ENGI 9621 Soil Remediation Engineering

Spring 2015 Faculty of Engineering & Applied Science

Lecture 7: Soil Flushing

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Also called as “soil washing” if the contaminants are treated on-situ An innovative treatment technology that floods soils with a solution to move the contaminants out A developing technology that has had limited use Accomplished by passing the flushing solution through in-place soils using an injection or infiltration process Extraction fluids must be recovered from the underlying aquifer and, when possible, they are recycled

7.1 Introduction (1) Definition of soil flushing

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The flushing solution typically one of two types of fluids: 1) water only; or 2) water plus additives such as acids (low pH), bases (high pH) or surfactants (like detergents) If injecting a solvent mixture into either vadose zone, saturated zone, or both to extract organic contaminants cosolvent flushing The cosolvent mixture normally injected upgradient of the contaminated area, and the solvent with dissolved contaminants is extracted downgradient and treated above ground

(2) Cosolvent flushing

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(3) Operation processes Drilling of injection /extraction wells into the contaminated site number, location, and depth of the wells depend on geological factors and engineering considerations Transportation or built up the site equipments (such as a wastewater treatment system) Pumping the flushing solution into the injection wells the solution passes through the soil, picking up contaminants along its way as it moves toward the extraction wells the extraction wells collect the elutriate (the flushing solution mixed with the contaminants) The elutriate is pumped out of the ground then treated by a wastewater treatment system to remove the contaminants

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Source: EPA, 1996 Typical in-situ soil flushing in vadose zone

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Recovered flushing fluids and groundwater with the desorbed contaminants need treatment to meet appropriate standards before reuse in the flushing process or discharge Separation of surfactants from recovered flushing fluid for reuse a major factor in the cost of soil flushing Treatment of the recovered fluids results in process sludges and residual solids, such as spent carbon and spent ion exchange resin must be appropriately treated before disposal Air emissions of volatile contaminants from recovered flushing fluids should be collected and treated to meet applicable regulatory standards

(4) Recovered fluid treatment

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The target contaminant group for soil flushing inorganics including radioactive contaminants It can be used to treat VOCs, SVOCs, fuels, and pesticides, but it may be less cost-effective than alternative The addition of environmentally compatible surfactants may be used to increase the effective solubility of some organic compounds The technology offers the potential for recovery of metals and can mobilize a wide range of organic and inorganic contaminants from coarse-grained soils

7.2 Applicability

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Contaminants Considered for Treatment by In Situ Soil Flushing

Source: EPA, 1996 8

Since in situ soil flushing is tailored to treat specific Contaminants it is not highly effective with soils contaminated with a mixture of hazardous substances, for example, metals and oils It would be difficult to prepare a flushing solution that would effectively remove several different types of contaminants at the same time

7.3 Limitations

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Low permeability or heterogeneous soils difficult to treat Surfactants can adhere to soil and reduce effective soil porosity Reactions of flushing fluids with soil can reduce contaminant mobility Permits are required for wastewater and air treatment systems Aboveground separation and treatment costs for recovered fluids can drive the economics of the process

More limitations…

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Approximate costs: $50 to $200 per ton Key cost drivers (1) Soil Permeability soils with lower permeability are more recalcitrant to soil flushing thus remediation time can be significantly increased which increases costs (2) Depth to Groundwater soils with a deeper water table causing a higher cost to complete

7.4 Economic consideration

Scenarios A B C D Site sizes Small Large Site conditions Easy Difficult Easy Difficult Cost per cubic yard $32 $49 $18 $27

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Spring 2015 Faculty of Engineering & Applied Science

Lecture 8: Soil Fracturing

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ENGI 9621 Soil Remediation Engineering

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Fracturing creating fractures in dense soils and making existing fractures larger to enhance the mass transfer of contaminants The fractures increase the effective permeability and change paths of fluid flow, thus making in situ remediation more effective and economical Fracturing also reduces the number of extraction wells required, trimming labor and material costs Two types of fracturing Pneumatic fracturing + Hydraulic fracturing

8.1 Introduction (1) Fracturing

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injects highly pressurized air or other gas into consolidated, contaminated sediments to extend existing fractures and to create a secondary network of fissures and channels accelerates the removal of contaminants by soil vapor extraction, bioventing, and enhanced in situ biodegradation

(2) Pneumatic fracturing

(3) Hydraulic fracturing involves injecting a fluid, usually water, at modest rates and high pressures into the soil matrix to be fractured a slurry mixture of sand and biodegradable gel is then pumped at high pressure to create a distinct fracture as the gel degrades, it leaves a highly permeable sand-lined fracture with the sand acting as a propping agent preventing the fracture from collapsing 14 14

Source: Sharma and Reddy, 2004

Two types of soil fracturing

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Fracturing is most appropriately applied to soils where the natural permeability is insufficient to allow adequate movement of fluids to achieve the remediation objectives in the desired time frame.

8.2 Applicability

• silty clay/clayey silt • sandy silt/silty sand • clayey sand • sandstone • siltstone • limestone • shale

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Fracturing techniques are equally applicable to both vadose zone (unsaturated) soils and saturated zone soils to improve the flow of air and water, respectively fracture formation in the range of from 20 to 35 ft or more is possible for near-surface soils

Fracturing, by itself, is not a remediation technique has to be combined with other technologies to facilitate the reduction of contaminant mass and concentration e.g.

• in situ biodegradation (by enhancing the delivery of oxygen and nutrients into inaccessible locations) • in situ air sparging (by creating fractured pathways to collect the injected air laden with contaminants)

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The selection between hydraulic (water-based) and pneumatic (air-based) fracturing are based on the following considerations:

8.3 Description of the process

• soil structure and stress fields • the need to deliver solid compounds into the fractures • target depth • desired areal influence • contractor availability • acceptability of fluid injection by regulatory agencies

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Hydraulic Soil Fracturing Pneumatic Soil Fracturing Effective in soils and rock Primarily effective in rock

Long term permeability enhancement Short term permeability enhancement in unconsolidated sediments

Specialized equipment and fluid chemistry expertise required

Less equipment and expertise required

Low leak-off prevents spreading of subsurface contaminants

Injected air can potentially spread soil vapour phase contaminants

Fracture clogging by fines is minimized because frac sand is designed to act as a geotechnical filter while maintaining enhanced permeability

Fractures are unsupported; migration of fines quickly clogs fractures

Greater range of adaptability with remediation technologies (e.g. SVE, Bioremediation)

Not readily adaptable to many remediation technologies

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injecting a fluid into a borehole at a constant rate until the pressure exceeds a critical value and a fracture is nucleated

The most widely used fracture fluid for environmental application the continuous mix grade of guar gum

The injection pressure required to create hydraulic fractures is remarkably modest (less than 100 psi)

(1) Hydraulic fracturing

Injection pressure as a function of time during hydraulic fracturing

Source: Suthersan, 1997 20 20

Method for creating hydraulic fractures in soil Source: Suthersan, 1997

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Hydraulic fractures are generally created beneath a casing into which a lance is advanced and withdrawn to the required depth with a hammer. Lateral pressure of the soil seals the casing during the controlled injection of the fracturing fluid and the proppants. The casing can be driven deeper to create another fracture.

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Hydraulic Fractures Source: Slack , 1998 22 22

advancing a borehole to the desired depth of exploration and withdrawing the auger positioning the injector at the desired fracture elevation sealing off a discrete 1 or 2 ft interval by inflating the flexible packers on the injector with nitrogen gas applying pressurized air for approximately 30 s repositioning the injector to the next elevation and repeating the procedure a typical fracture cycle approximately 15 min a production rate with one rig 15 to 20 fractures per day

(2) Pneumatic fracturing

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Schematic of pneumatic fracturing process Injection rates of up to 1000 scfm sufficient to create satisfactory fracture networks in low permeability formations

Source: Suthersan, 1997

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8.4 Limitations The technology should not be used in areas of high seismic activity Fractures will close in non-clayey soils Investigation of possible underground utilities, structures, or trapped free product is required The potential exists to open new pathways for the unwanted spread of contaminants (e.g., dense nonaqueous phase liquids)

Pneumatic fracturing $8 to $12 per ton Hydaulic fracturing 160 to $180 per ton for remediation in a 1-year treatment and $100 to $120 per ton in a 3-year remediation

8.5 Cost

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Spring 2015 Faculty of Engineering & Applied Science

Lecture 9: Phytoremediation

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ENGI 9621 Soil Remediation Engineering

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Use of plants remediate contaminated soil or groundwater Most of the activity in phytoremediation takes place in the rhizosphere – in other words, the root zone Can be used for the remediation of inorganic contaminants as well as organic contaminants Most suited for sites with moderately hydrophobic contaminants e.g.benzene, toluene, ethylbenzene, xylenes, chlorinated solvents, PAHs, excess nutrients such as nitrate, ammonium, and phosphate, and heavy metals

9.1 Introduction

(1) Definition of Phytoremediation

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low capital costs the operational cost of phytoremediation is substantially less and involves mainly fertilization and watering for maintaining plant growth aesthetic benefits minimization of leaching of contaminants and soil stabilization

(2) Advantages

(3) Limitations contaminants present below rooting depth will not be extracted plant may not be able to grow in the soil at every contaminated site due to toxicity remediation process can take years for contaminant concentrations to reach regulatory levels requires a long-term commitment to maintain the system

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Plants remove organic contaminants utilizing two major mechanisms (1) direct uptake of contaminants and subsequent accumulation of nonphytotoxic metabolites into the plant tissue + (2) release of exudates and enzymes that stimulate microbial activity and the resulting enhancement of microbial transformations in the rhizosphere (the root zone)

9.2 Phytoremediation mechanisms of organic contaminants

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Not all organic compounds are equally accessible to plant roots in the soil environment The inherent ability of the roots to take up organic compounds can be described by the hydrophobicity (or lipophilicity) of the target compounds Hydrophobicity = log KOW (KOW octanol–water partitioning coefficient) The higher a compound’s log KOW the greater the root uptake If compounds are quite water soluble (log KOW <0.5) they are not sufficiently sorbed to the roots or actively transported through plant membranes

(1) Direct uptake (Phytotransformation) -- Prerequisite

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Wood is composed of thousands of hollow tubes, like the bed of a hollow fiber chromatography column, with transpirational water serving as the moving phase Lignification Once an organic chemical is taken up, a plant can store (sequestration) the chemical in new plant structures Metabolism Detoxificate a parent chemical to nonphytotoxic metabolites, including lignin, that are stored in plant cells different plants exhibit different metabolic capacities Mineralization Mineralize the chemical to carbon dioxide, water, and chlorides

(1) Direct uptake (Phytotransformation) -- Mechanism

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(2) Degradation in rhizosphere (Rhizosphere bioremediation) Roots of plants exude a wide spectrum of compounds including sugars, amino acids, carbohydrates, and essential vitamins may act as growth and energy-yielding substrates for the microbial consortia in the root zone Exudates may also include compounds such as acetates, esters, benzene derivatives, and enzymes In situ microbial populations in rhizosphere enhanced degradation by provision of appropriate beneficial primary substrates for cometabolic transformations of the target contaminants Typical microbial population in rhizosphere 5 ×106 bacteria, 9×105 actinomycetes, and 2×103 fungi per gram of air-dried soil

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Oxygen, CO2, water, and contaminate cycling through a tree

Source: Suthersan, 1997 33 33

Most heavy metals have multiple chemical and physical forms in soil all forms are not equally hazardous, nor are all forms equally amenable to uptake by plants

Phytoremediation of heavy metal contaminated soils can be divided into phytostabilization, phytoextraction, phytosorption and phytofiltration approaches

9.3 Phytoremediation mechanisms of heavy metals

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It involves the reduction in the mobility of heavy metals by minimizing soil erodibility, decreasing the potential for wind-blown dust, and reduction in contaminant solubility by the addition of soil amendments Eroded material is often transported over long distances extending the effects of contamination and increasing the risk to the environment Planting of vegetation at contaminated sites significantly reduce the erodibility of the soils both by water and wind effectively hold the soil and provide a stable cover against erosion

(1) Phytostabilization

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The use of unusual plants that have the ability to accumulate very high (2 to 5%) concentrations of metals from contaminated soils in their biomass metals are translocated to the shoot and tissue via the roots Hyperaccumulator plants they exhibit the ability to tolerate high concentrations of toxic metals in above-ground plant tissues After harvesting a biomass processing step or disposal method that meets regulatory requirements should be implemented

(2) Phytoextraction

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Source: Suthersan, 1997

Phytoextraction of heavy metals 37 37

Hybrid poplar tree for phytoextraction Source: Chappell, 1997 38 38

Aquatic plants and algae are known to accumulate metals and other toxic elements from solution Plant roots acting to sorb, concentrate, or precipitate metals e.g. one blue-green filamentous algae of the genus Phormidium and one aquatic rooted plant, water milfoil (Myriophyllum spicatum) exhibited high specific adsorption for Cd, Zn, Ph, Ni, and Cu

(3) Phytosorption and phytofiltration

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More information: http://www.abydoz.com/tech.html

9.4 Filed application: a case study Abydoz technology Abydoz systems uses plants capable of purifying a wide variety of domestic, municipal and industrial wastewater's. The treatment area is a stable, engineered ecosystem and is based on complex inter relationships between plants, soils and microorganisms.

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ENGI 9621 – Soil Remediation Engineering

Lecture 10: Surface Capping

Spring 2015 Faculty of Engineering & Applied Science

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Surface capping also called covers or surface barriers constructed over the buried wastes preferred remedial action for landfills and widespread soil contaminants prevent infiltration of precipitation, thereby minimizing the generation of leachate prevent transfer of contaminants to the atmosphere, reduce erosion, and improve aesthetics may range from an one-layer to multilayer system of soils and geosynthetics

10.1 Introduction

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Source: Federal Remediation and Technologies Roundtable, 2003

A multilayer surface capping system

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Purpose support vegetation + protect underlying layers

Typically 60-cm thick

Topsoil the most commonly used material

Crushed stone or cobbles may substitute in arid environments

10.2 Configuration of a multilayer cap

(1) Surface soil layer

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Also called “biotic barrier”

90-cm layer of cobbles to stop burrowing animals and deep roots

Not always included in most cases, the surface soil layer and protection layer are combined to form a single layer

(2) Protection layer

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Prevents clogging of drainage layer by fines from soil layer

May be geosynthetic filter fabric or 30-cm sand

(3) Filter layer

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Reduce the head of water on the underlying barrier layer minimized percolation of water through the cap Drain the surface water infiltrated from overlying layers increase water storage capacity and help to minimize erosion of these layers Reduce pore water pressure in the cap materials At least 30 cm of sand with K = 10-2 cm/sec or equivalent geosynthetic materials

(4) Drainage layer

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“Composite liner” both geomembrane and low-K soil (clay) Restrict upward movement of any gases of volatile constituents + prevent infiltration of water into waste: hydraulic barrier Geomembrane at least 0.5 mm thick varying thicknesses (20 to 140 mils), widths (15 to 100 ft), and lengths (180 to 840 ft) Compacted clay at least 60 cm with K ≤ 10-7 cm/s

(5) Barrier layer

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Needed if waste will generate methane (explosive) or toxic gas Similar to drainage layer 30 cm of sand or equivalent geosynthetic materials Connected to horizontal venting pipes (minimal number to maintain cap integrity)

(6) Gas vent layer

Not all of these layers are needed at all sites 49

With good construction QA/QC FML has one hole per acre (one hole per 0.4 hectare)

Compacted clay layer provides hydraulic and diffusional barrier at holes

10.3 Characterization of the barrier layer

The barrier layer = Geomembrane (or FML: flexible membrane liner) + Compacted clay layer

FML Clay

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Through the hole water from the drainage layer will enter the barrier layer if the compacted clay layer is saturated a failed barrier layer of the surface capping system

FML

Drainage layer

Clay

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If the landfill liner layer = compacted clay layer no geomembrane (or FML: flexible membrane liner)

Where i hydraulic gradient h depth of drainage water D depth of clay layer K hydraulic conductivity

If h = 30cm and D = 90cm 52

If the landfill liner layer = geomembrane (or FML: flexible membrane liner) no compacted clay layer

Orifice equation

Where CB orifice coefficient ≈ 0.6 a hole area g acceleration due to gravity = 9.8 m/S2

h depth of drainage water

If h = 30cm 53

If the landfill liner layer = geomembrane (or FML: flexible membrane liner) + compacted clay layer

Empirical formula by Giroud et al

Where C = 1.15 for poor seal between FML and clay = 0.21 for good seal h depth of drainage water a hole area K hydraulic conductivity

If h = 30cm, D = 60cm, K = 10-7 cm/sec

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the composite liner (even poor quality) is significantly better than soil or FML alone

Seal between FML and clay is important ensure FML is wrinkle-free + ensure clay is rolled smooth and free of stone etc.

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Source: Fernald Environmental Management Project, 2002 Geomembrane 56

Approximate costs: $175,000 per acre for non-hazardous waste $225,000 per acre for hazardous waste

The cost highly dependent on the local availability of soils suitable for construction and the requirements for monitoring, leachate collection, and gas collection

10.4 Economic consideration

Ref of this lecture: Shanahan, Waste Containment and Remediation Technology, 2004

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