Alice Springs town basin, review 2003
Read, R. E.
E-Publications; E-Books; PublicationNT; Report ; no. 42/2003
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Report ; no. 42/2003
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areas of thinner aquifer near the basin margin. A rise in water level will cause a proportionately larger increase in stored volume in these saline areas. There are three possible mechanisms that could have caused the large increase: As noted above gravity drainage of a portion of aquifer may leave about 65% of the water behind in the unsaturated zone. If this is within the root zone, most of the water could be removed by evapotranspiration, with the salt remaining in place. If the aquifer was then saturated again with water of the same quality as before the salinity will be about 65% higher. In 1975 water levels had risen to within 4 or 5 m of the surface over most of the Town Basin (Figure 11 ). This is well within the root zone, so this mechanism is possible. The extreme rainfall caused unusual diffuse recharge, leaching salt normally immobile in the unsaturated zone to the water table. Irrigation could have leached salt down to the where it was dissolved by the rising water table. These mechanisms are not exclusive, and it is likely that all of them have operated to some extent. Beyond this it is hard to draw firm conclusions about the relation between salt storage and water level. Selected hydrographs in Appendix H show that while TDS levels generally declined after 1976 TDS in some bores (e.g. RN 4957) had begun to increase again in the 1990s. This predates the large rainfall event in 2000. The variety of TDS trends displayed by bores in Appendix H suggests that a number of mechanisms are at work. 1960 1970 1980 1990 2000 2010 Year 0 10000 20000 30000 40000 T of sa lt be lo w w ate r t ab le 4000000 6000000 8000000 10000000 12000000 14000000 To ta l w at er v ol um e i n sa tu ra ted zo ne m 3 800 1200 1600 2000 2400 2800 M ea n m g/ L Total salt in saturated zone Total stored water in saturated zone Mean m g/L Figure 32 Relation between salt storage and stored volume 55
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