Spatial and temporal variation in the hydrochemistry of marine prawn aquaculture ponds built in acid sulfate soils, Queensland, Australia.

Many brackish water aquaculture ventures in Australia and overseas have established ponds in coastal regions with acid sulfate soils (ASS). Acid sulphate soils are known to leach relatively high concentrations of metals, acid (metal and H+ ion) and sulfur, however very little is known about how these leached elements affect the water quality of aquaculture ponds.The main objective of this thesis was to describe the hydrochemical processes controlling the water chemistry in the water column and sediment pore water in the studied aquaculture ponds over time and space.Water samples providing the spatio-temporal data were collected from the ponds with the use of adapted sampling methods commonly used in the groundwater environment. A transect of five nested piesometers was installed in two prawn ponds at Pimpama, south east Queensland, Australia. Each piesometer nest contained a multilevel with eight outtakes, a mini – horizontal, and a slotted piesometer. Water samples were collected from each nested piesometer on a bi-monthly basis over the prawn-growing season. The unstable elements and water quality variables (pH, Eh, DO, EC, water temperature) were measured in the field. Stable elements were analysed in the laboratory using ICP-OES and ICP-MS. Soil samples were collected at the end of the season for elemental analysis. A number of key sediment/water interactions and processes such as precipitation/dissolution reactions, oxidation-reduction reactions, photosynthesis, adsorption and seawater buffering were identified as important controls on pond water conditions. This is the first study to provide detailed hydrochemcial analysis of the pond water over time and space and aided in identifying that even shallow water bodies can be chemically heterogeneous.Analysis of the water and sediment highlighted the selection of metals that can be associated with ASS and that are mobilised from pond sediments under certain chemical conditions. In Pond 7 Al, As, Ni and Zn concentrations were generally higher at the beginning of the grow-out season. Variability of the metal concentration was observed between the water column (0 – 1500 mm) and the pore-water (0 - -1000 mm). The highest concentration of Al (1044 µg/L) and Zn (104 µg/L) were sampled in the water column (approximately 400 mm from the surface of the pond). The highest concentration of As (130 µg/L) and Ni (73 µg/L) were sampled in the pore water sediment (associated with ASS). Elevated Mn and Fe2+ concentrations were also associated with the sediment pore water. The highest concentrations of Mn and Fe2+ were 4717 µg/L and 5100 µg/L respectively. In Pond 10, Ni concentrations (167 µg/L) were the highest at the beginning of the grow-out season. However, As (97 µg/L), Al (234 µg/L) and Zn (308 µg/L) were most concentrated during the middle of the cycle. The highest mean concentrations of these elements are As (63 µg/L), Al (91 µg/L) and Zn (69 µg/L) which are each associated with the sediment-water interface. These metals are integral in degrading the pond water quality and lead to a loss of beneficial algal blooms, a reduction in pond water pH, poor growth rates and high mortality in shrimp. It is also possible that the dissolved ions and precipitated compounds that are leached from the ASS are discharged into the adjacent coastal estuary of Moreton Bay. With knowledge obtained from this PhD study, effective management and treatment systems can be developed and implemented to minimise the impact of these soils on the pond system and the water discharging into natural coastal ecosystem