Elsevier Encyclopedia of Geology

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Oneacre JW (1993) Subtitle D Regulations Impact on

Ground Water Monitoring. Geotechnical News 11(3):

49–52.

Oneacre JW and Figueras D (1996) Ground Water Variabil-

ity at Sanitary Landfills: Causes and Solutions, Uncer-

tainty in the Geologic Environment. Proceedings, ASCE,

Madison, WI, pp. 965–987.

Ramawsamy JN (1970) Effects of Acid and Gas Production

as Sanitary Landfills. Ph.D. Dissertation,West Virginia

University.

Rank, et al. (1992) Environmental Isotopes Study at the

Breitenqu Experimental Landfill (Lower Austria). Tracer

Hydrology. In: Hotzl and Werner (eds.) Proceedings of the

6th International Symposium on Water Tracing, Karls-

ruhe, Germany, Sept 21–26, pp. 173–177. Rotterdam:

Balkema.

Whiticar MJ and Faber D (1985) Methane Oxidation in

Sediment and Water Column Environments – Isotope

Evidence. Advances in Organic Geochemistry 10:

759–768.

ENVIRONMENTAL GEOCHEMISTRY

W E Dubbin, The Natural History Museum, London, UK

Introduction

Soils and sediments occupying the Earth’s surface lie

at the interface of the lithosphere, atmosphere, bio-

sphere, and hydrosphere. Within the weathered, com-

plex, and porous milieu of these Earth-surface

materials, myriad biogeochemical processes govern

the movement of both nutrients and pollutants from

the lithosphere to biota, where they are incorporated

into plant and animal tissues, or to groundwaters,

where they may be transported great distances to

streams, rivers, and oceans. This chapter describes

the most notable pollutants, both organic and inor-

ganic, and the dominant processes governing their

mobility and bioavailability in terrestrial and aquatic

environments.

Trace Elements

Trace elements are those that occur in the lithosphere at

concentrations typically less than 1g kg

1

(Table 1).

Among the trace elements are the micronutrients

(e.g., Cu, Ni, Zn), which are essential for the growth

and development of micro-organisms, plants, and

animals, and also the metalloids, which have charac-

teristics of both metals and non-metals (e.g., As, B)

(see Minerals: Arsenates). Heavy metals are defined

as those trace elements with densities >5.0 g cm

3

Virtually all trace elements, even the micronutrients,

exhibit toxicity to animals and plants when present at

excessive concentrations. Radionuclides are a separ-

ate but important class of inorganic contaminant that

may occur naturally (e.g.,

222

Rn,

226

Ra,

238

U) or as a

consequence of nuclear fission related to atomic

weapons testing and nuclear power generation (e.g.,

Sr,

137

Cs,

239

Pu).

Although trace elements occur naturally in all

terrestrial environments, anthropogenic inputs may in-

crease these concentrations considerably. The principal

anthropogenic sources of trace elements are mining and

smelting activities, fossil fuel combustion, chemical and

electronics industries, as well as the addition of fertil-

isers and biosolids arising from agricultural operations.

One notable example of severe trace metal pollution

caused by smelting activities is found near Karabash, in

the south Ural Mountains region of Russia, where for

decades smelting operations have deposited metals on

the surrounding landscape, destroying much of the

vegetation and so contributing to widespread soil

erosion (see Environmental Geology).

Toxic levels of trace elements (e.g., Cr, Ni) may also

occur naturally, as in soils derived from serpentinitic

rocks, which can lead to phytotoxicity and the conse-

quent lack of vegetation over large areas of the

landscape where the serpentine soils occur (see Clay

Minerals). Alternatively, these elevated trace element

concentrations may induce metal tolerance among

certain plant species (e.g., Thlaspi spp.) as biological

communities adapt to these metal rich environments.

Trace Element Bioavailability

and Speciation

It is well established that total trace element content

in soil or sediment is a poor indicator of toxicity.

A more reliable measure of ecotoxicity is trace elem-

ent bioavailability. A trace element is considered

bioavailable if it can be utilized by biota. Bioavail-

ability is therefore broadly equated with solubility,

although some plants and micro-organisms are

able to extract metals from solid phases normally

considered insoluble. The solubility, and hence bio-

availability, of a particular trace element is deter-

mined largely by its solid-phase speciation and

mode of surface complexation. The main parameters

ENVIRONMENTAL GEOCHEMISTRY 21

Oneacre JW (1993) Subtitle D Regulations Impact on

Ground Water Monitoring. Geotechnical News 11(3):

49–52.

Oneacre JW and Figueras D (1996) Ground Water Variabil-

ity at Sanitary Landfills: Causes and Solutions, Uncer-

tainty in the Geologic Environment. Proceedings, ASCE,

Madison, WI, pp. 965–987.

Ramawsamy JN (1970) Effects of Acid and Gas Production

as Sanitary Landfills. Ph.D. Dissertation,West Virginia

University.

Rank, et al. (1992) Environmental Isotopes Study at the

Breitenqu Experimental Landfill (Lower Austria). Tracer

Hydrology. In: Hotzl and Werner (eds.) Proceedings of the

6th International Symposium on Water Tracing, Karls-

ruhe, Germany, Sept 21–26, pp. 173–177. Rotterdam:

Balkema.

Whiticar MJ and Faber D (1985) Methane Oxidation in

Sediment and Water Column Environments – Isotope

Evidence. Advances in Organic Geochemistry 10:

759–768.

ENVIRONMENTAL GEOCHEMISTRY

W E Dubbin, The Natural History Museum, London, UK

Introduction

Soils and sediments occupying the Earth’s surface lie

at the interface of the lithosphere, atmosphere, bio-

sphere, and hydrosphere. Within the weathered, com-

plex, and porous milieu of these Earth-surface

materials, myriad biogeochemical processes govern

the movement of both nutrients and pollutants from

the lithosphere to biota, where they are incorporated

into plant and animal tissues, or to groundwaters,

where they may be transported great distances to

streams, rivers, and oceans. This chapter describes

the most notable pollutants, both organic and inor-

ganic, and the dominant processes governing their

mobility and bioavailability in terrestrial and aquatic

environments.

Trace Elements

Trace elements are those that occur in the lithosphere at

concentrations typically less than 1 g kg

1

(Table 1).

Among the trace elements are the micronutrients

(e.g., Cu, Ni, Zn), which are essential for the growth

and development of micro-organisms, plants, and

animals, and also the metalloids, which have charac-

teristics of both metals and non-metals (e.g., As, B)

(see Minerals: Arsenates). Heavy metals are defined

as those trace elements with densities >5.0 g cm

3

Virtually all trace elements, even the micronutrients,

exhibit toxicity to animals and plants when present at

excessive concentrations. Radionuclides are a separ-

ate but important class of inorganic contaminant that

may occur naturally (e.g.,

222

Rn,

226

Ra,

238

U) or as a

consequence of nuclear fission related to atomic

weapons testing and nuclear power generation (e.g.,

Sr,

137

Cs,

239

Pu).

Although trace elements occur naturally in all

terrestrial environments, anthropogenic inputs may in-

crease these concentrations considerably. The principal

anthropogenic sources of trace elements are mining and

smelting activities, fossil fuel combustion, chemical and

electronics industries, as well as the addition of fertil-

isers and biosolids arising from agricultural operations.

One notable example of severe trace metal pollution

caused by smelting activities is found near Karabash, in

the south Ural Mountains region of Russia, where for

decades smelting operations have deposited metals on

the surrounding landscape, destroying much of the

vegetation and so contributing to widespread soil

erosion (see Environmental Geology).

Toxic levels of trace elements (e.g., Cr, Ni) may also

occur naturally, as in soils derived from serpentinitic

rocks, which can lead to phytotoxicity and the conse-

quent lack of vegetation over large areas of the

landscape where the serpentine soils occur (see Clay

Minerals). Alternatively, these elevated trace element

concentrations may induce metal tolerance among

certain plant species (e.g., Thlaspi spp.) as biological

communities adapt to these metal rich environments.

Trace Element Bioavailability

and Speciation

It is well established that total trace element content

in soil or sediment is a poor indicator of toxicity.

A more reliable measure of ecotoxicity is trace elem-

ent bioavailability. A trace element is considered

bioavailable if it can be utilized by biota. Bioavail-

ability is therefore broadly equated with solubility,

although some plants and micro-organisms are

able to extract metals from solid phases normally

considered insoluble. The solubility, and hence bio-

availability, of a particular trace element is deter-

mined largely by its solid-phase speciation and

mode of surface complexation. The main parameters

ENVIRONMENTAL GEOCHEMISTRY 21

governing trace element speciation are pH and pe, the

so-called ‘master variables’, as well as the total con-

centration of the ion in solution. These variables

determine whether the ion in question will undergo

hydrolysis, precipitation, redox reactions, or any

number of complexation reactions at surfaces or in

aqueous solution. Generally, metal cations are most

soluble and available at low pH, principally pH <5,

where hydrolysis is minimal and sorption to layer

silicates and metal oxides is limited. Conversely, oxy-

anions such as chromate (see Minerals: Chromates)

(HCrO



) and selenite (HSeO



) show greatest solubil-

ity at high pH, where electrostatic repulsion with

negatively charged colloids minimizes surface com-

plexation.

Redox-sensitive elements such as As, Cr, Fe, and

Mn are subject to oxidation state changes under

Earth surface conditions, with important impli-

cations for solubility and toxicity. Of the two As

oxidation states predominating under Earth surface

conditions (i.e., As(III) and As(V) ), As(III) is more

problematic because of its greater mobility and

bioavailability. Given the ubiquity of manganese

oxides (e.g., d-MnO

) in soils and sediments, these

minerals have been proposed as key agents in

the natural attenuation of As(III) contamination, as

the oxidation of As(III) to As(V) by Mn(IV) is both

rapid and thermodynamically favourable. The human

health implications of As bioavailability is illustrated

most clearly on the deltaic plains of Bangladesh

and West Bengal, where groundwater As concentra-

tions may exceed 400 mgl

1

as a consequence of bio-

logically mediated reductive dissolution of Fe(III)

oxides, leading to the release to solution, in bioa-

vailable form, of the previously sorbed As. Manga-

nese(IV) oxides can also mediate the oxidation of

Cr, from Cr(III) to Cr(VI). However, this redox reac-

tion is undesirable because, unlike As, the oxidized

form of Cr, Cr(VI), is the form of Cr most mobile

and toxic.

Table 1 Abundance, speciation, and toxicity of trace elements in soil and sediment

Element

Median soil content

(mg kg

1

) Dominant solution species

Function; Toxicity

Ag 0.05 Ag

, AgCl None known; Plant and animal toxin

As 6 As(OH)

, AsO

3

AsO



, HAsO

2

None known; Plant and animal toxin

B20 H



Plant nutrient; Phytotoxin

Be 0.3 Be

2þ

, Be(OH)



, Be(OH)

2

None known; Plant and animal toxin

Bi 0.2 Bi

3þ

None known; Plant and animal toxin

Cd 0.35 Cd

2þ

, CdSO

, CdCl



, CdHCO

None known; Animal toxin

Cl 100 Cl



Plant nutrient; Phytotoxin

Co 8 Co

2þ

, CoSO

, Co(OH)

Plant and animal nutrient; Plant and animal

toxin

Cr 70 Cr(OH)

, Cr(OH)



, HCrO



Animal nutrient; Cr(VI) is a plant and animal

toxin

Cs 4 Cs

None known; None known

Cu 30 Cu

, CuCl



, CuCO

, CuHCO

, Cu(OH)

Plant and animal nutrient; Plant and animal

toxin

Hg 0.06 Hg

2þ

, HgCl

,CH

, Hg(OH)

None known; Plant and animal toxin

Mo 1.2 H

MoO

, HMoO



, MoO

2

Plant and animal nutrient; Plant and animal

toxin

Ni 50 Ni

2þ

, NiSO

, NiHCO

, NiCO

Plant and animal nutrient; Plant and animal

toxin

Pb 19 Pb

2þ

, PbSO

, PbHCO

, PbCO

, PbOH

None known; Plant and animal toxin

Rb 67 Rb

None known; Phytotoxin

Sb 1 Sb(OH)

, Sb(OH)



, Sb(OH)



None known; Plant and animal toxin

Se 0.4 HSeO



, SeO

2

, SeO

2

Animal nutrient; Plant and animal toxin

Sn 4 Sn

4þ

None known; Plant and animal toxin

Tl 0.2 Tl

None known; Plant and animal toxin

U 2.7 UO

2(s)

,UO

2þ

,UO

(CO

)

4

, (UO

)

(OH)



None known; Animal toxin

V90 VO

2þ

,VO

(OH)



,VO

(OH)

2

Plant and animal nutrient; Plant and animal

toxin

W 1.5 WO

2

None known; None known

Zn 70 Zn

2þ

, ZnSO

, ZnHCO

, ZnCO

, Zn(OH)

Plant and animal nutrient; Plant and animal

toxin

Bowen (1979).

Hayes and Traina (1998).

22 ENVIRONMENTAL GEOCHEMISTRY

Organic Contaminants

The environmental geochemistry of organic contam-

inants primarily concerns the sources, movement, and

fate of petroleum hydrocarbons and their by-

products, as well as the halogenated hydrocarbons,

the group to which many pesticides belong. Petroleum

hydrocarbons and associated compounds such as the

oxygenates (e.g., methyl t-butyl ether (MTBE)) con-

stitute a significant environmental risk by virtue of

their widespread occurrence, mobility, and ecotoxi-

city (see Geochemical Exploration). Following release

of hydrocarbons to the environment by multiple path-

ways, the low molecular weight volatile fraction

(<C

), often containing the carcinogenic benzene

and polycyclic aromatic hydrocarbons, is largely lost

to the atmosphere through volatilisation, leaving a

relatively small but environmentally important por-

tion of the light hydrocarbon pool to react with soil or

sediment, or to enter groundwater. The less soluble,

more chemically inert higher molecular weight hydro-

carbons (>C

) are potentially more disruptive to

ecosystems, as illustrated by the spill in 1989 of

crude oil from the Exxon Valdez oil tanker, which

contaminated nearly 1750 km of Alaskan shoreline.

Halogenated hydrocarbons have both natural and

anthropogenic origins, and many belong to the class

of contaminant known as persistent organic pollutants

(POPs), recalcitrant organic compounds that bioac-

cumulate and exhibit animal toxicity. POPs are domin-

ated by the chlorinated hydrocarbons, which include

many of the pesticides, such as aldrin, atrazine, ch-

lordane, DDT, heptachlor, and the polychlorinated

biphenyls. Despite the relative recalcitrance of POPs,

their degradation can be mediated both by abiotic pro-

cesses (e.g., oxidation by d-MnO

)aswellasbythe

native soil microflora, particularly the fungi, which

employ hydrolytic, reductive, or oxidative reactions

to induce molecular dehalogenation. The hydroxy-

lated compound so produced is thus rendered more

susceptible to the degradation reactions of other soil

microbiota.

The tendency of certain POPs to migrate from

tropical and temperate climates to the colder polar

regions has been the subject of study and debate

for decades. A model of redistribution described

as ‘global distillation’, involving POP evaporation

followed by transport and condensation in colder

regions, has received wide acceptance. Fractionation

of POPs, during redistribution to higher latitudes, is

driven by differential migration rates arising from

variable POP vapour pressures and partition coeffi-

cients, with POP transport occurring in distinct jumps

which are closely coupled to diurnal and seasonal

temperature cycles.

Acidification of Terrestrial and

Aquatic Environments

An important aspect of environmental geochemistry

is acid deposition and the related acidification of

Earth surface environments through both anthropo-

genic and natural processes. The burning of fossil

fuels releases SO

and NO

compounds which com-

bine with atmospheric water to yield H

and

HNO

that may be carried great distances before

deposition as rain, mist, fog, or snow. Deposition of

these acidic materials impacts negatively on soils,

vegetation, and water bodies, particularly lakes

which are poorly buffered and whose aquatic organ-

isms are therefore at risk of increased soluble Al

concentrations following a significant decrease in

lake pH. Monuments and buildings constructed of

limestone and marble, and which frequently represent

much of our cultural heritage, are also at risk from

acid deposition through dissolution of their constitu-

ent carbonate minerals (see Minerals: Carbonates).

The effects of acid deposition are not entirely nega-

tive, however, as the additions of N and S to soils are

beneficial, and these added nutrients frequently com-

prise a significant portion of the available soil N and

S in highly industrialized regions.

An aspect of acidification of growing environmen-

tal importance concerns the oxidation of mining

waste rich in reduced S, principally in the form of

pyrite (FeS

)(see Environmental Geology, Minerals:

Sulphides). Oxidation of this S can yield vast amounts

of H

, giving rise to highly acidic waters, known

as acid mine drainage (AMD) waters, containing

toxic levels of soluble metals (Figure 1). The environ-

mental significance of these acidic, metal-rich waters

was emphasised in dramatic fashion with the col-

lapse in 1998 of the Aznalco

llar mine tailings dam

in southwest Spain. Failure of the dam led to the

release of 1.3 million cubic metres of AMD waters,

laden with Ag, As, Cd, Cu, Pb, and Zn, and the

subsequent flooding of nearly 4,600 hectares of land

with this toxic effluent.

Environmental Restoration

Decontamination of terrestrial environments is often

costly and time-consuming, owing to the frequent

occurrence of multiple pollutants, as well as the

complexity of the contaminated matrices (e.g., soil

or sediment). Strategies employed to remediate con-

taminated environments involve both in situ and ex

situ techniques (Table 2), with the former generally

receiving wider acceptance because of greater efficacy

and lower implementation costs. In situ remediation

may simply involve introducing a liming material

ENVIRONMENTAL GEOCHEMISTRY 23

(e.g., CaO, Ca(OH)

, CaCO

) to raise pH and

thus decrease metal bioavailability through sorption

and precipitation. The addition of smectites or zeo-

lites (see Minerals: Zeolites) serves to remove

pollutants by means of ion exchange reactions,

whereas apatite addition induces the formation of

sparingly soluble metal phosphate precipitates.

Phytoremediation encompasses a variety of in situ

strategies involving the use of plants to remove or

render environmental pollutants harmless. The most

common of these strategies is phyto-extraction,

which involves growing plants capable of hyperaccu-

mulating the contaminant of interest, followed by

plant harvest to remove both plant and contaminant

from the site. Among the hundreds of hyperaccumu-

lators recently identified are the brake fern (Pteris

vittata), an effective As accumulator, and also the

basket willow (Salix viminalis), which effectively se-

questers both Zn and Cd. Biodegradation is a related

in situ remediation strategy that utilises native micro-

bial populations to degrade multiple organic contam-

inants. This microbial degradation is optimal within

aerobic rhizosphere communities at circumneutral

pH, particularly in the presence of abundant nutri-

ents, which may be augmented with external sources

to enhance microbial activity.

The more costly ex situ techniques are generally

applied only to the most high-value contaminated

sites. The simplest of the ex situ remediation strat-

egies involves direct excavation of the contaminated

material followed by landfill disposal. Soil washing

(see Soils: Modern) is more complex, involving ex-

cavation followed by mechanical screening to obtain

coarse (>50 mm) and fine (<50 mm) fractions which

are subsequently treated with a series of surfactants

and chelates to remove both organic and inorganic

contaminants, including radionuclides. The aim of

soil washing is to concentrate the pollutants within

the fine fraction, which is then treated with a solidifi-

cation/stabilizing agent (e.g., Portland cement, lime,

fly ash) prior to landfill disposal.

Figure 1 Acid mine drainage waters near the Kristineberg

Cu and Zn mine, Sweden (photo courtesy L. Lovgren, Umea

University).

Table 2 Summary of in situ and ex situ environmental restoration techniques

Method Contaminant Processes Limitations

In situ

Lime addition Metals, radionuclides pH increase causing sorption,

precipitation

Ineffective for oxyanions

Smectite, zeolite, apatite

addition

Metals, radionuclides Ion exchange, sorption, precipitation Selective, short term remediation

Phytoremediation Metals, organics Phytoaccumulation, phytodegradation Unsuitable for highly

contaminated sites

Biodegradation Organics Microbial degradation Long-term remediation

Volatilization Volatile organics Evaporative loss of volatile pollutants Contaminant must be volatile

Ex situ

Excavation and disposal All Removal and disposal Costly, risk of pollutant dispersal

Soil washing Metals, organics,

radionuclides

Excavation, leaching of contaminants Costly

Solidification/stabilization Metals, radionuclides Addition of stabilising agent Volatile organics not immobilized

24 ENVIRONMENTAL GEOCHEMISTRY

Further Reading

Adriaens P, Gruden C, and McCormick ML (2004) Biogeo-

chemistry of halogenated hydrocarbons. In: Lollar BS

(ed.) Environmental Geochemistry, vol. 9, pp. 511–539.

Treatise on Geochemistry Holland HD and Turekian KK

(eds.). Oxford: Elsevier-Pergamon.

Adriano DC (1986) Trace Elements in the Terrestrial Envir-

onment. New York: Springer-Verlag.

Alexander M (1999) Biodegradation and Bioremediation,

2nd edn. London: Academic Press.

Alloway BJ (1995) Heavy Metals in Soils, 2nd edn. London:

Blackie Academic and Professional.

Blowes RJ, Ptacek CJ, Jambor JL, and Weisener CG (2004)

The geochemistry of acid mine drainage. In: Lollar BS

(ed.) Environmental Geochemistry, vol. 9, pp. 149–204.

Treatise on Geochemistry Holland HD and Turekian KK

(eds.). Oxford: Elsevier-Pergamon.

Bowen HJM (1979) Environmental Chemistry of the Elem-

ents. London: Academic Press.

Cozzarelli IM and Baehr AL (2004) Volatile fuel hydrocar-

bons and MTBE in the environment. In: Lollar BS (ed.)

Environmental Geochemistry, vol. 9, pp. 435–474. Trea-

tise on Geochemistry Holland HD and Turekian KK

(eds.). Oxford: Elsevier-Pergamon.

Hayes KF and Traina SJ (1998) Metal ion speciation and its

significance in ecosystem health. In: Huang PM (ed.) Soil

Chemistry and Ecosystem Health, pp. 45–84. Madison,

USA: Soil Science Society of America.

Schwarzenbach RP, Gschwend PM, and Imboden DM

(1993) Environmental Organic Chemistry. New York:

Wiley.

Sposito G (1996) The Environmental Chemistry of Alumi-

num, 2nd edn. London: CRC Press.

a0005 ENVIRONMENTAL GEOLOGY

P Doyle, University College London, London, UK

Introduction

Environmental geology has grown in stature as a dis-

cipline over the past 40 years, as considerations of

economic geology have moved away from the simple

exploitation of the late nineteenth and early twentieth

centuries. Today, environmental geology is a broad

area of geological endeavour and a major industry in

its own right.

Defining Environmental Geology

Environmental geology may be defined as the

interaction of humans with their – fundamentally

geological – environment. The environment can be

considered to consist of both the constituents of the

Earth itself (rocks, sediments, and fluids) and its surface

and the processes that operate to change it through

time.

Environmental geology is a subset of environmental

science, which is the study of the interaction of

humans with all aspects of their environment – phys-

ical, atmospheric, and biological – and is linked dir-

ectly with engineering geology (see Engineering

Geology: Overview). This definition clearly indicates

that it is the introduction of the human element to the

equation that defines the concept of the environmen-

tal sciences – and, therefore, environmental geology –

and it is a consideration of both the debits (impacts)

and credits (benefits) of our existence. Environmental

science is a way of managing our existence so as to

maximize human success while minimizing the nega-

tive aspects. At the heart of environmental geology, as

with all environmental sciences, is the concept of sus-

tainable management – working with natural systems

to sustain development but not at an unacceptable

environmental cost.

Scope of Environmental Geology

It has been estimated that around 50% of the Earth’s

population live in urban centres, centres that cover just

1% of the Earth’s surface. Given that environmental

geology is the interaction of humans with their environ-

ment and given that the majority of humans live in

cities, it follows that environmental geology can be

considered as primarily an urban issue, with the most

challenging problems occurring within the immediate

hinterland of urban centres.

Environmental geology has been defined as an

urban concept, a ‘machine’ that balances inputs, out-

puts and maintenance, most if not all of which have a

geological component (Figure 1). The inputs include

water (derived locally or from more distant locations,

but feeding into the city), raw materials (in the form of

ENVIRONMENTAL GEOLOGY 25

mineral resources for industry and construction), food

resources (both locally produced and transported),

and energy (the result in many cases of mineral re-

sources such as coal, gas, and uranium). Outputs from

the ‘machine’ include products from industry, wastes

(in the form of worn-out materials, by-products of

industry, and day-to-day wastes from domestic, indus-

trial, and commercial sources), and pollution from

poor waste-management strategies. The urban ma-

chine needs constant maintenance to replace its infra-

structure and foundations, and to protect it from

natural hazards.

The demands of the machine drive our demands on

the environment, with the need for exploration and

exploitation of mineral, water, and soil resources to

provide the inputs; the need to combat environmental

stress and natural hazards in protecting the workings

of the machine, and the requirement to deal with the

outputs, particularly wastes and their potential to

pollute if not handled with appropriate sensitivity.

All these aspects (and more) fall within the broad

scope of environmental geology, and they can be

distilled into the following themes.

1. Geology of resource management, including ex-

ploration and exploitation of resources (e.g. fuels,

industrial minerals, and water) and the mitigation

and limitation of associated adverse environmental

impacts.

2. Geology of the built environment, particularly

the constraints that ground conditions place on

development.

3. Geology of waste management, specifically the

disposal of wastes in the physical environment.

4. Geology of natural hazards.

Geology of Resource Management

Resources may be defined in the geological context as

naturally occurring solids, liquids, or gases that are

known or thought to exist in or on the Earth’s crust in

concentrations that make extraction economically

feasible either at present or at some time in the future.

This definition includes those geological materials

that can be extracted with currently available tech-

nology (reserves) as well as those that are thought

to exist but that will require further technological

development to remove them.

In the short term, resource-management options

focus on reserves, and environmental considerations

have more power than at any other time in the past

to reduce available reserves by altering the economics

of their exploitation. Long-term planning must, of

course, be based on the probability of new resources

being discovered. The size of the available reserves

relative to the total resources can be altered by several

factors, such as commodity price, exploration, increas-

ing the extent of the known resource, technological

developments, and changes in regulation.

Economic Mineral Resources

Economic mineral resources are varied and can be

defined in their broadest sense as any geological ma-

terial that is of commercial value to society. This broad

definition includes such diverse materials as fuels (e.g.

coal, gas, uranium), construction materials, industrial

minerals, metals, and precious minerals. Extraction of

these materials varies from bulk extraction to special-

ist mining, and, although the basic concepts of explor-

ation and exploitation may have many parallels, the

environmental impacts posed by each one may well

differ strongly.

Environmental Impacts of Mineral Extraction

The environmental impacts of mineral extraction vary

according to the type of mineral and the extent of its

deposit, with impacts varying throughout the working

life of a mine or quarry, and with the issues often

continuing long after the deposits worked are no

longer economically viable. Typical issues are aspects

of mine operation, mining subsidence, tackling mine

wastes, and quarry or mine restoration.

There are a wide range of issues associated with

mining. Quarries are troubled by blast noise and

vibration, which can lead to increased rock-fall and

Figure 1 The urban machine. (Reproduced with permission from Bennett MR and Doyle P (1997) Environmental Geology: Geology and

the Human Environment.

Chichester: John Wiley.)

26 ENVIRONMENTAL GEOLOGY

landslip activity. In deep mines, the emission of me-

thane and other gases is a problem, while wastewaters

may be charged with iron and may be acidified by the

breakdown of pyrite and other sulphides. Acid mine

drainage is a growing problem in many disused metal

mines and has adverse affects on groundwater supplies

and water bodies, such as estuaries, rivers, and lakes.

Mining subsidence is a result of the removal of geo-

logical materials underground, creating a void space,

which subsequently collapses (Figure 2). Similar situ-

ations occur when voids are created for purposes other

than mineral extraction. Longwall mining is associ-

ated with deep coal extraction and generally involves

the planned subsidence of a relatively large area.

Pillar-and-stall mining, which dates back centuries,

can lead to the differential collapse of the pillars,

giving rise to graben-like failure structures. In other

cases, removal of deep salt deposits through the

process known as brining – the pumping out of

brine-charged waters – can lead to severe subsidence.

Large-scale subsidence is often difficult to manage,

and the possibility of constructing buildings over an

unknown area of mining activity is a very live issue,

even where the mining is known to date back to an-

tiquity, as in Rome. Raft foundations, grouting, deep

piling, and excavation and backfill are all possibilities,

depending on the depth and extent of the problem.

Mine wastes are produced at two stages during

the mineral-extraction process: during mining, when

waste rock or spoil is produced; and during further

processing of the materials extracted, which creates a

further set of mineral wastes, generally much finer

than simple spoil and referred to as tailings. Spoil is

of variable grain size and generally accumulates in

open tips close to the mine workings. Tailings pro-

duced by mineral processing tend to be much richer

in minerals and more uniform and of a finer grain

size owing to milling. In some cases these materials

have a high toxicity, as hazardous chemicals are used

in the separation of mineral particles. Tailings are also

more likely to be distributed by weathering and

erosion, raising the consequent possibility of hazard-

ous air-borne particulate matter. The long-term sta-

bility of mine wastes is also of concern, as with

the South Welsh Aberfan disaster in 1966, where

111 000 m

of debris moved down a 13



slope,

enveloping a school with tragic loss of life.

Dealing with waste tips is a matter of concern for

environmental geologists, and the solution depends

very much on the nature of the material. In the UK,

tax on aggregate extraction led to the reuse of some

tax-exempt spoil, particularly slate and shale, as poor-

quality aggregate. Other mineral spoil may be reused

for rock fill or inert landfill or even in dealing with

mine subsidence and quarry-restoration projects. Tail-

ings, on the other hand, given the potentially high level

of toxicity, have to be treated with caution. Wet tail-

ings have to be ponded, treated, and removed, while

dry tailings have to be carefully monitored so that

water passing through the wastes does not lead to

Figure 2 Subsidence associated with longwall coal mining.

ENVIRONMENTAL GEOLOGY 27

groundwater pollution. Restoration and stabilization

of wastes is essential, but this can be a tall order when

vegetation that could be useful in binding surfaces is

inhibited by the toxicity of the waste material itself.

Quarry and mine restoration is a major task. For

mines, it is important first of all to ensure the safety of

the underground workings themselves and to monitor

such issues as the build-up of gases and acid mine

drainage. In other cases, the mine workings may

present a hazard in the form of future subsidence.

Pit-head machinery and processing works, which may

have used chemical processes requiring specialist atten-

tion, will also be an issue. Finally, the mine wastes

themselves must be dealt with. For quarries, restoration

issues reflect the nature of the materials quarried, with

production-blasted hard-rock quarry faces represent-

ing an unsafe and unstable option. Restoration depends

very much on the projected end use of the quarry; if left

as a void, then faces will require engineering attention.

This is particularly so given that many quarries are now

being used to house industry or commercial concerns.

In other cases, the void will be filled, most often as

landfill.

Water Resources

Water is a vital geological resource that sustains life,

yet most of the Earth’s water is unavailable, with

98% being in the oceans and 1.6% in ice-sheets,

leaving only 0.4% for drinking. Suitable drinking-

water resources are distributed unevenly across the

globe, and this, together with the predictions of

environmental change, means that the search for

drinking-water supplies is a major preoccupation of

environmental geologists. Water-resource manage-

ment is essentially a combination of three factors:

acquisition, redistribution, and the treatment and

disposal of wastewaters. The potential to pollute

existing supplies through inadequate waste-disposal

strategies is also a major issue, and this will be dealt

with later when considering waste management.

Acquisition of water is primarily a process of ex-

traction from groundwaters, which are controlled by

rainfall and the porosity and permeability of the rocks

present in a given area (Figure 3). These can be highly

variable, and, even within a small area such as the

UK, the percentage of water derived from this source

is variable. Where groundwater is not an option, reli-

ance on surface waters – rivers, lakes, and reservoirs –

becomes much more significant, with desalination of

seawater also possible.

The redistribution of supplies is an enormous under-

taking and has lead to major international disputes

when some countries have sought to dam rivers to the

detriment of others downstream. Such international

tensions are unlikely to recede in the future and could

lead to ‘water wars’ if diplomacy breaks down. The

creation of dams to contain waters and generate hydro-

electric power has been an attractive but often highly

controversial approach in some water-poor countries.

However, inadequate site assessment has led to the fail-

ure of some dam sites, with leakage of waters or failure

of the dam structure itself, so the high-cost deployment

of such schemes requires a complete understanding and

assessment of the issues involved.

Soil Resources

Together with water, soils are a fundamental geological

resource, allowing us to grow sufficient crops to supply

the food required to sustain the planet’s growing popu-

lation. The distribution and formation of soils across

Figure 3 Schematic diagram of the main water-bearing units. (Reproduced with permission from Bennett MR and Doyle P (1997)

Environmental Geology: Geology and the Human Environment. Chichester: John Wiley.)

28 ENVIRONMENTAL GEOLOGY

the world is contingent on climate, rainfall, biological

activity, topography, and the underlying geology. Soils

vary considerably, with an infinite variety – there are

at least 19 000 different types in the USA alone – and

widely differing classification schemes across the world

(see Soils: Modern). The potential to erode and denude

soils depends on the intensity of use, the ground cover,

and so on. Soil conservation is therefore a major

issue and can vary with land-use changes, new

technologies, and colonization.

In the UK, raising the awareness of soils and the need

to conserve them has led to the construction of five

basic principles: that soils are an essential part of life

support; that soils should enjoy the same level of pro-

tection as water and air; that integrated environmental

management should include soils; that contaminated

soils should be remediated; and that contamination

should be avoided. These issues obviously have a global

relevance.

Aesthetic and Scientific Resources

Conservation of aesthetic and scientific resources in-

volves the conservation of areas of landscape and/or

specific geological sites – known as geodiversity – for

future generations (see Geological Conservation). The

concept is based on four basic convictions: that they

should be conserved for their own sake; that they form

the basis for exploitation; that they form the basis for

research and training; and that they have aesthetic and/

or cultural value. This approach is growing in stature

across the world, leading to a wider awareness of the

value of geological features.

Geology of the Built Environment

The built environment – the towns and cities in which

the vast majority of people live – is the central pillar

of the urban machine, and, in many cases, the cause

of many environmental issues. Built from geological

resources – geomaterials – towns and cities are in

constant need of renovation, renewal, and develop-

ment, with the consequent need for greater explor-

ation for aggregates, stone, and other materials

(Figure 4). Engineering geology is an important factor

in the construction of the built environment, particu-

larly in the provision of sound foundations. Many of

the older cities of the world are founded on ancient

settlement sites, and this in turn creates its own issues

of long-term stability.

Geomaterials

Construction materials, known as geomaterials, are

quarried and mined from a wide variety of settings.

The pressure to discover new resources is increasing,

as the pressure to house the world’s burgeoning popu-

lation also increases. Stone and clay for adobe bricks

have been the main building resources of the world

since antiquity, and concrete has a long and venerable

history. These materials are still in demand, and

quarrying for them creates much the same environ-

mental issues as any other type of extractive industry

(see Aggregates).

Another important role for environmental geolo-

gists is in assessing the appropriateness of geomater-

ials for the job for which they are intended. This

might include, for example: a durable, structurally

strong, easily worked, and attractive stone for con-

struction; large relatively uniform blocks of stone that

can take a high degree of pounding without breaking

down for use as armourstone blocks in coastal de-

fences; or the provision of aggregates for roads that

do not polish easily and can take a fair amount of

crushing from large vehicles. Durability of stone and

the effects of stone cleaning are major issues in some

of the major cities of the world, needing constant

monitoring (see Building Stone).

Figure 4 The main geomaterials and their uses. (Reproduced with permission from Bennett MR and Doyle P (1997) Environmental

Geology: Geology and the Human Environment.

Chichester: John Wiley.)

ENVIRONMENTAL GEOLOGY 29

Elsevier Encyclopedia of Geology - vol I A-E

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