Turner, J.V., Townley, L.R., and Barr, A.D. (1994), Progress Report to MERIWA and AMIRA for the period January to June 1994, on Groundwater Supply to the Mining Industry on the WA Goldfields, Project M217/P321A, CSIRO Division of Water Resources, Consultancy Report, 64pp.

Hydraulic properties of bounding formations

Measurements of hydraulic conductivity of low permeability aquitards overlying the paleochannels has been completed using slug tests. Results of the slug tests indicate an overall range of values for hydraulic conductivity from 10^-6 to 10^-9 m/s. Several factors dictate that care should be taken not to over-interpret the results of slug tests and consequently the results are regarded as operational guides. However, we believe that the results are a true reflection of actual aquitard hydraulic properties. The results are two to three orders of magnitude higher than those obtained by triaxial tests reported earlier, and is probably due to the effective scales over which the different measurements are made. Piezometers have been completed and screened in the weathered basement formations underlying the producing aquifers. Well recovery tests reveal that these materials are more permeable than the aquitard clays, suggesting that upward leakage should not be neglected in our resource assessments and modelling. The range of transmissivities determined from three tests was 0.1 to 3 m²/day. This range is at least two orders of magnitude less than that determined for aquifer transmissivities which are typically in the range 100 to 1000 m²/day. These results will be incorporated into the ongoing modelling investigations.

Aquifer-aquitard modelling

Methods for determining the movement of an aquitard water table in response to aquifer stress were investigated using a model of the unsaturated zone coupled to one of the saturated zone. During the previous reporting period, progress was being made in coupling this model with the one-dimensional vertical saturated model. In developing this modelling capability it was recognised that the question of the significance of the water yield by the process of aquitard consolidation needed to be investigated.

An analysis of aquitard consolidation due to aquifer depressuring based on data from the triaxial test results concluded that the total groundwater yield due to consolidation was between 0.02 and 0.32 m of water. The regional significance of consolidation was investigated by estimating the surface area of contact between aquifer and aquitard as 72 km² and combining this estimate with the consolidation lengths to give an estimated total yield of between 1.4 x 10⁶ and 23 x 10⁶ m³. By comparison, the annual production of groundwater from the paleochannel aquifers in 1990 was 14.3 x 10⁶ m³. Thus the range of values for the volume of groundwater yielded by consolidation, when placed in the context of the total annual groundwater production is determined to be of marginal significance over the expected lifetime of the paleochannel aquifers.

A model of the behaviour of a pumping well in an aquifer overlain by a water table aquitard is now complete. This stage of the modelling completes the coupling between the saturated and unsaturated models and describes the fall in the aquitard water table in response to aquifer depressuring. Linking a series of such models to represent pumping wells in a quasi-linear paleochannel is being developed.