Hydrogeology
A. Features of the Aquifer Systems
The main groundwater resources of the Lockyer Valley are within the alluvial deposits of the drainage system. Within the valley, these deposits cover around 28,000 ha.
The alluvium directly overlies the sedimentary formations that form the bedrock. In the central part of the drainage system water bores show the alluvium to be between 20 and 30 m thick. The shape of the base of the alluvium depends on the form of the sedimentary rocks; in some areas it is level, in others irregular. In medium size streams the alluvial cross-section may be U-shaped, but in smaller, narrower valleys it is V-shaped.
The alluvial deposits are composed of well-graded gravel, sand and silt with a clayey matrix; cobbles also occur in channel deposits. Clay-rich layers are common in the broad alluvium of the central sections of Lockyer Creek. Typically, the lower (deeper) section of the alluvium is composed of coarser sands, and is the main water-bearing layer. These coarser sands have the highest porosity. Some of these gravels are from the surrounding ranges, and are older material. In some broader sections of the drainage system, these lower gravels can be semi-confined by lower porosity silt-clay layers. Fine-grained silty sediments are more common in the upper, shallower parts of the alluvium.
Sedimentary rock formations form the hills and ridges within the valley and the divides between the smaller valleys and sub-catchments. These formations continue at depth below the alluvium. The formations have different geological names; we use the most recent naming scheme, of,
- upper: Koukandowie Formation (exposed in sides of LV and upper slopes)
- middle: Winwill Conglomerate
- lower: Gatton Sandstone (exposed in central valley floor along Lockyer Creek)
- lowest: Helidon Sandstone (exposed in northwest of LV)
Note: Koukandowie Formation used to described as two formations, Ma Ma Creek Sandstone, and Heifer Creek Sandstone.
Groundwater does occur within the sedimentary formations, but in most cases this is within fractured zones, old gravels beds and bedding planes. In some places this groundwater can be seen as springs on valley sides. Usually the bedrock groundwater is more saline than the alluvial groundwater.
In some places very deep sedimentary basin water discharges (e.g. Helidon). This groundwater leaks up faults from the Great Artesian Basin has a high CO2 content. It is found in several places throughout the Lockyer Valley.
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B. Recharge to Lockyer Valley Aquifers
"Recharge" is the process by which groundwater in aquifers is replenished. Such recharge can be direct rainfall or indirect.
In the Lockyer catchment most of the recharge to the alluvial aquifers is indirect and is stream flow from the upper catchment and surrounding ranges. This water is largley from rainfall in those areas that,
- directly runs off into the streams, or
- infiltrates through the soils into the basalt aquifers on the ranges, then seeps out into the streams.
Flow in most of the tributary streams of the Lockyer Valley catchment are in response to rainfall (on the ranges and within the valley). As the waters flow down the streams they gradually soak into the alluvial material, until the stream flow ceases. It requires prolonged heavy rain on the ranges for the Lockyer streams to have continuous flow.
The sedimentary formations of the Lockyer Valley, especially the sandstones, provide minor leakage of groundwaters into the alluvium.
There is very little direct recharge to the alluvium via infiltration of rainfall through soil, due to the slow rate and fine grain silty layers at depths around 0.5-0.8 m.
Small weirs to increase alluvial recharge have been built on most tributaries. Although they have experienced some saltation, most of the weirs achieve an annual recharge of around 700 ML (DPI-WR, 1993). [This weir recharge is of course dependent on streams flowing.] There are now 16 artificial recharge weirs on streams throughout the valley.
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C. Drillholes and Pump Tests
Drillholes
Obviously to explore for groundwater and to test it drillholes are needed. The most common type of drilling for groundwater in alluvial aquifers is rotary, using water to return cuttings to the surface. In cases where the material is very sandy, and the sides of holes collapse a drilling mud (bentonite) is used to “thicken” the water and support the hole.
After the hole is drilled, it is essential that it is established following accepted requirements of construction.
When the hole is completed it needs to be "developed". This means cleaning out the fine material from the screen and the gravel pack around the screen, so water can freely flow in. Development is often done by using a compressor and blowing air under pressure into the hole through a pipe.
Pump Tests
Also called hydraulic tests, these are required to determine the field permeability around the bore. This information is needed to (a) assess the potential of an aquifer, and (b) use in computer groundwater flow models.
The basis of a pump test is to remove water from a bore, and measure (a) how much the water level drops over time, and (b) when the pumping stops, how the bore recovers (water flows back in) over time.
These tests can be done on one bore, or using two bores where one is pumped, and the other measured. Pump tests can go for different lengths of time, depending on such things as size of aquifer and type, diameter and depth of bore, and size of pump.
Commonly, pump tests in alluvium use a 50 mm diameter bore casing, with a downhole submersible pump. For our research testing we use a Grundfos with a pumping rate of 0.3 L/sec for 1 -2 hours; this is quite a bit lower that some tests for aquifer production. Data are then plotted on a time / drawdown curve and several methods can then be used to calculate the aquifer properties. These values are an average of the aquifer around the bore, or between the two bores.
The important property determined is hydraulic conductivity, K, in m/day. This is a "field permeability". A study by Wilson (2005) showed that for the Tenthill-Ma Ma Creeks floodplain K values were: for the productive coarse sands and gravels at the base of the alluvium 50 – 80 m/day; for finer grained alluvium much lower (e.g. 2 – 10 m/day), and the sandstone bedrock lower still (< 2 m/day).
In the table of hydraulic conductivities (below) are typical K values for different sediment types, but note that within aquifers these materials are mixed, and can also be compacted.
| material | hydr. conductivity [m/day] |
|---|---|
| gravel, coarse | 305 |
| gravel, medium | 286 |
| gravel, fine | 265 |
| sand, very coarse (fine gravel) | 204 |
| sand, very coarse to coarse | 143 |
| sand, coarse | 73 |
| sand, medium to coarse | 33 |
| sand, medium | 16.5 |
| sand, fine to medium | 8.2 |
| sand, fine | 4.1 |
| sand, very fine to fine | 1.7 |
| sand, very fine | 0.8 |
| clay | 0.4 |
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Further information
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A1: simple cross-section of Lockyer Valley [114.35 kB] - simplified valley cross-section, showing relation of groundwater to seasonal rainfall and stream |
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A2: 3-D cross-section of a SEQ valley [98.89 kB] - general hydrogeological model for a typical catchment in SE Queensland (DNR). This generally applies to the main Lockyer Valley, although varies in detail. (Note: for the Lockyer case, the basalts are on top of the ranges) |
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A3: photo: gravels in flowing river [104.05 kB] - poorly sorted gravels (i.e. many sizes, plus a lot of fine material) in bars and on banks. These types of alluvial deposits from ancient streams are buried at depth in the alluvium of the Lockyer Valley. These coarse materials are highly transmissive and form the main alluvial aquifers. This is the Dumaresq River near Texas (Qld). |
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B1: diagram: recharge model [70.53 kB] - sketch showing the main processes involved in recharge of groundwaters to the alluvial aquifers |
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C1: diagram: bore construction [113.67 kB] - good (right) and bad (left) design of boreholes. This is important so there is not contamination of the water bearing layer from the surface or other water bearing zones. [DNR Fact Sheet] |
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C2: developing the bore [106.13 kB] - when new bores are drilled, they need to be cleaned out to remove drilling mud or fine materil from the screen or gravel pack, to allow easy flow of water into the bore. [DNR Fact Sheet] |
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C3: photo: pump test Laidley [71.91 kB] - pump test of a QDNR observation bore at Laidley |
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C5: plot: pump test evaluation [66.88 kB] - result of a pump test (drawdown plot) - used to determine aquifer properties (K) and how well the bore will flow. These can also be important to establish the best pumping rate for a particular bore |
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C6: plot: pump test evaluation - different cases [91.15 kB] - here are some examples of different time-drawdown plots for different bores, that reflect different aquifer conditions |
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C7: results of pump tests [68.37 kB] - pump test assesment at Tenthill - Ma Ma (Wilson, 2005) - estimated K values |
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