Remediation of Contaminated Soil Environmental remediation deals with

Remediation of Contaminated Soil • Environmental remediation deals with the removal of pollution or contaminants from environmental media such as soil, groundwater, sediment, or surface water for the general protection of human health and the environment. • Remediation work is usually describes as: - ‘On-site’ activities describes work being carried out within the confines of a remediation project. - ‘Off-site’ activities are those that are carried out away from the site such as disposal of material to a landfill site or treatment centre. • Remediation treatment is usually describes as: - In-situ remediation techniques involved leaving the soil in its original and bringing the treatment processes to the soil. - Ex-situ remediation techniques involved removing the soil from the subsurface and treat it ‘on-site’ or ‘off-site’. • In-situ remediation methods cause fever disturbances to the site, less contaminant exposure to public and less expensive than ex-situ methods.

Excavation • Excavation of contaminated soil followed by transportation and disposal in landfills is the most common practice of remediating these soils. • The soil may require pretreatment to reduce concentration below the land disposal restrictions by the regulation. • Excavation is relatively simple, fast and cost-effective for small volumes under any soil and contaminant conditions.

Vadose Zone (Unsaturated Zone) • A subsurface zone of soil or rock containing fluid under pressure that is less than that of the atmosphere. Pore spaces in the vadose zone are partly filled with water and partly filled with air. The vadose zone is limited by the land surface above and by the water table below. Vadose zone divided into: i) Soil water or root zone ii) Intermediate vadose iii) Capillary zone

What does a typical soil remediation system look like?

Remediation techniques for contaminated soil: i) Physical ii) Chemical treatment iii) Biological treatment Physical and chemical treatment: i) Soil vacuum extraction/soil vapor extraction (SVE) ii) Soil washing iii) Soil flushing iv) Neutralization v) Oxidation vi) Precipitation vii) Reduction viii) Carbon adsorption ix) Ion exchange Biological treatment: i) iii) iv) Aerobic bioremediation Anaerobic bioremediation Biological seeding Composting

Bioremediation • Bioremediation refers to the use of microorganisms to remove undesirable compounds from soil, sludge, groundwater or surface water so that these sources will be returned to their "clean & natural" state. • It can be applied as in-situ treatment by using indigenous microorganisms to treat contaminated soil and ground water in place without moving the soil or ground water. • Bioremediation technology : natural attenuation, biostimulation, and bioaugmentation.

• Bioremediation which occurs without human intervention other than monitoring is often called natural attenuation. This natural attenuation relies on natural conditions and behavior of soil microorganisms that are indigenous to soil. • Biostimulation also utilizes indigenous microbial populations to remediate contaminated soils. Biostimulation consists of adding nutrients and other substances to soil to catalyze natural attenuation processes. • Bioaugmentation involves introduction of exogenic microorganisms (sourced from outside the soil environment) capable of detoxifying a particular contaminant, sometimes employing genetically altered microorganisms

• Bioremediation can be implemented in a number of treatment modes: - aerobic - anoxic - anaerobic - co-metabolic • Three primary ingredients for bioremediation are: - presence of a contaminant, - an electron acceptor, - presence of microorganisms that are capable of degrading the specific contaminant. Microbes + Electron Donor (Energy & Carbon Source) + Nutrients + Electron Acceptor → More microbes + Oxidized End Products Electron donor : waste contaminants as energy source Electron acceptor: O 2, NO 3, SO 4, CO 2, organic carbon

Aerobic and anaerobic bacteria can be identified by growing them in liquid culture: 1: Obligate aerobic (oxygen-needing) bacteria gather at the top of the test tube in order to absorb maximal amount of oxygen. 2: Obligate anaerobic bacteria gather at the bottom to avoid oxygen. 3: Facultative bacteria gather mostly at the top, since aerobic respiration is the most beneficial one; but as lack of oxygen does not hurt them, they can be found all along the test tube. 4: Microaerophiles gather at the upper part of the test tube but not at the top. They require oxygen but at a low concentration. 5: Aerotolerant bacteria are not affected at all by oxygen, and they are evenly spread along the test tube.

• In situ bioremediation causes minimal disturbance to the environment at the contamination site. In addition, it incurs less cost than conventional soil remediation or removal and replacement treatments because there is no transport of contaminated materials for off-site treatment. • in situ bioremediation has some limitations: • 1) it is not suitable for all soils, • 2) complete degradation is difficult to achieve, and 3) natural conditions (i. e. temperature) are hard to control for optimal biodegradation. Ex situ bioremediation, in which contaminated soil is excavated and treated elsewhere, is an alternative.

• Ex situ bioremediation approaches include use of bioreactors, landfarming, and biopiles. In the use of a bioreactor, contaminated soil is mixed with water and nutrients and the mixture is agitated by a mechanical bioreactor to stimulate action of microorganisms. This method is better-suited to clay soils than other methods and is generally a quick process • Microorganisms have limits of tolerance for particular environmental conditions, as well as optimal conditions for optimum performance. Factors that affect success and rate of microbial biodegradation are nutrient availability (N, P, trace metal), moisture content, p. H, oxygen level and temperature of the soil matrix. Inorganic nutrients including, but not limited to, nitrogen, and phosphorus are necessary for microbial activity and cell growth

Environmental factor affecting bioremediation • Microbial population - An acclimated indigenous population of microbes capable of degrading the compounds of interest must exist at the site. If these microbes does not exist, inhibitory or toxic compounds at the site should be suspected and alternatives remediation techniques should be considered. • Oxygen - O 2 is the preferred electron acceptor because it yield maximum energy to the microorganism, thus higher cell production and organism growth per unit electron donor utilized. - It need for aerobic biodegradation process: > 1 mg/l in aq phase; > 2% vol. in gas phase for vapor systems to ensure that O 2 is not limiting factor. - Clay content of soil may affect oxygen content in soil. Higher moisture content in clay restrict O 2 diffusion. - Loss of O 2 due to aerobic biodegradation induces a change in the activity of microbial population. Obligate anaerobic and facultative anaerobic microorganisms become the dominant population.

Comparison of Free Energy Values for Metabolism of Glucose in the Presence of Various Electron Acceptors Equation kcal/electron equivalent

• Soil moisture - is an important factor affecting the effectiveness of using bioremediation for contaminated soil because microbes rely on soil moisture for their growth and survival. - Soil water provide as media for transfer of contaminants from solid phase to microorganisms. - Soil water content ranges 25 – 85 % of field capacity (water content of soil after freely drains by gravity) is needed to sustain microbial activity. Example: Bioremediation of PAH at different soil moisture content PAH Antracene Fluoranthene Moisture content 60 -80 % 20 -40 % Half life 37 d 43 d 231 d 559 d

• p. H - p. H 7 is the optimal condition for biological treatment performance. Because of p. H in soil is difficult to modify, it can be used as indicator in assessment for using bioremediation technique. • Temperature - Biological system can be operated in a wide range of temperature 5 – 60 deg C - 3 temperature ranges were identified based on the growth of microbes: Psychrophilic (< 15 deg C), Mesophilic (15 – 45 deg C), Thermophilic (>45 deg C)

• Nutrients - Major nutrient: N, P - Minor nutrient: Na, K, Ca, Mg, Fe, Cl, S - Trace nutrient: Mn, Co, Ni, Va, Cu, Zn - Ratio of nutrient require is C: N: P = 100: 1 (the ratio in cell ~ 50: 1) with assumption that half of C is used for cell production and half for energy production by the cells. • Toxicants in waste - Any material can disrupt the biochemical process in microorganisms employed in the treatment system, will cause failure of the system. - The microorganisms presence within the treatment system can acclimate to some of the pollutants or by design like blending the contaminated soil with uncontaminated soil to reduce the toxicity level (in a soil pile or land farm system).

• Bioventing – in situ aeration of soil • Composting – addition of moisture and nutrients, regular mixing for aeration • Biopiles – ex situ aeration of soil • Land farming/treatment – application of organic materials to natural soil followed by irrigation and tilling

Bioventing • Bioventing is an in-situ remediation technology that uses indigenous microorganisms to biodegrade organic constituents adsorbed to soils in the unsaturated zone. • In bioventing, the activity of the indigenous bacteria is enhanced by inducing air (or oxygen) flow into the unsaturated zone (using extraction or injection wells) and, if necessary, by adding nutrients.

• • Air delivery from atmosphere to the soil above water table through injecting well. Air blower may be used to push air into the soil through injection wells. Air flow through the soil, and the oxygen present in the air is used by microorganism. When extraction wells are used for bioventing, the process is similar to soil vapor extraction (SVE). However, while SVE removes constituents primarily through volatilization, bioventing systems promote biodegradation of constituents and minimize volatilization (generally by using lower air flow rates than for SVE). In practice, some degree of volatilization and biodegradation occurs when either SVE or bioventing is used.

• Applicable for BTEX, PAH, some chlorinated aliphatic compounds (TCE) • High molecular weight and less volatile hydrocarbons like diesel, kerosene are better treatment by bioventing than SVE An initial screening of bioventing effectiveness, which will allow you to quickly gauge whether bioventing is likely to be effective, moderately effective, or ineffective. These factors are: (a) The permeability of the petroleum contaminated soils. This will determine the rate at which oxygen can be supplied to the hydrocarbon-degrading microorganisms found in the subsurface. (b) The biodegradability of the petroleum constituents. This will determine both the rate at which and the degree to which the constituents will be metabolized by microorganisms.

A screening tool that may use as an initial assessment of the potential effectiveness of bioventing.

Soil Texture How to use the diagram? Example: A soil containing 35 % clay, 30 % silt and 35 % sand – clay loam

Detailed Evaluation Of Bioventing Effectiveness

Site Characteristics Intrinsic Permeability • Intrinsic permeability is a measure of the ability of soils to transmit air and is the single most important factor in determining the effectiveness of bioventing because it determines how much oxygen can be delivered (via extraction or injection) to the subsurface bacteria. • To degrade large amounts of petroleum hydrocarbons, a substantial bacterial population is required which, in turn, requires oxygen for both the metabolic process and the growth of the bacterial mass itself. Approximately 3 to 3½ pounds of oxygen are needed to degrade one pound of petroleum product.

• Coarse-grained soils (e. g. , sands) have higher intrinsic permeability than fine-grained soils (e. g. , clays, silts). The ability of a soil to transmit air, which is of prime importance to bioventing, is reduced by the presence of soil water, which can block the soil pore and reduce air flow.

Soil Structure And Stratification • Soil structure and stratification are important to bioventing because they affect how and where soil vapors will flow within the soil matrix when extracted or injected. • Structural characteristics such as microfracturing can result in higher permeabilities than expected for certain soils (e. g. , clays). Increased flow will occur in the fractured but not in the unfractured media. • Stratification of soils with different permeabilities can dramatically increase the lateral flow of soil vapors in more permeable strata while reducing the soil vapor flow through less permeable strata. This preferential flow behavior can lead to ineffective or extended remedial times for less-permeable strata or to the possible spreading of contamination if injection wells are used.

Microbial Presence • Soil normally contains large numbers of diverse microorganisms including bacteria, algae, fungi and protozoa. In well aerated soils, which are most appropriate for bioventing, these organisms are generally aerobic. • Bacteria require a carbon source for cell growth and an energy source to sustain metabolic functions required for growth. • Microbes are classified by the carbon and TEA sources they use to carry out metabolic processes. Bacteria that use organic compounds (such as petroleum constituents and other naturally occurring organics) as their source of carbon are called heterotrophic; those that use inorganic carbon compounds such as carbon dioxide are called autotrophic.

• For bioventing applications directed at petroleum products, bacteria that are both aerobic (or facultative) and heterotrophic are most important in the degradation process.

Soil p. H • The optimum p. H for bacterial growth is approximately 7; the acceptable range for soil p. H in bioventing is between 6 and 8. Soils with p. H values outside this range prior to bioventing will require p. H adjustments during bioventing operations.

Moisture Content • Bacteria require moist soil conditions for proper growth. Excessive soil moisture, however, reduces the availability of oxygen, which is also necessary for bacterial metabolic processes, by restricting the flow of air through soil pores. • The ideal range for soil moisture is between 40 and 85 percent of the water-holding capacity of the soil. • The capillary fringe usually extends from one to several feet above the elevation of the groundwater table. Moisture content of soils within the capillary fringe may be too high for effective bioventing. • Depression of the water table by groundwater pumping may be necessary to biovent soils within the capillary fringe.

Soil Temperature • Bacterial growth rate is a function of temperature. Soil microbial activity has been shown to decrease significantly at temperatures below 10 C and essentially to cease at 5 C. • Microbial activity of most bacteria important to petroleum hydrocarbon biodegradation also diminishes at temperatures greater than 45 C. Nutrient Concentrations • Bacteria require inorganic nutrients such as ammonium and phosphate to support cell growth and sustain biodegradation processes. Nutrients may be available in sufficient quantities in the site soils but, more frequently, nutrients need to be added to soils to maintain bacterial populations.

Depth To Groundwater • Bioventing is not appropriate for sites with groundwater tables located less than 3 feet below the land surface. Special considerations must be taken for sites with a groundwater table located less than 10 feet below the land surface because groundwater upwelling can occur within bioventing wells under vacuum pressures, potentially reducing or eliminating vacuum-induced soil vapor flow. • This potential problem is not encountered if injection wells are used instead of extraction wells to induce air flow.

Constituent Characteristics Chemical Structure • The chemical structures of the constituents present in the soils proposed for treatment by bioventing are important for determining the rate at which biodegradation will occur. • Although nearly all constituents in petroleum products typically found at UST sites are biodegradable, the more complex the molecular structure of the constituent, the more difficult and less rapid is biological treatment. • Most low-molecular weight (nine carbon atoms or less) aliphatic and mono aromatic constituents are more easily biodegraded than higher-molecularweight aliphatic or polyaromatic organic constituents.

• Evaluation of the chemical structure of the constituents proposed for reduction by bioventing at the site will allow you to determine which constituents will be the most difficult to degrade.

Vapor Pressure • Vapor pressure is important in evaluating the extent to which constituents will be volatilized rather than biodegraded. • Constituents with vapor pressures higher than 0. 5 mm Hg will likely be volatilized by the induced air stream before they biodegrade. • Constituents with vapor pressures lower than 0. 5 mm Hg will not volatilize to a significant degree and can instead undergo in situ biodegradation by bacteria.

Product Composition And Boiling Point • Boiling point is another measure of constituent volatility. • Nearly all petroleum-derived organic compounds are capable of biological degradation, although constituents of higher molecular weights and higher boiling points require longer periods of time to be degraded. • Products with boiling points of less than about 250 C to 300 C will volatilize to some extent and can be removed by a combination of volatilization and biodegradation in a bioventing system.

Henry*s Law Constant • Another method of measuring the volatility of a constituent is by noting its Henry*s law constant. • Henry*s law constants for several common constituents found in petroleum products are shown in table. Constituents with Henry*s law constants of greater than 100 atmospheres are generally considered volatile and are more likely to be volatilized rather than biodegraded.

Components Of A Bioventing System 1) Extraction Wells i) Well Orientation • A bioventing system can use either vertical or horizontal extraction wells. Orientation of the wells should be based on site-specific needs and conditions.

ii) Well Placement and Number of Wells • The number and location of extraction wells can be determined by using several methods. a) In the first method, divide the area of the site requiring treatment by the area corresponding to the design ROI of a single well to obtain the total number of wells needed. Then space the wells evenly within the treatment area to provide areal coverage so that the areas of influence cover the entire area of contamination. Area of influence for single extraction well = Π (ROI)2 Number of wells needed = Treatment area (m 2) Area of influence for single extraction well (m 2/well)

ROI of Bioventing system • The ROI is the radial distance from an extraction well that has adequate air flow for effective removal of contaminants when a vacuum is applied to the extraction well.

b) In the second method, determine the total extraction flow rate needed to exchange the soil pore volume within the treatment area in a reasonable amount of time (3 to 7 days). Determine the number of wells required by dividing the total extraction flow rate needed by the flow rate achievable with a single well. Number of well needed = (µV/t)/q µ = soil porosity (m 3 vapor/m 3 soil) V = volume of soil in treatment area (m 3 soil) q = vapor extraction rate from single extraction well t = time for exchange pore volume (hr) In the example below, an 7 d exchange time is used, Number of well needed = m 3 vapor m 3 soil 168 h m 3 vapor h

• Consider the following additional factor in determining well spacing: - use closer spacing in areas of high contaminant concentration to increase oxygen flow and accelerate biodegradation rate - at sites with stratified soils, wells that are screened in strata with low intrinsic permeabiliies should be spaced more closely than wells that screened in strata with higher intrinsic permeabilities - if surface seal exists or is planned for the design, space the well slightly farther apart. A surface seal increase the ROI by forcing air to be drawn from a greater distance by preventing short-circuiting from land surface. However, passive vent wells or injecting wells may be required to supplement flow of air in the subsurface

iii) Well construction • a) Vertical extraction wells are usually constructed of PVC casing and screening. Extraction well diameters typically range from 2 to 12 in

• Vertical extraction wells are constructed by placing the casing and screen in the center of borehole. Filter pack material is placed in the annular space between casing/screen and the walls of the borehole. • The filter pack material extends 1 to 2 ft above the top of the well screen and is followed by a 1 to 2 ft bentonite seal. Cement-bentonite grout seals the remaining space up to the surface. • Filter pack material and screen slot size must be consistent with the grain size of the surrounding soils.

• The location and length of the well screen in vertical extraction or injection wells can vary and should be based on the depth to groundwater, the stratification of the soil, and the location of contaminants. • The ROI is affected by the intrinsic permeability of the soils in the screened interval (lower intrinsic permeability will result in a smaller ROI, other parameters being equal), the placement of the screen can affect the ROI. • At a site with homogeneous soil conditions, ensure that the well is screened throughout the contaminated zone. The well screen may be placed as deep as the seasonal low water table. A deep well helps to ensure remediation of the greatest amount of soil during seasonal low groundwater conditions. • At a site with stratified soils, the screened interval can be placed at a depth corresponding to a zone of lower permeability. This placement will help ensure that air passes through this zone rather than merely flow through adjacent zones of higher permeability.

b) Horizontal extraction well systems are generally used in shallow groundwater conditions. • Horizontal extraction wells are constructed by placing slotted PVC piping near the bottom of an excavated trench. Gravel bedding surrounds the piping. A bentonite seal or impermeable liner is added to prevents air leakage from the surface.

• When horizontal wells are used, the screen must be high enough above the groundwater table so that normal groundwater table fluctuations do not submerge the screen. • Additionally, if vacuum extraction is used, pressures should be monitored to ensure that induced groundwater upwelling does not closed up/occlude the screen(s).

2) Air Injection Wells • Air injection wells are similar in construction to extraction wells. Horizontal wells are also applicable for air injection. • Active injection wells force compressed air into soils. Passive injection wells, or inlets, simply provide a pathway that helps extraction wells draw air from the atmosphere into the subsurface. • Air injection wells can be used alone or, more commonly, in conjunction with extraction wells. The injection well/extraction well combination is often used at sites. • Air injection wells are seldom used by themselves primarily because the contaminated offgas can not be collected. Without the ability to collect the offgas, contaminated vapor may spread to previously uncontaminated areas.

3) Vapor Pretreatment • Extracted vapor can contain particulates that can damage blower parts and inhibit the effectiveness of downstream treatment systems. • In order to minimize the potential for damage, vapors are usually passed through a moisture separator and a particulate filter prior to entering the blower.