The effect of ochreous sludge from blocked irrigation bores on the oxi
dation rate of soluble Fe(II) was studied. Solutions of Fe(II) at an i
nitial concentrations of 12-15 ppm were held at 19 degrees C under an
oxygen partial pressure of ca 1.4% and the decline in Fe(lI) concentra
tion observed. Solutions were buffered with 2-(N-morpholino) ethanesul
fonic acid (MES) at pH values of 5.8, 6.0, 6.3 and 6.5. Sterile flasks
showed agreement with the expected 1st order behaviour (with respect
to [Fe(II)]) while flasks inoculated with ochreous sludge deviated sig
nificantly from this pattern. Rates of Fe(II) oxidation were significa
ntly enhanced in the inoculated flasks with the greatest increase obse
rved at pH 5.8.