air, water and land pollution chapter 4: soils and land contamination copyright © 2009 by dbs

Air, Water and Land Pollution

Chapter 4:Soils and Land Contamination

Copyright © 2009 by DBS

Contents

• Introduction• Soil resources• Soil constituents• Soil properties• Contamination threats to soils• Land contamination and regeneration• Managing soil for sustainable communities

Soils and Land ContaminationIntroduction

Soil as a Living System

Definitions:

1. “The unconsolidated mineral material on the immediate surface of the earth that serves as a natural medium for the growth of plants.”

2. “The unconsolidated mineral matter on the surface of the earth that has been subjected to and influenced by genetic and environmental factors of parent material, climate, macro- and micro-organisms, and topography, all acting over a period of time and producing a product – soil – that differs from the material from which it is derived in many physical, chemical, biological and morphological properties and characteristics.”


Environmental Compartments


Soil as a Living System

• One of the three key compartments of the terrestrial environment

• Living system made up of complex communities of organisms

• Use carbon from plants as substrate

• Soil organisms process 60 x 109 metric tons per year, big part of C-cycle


The Soil Habitat

• Biomass soils >>> above ground habitat

• Extreme diversity, increases with decreasing spatial scale


The Soil Habitat

• Diversity supports the multifunctionality of the soil system

• Ecosystem services – food, fiber and energy production, water retention etc.


The Soil Habitat

• “Soil Architecture” - a key factor controlling the type and extent of life in soil

– Physical network and size distribution of pores

– Behavior of water and dissolved solutes

– Physical and chemical conditions of soil determine the types and conditions for biological communities and soil quality/health


The Soil Habitat

• Soils with open structures that allow free movement of water, solutes, and gases will support a richer biological community

Soil is a 3-phase material


The Soil Habitat

• Subject to physical and chemical stresses

• Measure of stress - response and resilience

Resistance = change in state for a given level of perturbatione.g. respiration rate reduction arising from compaction

Resilience = how far a system is able to return to its original state ‘bounce back’

Ritz, 2003


Soil Functions

• Supports ecosystem services

• Platform for both the natural as well as the built environment

• Provisioning services– foods and fiber– precursors to pharmaceutical and industrial

products– energy

• Regulating services– carbon sequestration and climate regulation– waste decomposition and detoxification– nutrient dispersal and cycling

• Supporting services– purification of air and water– crop pollination and seed dispersal– pest and disease control

• Cultural services– cultural, intellectual and spiritual inspiration– recreation– scientific discovery

• Preserving services– genetic and species diversity for future use

‘natural capital’– accounting for uncertainty– protection of options

De Groot et al., 2002


Soil Protection

• Maintenance of the capacity of soil to delivery of this wide range of services requires that the soil system remains intact and its performance is not compromised by imposed stresses

• Threats include:

– Surface sealing by construction

– Erosion

– Contamination

– Loss of organic matter

– Increased salinity

– Compaction

Soils and Land ContaminationSoil Resources

Soil Formation

• ‘Pedogenesis’ – physical, chemical and biological processes

• Important factors in soil formation:

– Parent material

– Climate

– Landscape topography

– Biotic Factors – vegetation, animals

– Timescale

Results in a large variety of soils in different locations, each with a set of distinct layers (horizons) that make up a soil profile


Soil Formation

• Soil water regime greatly influences formation

Precipitation >>> evapotranspiration

– Materials leach down through soil profile to lower horizons

Arid soils

– Net movement is up, leads to salt accumulation in surface horizons

Waterlogged soils

– Reducing conditions lead to solubilization of iron and manganese


Types of Soil

• Differences in pedogenesis result in high degree of heterogeneous character

• Soils become very identifiable (difficult when modified by human activity)

e.g. Pozols vs. Solonetz

PODZOLSAcid soils with a subsurface accumulation of Fe-Al-organic compounds(form in temperate regions with higher rainfall)

SOLONETZSoils with subsurface clay accumulation, rich in sodium(form in arid regions with intermittent rainfall)


Types of Soil

• World Reference Base (WRB) soil classification system

– National and local use

– Defines 30 distinct soil groups and many more sub-groups

– Useful as it provides a basis for inter-comparison of results

– May be modified by human activities


http://www.fao.org/ag/agl/agll/wrb/

http://www.fao.org/ag/agl/agll/wrb/


Spatial variation

• Soil exhibits a large degree of spatial variability in physical, chemical and other properties at all scales

• Not as simple as studying maps!


Spatial variation

• Scientific study of soils must take this into account

• Requires careful descriptions of soils under study and assessment of how representative they are of the area under study

e.g. 1: a study of the levels of contaminants at a site under investigation need to be made on the basis of a chemical analysis of a sufficient no. of samples taken in a spatially representative manner

e.g. 2: estimating the background distribution of an element requires a sampling pattern that takes into account the presence of different soil types

Soils and Land ContaminationSoil Constituents

The Mineral Fraction

• Texture

– Particle size distribution of mineral components in a soil is described by its texture

sand (0.02-2.00 mm) silt (0.002-0.02 mm) clay (< 0.002 mm)



• Clay Minerals– Relative surface area – It is on the surface that many chemical and

physical processes take place. – Smaller = more surface area (clay is tiny!)


The Mineral Fraction• Textural Classes

– Particle size distribution:

sand (0.02-2.00 mm)

silt (0.002-0.02 mm)

clay (<0.002 mm)

– Loam = good combination of soil

– Sci. explanation = good mix of fine particles


The Mineral Fraction• Clay Minerals

– Aluminosilicates (produced by weathering rocks)– Sheet silicates formed from tetrahedral silica and octahedral alumina

0.26 nm

oxygen

silicon

Silicon tetrahedron

0.29 nm

aluminium or magnesium

hydroxyl or oxygen

Aluminium Octahedron


The Mineral Fraction• Clay Minerals

– Aluminosilicates (produced by weathering rocks)– Sheet silicates formed from tetrahedral silica and octahedral alumina

tetrahedron



• Clay Minerals– For simplicity, let’s represent silica tetrahedral sheet and alumina

octahedral sheet by:

Si

Al



• Clay Minerals– Different combinations of tetrahedral and octahedral sheets form different

clay minerals:

1:1 Clay Mineral (e.g., kaolinite):


Si

Al

Si

Al

Si

Al

Si

Al

joined by strong H-bond no easy separation

0.72 nm

Typically 70-100 layers

joined by oxygen sharing

1:1 Clay Mineral (e.g., kaolinite):



• Clay Minerals– Different combinations of tetrahedral and octahedral sheets form different

clay minerals:

2:1 Clay Mineral (e.g., montmorillonite)

30


Si

Al

Si

Si

Al

Si

Si

Al

Si

0.96 nm

joined by weakvan der Waal’s bond

easily separated by water

2:1 Clay Mineral (e.g., montmorillonite)



• Clay Minerals

– Various types, depends on parent materials

– Aluminosilicates (produced by weathering rocks)

– Sheet silicates formed from tetrahedral silica SiO2 and octahedral alumina (Al2O3)

Clay mineralsphotographedwith an electronMicroscope.

Note: they are plateor flake like andare stacked on topof each other.



• Clay Minerals – Surface Charge

– Isomorphous substitution, of Si4+ or Al3+ by cations of lower valence leads to deficiency of +ve charge, generates permanent –ve charge at the clay surface

– Edge charge, depends on pH, ionization of OH group to O-, may also become +ve with addition of H+

– Permananet excess –ve charge contributes to cation exchange capacity (CEC) of the soil (may pull +ve ions from solution)

– ‘Nutrient storehouse’ (also between plates)

– Also absorbs water, hydration spheres



• Clay Minerals – colloidal behavior



• Clay Minerals

– Some are non-expansive whilst others are expansive (shrink and swell) with water content

– Shrinking and swelling has strong influence on formation of soil aggregates and structure



• Clay Minerals

e.g. non-expansive kaolinite (silica: alumina ratio of 1:1)low surface area and CEC 2-20 cmol(+) kg-1 (or mEq/100 g)

e.g. expansive Montmorillonite (silica:alumina ratio 2:1)large surface area and high CEC 80-120 cmol(+) kg-1



• Metal Oxides (Fe/Mn/Al)

– Quantities depend on parent material

– Prevailing redox conditions

– pH and pe conditions will determine speciation



• Metal Oxides (Fe/Mn/Al)

– pH dependent solubility leads to dissolution and precipitation

– Hydroxides generally insoluble

– Fe(II) favored at low pe(reducing conditions)

– Fe(III) favored at high pe

– Fe(III) oxides and oxyhydroxides give rise to characteristic brown color of soil



• Carbonates

– CaCO3 occurs in soils with limestone parent material

– Also formed chemically in soils

– Produces natural alkaline conditions

– Liming is used to modify soil pH and control solubility of toxic contaminants, e.g. Al3+ and H+



• Sulfides and Sulfates

– Speciation changes with redox conditions

– Mediated by micro-organisms in soil

– Sulfides occur in anaerobic conditions

– Sulfate moves up or down soil profile based on net water movement



• Trace Elements

– Depend on parent material, land-use and atmospheric deposition

– Urban soils frequently contain more than rural soils


Organic Matter

• SOM = Soil Organic Matter

• Derived from plant residues and MO decomposition products

• Arid soils contain less SOM (<1 %)

• Humid regions SOM (1-10 %)

• Far more C in soil than the atmosphere or above ground biota


Organic Matter

• 100 years of study…

Proteins

Lipds

Carbohydrates

Porphyrins

Plant pigments

SOM (humus)

MO’s and abiotic reactions

1000’s of different structures 500 – 5000 amu


Organic Matter

• Defined operationally by a alkali-based extraction – humic substances

– Acid soluble fraction = fulvic acid

– Acid insoluble fraction = humic acid

– Left overs = humin

• No sharp divisions between these substances, all part of a heterogeneous supramolecular system of transformed biomolecules

Soils and Land ContaminationSoil Properties

The Soil Habitat

• Soils with open structures that allow free movement of water, solutes, and gases will support a richer biological community

Soil is a 3-phase material


Water Retention and Drainage

• The Soil Pore System

– Porosity (ϕ) = volume void-space (fluids) / total volume material

– Typically decreases as particle size increases due to aggregate formation



• The Soil Pore System

– Controls movement of water and air into and from the soil profile

– Which in turn controls the mass transfer of gases, liquids, sediment, particles and dissolved substances



• Soil Water

– Inter-particle pores

– Macropores – worm burrows, root channels, cracks, tillage

Macropores – water cannot be held against force of gravity

Micropores – small enough to hold water against force of gravity



• Soil Water

– Volumetric water content of soil = Vw / VT

Where Vw = volume water, VT = total volume (soil+water+pore space)

– Gravimetric water content of soil = mw/mb

Where mw = mass water, mb = bulk material mass



• Soil Water

– Water holding capacity depends on:capillary action, and size of the pores

– Sandy soils have large particles and large pores. However, large pores do not have a great ability to hold water. As a result, sandy soils drain excessively

– Clay soils have small particles and small pores. Since small pores have a greater ability to hold water, clay soils tend to have high water holding capacity



• Soil Water

– Water-retention curves

– Pressure vs. water content Finer particles, and

hence pores hold water more strongly against applied pressure than coarse ones



• Soil Water

– Amount of soil water content held in soil after excess has drained away (2-3 days) after rain event - “field capacity”

Pores > 30 μm diameter drain under gravity

Pores < 30 μm diameter hold water via capillary action


Soil pH and Redox

• Soil pH

– Concentration of H+ ions in soil solution

– Normally in equilibrium with –vely charged soil particle surfaces

– Best measured in field but cannot measure dry soils

– Measured in lab by equilibrating soil with DI water

– Sometimes low conc. CaCl2 solution to mimic field conditions

Why is soil pH sometimes measured via dilution with low concentration CaCl2?

When the soil is diluted with water , most of the H+ ions tend to remain attracted to the soil particles and are not released into the soil solution.

The addition of Ca2+ ions to replaces some of the H+ ions on the soil particles, forcing the hydrogen ions into the solution.

The pH measured in CaCl2 is almost always lower than pH of the same soil measured in water due to the higher concentration of H+.

The procedure gives a value similar to that for natural soil solution because the soil solution also contains dissolved Ca2+ and other ions.


Soil pH and Redox

• Soil pH

– The most important physiochemical parameter

– Influences the chemistry of the soil; Plant growth; microbial ecology and activity; contaminant chemistry

– Normal: 4-9


Soil pH and Redox

• Redox Potential

– High O2 demand maintained by aerobic organisms and plant root respiration

– Rate of exchange of O2 depends on porosity and water content

– Saturated soil with fresh OM quickly becomes reducing (low pe)

– Fully drained, sandy soil with little fresh OM will remain aerobic (high pe)


Adsorptive Properties

• Soil Surfaces – physical and chemical

– SA is large, greatest in fine clays, all heterogeneous

– Physical adsorption to:

Clay surfaces

Organics

Metal oxides



• Physical (Non-Specific Adsorption)

– Ion-exchange – exchange of counter-ions balancing surface charge on soil particles and ions in solution

– CEC increases with increasing soil pH (removing H+)

– Negative charge on soil colloids (and hence CEC) is due to:(a) isomorphous substitution, (b) dissociation of carboxylic and phenolic groups on SOM, (c) pH dependent charge on metal oxides



• Chemisorption (Specific Adsorption) of Metals

– Stronger than physical adsorption

– Forms chemical bonds with metal oxides and organic matter

e.g. metal oxy-hydroxide complexes (FeOOH), phosphate (PO43-) and

arsenate (AsO43-) form strong bonds with Al and Fe oxides

Chemisorption of phosphate and Cu ionsonto metal oxide surface



• Chemisorption (Specific Adsorption) of Metals

– SOM carries wide range of functional groups able to complex metal ions

e.g. carboxylic acid (–COOH) and phenol (-OH)

–COOH + M+ -COO-M + H⇌ +



• Adsorption of Organic Compounds

– Anionic organic contaminants (e.g. phenoxyalkanoic herbicide) less strongly bound to soil than non-polar (e.g. PAHs) or cationic (e.g. bipyridal herbicides)

– Adsorption of non-polar organic contaminants occurs mainly to SOM

– Partitions into hydrophobic centers in SOM molecules and hydrophobic mineral surfaces

– Can be predicted by determining Octanol-Water Partition Coefficient (KOW)

Low KOW = hydrophillic (e.g. 2,4-D)

High KOW = hydrophobic, adsorb strongly to SOM (e.g. DDT)

Equilibrium Sorption Phenomena

Measuring Adsorptive Properties

• Constants (Ks) are an equilibrium concentration of a chemical in one phase divided by the concentration of the same chemical in a different phase (working at equilibrium is a simplification)

surface

Metals Organics (hydrophobic)

adsorption(ion-exchange process)

partitioning (solvation process)

+adsorption if polar

Sorption = attraction = adsorption (metals) = partitiioning (organics)

Equilibrium Sorption Phenomena

Soils and Land ContaminationContamination Threats to Soil

Introduction

• Soils have been subject to a number of contamination threats:

– Heavy metals and organic contaminants from old industrial sites and underground storage tanks

– Oil and fuel dumping

– Leaching of waste from landfills

– Potentially toxic elements (pte) and pathogens in sewage sludge applied to agricultural land

– Industrial sludge and dredgings applied to land (waste disposal)

– Radioactive releases from industrial accidents

– POPs deposited on soils following aerial release from industry

– Agricultural residues (pesticides, fungicides, herbicides)

– Spent munitions and explosives


Introduction

• Risk assessment and ultimately risk management are used to manage threats

Risk Assessment

Risk Prioritization

Risk Management


Introduction

• Risk assessment and management requires understanding the environmental capacity of soils and the impact of hazardous agents


Introduction

• Ultimate Goal - Managing Risk

– What is specifically at risk?Soil function, soil microorganisms, crops, humans

– What is the risk from?

– Loss of buffering capacity, loss of soil structure, reduction in soil quality


Introduction

• Since soil is a 3-phase material it is susceptible to harm from waterborne, airborne and direct application routes

• ‘Environmental sink’ for some contaminants

• Act as sources of contamination for other environmental compartments

• Multifunctionality of soils:

Source of contamination

Pathway by which contamination reaches other compartments

Receptor of contamination


Introduction

• Managing Risks:

– Critical to understand major contributing factors and control mechanisms

– Able to prioritize between issues and target management strategies and resources towards the most significant risks and contributing factors

– Predicting consequences of soil pollution (what might happen)

– Predicting the likelihood of occurrence (probability)

– Requires modelling!

Where Next?

Next step:

Convert chemical process into a ‘mathematical form’ that can be integrated into an environmental model

Cannot fit all chemistry into models

…has to be simplified


Introduction

• Modeling of pollutant transport processes and exposure assessment

• Many simplifications have to be made since:

– Heterogeneous

– Multiple phases (pore fluid, mineral, organic matter, biota, etc.)

– Analytical difficulties – can’t measure everything!

Soils and Land ContaminationLand Contamination and Regeneration

Introduction

• How to manage historic industrialization?

• Post-industrialized world…a sustainable world?

• Globalization of national companies has led to former industrial sites becoming vacant

• Redevelopment of these sites (brownfields) for housing or other uses requires risk assessment


Introduction

• Comprehensive Environmental ResponseCompensation + Liability Act

(CERCLA)

or ‘Superfund’

NPL list

Creosote Contaminates Louisiana Community for Generations

Behind the For Sale sign liesthe site of the old LincolnCreosote plant. Low incomesubsidized housing built inthe 1970s is across the ditch

BOSSIER CITY, Louisiana, September 5, 2001(ENS) - A small neighborhood in Bossier City,Louisiana has some of the highest levels ofchemical contamination, cancers and birthdefects ever documented in the United States,according to National Institutes of Health (NIH)scientists

Source: http://www.ens-newswire.com/ens/sep2001/2001-09-05-01.asp

Lincoln Creosote

• Operated 1935-1969• NPL Superfund site

‘Cleaned up’ 1996

…incidence of leukemia from the late 1970s to the mid-1990s is as much as 40 times higher than normal populations

Incidences of birth defects are 300 percent higher that those recorded during a comparable time period in Osaka, Japan which is near Hiroshima where an atomic bomb was dropped in 1945 to end World War II.

Superfund SitesSuperfund sites are uncontrolled or abandoned places where hazardouswaste is located, possibly affecting local ecosystems or people

Years ago, people were less aware of how dumping chemical wastes might affect public health and the environment. On thousands of properties where such practices were intensive or continuous, the result was uncontrolled or abandoned hazardous waste sites, such as abandoned warehouses and landfills. Citizen concern over the extent of this problem led Congress to establish the Superfund Program in 1980 to locate, investigate, and clean up the worst sites nationwide.

Source: http://www.publicintegrity.org/Superfund/

Huai River Basin

http://www.nytimes.com/packages/khtml/2004/09/11/international/20040912_CHINA_FEATURE.html

Pollution in China and India


Risk-Based Land Management

• Environmental Fate and Transport

– Physiochemical properties of pollutants in soils

Fate in soil is determined by:

1. Adsorption - Binding by soil is critical

2. Leaching

3. Volatilization

4. Uptake

5. Run off

6. Degradation

Environmental Fate

ADSORBED

DISSOLVED

MO DECOMPOSITION

LEACHING

CHEMICAL DECOMPOSITION

VOLATILIZATIONWORMS ETC.

PLA

NT

S

WATER TABLE

PHOTO DECOMP.

UPTAKE

RUN OFF

-


P

P

P

P

P

P

P -

1. Adsorption: To clay or organic matter:

Pollutant is bound to or adsorbed onto surface of clay, can be released as free or solvated pollutant

Equilibrium is influenced by:1) Nature of pollutant2) Water content3) Soil pH, pe and temperature

--

--

- P

P

P

P

P

P

P


2. Leaching:

Lower solubility = slower leaching = greater persistence

3. Volatilization:

Influenced by chemical structure (V.P.) and atmospheric conditions

4. Uptake:

By plants etc.

Warm, moist conditions, acidic soil, high solubility = greater uptake

5. Run off / soil erosion:

Effect is greater with least adsorption, erosion caries particle away

6. Degradation:

Microbial, chemical and light induced




e.g. organic pollutants at sites contaminated with wood preserving chemicals




– Physiochemical properties used to:

(i) direct site investigation techniques – identify key phases for sampling and analysis

(ii) inform selection of remediation technologies – principal focus should be key compartment within which risk-critical compounds are present


Risk-Based Land Management• Case Study 3: e.g. benzo(a)pyrene (PAH)

– Principal compartment we can expect to find contaminants

Low VP, low solubility, high Koc, high t1/2 – expected to be found in soil

– Proposed sampling and analytical methods

Involve extraction from soil matrix, gas chromatography– Key exposure pathways

Exposure to soil: dermal, ingestion, inhalation– Risk management approach

Relative importance of each route depends on end use of site

Inhalation of dust may only be important if soils are left exposed

Ingestion may be less of a concern if end use is an industrial park

For housing important options include bioremediation or removal of soil



• Using Risk Assessment Tools

– Risks to humans and ecosystems are assessed based on (i) ‘acceptable’ risk, a level below which further remediation has marginal benefit

(ii) Toxicologically-based site assessment criteria




– Standard exposure pathways used by risk assessment software packages

– Can then compare these to acceptable daily intake or reference doses

– Most models adopt generic exposure scenarios

– Possible to quickly identify key pollutants, exposure pathways and receptors that contribute most to the site risk

[pollutant] in soil mg kg-1

[pollutant] in water, air, soil, biota mg kg-1

Dose: [pollutant] in human mg kg-1 d-1




– Probabilistic models that account for uncertainties using stochastic methods (element of randomness)

– Identify principal sources of variance in estimates of exposure

– More credible risk assessment



• Scientific Rigor

– Case study 4: assumptions in estimating exposure (ingestion of soil)




– Decision to remediate contaminated sites has to be based on high quality data

– Risk assessment with ‘made up’ numbers will provide false sense of security in estimate of the magnitude of the risk




– Central to understanding fate of pollution is the characterization of its source

– Understanding the source allows the nature of the hazard to be identified

e.g. Benzene is a human carcinogen by inhalation (high VP), slight solubility so may contaminate ground water

– Quantifying the source is important - risk estimates rely on modeled exposures rather than measured




– Site risk assessments have associated geochemical and analytical uncertainties

e.g. reference soil [As] = 33 mg kg-1

– Variance must be considered by regulators,in this case for a soil guideline value of 20 mg kg-1



• Bioavailability

– Risk estimates based on dose received at exposure points do not include processes occurring in the body



• Communicating Risk, Environmental Justice and Participation in Decision-Making

– Make accessible to non-expert audiences

– Providing info., consulting with others, involving stakeholders in decision making

– Development and exploration of options rather than single solutions

– Clearly identifying the problem

Soils and Land Contamination Summary

Soils and Land Contamination Summary

• Question 9: Coal gas = Town gas in USA

use http://en.wikipedia.org/wiki/Town_gas

References

• Andreae, M.O. and Schimel, D.S. (eds.) (1989) Exchange of trace gases between terrestrial ecosystems and the atmosphere. Wiley.

• Alloway, B.J. (ed.) (1990) Heavy Metals in Soils. Blackie.Harrison, R.M. (2006) Introduction to Pollution Science. The Royal Society of Chemistry, London.

• White, R.E. (1987) Introduction to the Principles and Practice of Soil Science, 2nd ed. Blackwell.

• Wild, A. (1993) Soils and the Environment. Cambridge University Press.

air, water and land pollution chapter 4: soils and land contamination copyright © 2009 by dbs

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