Ochre deposits and associated bacteria in some field drains in Scotland

Drainage systems installed in highly organic soils, with the ground water pH near to neutral and affected by iron ochre deposition, were studied at six sites in Scotland. The iron ochre was composed of iron–encrusted filamentous bacteria entrapping amorphous material. The filamentous bacteria were identified as Gallionella spp., Sphaerotilus natans, Leptothrix ochracea and Leptothrix pseudo–ochracea . Heterotrophic iron–oxidizing bacteria were also present in the iron ochre, but Thiobacillus ferroxidans was not isolated. Differences in the proportion of species could not be correlated with peat type or the age of the drainage system. Sulphate reducing bacteria and bacteria capable of reducing Fe³⁺ were also isolated from the drainage water.
A preliminary investigation of the importance of biotic factors in the kinetics of Fe²⁺ oxidation showed that the rate of Fe²⁺ oxidation in unfiltered drainage water, was about twice the rate in filtered water, at 10°C. Incubations over a range of temperatures showed that the differences in rates were greatest at low temperatures.