Bribie Island is a large barrier island within Moreton Bay, situated 60 km north of Brisbane and separated from the mainland by Pumicestone Passage, a shallow estuary. Groundwater and soils of the island are naturally very acidic and low in nutrients, as is typical of the type of landform in the region. In its natural state, the Island supports coastal heath, melaleuca and mixed eucalypt woodland. Approximately 40 years ago, commercial pine plantations began to be established on Bribie Island and the entire plantation resource was recently cleared over a period of a few years, mostly in preparation for further plantings.
There have been hydrological studies on the island in relation to groundwater levels, water supply and wastewater recharge. Also, there is current interest in solute transport from the island and possible contamination of surrounding marine ecosystem. The sandy soils mean that most rainfall enters a near surface aquifer and then percolates into a deeper aquifer through various configurations of semi-permeable "coffee rock".
The establishment and recent removal of pine plantations in central parts of the Island may be affecting the physical and chemical nature of the shallow and deep aquifers. As a preliminary investigation, groundwater quality and acidity have been compared between areas of cleared pines and native vegetation. Because no mature pines have been left on the Island, there was no opportunity to consider conditions prior to clearing.
Water samples and heights were taken at 38 wells with the main focus on replicated samples from 8 wells from each category of cleared pine or native vegetation. Samples were analysed for total acidity, dissolved organic carbon, various forms of phosphorus and nitrogen, and a range of elements including iron, aluminium, potassium, and sulphur.
Less dissolved organic material and higher pH, but not necessarily lower total acidity, is characteristic of the deep aquifer compared to the surface aquifer. The deep aquifer tended to have lower levels of most cations but there were a number of exceptions.
Regardless of land use, high acidity of groundwater is primarily related to a high content of dissolved organic material in the form of humic and fulvic acids.
There were statistically significant raised levels of nitrogen, phosphorus, potassium and iron in groundwater beneath areas of cleared pines. At least in the case of phosphorus and nitrogen the difference was likely the result of application of di-ammonium phosphate fertiliser.
For groundwater samples taken under native vegetation, those from areas of melaleuca woodland where groundwater is discharged and soils have higher clay content, had the highest levels of nutrients.
High acidity facilitates solute transport and these results indicate land use is influencing the quality of the groundwater. It is recommended that monitoring and study of the transport of solutes offsite be undertaken and that the possible consequences of raised fertility in an environment of naturally low fertility be assessed.