Land Disposal of waste, Dubbo, NSW, Australia

The city of Dubbo, NSW, has set up a waste water re-use woodlot, with the water applications being monitored with various soil moisture sensors, including a neutron probe. Initially, the automatic irrigation system was set to apply water for 1 hour/day.

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fig 3 Soil moisture profiles after application of waste water

The two readings on 16/1 were taken before and after the 1 hour irrigation - note that there is almost no change in the profile indicating an ineffective application of effluent. The irrigation time was increased to 3 hours/day and the water profiles started to increase, with infiltration down to 40 cm by 19/1 and by the 24/1 an increase in water content at 120 cm. There is a possibility that efflluent water may be draining through the profile.

The same data can be plotted as a time graph, with the through-drainage showing as an increase in water content at 100cm on 22/1.

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fig 5 changes in soil moisture with time at different depths

Quantitative measurement of soil moisture, combined with appropriate software, allows for routine monitoring of the woodlot. The irrigation schedule can now be adjusted to optimise tree production, while at the same time ensuring that effluent does not contaminate the aquifer.

Solid waste disposal using landfill can also result in through-drainage of contaminants. California environmental protection regulations are now calling for environmental monitoring to guarantee that through-drainage is not occuring. This results in a very different design problem, as the assumption that isolated monitoring is indicative of overall behaviour is no longer appropriate. Some landfill sites have had impervious liners installed, with several horizontal access tubes below the liners for neutron probe monitoring of possible leaks.