Effect of sulfuric acid discharge on river water chemistry in peat swamp forests in central Kalimantan, Indonesia

To estimate the range of area that is affected by sulfuric acid pollution after pyrite oxidation, the surface water chemistry of two rivers in peat swamp forests in central Kalimantan, Indonesia, was surveyed at 1.0- to 3.0-km intervals in September 2003 and 2004 (dry season) and March 2004 and 2005 (rainy season). Water discharged from canals into the main stream of the Sebangau River and the Kahayan River showed lower pH compared to the mainstream water of the rivers, implying sulfuric acid loading from the canal to the main stream of the rivers. The ratio of concentrations of sulfate ion/chloride ion, which was used as a parameter for estimating the contribution of pyritic sulfate to river water chemistry, showed that sulfuric acid loading from pyrite oxidation occurred from the river mouth up to 150 km upstream in both rivers. Water of the main stream of the rivers as well as water discharged from artificial canals into the main stream in the rainy season showed much higher acidity and a higher ratio of sulfate ion/chloride ion than that in the dry season. This result implies that the discharge of pyritic sulfate from peat swamp forests to the limnological system is much higher in the rainy (high water table) season than the dry (low water table) season. Water in the canal in the rainy season was found to be highly acidic (pH = 2.0–3.0). Pyrite oxidation after peatland development causes not only acidification of soil but also acidification of the limnological ecosystem.