|
Like many other underground fuel
storage tanks and associated piping, those at the NEX gasoline
service station at Ventura County Naval Base in Port Hueneme,
California caused contamination of soil and groundwater. The
contaminants include gasoline and its octane-boosting
additive, MTBE, which leaked into the soil and moved through
the subsurface until being intercepted by shallow and
relatively fast-moving groundwater. The groundwater carried
the gasoline in its direction of flow, creating a plume which
crossed into the path of a large eucalyptus tree.
The eucalyptus tree is 60 feet
tall and from 50-60 years old. One of over 600 species,
eucalyptus in general are adept at growing under adverse
conditions such as low-nutrient soil and drought. The ability
to withstand drought conditions is partly due to the tree’s
growth habit of developing a taproot which seeks out deeper
water, including groundwater. The tree growing at the subject
site did not have to grow a very deep taproot since
groundwater is available at nine or ten feet below the ground
surface. Once the taproot encounters groundwater it sends out
a web of lateral branches which act to extract water from the
capillary fringe zone of the water source – in this case the
groundwater table. Water is carried from the roots to the
leaves, where most is released to the atmosphere as vapor,
part of the process called transpiration. While the amount of
water a tree transpires varies greatly depending on the
species, age, and site conditions, studies have shown that
some adult trees transpire thousands of gallons per day.
The use of plants in certain
environmental applications is called phytoremediation.
Phytoremediation is actually a diverse field, encompassing a
variety of applications such as the use of plants in erosion
control (phytostabilization), extraction of heavy metals from
soil (phytoextraction), and the growth of floating water
plants to remove dissolved contaminants (rhizofiltration).
Where contaminated groundwater is relatively shallow (less
than 20-30 feet) and suitable land area is available, groves
of trees have been planted to remediate groundwater. In this
application the trees uptake groundwater and, often, the
dissolved contaminants; depending on the contaminant, the tree
species, and environmental conditions, the contaminants are
transformed by metabolic pathways of the tree (phytotransformation),
adsorbed to tree tissue, or are released to the atmosphere (phytovolatilization)
- or a combination of these.
But it is not the tree doing all
the work. Microbes living in the soil or on the fine roots of
the tree are responsible for much of the removal or
transformation of the contaminants. Bacteria and fungi in the
soil, many associated with the tree roots, use the contaminant
as a nutrient source. The tree supplies oxygen to the
subsurface via leakage through the root hairs (small
structures on fine roots responsible for most nutrient
uptake), as well as nutrients, co-factors, and other
substances, stimulating microbial growth in the vicinity of
the roots. The area immediately surrounding the roots becomes
aerobic, which allows the growth of aerobic microorganisms.
Beyond this small aerobic zone, in the more oxygen-limited
region, facultatively aerobic (those that require a minimal
concentration of oxygen) organisms grow, and beyond this the
anaerobic organisms grow. The various aerobes, facultative
aerobes, and anaerobes share metabolites, creating a complex
and synergistic consortium which serves to break down a wide
variety of contaminants. This plant-assisted breakdown of
contaminants, typically organics, by the microbiota in the
vicinity of roots - or the rhizosphere - is called enhanced
rhizosphere biodegradation.
MTBE is a contaminant of
particular concern because of its suspected health-effects and
its persistence and mobility in the subsurface environment. It
is highly soluble in water compared to the other components of
gasoline (and it comprises a significant volume of gasoline,
from 11-15%), and has a relatively low affinity to bind to
naturally occurring compounds of soil that normally bind
organic contaminants which limit their spread. Compounding
this, MTBE does not biodegrade readily under natural
conditions. Because of its persistence, mobility, and
suspected health-effects, MTBE has become a contaminant of
concern at a large number of sites.
Groundwater monitoring wells
throughout the base are regularly sampled so that the movement
of the MTBE/fuel plume can be tracked. Groundwater samples
collected on the downgradient side of the subject eucalyptus
tree had unexpectedly low MTBE and fuel contaminant
concentrations. As a result, the tree was more closely studied
and samples were collected around the tree on a finer scale.
The studies included measurement of tree tissue samples for
MTBE and metabolites, transpiration sample collection and
analysis, sap flow measurements, and additional sampling and
analysis of soil and groundwater around the tree. In addition,
laboratory studies were carried out on young trees to
determine whether the trees uptake and volatilize MTBE.
The laboratory studies used
seedling trees in isolation chambers fed a solution of
radiolabelled MTBE. Radioactivity of various media, including
soils and vapors from different parts of the chamber, was
measured. The results indicated that the young eucalyptus
trees took MTBE from the soil, bound some radiolabel in their
tissues, and transpired about 17% of the radiolabel through
their leaves, though none was recovered as completely
mineralized CO2.
Field studies involving the
subject eucalyptus tree were carried out on two occasions.
Leaves, branches and trunk corings were collected and analyzed
for the presence of MTBE. No MTBE was detected in any of the
tree tissue. It is possible that the tree uptakes MTBE, but
either transforms it to other compounds or binds it in the
roots or other tissue not sampled. Leaf vapors were also
collected to determine whether the tree was volatilizing MTBE,
but no MTBE was detected here either.
To further understand the tree’s
effect on soil and groundwater contaminant concentrations, a
fine sampling grid consisting of 14 sampling points centered
on the tree was set up. Soil and groundwater samples were
collected at two depths in each of the 14 borings, at 9 and 16
feet bgs. Results for the 9-foot sampling interval only are
shown, though similar attenuation down-gradient of the tree is
seen at the 16-foot interval. Samples were analyzed for MTBE,
TPH (as gasoline) and BTEX. The results were consistent with
observations from the coarser base-wide sampling grid: the
eucalyptus tree attenuates down-gradient contaminant
concentrations. Soil concentrations are also similarly
attenuated.
The isoconcentration maps shown
in the isoconcentration maps demonstrate that downgradient
concentrations of MTBE are 10-100X lower than upgradient MTBE
concentrations. It appears that the MTBE is being drawn to the
root zone of the tree in the upgradient area, and is then
either biodegraded as it flows through the roots or is taken
up by the tree. The removal occurs relatively rapidly as the
movement of water through the coarse aquifer materials is on
the order of 1-foot/day. MTBE concentrations remain depressed
for some distance downgradient of the tree before mixing from
adjacent, untreated areas cause the concentration to rise
again. Since the tree is located close to one edge of the
contaminant plume, concentrations on the southern edge of the
study area are consistently lower, both upgradient and
downgradient of the tree.
The results suggest that TPH,
BTEX, and MTBE are being degraded in the root zone of the
eucalyptus tree via rhizosphere bioremediation, though it is
possible that the tree itself is also transforming some of the
contaminants in the root zone (or in other tissue where it is
below detectable levels). In any event, the tree is, at a
minimum, providing favorable conditions for the enhanced
biodegradation of contaminants flowing through its roots zone.
The application of trees to
remediate groundwater contamination has many advantages, most
of which are consistent with other types of biodegradation
technologies, but is not without limitations. Most notably,
contaminants must be located near the surface, where roots can
reach them. Suitable growing conditions must be available
(soil, climate), and land area must be available for a
sufficient number of trees to treat the plume. Lastly, the
time needed to grow the trees before they are fully effective
can be a problem, though the trees can be planted closely when
young and thinned later. A wide variety of trees, adapted to
nearly every climactic zone, are available which grow quickly,
are disease-resistant, easy to propagate (can be planted as
cuttings) and are phreatophytic (meaning they send roots to
groundwater), such as hybrid poplars, willows, eucalyptus and
other phreatophytes. Where site conditions and other project
factors favor phytoremediation, it can be a most
cost-effective and elegant solution.
The tree at the subject site just
happened to be located in the path of the contaminant plume
and was well established before being exposed to the plume.
The large size of the tree is a significant factor in the
ability of a single specimen to effect the attenuation seen
here; the root mass of this specimen is probably equal to tens
of younger trees such as those planted for purposes of
remediation. In any event, the tree, fortuitously located as
it is, is having a significant impact on downgradient
contaminant loadings and is protecting and enhancing the
environment in a variety of ways, some of which may not be
obvious to the casual observer.
Top |
