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Area underlain by shallow watertables, and areas where watertables are rising. This indicator is a direct measure of the condition. The depth of the watertable is important because if it is too shallow there is a risk from salinisation. A shallow watertable is typically defined as shallow when the watertable reaches between one to four metres from the ground surface. When a watertable is within two meters of the surface there is a potential for water to move to the surface by capillary rise carrying salt with it. The water evaporates and leaves salt behind. Trends in watertable depth over time e.g. rising or falling watertables indicates the potential for increased salinisation. This is a very direct measure of the sustainability of a groundwater system, and generally needs a long-term record (e.g. 5 to 10 years) in order to account for the effects in year-to-year variations in rainfall. Rates of rises of centimetres per year can be a concern where the watertable is shallow. However, rising watertables in Tasmania do not necessarily equate to rising salinity. It is a good indicator of potential risk but is not a direct measure as it is in other states of Australia. In Tasmania some salinity may be caused by the flushing of salt laterally during the short periods of temporary waterlogging with no permanent water influencing the process.
In Tasmania there has been limited groundwater monitoring systems suitable for assessing groundwater levels and trends. The only Statewide groundwater level data available is from 2,903 production groundwater bores compiled by Mineral Resources Tasmania. The table below displays the groundwater level data mainly collected at the time of drilling and reflects the average of samples collected over 80 years (1922-99). This data should be treated with caution as the bores do not represent a structured sampling of Tasmania's hydrogeology. Ground water depth to water struck (DWS) - selected records Statewide (1922-99) DWS (m) Number % 0-2 49 1.7 2-5 255 8.8 5-10 839 28.9 Over 10 1,760 60.6 Total 2,903 100.00 Source: Bastick and Walker (2000). The only groundwater trend data (i.e. whether groundwater is rising, falling or flat) available is from a series of 54 piezometers (a bore specifically designed to measure the ground water surface) and bores monitored by the DPIWE in the salt affected areas in Cressy Longford area and Coal River Valley irrigation scheme. These areas were monitored for five years or more. The data showed that the standing water levels in the 54 piezometers and/or bores that 35% (19) were falling, 45% (24) were flat, and 20% (11) were rising. Based on salinity risk classification for standing water levels, Bastick and Walker (2000) calculated that over half (54%) the bores were at high risk of salinisation, just under half (44%) were at moderate risk and only 2% were at low risk of salinisation. Again this data should be interpreted with caution as these bores were deliberately located as demonstrations for farmers next to existing saline scalds and so do not represent what is regionally happening in these areas. Core Land Indicator L 4 (ANZECC 2000). |
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