Groundwater flow modelling of the Grant Group and Poole Sandstone aquifer – Fitzroy Trough, Western Australia

[摘要] A numerical groundwater flow model has been developed using the industry standard MODFLOW code to for the first time estimate the water balance for the most regionally extensive and high-yielding aquifers of the Fitzroy Trough, a sub-basin of the geological Canning Basin. The model domain covers an area of more than 600 km x 400 km, running from the edge of the Kimberley Plateau in the east to the Indian Ocean in the west, and from the Great Sandy Desert in the south to King Sound in the north. This is the first time such an ambitious modelling study has been attempted in what is a very data sparse region. Accordingly, the model should be viewed as providing a highly simplified, broad-scale assessment and a solid foundation for future studies.The primary focus of the model development and calibration was the inter-connected Grant Group and Poole Sandstone aquifers, however two overlying model layers were also implemented to represent – from oldest to youngest – the Noonkanbah Formation aquitard, and an aggregation of the mixed aquifers/aquitards comprising the Liveringa Group, Erskine and Wallal sandstones, and Fitzroy River Alluvium. The geological structure has been interpolated using stratigraphic information from a combination of mineral exploration and stratigraphic drill holes, as well as groundwater bores in the area. The thickness of the three aggregated model layers was independently calculated and the layering computed by successively subtracting each layer from the land surface. The hydraulic properties of the aquifer/aquitard layers have been set to conform to a range defined by limited existing measurements of these values within each layer.Recharge to the groundwater system has been conceptualised by an up-scaled version of point estimates of recharge determined using the chloride mass balance method. Using multiple linear regressions of rainfall and clay content, and modifiers for local slope and regolith thickness, recharge estimates have been made across the model region.The model was run to a steady-state by running for 500 years in transient mode with constant inputs, which was determined to be sufficient to ensure the change in aquifer heads and water balance components over time was effectively zero. Steady-state calibration was then obtained by manually adjusting hydraulic conductivity in three simplified zones based on historical gamma logs and within ranges constrained by previous aquifer pumping tests. Adjustments were made until simulated groundwater levels provides an adequate match to observed values taken as single measurements spread over approximately 50 years, which were also used to produce a potentiometric surface for the aquifer system. Groundwater levels for the amalgamated shallow aquifers in layer 1 were not a focus of model calibration, although a key objective was to ensure simulated water levels provided a sensible representation of the water table, following a subdued form of the topography. An additional loose calibration target was to ensure the model produced groundwater discharge fluxes to known gaining reaches of the Fitzroy River that were of similar order of magnitude compared to historical dry season stream-flow gauging records. Whilst this was achieved, the model could not represent one known losing reach of the Fitzroy River, as such conditions cannot be simulated with a Drain-type boundary condition. In addition to the general paucity of aquifer/aquitard hydraulic properties, a lack of historical time-series groundwater level measurements meant the MODFLOW model could not be calibrated in transient mode. Accordingly, this model is consistent with a Class 1 model according to the Australian Groundwater Modelling Guidelines.

[发布日期] 2018-07-07 [发布机构] CSIRO

[学科分类] 地球科学(综合)