The hydrology and hydrogeology of Heron Island, the Great Barrier Reef: modelling natural recharge and tidal groundwater flow in a Coral Cay

Chen, Delton B. (2000). The hydrology and hydrogeology of Heron Island, the Great Barrier Reef: modelling natural recharge and tidal groundwater flow in a Coral Cay PhD Thesis, Department of Chemical Engineering, The University of Queensland.

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Author Chen, Delton B.
Thesis Title The hydrology and hydrogeology of Heron Island, the Great Barrier Reef: modelling natural recharge and tidal groundwater flow in a Coral Cay
School, Centre or Institute Department of Chemical Engineering
Institution The University of Queensland
Publication date 2000-01
Thesis type PhD Thesis
Supervisor Krol, Andre
Total pages 450
Language eng
Subjects 0904 Chemical Engineering
Formatted abstract
 Heron Island is a coral cay located on a platform reef, Heron Reef, in the southern region of Australia's Great Barrier Reef. Although the island is only 0.2 km2 in area, it is ecologically significant because it supports a stand of the now uncommon Pisonia grandis tree and is a seasonal breeding site for seaturtles and many thousands of seabirds. It also supports concentrated social and economic activity being the site of a tourist resort and a scientific research station. Whilst Heron Island's groundwater is brackish and non-potable, the hydrology and hydrogeology of the island has attracted recent interest from researchers pioneering the study of nutrient dynamics on coral cays (circa 1990) and from government agents concerned with the disposal of treated sewage in the Great Barrier Reef.

Previous research has identified an ecologically-significant store of water-soluble nutrients, particularly nitrates and phosphates, in the cay's soil and groundwater. Most of this nitrate was leached from fresh bird guano that was deposited at the island. To better understand the dynamics of water-soluble nutrients at Heron Island an improved understanding of natural recharge and tidally-influenced groundwater flow is required. This study examines these processes with particular emphasis given to field investigations and the development of conceptual and mathematical models. The study is therefore comprised of two major parts: Part I examines natural recharge hydraulics, and Part II examines tidal groundwater hydraulics.

In Part I, hydrometeorological data, including throughfall, soil moisture levels and soil matric potentials, were collected intensively over a ten month period from an instrumented study site in the Pisonia grandis forest. This data, in combination with data obtained from laboratory and field tests on soil samples, are used to estimate the water retention and the unsaturated hydraulic conductivity functions of Heron Island soil at the site. A finite-difference numerical model is developed from Richard's equation and the Moving Mean Slope (MMS) method to simulate non-hysteretic soilwater redistribution in the vertical dimension and thus help interpret the unsaturated hydraulic conductivity functions of the soil.

By applying approximate techiques to the hydrometeorological field data, an estimate of the local hydrologic balance is obtained; including throughfall, soil-water storage, deep drainage, evapotranspiration and surface runoff estimates. This tenmonth-long hydrologic balance is used to calibrate a one-dimensional hydrologic model for Heron Island: called the Soil-Water Transport and Evapotranspiration Model (STEM). The STEM model was used to simulate natural recharge as a function of daily rainfall, and as such is applied to a ten-year-long historical rainfall record. The model results are examined at hourly, daily, and yearly time-scales to improve understanding of natural recharge at Heron Island. Because field measurements of natural recharge were beyond the resources of the current study, the accuracy of STEM's predictions is not established and the model is only used interpretatively.

In Part II, groundwater tides at Heron Island and ocean tides at Heron Reef were monitored intensively over a number of weeks using piezometers, pressure transducers and electronic data-logging equipment. The amplitude-decay and phaseshift of the tidal signal are determined for fixed positions in the island's aquifer. The data also indicate that the groundwater movement directly below Heron Island was unconfined and essentially vertical. Analytical and semi-analytical expressions for this type of flow are derived and applied to the field data to estimate the aquifer's effective vertical hydraulic diffusivity at a number of locations. At one location near the shoreline, distortions of the tidal-pressure signal are interpreted as a seawater intrusion process that involves the beach face and the tide over the reef flat.

A conceptual hydrogeological model for Heron Island and Reef is proposed based on the 'dual-aquifer' concept which involves a moderately permeable upper-layer of Holocene-age limestone overlaying a highly permeable lower-layer Pleistocene-age limestone: a concept that is commonly associated with low atoll islands. Further, to validate the conceptual hydrogeological model, a two-dimensional finite-element groundwater model is developed using a general-purpose groundwater transport simulation model (SUTRA). The results of the SUTRA simulations are compared with the field observations made at the centre of the cay to show that the simulations and the conceptual hydrogelogical models are reasonable.

The results of this study will be of interest to hydrologists, hydrogeologists and biogeochemists who study low carbonate islands, and scientists interested in nutrient dynamics in the vadose and phreatic zones at Heron Island. Furthermore, environmental implications are highlighted by the study which should be of interest to island managers. These include: (a) a high potential for nutrient enrichment of the reef ecosystem as a result of disturbing natural groundwater flow, especially in the vicinity of the cay's shoreline, and (b) a low immediate risk associated with the current approach to aquifer-disposal of treated sewage effluent.
Keyword Groundwater flow

Document type: Thesis
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