What lies beneath?

Thirty-one soil moisture sensors from Sentek Technologies are being installed as part of the Mulloon Rehydration Initiative’s monitoring program. They are located at Mulloon Creek Natural Farms (Home Farm & Duralla), at Mulloon Farm North and at Palerang and are being paired with approximately half of the 75 previously installed piezometers.

During their installation, the auger being used to drill the holes where the sensors sit have encountered amazing soil profiles. Sometimes as the auger drove down through the soil layers it slowed considerably due to rock, heavy clay, large gravel, water from an aquifer, and even a fine sandy slurry, which we hope to extract using a special tool to remove the slurry as the auger is unable to retrieve it. As a result, a few of the sensor housings are less than the desired 2m depth depending on the subterranean landscapes, with one reaching only 70cm.

Aquifers have also been hit in 10 of the installation sites with some being quite significant, making it difficult to install the bung (plug) at the bottom of the pipe and set it firmly enough to seal the hole as it is hard pushing water back against so much pressure. (See videos below.)

Data from the fully installed sensors will initially be sent to Sentek and then transferred to HydroTerra where it will be stored in the integrated data management system. This data will accessible by the public in due course, via the HydroTerra portal.

The Mulloon Rehydration Initiative is jointly funded through the Mulloon Institute and the Australian Government’s National Landcare Program. The initiative is also assisted by the NSW Government through its Environmental Trust.

Soil profiles

Many of the sites had a medium to heavy clay layer in the B horizon of the soil that can start as shallow as 60cm depth and can be right through to the 2m depth. The A and B horizons are the top two layers of the soil. The A horizon (topsoil) is where there is most soil life while the B horizon (subsoil) is where clays and materials have washed down from the A horizon.

Many of the sites had a medium to heavy clay layer in the B horizon of the soil that can start as shallow as 60cm depth and can be right through to the 2m depth. The A and B horizons are the top two layers of the soil. The A horizon (topsoil) is where there is most soil life while the B horizon (subsoil) is where clays and materials have washed down from the A horizon.

Further down, this soil is mottled in colour with the ochre colour indicating seasonal inundation of the soil at this depth, with iron causing the rusty colour. This section of soil was a heavy clay with some sand and small gravel. This tends to indicate there may be an aquifer just below that at times rises into this section of soil.

Further down, this soil is mottled in colour with the ochre colour indicating seasonal inundation of the soil at this depth, with iron causing the rusty colour. This section of soil was a heavy clay with some sand and small gravel. This tends to indicate there may be an aquifer just below that at times rises into this section of soil.

The next sample down the hole revealed this heavy clay that could act as an aquitard – a zone within the Earth that restricts the flow of groundwater from one aquifer to another. Aquitards can absorb water and swell up and reduce the amount of water able to pass through them, causing any water above them to move laterally underground. This is turn can produce aquifers that connect to streams or deeper groundwater reserves or feed into the floodplain through old buried stream channels. The light grey mottled soil in this sample has been affected by water inundation and is leached of much of its colour.

The next sample down the hole revealed this heavy clay that could act as an aquitard – a zone within the Earth that restricts the flow of groundwater from one aquifer to another. Aquitards can absorb water and swell up and reduce the amount of water able to pass through them, causing any water above them to move laterally underground. This is turn can produce aquifers that connect to streams or deeper groundwater reserves or feed into the floodplain through old buried stream channels. The light grey mottled soil in this sample has been affected by water inundation and is leached of much of its colour.

Continuing down the hole, this sandy clay with small gravel contains much water from a shallow but small aquifer. The size and shape of the sand and gravel indicates that this was once an old, shallow waterway that has long since been buried by successive floods, depositing soil and organic matter onto the floodplain.  

Continuing down the hole, this sandy clay with small gravel contains much water from a shallow but small aquifer. The size and shape of the sand and gravel indicates that this was once an old, shallow waterway that has long since been buried by successive floods, depositing soil and organic matter onto the floodplain.  

Further down, the auger encountered sand with larger, more angular gravel in a slurry with some clay. This indicates that the buried old waterway (approx. 1.1m at depth) was at one stage having quite a flow of water to deposit such material. Due to the nature of this buried flow line when water enters this section it is able to move more easily than if it were clay.

Further down, the auger encountered sand with larger, more angular gravel in a slurry with some clay. This indicates that the buried old waterway (approx. 1.1m at depth) was at one stage having quite a flow of water to deposit such material. Due to the nature of this buried flow line when water enters this section it is able to move more easily than if it were clay.

Aquifers

At other times the auger hit a pressurised aquifer which forced water back up the pipe and needed to be pumped out and promptly plugged to prevent further inundation.

Sometimes augers hit aquifers that can be under pressure and force water up into the access tube which then has to be bailed out and promptly plugged to prevent further inundation. Some installations had to be abandoned completely and resituated when rock was encountered too close to the surface.

Kelly Thorburn