Monitored natural remediation (MNR) is the "natural" remediation of a contaminated site by
indigenous microorganisms and possibly abiotic processes. It is similar to passive or intrinsic
bioremediation but includes an agreed-upon monitoring plan to confirm that remediation
processes are occurring. For some of the contaminants present in groundwater, notably BTEX
and TCE, many of the degradative processes are well characterized and can be measured, and
groundwater systems are relatively well mixed compared to surface soils. For example, if
anaerobic respiration is using BTEX as its carbon source, NO and Fe may show a speciation
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difference concomitant with lower BTEX concentrations. A gradient of speciation of electron
acceptors, from oxidized to reduced forms, that coincides with lower BTEX concentrations may
be an indication of anaerobic respiration using BTEX as the carbon source and alternative
electron acceptors. An area of active research is transferring the philosophy of MNR to surface
soils, where the spatial distribution of the contaminant is typically heterogeneous, conditions are
not constant, and natural mixing does not occur.
4.4.2 Phytoremediation
Phytoremediation is an umbrella term describing the use of plants to remove, contain, or
transform contaminants.
Phytoextraction: Some species of plants take up significant amounts of nutrients. This
capability can be exploited to remove excess nutrients from soils. Some plants accumulate
compounds, such as metals, to a degree greater than the concentration in the soil solution. This is
termed hyperaccumulation, and is another example of phytoextraction. Plants with accumulated
metals can be harvested and disposed of or, in some cases, recovery of the metals may be
feasible.
Phytodegradation: Some plants are also capable of taking up and degrading relatively water-
soluble organic contaminants, such as TCE. In some plants, such as some hybrid poplars, TCE
can be degraded by enzyme systems in the plant.
tissue, perhaps in concert with transpiration.
Hydraulic Control: Some plants, such as poplar trees, can transpire sufficient water to influence
flow of shallow groundwater. This can be beneficial by limiting groundwater transport of
contaminants, and can be coupled with phytoextraction and phytodegradation.
Rhizodegradation or rhizosphere-enhance bioremediation: Carbon exudations and secretions
from roots stimulate microorganisms in the rhizosphere (zone of soil next to the roots). The
enhanced microbial activity in the rhizosphere in turn can enhance degradation of contaminants.
Phytostabilization: Plants, in concert with microorganisms, also influence the turnover and net
accumulation of organic matter into the soil, an overall process referred to as mineralization-
immobilization turnover (MIT). Some contaminants or their transformation products can be
chemically bound or incorporated into soil organic matter, a process known as humification, or
physically trapped in the soil humic or mineral fractions, a process known as sequestration.
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