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Geotechnical News • December 2013
www.geotechnicalnews.com
GROUNDWATER
Contamination of Till Aquitard by DNAPL:
Is it Actual, or Drilling Artefact?
Robert P. Chapuis
During my career as a consultant, and
then as a professor, I have had the
privilege to be an expert in several
cases. This is the second old case that
I present in Geotechnical News. It
explains a situation that many profes-
sionals may have suspected but not
documented. Over the past few years,
I have asked for authorization to
publish scientific issues on old (over
20 years) but still interesting cases.
Having no authorization to publish is
unfortunate for professional knowl-
edge. The owners and their current
legal counsels gave authorizations,
but they came with the request that all
names and legal issues be kept con-
fidential. In addition, no photograph
can be published, which would enable
identification of a site or person. The
actual year cannot be given.
Context
This case is about investigation of a
pollution problem. DNAPL spills had
sink below the water table where they
usually form pools and fingers that
dissolve slowly. The upper sand–and–
gravel aquifer, and also the fractured
rock aquifer below the glacial till
were contaminated. The several meter
thick, non–plastic, till was an aquitard
between the two aquifers. During the
first stages of investigation, many
boreholes were drilled and conven-
tional monitoring wells (MWs) were
installed in the two aquifers (conven-
tional = a single MW in a single bore-
hole), with 1.5 to 4.5 m long screens.
A few 6 or 8 in. boreholes lodged two
MWs, one in each aquifer. The annular
space between the MW solid pipes and
the till wall was sealed with benton-
ite pellets and grout. The field and
laboratory data were used to delineate
two contamination plumes, one for
each aquifer. The two plumes travelled
more or less horizontally through the
two aquifers.
The plume in the fractured rock had
high levels of contamination, which
had started the field investigation and
legal process. Also, it travelled, as
expected, much faster than the plume
in the sand–and–gravel aquifer. As
usually, the DNAPL concentrations
groundwater fluctuated in time and
space. Such fluctuations are produced
by irregular DNAPL emission at the
surface (spills, dumps) and internal
sources (pools and fingers), fluctua-
tions of infiltration and groundwater
table position, and most important,
aquifer heterogeneity (e.g., Johnson
and Pankow 1992).
During the investigation, a question
was raised about how high con-
centrations of DNAPL had rapidly
reached the lower confined aquifer.
The consulting company, owner, and
regulating body, believed that the
DNAPL had percolated through the till
aquitard, close to the DNAPL source
zones. Another phase of investigation
was designed to assess and quantify
the movement of DNAPL through the
till aquitard.
Till investigation
The non–plastic till contained boul-
ders: it was not possible to drive a
penetrometer system or a flush–joint
casing, which is the less disruptive
of non–destructive drilling methods
(Morin et al. 1988). At the time of
this case history, the sonic drilling
method was unknown. The till was
investigated using diamond drilling
and a flush–joint casing, with upwards
clean water jets as washing fluid for
the cored sediments. Split spoons
were used to recover closely spaced
very dense till samples. These are
known to be class–4 soil samples in
any geotechnical classification and not
intact samples as sometimes found in
geological papers. No field representa-
tive water samples could be recovered
in the process. The sampling recovery
was most often in the 35–75% range.
The soil samples were analyzed for
DNAPLs. It was important to know
whether the glacial till was micro–
fractured and how much, because a
DNAPL can form a free–product pool
on the uneven top on aquitard, and
then can easily move through down-
ward through near–vertical micro
fractures. This was the initial explana-
tion of high pollution in the fractured
rock aquifer.
The end result of the till investigation
is summarized as follows. Among all
the till samples, a few were highly
polluted, a few were moderately
polluted, but most were not polluted.
No relationship was found between
concentration and depth, till thick-
ness, distance to the known source of
DNAPL. Several explanations were
proposed to try to explain the find-
ings. None of them was convincing,
because all of them assumed unverifi-