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Oxygen component acts as an important factor of natural decomposition process inside pyrite soil. This phenomenon is believed occurred in the vadose zone of tidal lowland area when water table is declining. Meanwhile, the concentration of dissolved oxygen (DO) is considered sustainable in aqueous medium if the soil is in the saturated condition. This research aims to simplify the rate of the decomposition in a mathematical approach due to oxygen transfer inside porous media. Location of study was in the lowland area of Tabunganem district with B category of wetland typology in South Kalimantan Province which directly connects to the estuary of Barito river. Measurement of water sample was conducted in secondary and tertiary canals around Tabunganem district in the different location. The measurement itself was done in higher and lower tidal conditions due to tidal force from the estuary. In this term, the mechanism of the water intake was following Garpu system, special lowland water leeching management, with two ways of water flow and two acid ponds. The result shows that minimum concentrations of ferric-oxide and DO was measured averagely in different canals at 0.1 ppm and 1 ppm respectively, while the maximum amount was observed at 2 ppm and 4 ppm. Moreover, oxygen transfer inside the vadose zone was assumed to increase in the vertical direction, while DO consumption by aerobe bacteria gathering which starts at 2 ppm, was abandoned. This study is confirming the linear correlation between tidal force influence from the estuary of Barito river and the decomposition rate of the pyrite soil around Tabunganem district. However, the influence of the acid ponds was not explicitly declared in formulation.

oxygen; decomposition; pyrite; vadose; lowland