CHAPTER 14

Soil Pollution and Land ContaminationB. J. ALLOWAY

14.1 INTRODUCTION

Soil is an essential component of terrestrial ecosystems because the growth ofplants and biogeochemical cycling of nutrients depend upon it. Of the total areaof the world's land mass (13.07 x 109 ha), only 11.3% is cultivated for crops;permanent grazing occupies 24.6%, forest and woodland 34.1% and 'otherland' including urban/industry and roads, accounts for 3PZo.1 From a resourceperspective, soil is vitally important for the production of food and fibre cropsand timber and it is therefore essential that the total productive capacity of theworld's soils is not impaired. Pollution, along with other types of degradation,such as erosion, and the continuing spread of urbanization poses a threat to thesustainability of soil resources. Soil pollution can also be a hazard to humanhealth when potentially toxic substances move through the food chain or reachgroundwater used for drinking water supplies.

In comparison with air and water, soil is more variable and complex incomposition and it functions as a sink for pollutants, a filter which retards thepassage of chemicals to the groundwater, and a bioreactor in which manyorganic pollutants can be decomposed. As a consequence of its occurrence atthe interface between the land and the atmosphere, soil is the recipient of adiverse range of polluting chemicals transported in the atmosphere. Furtherinputs of pollutants to the soil occur as a result of agricultural and wastedisposal practises but, in general, the most severe pollution usually results fromindustrial and urban uses of land.

It is generally accepted that most of the soil in technologically advancedregions of the world is polluted (or contaminated), at least to a slight extent.2

However, in many cases the relatively small amounts of pollutants involved maynot have a significant effect on either soil fertility or animal and human health.More severe 'chemical pollution' which poses a greater hazard has beenestimated by a Global Assessment of Soil Degradation ('GLASOD') to affect atotal of 21.8 x 106 ha of land in Europe, Asia, Africa and Central America.3

Realistic estimates of areas affected by soil pollution are difficult owing tounreliable official figures and inadequate data for many parts of the world.

Industrially contaminated land tends to contain higher concentrations and agreater possible range of pollutants than other sources of pollution. It hasrecently been estimated that there are between 50 000 and 100 000 contaminatedsites in the United Kingdom which are estimated to occupy around 300 000 ha.In the USA, 25 000 contaminated sites have been identified; 6000 sites are beingcleaned-up in the Netherlands, there are known to be at least 3115 sites inDenmark and 40 000 suspect areas have been identified on 5000-6000 sites in theformer western part of Germany.4 Land contamination as a result of warfare,military training and the manufacture and storage of explosive materials, suchas TNT (trinitrotoluene), have resulted in very large areas of many countries,such as Germany, being contaminated with very persistent organic andinorganic chemicals.

Old industrial sites are generally characterized by being very heterogeneous,both with regard to the distribution of pollutants and also to the properties ofthe soil materials that control the behaviour of these chemicals. In contrast,atmospherically deposited pollutants tend to have more even distribution withgradual changes in concentrations, which tend to decrease with distance fromthe source. The upper horizons of the soil are contaminated to the greatestextent by atmospheric deposition.

All contamination/pollution situations comprise the following components:(i) a source of pollutant, (ii) the pollutant itself, (iii) a transport mechanism bywhich the pollutant is dispersed, and (iv) the receptor where the transport phaseterminates. Transport can be by moving air or water, by gravity movementdownslope, or by direct conveyance and placement such as the spreading ofwaste materials on land. Although this is a very simple conceptual model whichdoes not take account of variations in time and quantity, it does provide a usefulbasis from which to consider the pollution of soils and other environmentalmedia.

14.2 SOIL POLLUTANTS AND THEIR SOURCES

14.2.1 Heavy Metals

Heavy metals have a density of greater than 6 g cm" (but some authors use avalue of >5g cm"3) and an atomic number greater than 20, and occurnaturally in rocks and soils but concentrations are frequently elevated as aresult of pollution.5'6 The term 'heavy metal' is imprecise but is widely usedalthough others such as 'toxic metals', 'potentially toxic elements' and 'tracemetals' are possible alternatives. Heavy metals belong to the group of elementsdescribed geochemically as 'trace elements' because they collectively comprise< 1% of the rocks in the earth's crust. All trace elements are toxic to livingorganisms at excessive concentrations, but some are essential for the normalhealthy growth and reproduction by either plants or animals at low but criticalconcentrations. These elements are referred to as 'essential trace elements' or'micronutrients' and deficiencies can lead to disease and even death of the plantor animal. The essential trace elements include: Co (for bacteria and animals),

Cr (animals), Cu (plants and animals), Mn (plants and animals), Mo (plants),Ni (plants), Se (animals) and Zn (plants and animals). In addition, B (plants), Cl(plants), Fe (plants and animals), I (animals) and Si (plants and animals -probably) are also essential trace elements but are not dense enough to beclassed as heavy metals.

Other elements, including: Ag, As, Ba, Cd, Hg, Tl, Pb, Sb, have no knownessential function and, like the essential trace elements, cause toxicity above acertain tolerance level. The most important heavy metals with regard topotential hazards and occurrence in contaminated soils are: As, Cd, Cu, Cr,Hg, Pb and Zn.6

14.2.1.1 Sources of Heavy Metals, (a) Metalliferous Mining. This is animportant source of contamination by a wide range of metals, especially As, Cd,Cu, Ni, Pb and Zn, because ore bodies generally include a range of mineralscontaining both economically exploitable metals (in ore minerals) and uneco-nomic elements (in gangue minerals). Most mine sites are contaminated withseveral metals and accompanying elements (e.g. sulfur). Wind-blown tailings(finely ground particles of ore and country rock) and ions in solution from theweathering of ore minerals in heaps of tailings tend to be the major sources ofpollution from abandoned metalliferous mine sites.

(b) Metal Smelting. This is the process of producing metals from minedores and so can be a source of many different metals. These pollutants aremainly transported in air and can be in the form of fine particles of ore, aerosol-sized particles of oxides (especially important in the case of the more volatileelements, such as As, Cd, Pb, and Tl) and gases (SO2). In some cases, pollutionis directly traceable in soil < 40 km downwind of smelters.

(c) Metallurgical Industries. Pollution can include aerosol particles fromthe thermal processing of metals and solid wastes, effluents from the treatmentof metals with acids and solutions of metal salts used in electroplating.

(d) Other Metal-using Industries. These can be a source of metals ingaseous/particulate emissions to the atmosphere, effluents to drains and solidwastes. These include: the electronics industry where metals are used insemiconductors, contacts, circuits, solders and batteries; plating (Cd, Ni, Pb,Hg, Se, and Sb); pigments and paints (Pb, Cr, As, Sb, Se, Mo, Cd, Co, Ba, andZn); the plastics industry (polymer stabilizers such as Cd, Zn, Sn, and Pb); andthe chemical industry which uses metals as catalysts and electrodes includingHg, Pt, Ru, Mo, Ni, Sm, Sb, Pd, and Os.

(e) Waste Disposal. Municipal solid waste, special wastes and hazardouswastes from many sources can contain many different metals.

(f) Corrosion of Metals in Use. Corrosion and chemical transformation ofmetals used in structures, e.g. Cu and Pb on roofs and in pipes, Cr, Ni and Co instainless steel, Cd and Zn in rust preventative coatings on steel, Cu and Zn inbrass fittings, and Cr and Pb from the deterioration of painted surfaces.

(g) Agriculture. This mainly includes As, Cu, and Zn which are (or havebeen) added to pig and poultry feeds, Cd and U contaminants in some

phosphatic fertilizers, and metal-based pesticides (historic and current) such asAs, Cu, Mn, Pb and Zn.

(h) Forestry and Timber Industries. Wood preservatives containing As, Crand Cu have been widely used for many years and have caused contaminationof soils and waters in the vicinity of timber yards. Several organic chemicals,including tar derivatives (creosote) and pentachlorophenol, are also used aswood preservatives.

(i) Fossil Fuel Combustion. Trace elements present in coals and oils includeCd, Zn, As, Sb, Se, Ba, Cu, Mn and V and these can be present in the ash orgaseous/particulate emissions from combustion. In addition, various metals areadded to fuels and lubricants to improve their properties (Se, Te, Pb, Mo andLi).

(j) Sports and Leisure Activities. Game and clay pigeon shooting involvesthe use of pellets containing Pb, Sb and As but alternatives to these metals suchas steel, Mo and Bi are also being introduced.

14.2.2 Hydrocarbon Pollutants

Hydrocarbon pollutants from petroleum mainly comprise a range of satu-rated alkanes from methane (CH4), ethane (C2H6) and propane (C3H8)through straight and branched chains to C76Hi54. Aromatic hydrocarbonsand organic components containing nitrogen and sulfur can also be importantconstituents of some petroleum deposits. The hydrocarbons derived from coaland petroleum tend to form the main group of organic macropollutants insoils.

A commonly encountered group of aromatic molecule hydrocarbon pollu-tants are the BTEX compounds (benzene, toluene, ethyl benzene and xylene)which commonly occur in plumes in the groundwater beneath a wide range ofindustrial sites. Organic solvents are used widely in industry and can beimportant soil pollutants at industrial sites. These can include butane and n-hexane, benzene, toluene and organochlorine compounds such as vinyl chlor-ide, chloroform, carbon tetrachloride and trichloroethane. Apart from anytoxicity hazard associated with the ingestion or inhalation of hydrocarbons,there is also a high risk from fires and explosions.

On the basis of their behaviour in soils and groundwaters, hydrophobicorganic liquid contaminants, such as solvents, are often grouped as non-aqueous phase liquids (NAPLs) and depending on their density they aresubdivided into light non-aqueous phase liquids (LNAPLs) and dense non-aqueous phase liquids (DNAPLs).

14.2.2.1 Sources of Hydrocarbon Pollutants, (a) Fuel Storage and Distribu-tion. Leaking underground storage tanks, spillages at distribution depots andfrom road accidents can lead to pollution of soils and aquifers with petrol anddiesel fuels. It is possible that around 30% of filling stations in the UnitedKingdom may be causing some subsurface pollution though leakages fromunderground storage tanks. In view of the very large volumes of petroleum fuels

used, this source must account for a high proportion of soil pollution byhydrocarbons. However, despite their ubiquitous occurrence, hydrocarbons aremore readily degraded in soils and pose less of a toxicological risk than organo-micropollutants such as PAHs, PCBs, dioxins and many pesticide derivatives.However, Pb-containing petrol will continue to pose a long-term Pb contam-ination hazard.

(b) Disposal of Used Lubricating Oils. In addition to hydrocarbons andpowdered metal, used lubricating oils contain PAHs. Some do-it-yourself carmechanics sometimes dispose of used motor oils onto garden soils, and landaround garages, farm yards and scrap yards can be polluted with this material.

(c) Leakage of Solvents from Industrial Sites. Hydrocarbon solvents areused widely in industry and spillages/leaks into soils frequently occur. Inaddition to specialist manufacturers and distributors of solvents, an impor-tant source of pollution by these compounds are manufacturers of semicon-ductors and other electronic components which use solvents for degreasing.Organic solvents are DNAPLs which, although having a very low watersolubility, can cause serious groundwater pollution problems. Pollution ofgroundwater (in aquifers) necessitates remedial actions, such as 'pump andtreat' processes, including air stripping to maximize the volatilization of thesecompounds.

(d) Coal Stores. Coal is a solid form of hydrocarbon and the main hazardassociated with it is the risk of fires. Coal mines, and the sites of former coalstorage facilities in factories, railway depots and similar places are likely tocontain significant amounts of coal mixed into the soil which could constitutea combustion hazard. Some coals can contain significant amounts of ironpyrites (FeS2) which undergoes oxidation when exposed to the air resulting inthe formation of iron hydroxides and sulfate anions. These anions have amajor acidifying effect on soils which can result in any heavy metal contami-nants becoming more mobile and bioavailable. If the acidification is verypronounced, decomposition of clay minerals occurs at around pH 4 whichresults in the formation of free Al3 + and Al(OH)2

+ ions which are toxic tomost plants.

14.2.3 Toxic Organic Micropollutants (TOMPs) (also called PersistentOrganic Pollutants (POPs)) (see also Chapter 17)

The most commonly encountered toxic organic micropollutants include: poly-cyclic aromatic hydrocarbons (PAHs), polyheterocyclic hydrocarbons (PHHs),polychlorinated biphenyls (PCBs), polychlorinated dibenzodioxins (PCDDs),polychlorinated dibenzofurans (PCDFs) and pesticide residues and metabolites.Many of these organic pollutants are discussed in more detail elsewhere in thisbook.

Pesticides. These comprise a very large range of different types of organicmolecules which are used with the intention of destroying pests of various types,including: insects, mites, nematodes, weeds, and fungal pathogens. The types ofcompounds used as pesticides include (examples are given in parenthesis):

Insecticides: Organochlorines (DDT, BHC)Organophosphates (Malathion, Parathion)Carbamates (Aldicarb)

Herbicides: Phenoxyacetic acids (2,4-D, 2,4,5-T)Toluidines (Trifluralin)Triazines (Atrazine, Simazine)Phenyl ureas (Fenuron, Isoproturon)Bipyridyls (Diquat, Paraquat)Glycines (Glyphosate)Phenoxypropionates (Mecoprop)Translocated carbamates (Barban, Asulam)Hydroxyl nitriles (Ioxynil, Bromoxydynil)

Fungicides:Non-systemic

Inorganic and heavy metal compounds (Cu in Bordeaux Mixture)Dithiocarbamates (Maneb, Zineb)Phthalimides (Captan, Captafol)

SystemicAntibiotics (Cycloheximide, Blasticidin-S)Benzimidazoles (Carbendaim, Benomyl, Thiabendazole)Pyrimidines (Ethirimol, Triforine)

As a consequence of the variety of compounds involved, there are majordifferences in their behaviour in soil and toxicity to plants, animals, soilorganisms and humans. Many pesticides break down into toxic derivatives andmay cause phytotoxicity problems in sensitive crops. The most serious problemsassociated with pesticide pollution of soils are the contamination of surface andgroundwaters and entry into the food chain through crops or livestock.

Typical rates of pesticide application in agriculture are 0.2-5.0 kg ha"1 butfrequently higher rates of some pesticides may be used for non-agriculturalpurposes, such as weed clearance on rail tracks and urban paths.7 In the UnitedKingdom, the total tonnage of pesticide active ingredient used decreased by20% between 1980 and 1990 but the area treated with these substances increasedby 9%.8 Generally, less than 10% of the pesticide reaches its intended target; theremainder may reside in the soil, some will be volatilized and some will beleached through soils to groundwater or via under drainage to water courses.Most pesticides have water solubilities greater than 10 mg L~l and are thereforehighly prone to leaching through soils. The half-lives of many pesticidecompounds in fertile soils range from 10 days to 10 years and so, in manycases, there is sufficient time for some leaching to occur. Atrazine with a half-lifeof 50-100 days gives rise to widespread groundwater contamination. Theconcentrations of soil-acting pesticides in the soil solution are thousandsof times greater than the EC guideline concentration for potable waters(0.1 jug L - 1 per compound, total concentration 0.5 /ig L - 1) and so there is a

strong probability of groundwater contamination above EC limits.7 (See alsoChapter 1.)

14.2.4 Other Industrial Chemicals (see also Chapter 1)

It is estimated that between 60000 and 90 000 chemicals are in currentcommercial use and thousands of new compounds are being brought into use(and dispersed in the environment) each year. Although not all of theseconstitute potential toxicity hazards, many will cause pollution of soils as aresult of leakage during storage, from use in the environment, or from theirdisposal either directly, or from wastes containing them. Apart from industrialuses, a wide range of chemicals are used in domestic products and so their use isover a much larger geographical area and their disposal is less controlled thanmost industrial chemicals (which are subject to strict regulations).

The total world production of hazardous and special wastes was 338 x 106 tin 1990.9 Although the Red, Black and Grey Lists of hazardous chemicalscontain a large number of priority substances which can pollute soils, only a fewexamples can be given in Table 14.1.

14.2.5 Nutrient-rich Wastes

(a) Sewage Sludges (also called Bio solids) (see also Chapter 5). These are theresidues from the treatment of wastewater and large quantities are producedworldwide (6.3 x 106 t in the original 12 countries of the European Union in1990 and 5.4 x 106 t dry solids in the USA). This sludge has usually beendisposed of onto agricultural land (43% of total in UK and 22% in the USA),into the sea (30% in the UK), landfilled or incinerated. In the European Union,

Table 14.1 Priority hazardous chemicals (based on the UK Red List and ECLists I and II)

Mercury and its compounds DichlorvosCadmium and its compounds 1,2-Dichloroethaney-Hexachlorocylohexane (Lindane) TrichlorobenzeneDDT SimazinePentachlorophenol and its compounds Organotin compoundsHexachlorobenzene Cyanide, FluoridesHexachlorobutadiene TrifluralinFenitrothion Azinphos-methylAldrin, Dieldrin Organophosphorus compoundsEndrin EndosulfanCarbon tetrachloride AtrazinePolychlorinated biphenylsPersistent mineral oils and

hydrocarbons of petroleum

disposal at sea was banned from December 1998 and many other countrieshave discontinued oceanic disposal. As a consequence of this, the otherdisposal routes are being used to a much greater degree and disposal to landis increasing. Sewage sludge is a valuable source of plant nutrients (especially Nand P) and a useful source of organic matter which has beneficial effects on soilaggregate stability. However, its value is somewhat diminished by its content ofpotentially harmful substances which include heavy metals, especially Cd,Cu, Ni, Pb and Zn, and organic pollutants. The most important POPs insewage sludges include: (a) halogenated aromatics, e.g. polychlorinated bi-phenyls (PCBs), polychlorinated terphenyls (PCTs), polychlorinated naph-thalenes (PCNs) and polychlorinated benzenes, polychlorinateddibenzodioxins (PCDDs), (b) halogenated aliphatics, (c) polycyclic aromatichydrocarbons (PAHs), (d) aromatic amines and nitrosamines, (e) phthalateesters, and (f) pesticides.10 Sewage sludges can also contain some pathogenicorganisms which were not destroyed during the sewage treatment. Followingconcerns about the transmission of these pathogens to humans through foodcrops, an additional sterilization stage is being introduced into sewage treat-ment to overcome this problem.

(b) Livestock Manures. These contain large amounts of N, P and K and arevaluable sources of these nutrients for crops but they can also contain residuesof food additives which can include As, Cu and Zn, veterinary medicines fed topigs and poultry and some pathogens.

14.2.6 Radionuclides (see also Chapter 18)

Nuclear accidents like those at Windscale (UK) in 1957 and Chernobyl(Ukraine) in 1986 resulted in many different radioactive substances beingdispersed into the environment. The greatest long-term pollution problem isconsidered to be caused by 137Cs which has a half-life of 30 years and behaves ina manner similar to K in soils and ecosystems. Atmospheric testing of nuclearweapons dispersed large amounts of 90Sr which has a half-life of 29 years andbehaves similarly to Ca in biological systems and poses a hazard to humansbecause it is stored in the skeleton.

14.2.7 Pathogenic Organisms

Soils can be contaminated with pathogenic organisms (bacteria, viruses, para-sitic worm eggs) from various sources, including the burial of the dead bodies ofanimals and humans, manures and sewage sludges. The soil can act as areservoir of these pathogens which can reach groundwater, can infect livestockand humans through soil particles consumed directly by children or onunwashed hands or attached to herbage and vegetables. The latter is the mainreason for the introduction of the sterilization of sewage sludges which areapplied to agricultural land.

14.3 TRANSPORT MECHANISMS CONVEYING POLLUTANTS TOSOILS

Pollutants reach soils by four main pathways:

• atmospheric deposition of particulates (washout or dry deposition) (seeChapter 7),

• sorption of gases (e.g. volatile organic compounds) from the atmosphere,• fluvial transport and deposition/sorption from flood waters,• placement (agricultural amendments, dumping, injection, surface spread-

ing etc.).

Fluvial transport is important in land subject to flooding. This has been animportant pollution pathway in the United Kingdom in areas of metalliferousmining in the nineteenth century. Before pollution controls were introduced in1876, Pb-Zn mines discharged waters from ore dressing operations directly intostreams and rivers. This led to the alluvial soils in most flood plains of riversdraining mining areas being severely contaminated with Pb, Zn and othermetals.11 Soils on the flood plains of many major rivers in the world which drainindustrial and urbanized areas have been significantly contaminated with adiverse range of substances through flooding.

Volatilization involves a substance changing from a liquid to a gas and this isa very important mechanism by which many organic compounds becomedispersed in the atmosphere and, conversely, sorbed from the atmosphere ontosoils or plants.

Placement of pollutants can occur in many ways; the most obvious being thespreading of wastes, such as sewage sludges or metal-rich manures from pigs orpoultry. Phosphatic fertilizers can also contain significant concentrations of Cdand U contaminants and have been at least partially responsible for thesignificantly elevated concentrations of these elements in agricultural soils inmany parts of the world. The spraying of pesticides onto crops and soils is a goodexample of placement although they may be further dispersed in the environ-ment afterwards. Industrially contaminated land with its associated demolitionof old buildings and manufacturing plant, the redevelopment of sites, leakages ofstored chemicals, accumulations of wastes (as well as fires and even explosions)provides other examples of the placement of contaminants onto soils.

14.4 THE NATURE AND PROPERTIES OF SOILS RELATED TO THEBEHAVIOUR OF POLLUTANTS

14.4.1 The Nature of Soils

Soil is the geochemically and biochemically complex material which forms atthe interface between the atmosphere and the earth's crust and is highlyheterogeneous in composition and spatial distribution. Soil comprises a mixtureof mineral and organic solids, permeated by voids containing aqueous andgaseous components and a microbial biomass. Soils are usually differentiated

vertically into a series of distinctive layers, called 'horizons', which differ bothmorphologically and chemically from the layers above and below them. Thesehorizons collectively form the soil profile (or pedon) which is the unit ofclassification of soils. Soil formation results from interactions between theweathered geological material on which the soil has formed, and climaticconditions, vegetation cover, landscape position and the time over which thesoil has been forming. Soil formation is a dynamic process and major changes inany of the environmental factors (such as climate, drainage or vegetation) willresult in changes in the nature of the soil horizons. As a result of the wide rangeof rock types and environmental conditions around the globe, soils differmarkedly in physical, chemical and biological characteristics. Nevertheless,there are several properties which most soils have in common which relate to thebehaviour of pollutants.

All soils contain humus which is highly polymerized organic materialsynthesized by microorganisms from the decomposition products of deadplant material. The organic matter contents of most soils lie in the range 0.1-10% but peaty soils can contain more than 70% organic matter. Soils in hot, dryclimates tend to contain much lower amounts of organic matter than soils inhumid and cooler regions.

Soils contain varying amounts of different primary and secondary minerals.Primary minerals occur in unweathered fragments of igneous rock either fromthe parent material, or erratic stones deposited by ice or water, and theirweathering provides plant nutrients and gives soils distinctive colour andchemical properties. Secondary minerals have been synthesized from theproducts of weathered rocks and can include hydrated oxides of Fe, Al andMn (Fe oxides give soils their characteristic brown colour) and clay mineralswhich are thin layered forms of aluminium silicates. These secondary minerals(clays and precipitates of Fe oxides) and humus together form the colloidalfraction which provides soils with significant sorptive properties and is veryimportant in determining the fate of pollutants (and plant nutrients) in soils.Other secondary minerals, such as calcium carbonate, can be precipitated fromgroundwater in semi-arid and arid climates, but also occur as part of the parentmaterial of soils which have developed on limestones in humid climates. Ingeneral, as soils mature their content of primary minerals gradually decreasesuntil they contain only secondary minerals. Tropical soils tend to containmainly Fe and Al oxides and kaolinite (clay mineral).

It is important to stress that the soils found at many heavily contaminatedsites can differ markedly from natural (pedologically derived) soils. Very often,the use of rubble for hard core to act as foundations for buildings results in thepedological constituents being heavily diluted by other material, such as highlyalkaline concrete, mortar, wastes and product residues. These extraneousmaterials will have a major effect on the behaviour of pollutants at these sitesboth chemically, such as adsorption/desorption, and physically, especially withregard to porosity and the movement of chemicals downwards towards thegroundwater. A generalized comparison of contaminated agricultural andindustrial soils is given in Section 14.4.3 of this chapter.

14.4.2 Chemical and Physical Properties of Soils Affecting the Behaviour ofPollutants

Space does not allow a detailed consideration of the chemistry related to thebehaviour of pollutants in soils. (Readers are referred to texts by McBride,12

Sposito13 and White14 and others for a more detailed coverage.) However, themain considerations are the factors which control the sorption and desorptionof ionic and uncharged compounds in soils. Most heavy metals exist in the soilpredominantly as cations but important elements such as: As, B, Mo, V, Sb andSe occur as anions. Although some pesticides are ionic, most organic contami-nants are uncharged and tend to be hydrophobic.

Sorption of pollutants can be by several mechanisms, including:(a) Non-specific Cation and Anion Adsorption (also referred to as cation or

anion exchange). Cations are adsorbed onto negatively charged surfaces onthe soil colloidal fraction. This comprises the aluminosilicate clay minerals,hydrous oxides of Fe and Mn, and humic organic material. Anions are adsorbedon positively charged sites on the colloidal fraction which are mainly contrib-uted by hydrous oxides of Fe. These Fe oxides have a variable charge which isdependent on the pH of the soil. The point of zero charge (PZC) on the pureoxides is around pH 8.0 but when present in the soil it is around pH 7.0. Belowthis PZC (pH) value these oxides are positively charged and adsorb anions, butabove the PZC they are negatively charged and adsorb cations. Soil organicmatter also has a pH dependent surface charge but this tends to be predomi-nantly negatively charged above pH 2.5. The negative charges result from thedeprotonation of carboxy and phenolic groups on the surfaces of humicpolymers. Clay minerals possess permanent negative charges on their surfacesdue to charge imbalances where isomorphous substitution of a major constitu-ent has occurred in the crystal lattice of the mineral during its formationsubstitution {e.g. Al3+ replacing Si4 + , or Mg2+ replacing Al3 + ).

The soil pH is the most important single physico-chemical parametercontrolling the sorption-desorption of ions in soils. The normal range of pH insoils throughout the world is 4-8.5 owing to the buffering by Al at the lower endand by CaCO3 at the upper end. In general, soils in humid regions, which aresubject to leaching of bases, tend to have a pH range of 5-7 (although organicupland soils may have values of less the 4.0). Soils in arid regions tend to havepH values of 7-9 owing to the accumulation of CaCO3 and other salts in thepredominantly evaporating moisture regime.

Cation exchange involves a higher concentration of cations being held in thezone of attraction of the negative charges on the soil colloid surfaces. Ingeneral, it is found that the Cation Exchange Capacity (CEC) of a soil increaseswith a rise in pH, at least up to pH 7.0. These cations are in a dynamic state offlux dependent on the nature of the charged surface, the nature of the ion (itsvalency and hydrated size) and its concentration and the concentrations ofother ions in the soil solution. There is a general order of replacement wherebyit is found that those ions which are most strongly attracted replace othercations in the zone of attraction. This order tends to vary with the adsorbent

surface, but for the clay mineral illite the order of increasing selectivity wasgiven by Bittel and Miller:15

Mg > Cd > Ca > Zn > Cu > Pb.

Anions are retained on positively charged surface sites and these tend to ariseas a result of pH values below the PZC of hydrous oxides of Fe, Mn and Al, andby 'ligand exchange' where a surface complex forms between an anion and ametal, usually Fe or Al in a hydrous oxide or a clay mineral. The sorptivecapacity of a soil (i.e. the Cation or Anion Exchange Capacity) is expressed inunits of centimoles of charge per kg of soil (cmolc kg"1).

(b) Specific Adsorption. This occurs where metals such as Cd, Cu, Ni andZn form complex ions (MOH + ) on surfaces that contain hydroxyl groups,especially hydrous oxides of Fe, Mn and Al. These complex ions do not undergocation exchange but can be displaced by strong acids or complexing agents.Specific adsorption is strongly pH dependent and is responsible for the retentionof much larger amounts of metals than cation exchange. The general order ofincreasing strength of specific adsorption of heavy metals was given byBrummer:16

Cd > Ni > Co > Zn » Cu > Pb > Hg.

(c) Organic Complexation of Metals. This occurs when the solid statehumic material binds metals into a ring type structure, most commonly achelate. Humic compounds with hydroxy, phenoxy, and carboxy reactivegroups can form coordination complexes with metals. The stability constantsof chelates vary for different elements and different ligands. In general,the stability constants of humic complexes tend to decrease in the order:Cu > Fe = Al > Mn = Co > Zn. Organic ligands can render many metals,especially Cu and Pb, relatively immobile. However, low molecular weightcomplexes of metals, not necessarily of humic origin, tend to be soluble and canprevent metals from being adsorbed onto soil surfaces and thus render themmore mobile and possibly more available for uptake by plant roots.

(d) Sorption of Organic Contaminants on Humic Material. This is the mainmechanism by which non-polar, hydrophobic organic molecules are bound insoils. This may be by physical means or by chemical bonding.

(e) Chemisorption of Elements. This occurs when the element is incorpo-rated into the structure of the compound. The most common example of this iswhen metals, such as Cd, replace Ca in the mineral structure of calcite (CaCO3).

(f) Co-precipitation of Elements. This is the simultaneous precipitation of achemical agent in conjunction with other elements. The elements typically foundco-precipitated with secondary minerals in the soil include:17

Fe oxides: V, Mn, Cu, Zn, Mo;Mn oxides: Fe, Co, Ni, Zn, Pb;Calcite: V, Mn, Fe, Co, Cd;Clay minerals: V, Ni, Co, Cr, Cu, Pb, Ti, Mn, Fe.

(g) Precipitation. This occurs when the concentrations of metal andaccompanying ions exceed the solubility product of compounds such asCdCO3, CdS and Pb^PO^Cl.

Those contaminants which are sorbed tend to be held against leaching andare less readily available for uptake by crops than those remaining in unboundforms within the soil. Volatilization of organic molecules and methylated formsof certain inorganic elements (As, Hg and Se) is also important. Some of thevolatile compounds lost to the atmosphere may be decomposed by UV light(photolysis) or can also be sorbed onto the waxy cuticle of plant leaves andpossibly enter the food chain.

The relative balance of reduction and oxidation (redox status) of a pollutedsoil plays an important role in the behaviour of some pollutants. Firstly, it willdetermine whether there will be an appreciable concentration of Fe and Mnoxides present. These are especially important for the sorption of As, Mo, andCd. Secondly, some elements such as Cd, which readily form insoluble sulfideprecipitates (CdS) under strongly reducing conditions, will be very immobile inwaterlogged soils. However, if these soils become aerated due to drainage anddrying out, the sulfide will oxidize to form sulfuric acid and so the liberatedCd2+ ions will be highly mobile and available for uptake. This occurs incontaminated paddy soils used for growing rice.

Organic pollutants bearing electrostatic charges will also be adsorbed ontooppositely charged sites on the soil colloids {e.g. the herbicides paraquat anddiquat are strongly cationic). However, many organic pollutants are non-polarand uncharged and are normally bound to the soil organic matter by physicalmechanisms, such as hydrophobic bonding. In soils which have received sewagesludge, the sludge material acts both as a source of several organic and inorganicpollutants and also as the major adsorbent for them.

The sorptive properties of a soil for both inorganic and organic pollutants canbe described mathematically; in most cases sorption is found to fit either theLangmuir or the Freundlich adsorption isotherm equations but space does notpermit its coverage here.

14.4.3 Comparison Between Soils of Rural and Industrial Sites

Many contaminated soils tend to occur on derelict or active industrial sites andthese may differ greatly from a normal (pedological) soil in both constituentsand physico-chemical properties. A summary of the differences which may beencountered between soils in rural and industrial sites in Northern Europeanand other temperate climatic zones is given in Table 14.2.

14.4.4 Degradation of Organic Pollutants in Soils

Organic pollutants can be degraded in soils and, possibly, aquifers by either (a)non-biological mechanisms, including: hydrolysis, oxidation/reduction, photo-decomposition and volatilization, or (b) microbial decomposition ('biodegrada-tion').

Table 14.2 Comparison of typical soils from rural and industrial sites withregard to factors affecting the behaviour of pollutants

Parameter Rural soils Industrial sites

pH 5-8 2-13Organic matter (%) 1-10 < 1Clays and oxides Abundant VariablePlant nutrients Abundant LowCones of CP, SO4

2 ~ Low HighConcrete/brick rubble Absent HighHeavy metals < 0.05-0.1( < ̂ sp) < 1 % (> Ksp)Organic pollutants Low Often highSpatial heterogeneity Low-moderate Very highToxicity hazards Food chain Inhalation

Soil ingestion Soil ingestionEcotoxicity Ecotoxicity

Potable ground and surface waters

When a pollutant chemical comes into contact with microbial colonies inbiofilms lining the voids within the soil matrix, various extracellular enzymeswill be secreted. These may partially degrade the chemical which is thenabsorbed into the microbial cell where intracellular enzymes may catalysefurther decomposition reactions which bring about the release of energy andnutrients. Several species of bacteria and different enzymes may be involved atthe same time and bring about a sequence of degradation steps producingincreasingly simpler compounds which are either used in 'anabolic' (cellbuilding) or 'catabolic' (energy releasing) processes.18

The greatest energy yield is obtained when the catabolic decomposition of anorganic substrate by microorganisms occurs in the presence of free oxygen, thenext in order of decreasing energy yield is the reduction of Fe and Mn oxides,then the reduction of NO 3 " , followed by SO4

2" and finally the reduction ofCO2 to CH4.

Although most organic molecules are biodegradable, the rate at which thistakes place can vary greatly. The susceptibility of an organic molecule tobiodegradation depends largely on its structure. Compounds which are mostresistant to degradation tend to have halogen atoms in their structure, especiallya large number of halogens, or which are highly branched, have a lowsolubilility in water or an atomic charge difference.18 Straight chain aliphaticcompounds are easily degraded, but unsaturated aliphatics are less degradablethan saturated forms. Simple aromatic compounds are usually degradable byseveral ring-cleavage mechanisms, but the presence of halogens stabilizes thering and makes the compound less readily degradable. In general, unless thereare specifically adapted microorganisms present, there will be a tendency for themore readily degradable molecules to be catabolized first. Adaptation can occurin a population of microorganisms as a result of a selection pressure. Individualorganisms with the ability to degrade or tolerate a toxic compound may arise as

a result of mutations and gene transfer and these individuals will have acompetitive advantage if they can utilize an abundant supply of an organicpollutant as a source of energy and nutrients.18 There is a time lag whileadaptation occurs before there are sufficient microorganisms present whichhave the ability to degrade the pollutant; this tends to give a two-phase curve forconcentration in the soil against time. The first phase is normally a steepdecrease in concentration with time due to physical processes such as volatiliza-tion. The second phase is slower but goes on until the concentration approachesclose to zero when the microorganisms have become adapted. 'Co-metabolism'can also occur and this is the term applied to the degradation of the pollutant(secondary substrate) by enzymes secreted by microorganisms to degradeanother substance (the primary substrate).

Biodegradation requires appropriate conditions for the growth of the micro-organisms which include adequate moisture, a temperature between 10 and45 0C, a pH which is preferably in the range 6-8, and a supply of macronutrients(N and P). The redox conditions will depend on the types of microorganismsand pollutants involved. The anaerobic process of reductive dehalogenation canbring about the degradation of some halogenated pollutants such as perchloro-ethylene under reducing conditions. Some pollutant chemicals are highly toxicto soil microorganisms and so the inability to degrade may be linked to the lackof tolerance to the toxicity.18

The organochlorine pesticides have been used for more than 50 years andare regarded as the most persistent of all groups of pesticides. The order fordecreasing persistence is: DDT > dieldrin > lindane (BHC) > heptachlor >aldrin with half-lives of 11 years for DDT down to 5 years for aldrin.19 Themost persistent organic pollutants of all in soils are probably the more highlychlorinated PCBs and dioxins (PCDDs). However, the persistence of achemical in any soil is determined by the over-all balance between itsadsorption onto soil colloids (usually organic matter), the extent of volatiliza-tion, uptake into or binding on, plant roots, and transformation/biodegrada-tion processes. This balance depends on the nature of the pollutant, itsconcentration, the soil organic matter content, the soil pH and redox status,the available moisture, general soil fertility (level of nutrient supply, activity ofsoil microorganisms) and the time taken by microorganisms to adapt to thenew pollutant substrate.

Bioremediation of soils contaminated by organic chemicals exploits theprocesses of biodegradation which would occur naturally but often at a muchslower rate. The natural degradation processes are promoted by optimizingconditions for the soil microorganisms with regard to factors such as pH,aeration, moisture content and nutrient supply.

14.5 THE CONSEQUENCES OF SOIL POLLUTION

Soil pollution can give rise to toxicological problems in humans, livestock,crops, ecosystems and also in damage to structures and services. This canhappen directly through contact with the soil itself, or indirectly through soil

pollution causing groundwaters to become contaminated and these giving riseto toxicological and structural problems. As a consequence this pollution canrestrict the uses to which land is put and there is therefore a need to be ablepredict problems by the use of soil quality standards and guidelines. These arebased on the analysis of soils for the inorganic and organic contaminantsconsidered to constitute the greatest hazards.

14.5.1 Soil Analyses and Their Interpretation

14.5.1.1 Methods of Soil Chemical Analysis. Analysis of soils for contami-nants involves the collection of representative samples from suspected pollutedsites and local controls. These samples are subsequently prepared for analysis,usually by drying and grinding followed by sieving. Analytical proceduresinvolve either determination of the total concentration of the pollutant or apartial extraction procedure which can be correlated against a critical concen-tration. Space does not permit a detailed description of analytical procedurescommonly used, but a brief summary is given in Table 14.3.

Table 14.3 Summary of analytical procedures for soil pollutants

Pollutant Analytical procedure

Heavy metals (in cone, acid Flame atomic absorption spectrophotometry (FAAS),digests or partial extractants) inductively coupled plasma atomic emission

spectrophotometry (ICP-AES) or ICP-massspectrometry (ICP-MS)

As, Bi, Hg, Sb, Se, Sn, and Te Hydride generation atomic absorption(in acid digests or partial spectrophotmetry (HGAAS) using sodiumextractants) borohydride in NaOH

Borate (water soluble) ICP-AES (using quartz or plastic apparatus instead ofPyrex)

Organic pollutants (dissolved Gas chromatography with flame ionization detectorin appropriate organic (GC-FID) or with electron capture detectorsolvents) (GC-ECD) or GC combined with mass spectrometry

(GC-MS)

Cyanides Colourimetrically - using pyridine pyrazalone (blue)reaction

Sulfates (water soluble) Elution through exchange column followed bytitration with NaOH (or ion chromatography)

Sulfates (total) Dissolution in HCl, precipitation of Al and Fefollowed by gravimetric determination using BaCl2

Chlorides (total - in HNO3) Volhard's Method - back titration with ammoniumthiocyanate after initial precipitation with AgNO3(or ion chromatography)

Adapted from Alloway.20

Critical values for soils and sediments (jig g ])

Substance

MetalsArsenicCadmiumCopperMercuryNickelLeadZinc

Inorganic pollutantsCyanides (free)Cyanides (complex) (pH < 5~Cyanides (complex) (pH > 5Sulfur

Polycyclic aromaticsPAHs (total of 10)NaphthaleneAnthraceneBenzo[#]pyrene

Chlorinated hydrocarbonsCH (total)PCBs (total of 7)Chlorophenols (total)

Aromatic compoundsAromatics (total)BenzeneToluenePhenols

Other organic compoundsPyridineGasoilMineral oil

1986 values

A

201

500.5

5050

200

1) 5) 5

2

10.10.10.05

0.050.050.01

0.10.010.050.02

0.120

100

B

305

1002

100150500

10505020

205

101

111

70.531

5100

1000

C

5020

50010

500600

3000

100500

200

20050

10010

101010

705

3010

60800

5000

1994 values

TargetValue

290.8

360.3

3585

140

15

1155025

0.02

0.050.050.05

0.01

50

InterventionValue

5512

19010

210530720

20650

40

110

1130

1

1

500

Table 14.4 The Netherlands guideline values for selected pollutants in soils andsediments

1986 Values: A = reference value, B = test requirement, C = intervention value (clean-up!).21

1994 Values: Target Value is the concentration which ought to be aimed for in the longer-term;Intervention Value = action to clean-up (Netherlands Ministry of Housing, Spatial Planning andEnvironment, 1994).22

14.5.1.2 Soil Quality Standards for the Interpretation of Soil AnalyticalData. Having obtained concentrations of pollutants in soils, the interpretationof the data will be dependent upon national or international guidelineconcentrations or legal limits. These critical concentrations vary betweencountries and states. Perhaps the most widely known are the values used in theNetherlands ('Dutch Values') but these are relatively conservative being basedlargely on ecotoxicological data (which tend to have lower critical concentra-tions than human toxicological values) and were originally based on theprinciple of 'multifunctionality of use' whereby soil should be maintained, orcleaned up to a standard which will allow it to be used for any purpose includinggrowing food crops. This is not universally accepted, and other countries, suchas the United Kingdom, have used the principle of 'fitness for use'. This meansthat land to be used for domestic gardens where food crops will be grown shouldhave much lower concentrations of hazardous chemicals than land to be builton for non-residential purposes. However, owing to the problems of cost andtime necessary to clean up soils, a more 'fitness for use' approach has also beenadopted in the Netherlands. Values are given in Table 14.4 for selectedpollutants. This is not the complete list and the values are given for a standardsoil containing 10% organic matter and 20% clay. The A, B, and C values,originally introduced in 1986, have been superseded by Target Values andIntervention Values and the latter have been revised (usually downwards).

In the United Kingdom, the guideline values used are based on the ICRCLprovisional document of 1987.23 In due course, these will be replaced by adifferent set of values obtained from a quantitative risk assessment model(CLEA - Contaminated Land Exposure Assessment) based on exposure path-ways. The United Kingdom ICRCL values are based on fitness for purpose andhave maximum total concentrations of elements in domestic gardens of: 3 figg~x Cd, 130 fig g~x Cu and 500 fig g~x Pb. Space does not permit the variousguideline values to be given in detail and readers wishing to find out more aboutthis are recommended to consult the most recent values for their own countries.

14.5.2 Hazards Associated with Soil Pollutants

A wide range of possible harmful effects can be associated with polluted soilsand the more important types of possible hazard are summarized in Table 14.5.

14.5.3 Remediation of Contaminated Soils

Where the concentrations of pollutants exceed statutory or guideline qualitystandards there is a need to make a careful assessment of the risks at theparticular site and in many cases it will be necessary to carry out some form ofclean-up or remediation. In many cases, this will involve excavating the mostpolluted soil and either disposing of it safely in a licensed landfill (often referredto as 'dig and dump'), or cleaning it up off-site {ex situ remediation). However,with the high cost of haulage and of landfilling special wastes, there is increasing

Table 14.5 Examples of the hazards caused by soil pollutants

Hazard Pollutants

(1) Direct ingestion of contaminated soil As, Cd, Pb, CN~, Cr6 + , Hg, coal tars(by children, animals, gardeners and (PAHs), PCBs, dioxins, phenols, pathogenson unwashed vegetables)

(2) Inhalation of dusts and volatile Organic solvents, radon, methyl forms ofcompounds from contaminated soil Hg, metal-rich particles, asbestos

(3) Uptake by plants and movement As, Cd, 137Cs, Hg, Pb, 90Sr, Tl, PAHs,through the food chain to livestock various pesticidesand/or humans

(4) Phytotoxicity SO42", Cu, Ni, Zn, Cr, B, and CH4

(5) Toxicity to soil biota (especially Cd, Cu, Ni, Znmicrobial biomass)

(6) Deterioration of building materials SO42", SO3

2", C P , tars, phenols, mineraland services oils, organic solvents

(7) Fires and explosions CH4, S, coal and coke dust, residues ofexplosives {e.g. TNT), tar, rubber, plastics,high calorific value wastes {e.g. old landfills)

(8) Contact of people with contaminants Tars (PAHs), phenols, asbestos,during demolition or excavation of radionuclides, PCBs, TCDDs, pathogenssites

(9) Contamination of surface and CN~, SO42", metal salts, organic (LNAPLs

ground waters compounds and DNAPLs) surfactants, farm wastes,pesticides

Adapted from Becket and Sims,24 ICRCL23 and Alloway.20

use made of in situ remediation methods. The main types of remediationmethods in use are briefly outlined in Table 14.6.

In the case of organic liquid pollutants, these methods include: (a) Contain-ment - where the contamination is retained by physical barriers (such astrenches filled with impermeable clay, such as bentonite) or hydraulic barriersbased on maintaining fluid pressure differentials by the extraction or injection ofwater to confine the contaminant plume in aquifers (permeable undergroundstrata); (b) 'Pump and Treat' technique applied to contaminated groundwaterswhich are pumped up to the surface from specially drilled wells and then treatedto remove the contaminants and the cleaned water is then usually returned tothe aquifer. In the case of volatile organic compounds, such as chlorinatedsolvents, this can include bubbling air through the extracted water to enhancethe volatilization of the organic pollutants (called 'air sparging'). When water isinjected into contaminated soil to dissolve soluble contaminants and thenextracted, this technique is referred to as 'in situ soil washing' which can besuccessfully applied to the removal of a wide range of contaminants (not allnecessarily water soluble), including salts such as sulfates. In situ enhancedrecovery techniques include soil vapour extraction by soil venting, where air is

Table 14.6 Techniques available for the remediation of contaminated land

Method Contamination problem

Civil engineering based methods

(1) Excavation and safe disposal (usually 'Hot spot' contamination atto landfill) concentrations too high for safe in situ

treatment, especially inorganic chemicalssuch as heavy metals

(2) Physical containment - using in- Organic NAPLs, landfill leachatesground barriers and covers

(3) Hydraulic controls Supporting containment and/or fortreatment of contaminated surface orgroundwaters

Process-based remediation

(1) Thermal treatment - to remove, Incineration of soils contaminated withstabilize or destroy contaminants POPs such as PAHs, PCBs and PCDDs

(2) Physical treatment to separate Soil washing to remove sulfates, PAHscontaminants from soils or different metals and other pollutants adsorbed onfractions of contaminated media silt and clay particles(usually ex situ)

(3) Chemical treatment - reactions to Dechlorination of PCB contaminatedremove, destroy or modify soils, acid leaching of metal-contaminatedcontaminants (in situ, or ex situ) soils

(4) Biological treatment - using Organic contaminants, includingmicroorganisms to remove, destroy or hydrocarbons, pesticides, solvents, tars,modify contaminants (in situ or ex PAHssitu)

(5) Stabilization/solidification - where Heavy metals, persistent organicchemicals are rendered less available pollutantsto receptors (in situ or ex situ)

Adapted from Harris and Herbert.25

pumped into soil which has sufficient macropores to provide adequate airpermeability and collected from extraction wells from which the volatilecompounds can be either incinerated on site or adsorbed onto activated carbon.

Soil pollutants adsorbed to the soil solids are generally more difficult to treat.In the case of organic compounds, bioremediation, which involves optimizingthe conditions for the microbial decomposition of the chemicals, is increasinglybeing employed. For heavy metals, chemical treatment involving the use ofextractants, such as acids or chelating agents, is possible but not widely used.Excavation and disposal to a licensed landfill is the most common approach. Arelatively recent development is phytoremediation, which uses plants with ahigh capacity to accumulate metals to deplete the plant-available fraction ofmetals in the soil. However, although several hyperaccumulator plant specieshave been identified and show considerable potential, most do not produce

sufficient biomass to effectively remove significant amounts of metals fromcontaminated soil. Nevertheless, it is likely that this method will be developedfor use on soils with low to moderate levels of pollution but severely pollutedsoils will probably still need to be removed.

Apart from the application of the various possible remediation methods, it isoften found that some organic pollutants undergo natural processes of adsorp-tion and degradation, and this is referred to as 'natural attenuation'. There is anincreasing body of evidence to show that these natural processes account for agreat deal of diminution of risk from soil pollution at many types of sites.However, it does need careful monitoring to ensure that improvements aretaking place over an appropriate period of time. The range of availableremediation techniques is exemplified by Table 14.6.

14.6 CASE STUDIES OF CONTAMINATED INDUSTRIAL LAND

14.6.1 Former Gasworks Sites

By the end of the nineteenth century most towns and cities in North WestEurope, North America and many other parts of the world had their own gas-works to produce coal gas for heating and lighting in industrial and domesticproperties. When the total number of sites, including large municipal works andsmall units in some industrial premises are considered, there are probably manythousand sites polluted by gas manufacture throughout the world. In the UnitedKingdom, there are between 2000 and 5000 gas manufacturing sites and in theNetherlands 234 former gasworks sites have been identified as being in need ofmajor remediation. The production of the gas was based on the heating of coalin a non-oxidizing atmosphere. In addition to the main products of methaneand carbon monoxide, this process also produced tars, phenols, cyanides andvarious other impurities which had to be removed from the gas and haveconsequently accumulated at gas works sites. The contamination problemsassociated with gasworks include contamination of soils and both surface andgoundwaters by tarry wastes and phenols, and cyanides. Atmospheric pollutionby volatilization of naphthalene, benzenes, acenaphthalene and cycloalkenes,thioprene, pyridine, and hydrogen sulfide from contaminated soils at gasworkssites is also reported. In addition to the chemical pollution associated with gasmanufacture, additional environmental pollution can also arise from asbestosinsulation and Pb from old paintwork.26

In the UK, the National Millennium Exhibition was housed in a domestructure on the Greenwich Peninsula in the River Thames in East End ofLondon. The site had been occupied by one of the largest gasworks in Europe(125 ha) for about 90 years until the 1970s and had become heavily contami-nated with the usual materials and waste products from gas production, plusthose of ancillary works which included chemicals, tar distillation, and a benzolplant. The remediation works required before the Millenium Dome could beconstructed and opened to the public involved: the excavation and safe disposalof > 200 000 m3 of contaminated soil material, soil vapour extraction over

4.3 ha, the washing of > 30000 m3 of sand and gravel, and an on-site watertreatment facility (Griffin27 and Barry28).

14.6.2 Sites Contaminated with Solvents

Chlorinated hydrocarbon solvents such as trichloroethene, 1,1,1-trichloro-ethane, tetrachloromethane and tetrachloroethene are widely used in industryand are frequently found as soil pollutants at industrial sites. In most cases,pollution occurred as a result of spillages and this has resulted in the chlorinatedhydrocarbons being found in the following four types of sites: (i) as isolateddroplets within the pollution plume in the groundwater (aquifer) or trapped inpools in low permeability material, (ii) dissolved in porewaters, (iii) as vapour inthe unsaturated zone (above the aquifer), and (iv) sorbed onto solid phase soilorganic matter.29 Soil gas monitoring has revealed that some derelict industrialsites with relatively low concentrations in soil have given rise to markedpollution of groundwater in boreholes below the sites at depths down to187 m. The concentrations of chlorinated hydrocarbon solvents in soil gas atderelict industrial sites were found to vary by seven orders of magnitude up to amaximum value of 2000 / igL" 1 trichloromethane.29 Volatile compounds, suchas chlorinated solvents, can be removed from groundwaters by pump and treatmethods in which air is bubbled through the water to enhance the volatilizationof the chemicals which are then trapped and destroyed.

A fire at a solvent recovery works in the village of Carrbrook, in Cheshire,England, in 1981 caused severe contamination of soils in the local area withbenzene, PCBs, and dioxins.30 Concentrations of up to 304 mg kg" 1 totalsolvents, 1160 mg kg" 1 PCBs and 168 ^g kg" 1 dioxin (expressed as TEQ =toxic equivalents of 2,3,7,8-tetrachlorodibenzodioxin TCDD) were found insoils at the site. Domestic pets kept in houses and gardens near to the site werefound to show abnormal health effects. Worst affected were guinea pigs whichdeveloped a terminal wasting disorder.30 It is generally recognized that guineapigs have the lowest tolerance of all animals tested to dioxins. Concentrations of100-1000 ng kg" 1 TCDD are considered typical for industrially contaminatedsites but the levels at the solvent recovery works site were much higher than themaximum in this range. Perhaps the worst case of dioxin pollution occurred atTimes Beach in Missouri where waste oil contaminated with distillation residuesfrom organochlorine production had been used to reduce dust problems on drysoils. This contaminated oil gave rise to concentrations of up to 33 mg kg" 1

TEQ in the soils and caused the death of horses, cats, dogs, chickens and birdsexposed to the soil. Children playing in the area developed chloracne, acharacteristic symptom of exposure to dioxins.31

14.6.3 Lead and Arsenic Pollution in the Town of Mundelstrup in Denmark

Severe Pb and As contamination was found in housing in the town ofMundelstrup Stationsby west of Aarhus in Denmark in 1987. The pollutionhad arisen from the disposal of fill from a former fertilizer factory which had

used metal-rich pyrite ore for the manufacture of sulfuric acid. Concentrationsof up to 67 562 mg kg" 1 Pb and 5481 mg kg" 1 As were found in the soil ofgardens of the houses affected. A comprehensive survey was carried outinvolving the analysis of approximately 1000 samples. The contaminated areacovered 6700 m2 and varied in depth between 0.5 and 8 m. The worst affectedarea was at the site of the original factory. It was decided to remove all soil fromaround the houses which exceeded the Danish soil quality criteria of 40 mgkg" 1 Pb and 20 mg kg" 1 As. A borrow-pit was dug to supply clean soil toreplace the 50 000 m3 of soil excavated from around the houses with thecontaminated gardens. This same pit was used as a special landfill to receivethe contaminated soil after appropriate engineering with layers of lime toprevent leaching and the whole landfill was covered by a new motorwayroundabout. Only 30 houses were affected and these had their garden soilcompletely replaced and professional landscape gardeners created new gardens.This is an example of the ideal clean-up of an urban area. It was carried out atgreat cost (32 x 106 DKK) and this may not be possible in other countries witha greater legacy of historically polluted urban sites.32 It is interesting to notethat the Danish soil quality standard of 40 mg Pb kg" 1 is extremely conserva-tive. If this value was to be used in more highly industrialized countries, such asthe United Kingdom, almost all urban soils would be classed as having excessiveconcentrations of lead.

14.6.4 Pollution from a Pb-Zn Smelter at Zlatna, Romania

Acid precipitation from a Pb-Zn smelter at Zlatna in Transylvania, Romaniahas led to obvious toxic effects on vegetation and damage to buildings within anarea of at least 50 000 ha. The smelter lies in a valley and the fumes are trappedwithin the valley. The precipitation of SO2 fumes and metal aerosol particles haskilled many plant species and this has led to massive soil erosion and associateddeterioration of the soil structure due to the loss of the surface organic matter.A layer of colluvium up to 46 cm thick has developed on the top of the soilprofiles at the base of the valley side slopes. The metals deposited from thesmelter fumes are rendered relatively highly mobile in the acid soils but SO2 isthe most destructive pollutant. The most urgently required pollution controlmeasures are scrubbers to reduce the SO2 concentrations in the smelteremissions but this would be prohibitively expensive.

14.6.5 Cadmium Pollution in the Village of Shipham in England and the JinzuValley in Toyama Province, Japan

Zinc and Pb mining was carried out around the village of Shipham in Somerset,England during the nineteenth century and the land has been left highlycontaminated with these metals and Cd which was present in considerableconcentrations in the Zn ore. Garden soils used for the growing of vegetableswere found to contain up to 360 /ig g" l Cd, 37 200 fig g " ] Zn, and 6540 fig g" l

Pb and the mean Cd concentration in almost 1000 samples of garden vegetables

was 0.25 /ug g 1 which was nearly 17 times the national average of 0.015 jig g x

Cd (in the dry matter). The highest Cd concentrations occurred in leafyvegetables such as spinach, lettuce and cabbage which contained up to 60 timesmore Cd than the same species grown locally in uncontaminated soils. Althoughthe Cd concentrations in the vegetables were relatively high, no adverse healtheffects were found in the population and this was ascribed to the low percentageof home-grown vegetables in the diet, a generally varied diet and a public watersupply which conformed to national and international standards for Cd.33

In Japan, soils used for growing paddy rice in the Jinzu Valley had also beencontaminated with Cd and other metals from mining operations upstream but,in contrast to Shipham, many people had suffered ill-health effects. Twohundred elderly women who had given birth to more than one baby had beendisabled by a Cd-induced skeletal disorder know as 'itai-itai' disease and 65women had died of this condition. The disease occurred during and immediatelyafter the Second World War when diets were more deficient in Ca and proteinthan normal. Average Cd concentrations in rice (0.7 jug g~l DM) in thecontaminated soils were ten times greater than in local controls and themaximum Cd content in rice grown in the Jinzu Valley was 3.4 jug g~l DM. Inthe paddy soils the Cd is present as insoluble CdS during the flooded period butwhen the paddy fields are drained in readiness for harvest the sulfide oxidizes torelease Cd + ions and sulfuric acid which increases the bioavailability of theCd.34 In the Shipham soils, a high content of CaCC>3 from mining waste helps toreduce the bioavailability of the Cd which was present at total concentrationsmore than a hundred times higher than in the Jinzu paddy soils.

14.7 CONCLUSIONS

Soils and associated surface and groundwaters can become polluted by a widerange of chemicals arising from many human activities and in every case thebioavailability of the pollutant will depend on: (a) the combined effects of thetype and concentration of the polluting substance; (b) the composition of thesoil or site matrix (in the case of heavily developed industrial sites), especially itsorganic matter, sand, clay and free carbonate contents; (c) the soil physico-chemical conditions (pH and redox status); (d) the genotype of the plantsgrowing on the soil (species and varieties vary greatly in their capacity toaccumulate pollutants); and (e) the climate (soil moisture status, temperature).Although these interactions can be relatively complex, there is enough informa-tion becoming available to allow predictive and risk assessment models to bedeveloped. However, much more is currently known about the behaviour ofinorganic pollutants, such as heavy metals which are easier (and less expensive)to analyse for, than many of the organic pollutants. However, analyticaltechniques for persistent organic pollutants are becoming more rapid and lessexpensive and will thus provide more data for use in predictive models. In orderto be able to monitor both naturally attenuated and remediated sites effectively,a new generation of sensors and techniques to measure concentrations ofselected organic pollutants and their degradation products on-line in the field

(such as in wells) is being developed. From the food safety aspect, it is widelyconsidered that the priority pollutants of soils and crops are dioxins and PCBsamong the organic pollutants and As and Cd out of the inorganic pollutants.35

However, in addition to these elements and substances, the more ubiquitousPAHs and BTEX compounds (benzene, toluene, ethyl benzene and xylene) andheavy metals such as Hg and Pb will continue to pose major problems inpolluted soils and/or waters which will need to be attended to.

14.8 REFERENCES

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2. K. C. Jones, Environ. PoIIuL9 1991, 69, 311-325.3. L. R. Oldeman, R. T. A. Hakkeling and W. G. Sombroek, 'World Map of the Status

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3rd edn., Blackwell Science, Oxford, 1997.15. J. E. Bittel and R. J. Miller, J. Environ. QuaL, 1974, 3, 243-244.16. G. W. Brummer, 'The Importance of Chemical Speciation in Environmental

Processes', Springer Verlag, Berlin, 1986.17. G. Sposito, in 'Applied Environmental Geochemistry', ed. I. Thornton, Academic

Press, London, 1983.18. M. D. LaGega, P. L. Buckingham and J. C. Evans, 'Hazardous Waste Manage-

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and P. L'Hermite, Elsevier, Amsterdam, 1987, p. 181.20. B. J. Alloway, in 'Understanding Our Environment', ed. R. M. Harrison, Royal

Society of Chemistry, Cambridge, 1992.21. J. E. T. Moen, J. P. Cornet and C. W. A. Evers, in 'Contaminated Soils', eds. J. W.

Assink and W. J. van den Brink, Martinus Nijhoff, Dordrecht, 1986.22. Netherlands Ministry of Housing, Spatial Planning and Environment, 'Intervention

Values and Target Values - Soil Quality Standards', Directorate-General forEnvironmental Protection, Department of Soil Protection (625) Rijnstraat 8, TheHague, The Netherlands, 1994.

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