PHD is associated with the Phil LiDAR 1 Project. Hence we refer the reader to the USC Phil LiDAR 1 Narrative Technical Report (USC Phil LiDAR Team, 2014).


Water Quality Assessment of Mananga


 Rivers serve as the most important freshwater resource for human being storing about 2,000 km3 water globally. They present a continuously renewable physical resource used for domestic, industrial, and agricultural purposes (Rani, Sinha, Prasad, & Kedia, 2011). However, surface waters such as rivers are highly vulnerable to pollution attributing to their role in carrying off the municipal and industrial wastewater and run-off from agriculture in their vast drainage basins. Anthropogenic influences, as well as natural processes, deteriorate surface water and impair their use for drinking, industrial, agricultural, recreation or other purposes (Zhang, Guo, Meng, & Wang, 2009). Prevention and control of surface water pollution is essential in addressing threats posed towards the aquatic environment. Moreover, pollution control and prevention must depend on reliable information of water quality and identification of pollutant sources (Bu, Tan, Li, & Zhang, 2010).


  About thirty rivers are found in Central Visayas. Nineteen out of these thirty rivers are major rivers while the remaining ones are minor rivers. There are three main rivers in Metropolitan Cebu. These are: (1) Kotkot river, (2) Lusaran river and, (3) Mananga river. The Mananga river, principal river in the Mananga Watershed Forest Reserve, includes major tributaries such as Manipis river, Sinsin river, Napairan river, Managuksok river and Morga creeks (DENR-EMPAS, 2000).


  In the reconnaissance held on the 10th of May, preliminary quality assessment of the Mananga river was made. Mananga is crucial to Metro Cebu’s water supply. Water quality in the selected Mananga tributaries was described in terms of its physical parameters such as temperature, dissolved oxygen (DO) and hydrogen-ion activity (pH).


Temperature can be viewed as the main factor affecting almost all physico-chemical and biological reactions. It is well known that all physico-chemical “constants” vary with temperature. Several transformations or effects related to water will be favored by water temperature increase such as dissolution, solubilization, complexation, degradation, evaporation, etc. This phenomenon globally leads to the concentration increase of dissolved substances in water (Delpla, Jung, Baures, Clement, & Thomas, 2009). Dissolved oxygen (DO) is a crucial biogeochemical component in aquatic communities, and is therefore a primary indicator of ecosystem health. Dissolved oxygen concentrations are controlled by varying rates of gas exchange between the atmosphere and the dissolved phase vs. primary production by photosynthesis vs. consumption by respiration, driven by a variety of environmental conditions (e.g. light, temperature, nutrient availability, and substrate availability) (Poulson & Sullivan, 2010). Hydrogen-ion activity, conveniently expressed as pH, ranges between 6 and 8.5 at 25oC for natural waters. The pH 7 is considered the equilibrium state between hydrogen and hydroxyl ions.Occasionally, it can go as low as 2 in hydrothermal springs or as high as 12 in ultramafic rocks in which serpentinite is produced (DENR-EMPAS, 2000).


River Stations


River stations included in the ocular survey are downstream tributaries of the Mananga river.


First Station


This station is situated at barangay Jaclupan, City of Cebu. It is geographically located at 10°18.00 N Latitude and 123°48.51′ E Longitude with an elevation of 58 meters above sea level. As seen in Figure 7.1, vegetation is dense and the river is rocky.



Second Station


This station is situated at barangay Camp 4, City of Cebu. It is geographically located at 10°19’08″N Latitude and 123°49’10″E Longitude. Dominant vegetation surrounding the station is evident. Water is quite turbid. Water gauges are located on top of huge rocks, as observed in Figure 7.2, for flood monitoring.


Third Station


 This station (see Figure 7.3) is situated at barangay Bonbon, City of Cebu. It is geographically located at 10°21’85″N Latitude and 123°49’60″E Longitude with an elevation of 215 m. River had awful stench from organic matter buildup of livestock (i.e. pig) waste at the edge of the river.



Physical Parameters


  Physical parameters such as temperature, dissolved oxygen (DO) and hydrogen-ion activity (pH) were determined in situ from the sampling stations. This was done by dipping the sensors of the electronic devices into the water and recording the results, as seen in Figure 7.4.



  The average temperature of the river stations ranged from 29oC to 32oC. The highest temperature value (32.4oC) was recorded at station 3 of Mananga river while the lowest (29.1oC) was recorded at station 2, as presented in Table 7 1. The presence of vegetation canopies that abound in stations 1 and 2 is interesting to note since it could prevent the sun’s heat from reaching the water surface and may also provide better ventilation.



  It is important that aquatic resources contain sufficient levels of dissolved oxygen (DO) to sustain estuarine and coastal aquatic biota. When hypoxia (DO < 2 mg/L) occurs, a wide range of negative effects on aquatic organisms result (Zhang & Li, 2010). Hypoxic conditions (low DO) reduce or eliminate sensitive fish and invertebrate species, alter biogeochemical pathways, and reduce nutrient retention by sediments (Ancog, Andrade, Miasco, & Ortiz, 2010). Average dissolved oxygen values of the three stations range from 7.15 mg/L to 9.40 mg/L. The lowest DO value was recorded at station 2 while the highest DO was obtained at station 3, as observed in Table 7 2.



  The high DO reading in the Station 3 may be attributed to the presence of flowing water as compared to the stagnant water in Station 2. Vegetation surrounding the river bank of Station 2 may be relatively denser compared to that of Station 3. However, running water provides aeration to water. Higher concentration of oxygen in air is dissolved in water upon aeration, hence, results to an increase in the DO reading.


 The average pH levels of the three stations ranged from 7.98 to 8.77 indicating neutral to alkaline condition, as presented in Table 7 3. This range is found to be within the accepted water quality standard ranging from 6.0–9.0 (Ancog, Andrade, Miasco, & Ortiz, 2010).



  It can be inferred that downstream Mananga tributaries showed apparent vegetation. Hypoxia (DO < 2 mg/L) is not present which is indicative of the existence of fish and invertebrate species in the tributaries, undisturbed biogeochemical pathways, and an increase of nutrient retention by sediments. The average pH range revealed neutral to alkaline conditions and is also within the accepted water quality standard.