This study examined the water quality of the large young tropical Bakun hydroelectric reservoir in Sarawak, Malaysia, and the influence of the outflow on the downstream river during wet and dry seasons. Water quality was determined at five stations in the reservoir at three different depths and one downstream station. The results show that seasons impacted the water quality of the Bakun Reservoir, particularly in the deeper water column. Significantly lower turbidity, SRP, and TP were found during the wet season. At 3–6 m, the oxygen content fell below 5 mg/L and hypoxia was also recorded. Low
The creation of a large-scale dam and its associated reservoir often has significant negative impacts on the hydrological, biological, and chemical processes of the reservoir, upstream, and downstream of the dam [
The water quality of reservoirs has been observed to vary seasonally in tandem with changes in temperature and rainfall [
On the other hand, the reservoir outflow has a great influence on the downstream river. Studies have shown that the downstream river is subjected to major environmental impacts which range from downstream morphology changes to loss of biodiversity of the ecosystem [
As a young reservoir in a tropical country, changes continue to occur in the reservoir and it is important to monitor the water quality in order to evaluate its suitability for secondary purposes such as aquaculture and recreation. The knowledge of the seasonal variation of the reservoir’s water quality is important for dam operation and management decision. The impact of the dam on the water quality of its downstream river during the wet and dry seasons remains unknown. Hence, the aim of this study was to assess the water quality of the Bakun Reservoir and the influence of its outflow on the water quality of the downstream river during wet and dry seasons.
The present study was conducted at Bakun Reservoir and its downstream river in Sarawak, Malaysia, as illustrated in Figure
The study area and sampling stations in the present study.
Sampling was conducted in February and September 2014 corresponding to the wet and dry seasons in Sarawak (Table
The details of the sampling location and sampling regime in the present study.
Station | Coordinates | Date | Location |
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Bakun hydroelectric reservoir | |||
St. 1 | N 02°43′34.4′′ |
26 Feb. 2014, 1:15 p.m. |
Batang Balui |
St. 2 | N 02°39′32.2′′ |
26 Feb. 2014, 9:45 a.m. |
Linau River |
St. 3 | N 02°42′59.8′′ |
27 Feb. 2014, 12:55 p.m. |
Upper part of Murum River |
St. 4 | N 02°44′15.3′′ |
26 Feb. 2014, 3:06 p.m. |
Lower part of Murum River |
St. 5 | N 02°45′09.8′′ |
27 Feb. 2014, 3:00 p.m. |
Near the intake point and the dam |
Downstream river of Bakun hydroelectric dam | |||
St. 6 | N 02°46′21.8′′ |
26 Feb. 2014, 3:00 p.m. |
Long Baagu (4.3 km downstream of the Bakun hydroelectric dam) |
Depth profiles of temperature and dissolved oxygen (DO) were measured using a YSI 6820 V2 multiparameter water quality sonde during the first sampling in February 2014. The pH and turbidity were measured at 0 m, 10 m, and 20 m depths in Bakun Reservoir in both samplings by using a pH meter (EcoScan, Eutech) and a turbidity meter (Martini Instruments, Mi415), respectively. Triplicate water samples were collected at 0 m, 10 m, and 20 m depths in Bakun Reservoir (Stations 1 to 5) using a Van Dorn water sampler whereas triplicate water samples were collected at 0 m depth at the downstream river of the dam (Station 6). The depth of the reservoir was measured using a portable depth sounder (Speedtech). All sampling bottles were acid-washed, cleaned, and dried before use. Water samples were acidified to pH < 2 for total phosphorus (TP) analysis. All samples were placed in an ice box and transported to the laboratory for further analysis [
All the analyses were conducted according to standard methods [
Quality control steps were taken throughout the study. Sample bottles and glassware were washed using phosphate-free detergent followed by the standard acid wash procedure. Sample preparation and storage were performed according to the standard methods [
Comparison of water quality parameters between the stations and the depths in the Bakun hydroelectric reservoir was conducted using one-way ANOVA and Tukey’s pairwise comparisons with 5% significance level. Student’s
Figure
Depth profile of temperature and DO in Bakun Reservoir in February 2014.
Dissolved oxygen was relatively consistent in the surface water of the Bakun Reservoir, with a mean value of 7.22 mg/L. The DO level started to decrease rapidly from a depth of 2 m to less than 0.2 mg/L at a depth of 4 m at Station 1 which is located at Batang Balui. The DO level at Stations 2, 3, and 5 started to decrease rapidly from the depth of around 3 m whereas the DO level at Station 4 started to decrease from 5 m depth. In other words, the healthy level of DO content above 5 mg/L was only observed at the water column above 3–6 m in Bakun Reservoir. Similarly, [
The pH value of the Bakun Reservoir ranged from 4.93 ± 0.06 to 8.06 ± 0.05 during the wet season with the lowest and highest pH value being observed at Station 5 and Station 2, respectively. On the other hand, the pH value of the Bakun Reservoir is relatively consistent during the dry season with a mean value of 7.30. Vertical distribution of pH values in Bakun Reservoir differed between the wet and dry seasons although this was not significantly different (
Mean difference of water quality parameters of the Bakun Reservoir during the wet season and dry season conducted in February and September 2014, respectively (
Parameter | Sampling | Depth | Station | Mean | Difference |
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1 | 2 | 3 | 4 | 5 | ||||||
pH | Wet season | 0 m |
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6.67 | +0.20 | 0.422 |
10 m |
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20 m |
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Dry season | 0 m |
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6.47 | |||
10 m |
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20 m |
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Turbidity, FNU | Wet season | 0 m |
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38.89 | − |
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10 m |
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20 m |
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Dry season | 0 m |
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81.24 | |||
10 m |
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20 m |
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Chl |
Wet season | 0 m |
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0.93 | −0.18 | 0.655 |
10 m |
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20 m |
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Dry season | 0 m |
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1.11 | |||
10 m |
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20 m |
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TSS, mg/L | Wet season | 0 m |
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40.1 | −29.6 | 0.147 |
10 m |
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20 m |
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Dry season | 0 m |
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69.7 | |||
10 m |
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20 m |
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BOD5, mg/L | Wet season | 0 m |
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4.02 | −0.26 | 0.135 |
10 m |
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20 m |
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Dry season | 0 m |
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4.28 | |||
10 m |
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20 m |
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Wet season | 0 m |
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0.004 | +0.001 | 0.282 |
10 m |
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20 m |
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Dry season | 0 m |
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0.003 | |||
10 m |
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20 m |
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Wet season | 0 m |
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0.021 | +0.006 | 0.233 |
10 m |
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20 m |
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Dry season | 0 m |
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0.015 | |||
10 m |
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20 m |
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OKN, mg/L | Wet season | 0 m |
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0.34 | +0.01 | 0.783 |
10 m |
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20 m |
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Dry season | 0 m |
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0.33 | |||
10 m |
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20 m |
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SRP, |
Wet season | 0 m |
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8.6 | − |
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10 m |
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20 m |
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Dry season | 0 m |
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37.9 | |||
10 m |
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20 m |
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TP, |
Wet season | 0 m |
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112.4 | − |
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10 m |
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20 m |
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Dry season | 0 m |
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235.1 | |||
10 m |
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20 m |
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Means in the same row with the same letters or column with the same numbers are not significantly different at 5% level. The positive value of mean difference indicates that the parameter studied is higher during the wet season whereas the negative value indicates that the parameter studied is higher during the dry season. The significant difference at
The distribution of pH and turbidity at three different depths (0, 10, and 20 m) of Bakun Reservoir in February (a) and September (b) 2014.
Table
Correlation of water quality parameters of the Bakun Reservoir during the wet season (
Parameter | pH | Turbidity | Chl |
TSS | BOD5 |
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OKN | SRP | TP |
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pH | 1.000 | |||||||||
Turbidity | 0.005 | 1.000 | ||||||||
Chl |
0.314 | −0.254 | 1.000 | |||||||
TSS | −0.162 |
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−0.368 | 1.000 | ||||||
BOD5 | −0.345 | 0.242 | −0.263 | 0.310 | 1.000 | |||||
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−0.288 | 0.093 | −0.383 | 0.247 |
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1.000 | ||||
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0.161 | 0.067 | 0.389 | −0.011 | 0.272 | −0.002 | 1.000 | |||
OKN | −0.081 | 0.023 | 0.091 | −0.208 | 0.499 | 0.265 |
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1.000 | ||
SRP | −0.365 | 0.237 | −0.130 | 0.281 | 0.044 | 0.288 | 0.223 | 0.093 | 1.000 | |
TP | −0.128 | 0.474 | −0.144 | 0.217 | 0.400 |
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0.121 | 0.271 |
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1.000 |
Correlation of water quality parameters of the Bakun Reservoir during the dry season (
Parameter | pH | Turbidity | Chl |
TSS | BOD5 |
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OKN | SRP | TP |
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pH | 1.000 | |||||||||
Turbidity |
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1.000 | ||||||||
Chl |
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−0.385 | 1.000 | |||||||
TSS | −0.407 |
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−0.259 | 1.000 | ||||||
BOD5 | −0.378 | 0.217 | 0.090 | 0.346 | 1.000 | |||||
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0.465 | −0.008 | 0.041 | −0.048 | −0.479 | 1.000 | ||||
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−0.192 | 0.197 | −0.293 | 0.085 | −0.405 | 0.289 | 1.000 | |||
OKN | −0.085 |
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0.001 |
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0.283 | 0.166 | 0.276 | 1.000 | ||
SRP |
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−0.361 |
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−0.359 | −0.024 | −0.041 | −0.163 | −0.009 | 1.000 | |
TP |
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0.205 | −0.255 | 0.128 | 0.176 | −0.406 | 0.513 | 0.103 | −0.168 | 1.000 |
The surface turbidity value of the Bakun Reservoir was low (<6 FNU) and increased significantly (
The surface Chl
The distribution of Chl
Previously, the highest concentration of 7.25
All surface TSS values in Bakun Reservoir were classified as Class I which is less than 25 mg/L. The surface TSS concentration in the present study was lower than the previously reported surface TSS concentration (66.7–100.0 mg/L) in the year 2013 [
Surface BOD5 concentration at the Bakun Reservoir ranged from 3.24 ± 0.19 mg/L to 4.30 ± 0.15 mg/L and from 3.47 ± 0.06 mg/L to 4.43 ± 0.06 mg/L during the wet and dry seasons, respectively. Table
The
The distribution of
The
The concentration of SRP was low and relatively consistent in the Bakun Reservoir during the wet season with a mean value of 8.6
The distribution of SRP and TP at three different depths (0, 10, and 20 m) of Bakun Reservoir in February (a) and September (b) 2014.
Surface TP of the Bakun Reservoir ranged from 78.2 ± 3.6
Table
Summary of the mean and standard deviation of the in situ and ex situ water quality parameters in the downstream river of the Bakun Dam (Station 6) and the mean difference of the parameters between the wet and dry seasons (
Parameter | Wet season | Dry season | Mean difference |
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In situ | ||||
Temperature, °C |
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DO, mg/L |
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pH |
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Turbidity, FNU |
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Ex situ | ||||
Chl |
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+0.12 | 0.179 |
TSS, mg/L |
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BOD5, mg/L |
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+0.014 | 0.115 |
OKN, mg/L |
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+0.01 | 0.057 |
SRP, |
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TP, |
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The positive value of mean difference indicates that the parameter studied is higher during the wet season whereas the negative value indicates that the parameter studied is higher during the dry season.
Low pH values (≈6.1) were observed at the downstream river and classified as Class II according to NWQS [
Mean difference of in situ and ex situ water quality parameters between the intake point of the dam at 10 m (Station 5) and its downstream river (Station 6) during wet and dry seasons (
Parameter | Wet season | Dry season | ||
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Mean difference |
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Mean difference |
| |
In situ | ||||
pH | −0.03 | 0.469 | +0.01 | 0.288 |
Turbidity, FNU |
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Ex situ | ||||
Chl |
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−0.02 | 0.755 |
TSS, mg/L |
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BOD5, mg/L |
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+0.000 | 0.374 |
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+0.003 | 0.631 | −0.000 | 0.374 |
OKN, mg/L |
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SRP, |
+3.2 | 0.157 |
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TP, |
+10.5 | 0.189 |
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The positive value of mean difference indicates that the parameter studied is higher in the downstream river whereas the negative value indicates that the parameter studied is lower in the downstream river.
There was no significant difference in Chl
SRP and TP concentrations in the downstream river exhibited a similar trend where the concentration during the wet season was significantly lower than during the dry season (
Figure
Clusters of the five sampling stations located in Bakun Reservoir at three different depths (0 m, 10 m, and 20 m) and one station located at its downstream river collected at 0 m during dry and wet seasons in Sarawak, Malaysia.
The Bakun hydroelectric reservoir is a thermally stratified reservoir with a temperature gradient of approximately 5°C within the thermocline layer. The thickness of the well oxygenated water was around 3–6 m of the surface water, whereas the oxygen content of most of the water body was below 5 mg/L or even in hypoxia. The Bakun Reservoir showed signs of organic pollution with high BOD5, OKN, and TP concentrations observed in the reservoir. Acidification was observed in parts of the reservoir, particularly downstream of active logging activities and the Murum hydroelectric dam construction during the wet season. The water quality of the reservoir was influenced by the wet and dry seasons particularly in the deeper water column. SRP and TP concentrations were discovered to be higher during the dry season in the reservoir. This result suggests the necessity of management and conservation of the reservoir to prevent further deterioration in the reservoir’s water quality where different water quality parameters should be targeted during different seasons. The present study also demonstrated that the water discharged from the Bakun Reservoir has a great impact on the water quality at the downstream river. The water released from the reservoir decreased the temperature, DO, and pH of the downstream river whereas turbidity and TSS concentration increased in the downstream river. Nevertheless, the water quality of the downstream river, particularly BOD5, OKN, and TP concentrations, was also influenced by adjacent anthropogenic activities such as household wastewater. This result suggests that the downstream river of the Bakun Reservoir was not solely impacted by the reservoir’s outflow. Therefore, all factors should be taken into account in decision-making of the management of the downstream river for the health of sensitive aquatic organisms.
The authors declare that there are no conflicts of interest regarding the publication of this paper.
The authors appreciate the financial support provided by the Malaysian Ministry of Higher Education through Grant no. FRGS/STWN01(04)/991/2013(32) and the facilities provided by Universiti Malaysia Sarawak.