A two-year study of the relationships between water chemistry, hydrology and climatology in areas of acid sulfate soils has demonstrated that discharge of sulfuric acid to estuaries is not dependent solely on the magnitude of a rainfall event. Large rain events did not always produce significant changes to water chemistry, and small rain events could produce large changes if the prevailing conditions were suitable. The magnitude of changes to estuarine waters was found to be dependent on the position of the watertable, and therefore the available soil pore space, and the store of acidic water in floodgated drains at the time of rainfall. These results have significance for predicting the discharge of acid drainage to estuarine environments and the possible impacts on aquatic organisms. A water balance equation, which estimated acid discharge to surface waters as high as 317 t of H2SO4 in one month, can be used to predict monthly discharges for rain events of varying magnitude if prevailing weather conditions are known.

Acid sulphate soils form when iron pyrite oxidises to sulphuric acid on atmospheric exposure, lowering soil pH below 4. In the Richmond River estuary, northern New South Wales, flood mitigation, drainage works and floodplain excavations have augmented acid sulphate soil formation by increasing pyrite availability for oxidation. These engineering works have facilitated the transport of acidified water and have impeded recovery from tributary acidification. The increased frequency and duration of acidification are associated with elevated concentrations of inorganic monomeric aluminium and dissolved iron, and major iron hydroxide precipitation, with deleterious effects on exposed aquatic biota and habitat. Fish kills and an outbreak of epizootic ulcerative syndrome (EUS) have been recorded from acidified sites in the estuary.

SUMMARY The principles of colour perception are outlined and applied to the problems of soil colour measurement using ‘Munsell Soil Color Charts’. As care is needed in using these charts to make an accurate assessment of soil colour, recommendations for the correct methods for soil colour measurement are outlined. The three soil colour characteristics of Munsell Hue, Value and Chroma are commonly used in multivariate analyses as orthogonal, equi‐dimensional, linear variables that describe perceived colour space. Such an approach may be inappropriate because the angular separation of Hue and an approximately 2.5:1 scaling relationship between Value and Chroma should be accounted for in any model of perceived colour space. An appropriate and simple polar transformation of Munsell colour data is compared with some other coordinate systems for representing colour space, including the CIELAB system. This polar transformation is recommended for processing colour data in multivariate analyses.

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