Strong acid waters in tributaries of the Nishi River, Fukuoka prefecture

Acid mine waters of Nishikawa, Fukuoka Prefecture were studied. River water collected from several branch streams of Nishikawa is characterized by very low pH of 2.6-4.2 and enriched in Fe, Ca²⁺ and SO₄^2-, while the main stream of Nishikawa shows pH of 6.7-7.1. Yellowish brown precipitates are usually observed on a riverbed of the branch streams. They were identified to be such amorphous materials as iron hydroxide and iron hydrosulfate, including goethite- and schwertmannite-like minerals by means of XRD and EPMA analyses. In the study area, coal beds occur in the Paleogene Nogata Group and they were exploited in early 20th century. It is common that pyrite is hosted not only in a coal bed but also in the surronding host rocks. We detected pyrite grains in a silicified wood, which is frequently intercalated in coaly shale of this area. Oxidization of pyrite in the coal-bearing beds well explains the origin of Fe-rich strong acid river water in this area.