Despite being one of the largest aquifers of its type in Australia, the Burdekin River Delta (BRD) is an area that has received comparatively little research on its groundwater resources. This study conceptualises the hydrogeology of the BRD and characterises the relationships between the stratigraphic elements and the physical and chemical components of the groundwater system that influence the major governing processes. Importantly, a large amount of spatial and temporal groundwater information exists in database form, which enables an integrated conceptual model of the BRD aquifer to be developed from the key hydrogeological and hydrogeochemical relationships. Conceptualisation of the BRD aquifer is achieved by categorising four main aspects of the groundwater resource: 1. Surface characterisation; 2. Geologic characterisation; 3. Hydrogeologic characterisation; and 4. Groundwater System characterisation. The BRD is a large cuspate delta comprising a complex stratigraphy of Pleistocene to Holocene sediments of fluvial, deltaic and marine origin to a maximum depth of about 150 metres. The lower Pleistocene sediments lie predominantly below sea level and are typified by laterally discontinuous sands, silts and clays that have formed in response to fluctuating sea levels. The upper Pleistocene boundary is differentiated from the overlying Holocene sediments by a formerly exposed surface of semiconsolidated oxidised sandy clays and gravel. By contrast, the Holocene sediments comprise loose, uncompacted sequences of fluvial channel sands, interdistributary floodplain silts and marine incursions of estuarine clays and mangrove muds. The anastomosing array of fluvial sand bodies of former Burdekin River channels and levees is the setting for the main shallow aquifer units. Aquifer units of the lower Pleistocene sediments are in hydraulic connection with the Holocene units, effectively categorising the whole BRD as a single unconfined aquifer. Hydraulic gradients from both sides of the river divide the BRD into two broad flow regimes. Interpreted flow zones based on hydrograph patterns further subdivide the flow system based on seasonal recharge response to elevated river heights and flooding, and response to long-term rainfall patterns associated with La Niña episodes of the Southern Oscillation. Stable isotope data (2H and 18O) indicate that the dominant isotopic signature of groundwater throughout the BRD corresponds with intense rainfall activity, however high deuterium-excess values indicate that significant evaporation occurs prior to recharge. This infers dominant recharge by the Burdekin River that drains a massive catchment extending hundreds of kilometres inland. Direct recharge via rainfall infiltration is largely dependant on soil texture. More conductive soils are associated with the major levee systems that comprise the main shallow aquifers. Two evolutionary hydrogeochemical paths exist for the north and south sides of the river, and are constrained by the interpreted flow zones. In the south side, groundwater enters the main aquifer from river recharge and leakage out of weathered granite outcrops (exposed bedrock). Mineral hydrolysis and evaporative concentration of salts initially evolve groundwater in the weathered granite to a combination of Na-Cl and Na-HCO3 type. Leakage through clay-rich hillwash and marginal sediments causes reverse cation-exchange reactions where excess Na replaces Ca and Mg on ion-exchange surfaces. This leads to the formation of Mg,Ca-Cl type groundwaters into the southern parts of the main aquifer (supersaturated with respect to calcite and dolomite). Discharge towards the coast is characterised by seawater mixing where salinity increases with corresponding evolution to Na-Cl type waters. Recharge waters from the Burdekin River are fresh (<250mS/cm) Ca-HCO3 type, undersaturated with respect to calcite, and are easily distinguishable from the ion-exchange groundwater. In the north, only one smaller outcrop of bedrock exists, which hosts similar mineral hydrolysis reactions and base-exchange reactions. An absence of associated Na-Cl type waters means that reverse-cation exchange reactions are negligible, and so water types are predominantly Na-HCO3 type. Aquifer sands in the north are more widespread than in the south, so the fresh Ca-HCO3 recharge waters tend to dominate the overall groundwater composition, with Na-HCO3 types limited to the exposed bedrock areas. Towards the coastline, groundwater mixes with seawater towards Na-Cl type waters, similar to that observed in the south. The mangrove mud sequences that flank the coastline of the BRD are associated with high-Fe and low-pH groundwater formed by the oxidation of Fe-sulphides such as pyrite). SO4 is a product of this reaction, but does not achieve abnormally high concentrations, possibly due to the presence of sulphate-reducing bacteria. Carbonate dissolution is a possible side effect of acid sulphate generation, with possible gypsum dissolution as a secondary source of SO4. This study tested an alternative method to characterising groundwater to determine if the spatial extent of hydrogeochemical processes could be defined and comparable results achieved. This method involved discriminating discrete statistical groups of ionic ratios based on their cumulative frequency distribution. The statistical groups are bounded by critical values that distinguish different chemical processes, referred to as hydrogeochemical indicators. Various tested ionic ratios produced analogous indicators, proving their reliability as a valid method for the characterisation of groundwater chemistry. The significance of this research underlies the importance of groundwater use in the BRD as a primary source of irrigation supplies. Land use expansion and unregulated pumping pose a risk to future groundwater quality and sustainable volumes. The understanding of the relationship between the main geologic elements and the subsequent hydrogeochemical processes provides a scientific basis for conceptualising the groundwater resource. This establishes a framework for initiating future groundwater management options.
| Identifer | oai:union.ndltd.org:ADTP/264934 |
| Date | January 2004 |
| Creators | McMahon, Gerard Armstrong |
| Publisher | Queensland University of Technology |
| Source Sets | Australiasian Digital Theses Program |
| Detected Language | English |
| Rights | Copyright Gerard Armstrong McMahon |
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