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.
