MICROORGANISMS AND OCHRE DEPOSITS IN FIELD DRAINS OF ONTARIO

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Abstract
The effluents and ochreous deposits in several partly blocked field drains in Ontario were microbiologically and chemically assayed. The effluents contained several million per milliliter of the iron-oxidizing bacterium Gallionella. Sphaerotilus, a sheath-forming bacterium, was also numerous. Both microorganisms were generally encrusted with amorphous ferric iron oxides. They were also found in the ochreous deposits inside the tiles and also in the deposits adjacent to plugged filters surrounding the tiles. The iron-oxidizing bacterium Thiobacillus ferrooxidans was not found. Filamentous structures resembling the iron-oxidizing bacterium Metallogenium were frequently observed but the experimental evidence indicated them to be aggregates of colloidal Fe(OH)3. Calcite, dolomite, quartz and feldspar were common constituents of the effluents and the ochreous deposits. In some cases, one or more of gypsum, plagioclase, chlorite and illite were present. The ochreous deposits were generally high in iron and contained elements normally found in soil. Occasionally the materials were found to contain traces of magnetite. Except for the iron in the silicates the remaining iron was non-crystalline, present as amorphous ferric oxyhydroxide. Amorphous manganese oxide was also found. It was recognized that chemical oxidation might be an important agent in oxidizing the FE+2; however, the results and a literature survey indicated that Gallionella plays a major role. It is theorized that following oxidation, a ferric hydroxide sol is produced, which in the presence of electrolytes forms aggregates of colloidal Fe(OH)3 that combine with microbes and ions in the tile water to produce an amorphous precipitate. In association with carbonates, quartz and clay minerals, the precipitate forms ochreous substances. The paucity of dissolved ferrous and ferric iron, the presence of organic matter and adsorbed anions inhibits crystallization of the amorphous ferric oxyhydroxide. Roots in conjunction with large numbers of Sphaerotilus formed effective plugs.