A study on dissolved oxygen and its relation to organic matter load in the Cisadane River estuary.
Kajian ketersediaan oksigen dan kaitannya dengan beban organik di perairan estuari sungai cisadane
Date
2011Author
Hariyadi, Sigid
Adiwilaga, Enan Mulyana
Prartono, Tri
Hardjoamidjojo, Soedodo
Damar, Ario
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The Cisadane River estuary might be polluted by organic wastes from various anthropogenic activities in the catchment area, which may significantly increase its organic matter load, and eventually cause depletion of dissolved oxygen (DO). This research is intended to obtain information concerning DO and its relation to organic matter (BOD, COD, organic N) conditions in the Cisadane River estuary, in particular during the dry season when the river flow is limited, and to obtain information where the potential critical estuarine zone due to decreased DO. Dissolved oxygen, BOD, COD, Total Kjeldhal Nitrogen (to determine organic N) , salinity, temperature, pH, water transparency, and microbial content were studied at four stations, each representing riverine zone, mixing zone, and marine zone, from surface and bottom layers, during low and high tides. Observations were conducted in 2007 (September-October) and 2008 (June, July, August). BOD was determined by three days incubation at 30 ºC. BOD rate constant (k) was also determined using least-squares analysis. Primary productivity (photosynthetic oxygen production) of the estuary were studied at two stations in the mixing zone. Sediment oxygen demand from the mixing zone stations, were also studied in the laboratory (Nested design experiment). Dissolved oxygen in the estuary ranged between 0.0-7.6 mg/L with the maxima average of 2.58 ± 2.23 mg/L in the riverine and mixing zones, and the average values of 6.37 ± 0.80 mg/L in the marine zone. The DO in the riverine and mixing zones were classified as hypoxia (< 3 mg/L), and even anoxic at several observations in these two zones. The BOD ranged from 0.52 to 13.49 mg/L, the highest was in the riverine zone (5.0 ± 3.5 mg/L), followed by the mixing zone (4.1 ± 2.2 mg/L), and marine zone (2.5 ± 1.0 mg/L). The BOD/COD ratio varied between 0.005-0.95, with the highest fluctuation in the marine zone. The BOD rate constant also varied between 0.01-0.89 per day, meanings that the estuary’s degradation rate in decomposing organic matter varied from very slow to very fast. On the average, oxygen utilization rate per hour was highest in the riverine zone, followed by mixing zone. Oxygen utilization at the riverine and mixing zone were about 0.14-0.56 mg/L per hour, and these values were considerably high compared to the DO values available. There is no certain relation between DO and organic matter (BOD) at the estuary zone of the Cisadane River. The budget oxygen approach indicated that oxygen loads from reaeration and photosynthesis were not sufficient for the water column decomposition and respiration, and also for the sediment oxygen demand. Oxygen input carried by the ocean tide plays a more important role in supplying oxygen to the mixing zone compared to the oxygen carried by the river flow. Estuari Cisadane adalah muara dari S. Cisadane, salah satu sungai besar di Jawa, yang alirannya melintasi daerah pemukiman yang luas dengan beragam kegiatan, yaitu kota Bogor dan Tangerang. Limbah dari berbagai kegiatan perkotaan tersebut, diduga telah menyebabkan meningkatnya beban limbah organik mudah urai (biodegradable organic wastes) sehingga berakibat pada penurunan kualitas air, antara lain kondisi hypoxia (kadar oksigen terlarut ≤ 3 mg/L), di estuari. Kondisi hypoxia di estuari dapat terjadi sehubungan dengan penggunaan oksigen yang tinggi terutama untuk dekomposisi bahan organik. Tingginya beban bahan organik ini terutama akibat limbah berbagai kegiatan manusia (antropogenik) di sepanjang aliran sungai, yang terbawa aliran dan tertahan oleh arus pasang-surut laut, khususnya di musim kemarau saat debit aliran sungai rendah, sehingga dapat terakumulasi pada zona tertentu di estuari.
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