Page 208 - Materials Chemistry, Second Edition

Vadose Zone Soil Remediation 191

most popular format. This approach treats soil stockpiled on the site with per-
forated pipes embedded in the piles as the conduit for air supply. To minimize
fugitive emission and potential secondary contamination from leachates, the
stockpiles are usually covered on the top and lined at the bottom.
In situ treatment enhances the natural microbial activity of undisturbed
soil in place to decompose organic COCs. A nutrient solution is often per-
colated or injected into the subsurface to support the activities of the bio-
degraders. Run-on and run-off controls and waste containment are often
required. In a slurry bioreactor, impacted soil is mixed with a nutrient solu-
tion under controlled operating conditions (i.e., optimal pH, temperature,
dissolved oxygen, and mixing).
Microorganisms require moisture, oxygen (or absence of oxygen for
anaerobic biodegradation), nutrients, and a suitable set of environmental
factors to grow. The environmental factors include pH, temperature, and
absence of toxic conditions. Table 5.2 summarizes the critical conditions for
bioremediation.

5.4.2 Moisture Requirement

As shown in Table 5.2, the optimal moisture content for soil bioremediation
is 25%–85% of the water-holding capacity. In most cases, soil moisture will
be below or in the lower end of this range; therefore, addition of moisture is
commonly needed.
The moisture present in the vadose zone is often quantified by a term called
the volumetric water content or degree of saturation. Volumetric water content var-
ies from zero to the value of porosity, while degree of saturation varies from
zero to one and refers to the percentage of pore space occupied by moisture.
For complete saturation, the volumetric water content is equal to porosity, and

TABLE 5.2
Critical Conditions for Bioremediation
Environmental Factor Optimum Conditions
Available soil water 25%–85% water-holding capacity
Oxygen Aerobic metabolism: >0.2 mg/L dissolved oxygen, air-filled pore
space to be >10% by volume
Anaerobic metabolism: oxygen concentration to be <1% by volume
Redox potential Aerobes and facultative anaerobes: >50 millivolts
Anaerobes: <50 millivolts
Nutrients Sufficient N, P, and other nutrients (suggested C:N:P molar ratio of
120:10:1)
pH 5.5 to 8.5 (for most bacteria)
Temperature 15°C–45°C (for mesophiles)
Source: [6].