Temporal variations of mineral character of acid-producing pyritic coastal sediments, Southeast Queensland, Australia

The lower Pimpama River catchment possesses many features of morphology, geology and landuse, typical for Southeast Queensland. Pimpama River and its main tributary Hotham Creek meander over a coastal plain which developed during the last several thousand years as a result of sea level fluctuations and changing fluvial and estuarine regimes which provided ideal conditions for the formation of sedimentary pyrite. A complex mixture of natural and human factors triggers and controls the oxidation and hydrolysis of this pyrite. The consequent production of sulfuric acid and leaching of metals from the pyrite-rich sediments represent main environmental issues of this coastal setting. This study aimed to determine the lithological character of the coastal unconsolidated sediments, and identify changes produced by acidity over a long period of time in a natural system and over a short period of time in a laboratory system. The mineral composition of the estuarine sediments of the coastal plain reflects the lithology of their source, the geological basement, and also the enhanced weathering rate due to acid production. The primary minerals present in the sediments consist of quartz and feldspars (primarily albite, K-feldspars to a lesser extent and minor anorthite) the product of physical weathering of bedrock material, mainly sandstone. Kaolinite is the dominant clay mineral and represents the product of intense leaching of feldspars promoted by acid conditions. Kaolinite is fairly ordered in structure in fresh and weathered bedrock and it becomes disordered in the estuarine sediments. Illite is mainly present in bedrock-related samples and the highest amounts are associated with muscovite. In the tidal sediments, illite is present to a lesser extent and is not found in the lower estuary. The distribution of mixed layers of smectite–illite is highly variable and their deposition is mainly controlled by the hydrodynamic conditions of the environment. Only low energy tidal settings favour the deposition of smectites. Pyrite is present at many sites and is always accompanied by oxidation products such as jarosite and gypsum. The laboratory experiment showed that in the short-term, changes induced by acid production mainly involved the: (1) decomposition of oxidation products such as gypsum and jarosite, (2) weathering of feldspars (mainly plagioclase), and (3) increased amount of kaolinite and possible increased structural disorder. These observations are of significance when considering wetlands as a method of remediating acid-producing areas by re-establishment of reducing conditions. Jarosite is quite widespread and can represent a source of acid over a short period of time in water-saturated, partially oxidized pyrite-rich sediments. The initial several months of a wetland establishment may, however, be dominated by secondary acid production and metal leaching from preexisting oxidation products like jarosite and natrojarosite.