Galiwinku Groundwater Resource and Evaluation DLH-492
Yin Foo, Des
E-Publications; E-Books; PublicationNT; Report ; 6/1984
Made available via the Publications (Legal Deposit) Act 2004 (NT).
Dept. of Transport and Works
Report ; 6/1984
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Technical Report WRD84006 Viewed at 14:07:13 on 29/07/2010 Page 59 of 60. (a) ANNUAL SAFE YIELD CALCULATED FROM GROUNDWATER THROUGHFLOW Only an order of magnitude estimate of throughflow can be made on available data. As described in Section 2.3.2, groundwater movement is postulated to be directed away from the central ridge, discharging through the boundary delineated in Figure 2.3. This boundary is essentially at the base of the ridge, and throughflow is evidenced in the perennial lagoons featured throughout the area.. Where confined, throughflow from the aquifer is restricted along strike of the formation. The resulting perimeter of outflow is approximately 18 kID. The lack of water level data does not permit an accurate calculation of gradient. However, end of dry season water levels measured in 1981, following an average rainfall year, generally represented a 2.5 m potentiometric head above mean sea leve1. This is conservative when it is considered that the effect of extracting 1200 kL/day from the borefield is neglected. The gradient has been calculated to be approximately 1: 1200 by considering the 2.5 m head to be a maximum of 3 km from any outflow point. The transmissivity from Section 2.2.1 was 600 m2/day. Using the flow relationship Q = T i w where T - transmissivity i - gradient w - outflow width or perimeter, the estimated throughflow from the system at that time was 9000 m3 /day. This equates to an annual safe yield in the order of 3000 ML. It should be noted that this figure would be expected to represent a very conservative estimate of safe yield since throughflow would be expected to be much greater immediately after the 'wet' season. (b) ANNUAL SAFE YIELD CALCULATED FROM ESTIMATED RECHARGE This method here is used to give only an order of magnitude indication of safe yield. The mean annual rainfall for Galiwinku calculated from 45 years of record is 1377 mm, most of which is received during the wet season months from December to Apri1. Recharge to the aquifer is over the area where it is unconfined. This is represented by an area of app:-oximately 27 km2 The infiltration capacity of the strata has not been evaluated. However, given the sandy nature of the soil and weathered rock profiles, it could normally be expected to be as high as 20%. Using this figure for recharge, mean annual recharge is in the order of 7400 ML.
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