Soil Groundwater and Subsurface Contamination

CONTENTS

4.1 The Nature of Soils

Soil Formation

4.2 Soil Profiles

Soil Horizons

Steps in the Typical Development of a Soil and Its Profile (Pedogenesis)

4.3 Organic Matter in Soil

Humic Substances

Some Properties of Humic Materials

4.4 Soil Zones

Air in Soil

4.5 Contaminants Become Distributed in Water, Soil, and Air

Volatilization Sorption

4.6 Partition Coefficients

Air-Water Partition Coefficient Soil-Water Partition Coefficient Determining Kd Experimentally The Role of Soil Organic Matter The Octanol/Water Partition Coefficient, Kow Estimating Kd Using Solubility or Kow

4.7 Mobility of Contaminants in the Subsurface

Retardation Factor

Effect of Biodegradation on Effective Retardation Factor A Model for Sorption and Retardation Soil Properties

4.8 Particulate Transport in Groundwater: Colloids

Colloid Particle Size and Surface Area

Particle Transport Properties

Electrical Charges on Colloids and Soil Surfaces

4.9 Biodegradation

Basic Requirements for Biodegradation

Natural Aerobic Biodegradation of NAPL Hydrocarbons

4.10 Biodegradation Processes

4.11 California Study

4.12 Determining the Extent of Bioremediation of LNAPL

Using Chemical Indicators of the Rate of Intrinsic Bioremediation

Hydrocarbon Contaminant Indicator

Electron Acceptor Indicators

Dissolved Oxygen (DO)

Nitrate + Nitrite Denitrification

Iron (III) Reduction to Iron (II) Sulfate Reduction

Methanogenesis (Methane Formation) Redox Potential and Alkalinity as Biodegradation Indicators References

4.1 THE NATURE OF SOILS

Whereas water is always a potential conveyor of contaminants, soil can be either an obstacle to contaminant movement or a contaminant transporter. The stationary soil matrix slows the passage of groundwater and provides solid surfaces to which contaminants can sorb, delaying or stopping their movement. On the other hand, soil can also move, carried by wind, water flow, and construction equipment. Moving soil, like moving water, transports the contaminants it carries. Predicting and controlling pollutant behavior in the environment requires understanding how soil, water, and contaminants interact. That is the subject of this chapter.

Soil Formation

Soil is the weathered and fragmented outer layer of the earth's solid surface, initially formed from the original rocks and then amended by growth and decay of plants and organisms. The initial step from rock to soil is destructive "weathering." Weathering is the disintegration and decomposition of rocks by physical and chemical processes.

Physical Weathering

Physical weathering causes fragmentation of rocks, increasing the exposed surface area. Common causes of physical weathering are

• Expansion and contraction caused by heating and cooling

• Stresses from mineral crystal growth and freezing and thawing of water in cracks and pores

• Penetration of tree and plant roots

• Scouring and grinding by abrasive particles carried by wind, water, and moving ice

• Unloading: When confining pressures are lessened by uplift, erosion, or changes in fluid pressures, unloading can cause cracks at thousands of feet deep

Chemical Weathering

Chemical weathering of rocks causes changes in their mineral composition. Common causes of chemical weathering are

• Hydrolysis and hydration reactions (water reacting with mineral structures)

• Oxidation (usually by oxygen in the atmosphere and in water) and reduction (usually by microbes)

• Dissolution and dissociation of minerals

• Immobilization by precipitation, e.g., the formation of solid oxides, hydroxides, carbonates, and sulfides

• Loss of mineral components by leaching and volatilization

• Chemical exchange processes, such as cation exchange

Physical and chemical weathering processes often produce loose materials that can be deposited elsewhere after being transported by wind (aeolian deposits), running water (alluvial deposits), and glaciers (glacial deposits).

The next steps, after the rocks have been fractured and broken down, are the formation of secondary minerals (e.g., clays, mineral precipitates, etc.) and changes caused by plants and bioorganisms.

Secondary Mineral Formation

Secondary minerals are formed within the soil by chemical reactions of the primary (original) minerals. The reactions forming secondary minerals are always in the direction of greater chemical stability under local environmental conditions. These reactions are facilitated by the presence of water which dissolves and mobilizes different components of the original rocks and allows them to react to form new compounds.

Roles of Plants and Bioorganisms

Organic materials play many complex roles. Roots form extensive networks permeating soil. They can exert pressures that compress aggregates in one location and separate them in another. Water uptake by roots causes differential dehydration, initiating the shrinkage and opening of many small cracks.

The plant root zone in the soil is called the rhizosphere. It is the soil region where plants, microbes, and other soil organisms support one another. Soil organisms include thousands of species of bacteria, fungi, actinomycetes, worms, slugs, insects, mites, etc. The number of organisms in the rhizosphere can be 100 times larger than in non-rhizosphere soil zones. Root secretions and dead roots promote microbial activity that produces humic cements. Root secretions include mucigel, a gelatinous substance that lubricates root penetration, various sugars, as well as aliphatic, aromatic, and amino acids. These substances and dead root material are nutrients for rhizosphere microorganisms. The root structure itself provides surface area for microbial colonization.

4.2 SOIL PROFILES

A vertical profile through soil tells much about how the soil was formed. It usually consists of a succession of more-or-less distinct layers, or strata. The layers can form from aeolian or alluvial deposition of material, or from in situ weathering processes.

Soil Horizons

When the layers develop in situ by the weathering processes described above, they form a sequence of horizons. The horizons are designated by the U.S. Department of Agriculture by the capital letters O, A, E, B, C, and R, in order of farthest distance from the surface (see Figure 4.1).

O-horizon: Organic

• The top horizon: starts at the soil's surface

• Formed from surface litter

• Dominated by fresh or partly decomposed organic matter

A-horizon: Topsoil

• The zone of greatest biological activity (rhizosphere)

• Contains an accumulation of finely divided, decomposed, organic matter, which imparts a dark color

• Clays, carbonates, and most metal cations are leached out by downward percolating water; less soluble minerals (such as quartz) of sand or silt size become concentrated in the A-horizon

E-horizon: Leaching zone (sometimes called the A-2 horizon)

• Light-colored region below the rhizosphere where clays and metal cations are leached out and organic matter is sparse

B-horizon: Subsoil

• Dark-colored zone where migrating materials from the A-horizon accumulate C-horizon: Soil parent material

• Fragmented and weathered rock, either from bedrock or base material that has deposited from water or wind

R-layer: Bedrock

• Below all the horizons; consists of consolidated bedrock

• Impenetrable, except for fractures

Steps in the Typical Development of a Soil and Its Profile (Pedogenesis)

• Physical disintegration (weathering) of exposed rock formations. This forms the soil parent material, the C-horizon.

• The gradual accumulation of organic residues near the surface begins to form the A-horizon, which might acquire a granular structure, stabilized to some degree by organic matter cementation. This process is retarded in desert regions where organic growth and decay are slow.

• Continued chemical weathering (oxidation, hydrolysis, etc.), dissolution, and precipitation begin to form clays.

• Clays, soluble salts, chelated metals, etc. migrate downward through the A-horizon, carried by permeating water, to accumulate in the B-horizon.

• The C-horizon, now below the O, A, and B horizons, continues to undergo physical and chemical weathering slowly transforming into B and A horizons, deepening the entire horizon structure.

• A quasi-stable condition is approached in which the opposing processes of soil formation and soil erosion are more or less balanced.

4.3 ORGANIC MATTER IN SOIL

Soil organic matter influences the weathering of minerals, provides food for microorganisms, and provides sites to which ions are attracted for ion exchange. Only two types of organisms can synthesize organic matter from non-organic materials. These are certain bacteria called autotrophs and chlorophyll-containing plants. Organic matter is developed in soil from the metabolism, wastes, and decay products of plants and soil organisms. For example, soil fungi metabolism produces excellent complexing agents, such as oxalate ion, and citric and other chelating organic acids. These promote the dissolution of minerals and increase nutrient availability. Some soil bacteria release the strong organic chelating agent 2-ketogluconic acid. This reacts with insoluble metal phosphates to solubilize the metal ions and release soluble phosphate.

The amount of organic matter has a strong influence on the properties of soil and on the behavior of soil contaminants. For example, plants have to compete with soil for water. In sandy soils, pore space is large and particle surface area is small. Water is not strongly adsorbed to sands and is easily available to plants. However in sandy soils water drains off quickly. On the other hand, water binds strongly to organic matter in soil. Soils with high organic content hold more water; but the water is less available to plants.

FIGURE 4.1 Generalized soil profile showing the horizon sequence.

As another example, oxalate ion is a metabolite of certain soil organisms. In calcium soils, oxalate forms calcium oxalate, Ca(C2O4), which then reacts with precipitated metals (particularly Fe or Al) to complex and mobilize them. The reaction with precipitated aluminum is

3 H+ + Al(OH)3(s) + 2 Ca(C2O4) Al(C2O4)2(aq) + 2 Ca2+(aq) + 3 H2O.

Because hydrogen ions are consumed, this reaction raises the pH of acidic soil. It also weathers minerals by dissolving some metals and provides Ca2+ as a plant and biota nutrient. Similar processes with silicate minerals release K+ and other nutrient cations.

Rule of Thumb

Organic matter is typically less than 5% in most soils but is the main factor in plant productivity. Peat soils can be 95% organic. Mineral soils can be less than 1% organic.

Humic Substances

The most important organic substance in soil is humus, a collection of variously sized polymeric molecules consisting of soluble fractions (humic and fulvic acids) and an insoluble fraction (humin).

Fulvic Acid Fractions

FIGURE 4.2 Characteristic structural portion of an unionized humic or fulvic acid.

FIGURE 4.2 Characteristic structural portion of an unionized humic or fulvic acid.

Humus is the near-final residue of plant biodégradation and consists largely of protein and lignin. It is what remains after the more easily degradable components of plant biomass have degraded, leaving only parts that are most resistant to further degradation. Humic materials are not well defined chemically and have variable composition. Percent by weight for the most abundant elements are C: 45-55%, O: 30-45%, H: 3-6%, N: 1-5%, and S: 0-1%. The exact chemical structure depends on the source plant materials and the history of biodegradation.

Humic and fulvic acids are soluble organic acid macromolecules containing many -COOH and -OH functional groups that ionize in water, releasing H+ ions and providing negative charge centers on the macromolecule to which cations are strongly attracted (see Figure 4.2). Humic materials are the most important class of soil complexing agents and are found where vegetation has decayed.

Some Properties of Humic Materials Binding to Dissolved Species

Humic materials are effective at removing metals from water by sorption to negative charge sites, mainly at the structural oxygen atoms. They strongly adsorb heavy polyvalent metal cations. The cation exchange capacity of humic materials can be as high as 500 meq/100 mL. Humic materials may contain metals like uranium in concentrations 10,000 times greater than adjacent water. Humic materials also bind to organic pollutants, especially low solubility compounds like DDT and atrazine. Much of the utility of wetlands for water treatment arises from their high concentrations of humic materials. Figure 4.3 shows several ways by which metal cations bind to humic and fulvic acids.

FIGURE 4.3 Some types of binding metal ions (M2+) to humic or fulvic acids.

Light Absorption

Humic materials absorb sunlight in the blue region (transmitting yellow) and can transfer the solar energy to sorbed molecules, initiating reactions. This energy transfer process can be effective in degrading pesticides and other organic compounds.

4.4 SOIL ZONES

The soil subsurface is commonly divided into three zones, based on their air and water content (see Figure 4.4). From the ground surface down to an aquifer water table, soils contain mostly air in the pore spaces, with some adsorbed and capillary-held water. This region is called the water-unsaturated zone or vadose zone. From the top of the water table to bedrock, soils contain mostly water in the pore spaces. This region is called the saturated zone.

Between the vadose and saturated zones, there is a transition region called the capillary zone, where water is drawn upward from the water table by capillary forces. The thickness of the capillary zone depends on the soil texture — the smaller the pore size, the greater the capillary rise. In fine gravel (2-5 mm grain size), the capillary zone will be of the order of 2.5 centimeters thick. In fine silt (0.02-0.05 mm grain size), the capillary zone can be 200 centimeters or greater.

The saturated zone lies above the solid bedrock, which is impermeable except for fractures and cracks. The region of the subsurface overlying the bedrock is generally unconsolidated porous, granular mineral material.

Air in Soil

Air in soil has a different composition from atmospheric air because of biodegradation of organic matter by soil organisms. Biodegradation occurs in many small steps, but the net overall reaction is shown in Equation 4.2, where organic matter in soil is represented by the approximate generic unit formula {CH2O}. An actual molecule of soil organic matter would have a formula that is approximately some whole number multiple of the {CH2O} unit.

Oxygen from soil pore space air is consumed and CO2 released by microbial metabolism. Much of the soil air is semitrapped in pores and cannot readily equilibrate with the atmosphere. As a result, O2 content is decreased in soil air from its atmospheric value of 21% to about 15%, and CO2 content is increased from its atmospheric value of about 0.03% to about 3%. This, in turn increases the dissolved CO2 concentration in groundwater, making it more acidic. Acidic ground-water contributes to the weathering of soils, especially calcium carbonate (CaCO3) minerals.

When soil becomes waterlogged, as in the saturated zone, many changes occur:

• Oxygen becomes used up by respiration of microorganisms.

• Anaerobic processes lower the oxidation potential of water so that reducing conditions (electron gain) prevail, whereas oxidation conditions (electron loss) dominate in the unsaturated zone.

• Certain metals, particularly iron and manganese, become mobilized by chemical reduction reactions changing them from insoluble to soluble forms:

FIGURE 4.4 Soil zones in the subsurface region.

Groundwater, moving under gravity, can transport dissolved Fe2+ and Mn2+ into zones where oxidizing conditions prevail, e.g., by surfacing to a spring or lake. There, Equations 4.3-4.5 are reversed and the metals redeposit as solid precipitates, mainly Fe(OH)3 and MnO2. Precipitation of Fe(OH)3 often causes "red water" and red or yellow deposits on rocks and soil. MnO2 deposits are black. These deposits can clog underdrains in fields and water treatment filters.

4.5 CONTAMINANTS BECOME DISTRIBUTED IN WATER, SOIL, AND AIR

In the environment, contaminants always contact water, air, and soil. No matter where it originated, a contaminant moves across the interfaces between water, soil, and air to become distributed, to different degrees, into every phase it contacts. Partitioning of a pollutant from one phase into other phases serves to deplete the concentration in the original phase and increase it in the other phases. The movement of contaminants through soil is a process of continuous redistribution among the different phases it encounters. It is a process controlled by gravity, capillarity, sorption to surfaces, miscibility with water, and volatility.

Volatilization

The main partitioning process from liquids and solids to air is volatilization which moves a contaminant across the liquid-air or solid-air interface into the atmosphere or into air in soil pore spaces. Volatilization is an important partitioning mechanism for compounds with high vapor pressures. For contaminant mixtures such as gasoline, the most volatile components are lost first causing the composition and properties of the remaining liquid mixture to change over time. For example, the most volatile components of gasoline are also the smallest molecules in the mixture.

Hence, as gasoline "weathers" and loses these smaller molecules by volatilization, its vapor pressure decreases and its viscosity and density increase.

Sorption

The main partitioning process from liquids and air to solids is sorption which moves a contaminant across the liquid-solid or air-solid interface to organic or mineral solid surfaces. For example, contaminants dissolved in stream water may become bound to suspended and bottom sediments. Sorption from the water phase is most important for compounds of low solubility. Once a contaminant is sorbed to a surface, it undergoes chemical and biological transformations at different rates and by different pathways than if it were dissolved.

Example 4.1

Estimating relative air/water partitioning behavior

Suppose you need to compare the air/water partitioning behavior of the compounds tabulated below but are able to find only melting point data. Estimate their relative vapor pressures and water solubilities based on their structures.

Compound

Structure

Phenol

Melting temperature = 43.0° C

1,2,3,5-tetrachlorobenzene Melting temperature = 54.5oC

1,2,4,5-tetrachlorobenzene Melting temperature = 140oC

Answer: Solubility will vary with polarity. The more polar the molecule, the more soluble it will be, because of stronger attraction to polar water molecules. It also varies with molecular weight. Higher molecular weight tends to decrease solubility because London dispersion forces are stronger, attracting the compound molecules to one another more strongly. Vapor pressure will vary inversely with the melting point because a high melting point indicates strong intermolecular attractive forces, and vice versa.

TABLE 4.1

Measured Values for Melting Point, Vapor Pressure, and Solubility

Phenol

1,2,3,5-tetrachlorobenzene 1,2,4,5-tetrachlorobenzene

43.0 2.6 x 10-4 6.3 x 10-1 54.5 1.9 x 10-4 1.6 x 10-5 140.0 3.0 x 10-5 2.5 x 10-6

Note: Tm = melting point; Pv = vapor pressure; Sw = solubility in water.

Vapor pressure: (Lowest to highest vapor pressure is in the order of highest to lowest melting point.)

Solubility: (Lowest to highest solubility is in the order of lowest to highest polarity and highest to lowest molecular weight.)

1,2,4,5-tetrachlorobenzene (nonpolar because of symmetry) < 1,2,3,5-tetrachlorobenzene (less symmetrical, more polar) < phenol (most polar and lightest molecular weight of all).

The measured values in Table 4.1 confirm these relative vapor pressure and solubility estimates.

Example 4.2

Rank the four compounds below in order of increasing tendency to partition from water into air.

Answer: All the compounds are similar in molecular weight and differ only in the top functional group. The molecule having the group with the weakest attractive force to water will most readily partition from water into air. So, we want to rank them by their relative attractions to water.

I: Has no oxygen for hydrogen-bonding to water and is the least polar. It will most readily volatilize.

IV: Has an oxygen, but no hydrogen is attached to it for hydrogen-bonding to water. It will be next in volatility from water to air.

1,2,4,5-tetrachlorobenzene < 1,2,3,5-tetrachlorobenzene < phenol.

III.

II and III: Can both hydrogen-bond. II can form one H-bond, while III can form two H-bonds. So, II is third in volatility and III is the least volatile in water.

Tendency to volatilize from water: I > IV > II > III. Their Henry's Law constants are

I: Kh = 1.38; II: KH = 0.0034; III: KH = 0.0015; IV: KH = 1.17. A larger Henry's Law constant means greater volatility (see Section 4.6). 4.6 PARTITION COEFFICIENTS

The tendency for a pollutant to move from one phase to another is often quantified by the use of a partition coefficient, also called a distribution coefficient. Partition coefficients are chemical specific. They can be measured directly or, in some cases, estimated from other properties of the chemical. The simplest form of a partition coefficient is the ratio of the pollutant concentration in phase 1 to its concentration in phase 2:

concentration in phase 1 C

' concentration in phase 2 C2

This expression assumes that a linear relation exists between the concentrations of a substance in different phases and is often satisfactory for low to moderate concentrations. Using the water phase as the reference phase, a linear relation gives Equations 4.7-4.9 for partitioning between water and air, water and a pollutant free product, and water and soil.

Kh is the air-water partition coefficient, also known as Henry's Law constant. Ca and Cw are the pollutant concentrations in air and water, respectively.

Kp is the bulk pollutant-water partition coefficient. Cp and Cw are the pollutant concentrations in bulk pollutant and water, respectively. A bulk pollutant is the portion of a contaminant that remains in its original form, such as a layer of oil floating on a river or above the groundwater table.

Kd is the soil-water partition coefficient. Cs and Cw are the pollutant concentrations sorbed on soil and dissolved in water, respectively.

Each value of K depends on the properties of the particular pollutant and the temperature. Kd also depends on the type of soil.

Air-Water Partition Coefficient Example 4.3

Using Henry's Law

Henry's Law, Ca = KHCw, describes how a substance distributes itself at equilibrium between water

and air. The units of Henry's Law constant, KH = —- , depends on what units are used to express

Cw concentrations in air and water.* For the case of oxygen gas, O2, at 20° C

• When air and water concentrations both have the same units,

• For water concentration in mol/L or mol/m3, and air in atmospheres,

KH(O2, 20° C) = 635 Latm/mol = 0.635 atmm3/mol. (4.11)

• For water concentration in mg/L and air in atmospheres,

If soil pore water is measured to contain 3.2 mg/L of oxygen at 20° C, what is the concentration, in mg/L and in atmospheres, of oxygen in the air of the soil pore space?

Answer:

For O2 at 20°C, KH = 26 = -5-; Ca = (26)(3.2 mg/L) = 83.2 mg/L.

Also,

Kh = 0.0198 L atm/mg = -5-; Ca = (0.0198 Latm/mg)(3.2 mg/L) = 0.063 atm.

Since the normal atmospheric partial pressure of oxygen at sea level is about 0.2 atm, this result indicates the presence in the soil of microbial activity that has consumed oxygen.

Example 4.4

BOD and Henry's Law

A certain sewage treatment plant located on a river typically removes 100,000 lbs (4.54 x 107 g) of biodegradable organic waste each day. If there were a plant upset and it became necessary to

* Henry's Law constants are tabulated in many references, such as Handbook of Chemistry and Physics, Howard, P.H., Ed., CRC Press, Boca Raton, 1991; Handbook of Environmental Fate and Exposure Data for Organic Chemicals, Vols. I-III, Lewis Publishers, Chelsea, MI; Lyman, W.J., Reehl, W.F., and Rosenblatt, D.H., Handbook of Chemical Property Estimation Methods, 2nd printing, American Chemical Society, Washington, D.C., 1990; Mackay, D. and Shiu, W.Y., A critical review of Henry's Law constants for chemicals of environmental interest, J. Phys. Chem. Ref. Data, 10(4): 1175-1199, 1981. There are also computer programs that calculate Henry's Law constant from other chemical properties.

release one day's waste into the receiving river, how many liters of river water could potentially be contaminated to the extent of totally depleting the water of all oxygen?

Answer: An approximate chemical equation we have used before as being suitable for biodegradation of organic matter is

Assume the river water is saturated with oxygen from the air at 20° C and that no additional oxygen dissolves from the atmosphere, a worst case.

Necessary data

Atmospheric pressure at the treatment plant = 0.82 atm. Vapor pressure of water at 20° C = 0.023 atm. Percent O2 in dry air = 21%. From Equation 4.11, KH(O2) = 635 Latm/mol.

Calculation

Organic matter is biodegraded, consuming oxygen by Equation 4.2. This chemical equation shows that one mole of O2 is consumed for each mole of CH2O biodegraded. The molecular weight of CH2O is 30 g/mol. Therefore,

4 54x107 g moles of CH2O in sewage = = 1.5 x 106 mol = moles of O2 consumed.

Atmospheric pressure = Ptotal = 0.82 atm = PO + PN + PHO.

Pdry an- = atmospheric pressure - partial pressure of water vapor = Ptotal - PHO.

Therefore, PO = (0.21)(Ptotal - PHO) = (0.21)(0.82 atm - 0.023 atm) = 0.17 atm.

Use Henry's Law to find the concentration of dissolved O2 in the river:

635 atm • L/mol or [O2(aq)] = (2.7 x 10-4 mol/L) x (32 g/mol) = 8.6 mg/L.

In saturated water at 20° C and 0.82 atm total pressure, [O2, aq] = 2.7 x 10-4 mol/L.

Liters of river water depleted of O2 = --2- = 5.6 x 109 L.

Note that both vapor pressure and solubility of a pure solid or liquid generally increase with temperature, but vapor pressure always increases faster. Therefore, the value of KH increases with temperature, indicating that, for a gas partitioning between air and water, the atmospheric portion increases and the dissolved portion decreases when the temperature rises. This is consistent with the observation that the water solubility of gases decreases with increasing temperature.

Rule of Thumb

Estimating KH:

If a tabulated value for KH cannot be found, it may be estimated roughly by dividing the vapor pressure of a compound by its aqueous solubility. For some compounds, tabulated values of vapor pressure and solubility may be easier to find than KH values.

K _ Ca ( partial pressure in atmosphere) » vapor pressure (atm)

Cw (mol/L) aqueous solubility (mol/L)

In this case, the units of KH are atmLmot1.

Example 4.5

Estimate Henry's Law constants for chlorobenzene and bromomethane using vapor pressure and aqueous solubility.

Answer:

Chlorobenzene: Pv (25° C) = 1.6 X 10-2 atm; Cw (25°C) = 4.5 X 10-3 mol/L.

Kh (25°C) = Pv/Cw = 1.6 X 10-2 atm/4.5 X 10-3 mol/L = 3.6 Latm/mol.

This happens to match exactly the experimental value. To put KH into dimensionless form, divide by RT (R = universal gas constant, T = temperature in degrees Kelvin), equivalent to multiplying by 0.041 mol/Latm

Bromomethane: Pv (liq, 25°C) = 1.8 atm; Cw (1 atm, 25°C) = 0.16 mol/L

At 25°C, bromomethane has a vapor pressure > 1 atm, so it is a gas. Since the solubility is given at 1 atm partial pressure

Kh (25°C) = Pv/Cw = 1 atm/0.16 mol/L = 6.3 L atm/mol.

Soil-Water Partition Coefficient

The partitioning of a compound between water and soil may deviate from linearity. This is particularly true for organic compounds. To account for this, the corresponding partition coefficient is often written in a modified form called the Freundlich isotherm. The modification consists of introducing an empirically determined exponent to the Cw term.

where

Cs = concentration of sorbed organic compound in solid phase (mg/kg). Cw = concentration of dissolved organic compound in water phase (mg/L). Kd = Cs/Cwn = partition coefficient for sorption. n = empirically determined exponential factor.

When Cw << Cs (the common case of organic compounds of low water solubility), then n is close to unity for most organics. However, n typically is temperature dependent.

Equation 4.14 can be written as an equation for a straight line by taking the logarithm of both sides, as in log Cs = log Kd + n log Cw. (4.15)

It is important not to extrapolate the Freundlich isotherm too far beyond the range of experimental data.

Example 4.6

Use of the Freundlich isotherm

A power company planned to discharge its power plant cooling water into a small lake. Before purchasing the lake, it tested the water and found the pesticide 2,4-D (2,4-dichlorophenoxyacetic acid) at 0.8 ppt (0.8 parts per trillion, or 0.8 x 10-9 g/L) just a little below the permitted limit of 1 ppt. The company calculated that its operation would raise the water temperature in the mixing zone near its discharge from 5° C to 25° C. Should the company anticipate a problem?

Answer: 2,4-D has low solubility and is denser than water. It will sink in the lake and become sorbed on the bottom sediments. The potential problem is whether or not the expected increase in temperature will cause the 2,4-D limit to be exceeded because of additional 2,4-D partitioning into the water from the bottom sediments. It is a case for the Freundlich isotherm because the constant n is a function of temperature and will cause a change in Kd when the temperature changes. A study8 measured Freundlich isotherm values for 2,4-D at 5° C and 25° C, as given in Table 4.2.

TABLE 4.2

Values for Freundlich Isotherm Parameters of 2,4-D

Temperature

TABLE 4.2

Values for Freundlich Isotherm Parameters of 2,4-D

Temperature

(° C)

n

Kd

log Kd

5

0.76

6.53

0.815

25

0.83

5.20

0.716

At 5°C: Cs = (6.53)(0.8 x 10-9 g/L)0 76 = (6.53)(1.22 x 10-7) = 797 x 10-9 g/kg. There are 797 ppt of 2,4-D sorbed on the sediments at 5°C.

Note that the concentration of 2,4-D sorbed to sediments is about 1000 times greater than the concentration dissolved. Even if a temperature rise to 25°C causes a large percentage increase in the dissolved portion, the percentage loss from the sediment fraction will be one thousand times smaller. Assume that the sediment concentration at 25° C is essentially the same as at 5° C. This allows an approximate calculation of Cw at 25° C.

0.83

The expected temperature rise will cause 2,4-D to desorb from the bottom sediments and raise its water concentration well over the permitted limit.

Determining Kd Experimentally

The Freundlich Equation 4.15 can be used to determine Kd experimentally, as follows:

1. Prepare samples having several different concentrations of dissolved contaminant in equilibrium with soil from the site of interest.

2. Measure the contaminant concentrations in water, Cw, in each sample.

3. Measure the corresponding contaminant concentrations sorbed to soil, Cs.

4. Plot log Cs vs. log Cw to get a straight line with slope = n and intercept = log Kd.

Example 4.7

Prepared water samples containing different concentrations of benzene were equilibrated with soil from a site under study. Equilibrium concentrations of dissolved and sorbed benzene are shown in Table 4.3. Find Kd for benzene in this soil.

TABLE 4.3

Benzene Partitioning Data for Soil and Water in Equilibrium

TABLE 4.3

Benzene Partitioning Data for Soil and Water in Equilibrium

Dissolved Benzene

Sorbed Benzene

Cw (mg/L)

Cs (mg/kg)

6.59

2.2

10.00

3.1

33.28

8.1

34.57

9.6

68.31

15.00

88.89

26.00

183.74

44.00

340.54

89.00

452.30

119.00

674.79

130.00

819.56

188.00

955.95

1. Determine the base-10 logarithms of all the concentration values. The logarithmic values are given in Table 4.4.

TABLE 4.4

Logarithms of Cw and Cs

TABLE 4.4

Logarithms of Cw and Cs

log Cw

log Cs

0.819

0.342

1.000

0.491

1.522

0.908

1.539

0.982

1.834

1.176

1.949

1.415

2.264

1.643

2.532

1.949

2.655

2.076

2.829

2.114

2.914

2.274

2.980

2.393

2. Plot log Cw vs. log Cs and fit a straight line through the points. The formula of the line is log Cs = log Kd + n log Cw. Therefore, the slope of the line is equal to n and the y-axis intercept is equal to log Kd. The resulting plot is shown in Figure 4.5.

R2 = H QCMQ

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