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II. Position in the Periodic Table
ARSENIC IN THE SOIL ENVIRONMENT: A REVIEW
in electronegativity that is found on descending this group is not sufficient to give
As a metallic character, and it is often described as a metalloid. In soils, the chemical behavior of As is in many ways similar to that of phosphorus (P), especially
in aerated systems, where the As" ion generally resembles orthophosphate ion
closely (Walsh et al., 1977). However, under conditions normally encountered in
soils, As is more mobile than P and unlike P can undergo changes in its valence
III. BACKGROUND SOURCES
The main source of As in soils is the parent materials from which the soil is derived (Yan-Chu, 1994). The native As content may vary considerably within an
area and is often determined by the geological history of the region (Wild, 1993).
Arsenate and As"' are the dominant As species in soils (Deuel and Swoboda, 1972;
Walsh and Keeney, 1975), and anthropogenic sources of As pollution have enhanced the background concentrations of these species.
The As content of rocks depends on the rock type, with the sedimentary rocks
containing much higher concentrations than the igneous rocks (Bhumbla and
Keefer, 1994). Although discernible differences exist between rock groups, the
range of As concentrations within a rock type may vary considerably. Generally,
the mean As concentrations in igneous rocks range from 1.5 to 3.0 mg As kg-I,
whereas the mean As concentrations in sedimentary rocks range from 1.7 to 400
mg As kg-I.
Atmospheric deposition contributes significantly to the geochemical cycle of As
(O'Neill, 1990). Chilvers and Peterson (1987) estimated a global atmospheric flux
value of 73,540 t year-', with a 6 M O split between natural and anthropogenic
sources. This compares with Nriagu and Pacyna (1988), who estimated a ratio of
70:30, with an anthropogenic As input of 18,800 t year-'. About 60% of the atmospheric As flux has been estimated to be due to low-temperature volatilization,
with volcanic activity the next most important natural source (Chilvers and Peterson, 1987). However, on a localized scale, volcanic activity may be the dominant
source of atmospheric deposition (O'Neill, 1990).
OF As IN SOILS
The distribution of As in soils may vary with soil type, depending on the nature
of the parent material. Background concentrations do not generally exceed 15 mg
As kg-l (National Research Council of Canada [NRCC], 1978), although concentrations ranging from 0.2 to 40 mg As kg-l soil have been reported (Walsh et
E. SMITH ETAL.
Arsenic Concentrationsfrom Noncontaminatedand ContaminatedSoils in North Americaa
Total As content
soil (mg kg-')
"Reprinted with permission from Walsh and Keeney, 1975, 0 1975 American Chemical Society.
al., 1977). Dudas and Pawluk (1980) reported background As concentrations that
averaged 5 mg As kg-' in 78 chernonzemic and luvisolic soil samples in Alberta.
Much higher As concentrations have been reported in acid sulphate soils developed on pyritic parent material. For instance, Dudas (1987) attributed elevated As
concentrations that ranged from 8 to 40 mg As kg-I in Canadian acid sulphate
soils to the weathering of pyrites in the parent material.
Other studies have reported a similar variability among soils from various regions. Reviewing literature on As concentrations in nonpolluted and polluted soils,
Walsh and Keeney (1975) concluded that nonpolluted soils in North America
(Table I) rarely contain more than 10 mg As kg-' soil. Similarly, the NRCC (1978)
report on the effects of As in the Canadian environment concluded that background
As concentrations in soils rarely exceed 15 mg As kg-' soil.
A limited number of similar studies have been reported in Australia. Merry et
al. (1983) studied 15 surface (0-150 mrn) soils from South Australia and 6 from
Tasmania that were considered unlikely to have received anthropogenic sources
of As. The median As concentrations in South Australian and Tasmanian soils were
3.9 mg As kg-' (+2.0) and 0.6 mg As kg-' (+0.55), respectively. Tiller (1992)
ARSENIC IN THE SOIL ENVIRONMENT:A REVIEW
Total As (rng kg-I)
“‘Iiller, 1m;reprintedby permission of CSIRO Aushlia
has also compared background As concentrations from several studies of urban
soils (Table 11) demonstrating a wide range in soil-As concentrations. In contrast
to studies by Merry e?al. (1983) and Tiller ( 1992), Fergus ( I 955) reported elevated As concentrations in soils derived from weathered quartzite (70-100 mg As
kg-’ at 0-75 mm) that resulted in restricted growth and toxicity symptoms on the
leaves of banana palms in Queensland. These regional variations in As concentrations in soils highlight the wide variability in soil As.
IV. ANTHROPOGEMC SOURCES
Anthropogenic activities that contribute As to the soil environment originate
from primary and secondary industries. These varying sources add As that differs
widely in nature and composition. Such variations in the composition and nature
of As have implications for biological availability as well as the mobility of As in
soils. The following sections briefly consider the sources and forms of As entering the soil environment through anthropogenic activities.
Arsenic trioxide (As,O,) is the major form of As that is produced for industry.
Industrial uses include the manufacture of ceramics and glass, electronics, pigments and antifouling agents, cosmetics, and fireworks (Leonard, 1991). Arsenic
is also added as a minor constituent to Cu and Cu-based alloys to raise the corrosion resistance of the metal(s) (Nriagu, 1994).
Arsenic trioxide is recovered from the smelting or roasting of nonferrous metal ores or concentrates (Loebenstein, 1993). From the limited data available (Fig.
1 ), the world production of As,O, appears to have remained relatively constant
E. SMITH E T A .
g 60000 -
l! 40000 -s30000
3 20000 -
- 400 =
- 200 'j
Figure 1 World production of As,O, and Australian imports per year (after Loebenstein, 1993,
and Australian Bureau of Statistics, 1995).
from 1985 to 1990 at approximately 50,000 t year-' (Loebenstein, 1993). This
contrasts with the declining use of As compounds in agriculture, which in the late
1970s to the early 1980s made up approximately 70% of the world As consumption (Hillier, 1980). This decline in As use in the agricultural sector has probably
been offset by the increasing use of As in the timber treatment industry. Arsenic
has excellent wood-preserving properties and is used in the timber industry in conjunction with Cu and Cr. World usage of As as a wood preservative is increasing
at approximately I-2% a year (Loebenstein, 1993).
Tentative estimates of the anthropogenic As fluxes between land, oceans, sediments, and the atmosphere have been calculated (Nriagu and Pacyna, 1988; Chilvers and Peterson, 1987). Nriagu and Pacyna (1988) estimate that the total worldwide anthropogenic As discharge onto land was 64,000-132,000t year-' (Fig. 2).
They estimated the major sources of As discharged onto land originated from commercial wastes (about 40%), coal ash (about 22%), and atmospheric fallout from
the production of steel (about 13%). Other anthropogenic sources of pollution associated with the mining industry (about 16%)also greatly contribute to As emissions onto land.
Arsenic is a natural component of Pb, Zn, Cu, and Au ores. Consequently, contamination of the atmosphere, soils, sediments, streams, and groundwaters is possible during mining and/or smelting processes. Although As-contaminated agri-
ARSENIC IN THE SOIL ENVIRONMENT: A REVIEW
Figure 2 Worldwide As discharges onto soils (after Nriagu and Pacyna, 1988).
cultural soils have been the subject of numerous detailed investigations, limited
studies have been carried out on the detailed nature and dynamics of As in mine
Peterson et al. (1979) investigated the total As concentrations and nature of As
in grossly contaminated mine spoils of southwest England. Total As concentrations in the spoils were greater than 20,000 mg As kg- with the maximum concentration at a depth of 20-40 mm, and very high As concentrations being detected to the lowest depth sampled (33,750 mg As kg-' at 250-300 mm). These
investigators also assessed bioavailability of As with deionized water. They found
that water-soluble As concentrations (0.3 1 mg As kg- at 0-20 mm) were generally less than 1% of total As. Arsenate and As"' were the major As forms present
in the water-soluble soil extracts, although dimethylarsine was also detected in surface samples. Although As" was the predominant form throughout the soil profile,
similar concentrations of As"' were reported in the surface samples. Even though
these soils had been relatively undisturbed for 70 years, the contaminated sites
were largely barren and supported only a limited number of plant species that generally covered less than 1% of the contaminated area. Grasses were the predominant species present (Agrostis stolonifera and A. tenuis),with As concentrations in
leaves greater than I000 mg As kg- (dry weight). Wild ( 1 973-74) reported that
72 plant species were found on 15 Rhodesian arsenical mine dumps with total As
concentrations ranging from 200 to 30,000 mg As kg-'. As may be expected, the
number of plant species and their density was found to increase as As content decreased. Of the mine dumps surveyed, the Banshee mine dump (30,000 mg As
E. SMITH ETAL.
kg-l) was affected the worst and was incapable of supporting vegetation. Generally, weed species were found to be the most important species in terms of plant
numbers on the mine dumps. Ffaveriatrinervia (Gaika Weed) was often the dominant or most important weed species present on the mine dumps. Of the grass
species, Cynodon dactylon was the most important species, being present in soils
with As concentrations ranging from 200 to 30,000 mg As kg- *.While As and other heavy metal concentrations at some mine dumps may inhibit stabilizing soil
vegetation from establishing, some plants, such as Cydon dactylon, can tolerate
high As concentrations and may be useful for stabilizing mine dump soils. They
may also offer a long-term, low-cost solution for remediating mine dumps when
other remediation techniques are impractical.
There have been a number of reported incidences of atmospheric As release during the smelting of Pb, Zn, Au, and Cu ores (Crecelius et al., 1974; Ragaini et al.,
1977; Li andThornton, 1993). Crecelius et aE. (1974) reported that a large Cu mine
near Tacoma, Washington, released approximately 300 t of particulate matter into
the atmosphere per year. Dust containing approximately 20-30% As contaminated the soil (0-30 mm) within a 5-km radius of the smelter, with up to 380 mg As
kg-' occurring at some of the sites sampled. Li and Thornton (1 993) studied the
As contamination of soil from three ore smelting areas in England-Derbyshire,
Cornwall, and Somerset. They reported that As concentrations in the topsoil
(0-1 50 mm) were elevated above background measurements (7.69-8.97 mg As
kg-l) and ranged between 16 and 925 mg As kg-', depending on the sampling
area. Although most mining and smelting in these regions ceased at the end of the
19th century, As contamination in some areas is still particularly high. This emphasizes the general long-term problem posed by the recalcitrant nature of compounds associated with soil contamination from industrial sources.
Unlike many heavy metals such as Cr, Cd, and Hg, As has been detected in
groundwaters especially at sites contaminated by mill tailings. Bernard (1 983) investigated the contamination of groundwater and the subsequent contamination of
Lake Moira, Canada, and found that haphazard disposal of mill tailings and other
slag wastes resulted in considerable leaching of As from these sites. Water samples collected from around the tailings and As storage areas in a hydrological investigation of groundwater had As concentrations ranging from 600 to 2200 mg
As liter-'. Extensive mitigation methods have been required to alleviate the high
As concentrations in the lake. Similarly, Leblanc et a f .(1996) reported that the dissolved As content of an acidic stream (pH 2.2-4) originating from a waste mine
dump of the Camoulbs Pb-(Zn) mine in Gard, France, was extremely high (average 250 mg As liter-'). Leblanc et al. (1996) also observed that As was precipitated and concentrated in Fe-As bacterial stromatolites. It was proposed that the
accumulation of As was through direct or induced microbial action. Rittle el al.
(1995) also reported that the immobilization of As"' into a Fe-As-S solid phase
was also linked to microbial activity.
ARSENIC IN THE SOIL ENVIRONMENT: A REVIEW
The mitigation of polluted As sites is an important aspect of the remediation
process and is a research area that is receiving considerable attention. However,
discussion of these remediation processes is beyond the scope of this review.
Fly ash presents an increasing waste problem worldwide because of the continuing demand for coal-fired power stations. In addition to directly releasing As into
the atmosphere, coal combustion produces fly and bottom ash containing As. The
physical and chemical properties of fly ash restrict the general utilization of fly ash,
resulting in a large proportion of the ash being used as land fill (Beretka and Nelson, 1994).
Generally, As concentrations in coal vary from 2 to 82 mg As kg-I, depending
on geological origin (Adriano et al., 1980). However, very high concentrations of
As (1500 mg As kg- I ) have been recorded in brown coal from the former Czechoslovakia (Bencko and Symon, 1977).
An important feature of fly ash is the variation of elemental concentration with
particle size. Natush er al. (1975) have observed that the concentrations of As and
other metals in fly ash tend to increase as particle size decreases. Smaller particles
of fly ash may escape emission-control devices and therefore may have greater impacts on biological systems in the vicinity of the emission source.
The oxidation state of As in coal ash and its leachate are of concern, since
As"' is considerably more toxic than As". However, there is a lack of information
on the dominant redox state of As present in fly ash. Turner (1981) reported that
pond effluent from 12 ash disposal systems contained quantities of total dissolved
As of less than 0.5-1 50 pg As literp1. Arsenate was the dominant As species present, but the more mobile and toxic As"' accounted for between 3 and 40% of the
dissolved As. This highlights the fact that, although As" may be the dominant As
species present, conditions may be favorable for the presence of As"' as well. In
contrast to the relatively low As"' concentrations in the ash porewater, interstitial
As"' concentrations collected from two wells ranged from 1.2 to 550 pg As liter-'
at4-6 m and43-1480 pg As liter-' at 10-12 mdepth, respectively (Turner, 1981).
These relatively high concentrations of As"' may be of environmental and human
and animal health concerns if contamination of surface andor groundwater were
The disposal of fly ash in Australia is typical of that found in many Western
countries. Australia currently produces approximately 7.7 X lo6t (1992) of fly ash
annually from major power stations (Beretka and Nelson, 1994). Of this, approximately 7.7 X lo5 t were used in the production of cement, sand replacement in
E. SMITH ETAL.
concrete, and the manufacture of blended Portland fly-ash cement. Small quantities are also used as a mineral filler in asphalt, and the remaining fly-ash (about
90%) is disposed of either as mine fill or as a codisposal material in waste dumps
(Beretka and Nelson, 1994). This cheap and therefore very attractive form of
disposal may be of environmental concern given the relatively high concentrations
of potentially toxic and mobile As"' in interstitial porewaters that have been reported.
2. Tannery Wastes
Arsenic was historically used as a pesticide in the treatment of animal hides
(Sadler et al., 1994). Sadler et al. (1994) investigated the As status of surface and
subsurface soil contamination by long-term disposal of tannery wastes in Queensland, Australia. For over 81 years, liquid wastes from a tannery were pumped or
transported by tanker to a site in Brisbane. The liquid waste was disposed of either
by burial or spray irrigation methods. Surface-soil contamination (0-125 mm) displayed considerable variation, ranging from less than 1 to 435 mg As kg- soil
across the site. Similarly, subsurface soil (250-375 mm) contamination also varied considerably, ranging from less than 1 to 1010 mg As kg-' soil. Although the
amount of As contamination of the soil is not as high compared with mining and
other reported sites, contamination at this site extended to considerable depth
(600-725 mm) in the soil. This may be due both to the nature of As in the wastes
and to the soil characteristics. Sodium arsenite was the active ingredient in the pesticide formulation used extensively to treat animal hides (Sadler ef al., 1994). Presence of As at considerable depth in sandy soils may be attributed to the high mobility of As"' in such soils ( T a m e s and de Lint, 1969).
The wood preservative industry is the major market for As in the United States
(Loebenstein, 1993), and in 1990 this industry accounted for approximately 70%
of the domestic As demand in the United States (Loebenstein, 1993).Although the
wood preservative industry is a major end user of imported As,O,, there are few
reported incidences of contamination. Nevertheless, Lund and Fobian ( 1991 ) reported elevated As concentrations in two soil types (typic haplorthod and typic
hapludalf) due to spillage of chemicals used in impregnating wood. Arsenic concentrations in the haplorthod were highest in the surface soil (3290 mg As kg-'
soil) and showed a general decline with increasing soil depth. Similar trends were
evident for the hapludalf (surface sample approximately 380 mg As kg- soil), but
there were large variations in the profile that could not be explained by the composition of the soil horizons (Lund and Fobian, 1991). Generally, As was retained
in the A and B horizons of both profiles. In the A horizon, the retention of As was
ARSEMC IN THE SOIL ENVIRONMENT: A REVIEW
associated with high organic-matter content, whereas retention in the B horizon
may be associated with adsorption by Mn, Fe, and A1 oxides (Lund and Fobian,
1991). The mechanism of As adsorption and the role of oxidic materials is further
discussed in the section on adsorption mechanisms.
McLaren et al. ( 1 994) investigated the leaching of Cu, Cr, and As (CCA) solution through free-draining, coarse-textured surface and subsurface soils (typic
ustipsamment and udic ustochrept) using undisturbed soil lysimeters. Cumulative
amounts of As leached through the lysimeters ranged from 4 to 30% of the total
CCA solution applied (90 mg Cu, 157 mg Cr, and 130 mg As). The large amount
of As leached is probably due to As being present as a simple salt (H,AsO,) in the
CCA solution and therefore represents an increased leaching potential in comparison with metals in sewage sludge, which are in relatively immobile forms
(McLaren et af.,1994).
Agricultural inputs such as pesticides, desiccants, and fertilizers are the major
sources of As in soils (Jiang and Singh, 1994). Numerous cases of As contamination of agricultural soils have been recorded (Bishop and Chisholm, 1961; Woolson et al., 1971a; Hess and Blanchar, 1976; Merry et al., 1983).
From the late 1800s and until the introduction of dichlorodiphenyltrichloroethane (DDT), Pb arsenate (PbAsO,), calcium arsenate (CaAsO,), magnesium
arsenate (MgAsO,), zinc arsenate (ZnAsO,), zinc arsenite (Zn(AsO,),), and Paris
were used extensively as pesticides in orchards (Anastasia and Kender, 1973; Merry et al., 1983). The resultant pollution
of orchard soils by inorganic Pb and As pesticides has been extensively reported
in the literature(BishopandChisholm,1961;Franketal., 1976;Merryetal., 1983;
Peryea and Creger, 1994).
Bishop and Chisholm (1 96 1) investigated As soil pollution on 25 Annapolis Valley orchards (mostly sandy loams; pH 6.2-6.7). It was found that the use of of arsenical pesticides had resulted in the accumulation of 9.8-124 mg As kg-' in the
topsoil (0-1 SO mm) (Table 111). The considerable variations in As concentrations
were attributed to different spraying practices at each orchard. Frank et al. ( 1976)
reported similar findings in apple orchards to which PbAsO, sprays were applied
for periods ranging from S to 70 years. The mean As concentrations were 54.2 2
25.8 mg As kg-' and 20.9 ? 13.6 mg As kg-*, respectively, in the 0-150-mm and
150-300-mm layers of soil. A comparison of the age of orchards versus As concentration in the surface soil showed an increase of 7-121 mg As kg-' after 70
E. SMITH ETAL.
Arsenic Contaminationof Orchard Sites
Total soil As (&I50 mm)
Mean of 3 1 orchards
Mean of 98 orchardsb
Bishop and Chisholm, 1961
Frank et al., 1976
Merry et al., 1983
~ ~ _ _ _ _ _ _ _~
~ _ _ _ _ _ _ _ _ ~
"No data available.
bTen apple and pear orchards from South Australia and 60 from Tasmania.
CMeanof 15 soils from South Australia.
%lean of 6 soils from Tasmania.
'Samples from depth of 0-100 mm.
years of pesticide applications. Increases in the As concentration were also evident
in the 150-300-mm layer, although the concentrations were much lower. However, comparison of the Pb-As ratio between the untreated and treated topsoils indicates that there may have been considerable loss of As from the surface soil (Frank
et af.,1976). This loss is reflected to some extent in the accumulation of As in the
150-300-mm horizon. Merry et af. (1983) reported that although there was considerable accumulation of As in 98 surface soils of apple and pear orchards in
South Australia and Tasmania (Table III), there was evidence of loss of As from
the surface soil at some sites. Translocation by leaching in soil solution or colloids
in suspension were suggested as possible mechanisms for losses (Merry et al.,
1983), although losses ofAs compounds through volatilization may also be an important but difficult pathway to quantify. Barrow (1974) and Davenport and Peryea
(1 99 1) have reported that P amendments to soil may contribute to the displacement
of As in soils, and leaching may be accentuated in sandy soils (Tammes and de
Lint, 1969). Peryea and Creger (1994) found that the movement of Pb and As was
greater in soils with low clay and organic contents, high irrigation rates, and high
application rates of Pb arsenate pesticide. This study highlights that As mobility is
a result of complex interactions between soil and solution factors that influence the
leaching of As from the surface soil.
Arsenical pesticides were also widely used in livestock dips to control ticks,
fleas, and lice (Vaughan, 1993). In Australia, As-based pesticide solutions were
widely used in Queensland and northern New South Wales from the early 1900s
ARSENIC IN THE SOIL ENVIRONMENT: A REVIEW
Average As Residues in Aggregated Soil
in and around Cattle-Dip Sites"
As (mg kg-' soil)
Location within dip site
Adjacent to dip bath
"After DIPMAC Report, 1992.
to 1955 (DIPMAC Report, 1992) to control ticks in cattle. Investigations of possible polluted sites have identified 1607 known cattle dip sites, of which 1041 sites
were still in operation in 1990 (DIPMAC Report, 1992). High concentrations of
As residues have been identified at some dip sites (Table IV), especially in the immediate area around the dip bath and draining pen (DIPMAC Report, 1992; Barzi
et al., 1996).
Arsenic residues (<50 to >3000 mg As kg- l ) have also been identified in subsurface layers at depths exceeding 500 mm (Naidu et al., 1995). Some of the As
concentrations present in the contaminated soil around livestock dips are comparable to As concentrations present in mine spoils but are perhaps more toxic because of the soluble nature of the As compounds present. Residential development
of the contaminated sites may pose a considerable risk to human health. Similar
problems also exist at former sheep dip sites due to the use of As-based pesticides
by pastoralists in Australian Capital Temtory, Australia. Similar contaminated
sites, resulting from animal dips, exist in Africa and many parts of the United
Since the late 19th century, inorganic arsenical compounds were used as nonselective soil sterilants and weed killers (Vaughan, 1993). Because of their persistence in the soil and toxicity to humans and stock they were superseded by
organoarsenical herbicides (McMillan, 1988).
Monosodium methanearsonate (MSMA) and disodium methanearsonate
(DSMA) have been used extensively as preemergence and postemergence herbicides in cotton and turf grasses (Sachs and Michael, 1971). Although both MSMA
and DSMA are effective as selective grass suppressors, MSMA has been used almost exclusively in Australia (McMillan, 1988).The use of MSMA and DSMA in