The soil is a primary recipient by design or accident of a myriad of waste products an:
chemicals used in modern society, It has always been convenient to "throw thing:
dwdf ," and the soil has been the recipient of most of these things, Every year million,
of tons of products from a variety of sources-industrial, domestic, and agriculturalfind
their way into the world's soils. Once these materials enter the soil, they becon:.
part of biological cycles that affect all forms of life. One of the challenges facir:.
humankind is to better understand how wastes affect these cycles and, in turn, the n-el,being
of all plant and animal life.
In previous chapters we highlighted the enormous capacity of soils to accomm. _
date added organic and inorganic chemicais. Tons of organic residues and anima_
manures are broken down by soil microbes each year (Chapter 12), and large quant:ties
of inorganic chemicals are fixed or bound tightly by soil minerals (Chapter 14). Bu:
we also learned of the limits of the soil's capacity to accommodate these chemicals
and how environmental quality suffers when these limits are exceeded.
We have seen how soil processes affect the accommodation and release of wast.
products. For example, the production and sequestering of greenhouse gases/ such a,
nitrous oxide, methane, and carbon dioxide (see, e.g., Sections 12.17 and 13.10), al;
very much influenced by soil processes, Other nitrogen- and sulfur-containing gases
coming from domestic and industrial sources/ as well as from the soil, acidify th.
atmosphere, and come to earth in acid rain (see, e.g., Section 9.5), Mismanaged irl_gation
projects result in the accumulation of salts, especially in arid-region soils (se=
Section 10.3).
We have also seen how fertilizer and manure applications that leave excess quant__
ties of nutrients in the soil can result in the contamination of ground and surface \,vatei:
with nitrates (Section 13.9) and phosphates (Section 14.2). The eutrophication c_
ponds, lakes, and even slow-moving rivers is evidence of these nutrient buildups. Hug.
"animal factories" for meat and poultry production produce mountains of manure the.
must be disposed of without loading the environment with unwanted chemicals an:
with pathogens that are harmful to humans and other animals (Section 16.4).
In this chapter we will focus on chemicals that contaminate and degrade soi.:
796
including some whose damage extends to water, air, and living things, The brief review
of soil pollution is intended as an introduction to the nature of the major pollutants,
their reactions in soils, and alternative means of managing, destroying, or inactivating
them.
l 8.1 Toxlc oRGANlc cHEMlcALs
[-omprehensive information
,n clea n itts ( ontam ina tťd
soils at abandoned jndrrstrial
gov/browníields/
Modern industrialized societies have developed thousands of synthetic organic com_
pounds for thousands of uses. An enormous quantity of organic chemicals is manufactured
every year-about 60 million Mg in the United States alone. Included are plastics
and plasticizers, lubricants and refrigerants, fuels and solvents, pesticides and preservatives.
Some are extremely toxic to humans and other life. Through accidental leakage
and spills or through planned spraying or other treatments, synthetic organic chemicals
can be found in virtually every colner of our environment-in the soil, in the groundwateí,
in the plants, and in our own bodies.
Enuíronmental Damage from Organíc Chemícals
These artificially synthesized compounds are termed xenobiotics because they are
unfamiliar to the living world (Greek xeno, stratge). Being nonnatural, many xenobiotics
are both toxic to living organisms and resistant to biological decay. The chemical
structures of xenobiotic compounds may be quite similar to those of naturally occurring
compounds produced by microorganisms and plants. The difference is commonly
the insertion of halogen atoms (Cl, F, Br) or multivalent nonmetal atoms (such as S and
N) into the structure (see Figure 18.1).
Some xenobiotic compounds are relatively inert and harmless, but others are biologically
damaging even in very small concentrations. Those that find their way into
soils may inhibit or kill soil organisms, thereby undermining the balance of the soil
community (see Section 11.15). Other chemicals may be transported from the soil to
the air, water, or vegetation, whete they may be contacted, inhaled, or ingested by any
number of organisms, including people. It is imperative, therefore, that we control the
release of organic chemicals and that we learn of their fate and effects once they enter
the soil.
Organic chemicals may enter the soil as contaminants in industrial and municipal
organic wastes applied to or spilled on soils, as components of discarded machinery, in
Iarge or small lubricant and fuel leaks, as military explosives, or as sprays applied to con_
trol pests in terrestrial ecosystems. Pesticides are probably the most widespread organic
pollutants associated with soils. In the United States, pesticides are used on some 150
million ha of land, three-fourths of which is agricultural land. Soil contamination by
other organic chemicals is usually much more localized. We will therefore emphasize
the pesticide problem.
The Nature of the PestícídeProblem
Pesticides are chemicals that are designed to kill pests (that is, any organism that the
pesticide user perceives to be damaging). Some 600 chemicals in about 50,000 formula_
tions are used to control pests. They are used extensively in all parts of the world. About
350,000 Mg of organic pesticide chemicals are used annually in the United States, with
similar amounts used in Western Europe and Asia. Although the total amount of pesticides
used has remained relatively constant or even dropped since the 1980s, formulations
in use today are generally more potent, so that smaller quantities are applied per
hectare to achieve toxicity to the pest.
Brnrrlrs or Prsrlclors. Pesticides have provided many benefits to society. They have
helped control mosquitoes and other vectors of such human diseases as yellow fever
and malaria. They have protected crops and livestock against insects and diseases. Without
the control of weeds by chemicals called herbicides, conservation tillage (especially
no-tillage) would be much more difficult to adopt; much of the progress made in controlling
soil erosion probably would not have come about without herbicides. Also, pes_
ticides reduce the spoilage of food as it moves from farm fields to distant dinner tables.
Toxtc OnenNlc CHEMIcALs 797
Pnoslrt4s wlrH Prsnoors. While the benefits to society from pesticides are gleat, so are
the costs (Table 18.1). Widespread and hear,y use of pesticides on agricultural soils and
suburban landscapes has led to contamination of both surface and groundwater. Therefore,
when pesticides are used, they should be chosen for low toxicity to humans and
wildlife, Iow mobili§ on soils, and low persistence (see Section 18.3). Even then, the use
of pesticides often has wide-ranging detrimental effects on the microbial and faunal
communities. In fact, the harm done, though not always obvious, may outweigh the
benefits. Examples include insecticides that kill natural enemies of pest species as well
as the target pest (sometimes creating new major pests from species formerly controlled
by natural enemies) and fungicides that kill both disease-causing and beneficia] mycorrhizal
fungi (see Section 11.9). Given these facts, it should not come as a surprise that
despite the widespread use of pesticides, insects, diseases, and weeds still cause the loss
of one-third of the crop production, about the same proportion of crops lost to these
pests in the United States, before synthetic organic pesticides were in use.
AlrrnNnrvrs ro Prsrlctors. Pesticides should not be seen as a pallacea| or even as indispensable.
Some farmers, most notably the small but increasing number who practice
organic farming,1 produce profitable, high-quality yields without the use of pesticides.
In managing the effects of pests in any type of plant community (agricultural, ornamental,
or forest), chemical pesticides should be used as a last resort, rather than as a
frsf resort. Before resorting to the use of an insecticide or herbicide/ eveíyeffort should
be made to minimize the detrimental effects of insects and weeds by means of crop
diversification, establishment of habitat for beneficial insects, application of organic
soil amendments, implementation of cultural practices to reduce weed competition,
and selection of pest-resistant plant cultivars. Too often, because pesticides are available
as a convenient crutch, these more sophisticated approaches to plant management ale
not explored.
NoNrnnerr Dnrveers. Although some pesticides are intentionally applied to soils, most
reach the soil because they miss the insect or plant leaf that is the application target.
When pesticides are sprayed in the field, most of the chemical misses the target organism.
For pesticides aerially applied to forests, about 25o/o reaches the tree foliage, and far
less than 1olo reaches a talget insect. About 30o/o may reach the soil, while about half of
the chemical applied is likely to be lost into the atmosphere or in runoff water.
Designed to kill living things/ many of these chemicals are potentially toxic to
organisms other than the pests for which they are intended. Some are detrimental to
nontarget organisms, such as beneficial insects and certain soil organisms. Those chem-
1The term organic farming has littie to do with the chemical definition of organic, which simply indicates
that a compound contains carbon, Rather, it refers to a system and philosophy of farming that eschews
the use of synthetic chemicals whi]e it emphasizes soil organic matteí and biological inteíactions to
manage agroecosystems.
TABLE l 8.1 Total Estimated Environmental and §ocial Cost
from pesticide use in the united states
The death of an estimated 60 million wild birds may represent
an additional substantial cost in lost revenues from hunters,
bird watchers, and so forth.
Type of impact Cost, $ million/v
Public health impacts
Domestic animai deaths and contamination
Loss of natural enemies
Cost of pesticide lesistance
Honeybee and pollination losses
Crop iosses
Fishery 1osses
Groundwater contamination and cleanup costs
Cost of govelnment regulations to píevent damage
Total
FIom Pimental, et al. (1992). O ,{merican Institute of Biological Sciences
787
30
520
1400
320
942
z4
1800
2oo
6023
Toxtc OnenNlc CHEMlcAls 799
icals that do not quickly break down may be biologically magnified as they move up the
food chain. For example, as earthworms ingest contaminated soil, the chemicals tend tc
concentrate in the earthworm bodies. When birds and fish eat the earthworms, the pesticides
can build up further to lethal levels. The near extinction of certain birds of prer
(including the American bald eagle) during the 1960s and 1970s called public attentior-_
to the sometimes-devastating environmental consequences of pesticide use. Mor.
recently, evidence is mounting to suggest that human endocrine (hormone) balanc.
may be disrupted by the minute traces of some pesticides found in water, air, and fooc
l8.2 KIND§ oF oRGANlc coNTAMlNANTs
lndustríal Organícs
Industrial organics that often end up contaminating soils by accident or neglect incluc.
petroleum products used for fuel [gasoiine components such as benzene, and tno..
complex polycyclic aromatic hydrocarbons, (PAHs)], solvents used in manufacturil:
processes fsuch as trichloroethylene (TCE)], and military explosives such as trinit:_toluene
(TNT). Several examples of their structures are shown in Figure 18.1. Pol;,ch,_
rinated biphenyls (PCB' constitute a particularly troublesome class of widely disper:.:
compounds. These compounds can disrupt reproduction in birds and cause cancel a::
hormone effects in humans and other animals. Several hundred varieties of liquli ,
resinous PCBs were produced from 1930 to 1980 and used as specialized lubrical.,
hydraulic fluids, and electrical transformer insulators, as well as in certain epoxy pa_:,,
Alternative cleanttp and many other industrial and commercial applications. Because of their extleme Ier.--
technologies for lcaking tance to natural decay and their ability to entel food chains/ even today soil and lr,ů,.r,rnderground
storage tanks: all over the siobe contain at least traces of pcBs.
WWW,ePa,§,ovllsweru§tl/Pubs The sites most intensely contaminated with organic poilutants are usually 1oc;:.
index'htm#tLrm' ,-rear chemical manufacturing plants or oil storage facilities, but railway, shipping,
":
_
highway accidents also produce hot spots of contamination. Thousands of neigh: , :
hood gas stations lepresent potential or actuai sites of soi] and groundwater conti]]
nation as gasoline ieaks from old, rusting underground storage tanks (Figure 1\ However,
as already mentioned, by far the most widely dispersed xenobiotics are t:
designed to kil1 unwanted organisms (i.e., pests).
pestícídes
Pesticides aíecommonly ciassified according to the group of pest organisms targr.:_
(l) insecticides, (2) fungicides, (3) herbicides (weed killers), (4) rodenticides, and (5) rlci,," "
cides,In practice, all find their way into soils. Since the first three are used in the la::_
quantities and are therefore more likely to contaminate soils, they will be given pr::-,: FIGURE
l8.2 Leaking underground stora.. ,.: ,
(LUST) replacement at a gas station in Califorl.. old
rusting steel tanks have been lemol-. - _ ]
replaced by more corrosion-resistant fiberglas: -" ,
which are set in the ground and covered li - , ,,i
gravel. The soil and groundwater aquifer ben.., - :, ,
tanks were cleaned up using special technl;_,
stimulate soil microorganisms and to pu. :
volatile organics such as benzene vapors, F..::-_"l,,
tion and replacement typically costs $500,[t ,
single gas station. (Photo courtesy of R. Wei1
8O0 SgíIs and Chemícal Pollutíon
!ó,l
i §.3
considelation. Figure 18.1 shows that most pesticides contain aromatic rings of some
kind, but that there is great variability in pesticide chemical structures.
lnsrcttciprs. Most of these chemicals are included in three general groups. The chlorinated
hydrocarbons, such as DDT, were the most extensively used until the early l97Os,
when their use was banned or severely restricted in many countries due to their low
biodegradability and persistence, as wel1 as their toxicity to birds and fish. The
orsanophosphate pestícides are generally biodegradable, and thus 1ess likely to build up
in soils and y/ater. However, they are extremely toxic to humans, so great care must be
used in handling and applying them. The carbttmates are considered least dangerous
because of their ready biodegtadability and relatively low mammalian toxicity. Howeve1
they are highly toxic to honeybees and other beneficial insects and to earthworms.
Funetclpps" Fungicides are used mainly to control diseases of fruit and vegetable crops
and as seed coatings to plotect against seed rots. Some are also used to plotect harvested
fruits and vegetables from decay, to prevent wood decay, and to protect clothing from
mildew. Organic materials such as the thiocarbamates and triazoles are currently in use.
HrRstclor§, The quantity of herbicides used in the United States exceeds that of the
other types of pesticides combined. Starting with 2,4-D (a chlorinated phenoxyalkanoic
acid), dozens of chemicals in literally hundreds of formulations have been placed on
the market (see Figure 18.1). These include the triazines, used mainly for weed control
in corn; sLtbstítutecl ureas; some carbamates; the relatively new sttlfonylureas, which are
potent at very low rates; clinitroanilínes; and acetttnilitles, which have proved to be quite
mobile in the environment. One of the most widely used herbicides, glyphosate
(Roundup), does not belong to any of the aforementioned chemical groups. Unlike
most herbicides, it is nonselective, meaning that it will kill almost any plant, including
crops. However/ a gene that confers resistance to its effects has been discovered and
engineered into several major crops. These genetically engineered crops can then be
gro\^/n with a very simple, convenient method of weed control thai usually consists of
one oI two sprayings of glyphosate that will kill all plants other than the resistant crop.
As one might expect, this wide variation in chemica1 makeup provides an equally
wide variation in properties. Most herbicides are biodegradable, and most of them are
relatively 1ow in mammalian toxicity. However, some are quite toxic to fish, soil fauna,
and perhaps to other wildlife. They can also have deleterious effects on beneficial
aquatic vegetation that provides food and habitat for fish and shellfish.
Nrntnroctnrs. Although nematocides are not as widely used as herbicides and insecticides,
some of them are known to contaminate soils and the water draining from
treated soils. For example, some carbamate nematocides dissolve readily in water, are
not adsorbed onto soil surfaces, and consequently easily leach downward and into the
groundwater. Other nematicidal chemicals are volati]e soil fumigants thai kill virtually
all life in the soil, both the helpful and the harmful (Section 18.4). Methyl bromide,
once the most commonly used of these t'umigants, has been banned because of its
adverse effects on the atmosphere and parts of the environment. Happiiy, the search for
substitutes has lead to the development of many nonchemical means to manage the
pests once controlled by this highly toxic chemical (e.g., see Figure 11.14).
EE!-§AV§OR cF $R6AN§e CHEMlcAL§ lN §olL2
Once they reach the soil, organic chemicals, such as pesticides or hydrocarbons, move
in one or more of seven directions (Figure 18.3): (1) they may vaporize into the atmosphere
without chemical change; (2) they may be absorbed by soils; (3) they may move
downward through the soil in liquid or solution form and be lost from the soil by leaching;
(a) they may undergo chemica1 reactions within or on the surface of the soil; (5)
they may be broken down by soil microorganisms; (6) they may wash into streams and
rivers in surface runoff; and (7) they may be taken up by plants or soil animals and
]For reviews on organic chemicals in the soil envilonment, see Sawhney and Brown (eds.) (1989) arrd
Pierzynski, ct al. (2000); for pesticides, see Cheng (ed.) (1990).
rk
:le
rd
ŇS,
\ea
,he
to
_lut
]ia-
.ía
D
BrHRvton oF ORGANIc CHrnllcnls lN §oll 801
4\ulalÝ,c
,a_ \
Detoxication
o
š-
*lel<-
*:ř
§l
OC:]}
\
i'move
uP the food chain. The specific fate of these chemicals will be determined at 1e,:
in part by their chemical structures, which are highly variable.
Organic chemicals vary greatly in theil volatility and subsequent susceptibility to atrl- ,
sPheric loss. Some soil fumigants, such as methyl bromide (nbw banneďfrom most use ,
were selected because of their very high vapor pressure, which permits them to peneti;.:
soil Pores to Contact the talget organisms. This same characteristic encoulages iapid , _ ,
to the atmosphere after treatment, unless the soil is covered or sealed. a few heiuicic:
(e.g., trifluralin) and fungicides (e.g., PCNB) are sufficiently volatile to make vaporizat_ _ .
a Primary means of their loss from soil. The lighter fractions of crude oil (e.g, gaso1_:,.
and diesel) and many solvents vaporize to a large degree when spilied on thďsořt,
The assumption that disappearance of pesticides from soits is evidence of th. :
breakdown is questionable. Some chemicals lost to the atmosphele are known to leti]:
to the soil or to surface v/aters with the rain.
The adsorption of organic chemicals by soil is determined largely by the characterist::
of the comPound and of the soils to which they are added. Soif organic mattel and hi5:
surface-area clays tend to be the strongest adsorbents for some compounds (Figure 18i=
\
F|GURE l 8.3 Processes affecting the dissipation of organic chemicals (OC) in soils. Note that the oC symbol is split up by decomp ,
tion (both bY light and chemical reaction) and degradation by microorganisms, indicating that these pro."rr.i altei or destror- ::
organic chemical. In transfeí processes, the oC remains intact, lFrom Weber and Milleí (19sĎ)]
ValatíIítry
Adsarptíon
Absorption
and exudatíon
t-
-"
802 Soílsorrd ťlhemíealFoíluííon
Lakeland
sand
a
HlOr-
treated
lakeland
sand
\o.
E ]. l
,i )L]
§,],
o- ].
pH 2.0
pH 4,5
pH 7.0
pH l 1.5
F|GURE l 8.4 Adsorption of polychlorinated biphenyl (PCB) by different
soil mateíiais, The Lake]and sand (Typic Quartzipsamments)
lost much of its adsorption Capacity \^/hen tleated v/ith hydlogen
peíoxide to lemove its organic mattel. The amount of soil material
required to adsorb 50o/o of the PCB was approximately 10 times as
8íeat for montmorillonite (a 2:I clay mineral) as for soil organic
matter/ and 10 times again as great foí Hzo2-treated Lakeland sand,
Later tests sho\^/ed that once the PCB was adsorbed, it was no longer
available for uptake by plants. Note that the amount of soil material
added is sho\^/n on a 1og scale, [From Strek and Weber (1982)]
F|GURE l 8.5 Theeffectof pHof kaoliniteontheadsorptionoí
glyphosate, a \ťidely used herbicide (Brand name Roundup@).
[Reprinted with permission flom J. S. McConnell and L. R.
Hossner, |. Agric. F oo d Chem. 33:707 5-7 B ( 1 9B5); copyright 1 9B5
American Chemical Society]
oj0: í:,,] :.]',:]]] ] :,l: ]l]]:]l]]]:]::]:]:] :,]:l :l l:orlll1.1l
&nouňt:,óí,,ad§orUetit:,(é)
Leachíng and Runoff
while oxide coatings on soil particles strongly adsorb others. The presence of certain
functional groups, such as
-OH, -NHz, -NHR, -CONH2, -COOR,
and
-*NR3,in the chemical structure encouíages adsorption, especially on the soil humus. Hydrogen
bonding (see Sections 5.1 and 8.3) and protonation [adding of H* to a group such as an
-NH,
(amino) gloup] probably promotes some of the adsorption. Everything else being
equal, larger organic molecules with many charged sites are more strongly adsorbed,
Some organic chemicals \^/ith positively charged groups, such as the herbicides
diquat and paraquat, are strongly adsorbed by silicate clays. Adsorption by clays of
some pesticides tends to be pH-dependent (Figure 18.5), with maximum adsorption
occuning at loy/ pH levels, which encoulages protonation. Adding an H* ion to functional
gíoups (e.g.,
-NHz)
yields a positive charge on the herbicide, resulting in greater
attíaction to negatively charged soil colloids.
The tendency of organic chemicals to leach from soils is closely related to their solubility
in water and theií potential for adsorption. Some compounds/ such as chloroform
and phenoxyacetic acid, are a million times more water-soluble than others, such as
l14
l)
:],d
Jo
m
l
;b8:i -,,
€ ,,6
o,,
o.
E4
4
],§,,1,1,.,:],]p,l,,;:,.],;,{§,,:.::".,J:§::,|,.:.:Q§,::':..'.:,:l0,]]',]:,]],]]8o:
1,1l :,,::',],,,,:HéibteiiJé:]cainaéntíáiióň]](iiiElL)]].]]:]]:,l ,]:,., ]
1,0 90 1,0o
BrHnvton oF ORGANIC CHeulcRl-s IN SolL 803
DDT and PCBs, which are quite so}uble in oil but not in water. High water-solubi1;favors
leaching losses.
Strongly adsorbed molecules ale not likely to move down the profile (Table i$*I}ur
Likewise, conditions that encourage such adsorption will discourage leaching. Leacr_ry
is apt to be favored by water movement, the greatest leaching hazatd occurring r::
highly permeable, sandy soils that are also low in organic matter. Periods of high rainfáll
around the time of application of the chemical promote both leaching and runott
l§es Qable 18.3). With iome notable exceptions, herbicides seem to bésomewhat
morě-mobile than most fungicides or insecticides, and therefore are more likely to find
their way to groundwater supplies and streams (Figure 18.6).
Contamínatíon of Groundwater
Experts once maintained that contamination of groundwater by pesticides occurred
only from accidents such as spills, but it is now known that many pesticides reach the
groundwater from normal agricultural use. Since many people (e.g., 4Oo/o of Americans,
depend on groundwater for their drinking supply, leaching of pesticides is of wide concern.
Table 18.4 lists some of the 46 pesticides found in a national survey of well wateĎ
in the United States. The concentrations are given ínparts per billion (see Box 18.1). In
some cases, the amount of pesticide found in the drinking water has been high enough
to raise long-term health concerns.
Chemícal Reactíons
Upon contacting the soil, some pesticides undergo chemical modification independent
of soil organisms. For example, iron cyanide compounds decompose within hours or
days if exposed to bright sunlight. DDl diquat, and the triazines are subject to sloltphotodecomposition
in sunlight. The triazine herbicides (e.g., attazine) and organophosphate
insecticides (e.g., malathion) are subject to hydrolysis and subsequent degradation.
While the complexities of molecular structure of the pesticides suggest different
mechanisms of breakdown, it is important to realize that degradation independent ol
soil organisms does in fact occur.
MícrobíalMetabolísm
Biochemical degradation by soil organisms is the single most important method trr
which pesticides are removed from soils. Certain polar groups on the pesticide molecu]e:
such as
-OH, -COO-/
and
-NH2,
provide points of attack for the organisms.
DDT and other chlorinated hydrocarbons, such as aldrin, dieldrin, and heptachlo:
ale very slowly broken down, persisting in soils fot 2O oI more years. In contrast, the
organophosphate insecticides, such as parathion, are degraded quite rapidly in soils
TABLE l 8.2 The Degree of Adsorption of §elected Herbicides
Weakly adsorbed herbicides are mole susceptible to movement
in the soil than those that are more tightly adsorbed,
Common name or designation Trade name Adsorptivity to soil colloids
Dalapon
Ch]oramben
Bentazon
2,4-D
Propachlor
Atrazine
Alachlor
EPTC
Diuron
Glyphosate
PaIaquat
Trifluralin
DCPA
Dowpon
Amiben
Basagran
Severa1
Ramrod
AAtrex
Lasso
Eptam
Karmex
Roundup
Paíaquat
Treflan
Dactha]
None
Weak
Weak
Moderate
Moderate
Strong
Strong
Strong
Strong
Very strong
Very strong
Very stron8
Very strong
804 SoíIs and Chemícal Pollutíon
TABLE l8.3 §urface Runoff and Leaching Losses (Through Drain Tiles) of the
Herbicide Atrazine from a Clay Loam Lacustrine Soil (Alfisols) in Ontario, Canada
The herbicide was applied at 1700 g/ha in late May. The data are the avera&e of three
. tillage methods. Note that the rainfall for May and |une is related to the amount
of herbicide lost by both pathways,
Atrazine loss, g/ha
Year of study
Surface runoff
ioss
Drainage
water loss
Total dissolved
loss
Percent oftotal Rainfall,
applied, o/o May-|une, mm
1
2
)-)
4
18
1
51
13
9
2
61,
1,.6
o.2
6.6
2.6
77o
30
255
16-5
27
3
113
45
Pf*mť Absorpúíorn
l0,000
|,000
l00
l0
I
0.0l
0.00l
Data abstracted from Gaynor, et al. (1995).
apparently by a variety of organisms (Figure 18.7). Likewise, the most widely used herbicides,
such as 2,4-D, the phenylureas, the aliphatic acids, and the carbamates, are
readily attacked by a host of organisms. Exceptions are the triazines, which are slowly
degraded, primarily by chemical action. Most organic fungicides are also subject to
microbial decomposition, although the rate of breakdown of some is slow, causing troublesome
residue problems.
Pesticides are commonly absorbed by higher plants. This is especially true for those pesticides
(e.g., systemic insecticides and most herbicides) that must be taken up in order
to perform their intended function. The absorbed chemicals may remain intact inside
the plant, or they may be degraded. Some degradation products are harmless, but others
are even more toxic to humans than the original chemical that was absorbed. Understandably,
society is quite concerned about pesticide residues found in the parts of
plants that people eat, whether as fresh fruits and vegetables or as processed foods. The
use of pesticides and the amount of pesticide residues in food are strictly regulated by
law to ensure human safety. Despite widespread concerns, there is little evidence that
the small amounts of residues permissible in foods by law have had any iII effects on
public health. However, routine testing by regulatory agencies has shown that about 1
to 2o/o of the food samples tested contain pesticide residues above the levels permissible.
J
1
.9
e
o
o
U
2o0 300 4o0 500 600
Days after application
Fl6!.jRE i$.6 Concentlation of two widely used herbicides,
atrazine and alachlor, in the runoff from watersheds
in Ohio planted to corn, along with the allowed Maximum
Contaminant Level (MCL) for drinking water. Note
that the concentration far exceeds the MCL, especially for
attazine, during the first 50 to 100 days after application.
If this runoff is not diluted with less-contaminated watel,
it would not be suitable for consumption by downstream
users. [Redrawn from Shipitalo, et a1. (1997)]
Atrazíne
L:_..\
Atrazine
McL lMLL --l--------Ě: = =: = =: = = =; =: =::--\--------
Alachlor
"7"
Alachlor
I00 700 800
§EFiA,lloR ůFů{iGANie enr,tttep"rE lN §oll E05
TABLE ; 8.4 Pesticides Present in Groundwater from NormalAgricultural Use
Note the wide range in concentrations which are considered to be risky to health. The great majoriý
of wells samPled were uncontam,:;::í:;rr::ír,x:;;;::ff
,wu,e
detected ťhey were often near
pesticide
Level fouttd, parts per billbn
Use' Median Ievel fotmdb Maximum level foLutd HeaLth-advisory level'
Alachlor
Aldicarb
Atrazine
Bromaci1
carbofuran
Cyanazine
2,4-D (Z,4 Dichlorophenoxyacetic acid)
DBCP d
DCPA
Dinocebd
EDB
Fonofos
Malathion
Metolachlor
Metribuzin
Oxamyl
Trifluran
10
200
i00
2o0
200
5
H
I
H
H
I
H
H
FUM
H
H, I,F
F
I
I
H
H
I
H
1
9
1
9
5
0.4
1
0.01
109
1
1
0.1
42
0.4
1
4
0.4
113
315
40
22
1,76
7
50
0.o2
L040
37
74
0.9
53
)a
7
395
2.2
10
3
90
40
lo
70
1000
7
Data from General Accounting office (1991).
" H = herbicide; I = insecticide, F = fungicide; FUM = fumigant.
b
Fifty percent above and 50oll below this value.
c
Health-advisory level is the concentration that is suspected of causing health problems over a 7O-year lifetime. Blank means no advisory ]el,t
has been set.
d
Most uses of this pesticide have been banned in the United States.
persísťegiceířc §oóřs
The persistence of chemicals in the soii is the net result of all their reactions, nl.-:
ments, and degradations. Marked differences in persistence ale the rule (see Fi:,'.:-
18.7). For example/ organophosphate insecticides may last only a few days in soils. . --
widely used herbicide 2,4-D persists in soils for only two to four weeks. PCBs, DDT :: _
other chlorinated hydrocarbons may persist for 3 to 20 years or 1onger (Table 18.5 . ., ,
persistence times of other pesticides and industrial organics fall generaliy betweer .,,
extremes cited. The majority of pesticides degrade rapidty enough to prevent bui]du:soils
having normal annual applications. Those that resist degradation have a $ii:,:
potential to cause environmental damage.
Continued use of the same pesticide on the same land can increase the rate of n_ _:
bial breakdown of that pesticide. Apparently, having a constant food source allc:, ,
population build up of those microbes equipped with the enzymes needed to
-:
. down
the compound. This is an advantage with respect to environmental qualitl, ;.:.,
a principle sometimes applied in environmental cleanup of toxic organic compo.-_: _
but the breakdown may become sufficiently rapid to reduce a pesticide's effectir-el:.
á?egá*r*m$ Vruďmeg,m&í §aty ta Fes fíeíde§-eaakínE
The vulnerability of groundwater to contamination by pesticide leaching varies g:..
from one area to another. Highest r,"ulnerability occurs in regions with high rainfa_ ,
abundance of sandy soils, and intensive cropping systems that involve high usa;.
those types of pesticides that are most soluble and 1east strongiy adsorbed by th. ,
colloids. For example, the southern Atlantic Coast of the United States is an aiea,.,,:-_sandy
soils are prominent, and where pesticide-intensive cropping systems (for _ ,
vegetables, peanuts, and cotton) are used. Likewise, vulnerability to leaching of : ,
pesticides and nitrates íshigh in the Corn Belt, where much of the 1and is under :
tinuous corn production with its high herbicide and nitrogen fertilizer use.
It should be pointed out that pesticide hazards are site-specific, and that : regional
generalizations might mask Iocalized areas of vulnerability. For examp_1
arid regions irrigated areas of intensive vegetable crop production may experience : "
E$6 "§oíísclncí ť'kemícal Polluťíom
As analytical instrumentation becomes more sophisticated, contaminants can be detected at much lower leve|s than was the
case inthe past. Since humans and other organisms can be harmed by almost any substance ií large enough quantities aíe
involved, the subject oí toxicity and contamínation must be looked at quantítatíuelg. That is, we must ask houl much, nol
simply urhat, is in the environment. Many highly toxic (meaning harmíul in very small amounts) compounds are produced
by natural processes and can be detected in the air, soil, and water-quite apart from any activitie§ oÍ humans.
The mere pre§ence oía natural toxin or a synthetic contaminant may not be a problem. Toxicity depends on ( l ) the concentratíon
of the contaminant, and (2) the level oí exposure oíthe organism. Thus, low concentrations oÍ certain chemicals
that would cause no observable effect by a síngle erposure (c,g.. one glass oí drinkíng warer) may cause harm (e.g., cancer,
birth defects) to individuals exposed to these concentrations over a long period of time (e.g,, three glasses oí water a
day íor many years).
Regulatory agencies atlempt to estimate the eÍÍectsof long-term eYposure when they §et standards Íor no-observableeííectlevels
(NOEL) or health-advisory levels (see Table l 8.'l). Some species and individuals within a species will be much more
sensitive than others to any given chemical. Regulators atlempt [o consider the risk to the most susceptible individual in any
particular case. For nitrate in groundwatel this individual might be a human iníant whose entire diet consists oí iníant formula
made with the contamlnated water, For DDT, the individual at greate§t risk might be a bird of prey that eat§ fish that eat worms
that ingest lake sediment contaminated with DDT. For a pesticide taken up by plants from the soil, the individual at 8reatest
risk might be an avid gardener who eats vegetables and íruits mainly írom the treated garden over the course of a liÍetime.
lt is important to get a íeel íor the meaning of the very small numbers used to express the concentration oí contaminants
in the environment. For instance. in Table l 8.4, the concentrations are given in parts per billion (ppb). This is equivalent
to microgram§ per kilogram or pglkg. ln water this would be pg/L. To comprehend the number l bíllíonimagine a
billion golí balls: lined up. they would stretch completely around the earth. One bad ball out oí a bíllion (l ppb) seems like
an extremely small number. On the other hand, l ppb can seem like a very large number. Consicler water contaminated with
l ppb oí potassium cyanide, a very toxic §Ubstance consi§ting of a carbon, a potassium. and a nitrogen atom linked together
(KcN). lí you drank iust one drop oí this Water, you would be ingestíng almost I fríllíorrmolecule§ of potassíum cyanide:
6,023 x l0]3 molecules 9.3 x l0ll molecules
l mol
lmol lpKCN lusKCN L cmj
,1 Lv'"w_=
" 65 g KCN " l06/pg KCN
" L ' loj cm]
" ludrops drop
ln the case oí potassium cyanide, the molecules in this drop of water would probably not cause any observable eííect.However,
íorother compounds. this many molecules may be enough to trigger DNA mutations or the beginning of cancerous
8rowth. Assessing these risks is still an uncertain business.
Atrazine (triazine)
Triíuralin (dínitroaniline)
@
d
.E
,6
Eq
c
.9
o
oo
;.ž
FIGURE l8.7 Degradation of four herbicides (alachlor, atrazine,
2,|-D, and trifuralin) and two insecticides (parathion and
carbaryl), all of which are used extensively in the Midwest of
the United States. Note that atlazine and alachlor are quite
slowly degraded, whereas parathion and 2,4-D are quickly broken
down. [Reprinted with permission from R. G. Krueger and
J. N. Seiber, Treatment and Disposal of Pesticide iťasfes, Symposium
Series 259; copyright 1984 American Chemical Society]
;
Brunvton oF ORGANIc Currr.llcnrs lN §oll 807
TABLE l8.5 Common Range of Persistence of a Number
of Organic Compounds
Risks of environmental pollution are highest with those chemicals
with gre ate st p er sistenc e.
Organic chemical
persistence
in soils
Chlorinated hydrocarbon insecticides (e.g., DDf, chlordane, and dieldrin)
PCBs
Triazine herbicides (e,g., atrazine and simazine)
Glyphosate herbicide
Benzoic acid herbicides (e.g., amiben and dicamba)
Urea herbicides (e.g., monuron and diuron)
Vinyl chloride
Phenoxy herbicides (2,4-D and 2,4,5-T)
Organophosphate insecticides (e.g,, malathion and diazinon)
carbamate insecticides
Carbamate herbicides (e.g,, barban and CIPC)
3-2O yr
2-7O yr
1,-2 yt
6-20mo
2-1-2mo
Z-1-0 mo
1-5 mo
1-5 mo
7-72wk
1-8 wk
2-8 wk
l8.4 EFFECTS oF
siderable leaching of both pesticides and nitrates. Likewise, application of certain \^řatersoluble
pesticides may result in groundwatel contamination even where the soil may
not be coarse in texture.
PEsTlclDEs oN §oll oRGANlsMs
Since pesticides are formulated to kill organisms, it is not surprising that some of these
compounds are toxic to specific soil organisms. At the same time/ the diversity of the
soil olganism population is so great that, excepting a few fumigants, most pesticides do
not kill a broad spectrum of soil organisms.
Fumigants are compounds used to free a soil of a given pest, such as nematodes. These
compounds have a more drastic effect on both the soil fauna and flora than do other
pesticides. For example ,99o/o of the microarthropod population is usually killed by such
fumigants as DD and vampam, and it may take as long as t\^/o years for the population
to fully lecover. Fortunately, the recovery time for the microflora is generally much less.
Fumigation reduces the number of species of both flora and fauna, especially if the
treatment is repeated, as is often the case where nematode control is attempted. At the
same time, the total number of bacteria is frequently much greater following fumigation
than before, This increase is probably due to the relative absence of competitors
and predators following fumigation and to the carbon and energy sources left by dead
organisms for microbial utilization.
The effects of pesticides on soil animals varies greatly from chemical to chemical and
from organism to organism. Nematodes are not generally affected, except by specific
fumigants. Mites are generally sensitive to most organophosphates and to the chlorinated
hydrocarbons/ with the exception of aldrin. Springtails vary in their sensitivity to
both chlorinated hydrocarbons and organophosphates, some chemicals being quite
toxic to these organisms.
ERnrHwonrus, Fortunately, many pesticides have only mildly depressing effects on earth\^,/orm
numbers/ but there are exceptions. Among insecticides, most of the carbamates
(carbaryl, carbofuIan, aldicarb, etc.) are highly toxic to earthworms. Among the herbi
cides, simazine is more toxic than most. Among the fungicides, benomyl is unusually
toxic to earthworms. The concentrations of pesticides in the bodies of the earthworms are
closely related to the levels found in the soil (Figure 18.8). Thus, earthworms can magnify
the pesticide exposule of birds, rodents, and other cleatures that prey upon them.
Fumígants
Effects on §oíl Fauna
808 Soils and Chemícal Pollutíon
a
a
a a aa
aa
aa
aa
a"
3""a aaa a
Ooo
aa
a
aa
a
a
"§a
aa
aa
a
aa
aaa
a
aa
aa aa
E
o
;o
l=
§
co
o
U
l00.o
l0.0
o.0 I
0.00 l
0.0l 0.1 1.0
concentration in earthworms
lo.0 l00.o
(ppr)
F!6URE l 8.8 Ěffect of concentlation of pesticides in soil
on their concentlation in earthworms. Birds or rodents eating
the earthworms at any level of concentIation would further
concentlate the pesticides. [Data from several sources
gathered by, Thompson and Edwards (1974); used with permission
of Soil Science Society of America]
Pesticides have significant effects on the numbers of certain predators and/ in tuln/
on the numbels of Prey olganisms. For example, an insecticide ihat reduces the numbers
of PredatorY mites may stimulate numbérs of springtaits, which ,.r"" i, p."y ro.
the mites (Figure 1B.9). Such organism interaction is norňral in most soils.
Effects orr 5oííMícroorganísms
The overall levels of bacteria in the soil are generally not too seriously affected by pesticides.
However, the org_anisms responsiblďfor nitiification and nitiogen fixatíon are
sometimes adversely affected. Insecticides and fungicides affect both"procesr", -o.Óthan do most herbicides, although some of the latteř can reduce the numberu or o.gi.risms
carrYing out these two reactions. Recent evidence suggests that some pesticideňan
enhance biological nitrogen fixation by reducing trre řřirrity of protozba and other
organisms that are Competitors or predators of the nitrogen-fixing bacteria. These findings
illustrate the complexity of life in the soil.
+l20
+l o0
+80
+60
+40
+20
_20
_4o
_60
_80
Springtails
/
Predatory
mítes
\.* lnsecticide residues
3 4 5 6 7 8 9 lo ll 12
Months after insecricide application
F|GL|RE l8.9 The direct effect of insecticide on predatoíy
mites in a soil and the indirect effect of reducing mite
numbers on the population of springtails (tiny iruects.t
that seíve as prey for the mites. lReplotted from Edwards
(1978); used with permission of Academic Press, Inc,,
Londonl
l00
80
60
40
20
@
.=
.=
@^
-96
.!c
c
§
E
f
tr
E
.!
c6
@
o
.E
@
c
-U
l4
Errrcrs oF PEsTlclDEs oN §oll Oncnntsnts E09
Fungicides, especially those used as fumigants, can have marked adverse effects on
soi1 fungi and actinomyÓ.t.r, thereby slowin§down the humus formation in soils, Inter_
;;ii;áďh"*ever, the iro."r, oiu-i"onifica'tion is often stimulated by pesticide use,
The negative ertect's-ái most pesticicles on soil microorganisms are temporary, and
after a few days o, -."kr, organism numbers generally recov^er, But exceptions are com_
mon enough to aictaá cautián in the use of tÉechemicals. Care must be taken to appll
ilr*Á
""rý,*hen
alteI;ate means of pest managemerrt are notavailable.
This brief review of the behavior of organic Čhemicalsin soils reemPhasizes.the complexity
of the chang", iúttut. place whěn new and exotic substances are added to our
environment. our máwi"ag" orirr" soil processes involved certainlv reaffirms the neces_
sitv for a thorough evaluation of potential environmental impacts prior to approval and
"rá
ár
".*
cherňicals for extensive use on the land,
_,,_ __ *, *-***-"_*-íťllť
I
l8.6 coNTAMlNATloN VíITH Toxlc lNoRGANlc suB§TANcEs4
aFor a review of this subject, see Kabata-Pendias and Pendias (1992)
The toxicitY of inorganic contaminants released into the environment every year is now
estimated to exceed that from organic and radioactive sources combined. Á fát share of
these inorganic substances ends up contaminating soils. The greatest problems most
likelY involve merculý cadmium, lead, arsenic, nicŘel, coppel/ zinc, chrómium, molybdenum,
manganese/ selenium, fluorine, and boron. To a greater or lesser degree, alíof
these elements are toxic to humans and other animals-. cadmium and a-rsenic are
extremelY Poisonous; mercury/ lead, nickel, and fluorine are moderately so; boron, copPeI,
manganese, and zinc ate relatively lower in mammalian toxicity. Table 18.7 prbvides
background information on the uses, sources, and effects of some of tňese
elements. Although the metallic elements (see periodic table, Appendix B) are not all,
stťctlY sPeaking, "heaw" metals, for the sake Óf simplicity thij term is often used in
referring to them.
818 SoíIs and Chemícal Pollutíon
TAELE l8.7 Sources of §elected lnorganic Soil Pollutants
Chemical Mctjor uses atttl sources of soil cottaminatiott
Orgatústtts
principally httrrned" Hurnan health effěcts
.\rSeniC
(]adnrium
Chromium
Copper
Lead
\,íercury
\ickel
Se]cnium
Zinc
Pesticides, plant clesiccants, animal fced additives, coal
and petroieum, mine tailings, deteIgents, and irrigation
WateI
Electroplating, pigments for plastics anc1 paints, plastic
stabilizers, batteries, and phosphate fertilizers
Stainless stee1, chíome-plated metals, pigments,
refractory brick manufacture, anc1 leather tanning
Mine tailings, fly ash, fertilizers, windblown
coppel-containing dust, and \^/ateť pipes
Conibustion of oil, gasoline, and coal; iron and steel
production; solder in wateI-pipes; paint pigment
Pesticides, catalysts for synthetic polymers, metallurgv, and
thermometeIs
Combustion of coal, gasoline, arrc1 oil; alloy
manufactute; electroplating; batteries; and mining
High Se geological formations and irrigation \,vastewatel in
which se is concentratecl
Galvanized iron anc1 stee1, alloys, batterics, brass,
rubber manufacture, mining, and old tires
H,A,F,B
H,A,F,B,P
H, A, F], B
lP
H,A,lB
H,A,F,B
RP
H,A,F,B
I,P
Cumulative poison,
cancer, skin iesions
Healt and kidney disease,
bone embrittlement
Mutagenic; also
essential nutíient
Rare; essentia1
nutlient
Brain damage,
convulsiclt-ts
Nerve clamage
Lttng cancer
Rare; loss oi l-rair and nail
defornities; e5Sential
nutrient
Rare; essentiai tlLltrient
- H = humans, A -animals/ F = fish, B -birds, P = plants,
l)ata selecte(l from Moore and Rarnan]oorthv (i984) and nurncrous otheI SouICes
§ources and Accwmw§atíon
There are many sources of the inorganic chemical contaminants that Can accumulate in
soils. The burning of fossil fuels, Šmelting(Figule 18.19), and other processing techniques
release intČthe atmosphere tons of řhesďelements, which can be Carried for miles
aná later deposited on the vegetation and soil. Lead, nickel, and boron are gasoline additives
that aró released into thě atmosphele and carried to the soi1 through Iain and Sno\^/.
Borax is used in detergents, fertilizers, and forest fire retardants, all of which commonly
reach the soil. superphosphate and limestone, two widely used soii amendments,
usualÍy contain small quaniitiesbf cadmium, copper/ manganese/ nickel, and zinc, Cad_
mium'is used in platin§ metals and in the manufacture of batteries. Arsenic WaS for many
yeals used as an insecticide on cotton, tobacco, fruit crops, lawns, and aS a defoliant or
vine kilter. Some of these mentioned elements are found as constituents in sPecific
organic pesticides ancl in domestic and industria1 sewage sludge. Additional localized
F|6URE I8.19 A partially denuded hiliside just downwind
from a copper smelter in Anaconda, Montana,
The heavy metal-laden fumes have contaminated this
area with copper, zinc, nickel, and other metals to levels
that are highly toxic to most plants and many othel
olganisms, decirnating the ecosystems of the aíea. Note
the serious erosion that has resu]ted from the devegetation,
despite the lecent invasion of the area by a few
meta]-tolirant plant species. (Photo coultesy of R. Wei1)
CoNr,q,ttlnerloN wlTH Toxtc lNoRaANlc §uBsTANcEs 819
Lead contamination is one of the most serious and widespread problems oíinorganic soil pol|ution. Long-term exposure
to low levels of lead can have proíoundly deleterious efíects on child development and neuro|ogical functíoning,
including intelligence. Lead poisoning has been shown to contribute to mental retardation, poor academic performance,
and juvenile delinquency. The U.S. EPA reports that nearly l million chi|dren in America today have dangerously
elevated levels oílead in their blood. ln the past (and, uníortunately, in the present in many developing countries
that still use leaded gasoline), much oíthe lead came írom burning leaded íuels. The content oí lead in soils commonly
increases with proximity to maior highways. The lead content oí soíls also usually increase§ as the distance írom the
center oí a major city decreases. Residents of inner cities generally live surrounded by lead-contaminated soils. The soil
on the windward side oí apartment buildings often shows the highest accumulations of lead, as it is there that the
wind-carried particulates tend to settle out oíthe air.
A second reason íor high lead concentrations in urban soíls is related to the lead,based pigments in paint írom prel
970 buildings. Paint chips, ílakes, and dust írom sanding painted suríaces spread the lead around, and eventually
much oíit ends up in the soi|. During dry weather, soi| particles blow about, spreading the lead and contributing to
the dust that settles on floors and windowsills. Although plants do not readíly take up lead through their roots, leadcontaminated
dust may stick to foliage and Íruits. Eating these garden products and breathing in lead-contamínated
dust are two pathways for human lead exposure (see Figure l 8.20).
However, the most serious pathway, at least íor young chi|dren, is thought to be hand-to-mouth activity-basically,
eating dirt. Anyone who has observed a toddler knows that the child's hands are continually in its mouth, Leadcontaminated
dust on suríaces in the home can thereíore be an important source oí lead exposure foryoung children;
so, too, can lead-contaminated soil in outdoor play areas. Having children wash their hands írequently can signiíicantly
cut down their exposure to this insidious toxín.
ln 200 l , the U.S. EPA set new standards íor the cleanup oí lead in soil around homes: 400 part§ per míllion (ppm)
of lead in bare soil in children's play areas or l ]00 ppm average íor bare soil in the rest oíthe yard. Soils with lead
leve|s higher than these standards require some remediation.
Until suitab|e phytoremediation techniques are íound, protecting children írom lead in soil around the home will
continue to be largely a matter oí stabilizing the lead away írom the reach oí children and dust-creating winds. Excavation
oísoi| around homes is extremely expensive and, given the low mobility of lead in soils, probably not necessary.
lnstead, some areas may be covered with a thick |ayer oíuncontaminated topsoil; others may be paved over, or a
wooden deck may be built over them. A well-maintained cover oíturfgrass will prevent most dust formation and soil
ingestion. Removal oílead-based paints is likewise very expensive and diííicult,so isolating the lead-based paint under
severa| coats of íresh paint may be a better way to avoid expo§ure.
F|GURE l8,20 Sources of heavy meta]s and their cycting in
should be noted that the content of metals in tissue generally
l.ulnerability of humans to heav.v metal toxicity,.
the soil-water-air-organism ecosysten]
builds up from left to íight, indicating
82o §oíls ond chemícal pollutíorr
contamination of soils with metals results from ore-smelting fumes, industrial wastes,
and air pollution.
Some of the toxic metals are being released to the environment in increasing
amounts, while others (most notably lead, because of changes in gasoline formulation)
are decreasing. All are daily ingested by humans, either through the air or through food,
watel, and-yes-soil (see Box 18.2).
Concentraúon ínOrganísm Tíssue
l8.7 foIENtTlAl
Irrespective of their sources, toxic elements can and do reach the soil, where they
become part of the food chain: soil-+plant-+animal-+human (Figure 18.20). Unfortu_
nately, once the elements become part of this cycle, they may accumulate in animal and
human body tissue to toxic levels. This situation is especially critical for fish and other
wildlife and for humans at the top of the food chain. It has already resulted in restrictions
on the use of certain fish and wildlife for human consumption. Also, it has become
necessary to curtail the release of these toxic elements in the form of industrial \^/astes.
l8.8 REAcTloNs oF lNoRGANlc CoNTAMINANT§ !N soIls
Heaug Metals in Sewage Sludge
Concern over the possible buildup of hear,y metals in soils resulting from large land
applications of sewage sludges has prompted research on the fate of these chemicals in
soils. Most attention has been given to zinc, copper, nickel, cadmium, and lead, which
are commonly present in significant levels in these sludges. Many studies have suggested
that if only moderate amounts of sludge are added, and the soil is not very acid
(pH > 6.5), these elements aíegenerally bound by soil constituents; they do not then
easily leach from the soil, and they are not then readily available to plants. Only in
moderately to strongly acid soils have most studies shown significant movement down
the profile from the layer of application of the sludge. Monitoring soil acidity and using
judicious applications of lime have been widely recommended to prevent leaching into
groundwaters and minimize uptake by plants.
More recently, studies using large amounts of sludge (up to and exceeding what is
permitted by U.S. EPA regulations) have suggested that metals from sludge may initially
be more mobile in soils than was previously thought. In fact, several studies have
reported that from 20 to 80 percent of the metals applied with sludge at high rates have
been leached from the root zone and, in all likelihood, Iost to the groundwater. The
metals in these studies probably moved as soluble organic complexes while the sludgesoil
mixture was still fresh. Over time, the metals remaining in the soil appear to be stabilized
in various low-solubility soil fractions (Table 18.12).
Fonlr.ls FouNo lN §olls Tnrnrro WITH StuDGE. By using a sequence of chemical extractants,
researchers have found that hear,lz metals are associated with soil solids in four maior
ways (Table 78.72). First, a very small proportion is held in soluble or exchangeable forms,
which are available for plant uptake. Second, the elements are bound by the soil organic
matter and by the organic materials in the sludge. High proportions of the copper and
chromium ale commonly found in this form, while lead is not so highly attracted,
Organically bound elements are not readily available to plants, but may be released over
a period of time.
Rr,ccrloNs oF lNoRGANlc CoNTAMINANTs lN §olls 823
TAB!-E í8.12 Forms oíSix Heavy Metals Found in the Ap Horizon of a Metea Sandy Loam (Typic Hapluda|fs) in
Michigan That Received 870 Mg/ha (Dry Víeight) of a "Dirty" Sewage Sludge Over l0 Years
The sludge application rate far exceeded that required to supply nitrogen to the crops grown, suggesting that the
purpose was disposal rather than utilizatíon. The sludge was incorporated into the soil betyveen 7977 and 1986,
prior ťo the implementation of source reduction programs to reduce the metal contents of most sewage
sluclges. The data are for soil samples taken 4 years a/ter the last sludge application. The soil CEC
was 7 cmol,/k&, the organic matter content was 7o/o, and the pH was 6.9.
Metal Content, mg/kg
Forms in soil SolubiIi|. Cd ()u Nl
Exchangeable and dissolved
Acid soluble (carbonates, some organic)
()rganic matter
F'e and Mn oxides
Residual (vcry insoluble sulfides, etc.)
Total of ail forms
4
140
56
96
11
:]o7
Most
I
V
Least
3
<1,,
<1
<1
=4.5
<1
3B
200
4B
6l7
<4
1,9
35
28
99
=l8-1
62
77o
31
1B0
24
467
52[t
19,1[,
tJ9
37(,
5ó
297Metal
Content, kg/haD
TotaLs
Total measured in Ap horizon
'lota] content in sludge applied
Apparent recovery, 0/o
=12
z1
=60
1,728
3000
5B
=507
4B0
=106
1 308
2100
62
B33l.
1130t.
;-
B59
1B00
4t]
nNumbers prcceded by < indicate that the 1eve1 present Was less than the lowest concentlation detectable bv the anall,tical method used.
t'Thc conversion frorn mg/kg to kg/ha assulnes a bulk density of 1.4 \,íg/m]r ancl a samplinS depth of 20 cm, Metal Concentlation clata from Beíti andJaco:
(1996). See also McBride, et al. (1999) for thIthel el,idence of sludge-borne metal mobilitv in soils.
The third and fourth associations of hear,y metals in soils are with cgrbonates anc
with oxides of iron and man1anese. These forms are less available to plants than either tht
exchangeable or the organically bound forms, especially if the soils ale not allowed tc
become too acid. The fifth association is commonly known as the residual fonn, whiclconsists
of sulfides and other very insoluble compounds that ale less available to plant:
than any of the other forms.
It is fortunate that most soil-applied hear,y metals are not readily absorbed by plant:
and that they are not easily leached from the soil. However, the immobility of the met_
als means that they wil1 accumulate in soils if repeated sludge applications are ma(lÉ
Care must be taken not to add such large quantities that the capacity of the soil to reac:
with a given element is exceeded. It is for this Ieason that regulations set maximu]l
cumulative loading limits for each metal (see Table 18.9).
ť&l*rffi sc{B§s fr*rm #rflaen .§cc.ó§,*als
Arsenic has accumulated in orchard soils following years of application of arseniccontaining
pesticides. Being present in an anionic form (e.g., HrAsO,, ), this element ,:
absorbed (as are phosphates) by hydrous iron and aluminum oxides, especially in aci:
soils. In spite of the capacity of most soils to tie up arsenates, long-term additions c.
arsenical splays can lead to toxicities for sensitive plants and earthworms. The arsen._
toxicity can be reduced by applications of sulfates of zinc, iron, and aluminum, whic:
tie up the arsenic in insoluble forms.
Contamination of soils with lead ís primarily from airborne deposition and com.]
from automobile exhaust and from paint chips and dust from woodwork coated wit]
old lead-pigmented paints. Most of the lead is tied up in the soil as insoluble carbonat€:
sulfides, and in combination with iron, aluminum, and manganese oxides (see Tab,.
l8.12). Consequently, the lead is largely unavailable to plants and is unlikely to co:-_
taminate groundwater, but may injure children who put contaminated soil in the
mouths (Box 18,2).
SoiI contamination by boron can occur from irrigation water high in this elemen:
by excessive fertilizer application, or by the use of po\^/er plant fly ash as a soil amen;.
ment. The boron can be adsorbed by organic matter and clays but is still available :
plants, except at high soil pH. Boron is relatively soluble in soils, toxic quantities beir_:
leachable, especially from acid sandy soils. Boron toxicity is usually considered a loca,ized
problem and is probably much less important than the deficiency of the elemen,
&24 §oíísorrd Chernícal Falšutían
Fluorine toxicity is also generally localized. Drinking water for animals and fluoride
fumes from industrial processes often contain toxic amounts of fluorine. The fumes can
be ingested directly by animals or deposited on nearby plants. If the fluorides are
adsorbed by the soil, their uptake by plants is restricted, The fluorides formed in soils
are highly insoluble, the solubility being least if the soil is well supplied with lime.
Mercury contamination of lake beds and of s\^/ampy areas has resulted in toxic levels
of mercury among certain species of fish. Insoluble forms of mercury in soils, not
normally available to plants or, in turn, to animals, are convelted by microorganisms
to an organic form, methylmercury, in which it is more soluble and available for plant
and animal absorption. The methylmercury is concentrated in fatý tissue as it moves
up the food chain, until it accumulates in some fish to levels that may be toxic to
humans. This series of transformations illustrates how reactions in soil can influence
human toxicities.
l8,
cH
Rec
lmt
! s. íl RAD§ONuCL§DE5 §N §OlL
U.:i, F.1',-\ tiivisiclrl clf
ratlilttiotl:
lťtvl,V. e|-}.1. goV, i,adi"rtíon
ffimdíoacťíuítgfram
Soils contain small quantities of "'U, 'oK,
s'Rb, 11C, and a number of other naturalhoccurring
radioactive isotopes (radionuclides) that are characterized by long half-tives
and give off minute amounts of radiation in the form of alpha particles (bundles of tlvo
neutrons and two protons) and beta particles (positive or negatively chargecl particles).
As a radionuclide decays, its nucleus discharges these particles, transforming the atonl
into a different isotope or element with a lighter nucleus. The time it takes for one-half
of the atoms of a particular radioactive isotope to undergo such decay is termed the hal7,
life of the isotope. After 10 half-lives, 99.7% of the original atoms wiII have decayed. The
intensity of radioactivity present-ol/ more precisely, the rate of radioactive decay-is
expressed using the SI unit becquerel (Bq), which replesents one decay per second. Arl
older metric unit, still in wide use, is the utrie (Ci), which equals 3.7 x 1010 Bq.6
lvueťeeir Físsíom
The process of nuclear fission, in connection with atomic y/eapons testing and nuclea:
po\ver generation, has contaminated soils with a number of additional radionuclides.
However, only two of these are sufficiently long-llved to be of significance in soils:
strontium 90 (half-life = 28 yr) and cesium 137 (tralf-life = 30 yr). The average leve1 o_
ouSr in soi] in the United States is about 14,4 kilobecquerels per squale metei (kBqinl
or 38B millicuries per square kilometer (mCi)ikm'. The average level for 137Cs is abou.
22,9 kBqlm2 (620 mCiikm2). The levels of radioactivity caused by fallout are quite sma.,
compared to that for naturally occurring radionuclides. For example, for natural1,,
occurring 10K
the average ]evel is about 1900 kBq/m2 (51,8O0 mCi/kmf).
Partly because of the cation-exchange properties of soils, the ievels of these fissio:
radionuclides found in most soils are not high enough to be hazardous. Most of the !'
5]
and l37Cs reaching the soi1 is adsorbed by the soil colloids in exchange for other catio],)
previously adsorbed, with the result that plants take up mainly the replaced catiotl:
rather than the added radionuclides (see Section 8.8). Even during the peak periods .,
\Veapons testing in the early 1960s, soils did not contribute significantly to the level i_ .
these nuclides in plants. Atmospheric fallout directly onto foliage was the prima:
Source of radionuc]ides in the food chain. Consequently, only in the event of a cat_,_
strophic supply of fission products could toxic soil levels of 90Sr
and 137Cs
be expecte._
Such high levels of 90Sr
and 1:J7Cs,
as well as '3]I, did contaminate soils in Ukrainé, Sca: dinavia,
and Eastern Europe in the wake of the 19B6 reactol meltdown at Chernobv]
Ukraine (then part of the Soviet Union). The accident deposited more than 200 k Bqir_
of radioactivity on soils as far away as the United Kingdom. Fortunately, consicieral...
research has been accomplished on the behavior of these nuclides in the soil_plant sr :_
tem.
Srnonrlunt 90" In the soil-plant-animal system, 90Sr
behaves very much like calciunr. :
which it is closely related chemically (see periodic table, Appendix B). It enters the sc
from the atmosphere in solub]e forms and is quickly adsorbed by the colloidal fractic:
both organic and inorganic. It is taken up by plants and assimilated much like ca]ciur:,,
Contamination of forages and, ultimately, of milk by this radionuclide is of concern .
the 90Sr
could potentially be assimilated into the bones of the human body. Fortunate,when
it exchanges with aluminum oI hydrogen ions adsorbed on the colloids in an ac _
soil, it comprises such a minute fraction of the exchangeable cations that its availa|., _
ity is quite low. However, should these soils be limed, the large quantities of addecl c.,.
cium are likely to cause the desorption of the strontium from the exchange srtr,
making it more available for leaching. However, the preponderance of calcium in t_,.
limed soil solution would compete with strontium for uptake by plant roots, and :
reduce the amount of strontium entering the food chain.
Crstun,a l 37. Although chemically related to potassium, cesium tends to be less reac__
available in many soils. Apparently, 1:r7Cs
is firmly fixed by vermiculite and related int.:.
stratified minerals. The fixed nuclide is nonexchangeable, much as is fixed potassiull-i
oThe curie was rramed after Marie and Pierre Curie, Polish scientists who discovered radium, an clen:.
that decays at the late of 3.7 x 101'] Bq/g. The becquerel was named aftcl Antoine Henrý Becquer.
French scientist who discovered radioactivity in uranium.
832 Soílsond Chemícal Pelíuťíorr
RadíoactíoeWastes7
some interlayers of clay (see Box 8.3 and Section 74.76). Plant uptake of '37Cs from vermiculitic
soiÍs is very limited. Where vermiculite and related clays are absent, as in some
tropica1 soils, 137Cs uptake is more rapid. In any case, the soil tends to damPen the
moiement of t37Cs into the food chain of animals, including humans.
loornr l 3 l. When it partially meted down, the nuclear reactor at Chernobyl emitted
significant quantities bf "'I, which accumulates in the human thyroid. People in the
arěa around the reactor have since suffered an increased incidence of thyroid cancer.
Because of its short halfJife (8.1 days), reactions in the soil and movement through the
soil-plant-animal food chain are less significant than contamination of drinking \ťater,
inhalation of contaminated dust particles, and contamination of edible plant foliage.
Research is underway to take advantage of plant uptake of radionuclides i_n PhYtoremediation
exercises. Plants, such as sunflowers, are being used to remove 90Sr and
137Cs from ponds and soils near the site of the Chernobyl nuclear disaster. Indian mustard
is also being used in nearby sites to remove such nucleotide contaminants.
In addition to radionuclides added to soils because of weapons testing and nuclear
poweí plant accidents, soils may interact with radioactive waste materials that have
ieaked irom their holding tanks or have been intentionally buried for disposal. Plutonium,
uranium, americiuh, neptunium, curium, and cesium are among the elements
whose nuclides occur in radioaclive wastes. These wastes are generated by research and
medical facilities (where the radionuclides are used in cancel therapy and the like), and
at po\^/er plants and weapons manufacturing sites.
'BecauŠe
of the secreryand lack of regulation associated with the latteq they constitute
some of the most polluted locations on Earth. For example, the U.S, Defense DePartment,s
now-abandóned plutonium-production complex at Hanford, Idaho, represents
one of the biggest envirohmental cléanup challenges in the world. Among the hazards
plaguing ttrařŠiteare hundreds of huge, in many cases leaking, underground tanks, in
irrricrr h"ighJevel radioactive \^/astes have been stored for decades. Billions of cubic meters
of soil aná water have been contaminated with radioactive \^/astes at U.S. \^/eapons manufacturing
sites and at similar, equally polluted sites in the former Soviet Union.
p1utoNtu1t Toxtclrv. Plutonium-239, a major pollutant at these sites, is dangerous both
because of its intense radioactivity and because of its high level of toxicity to humans.
The 239pu itself is quite immobile in soils, having a K7 estimated at about 1000. Nor is it
taken up readily by plants, so it does not accumulate along terrestrial food chains. It
does, hówever, accumulate in aigae. Furthermore, oily liquid wastes carrying 239Pu seeP
into the groundwater and nearĎy rivers, and contaminated surface soil blows in the
desert wiňd, spreading the radionuclides for many kilometers. CleanuP may be imPossible
at some of thesě former \^/eapons sites; the agencies responsible are struggling
merely to stabilize and contain the contamination. With a half-life of 24,4OO Years,
239Pu contamination is a problem that will not go away.
Low-Lrvrl Wnsrrs. Low-level radioactive v/astes also present some environmental challenges.
Even though the waste materials may be solidified before being placed in shaltoř
land burial |itr, so-e dissolution and subsequent movement in the soil are
possible. Nuclideďin wastes vary greatly in water solubility, uranium compounds being
quite solubte/ compounds of plutónium and americium being relatively insoluble, 1nd
cesium compoundi being intěrmediate in solubility, Cesium, a positively charged ion,
is adsorbed by soil colloids. Uranium is thought to occur as a U()22* ion thát is also
adsorbed uy s'oit. The charge on plutonium and americium appeaís to vaíy,dePending
on the nature of the complexes these elements form in the soil.
There is considerable variability in the actual uptake by plants of these nuclides from
soils, depending on such properties as pH and organic mattel content. The uptake from
soils by plants ii generallý lowest for plutonium, highest for neptunium, and intermeScience
(vo|.132) provides extensive technical information on radioactive
For a description of the environmental challenges at the Hanford site, see
7The
July 1981 issue of Soil
waste interaction with soils.
Zorpette (1996).
Rnolot,tuct-loEs lN SoIL 833
TABLE l 8. l 4 Concentrations oíSeveral Breakdown Products
of Uranium 238 and Thorium 232 (Nucleotides) in §ix Different
§oil Suborders in Louisiana
Note marked differences among levels in the different soils.
238[J
breakdown products, Bq,kg
2j2Th breakdown
products, Btl,kg
Soil suborder No. of samples Z26Ra 211Pb 211Bi 212Pb 137Cs laK
Udults
Aquults
Aqualfs
Aquepts
Aquolls
Hemists
22
aÁ
,rJ
93
57
1B
J/..)
30.4
51.1
92,2
90.4
136.3
27.7
36.7
3B.3
47.6
45.B
49.4
28.9
38.1
36.6
45.2
44.7
49.o
27.4
50.0
59.7
63.B
59.5
7 4.9
16,7 736
10.9 100
13.5 263
16.7 636
8.7 608
I9.4 783
From Meriwethel, et al. (1988).
diate for americium and curium. Fruits and seeds are generally much lower in these
nuclides than are leaves, suggesting that grains may be less contaminated by nuclides
than forage crops and leafy vegetables.
Since soils are being used as burial sites for low-level radioactive wastes, care should
be taken that soils are chosen whose properties discourage leaching or significant plaut
uptake of the chemicals. Data in Table 18.14 illustrate differences in the abitity o} air_
ferent soils to hold breakdown products of two radionuclides. It is evident that monitoring
of nuclear \^/aste sites will likely be needed to assure minimum transfer of ttre
4gc]ldes to other parts of the environment.