Browsing by Author "Mitry, WH"

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    Discrete interval sampling investigations into the relationship between upland swamps and siltstone aquitards within the Hawkesbury Sandstone, Sydney Basin
    (Australasian Environmental Isotope Conference, 2009-12-03) Peterson, MA; Mitry, WH; Waring, CL
    The Southern Coalfields of the Sydney Basin underlie large tracts of Sydney’s water catchment area. This area comprises a dissected plateau of near-horizontal strata, typically capped by the quartzose Hawkesbury Sandstone, with dams constructed across some of the deeper valleys. Many of the perennial tributaries arise within small upland swamps of several hectares size, which exist near the watershed divides. The swamps seem to be reliant on lateral groundwater flowpaths and iron-rich basal rock bars, both of which appear to be controlled by coherent siltstone aquitards within the Hawkesbury Sandstone. Here we present some results from a baseline study of one upland swamp, comparing the swamp with samples, geology and measurements from an open borehole on an adjacent ridge. Swamps and hillslopes were sampled from surface water and piezometers; including field measurements, stable and radioisotopes and general chemistry. Discrete intervals within the nearby borehole were selected based on drill core and other logging results that inferred naturally fractured layers of higher hydraulic conductivity, separated by the siltstones and other coherent sandstones. Five selected zones were isolated, sampled and measured using ANSTO’s zone-of- interest groundwater sampler (ZOIGS). Isolation of 5 m intervals is achieved using inflatable straddle packers, with vibrating wire piezometers measuring pressures above, within and below the isolated zone. Effective isolation by the packers and the siltstone aquitards was confirmed by head differences and independent recovery curves of adjacent zones during purging and sampling. Recovery curves were also used to infer effective hydraulic conductivities for each isolated zone. Tritium, deuterium, oxygen-18 and other results show that the lower portion of the swamp, just above the outcropping siltstone layer, had a higher contribution of groundwater. Exposure of the groundwater to the atmosphere at this point causes dissolved iron to oxidise and precipitate, which hardens the basal rock bar of the swamp against nick-point erosion. The basal rock bars therefore maintain a low hydraulic gradient and perched water table within the swamps. The two significant siltstone layers in the borehole were replicated at similar elevations within the swamp and in two previous boreholes in the area, implying that they may be quite extensive. Examination of the elevations of basal and mid-swamp rockbars of over 20 swamps in the local area indicates that many coincide with the level of these two siltstone layers. The implication is that the aquitard properties of the siltstone may be responsible for both the formation and survival of the swamps. Longwall coal mining often leads to subsidence and fracturing of the strata, sometimes resulting in changes to the surface-groundwater and aquifer connectivities. Fracturing of the aquitards and/or rock bars could dewater upland swamps if mining is not managed carefully. BHP Billiton Illawarra Coal Pty Ltd supported this and other baseline studies to better understand the upland swamps before undermining proceeds.