Impacts of Construction Activities on Aquatic Environment: A Case of the Kohelia River, Moheshkhali, Cox’s Bazar

With almost 700 rivers and their tributaries, Bangladesh has a lot of potential for the fishing industry [1]. It contributes 3.65% of the country's GDP, which is a significant amount, and makes up 23.81% of the GDP from agriculture. Additionally, fish and fisheries products are Bangladesh's second-largest export commodity, accounting for 1.97% of the nation's foreign exchange profits in the 2015-2016 period [2]. Fisheries resources are still essential for supplying animal protein needs and advancing the nation's socioeconomic growth. More than sixteen million people, or roughly 11% of the country's population, depend on the fishing industry directly or indirectly for their livelihoods [3]. The majority of our nation's rivers are currently in danger because of a few particular factors. On river banks, facilities such as power plants, bridges, flood control systems, etc. are constructed without taking the general state of the river into account. Old industrial contaminants that have accumulated at the riverbed may be disturbed by dredging. The Kohelia, the study river, is a significant coastal river that flows eastward and is part of the Moheshkhali Upazila's Dhalghata and Matarbari unions in the Cox's Bazar region.  This power plant region is located in Matarbari and Dhalghata Union under Maheshkhali Upazila, on the banks of the Bay of Bengal and Kohelia River. Along with other massive activities, a coal power plant's infrastructure is now being built. The primary objective of this study is to investigate and analyze the industrial development related impact aspects affecting the aquatic environment, fish diversity and catch composition in the Kohelia River. There is evidence that during their service life, construction materials leach certain chemicals when they come into contact with water. A variety of building materials, such as treated wood, bituminous sheets [4], facades [5], fiber cement sheets [6], paints, textiles [7], anti-corrosion coatings [8], concrete [9], cement pastes [10], fire-retardant coatings [11], roof membranes [12], and roof paints, have been found to exhibit this behavior.

The Fish Catch Assessment Survey is a thorough endeavor aimed at understanding the status of fish and fisheries resources in the surveyed area. By employing a random selection of fishing gear and calculating the capture per unit effort (CPUE), the survey can estimate the overall catch, providing valuable insights into fish populations and the effectiveness of different fishing methods. Collecting sediment samples of macro benthic faunal communities using an Ekman grab allows for the assessment of benthic organisms, which are crucial components of aquatic ecosystems. Similarly, collecting zooplankton water samples using a Zooplankton net with specific mesh size ensures the accurate representation of zooplankton populations, providing important data on the lower levels of the food chain. Utilizing an Olympus binocular microscope and the Sedgwick-Rafter counting chamber method for estimating phytoplankton species abundance indicates a meticulous approach to analyzing microscopic organisms. This method is commonly used in environmental studies due to its accuracy and reliability. Adhering to APHA guidelines for analyzing dissolved oxygen, salinity, and other parameters ensures the accuracy and consistency of the survey's results. These parameters are critical for understanding the overall health and dynamics of aquatic ecosystems. The comprehensive methodology outlined in the survey demonstrates a rigorous and scientific approach to assessing the status of fish and fisheries resources, as well as the broader aquatic ecosystem. Such surveys are essential for informed management and conservation efforts.

A case study on the culverting of Abram stream explores the effects of changing a stream to make room for the construction of a runway at an airport near Cleveland, Ohio, and looks at how construction affects water quality [13]. The study's findings showed there were no statistically significant differences in pH, conductivity, or TSS between the upstream and downstream phases suggest that the overall water quality may not be significantly impacted by construction projects that employ the proper best management practices (BMPs). Data were gathered using a verified questionnaire in a research conducted by Hasle et al. [14] on fisheries and fishing gear in the Patuakhali District, Bangladesh's Agunmukha River. The survey found 47 fish species spread throughout 28 families and nine main categories of fishing gear. Finfish species that are vulnerable, endangered, or critically endangered have also been found, underscoring the necessity of conservation measures to protect the diversity of fisheries.

In Khulna, Bangladesh, Janssen et al. [15] studied the variety of fish and shellfish in the Rupsa River. There are currently 62 species, but nine have vanished in the previous ten years, according to the study. Both naturally occurring and man-made issues, including overfishing, urbanization, pollution, illicit fishing gear, and navigation, have been linked to the reduction in biodiversity.

In their investigation of fish abundance on Hatiya Island, Bangladesh, Manoylov [16] identified 38 species, belonging to 9 orders and 21 families. The fact that a sizable portion of species were classified as Vulnerable, Endangered, or Locally Threatened highlights the importance of conservation initiatives.

In a Bangladeshi coastal river, Lupsea et al. [17] found 86 fish species in 61 genera and 31 families during their investigation of the ichthyofaunal diversity. Conservation problems were highlighted by the classification of thirteen species as vulnerable, ten as endangered, and eight as severely endangered.

The fishing techniques and equipment utilized to catch Asian seabass along Bangladesh's Bay of Bengal coast were examined by Mahmood et al. [18]. The study determined which fishing nets and equipment work best for catching Asian seabass at various stages of their life cycle. Using capture per unit effort (CPUE), Märkl et al. [19] investigated species abundance in Chalan Beel, Bangladesh. 38 distinct species were found in the study, with CPUE varying across various locations and types of gear in Chalan Beel.

3.1 Projects wastewater discharge quality

Labor camp wastewater (treated and untreated domestic and sewage wastewater) and overflow dredging water quality have been monitored twice during the months of January and March 2022.

Table 3. Labor camp wastewater quality test results

Note: Survey conducted by author. Sampling and lab test performed by the SGS BD Ltd. (https://www.google.com.bd/books/edition)

Table 4. Overflow dredging water quality test result

Note: Survey conducted by author. Sample collection and lab test done by the SGS BD Ltd. (https://www.google.com.bd/books/edition)

Lab test results in Table 3, consolidated with national standards (ECR’ 97), indicate that maximum temperature measured 30°C in March, with higher pH levels in January. TSS concentration recorded 36 mg/L in March, exceeding January levels. Maximum BOD and COD levels measured 40 mg/L and 215 mg/L in January, while Nitrate concentration was higher at 1.9 mg/L in January. Fecal Coliform count was higher at 16000 MPN/100ml in March. Comparing with national standards, pH, TSS, BOD, and Nitrate levels are within limits, but COD and Fecal Coliform levels exceed standards. Key informant interviews revealed that only 3 out of 21 labor camps have sewage treatment plants, and the quality of treated sewage water is not monitored before discharge. This suggests that questionable treated and untreated wastewater discharge may be causing elevated COD and Fecal Coliform concentrations in the water.

Table 5. Kohelia River water quality monitoring results

Note: Survey conducted by author. Sampling and lab test performed by the SGS BD Ltd

Analysis of test results in Table 2 indicates a maximum temperature of 27°C in March, with higher pH levels observed in January. Turbidity and TSS concentrations peaked at 1400 NTU and 1640 mg/L in March, significantly exceeding January levels. Maximum BOD level was 2 mg/L in March, while COD concentrations remained at 32 mg/L in both months. Nitrate levels showed slight variation, and Fecal Coliform counts were insignificant. pH, BOD, COD, and Nitrate concentrations met national standards, but TSS levels exceeded the limit. No standards exist for Turbidity and Fecal Coliform. Key informant interviews revealed that the power plant project did not discharge overflow dredging water in January, resulting in lower turbidity and TSS levels. However, in March, inadequate open space hindered proper treatment of overflow dredging water, leading to elevated turbidity and TSS levels (Tables 4 and 5).

3.2 Aquatic environment physico-chemical characteristics

In January and March 2022, surface water quality was monitored at three locations along the Kohelia River. Notable findings include the highest water levels at RW1, a maximum temperature of 30°C at RW3 in March, and higher salinity levels in January (21-22 ppt). Dissolved Oxygen (DO) levels were lower in March, and Biological Oxygen Demand (BOD) showed higher values during that month. Chemical Oxygen Demand (COD) varied slightly, while Turbidity and Total Suspended Solids (TSS) concentrations were significantly higher in March. Nitrate concentrations showed slight variations. Fecal Coliform counts were insignificant, and measured levels of pH, DO, BOD, and Fecal Coliform were within national standards (ECR’97). Comparative analysis with baseline data from October 2015 revealed a drastic decrease in water levels, potentially attributed to sediment-laden overflow dredging water causing riverbed filling. Despite increased salinity, TSS, and Turbidity levels, navigable water areas were found between RW1 and RW2. Significant variations in COD levels were observed, potentially due to different testing methods. Labor camp wastewater discharge had no observable impact on surface water quality. The study suggests adverse effects on fish diversity due to increased salinity from rapid siltation and overflow dredging water but highlights the complex dynamics influencing surface water quality in the Kohelia River.

3.3 Biological characteristics

Table 6. Benthos status observed at Kohelia channel monitoring month - Jan. 22

Note: Survey conducted by author. Sampling and analysis performed by the MFTS Laboratory. Unit: Individuals/m³

To cover the dry season, status of benthos, phytoplankton, and zooplankton communities have been monitored once in January 2022 at three locations. Survey findings are presented in Tables 6, 7, and 8.

The study reveals that the maximum abundance of the phytoplankton community was observed at Location-RW-2 (26,110 cells/L), followed by RW-1 (25,010 cells/L) and RW-3 (21,250 cells/L). A total of 56 phytoplankton species were documented under 5 groups. The highest abundance of the zooplankton community was found at Location-RW-1 (450 individuals/m³), followed by RW-2 (340 individuals/m³) and RW-3 (290 individuals/m³), with a total of 27 identified species under 6 groups. In sediment samples, the highest abundance of benthic faunal communities was observed at RW-1 (215 individuals/m³), followed by RW-2 (183 individuals/m³) and RW-3 (161 individuals/m³), with a total of 5 groups identified. Comparing with the baseline study (EIA 2013), both the abundance and species diversity of the phytoplankton community were significantly higher in the current study period. The abundance and species diversity of zooplankton and benthic faunal communities were slightly higher. Key informant interviews suggested that the discharge of overflow dredging water contains a high load of silt and clay, acting as nutrients, leading to the increased abundance of the phytoplankton community. Phytoplankton is a unicellular plant organism. So, its measuring unit is cells per liter. On the other hand, zooplankton and benthos are animal organisms. So, its measuring unit is different from phytoplankton. Zooplankton measuring unit is individual per meter cube, and it is internationally accepted unit and counting formula also different in both.

Table 7. Phytoplankton status observed at Kohelia channel monitoring period-semiannual (dry season) monitoring month - Jan’ 22 Unit: cells/L

Note: ++ most dominant species (above 70% among total species in the sample), + less dominant species (less than 10% among total species in the sample) Source: Survey conducted by author. Sampling and analysis performed by the MFTS Laboratory

3.3 Fisheries status

To capture impacts (of construction activities) scenario, over all river area has been splited in two parts. Area-1 covers 5.5 km from upstream up to construction site and Area-2 covers rest of the downstream areas. Fish Catch Assessment Survey has carried out during the months of January and March 2022 to cover dry season.  Survey findings are presented in Table 8.

Table 8. Type of fishing gears

During Fish Catch Assessment Survey, fish catch of total 11 units of 6 different types fishing gears have been assessed. Survey covered at least 30% of total number of each type of fishing gear found in operation during survey days. Observation of fish Catch effort included 4 hauls of two Estuarine Set Bag nets, 20 hauls of three Seine nets (Ber / Tana Jal), 21 hauls of two Gill nets, 4 hours catch of a single cast net, and 8 hours catch effort of two long line hooks. Observation also included 7 hours catch hand picking catch effort of two persons. Within the survey Area-1, fishing with aid of only one cast net, two long line hook, and hand picking has been recorded. Rest of the fishing gears found within the Area-2. In January, no fishing activity with aid of Estuarine Set Bag net has observed. In March, no fishing activity within the area of Area-1 has observed.

Identification of a total 41 fish species has been documented and estimation of catch of all surveyed gears has been weighted total 158.2 kg of which, only seven species weighted 4.1 kg has been documented during survey at Area-1. A maximum of 21 fish species have been caught in Estuarine Set Bag nets followed by 15 and 13 fish species has been documented from during observation of fish catch of Seine net (Ber / Tana Jal) and Gill Net respectively. From observation of fish catch of Cast net and long line hook, total 5 and 2 fish species have been documented respectively. A single species has been documented as catch effort of Hand Picking.

3.4 Species diversity by surveyed gears

Observations from 20 hauls of 3 seine nets recorded a maximum weight of 113.1 kg, with a Catch Per Unit Effort (CPUE) of 45.6 kg/net/day. Additionally, 4 hauls of 2 Estuarine Set Bag nets yielded a total weight of 32.5 kg and a CPUE of 16.3 kg/net/day. Two Gill nets and a single Cast net caught 9 kg and 0.5 kg, with CPUE estimates of 4.5 and 1 kg/net/day. Two sets of long line hooks, with an 8-hour catch effort, weighed 1.1 kg, and CPUE was estimated at 1.1 kg/net/day. A total of 2 kg was caught during a 7-hour hand-picking effort by two persons, with a CPUE of 2 kg/person/day. Comparative analysis with the baseline study (EIA-2013) in the dry season revealed a significant decline in fish species diversity and total harvest. The use of Estuarine Set Bag nets, seine nets, and Chat Jal has notably decreased since the baseline study.

Fishermen reported a decline in fish harvest in the Kohelia River since the start of the construction project in 2018. Previously, catches with Estuarine Set Bag nets and Seine nets were around 30-40 kg and 60-70 kg, respectively. However, the current maximum catch is 20 kg with Estuarine Set Bag nets and 40 kg with Seine nets. Several fish species, including Hilsha and Baim, have almost vanished, and the size of the remaining fish has reduced. Fishermen used to catch 10-15 Hilsha fish weighing 800 gm to 1.2 kg, but now they can hardly catch 1-2 fish weighing a maximum of 400 gm. The construction project, filling of the Kohelia River, and the construction of an access road over the foreshore have led to a 30-40% reduction in the potential fishing area. During the dry season, fishing is only possible during high tide, which lasts 5-6 hours, compared to the previous 8-10 hours a day. Fishing is restricted from Rajghat Bridge to the construction site (Survey Area-1) due to low water levels. The number of fishing gears, especially Estuarine Set Bag nets and Seine nets, has decreased by 50%. Fear of gear damage by construction vessels has led to a cessation of nighttime fishing. Fishermen also cited restrictions from the coast guard, increased river traffic, dredging activities by the Bangladesh Economic Zone Authority (BEZA), and the occupation of potential fishing spots by sand barges and dredgers as factors limiting their fishing activities (Table 9).

Table 9. Fish Catch Assessment Survey findings

3.5 Impacts on aquatic environment of the Kohelia River physico-chemical characteristics

The study conducted surface water quality monitoring in the Kohelia River during January and February 2022 to assess the impact of construction-related pollution on physico-chemical characteristics, particularly during the dry season. Parameters such as water level, temperature, salinity, pH, DO, BOD, COD, Turbidity, TSS, Nitrate, Oil and grease, and Fecal Coliform were monitored. The discharge of labor camp wastewater and overflow dredging water led to significant deterioration in water level, salinity, Turbidity, and TSS levels. Concentrations of pH, DO, BOD, and Fecal Coliform met national standards, while no standards were set for other parameters. Labor camp wastewater monitoring parameters (pH, TSS, BOD, and Nitrate) adhered to national standards, but COD and Fecal Coliform levels exceeded limits. Despite the discharge of wastewater with high COD and Fecal Coliform concentrations, no noticeable impact on the aquatic environment of the Kohelia River was observed. Overflow dredging water quality adhered to national standards, except for TSS concentrations, which were significantly higher. Turbidity and TSS levels increased during March 2022, impacting the Kohelia River Water. A study in Cleveland, Ohio, conducted by [13] on culverting a creek for airport runway construction showed significant variations in water level, turbidity, and TSS between upstream and downstream samples throughout baseline, construction, and post-construction phases. Significant siltation was observed, affecting areas up to 1 km upstream and downstream at a yearly rate of 0.5 to 0.8 m. The study suggests that discharged sediment from overflow dredging water increased turbidity and TSS levels, likely settling in the riverbed during slack tide, leading to the filling of the upstream Kohelia River. A definitive conclusion requires grain-size analysis of discharged sediment and mathematical model runs, which should be included in future studies to determine sediment deposition potentiality and rate.

3.6 Biological characteristics

Total 56 phytoplankton species have been documented under 5 groups. Study found a maximum abundance of phytoplankton community 26,110 cells/L at monitoring location Location-RW-2. Highest abundance of zooplankton community has found 450 individuals/m3 at monitoring location Location-RW-1. Total 27 zooplankton species have been identified under 6 groups. In sediment samples of survey areas, highest abundance of benthic faunal communities has been counted 215 individuals/m³ at monitoring location RW-1. Total 5 groups of benthic faunal species have been recorded during the study period. In 2016, the abundance of zooplankton ranged from 240.64 individuals/m3 to 45.48 individuals/m³ in the Station 3 during the investigation period. In the monsoon, the lowest value (111.99 individuals/m³) was recorded at Station 3 and the highest value (240.64 individuals/m³) was found at Station 1, which was changed in post monsoon with the lowest value of 45.48 individuals/m3 at Station 2 and the highest value 140.92 individuals/m³ at Station 1. In winter, the lowest value (116.12 individuals/m³) of zooplankton was recorded in Station 3 and highest value 128.6 individuals/m³ was found in Station 1, which changed in the pre monsoon when the lowest value 58.26 individuals/m³ was recorded at Station 2, and highest value 66.3 individuals/m³ at Station 3.

Study has found both abundance and species diversity of phytoplankton community recorded significantly higher than baseline study (EIA 2013) period. Abundance and species diversity of zooplankton and benthic faunal communities have found slightly higher than study carried out in 2016. Discharged overflow dredging water contains high load of silt and clay, which are nutrients. That’s why higher abundance of phytoplankton community has found. ‘Effect of Turbidity and Suspended Material in Aquatic Environments’ revealed that Turbidity and suspended material may reduce photosynthetic activity by interference with light penetration, but primary production and also be stimulated through the addition of nutrients associated with particulates.