Alice Springs town basin, review 2003
Read, R. E.
E-Publications; E-Books; PublicationNT; Report ; no. 42/2003
Made available via the Publications (Legal Deposit) Act 2004 (NT).
Department of Infrastructure, Planning and Environment
Report ; no. 42/2003
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5.7.2 Major recharge zones Water quality maps indicate that recharge from the Todd mainly occurs: Upstream from the Will Terrace causeway in the Northern Zone. Downstream of the Tuncks Road causeway. To a lesser extent downstream of the Casino causeway. The area downstream of the Tuncks Road causeway corresponds to where the Todd crosses an area of relatively thick permeable aquifer. 5.8 Discharge through Heavitree Gap 5.8.1 Hydraulic gradients Although an array of bores was drilled at Heavitree Gap in 1957 only one (RN 3064) has been monitored since. There is therefore no direct record of hydraulic gradients in the gap, and gradient had to be inferred from the head difference between RN 3064 and RN 3667 on the downstream side in the Inner Farm Basin (Appendix H). 5.8.2 Outflow Wilson (1958) estimated outflow to be 130 ML/year. There were some errors in his assumptions (see Macqueen (1976)) and this figure was revised to 100 ML/year. The hydrograph in Appendix H shows that the average head difference across Heavitree Gap since about 1973 is about 40% of that in the earlier monitored period, a result of the large decline in pumping in the Inner Farm area. There has been a substantial reduction in average outflow. Calculations in Appendix H show that since 1975 outflow has ranged from 40 to 90 ML/year, averaging 80 ML/year. The low permeability of the aquifer in Heavitree Gap relevant to that upstream is counter-intuitive. The following explanation is offered for it. The aquifers in the Town Basin were deposited in the Quaternary. Palaeoflood studies (Patton et al. 1993) have indicated that very large floods occurred in the Todd catchment at about 700 and 1 500 y B.P.. Such a large flood would have scoured out the unconsolidated material to bedrock, leaving a deep waterhole in the gap. This waterhole was slowly filled with silty sand by small flow events. Some later scouring by smaller floods partly removed some of the clay layers and deposited more permeable sand in the upper part of the section. Hydrographs and results are in Appendix H. Since 1975 outflow has ranged from about 40 to 90 ML/year with an average of 77 ML/year. Since a number of assumptions have had to be made this estimate is probably only accurate to + 20%. Previously one of the constraints on management of the Town Basin was the need to maintain outflow to the Inner Farm. The quality of the outflow is now so poor (see section below), that this is no longer an issue and it would benefit the Inner Farm basin if pumping in the Town Basin stopped outflow. 5.8.3 Water quality Unfortunately water sampling at the gap has been inconsistent, there are no bores with data over a long period, and no samples were taken between 1965 and 1989. Chloride 42
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