Water and Sediment Quality of Rajang River Downstream of Bakun Dam, Sarawak.

Water and sediment quality in the downstream river may be affected by the of the Bakun Hydroelectric dam. Knowledge on water and sediment quality in the regulated Rajang River, downstream of the Bakun Hydroelectric Dam is still scarce, especially for a longer distance up to Kapit. As more hydroelectric dams are to be built in the future, it is important to collect baseline data for future references. The main concerns in Rajang River are the differences of water quality between two operations, which are, during the opened spillway (water discharged through turbine and spillway) and the closed spillway (water discharged through turbine only). The water quality potentially changes with the discharge from the spillway. Therefore, in this study, the water and sediment quality were conducted along the Rajang River up to 210 km long. A total of 15 stations were selected for this study for a distance of 210 km, and six sampling trips were conducted from April 2016 to May 2018, covering the opened and closed spillway of the Bakun Hydroelectric Dam. Results show that pH is significantly lower at stations nearer to the dam, regardless of spillway operations, and gradually increasing as distance from the dam increases. At station 1, nearest to the dam during closed spillway, pH was less than 6.0, significantly lower than opened spillway. The DO concentration in the river deteriorated when there was no spillway discharge. Multiple regression modelling was applied to determine the minimum discharge required to raise DO and pH at station 1 to comply with Class II NWQS. Rainfall event before and during sampling increased turbidity and TSS; and decreased transparency. The means of COD, NO2-N, NO3-N, OKN, PO4-P, TS and H2S were significantly higher during opened spillway, especially at the stations nearer to the dam. The TP concentration in the river was mainly affected by the wastes from longhouses, logging camps and agricultural activities. In terms of water quality index (WQI), the classification ranged from Class II to III, and the status is either Slightly Polluted (94.4%) or Polluted (5.6%). Compared to the condition prior to impoundment, parameters such as temperature, pH, DO, NO3-N, PO4-P and TP declined with the impoundment of Bakun Dam. The heavy metals in water were mainly affected by the surrounding activities. The range of heavy metals for Al were from undetected to 0.651 mg/L; Cu, 0.008 mg/L to 0.015 mg/L; Fe, 1.070 mg/L to 3.417 mg/L; Mn, 0.007 mg/L to 0.113 and Zn, from undetected to 0.626 mg/L. The NWQS classifications for heavy metals in water ranged from Class II to III. Sediment quality analysis showed that the percentage of sand was 6.24% to 99.53%; silt, 0.22% to 93.55%; and clay, 0.21% to 20.88%. The textural classes in Rajang River were sand, loam, sandy loam, loamy sand and silt. The sediment quality results for pH, OM, TOC, TP and TKN were 6.06 to 6.43, 0.73% to 4.07%, 0.02% to 0.21%, 145 mg/kg to 381 mg/kg and 1099 mg/kg to 2289 mg/kg respectively. Based on sediment guideline for the protection and management of aquatic sediment quality in Ontario, the TOC and TP were categorized as below the lowest effect level, while TKN was categorized as exceeding the lowest effect level. Meanwhile, the distribution of TOC, OM, TP and heavy metals in sediments were significantly correlated with clay. The range of heavy metals in sediments for Al were 299.4 mg/kg to 1326.0 mg/kg; Cu, 3.933 mg/kg to 17.267 mg/kg; Fe, 4333.0 mg/kg to 16807.3 mg/kg; Mn, 61.0 mg/kg to 410.7 mg/kg; and Zn, 14.4 mg/kg to 41.4 mg/kg. Overall, most parameters were influenced by land-use activities besides spillway and turbine discharge; and large tributaries inflow. The concentration of DO should be raised during closed spillway in order to ensure the healthy growth of sensitive aquatic organisms.

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