Groundwater and Salinity


What are salts?

Salts are natural mineral components of soil that occur in various quantities throughout the landscape.

Soil and water are classed as saline when these salts become present in quantities sufficient to adversely affect the growth and productivity of plants and animals.

What causes salinity?

Millions of hectares of land across Australia are affected by naturally occurring salinity. However, many more thousands of hectares of land are also affected by man induced salinity.

The salts present in soils can be easily mobilised and transported by the movement of groundwater, capillary rise and evaporation, and leaching and biological activity. Ultimately, this may lead to the accumulation or depletion of salts in different parts of the landscape.

Since the advent of European settlement, general land clearing and the introduction of wide-scale agricultural practices (in particular irrigation) has in many cases exacerbated these naturally occurring processes leading to the more rapid development of salt affected soils and water. Such radical changes in land use have resulted in considerable changes to the overall water balance.

With less vegetation available to intercept and utilise rainfall, the more water there is available to enter the groundwater system dissolving soluble salts and raising groundwater levels (recharge).

Once the watertable reaches a critical depth below the ground surface (about 2m), evaporation of this water can occur via capilllary rise, transporting soluble salts with it to the soil surface. Over time, this leads to an accumulation of salts in the root zone of crops and pastures and subsequent losses in production.

Why is monitoring groundwater important?

Groundwater has the ability to mobilise and transport soluble salts when it moves through the regolith. It is very important to know where the watertable is, and understand its movements, in particular with respect to rainfall events and certain agricultural practices (ie irrigation).

In order to manage land to avoid salinisation the answers must be known to the following questions:
  • At what depth is the groundwater?
  • Is the groundwater rising over time?
  • What effect does irrigation or other management practices have on watertable movement?
  • How saline is the groundwater?
A watertable close to the ground surface may help contribute to accumulations of salt in the topsoil, along with other problems such as waterlogging during wet seasons.

By understanding and monitoring groundwater movement and quality, agricultural land may be managed to help suppress and avoid such problems. Groundwater can be monitored easily and cheaply through the installation of shallow observation bores.

Shallow observation bores

Installing an observation bore is the cheapest means of monitoring groundwater movement and associated salinity problems. Groundwater monitoring can effectively buy you time in the prevention of salinity, that otherwise might be a very expensive problem to cure at short notice.

How to monitor groundwater

Install an observation bore on your property. Measure watertable levels and groundwater quality on a regular basis. Weekly monitoring will provide you with more accurate data, however monthly monitoring is still acceptable.

Groundwater levels can be monitored using a measuring tape with a metal "plopper" attached to the end of it. When jiggled up and down, you will hear it hit the water surface. Alternatively, a fox whistle (obtained from gun shops) can be attached to the tape, which will produce a whistle as it reaches the groundwater.

Once you are satisfied the measuring apparatus has reached the groundwater, record the result. Don't forget to subtract the height of the pipe (above groundsurface) from the measurement in order to get the true level of the groundwater below ground surface.

To determine the quality of the groundwater, collect a sample from within the bore. This can be achieved by attaching a small up-turned cup to the end of a string or another measuring tape. Collect two samples of the water and throw it out. Then collect one more sample and keep it for testing. This helps mix the water and prevents the collection of stagnant surface water. The conductivity, or salt concentration of this water can then be measured using a small hand-held conductivity meter.

Note: It is important to keep the bore covered when not in use.

For information on installing shallow observation bores please contact:

Contact

Land Resource Assessment
Rob Moreton
171 Westbury Road
PROSPECT TAS 7250
Phone: 03 6777 2240
Fax: 03 6336 5365
Email: Rob.Moreton@dpipwe.tas.gov.au

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