Water Air Soil Pollut (2010) 206:263–272 DOI 10.1007/s11270-009-0103-z
Evaluation of Sediment Contamination and Effectiveness of Dredging in Mid-to-lower Han River Kyung-Ik Gil & Lee-Hyung Kim & Gye-Chun Cho & Jaeyoung Yoon
Received: 16 November 2008 / Accepted: 20 May 2009 / Published online: 20 June 2009 # Springer Science + Business Media B.V. 2009
Abstract The Han River, which is the largest river in Korea, is the primary source of drinking water for the 20 million people that live in the Seoul metropolitan and surrounding areas. The sediments in the river are highly polluted due to pollutant inputs from upstream tributaries as well as from partially treated municipal wastewaters. To characterize the contamination of the sediments, disturbed and undisturbed sediment samples were periodically collected from eight locations of the mid-to-lower Han River. They were analyzed for pH, water content, total solids, ignition loss (IL), total phosphorous (TP), total Kjehldahl nitrogen
K.-I. Gil Department of Civil Engr, Seoul National University of Technology, Seoul 139-743, South Korea L.-H. Kim Department of Civil & Environ. Engr., Disaster Prevention Research Center, Kongju National University, Kongju, Chungnamdo 314-701, Korea G.-C. Cho Department of Civil & Environ. Engr., Korea Advanced Institute of Science and Technology, Daejeon 305-701, Korea J. Yoon (*) Department of Environ. Engr., Korea University, Jochiwon, Chungnamdo 339-700, Korea e-mail:
[email protected]
(TKN), and chemical oxygen demand (COD). The mean values of pollutant concentrations in disturbed sediment were determined to be 6.9% for IL, 1,700 mg/kg for TP, 3,350 mg/kg for TKN, and 65,710 mg/kg for COD. Pollutant concentrations of undisturbed samples were found to decrease with sediment depth and time due to the removal mechanism. Monitoring of pre- and post-dredging conditions was also performed, and the results show that the pollutant concentrations decreased from those for the pre-dredging condition to 33–57% for TP, 51– 64% for TKN, and 30–62% for COD. It is concluded that dredging was an effective means to reduce the internal pollutant source. Keywords Dredging . Forced-release rates . Han River . Sediments . Sedimentation rate . Undisturbed sample
1 Introduction The Han River is 469.7 km long with a watershed of 26,219 km2. It begins in North-East Kangwon province and travels to the Yellow Sea through the Seoul metropolitan areas. It is the primary source of drinking water for the city of Seoul and for the surrounding provinces of Gangwon and Gyeonggi. Of the various pollution sources, nonpoint sources are the cause of many of the problems in the river.
264
Nonpoint pollutants are widespread because they can occur at any time under any type of land use (Bostrom 1988; EPA 1994). In spite of the billions of dollars that have been spent on the clean-up of municipal and industrial wastewaters, many water bodies including the Han River remain polluted due to nonpoint pollution. Currently, nonpoint sources are responsible for approximately up to 45% of the total suspended solids (TSS), 50% of the chemical oxygen demand (COD) and 80% of the total phosphorus (TP) inputs (Kim et al. 2004). Monsoon weather typical of Asian countries creates a concentrated rainy season during summer and washes off significant amounts of pollutants from nonpoint sources including rice paddy fields that are prone to the heavy use of phosphate and nitrate fertilizer. Subsequently, the receiving water bodies suffer from eutrophication. Pollutants entering water bodies may be adsorbed into the sediments and accumulate on the bottom of rivers or lakes (To 1974; Hieltjes and Lijklema 1980; Lijklema et al. 1993). These sediments may accumulate over long periods and can act as internal pollutant sources in the future. The pollutants may be transferred from the water column to the sediment layer through biochemical and physical reactions such as ion exchange, adsorption, and precipitation (Abrams and Jarrell 1995; Stumm and Morgan 1996). The pollutants may also be released from the sediments if the overlying water quality changes (Williams et al. 1980; Wetzel and Likens 1992; Jin et al. 2006). According to Kim et al. (2003, 2004), the seasonal phosphorus release mass from sediments in the Jamsil submerged dam area (midstream of the Han River) were determined to be in the ranges of 200–400 ton/y for TP and 23–40 ton/y for PO4-P in the winter, and 3,400–5,100 ton/y for TP and 225–440 ton/y for PO4-P in the summer, respectively. The mechanisms for reintroduction include advection, ion exchange, molecular diffusion, and biologically mediated changes. In order to protect water quality from these internal pollutant sources, various control methods such as sediment capping, vegetating, and dredging, etc., have been studied. Of these methods, dredging is typically used as a general sediment controlling method. In the Han River, dredging was carried out over a period of 5 years, from 1994 to 1999 in order to improve the water quality by removing the polluted sediments.
Water Air Soil Pollut (2010) 206:263–272
In spite of the magnitude of the problem of sediments as an internal source of pollution, minimal research has been carried out that has dealt with the pollutant characteristics of riverbed sediments with depth by analyzing undisturbed sediments with the intention of finding out its implication on remediation dredging. This is due to the difficulty involved in the measurement and analysis of samples. This present research was performed in order to more deeply understand the seasonal pollutant concentration changes occurring to the sediments and in order to characterize the sedimentation and release characteristics of river sediments. In addition, a comparison is carried out of the pollutant concentrations in the sediments before and after dredging in order to quantitatively assess its effectiveness as a measure of internal pollutant source management.
2 Materials and Methods As shown in Fig. 1, eight monitoring locations were chosen from the mid-to-lower reaches of the Han River as it passes through Seoul. The Jamsil submerged dam is located in the research area, in the eastern part of Seoul (M1 and M2 monitoring locations). The weir maintains a minimum water level and prevents back mixing from more polluted sections of the lower river. Maintaining a minimum water level is necessary to sustain a stable volume for drinking water treatment plants. Four major tributaries are located in the downstream research area of the Jamsil submerged dam; the Tancheon (near M3 and M4), the Jungrangcheon (near M5 and M6), the Banpocheon (near M7), and the Ukcheon (near M8). These tributaries are more polluted compared to the mainstream Han River. Water samples, as well as disturbed and undisturbed sediment samples, were collected from eight monitoring locations from May 1997 to April 1998. The sampling frequency at locations M1 and M2 was one per 2 months for disturbed sediment and one per 4 months for undisturbed sediment. At locations M3 through M8, the sampling frequency was one per 3 months for disturbed sediment and one per 4 months for undisturbed sediment. Water samples were collected using a Van Dorn sampler and were analyzed for temperature, as well as for concentrations of DO, TSS, COD, total nitrogen (TN), and TP, using
Water Air Soil Pollut (2010) 206:263–272
265
Fig. 1 Monitoring locations
Sampling Locations
N
Han River M8
M5 M2
M6
M1 M4
M3
M7 Submerged Dam
Seoul
JAPAN
KOREA
Standard Methods (AWWA 1995). Disturbed sediment samples were collected with a Ponar Grab style sampler, while undisturbed sediment samples were collected by divers with acryl tubes (4 cm diameter by 50 cm long). It is believed that disturbed sediment samples more significantly represent summer flood conditions, whereas undisturbed samples represent non-rainy seasons. The collected sediment samples
were analyzed using Standard Methods (AWWA 1995) for pH levels, water content (%), total solids (TS), IL, TP, TKN, COD, and sulfates. Sediment size distribution was measured using the sieve analysis method (Das 1990). This method separates the sediment into clay (smaller than 0.002 mm), silt (0.002 to 0.05 mm), and sand fractions (0.05 to 2 mm).
266
Water Air Soil Pollut (2010) 206:263–272
Fig. 2 Particle size distributions of sediments from submerged dam (M1 and M2; hollow dot) and downstream (solid dot)
then analyzed to determine the concentration of chemical constituents at each sample location in time. Also, forced-release tests were carried out in order to estimate the pollutant release masses during flood from sediments to the water column. The test was performed by mixing 200 g of wet sediment with 500 ml of distilled water (nitrogen gas stripping followed by sodium sulfite addition), which was then shaken at 20°C for 48 h. The details of the method are available in previously documented research (Kim et al. 2003).
A column test with the dimensions of 180 cm height and 18 cm diameter was performed in order to find the sedimentation rates of the samples from water column to sediments. Homogeneous suspension (Aiguier et al. 1996) was used, which is a technique that determines the settling velocities of solids. The hand flipping of the column creates a uniform distribution of solids which are able to settle throughout the column before the test. The samples are then withdrawn from four ports at equally spaced intervals. They are Table 1 Statistical analysis of disturbed sediments Parameters
pH (–)
No. of cases
36
Water content (%) 36
TS (%) 36
IL (%) 36
TP (mg/kg)
TKN (mg/kg)
TCOD (mg/kg)
SO4 (mg/kg)
36
36
36
36
Min.
6.8
40.9
3.9
2.5
250
880
8,720
350
Max.
7.4
96.1
59.1
14.1
3,840
6,810
121,100
2,770
Median
7.2
58.1
41.9
6.9
1,885
3,350
75,600
2,090
Mean
7.1
65.5
34.5
6.9
1,700
3,350
65,710
1,850
95% CI upper
7.2
71.6
40.6
8.0
1,990
4,000
76,920
2,130
95% CI lower
7.1
59.4
28.4
5.8
1,410
2,700
54,510
1,570
Standard deviation
0.14
17.9
17.9
3.2
1,915
33,120
866.8
820.5
Water Air Soil Pollut (2010) 206:263–272
267
3 Results and Discussions 3.1 Statistical Analysis of Disturbed Sediments
TP TKN COD SO4
IL
TP
TKN
COD
SO4
Matrix of Bonferroni Probabilities
Pearson correlation matrix
Fig. 3 Matrix of correlations and Pearson coefficients
IL
The particle size distributions of the sediment are important because smaller particles have a greater specific surface area for pollutant adsorption. As shown in Fig. 2, the sediments of the Han River are classified as silty clay, clay loam, or clay, using the U.S. Department of Agriculture textural classification (Das 1990). The scheme was intended to be used for a possible
application of the dredged sediments for agricultural lands. When compared to previous results (Kim et al. 2003) obtained in upper streams, the particles for the downstream Han River were considerably smaller than those in the upstream area, suggesting the potential presence of a high concentration of pollutants. Table 1 shows a summary of a statistical analysis for pollutants in disturbed sediments. Of the pollutant parameters, IL refers to the organic portion of the sediments. The ranges of IL are from 2.5% to 14.1% and the mean value is 6.9%. The mean concentrations
IL
TP
TKN
COD
SO4
IL
1
0
0
0
0
TP
0.92
1
0
0
0
TKN
0.84
0.81
1
0
0.003
COD
0.76
0.84
0.83
1
0
SO4
0.68
0.83
0.56
0.82
1
268
Water Air Soil Pollut (2010) 206:263–272
Figure 3 shows the matrices of a Pearson correlation and Bonferroni probabilities showing the relationships between pollutant parameters in sediments. At the far left-hand side of the figure, a scattergram shows the data points and +/− 95% confidence intervals (smooth lines). The data distribution is shown as a histogram in the boxes on the far righthand side. The lower table shows correlation coefficients (R) below the diagonal and confidence values (P values) above the diagonal. In general, the pollutant parameters closely correlate with each other because they are strongly associated with sediment particles and have accumulated below the water column. Among these parameters, IL is the least expensive to measure and was found to correlate reasonably well with other parameters that require more expensive chemical analysis. It is thought that IL could be used as a good proxy for the estimation of other pollutant parameters once the proper statistical relationship is developed.
for other parameters are 1,700 mg/kg for TP, 3,350 mg/kg for TKN, and 65,710 mg/kg for COD. The 95% confidence intervals are 1,410–1,990 mg/kg for TP, 2,700–4,000 mg/kg for TKN, and 54,510– 76,920 mg/kg for COD. When compared to previous research results (Kim et al. 2003, 2004), the pollutant concentrations in the research areas are two to three times higher than those in the lakes in the upper stream of the Han River. This was expected because of the greater pollutant inputs into the research area from various pollution sources. The research areas have many pollution sites of point and nonpoint sources such as several large wastewater treatment facilities (the largest being the Jungrang wastewater treatment system that treats 1.7 million m3/day), as well as urban and agricultural diffuse pollution sources. They significantly affect the water quality of the river. The aforementioned smaller particle size distribution in the downstream may also have contributed to the poor water quality.
0-10 cm 10-20 cm 20-30 cm 30-36 cm
Seasons
Winter
Fall
Summer
Spring 0
1
2 3 Ignition Loss (%)
4
5
0
200
400
600
5000 10000 COD (mg/kg)
15000
TP (mg/kg)
Seasons
Winter
Fall
Summer
Spring 0
500 1000 TKN (mg/kg)
1500
Fig. 4 Seasonal concentration changes with sediment depth at site M2
0
3.2 Seasonal Concentration Changes with Sediment Depth The 24 samples of undisturbed sediment were collected by a diver in order to understand the vertical changing characteristics of pollutant concentrations. Generally, pollutant concentrations decrease with sediment depth in all monitored sites. Figure 4 shows example profiles of pollutant concentrations in sediment layers at site M2. According to the similar trends found in a previous study (Nowell et al. 1999), the differences in concentration may be caused by removal mechanisms. Anaerobic degradation can reduce COD and organic content (e.g., IL), and can accelerate the release of nitrogen and phosphorus into the water column. Generally, one expects pollutant concentrations to decrease with increasing sediment depth due to biodegradation. Figure 4 also shows the seasonal changes of pollutant concentrations of the undisturbed samples according to sediment depth at site M2. The concentrations of IL, TP, TKN, and COD decrease with sediment depth because of the removal mechanism, as previously stated. However, with the exception of IL, these concentrations do not show a continuously decreasing tendency with the different seasons. The main sources of TP and TKN in the research areas are known to come from fertilizers from agricultural areas during the spring season (Chang 2005). Particularly, the source of TP originates from phosphate fertilizers, which are usually precipitated on the bottom of the water column by adsorbing into sediments. Therefore, this mechanism causes the high TP and TKN concentrations during the spring season (March–June). The highest concentration of COD occurs in October due to the flooding from the upper watershed areas. Figure 5 shows a statistical concentration analysis for undisturbed sediments as a function of sediment depth. IL_10, IL_20, and IL_30 represent the IL concentrations in 0–10, 10–20, and 20–30 cm sediment depth, respectively. The differences between the levels are significant at the 95% confidence level in most cases, as shown by the notched box plots. Outliers are declared when the value is 1.5 times the interquartile distance away from the 25% box (McGill et al. 1978). The mean concentrations from 0–10 cm were determined to be 5% for IL, 1,090 mg/kg for TP, 1,680 mg/kg for TKN, and 40,710 mg/kg for COD. These values are 1.2–1.5 times greater than the mean
269
Concentrations (IL=%, Others=mg/kg)
Water Air Soil Pollut (2010) 206:263–272
10000.0 1000.0 100.0 10.0 1.0 10 20 40 10 20 40 10 20 40 10 20 40 IL_ IL_ IL_ TP_ TP_ TP_ KN_ KN_ KN_ OD_ OD_ OD_ T T T C C C
95 % lower confidence interval 95 % upper confidence interval Mean Min. Outlier
50 %
Max.
Outlier
Fig. 5 Statistical concentration analysis with depth for undisturbed sediments
concentrations obtained in previous research (Kim et al. 2003, 2004). The 95% confidence intervals for undisturbed sediment samples at a 0–10 cm sediment depth are 4–6% for IL, 720–1,470 mg/kg for TP, 1,320– 2,030 mg/kg for TKN, and 25,000–56,500 mg/kg for COD. The statistical analysis clearly shows that the mean pollutant concentrations decrease with sediment depth for all parameters. The mean values of concentration in a 10–20-cm sediment depth are 80% for IL, 65% for TP, 70% for TKN, and 80% for COD compared to those in a 0–10-cm sediment depth. The values in a 20–30 cm sediment depth are only 40% for IL, 45% for TP, 48% for TKN, and 54% for COD. The concentrations of disturbed sediment are 1.4–2.0 times higher than those from 0–10 cm of undisturbed sediment samples. This is thought to be due to the fact that disturbed sediments were sampled at the top few centimeters of the river bed where the concentration was found to be the highest, whereas the compared undisturbed sediment is for the first 10-cm depth which also includes the portion of decreased concentration giving a lower concentration on average.
270
Water Air Soil Pollut (2010) 206:263–272
Fig. 6 Sedimentation test using vertical column
0
Time (min) 20 30
10
40
50
0 Initial conc. : TSS = 2,880 mg/L
Depth of column (cm)
20
% Removal
40
10 % 20 % 30 % 40 % 50 % 60 % 70 % 80 %
60 80 100 120
0
10
20
30
40
50
60
70
80
90
0
Initial conc.: TSS= 2,088 mg/L
Depth of column (cm)
20 40 60 80 100 120
3.3 Estimation of Sedimentation and Release Rates
100.00 Concentrations(mg/kg)
Figure 6 shows examples of the sedimentation column test results. The plot depicts the timevariable removal characteristics of initially homogenously mixed sediment samples at different ports. The line at 10% refers to the time required to achieve a 10% reduction of initial sediment concentration when it is completely mixed. The depth and the corresponding time required for a particular removal ratio can be used to estimate sedimentation velocity. Usually, the time required to achieve 40% removal at 120 cm column depth is used to determine the sedimentation velocity. The tests show that sedimentation velocities range from 12.0 to 18.5 cm/min. The values can be used as a guideline for the calculation of the sedimentation rates from water column to
10.00
1.00
0.10
SCOD
SOL_N
SOL_P
Fig. 7 Statistical analysis for forced-release masses
100
Water Air Soil Pollut (2010) 206:263–272
271
sediment layers and for the design of the sedimentation basin for dredged material. Forced-release tests were also carried out in order to determine the pollutant release masses from sediments to water column (Fig. 7). The forced-release masses provide valuable information for estimating the maximum release masses from sediments during flooding periods in the Han River. The concentrations of soluble COD (SCOD), soluble nitrogen (Sol N), and Soluble Phosphorus (Sol P) were analyzed using Standard Methods (AWWA 1995). The minimum and maximum release masses were 14.8–215.2 mg/kg for SCOD, 0.09–134.3 mg/kg for Sol N, and 0.02–23.3 mg/kg for Sol P. The values for the 95% confidence interval were 42.8–59.8 mg/kg for SCOD, 4.8–12.5 mg/kg for Sol N, and 1.6–3.2 mg/kg for Sol P. Compared to the values reported in Kim et al. (2004) for the Jamsil submerged dam area before dredging, the post-dredging release masses reported in this study are much smaller by a factor of 50 and 37 for SCOD and Sol P, respectively, suggesting the effectiveness of dredging.
3.4 Pollutant Concentration Changes by Dredging While many efforts have previously been made to improve the water quality in the Han River areas, the water quality has still deteriorated because of various pollutant inputs originating from watershed areas and sediments. Therefore, the Seoul municipality decided to remove sediments from the Han River and, since 1997, dredging using a vacuum pumping system has been performed on the M5, M7, and M8 sites where major tributaries of the Han River enter. The estimated depth of dredging is about 9 cm based on the total depth for undisturbed samples before and after dredging. Figure 8 shows the pollutant concentration changes in sediment after dredging in these areas. After dredging, the decrease in pollutant concentrations for sites M5, M7, and M8 were 33–57% for TP, 51–64% for TKN, and 30–62% for COD, which were lower than those prior to dredging. In Fig. 8, it can be seen that dredging can be an effective means to control internal nonpoint sources. It is thought that this is due
2500
12.0
before dredging
IL concentrations (%)
10.0
TP concentrations (mg/kg)
before dredging after dredging
8.0 6.0 4.0
1500
1000
500
2.0 0.0
0 M5
M7
M5
M8
M7
M8
120000
5000 COD concentrations (mg/kg)
before dredging
4500 TKN concentrations (mg/kg)
after dredging
2000
after dredging
4000 3500 3000 2500 2000 1500 1000
before dredging
100000
after dredging
80000 60000 40000 20000
500 0
0 M5
M7
M8
Fig. 8 Pollutant concentration changes before and after dredging
M5
M7
M8
272
to the fact that the bottom sediment in the Han River has a vertical distribution of pollutant concentration that decreases with depth. 4 Conclusions In this investigation, sediment characteristics of the mid-to-lower Han River were evaluated, including particle sizes and seasonal change of pollutant concentrations with sediment depth. Also, the sedimentation and release rates were estimated using a sedimentation column and forced-release tests. Finally, the effectiveness of dredging in reducing the pollutant concentrations in the Han River sediments was evaluated. The following specific conclusions are made: 1. The soil types of sediments in the research area are classified as silty clay, clay loam, or clay. 2. The mean values of pollutant concentrations in disturbed sediments were determined to be higher than those in the top surface of undisturbed sediments. Pollutant concentrations of undisturbed samples decreased with sediment depth and time due to the removal mechanism. 3. The sedimentation velocities were determined by the column test, which can be used for the calculation of the sedimentation rates from water column to sediment layers and the design of a sedimentation basin for dredged material. Release rates were determined by the forced-release test, and the maximum release rates from sediments can serve as guideline values that can be expected for summer flooding conditions. 4. Our results indicate that dredging was valuable for decreasing the pollutant concentrations of the sediments.
References Abrams, M. M., & Jarrell, W. M. (1995). Soil-phosphorus as a potential non-point source for elevated stream phosphorus levels. Journal of Environmental Quality, 24, 132–138. Aiguier, E., Chebbo, G., Bertrand-Krajewski, J.-L., Hedges, P., & Tyack, N. (1996). Methods for determining the settling
Water Air Soil Pollut (2010) 206:263–272 velocity of profiles of solids in storm sewage. Water Science and Technology, 33(9), 117–125. doi:10.1016/ 0273-1223(96)00377-0. AWWA.APHA, WPCE. (1995). Standard methods for the examination of water and wastewater (18th ed.). Washington DC: American Public Health Association. Bostrom, B. (1988). Relations between chemistry, microbial biomass and activity in sediments of a polluted vs. a nonpolluted eutrophic lake. Verhandlungen der Internationalen Vereinigung fur Theoretische und Angewandte Limnologie, 23, 451–459. Chang, H. (2005). Spatial and temporal variations of water quality in the Han River and its tributaries, Seoul, Korea, 1993–2002. Water, Air, and Soil Pollution, 161, 267–284. doi:10.1007/s11270-005-4286-7. Das, B. M. (1990). Principles of geotechnical engineering (2nd ed.). Boston, MA: PWS-KENT. EPA. (1994). Nonpoint sources pollution control program. U.S. EPA Report 841-F-94-005, USA. Hieltjes, A. H. M., & Lijklema, L. (1980). Fractionation of inorganic phosphates in calcareous sediments. Journal of Environmental Quality, 9, 405–407. Jin, X., Wang, S., Bu, Q., & Wu, F. (2006). Laboratory experiments on phosphorous release from the sediments of 9 lakes in the middle and lower reaches of Yangtze River region, China. Water, Air, and Soil Pollution, 176, 233– 251. doi:10.1007/s11270-006-9165-3. Kim, L.-H., Choi, E., & Stenstrom, M. K. (2003). Sediment characteristics, phosphorus types and phosphorus release rates between river and lake sediments. Chemosphere, 50 (1), 53–61. doi:10.1016/S0045-6535(02)00310-7. Kim, L.-H., Choi, E., Gil, K., & Stenstrom, M. K. (2004). Phosphorus release rates from sediments and sediment characteristics in Han River, Seoul, Korea. The Science of the Total Environment, 321, 115–125. doi:10.1016/j. scitotenv.2003.08.018. Lijklema, L., Koelmans, A. A., & Portielje, R. (1993). Water quality impacts of sediment pollution and the role of early diagenesis. Water Science and Technology, 28, 1–12. McGill, R., Tukey, J. W., & Larsen, W. A. (1978). Variations of box plots. The American Statistician, 32, 12–16. doi:10.2307/2683468. Nowell, L. H., Capel, P. D., & Dileanis, P. D. (1999). Pesticides in stream sediment and aquatic biota: distribution, trends, and governing factors. Boca Raton, FL: Lewis. Stumm, W., & Morgan, J. J. (1996). Aquatic chemistry (3rd ed.). New York: Wiley. To, Y. P. S. (1974). Effect of Sediment on Water Quality in the Occoquan reservoir. Ph.D. Dissertation. Blacksburg, Virginia: Virginia Polytechnic Institute and State University. Wetzel, R. G., & Likens, G. E. (1992). Limnological analysis (2nd ed.). New York: Springer. Williams, J. D. H., Shear, H., & Tomas, R. L. (1980). Availability to Scenedesmus quadricauda of different forms of phosphorus in sedimentary materials from the Great Lakes. Limnology and Oceanography, 25, 1–11.
