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The Palingkau Swamp Irrigation Scheme SP1 SP2 SP3 extends tidal irrigation between Kapuas Murung River and Kapuas River, which local peoples developed. The development of the new agricultural land behind the old tidal irrigation system creates oxidation of potential acid sulphate (FeS2) soil. The result of the leaching process polluted local’s agricultural area causes the decrease land productivity. The field observation recorded the water pH ranges between 3.9 - 5.3; 3.7 - 4.31 at the handil; 2.5 - 4 at the collector channels; and 2.8 - 3.6 at the primary channels. Good water management is needed to overcome these problems, both at the new irrigation system and handil. The HECRAS mathematical model is used to evaluate water circulation in the system and its reliability. The Efforts to normalize the handil impact increasing the amount of water entering the handil although it is not significant. Moreover, it still cannot meet the water demand for the leaching process in the network system by 2.06 million m3/day. It is necessary to increase the capacity of the channel to allow freshwater flow from the river for the leaching process in the network system.

The Palingkau SP1 SP2 SP3 swamp irrigation is an expansion of handil’s agricultural area, which is influenced by tides from the Kapuas River on the left and the Kapuas Murung river on the right. The new area that exploits backswamp and the buffer zone upstream of Handil, gives an additional burden of dispersion of toxic materials resulting from the oxidation of acid sulfate soils and the loss of fresh water supply from backswamp. The actual leaching process is indicated by the water pH value in the collector and primary channels ranging from 2.58 ~ 3.6. Meanwhile, the water quality of Handil, Kapuas and Kapuas Murung rivers is better, ranging from 3.7 ~ 5.34. The confluence of tidal from two opposite sides has resulted in the disruption of pollutant discharge. Instead of discharged into rivers, its flow to Handil’s agricultural area and reduce agricultural productivity. The HEC RAS software simulation model is used to evaluate the optimal water management system. One-way flow system is introduced hydraulic structures for regulating the flow of freshwater water at high tide from the left handil into the new irrigation area and regulating the drainage at low tide through the right handil.

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