Groundwater studies
A. Variations in Groundwater Levels
Alluvial groundwater levels respond rapidly to storm stream flow, which mostly occurs in summer (November – March).
Alluvial groundwater levels are also strongly affected by prolonged dry periods, and the associated increase in irrigation extraction that usually accompanies them. Under these conditions (which now currently exist) water tables are commonly well below stream beds.
Plots of bore hydrographs (i.e. water levels) show that over the last 15-20 years water levels show an overall downwards trend.
Recharge, when it does occur is related to storm-based stream flow, and the hydrological sequence can be depicted in a series of cross-sections (A6).
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B. Example from Southwest Area (upper) Lockyer - mouth of Tenthill-Ma Ma Creeks
This area has a broad expanse of alluvium where Ma Ma and Tenthill Creeks join with Lockyer Creek (B1). There is a large number of bores here, including some transects drilled by NR&M (as DNR).
This area were studied in detail by Andrew Wilson (B6), who looked at variations in water level, and produced a groundwater model for the local area (MAppSc, 2005). See Groundwater Modelling Page for model features. Andrew’s study incorporated the area further up Tenthill Creek also (B7).
Hydrogeological cross sections (B2) show the shape of the sandstone bedrock below the alluvium (and filled with alluvium). Levels of groundwater are shown for some different years.
Figure B3 is a hydrograph of one bore here, for a 40 year period (1960-2002). This is very interesting as it shows (a) the response of the bore to recharge from heavy rainfall events, (b) a fairly consistent trend to around 1985, and (c) the trend of lower rainfall and water levels since 1985.
Bore 516 between Gatton and Grantham has an automated water level monitor (B4). The plot shows water levels 1998-2003, and the response to heavy rain in May, 1996. Plot B5 shows the detail of the May, 1996 flood event and the daily response of groundwater levels.
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C. Example from East Area (lower) Lockyer - Woolshed to Buaraba Creeks
This area is a wide zone of alluvium along Lockyer Creek, and is immediately downstream of the “declared” groundwater zone. This area also has a large number of bores, including DNR observation bores (C3). The Lockyer channel here is incised within the lower formations of the valley (i.e. Gatton Sandstone) (C1). This area was studied by Julie Picarel (BAppSc Hon) (C7).
Photo C2 shows the dry creek bed, and the exposed alluvial bank. By using the drillers geological logs (NR&M database) for the bores in this area it is possible to contour the depth to the sandstone bedrock. This gives the shape of the bedrock below the alluvium (C4). In this area the alluvial ground surface is around 60 m ASL; the alluvium is well over 25 m thick in places.
A hydrogeological cross section (C5) shows the alluvium infill [there is a 40 x vertical exaggeration in scale]. Following are some points to note: (a) the thickness of the alluvium is much greater than the Lockyer Creek channel, (b) the coarse sands/gravels at the base of the alluvium, (c) 5 bores all screened at the base (some go into sandstone also), (d) the hole to west is within sandstone.
In C5 it is clear the water levels for both 1995 and 2004 are both well below the creek bed (20 m) and the volume of groundwater available is in the lowest section of the alluvium. Conductivity for these bores is quite good (~1000 µS/cm), but many bores downstream are 2-5 x this. Note bores into sandstone are more saline.
Groundwater levels for the period May-June, 2004 were plotted (C6); these show not only that the watertable is drawn well down, but that the contour lines strongly curve upstream. That means groundwater extraction here is trying to pull water down from further upstream; further evidence of mining water.
In C6 it is also interesting to note that the deepest section of the groundwater body (shown by the main flowline) does not follow the channel of Lockyer Creek.
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Further information
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