Networks of small tunnels in acid sulfate soils, created by old roots and soil ripening processes, have been found to play a significant role in the export of acid from these soils on the North Coast, according to Dr Peter Slavich, NSW DPI soils research leader.

"NSW DPI scientists observed that acidified groundwater can escape or seep into the drainage system up to 1000 times faster when these tunnels, called macropores, are present," Dr Slavich said.

"This massive variability in the rate at which water moves through acid sulfate soils is a major factor in how they are managed."

Due to having limited scientific data about the variable rates at which water moves through these coastal acid sulfate soils, the Northern Rivers Floodplain Network in association with NSW DPI, Southern Cross University and the Northern Rivers Catchment Management Authority initiated a survey project.

The project investigated saturated hydraulic conductivity (Ksat), or the lateral seepage of water through the soil profile, as a critical variable affecting the acid export from drained acid sulfate soils.

Results of the survey of coastal acid sulfate soil sites indicate that 48 per cent of test pits dug were in the high to extreme range for hydraulic conductivity.

Six of the seven floodplains surveyed had sites with lateral seepage of greater than 100 metres a day, which is in the extreme range.

Assessing hydraulic conductivity is important in order to design appropriate management strategies for broadacre remediation projects.

It has a controlling influence on the lateral movement of acidic groundwater into nearby drainage systems.

Co-ordinator of the Floodplain Network, Simon Walsh from NSW DPI at Wollongbar, says the network group is made up of mainly local government resource managers who work on the floodplains from the Manning River to the Tweed River.

The group reviews technical on ground works, floodplain management issues and current and future research needs.

"This hydraulic conductivity information is a valuable tool for councils in large scale remediation projects," Mr Walsh said.

The study was funded by the six North Coast participating member councils and NSW DPI.

The study area encompasses six catchments (Tweed, Richmond, Clarence, Hastings, Macleay, and Manning) and a number of high risk sites were selected within each catchment, in consultation with local council floodplain officers.

The study also provided an opportunity to train officers in the new simple semi-quantitative pit test and to show them how to analyse and interpret the data.

New acid sulfate field test

A simple new field test can now help with the management of acid sulphate soils.

The test, an outcome from a floodgate hydrology project at Shark Creek near Grafton, helps determine soil porosity, a key element in managing groundwater in coastal acid sulfate environments.

Soils at the property owned by Dave Maloney were found to be highly permeable.

This discovery led NSW Department of Primary Industries (DPI) scientists to devise a way of slowing the rate of groundwater seepage to his drainage system and constraining the acid in the backswamp soils.

Curbing movement of shallow groundwater also increased soil moisture and led to productivity improvements.

The field test relies on constructing a pit within certain parameters, and is reproducible from site to site.

Step one is to dig the pit and let it settle.

Then bail remaining groundwater before timing how quickly it refills.

The rate of refill is described as saturated hydraulic conductivity (Ksat).

The full technique is available on the web.

Contact Simon Walsh, Wollongbar, (02) 6626 1256, simon.walsh@dpi.nsw.gov.au or Phillip Hirst, Grafton, (02) 6640 1673, phillip.hirst@dpi.nsw.gov.au