Researchers at The Commonwealth Scientific and Industrial Research Organisation (CSIRO), Australia's national science agency, have discovered that microorganisms that help break down contaminants under the soil actually can get too hot for their own good.

While investigating ways of cleaning up ground water contamination, scientists examined how microbes break down contaminants under the soil's surface. They found that subsurface temperatures associated with microbial degradation can become too hot for the microbes to grow and consume the ground water contaminants, which can slow down the clean up of the ground water and even continue the spread of contamination.

The new findings mean that researchers now have to rethink the way ground water remediation systems are designed.

CSIRO Water for a Healthy Country Flagship scientist Colin Johnston says the researchers were investigating how temperatures below the soil's surface could be used as an indicator of the microbial degradation process associated with biosparging.

Biosparging is a technique that injects air into polluted ground water to enhance the degradation of contaminants.

The contaminants are food to the microbes, and the oxygen in the air helps the microbes unlock the energy in the food so that they metabolize and grow, consuming more contaminants and stopping the spread of the contamination.

"Observations of diesel fuel contamination showed that, at [approximately 11 feet] below the ground surface, temperatures reached as high as [116 degrees F]," Johnston says. "This is close to the [125 degrees F] maximum temperature tolerated by the community of microorganisms that naturally live in the soil at this depth, and within the range where the growth of the community was suppressed."

The growth of the soil's microorganism community also can be helped by adding nutrients. However, computer modeling confirmed that any attempts to further increase degradation of the contamination through the addition of nutrients had the potential to raise temperatures above the maximum for growth.

"Although increasing the flow of air would reduce temperatures and overcome these limitations, a fine balance needs to be struck as the injected air can generate hazardous vapors that overwhelm the microorganisms, leading to unwanted atmospheric emissions at the ground surface," Johnston says. "This would be particularly so for highly volatile compounds such as gasoline. It appears that prudent manipulation of operating conditions and appropriate timing of nutrient addition may help limit temperature increases."

Johnston notes that further research is required to better understand the thermal properties in the subsurface, as well as the seasonal effects of rainfall infiltration and surface soil heating.