Groundwater systems

The Pedirka Basin is one of a series of stacked sedimentary basins extending from the Neo-Proterozoic Amadeus Basin through to the Quaternary surficial Lake Eyre Basin. Aquifers occur in all major basin sequences, though due to the significant burial depth there is a paucity of data associated with the Amadeus and Warburton basins where they underlie the Pedirka Basin. With respect to water quality, availability and existing extraction the Great Artesian Basin (GAB) is the most significant and extensive water resource in the region. At a local scale, aquifers within the shallow Quaternary and Tertiary deposits, and the Crown Point Formation within the Pedirka Basin, also provide an important supply of stock and domestic groundwater. Pedirka Basin

The behaviour of groundwater within the Crown Point and Purni formations is poorly characterised. An estimated 28 bores have been constructed in the Crown Point Formation and only two bores in the Purni Formation; all groundwater infrastructure is located on the western edge of the basin in the NT. The availability of data is too limited to determine if these formations operate as a single homogenous flow system or as separate, discrete, aquifers.

As previously mentioned, with respect to groundwater extraction and therefore information concerning groundwater characteristics from the Pedirka Basin, groundwater extraction occurs exclusively from the Crown Point Formation. Well yields range from 0.2 to 2.5 L/second, with an average of 1.3 L/second. It is worth noting that the majority of wells are used for stock water supply and the estimated well yield generally reflects the water requirement rather than the true yield potential of the aquifer. Petroleum well completion reports document a hydraulic conductivity range of 0.08 to 1.66 m/day and a porosity range from core analysis of 10 to 32%. No estimates of aquifer parameters from groundwater-focused investigations are available.

Information on the groundwater characteristics of the Purni Formation is extremely limited. There is currently no extraction from this formation and no published assessment of groundwater depths, bore yield or aquifer parameters. Petroleum and gas well completion reports estimate hydraulic conductivity for the Purni Formation at between 0.11 and 2.44 m/day with a porosity range from core analysis of 16 to 25%. Cenozoic aquifers

Important, local scale water resources occur in shallow, disconnected Cenozoic aquifers. These include the Hamilton sub-basin, Tertiary paleovalley systems and Quaternary alluvial aquifers associated with the present day surface water drainage (Figure 24). Of particular note are Cenozoic alluvials associated with the Finke River, given the relationship between the Finke River and recharge to the GAB. These aquifers are generally porous media, of limited thickness (<100 m) and not regionally extensive. Very limited information is available on aquifer parameters or bore yields for the Cenozoic aquifers. Groundwater extraction is limited and is predominantly used for stock and domestic supply.

Figure 24

Figure 24 Interpreted watertable contours for the Pedirka subregion

Note 1: Although the watertable is ubiquitous across the landscape and is commonly represented by groundwater in Cenozoic formations, this surface does not necessarily imply continuous groundwater movement between formations, nor is it completely restricted to Cenozoic formations.

Note 2: The watertable measurements displayed have not been corrected for density because density corrections were determined to have only a negligible effect on head. Great Artesian Basin

Where it overlies the Pedirka Basin the GAB consists of an upper sequence of Cretaceous marine clays, silts and shales known collectively as the Rolling Downs Group and a lower sequence of Cretaceous to Jurassic terrestrial sediments that form a significant aquifer unit (the J aquifer). Within the Rolling Downs Group the Mackunda Formation and lower portions of the Winton Formation form an upper aquifer unit (the K aquifer) (Habermehl, 1980). The Bulldog Shale, Oodnadatta Formation and lateral equivalents separate the upper and J aquifers and form the main confining beds for the GAB sequence.

The J aquifer is an unconfined to confined artesian aquifer, comprising the Algebuckina Sandstone, Cadna-owie Formation and their lateral equivalents. It forms the only regionally extensive water resource with a high beneficial use (Figure 25). The J aquifer is a porous media sandstone aquifer which shows dual porosity behaviour at a local scale (Fulton, 2013). Typical bore yields range from

1 to 5 L/second, though yields of up to 125 L/second have been reported in free flowing bores (Humphreys and Kunde, 2004). Regionally, the J aquifer has reported porosities of between 10 and 29% (average of 23%) with an observed decrease in porosity with greater burial depth of the aquifer (Radke, 2000). Within the Pedirka Basin, transmissivity for the J aquifer is in the order of 2500 m2/day, hydraulic conductivity 11 m/day and storage coefficients of 1.2 x 10-3 (Fulton, 2013). Water level depths range from over 150 mBGL on the western margin of the GAB to more than 60 m above ground surface (artesian) in the south-east of the Pedirka Basin.The K aquifer is an unconfined to confined sub-artesian aquifer, consisting of disconnected sandstone aquifers within the Rolling Downs Group. The K aquifer is discontinuous across the Pedirka Basin and water quality is much more variable than the J aquifer, ranging from <1000 to >100,000 mg/L (Keppel et al., 2013). In comparison to the J aquifer, extraction from the K aquifer is very limited. The aquifer is generally used where accessibility of water is a greater driver than water quality (e.g. road construction). Tight shales of the Oodnadatta Formation and Bulldog Shale form an aquitard with extremely low vertical hydraulic conductivities (1 x 10-13 to 4 x 10-14 m/s, Love et al., 2013a) that limits cross-formational flow between the J and K aquifers. However, potential for cross-formational flow exists where bore construction is poor or bore casing is corroded. Several such cases are documented in the NT portion of the GAB (Fulton, 2013).

In SA, all groundwater resources within the GAB and Far North Prescribed Wells Area (FNPWA) are subject to license and allocation regulations governed by the FNPWA Water Allocation Plan. In the NT, all water resources (groundwater in all aquifers and surface water) that fall within the geographic extent of the NT GAB Water Control District are managed under the GAB NT Water Allocation Plan.

Figure 25

Figure 25 Density and temperature corrected potentiometric surface of the J aquifer clipped to the extent of the Pedirka Basin showing some of the basin margin and key discharge features Amadeus Basin and Warburton Basin

The western Pedirka Basin overlaps the eastern extent of the Neo-Proterozoic to Cambrian aged Amadeus Basin. The hydrogeology in the centre and west of the basin, where it is used for town water supply (Alice Springs) and oil and gas production (Palm Valley), is better characterised than the eastern region, where data are limited to regional studies (Lau and Jacobson, 1991) and drilling records from the NT groundwater database. Approximately 26 bores have been constructed in the Langra Formation where the Amadeus Basin outcrops immediately to the west of the Pedirka Basin in the NT. The Langra Formation comprises layers of sandstone, siltstone and conglomerate; extraction from the formation is exclusively for stock and domestic supply. Average bore yields range from 2.2 to 12.6 L/second, with an average water level depth of 45 m.

Existing knowledge of the Warburton Basin is derived from the petroleum and gas industry, as the basin occurs at significant burial depth beneath the Lake Eyre, Eromanga and Pedirka Basin sequences. There are no groundwater wells screened in the Warburton Basin and as such no reliable information on the hydraulic behaviour of formations within the basin.

Last updated:
5 January 2018
Thumbnail of the Pedirka subregion

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