushan Bay, Meiliang Bay, Gonghu Bay, the eastern part of the lake, and the inlets of the lake were most serious, with a deposition depth of 0.8 m to 1.5 m, consisting of major internal sources of water pollution in the lake. The release of nitrogen from internal sources accounted for about 22.5% of the TN load of the whole lake, and the release of phosphorus from internal sources accounted for about 25.1% of the TP load of the whole lake.

4.1.4 Algal bloom

According to monitoring data from the lake from 1987 to 2007 (Ma et al. 2010), algal bloom in the lake occurred more frequently year by year. On the basis of the analysis of MODIS satellite images, algal blooms occurred only in one month of all the years before 1998, and in at least two months each year after 1998. From 2005 to 2009, algal blooms occurred in more than six months in each year. From 1987 to 1998, algal blooms generally occurred in summer, from May to July, less often in August, and never in spring and winter. But after 1999, the frequency of algal blooms increased and the duration was prolonged, especially in 2007, when algal blooms occurred in all months of the year except January and February. Therefore, whether in frequency or in scale, algal blooms have been becoming more and more serious in the last two decades. However, the scale of algal blooms in the lake has been reduced asintensive rehabilitation measures have been implemented since 2007.

The accumulated dead algae in areas along the lake shore after large-scale algal bloom, which were confirmed by simulation experiments, are the material base of BWA and the reason why BWA has occurred frequently in these water areas.

4.2 Distribution of sludge in Taihu Lake

The sediment in Taihu Lake is the main destination of pollutants (such as nutrients, heavy metals, and organic toxicants) from outside of the lake. Sludge containing abundant nutrients may become the potential pollution source of the lake, as nutrients can be released into the lake under appropriate conditions, increasing the nutrient load in the upper part of the lake water. Thus, close attention should be paid to the siltation, distribution, and content of pollutants in the sludge in the lake.

Sediment exists in most areas of Taihu Lake, affected by the inflow, outflow, wind-induced current, and terrain of the lake bottom. Since the 1980s, with the accumulation of a large amount of pollutants in sediment due to severe water pollution, the concentrations of nitrogen, phosphorus, and organic matters have been relatively high, and have been transported, dispersed, and deposited with the lake current. In addition, lake utilization and creature residues in the lake have also influenced the sediment.

Sediment in the lake consists of 88% silt and 12% sludge with various thickness, located in the upper layer of sediment. The sludge has a moisture content of 55% to 85% and a density of 1.5 g/cm3to 1.8 g/cm3, and the silt has a moisture content of 85% to 150% and a density of 1.2 g/cm3to 1.5 g/cm3.

Taihu Lake is a typical shallow lake. Sediment is likely to suspend due to the agitation of wind when the temperature is relatively high. Although the amount of sludge is much less than that of silt, the main direct material source of BWA is the pollutants from the sludge-concentrated region. The latest survey on the sediment in the lake reveals that the thickness of the sludge containing organic matter in the western part of the lake is 0.02 m to 0.31 m, with a total amount of 2.793 × 106m3.

Fig. 2 shows the distribution of sludge in the western and northern parts of the lake. It shows that the areas where BWA commonly occurred in recent years almost match the areas where sludge was abundant. The water areas with a large amount of sediment are the places where BWA occurred readily. Similar to the effect of the accumulation and death of algae, the existence of sediment provides large amounts of organics, which are the material bases for the occurrence of BWA.

4.3 Meteorological and hydrological conditions of BWA occurrence

Fig. 2 Isolines of depth of sludge in northern part of Taihu Lake (unit: m)

The air temperature, water temperature, and wind speed are the main causes of BWA. Based on data from seven automatic monitoring hydrological stations around Taihu Lake, Xishan, Guangjingkou, Shadungang, the Taipu River entrance, Dapukou, Jiapu, and Xiaomeikou, when BWA occurred, the air temperature in the western part of the lake was 25℃ to 35℃, the mean water temperature was 24℃ to 32℃, and the wind direction changed frequently. The average wind speed was about 3.1 m/s, and the maximum wind speed sometimes reached up to 15.7 m/s.

4.3.1 Wind direction

Based on the statistic analysis of the meteorological data in Yixing City, when BWA took place, the main wind direction was from the southeast and the south, and sometimes it was from the west and the northwest. The wind was between three and four knots on the Beaufort scale, leading to the transportation of algae moving from the southern and central parts of the lake to the western and northern parts of the lake, increasing the amount of algae in the northwestern part of the lake.

4.3.2 Temperature

Metrological data around the lake show that seven to ten days before BWA was observed in the western and northern parts of the lake, the sunshine was sufficient in Yixing City, with air temperature rising, and hence the water was heated, accelerating the death of algae and producing a large amount of organic residue of algae. The water area had a severe lack of oxygen, due to a large amount of oxygen being consumed by microbial degradation. On the other hand, toxic intermediates were produced in the course of the incomplete decomposition of algae fragments (Sun et al. 2007).

4.3.3 Water level

BWAs both in the western and northern parts of the lake from the 1990s to 2008 wereanalyzed, and the results show that the water level was lower than 3.30 m when they occurred, with only a few exceptions. It is assumed that a lower water level is favorable for the occurrence of BWA. Nevertheless, based on analyses of 15 BWA observations from 2009 to 2010 in the western and northern parts of the lake, only a minority of BWAs occurred when the water level was less than 3.30 m, and the highest water level reached during their occurrence was 4.10 m. It seems that the water level in Taihu Lake has no absolute relationships with the BWA occurrence.

4.4 Sensitive areas of BWA and investigations

Based on analysis, the water areas where BWA can be observed are (1) water areas with many inlets from severely polluted rivers, (2) water areas with thick silt and sludge, (3) water areas where algae readily accumulates, (4) water areas or bays with little water exchange, and (5) water areas where BWA has been observed before. The sensitive areas of BWA in Taihu Lake are shown in Fig. 3.

Fig. 3 Sensitive areas of BWA in Taihu Lake

In order to obtain better information about BWA, measures such as combining random investigation with regular observation of the lake, analyzing the data both from field monitoring in the lake and from laboratory testing, integrating daily reports with regular analysis, and integrating data from hydrology departments and environmental protection departments should be adopted. Water sampling and monitoring should be carried out, with a focus on the sensory quality of BWA and algae, transparency, water temperature (both in surface water and deep water), wind speed, wind direction, DO (both in surface water and deep water), pH, the density of algae, and so on, of which DO, pH, and the density of algae would be monitored in the field with multi-parameter water quality meters. In case the water area is covered by algae, we should note specific information about the location, and area, and send the samples to the laboratory if necessary.

4.5 Control and prevention measures of BWA

On the basis of preliminary analysis of the causes and mechanism of BWA, measures such as algae collection, ecological dredging, pollution control, and water diversion from the Yangtze River to the lake are effective in mitigating and controlling BWA. Meanwhile, more efficient investigations in relevant BWA-sensitive areas and the setup of monitoring-warning systems are helpful for early alert and handling of BWA emergencies.

It is possible to prevent and control BWA, as the frequency and areas of BWA have decreased significantly since large-scale measures including algae collection and ecological dredging were conducted for lake rehabilitation.

5 Conclusions

(1) BWA is a phenomenon resulting from water environmental deterioration and eutrophication caused by accumulation of pollutants in Taihu Lake. Thioether substances such as dimethyl trisulfide are the representative pollutants of BWA. Preliminary study shows that chlorophyll-a, DO, pH, and water temperature are sensitive indicators of BWA.

(2) BWA is caused by the integrated reaction of dead algae and polluted sediment, and the former is the material base of BWA, while the latter is an important factor for BWA. Hydrological and meteorological conditions such as sunlight, air temperature, wind speed, and water flow are inducements of BWA in Taihu Lake.

(3) BWA can be prevented and controlled by pollution control and improvement of water environment of the lake. Measures such as algae collection, ecological dredging, pollution control, and water diversion from the Yangtze River to the lake are effective, and strengthening aeration is an emergency measure to control BWA.

(4) Multidisciplinary and multi-domain studies are necessary for the analysis of the causes of BWA. Nevertheless, there is no mature theory and experience up to now to explain the mechanism of BWA formation, which still needs further exploration and study.

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This work was supported by the National Water Project of China (Grant No. 2008ZX07101-011).

*Corresponding author (e-mail:lugh@hhu.edu.cn)

Received May 31, 2011; accepted Oct. 16, 2011