Schwertmannitic coatings on subsoil macropores of coastal acid sulfate soil landscapes in eastern Australia and implications for groundwater geochemistry

Schwertmannite (Fe8(OH)5.5(SO4)1.25) has recently been found to be the dominant mineral of iron precipitate accumulations from surface environments surrounding waterways (e.g. the sides of drains, and soil surface horizons) in acid sulfate soil landscapes in eastern Australia (Sullivan and Bush, 2004). In this study the yellowish-brown, orange-brown, and reddish-brown coatings on macropores (such as channels and planar pores) in 21 subsoil layers all located within acid sulfate soil ‘hotspots' in eastern Australia, were isolated and examined by both XRD and SEM-EDS to determine if schwertmannite was present. This examination determined that schwertmannite was detected in these coatings in 14 of these 21 subsoil layers. The finding that schwertmannite is common in the acid sulfate subsoils within acid sulfate soil hotspots has implications for the behavior of these soil materials and in particular the properties of groundwater in these soil layers. Surficial accumulations of schwertmannite in similar acid sulfate soil landscapes have been shown to exert a strong influence on the geochemistry of surface waters (Sullivan and Bush 2004). Research presented here indicates that the presence of macropore coatings of schwertmannite in the acid sulfate subsoil layers within these landscapes can similarly exert a strong influence on the geochemistry of groundwaters. Interestingly, the World Reference Base (1998) has included the presence of “yellowish-brown schwertmannite mottles” as a defining feature of sulfuric subsoil horizons. However, the results here indicate that schwertmannite can exhibit a wide range of colors from yellowish-browns through to reddish-browns when located as coatings in the subsoils of such acid sulfate soil landscapes. Schwertmannite exhibited a similar a wide range of colors in the surficial schwertmannite accumulations in these landscapes (Sullivan and Bush 2004). These results further emphasize the conclusions of Scheinost and Schwertmann (1999) that color is not suitable property to identify schwertmannite due to 1) schwertmanite's high color variability and 2) schwertmanite's similar average colors to those other iron precipitate minerals.