The Citizen, 2006-03-16, Page 28Going down
Sinkholes are a geological phenomenon that are being
studied by the county planning department and the
conservation authority. Shown are examples in Tuckersmith
(top) and Hibbert. (Courtesy photos)
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the sinkhole, as the contaminants
would be caught.
The final category for best
management practices is retiring the
farmland.
"These are just good practices
whether you have sinkholes or not,"
said Reid.
Programs are in plac9 to help
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THE CITIZEN, THURSDAY, MARCH 16, 2006. PAGE A7.
Agriculture 2006
Or anizations study geological phenomenon
By Bonnie Gropp
Citizen editor -
Protecting our groundwater
sources is not just a primary focus
these days but on-going with the
potential for problems often
undetermined.
One of these has been the
identification of sinkholes. "It is
what's known as karst topography.
What's happened is the groundwater
has dissolved the bedrock,
essentially creating underwater
caves. The ground sinks in creating a
depression at the surface," said Scott
Tousaw, director of planning for
Huron County. "They don't exist
everywhere. If you have bedrock 60
metres below. till you're not likely to
have a sinkhole."
However, while the sinkhole may
not be located on your land it could
_still have an impact on your water
supply. "It will affect farmers within
the drainage basin of a sinkhole."
In 2003 the Ausable Bayfield
Conservation Authority received
funding from the Ministry of the
Environment to do an initial
investigation to determine where
sinkholes were located and what it
meant. "The significance of this
study was with regards to the control
of groundwater quality and what
could be done," said Alec Scott of
ABCA.
In total they documented fouf
clusters of 60 sinkholes ranging in
size from 100 feet across and 30 feet
deep to 20 or 30 feet across and 10
feet deep, They are primarily located
in the south-west corner of Hibbert
and the east side of Tuckersmith.
The total area of land that drains
into these, according to county
planner Susanna Reid, is 813
hectares.
The second phase of the study was
in 2004 with Ontario Geological
Survey funding. "This was to build
on the first study," said Scott. "They
were more concerned with the
characteristics of the rock around the
sinkhole, but we also realized the
important connection with - the
bedrock and groundwater."
In this phase bore holes were
drilled, which actually remove cores
from the rock without cutting off
pieces. "We pulled out solid chunks,
then looked at that material,
categorized it and described it."
One of the bore holeS was done up
around Brussels. "It doesn't have the
depressions, but there are areas
where the rock is close the surface."
Though not a geologist and quick
to point that out, Scott does attempt
to explain some of the findings.
"There are different layers of
bedrock laid down from different
time periods. This type of stuff
means a lot to geologists, They live
and breathe it. But what it basically
comes down to is that in the areas
where there are sinkholes the rock is
softer, more fractured and closer to
the surface."
• The significance of this, he says, is
that surface water seeps down to
these fractures and actually dissolves
some of the rock. "The fractures-get
bigger. And as they dissolve there is
a bigger channel. The dirt get
washed down in, then the dirt above
collapses."
Some of these depressions, he
said, actually have significant
drainage flowing into them. "Up to
300 hectares may drain by creeks
right into the sinkhole. Because the
channels are big the water goes
down and mixes with the
groundwater. This has been an
issue."
The challenge, according to
Tousaw is that whatever might be
contained in agricultural runoff has a
fairly direct connection to the
sinkhole.
This: is a problem in part because
of how quickly they drain. "They
can fill like a tub, but drain in 24
hours. It can actually develop a
vortex like a tub drain."
Other land will have percolation,
said Tousaw. The layers of tight clay
in the till can make it difficult for the
water to get through. "It can take
years, sometimes hundreds to get to
the deep or bedrock aquifer."
Places where the overburden, or
glacial till, which is the
unconsolidated material of sand,
gravel and clay silt that sits between
the surface and bedrock, is thickest
provides better protection over the
bedrock aquifer.
And with the concern for water
suddenly a factor, flags have been
raised. "There is a problem so we
next assume it could be a significant
problem," said Scott.
That rationale brought them to the
next part of the study conducted in
2005. Sodium bromide, a relatively
inexpensive, easily to detected, safe
substance was put down the
sinkholes. Monitoring of nearby
existing wells was done for the next
two months, with "really good co-
operation from the landowners," said
Scott.
What they were looking for was
how quickly the tracers got into the
sinkholes and wells. "Had it been
fast that would have been a huge
concern. However, it turned out they
never did."
Scott has several explanations for
this. The samplings were done of
nine wells within a two-kilometre
radius in the summer. "They could
have .gone between the samplings.
And had it been spring, with 'the
higher water, it might have travelled
different channels."
As a result, Scott felt that the test
didn't provide conclusive results but
did imply that the surface and
groundwater connection is much
more complex than thought.
"It could be good news. But
-having said that, we can't ignore the
fact that the way we treat the land
around here could someday be an
issue. We are still working towards
trying to look at what best
management practices might be
most suited for agriculture."
In that capacity, the planning
department serves as a facilitator for
those affected by sinkholes. Their
best management practices are
strictly voluntary, according to Reid.
"There is no legislation in place
that says someone must follow these
practices," said Tousaw. "It is more
about stewardship."
However, Tousaw said that under a
nutrient management plan he
suspects that sinkholes would be
taken into account.
The best management practices
have several components. The first is
drainage and filtering. • "This is to
address the drains that flow into the
sinkholes. Berms can be built to
channel the water elsewhere," said
Reid.
Filtration can be done through the
construction of wetlands that will
capture and filter the water through
plants before it it goes into the
surface water drain or that it goes
into the sinkhole after it's filtered.
Changing .the type of tillage
practice to no-till could also affect
the runoff as would buffers around