Baseline sediment oxygen demand measurement at selected sampling stations in Pasig river using bench scale benthic respirometer chamber

The Sediment Oxygen Demand (SOD) describes the rate of oxygen consumption exerted by the bottom sediments on the overlying water. Studies in other countries show that SOD can account for more than 50 percent of the total oxygen demand in bodies of water. In the Philippines, studies using standard water quality parameters (pH, temperature, and Total Suspended Solids) as stipulated in the Department of Environment and Natural Resources Administrative Order 34 (DAO 34 Series of 1990) are being used to describe used to describe the Water Quantity Criteria of Water bodies. However, none has been investigated on the effect of SOD. This study focused on the effect of the river sediments on the dissolved oxygen budget on the overlying waters of Pasig River, one of the major rivers in the Philippines. The sampling stations selected for this study were Marikina (upstream), Sanchez (middle stream) and Jones (downstream) stations. The dissolved oxygen (DO) uptake rates were measured by collecting river water and sediment samples from selected three sites per station. Laboratory methods for SOD rate determination were employed in this study with the use of bench-scale benthic respirometers for monitoring the concentrations of dissolved oxygen in a known volume of water that is recirculated above a known area of sediment. Sediment bed depth (2.5 cm and 5.0 cm) and chamber flow rate were varied and correlated to SOD rate. Important water quality parameters were correlated to SOD rate such as total suspended solids (TSS) and salinity while other parameters such as pH and initial DO were monitored. Sediments were visually classified into sand or silt/clay and the effect of sediment type on SOD rate was determined. SOD rates generally increased for practically all sites in the three stations of Pasig River when flow rates increased. In Marikina Station, a 34% increase in SOD rates were determined when flow rates were varied 10.6 to 20.22 ml/s, while the increase is SOD rates from a flow rate of 10.6 to 32.75 ml/s was about 260%. In Sanchez Station, there was a low decrease in SOD rates of 14% from a flow rate of 10.6 to 20.22 ml/s while increase of 180% from flow rate of 10.6 to 32.75 ml/s was determined. In Jones Station, the increase in SOD rate when the flow rate was varied from 10.6 to 20.22 ml/s was 8% while a 44% increase was determined when the flow rate was increased from 10.6 to 32.75 ml/s. No consistently significant correlation was found between SOD rates and salinity between sites and stations of Pasig River. Initial DO concentration measured during sampling revealed extremely low DO levels not greater than 3.3 milligrams/liter (mg/L) and pH readings within the range of 6.5 to 8.5 at all sites in all stations. Visual inspection of sediments in the range of 6.5 to 8.5 at all sites in all stations. Visual inspections of sediments in the three stations revealed sandy sediments in Marikina Stations and silt/clay sediments in Sanchez and Jones Stations. SOD rates measured in Marikina Station, with sandy sediments, were lower (0.1280 1.4894 g/m2 / day) compared to the SOD rates measured in Sanchez (0.584 7.573 g/m2/day) and Jones (1.120 3.745 g/m2/day) Stations, with silt/clay sediments