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Components
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UNDERSTANDING SOIL SYSTEMS |
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Before deciding which methods
to use in the remediation of contaminated soils, it is imperative
to first analyze the current site soil conditions. Since in situ bioremediation
depends heavily on naturally occurring microbial activity in restoring
soil systems, it is important to understand the environmental parameters
that affect microbial activity. The following portions of this article
will therefore discuss these parameters and how they affect in situ
bioremediation applications.
Environmental parameters affecting microbial activity: |
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2.1 The Soil Matrix
2.1.1 Intrinsic Permeability
2.1.2 Soil Structure & Stratification
2.2 Contamination Distribution
2.3 Soil pH
2.4 Soil Temperature
2.5 Soil moisture content
2.6 Nutrient supply
2.1 The Soil Matrix
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Intrinsic permeability,
soil structure, and soil stratification all are factors that affect
the efficacy of in situ bioremediation. |
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2.1.1
Intrinsic Permeability
The intrinsic permeability of soils is a measurement of their ability
to transmit air. This is the single most important factor influencing
the efficacy of in situ bioremediation. Certain types of soils with
a particular structure and grain size distribution are more favorable
for bioremediation than others. Below is a table that shows the different
levels of permeability among soils and their suitability for convective
transport (i.e. oxygen transport). |
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![](soilstruct1%20copy.gif) |
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Fig.2 |
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2.1.2
Soil Structure & Stratification
Understanding soil structure and stratification is also important
when applying bioventing and biosparging methods, as these also influence
air flow movement throughout the soil matrix. Structural characteristics
like microfracturing can improve permeability even in fine-grained
soils (e.g. clays) where high permeability levels would otherwise
be unlikely. Increased air flow would only occur, however, in fractured,
rather than unfractured, media. Furthermore, soil structure affects
the amount of pressure required in biosparging methods. For example,
fine-grained soils require higher biosparging pressures due to restricted
air flow through smaller pores. In addition, stratification of heterogeneous
soil systems may alter air flow behavior. For example, layers containing
coarse soils will exhibit dramatically higher levels of soil vapor
lateral flow than fine-grained soils, where air movement is minimal.
This altered flow behavior can result in the unanticipated migration
of constituents. |
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2.2 Contaminant Distribution
Contaminant distribution determines the feasibility
of using bioremediation methods. Contaminants can reside in the
unsaturated zone, also referred as the vadose zone, of soil systems
in one or more of the following forms: |
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![](process.gif) |
1) As gas vapor within soil pores
2) As pure mobile or immobile non-aqueous phase liquids (NAPL)
3) Attached to soil particles
4) In an aqueous state, i.e. dissolved in soil moisture or ground
water. |
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Since bioremediation relies
on degradative processes to metabolize contaminants, these contaminants
must be (bio)available to the soil microorganisms. Generally, contaminants
that have lower aqueous solubilities, enter soils as a pure product,
and/or are likely to be absorbed to soil particles, are not bioavailable
to soil microorganisms. In order for biodegradation to occur, contaminants
must be available within the soil in aqueous or gaseous form. |
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2.3
Soil pH
Microorganisms generally survive at pH levels ranging from 5 to
9. For bioremediation applications, the optimum pH is usually slightly
above 7, where microorganisms perform degradative processes at a
relatively fast and efficient rate. However, the optimum pH for
microbial growth varies among species of microorganism. Acceptable
pH levels for bioventing and biosparging range from 6 to 8. Soils
chosen for remediation with pH levels outside this range may need
to be modified before and during bioventing and/or biosparging.
However, pH modification may also lead to later, sudden changes
in pH that hinder bacterial activity. |
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2.4 Soil
Temperature
The temperature of soil systems also affects the
performance of bioremediation. Occurring between 10oC and 45oC,
microbial activity typically doubles for every 10oC increase in
temperature. However, temperatures outside this range most often
bring microbial activity to a standstill. |
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2.5
Soil Moisture Content
Microorganisms, i.e. bacteria, require moist soil
conditions for proper cell growth. Too much soil moisture, however,
can hinder the distribution and availability of oxygen to bacteria.
Ideal soil moisture conditions are therefore between 40-85 percent
of the soil’s water-holding capability. |
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2.6 Nutrient Supply
Bacteria also require inorganic nutrients to sustain
cell growth and the biodegradation process. These nutrients generally
include nitrogen, phosphorous, potassium, sulfur, magnesium, calcium,
manganese, iron, zinc, copper, and trace elements. However, in situ
bioremediation normally only requires the introduction of nitrogen
and phosphorous, usually in the form of ammonium (nitrogen) and
phosphate (phosphorous). |
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