Potential use of phreatophytes in passive management of groundwater seepage
By Greg Bickerton, M.Sc, P. Eng. Dale Van Stempvoort, Ph.D,
Marcos Alvarez, Ph.D, and Steven Rose, M.Sc., P. Eng.
Greg Bickerton installs the datalogger for automated measurement of sap flow in a willow at
Belle Park.
The overall intent of this
research was to investigate the
potential to use phreatophyte
tree species to control seepage
of contaminated groundwater from
an old landfill at Belle Park, located
along the waterfront in Kingston,
Ontario. Phreatophytes are terrestrial
plant species that thrive under shallow
water table conditions by extending
their roots to the phreatic (water-saturated)
zone and transpiring groundwater.
Conventional remediation technologies,
including the current pump
and treat approach being used at Belle
Park, are expensive.
Alternative, emerging remediation
approaches may be better choices, or
offer cost savings if used in combination
with conventional approaches.
Parallel investigations were conducted
at Belle Park by Malroz Engineering
Inc. and Environment Canada. This article
focuses on the study by
Environment Canada,
and also provides a
brief summary of
results by Malroz
Engineering.
Site specific information
and other phytoremediation
studies
were reviewed, with a
focus on information
relevant to 1) potential
for hydraulic control of
groundwater seepage
by phreatophytes (e.g.,
poplar and willow); 2)
potential for phreatophytes
to uptake the
two main contaminants
of concern at the study
site: ammonia and
iron. The high ammonia
concentration in the
groundwater at Belle
Park is typical of old
landfills. This review
indicated there is some
potential that a “passive”
technology, landbased
phytoremediation
using phreatophytes, could be used
effectively at Belle Park. In this approach
the seepage of ammonia and iron in
groundwater along the margins of the
site would be captured or reduced by
phreatophyte transpiration, a form of
solar pumping. Uptake of ammonia as a
nutrient by phreatophytes is anticipated.
To determine the potential of phreophytes
as a remediation alternative at
Belle Park, transpiration rates of two
mature black willows (Salix nigra)
located near the south shore of Belle
Park were investigated by: 1) monitoring
patterns in groundwater fluctuations
in the vicinity of the study trees,
and 2) direct sap-flow measurements
(measured in one willow). The City of
Kingston provided on-site meteorological
data. Groundwater was sampled
from selected wells on two occasions
to determine any effects of the trees on
the distribution of ammonia and other
contaminants.
The automated sap flow data collected
in May 2003 displayed a diurnal
pattern. Calculations indicated a range
in sap flow of approximately 1,200 L
to 3,000 L per day for the tree studied,
similar to some rates reported elsewhere.
Sap flow data were not available
after May 2003 due to theft of
equipment. Automated groundwater
levels collected near the willows also
indicated a diurnal pattern. These data
give a more direct indication of
groundwater uptake by willows during
transpiration. Clear diurnal signals
were observed over most of the active
growing season for 2003, but not during
the period when sap flow data were
collected (May 2003). We speculate
that in May 2003, the willows at the
study site were primarily transpiring
water derived from a wet soil profile,
related to a spring snowmelt event.
Analyses of groundwater samples
indicated anoxic, reducing conditions,
with high iron, ammonia, chloride and
methane. Some contaminant concentrations
tended to be highest in the
vicinity of trees, suggesting evapotranspiration
has left high residues in
groundwater. Distribution of ammonia
did not indicate a consistent ability of
willows to lower its concentration.
However, low ammonia concentrations
in the vicinity of one willow suggests
nitrogen uptake.
Numerical simulation of 2003 diurnal
fluctuations in groundwater suggests
transpiration rates of approximately
20,000 L/day for the single
mature willow studied in detail. These
rates are consistent with some studies of
mature willows, but appear to violate
the theoretical maximum evapotranspiration
rate based on thermodynamic
considerations and are not consistent
with other reports.
The simulation rates are an order in
magnitude larger than the sap flow rates
observed in May. Possible explanations
for this discrepancy include uncertainties
associated with sap flow measurements
(25%), uncertainties in parameters
used in numerical simulations (e.g.,
hydraulic conductivity of soil/wastes),
changes in transpiration rates associated
with leaf development (after May
2003), and a potential role of hydraulic
lift, whereby the willows might extract
water from a deep layer of the soil profile
and redistribute it to shallow, dry
soil layer (i.e. reduced amount of transpiration
per unit of groundwater
uptake by roots).
A simple numerical capture zone
analysis based on estimated rates of
transpiration (obtained from the sap
flow measurements) suggests that it may
be possible to arrange mature black willows
such that adequate hydraulic control
can be obtained during active growing
seasons, without interfering significantly
with current park uses. If this
hydraulic control by willows can be confirmed
for other locations at Belle Park,
this approach may be an economical,
“green” alternative to seasonally offset
or replace the conventional pumping
system currently in use.
In the parallel study by Malroz
Engineering Inc., two plots of young
bare root phreatophytes were simultaneously
planted to investigate their
ability to adapt to the landfill setting,
and to observe their early influence on
the shallow groundwater table. These
plantings (willows in particular) indicated
that these species can readily
establish themselves within the landfill
footprint.
It is anticipated that future research
will reattempt to obtain contemporaneous
monitoring of groundwater fluctuations
and sap flow to reduce uncertainty
in quantification of transpiration,
with ongoing monitoring of the
phreatophyte plantings.
Greg Bickerton, e-mail: greg.bickerton@ec.gc.ca and
Dale Van Stempvoort e-mail: dale.vanstempvoort@ec.gc.ca,
are with
the National Water Research Institute,
Burlington, Ontario
Marcos Alvarez is
with Environmental Biotechnology
Applications, Environment Canada,
Gatineau, Quebec e-mail: alvarezm@agr.gc.ca. Steven Rose is with Malroz Engineering, Kingston,
Ontario, e-mail: rose@malroz.com
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