Salinity monitoring of runoff at a Hunter Valley coal mining site is showing very clean results.

Land rehabilitated after open cut coal mining is currently returned to either agricultural pasture or woodland and mining in the Hunter occurs within salinity prone landscapes.

At the Hunter Key site, one of eight key monitoring sites established around the State, NSW Department of Primary Industries began monitoring salinity and runoff at two locations in 2004.

The locations are a rehabilitated mine spoil at Rio Tinto Australia’s Coal and Allied west pit and, simultaneously, a native vegetation site chosen for its relatively pristine, non-farming condition, owned by Macquarie Generation power company.

Approximately 18 years ago Rio Tinto planted the rehabilitated site with a mixture of grasses and legumes, including rye grass, Rhodes grass, couch, Marra sub clover and lucerne, plus interconnecting tree belts and blocks of eucalyptus and acacias.

"We have found that on average, six per cent of rainfall runs off the rehabilitated mine site," NSW Department of Primary Industries (DPI) research hydrologist, Tony Bernardi, said.

"The quality of the water that runs off is very good, averaging 70 micro Siemens per centimetre, (µS/cm), while initial spikes can reach 230 µS/cm.

"The rehabilitated site compares with an average electrical conductivity of 5000 µS/cm in the stream in the undisturbed native vegetation site.

According to Rio Tinto Coal Australia’s environmental specialist at the site, Andrew Speechly, the ability to have runoff coefficients from a rehabilitation site will enable more accurate and effective design, to build on site water storages.

The DPI Hunter Key site results go a step beyond what was known from previous rehabilitation of Queensland coal mining sites.

The Queensland experience showed in the first few years of site rehabilitation, as much as 60pc of the rain ran off, carrying water of 2900 µS/cm, but then dropped off to 300 µS/cm.

Measuring salinity

The measurement of saltiness in water is expressed in terms of its capacity to conduct electricity, defined as "electrical conductivity" and expressed in this context in micro Siemens per centimetre.

The saltier the water, the more electrical current it can conduct, hence the higher the reading.

The World Health Organisation’s highest preferred limit of water for human consumption is 800 micro Siemens per centimetre (µS/cm).

Water starts to taste salty at 1700 µS/cm.

Seawater is approximately 54,500 µS/cm.

Contact Tony Bernardi, Queanbeyan, (02) 6298 0805, tony.bernardi@dpi.nsw.gov.au