Comparision between background concentration of arsenic in urban and non urban areas of florida

Comparision between background concentration of arsenic in urban and non urban areas of florida

1093-0191/03/$ - see front matter 䊚 2003 Elsevier Science Ltd All rights reserved.

doi:10.1016/S1093-0191(02)00138-7

Comparison between background concentrations of arsenic in

urban and non-urban areas of Florida Tait Chirenje *, Lena Q Ma , Ming Chen , Edward J Zilliouxa, a a b

Soil and Water Science Department, University of Florida, Gainesville, FL 32611, USA

a

Florida Power and Light, 700 Universe Boulevard, Juno Beach, FL 33408, USA

b

Received 10 April 2002; received in revised form 5 November 2002; accepted 17 November 2002

Abstract

Arsenic contamination is of great environmental concern due to its toxic effects as a carcinogen Knowledge of arsenic background concentrations is important for land application of wastes and for making remediation decisions The soil clean-up target level for arsenic in Florida (0.8 and 3.7 mg kgy 1 for residential and commercial areas, respectively) lies within the range of both background and analytical quantification limits The objective of this study

was to compare arsenic distribution in urban and urban areas of Florida Approximately 440 urban and 448 non-urban Florida soil samples were compared For non-urban areas, soil samples were collected from three land-use classes

(residential, commercial and public land) in two cities, Gainesville and Miami For the non-urban areas, samples

were collected from relatively undisturbed non-inhabited areas Arsenic concentrations varied greatly in Gainesville, ranging from 0.21 to approximately 660 mg kgy 1with a geometric mean(GM) of 0.40 mg kg , which were lowery 1

than Miami samples (ranging from 0.32 to 112 mg kg ; GMs2.81 mg kg ) Arsenic background concentrationsy 1 y 1

in urban soils were significantly greater and showed greater variation than those from relatively undisturbed non-urban soils(GMs0.27 mg kg ) in general.y 1

䊚 2003 Elsevier Science Ltd All rights reserved

Keywords: Background concentration; Natural and anthropogenic; Arsenic; Florida

1 Introduction

Arsenic occurs naturally in a wide range of minerals

in soils This, coupled with the once widespread use of

arsenic pigments, insecticides, herbicides, and industrial

wastes, makes it a common trace constituent of most

soils In fact, arsenic is the 20th most abundant element

Abbreviations: AM, arithmetic mean; ASD, arithmetic

standard deviation; CEC, cation exchange capacity; FCSSP,

the Florida Cooperative Soil Survey Program; GM, geometric

mean; GSD, geometric standard deviation; OC, organic carbon;

SCTL, soil clean-up target level.

*Corresponding author Tel.: 352-392-1951; fax:

q1-352-392-3902.

E-mail address: tchirenj@ufl.edu(T Chirenje).

in the earth’s crust and is a major constituent of )245 different minerals with sulfur deposits being the most common culprits (Woolson, 1983) Arsenic

concentra-tions are variable even in virgin components of the environment including soils, sediments, bodies of water, animals, and plants

Since arsenic is a known human carcinogen, its distribution and behavior in soils needs to be docu-mented to better understand its human exposure The United States Environmental Protection Agency

(USE-PA) has set the levels of arsenic allowed in oral intake,

drinking water and breathing air at 0.0003

mg kgy 1dy 1, 0.050 mg ly 1and 0.0043 mg my 3, respec-tively,(USEPA, 1998) The World Health Organization (WHO) has, in fact, recommended lowering the primary

drinking water standard to 0.010 mg ly 1

Arsenic was widely used in Florida during the early

part of the 20th century as an insecticide to control

disease-carrying ticks on cattle Arsenic was also used,

along with copper and chromium as a wood preservative

(CCA, Grant and Dobbs, 1977) The most common

present day uses of arsenic compounds include

pesti-cides, wood preservatives and as growth promoters for

poultry and pigs (O’Neill, 1990) Mining activities,

smelters and fuel combustion also contribute significant

amounts of arsenic to the environment

Arsenic distribution in Florida soils is likely to

encompass at least three populations of concentrations,

which may or may not be easily distinguishable These

include (1) natural background, (2) a diffuse

anthro-pogenic influence, or ‘anthroanthro-pogenic background,’ and

(3) localized point sources The relative proportion of

each population varies between urban and non-urban

areas Therefore, knowing the distribution of arsenic in

these three populations in both urban and non-urban

soils aids our understanding of the impacts of human

activity on natural concentrations of arsenic in soils

(O’Neill, 1990)

Significant land-use changes have occurred over the

decades due to the migration of people to Florida in

search of warmer climate and better economic

oppor-tunities Currently, 11% of the total land area in Florida

(total area 14 258 000 ha) is considered urbanized

(Nizeyimana et al., 2001) and this urbanization trend

continues to increase This is relatively greater than the

national urbanized area of 3%

Unlike natural areas, arsenic concentrations in urban

soils vary considerably over short intervals Urban soils

are complex and heterogeneous in their structure and

composition(Craul, 1985; Davies et al., 1987) Human

activity is the predominant active agent in the

modifi-cation of these soils(Barrett, 1987) A fitting definition

of an urban soil is, a soil material having a

non-agricultural, usually manmade surface layer more than

50 cm thick, that has been produced by mixing or

filling of the land surface in urban and suburban areas

(Craul, 1985) There is a greater probability of historic

anthropogenic contamination, vertical mixing during

development, use of fill from different geologic areas,

deposition andyor contributions from the use of

pesti-cides or amendments from other sources in urban areas

than non-urban areas (Craul, 1985; Thornton, 1987)

Intensive human activity significantly alters the original

native soils, making it difficult to describe urban soils

using typical soil classification schemes

Arsenic concentrations in relatively undisturbed areas

can still be attributed to purely geological factors with

a few exceptions where non-point sources due to

agri-cultural use of arsenic-containing pesticidesyherbicides

and aerial deposition are significant It may still be

reasonable to consider the arsenic concentrations in

these soils as the true natural arsenic background

con-centrations Areas that have had significant human activity (urban soils in general) are likely to exhibit

what we may call ‘anthropogenic background concen-trations’ of arsenic

Background concentrations of arsenic in relatively undisturbed Florida soils are established and they vary from 0.01 to 61.1 mg kgy 1, with a geometric mean

(GM) of 0.27 mg kgy 1(Chen et al., 1999) Typical soil

arsenic concentrations range between 0.1 and 40

mg kgy 1 worldwide, with an arithmetic mean (AM)

concentration of 5–6 mg kgy 1 (Kabata-Pendias and

Pendias, 1992) A survey of soils in the USindicated

that arsenic levels for undisturbed soils ranged from

-0.1 to 97 mg kgy 1with a GM arsenic concentration

of 5.2 mg kgy 1(Shacklette and Boerngen, 1984)

This investigation was conducted to (i) compare

arsenic background concentrations in urban and non-urban soils in Florida, and(ii) investigate the

relation-ship between arsenic background concentrations and the extent of human activity and other soil properties A medium-sized city (Gainesville) and a relatively large

city(Miami, in terms of population and level of

devel-opment) were used to represent urban areas

2 Methodology

Three different sets of samples (i) urban soils

col-lected from a medium-sized city, Gainesville

(popula-tion, 96 000; size, 93 km2), (ii) urban soils collected

from a relatively large city, Miami(population, 370 000;

size, 91 km2), and (iii) natural soils from relatively

undisturbed non-urban soils, were used

2.1 Soils from undisturbed areas

The non-urban soils used in this study were sampled and characterized as a part of the Florida Cooperative Soil Survey Program conducted jointly by the University

of Florida Soil and Water Science Department and the United States Department of Agriculture–Natural Resources Conservation Service (USDA–NRCS)

Dur-ing samplDur-ing, great care was taken to select sites without known sources of anthropogenic contamination Soil horizons were delineated and sampled using USDA guidelines(Soil Survey Division Staff, 1993) Based on

the mean coefficient of variation from a previous study

(Ma et al., 1997), a minimum of 214 soil samples were

required to establish a statistically valid database for Florida soils (with 95% confidence level and 20%

accepted variability between samples) However, a total

of 448 archived soil samples were selected to assure both taxonomic and geographic representation

The overall taxonomic representation was achieved

by weighting the number of samples for each soil order

by their estimated areal occurrences in Florida The total mapped area was 11 265 530 ha and covered

approximately 80% of Florida’s total land area Seven

soil orders were identified from 51 to 67 counties and

their approximate coverage was: Spodosols(28%),

Enti-sols (22%), Ultisols (19%), Alfisols (14%), Histosols

(10%), Mollisols (4%), and Inceptisols (3%) Based on

the areal occurrence of each soil order, the samples

included surface horizons from 122 Spodosols, 107

Entisols, 90 Ultisols, 60 Alfisols, 39 Histosols, 17

Mollisols, and 13 Inceptisols

2.2 Soils from urban areas (Gainesville and Miami)

The Gainesville study was done as a pilot test to

develop a comprehensive sampling protocol for other

cities The number of samples collected was based on

soil heterogeneity and determined using the following

Eq.(1):

2

where N is the number of samples, S is the estimated

standard deviation of the AM of all single values (in

this case,S was calculated from the 25 samples collected

from the University of Florida campus in Gainesville),

t is the Student t value for a given confidence intervala

(1.96 for the 95% confidence interval) and R is the

accepted variability in mean estimation (usually 10–

20% depending on the scale and budget of project) A

value of 20% was used and the minimum number of

samples needed for Gainesville was determined to be

130

Three land-uses were selected for sampling These

were residential, commercial and public land These

were chosen because they cover the largest area in most

urban settings Differentiating the samples from these

three land-use classes enabled us to test for differences

among them The number of categories selected from

these three land-uses depends on the depth of detail

required in the final sample

Five categories were chosen from the three land-uses

in Gainesville (i.e residential right-of-way, residential

yards, public buildings, public parks and commercial

areas) Forty surface samples (0–20 cm depth) were

collected in May 2000 from each category, resulting in

a total of 200 samples One out of every 5 samples

taken from each category was duplicated(for

compari-son of reproducibility), bringing the total number of

samples to 240 However, at least three cores were

taken and composited at each of the remaining sites

The sites for sample collection were randomly selected

within each category of land-use using a set of strict

exclusion criteria to avoid any potentially contaminated

areas Chirenje et al (2001) discuss both the

randomi-zation process and the exclusion criteria in detail

Based on the pilot study, no significant difference was observed in arsenic concentrations between soils in residential-yard and residential-right-of-way, thus the latter was used to represent residential soil, reducing land-use categories to four for all subsequent studies It was also later determined that the focus of such back-ground studies should produce a good estimate of the overall concentration distribution in each stratum with-out primarily focusing on the central tendency of each stratum Therefore the precision target would be set on

an upper percentile of the concentration distribution Conover(1980) described a method for calculating the

minimum number of samples needed for a given per-centile of a distribution to be exceeded by the maximum observed sample value with a given confidence level For example, the sample size needed to assure exceed-ence of the upper 95th percentile with 95% confidexceed-ence

is 59 Based on this, 60 samples(0–10 cm depth) were

collected in January, 2001 from four land-use categories

in the Miami study(residential areas, commercial areas,

public parks and public buildings) The change in depth

was instituted after the revision of the sampling protocol and depths of 0–10, 10–30 and 30–60 cm were subse-quently sampled in Miami and other cities that followed However, results from the top 10 cm only are discussed

in this publication These changes are discussed in detail

by Chirenje et al.(2001)

2.3 Sample preparation and trace element analysis

All soil samples were air dried, ground, and passed through a 2-mm sieve The screened samples were stored in sealed polyethylene containers before analysis The non-urban soils were digested using USEPA

Meth-od 3051a whereas for the urban soils, USEPA MethMeth-od

3051 was used A simpler protocol, USEPA Method

3051, was instituted after the non-urban soils study, therefore the new method was used for the urban soils study The soils were digested in a microwave digester using USEPA Method 3051(or 3051a), which is

com-parable to USEPA Method 3050, the hotplate digestion method (USEPA, 1996) In summary, 0.5–2 g of soil

samples were weighed into 120-ml Teflon tubes and digested in 9 ml of concentrated HNO for Method3

3051 (or 9 ml of concentrated HNO plus 3 ml of3

concentrated HCl for Method 3051a) in a CEM

MDS-2000 microwave digester (Matthews, NC) For

Histo-sols rich in organic matter, only 0.5 g of sample was used and 1.0 ml of H O was added prior to digestion.2 2

The resulting solution was filtered through a Whatman

No 42 filter paper and made up to 100 ml Arsenic concentrations in the digests(or digested samples) were

determined on a SIMAA 6000 graphite furnace atomic absorption spectrophotometer (GFAAS, Perkin-Elmer,

Norwalk, CT) using USEPA method 7060A (USEPA,

1995)

Table 1

Summary statistics for soil arsenic concentrations in different land-uses in Gainesville and Miami (all concentrations in mg kg ) y 1

Residential Commercial Public parks Public buildings Combined South Florida c North Florida c

Miami

Gainesville

AM, arithmetic mean.

a

GM, geometric mean; GSD, geometric standard deviation.

b

South Florida includes Miami and North Florida includes Gainesville.

c

In addition, soil properties that have been shown to

affect arsenic concentrations (pH, clay content, total

organic carbon(OC), and total Fe and Al) were

meas-ured using internationally accepted standard procedures

(Page et al., 1982) The concentrations of Fe and Al

were determined using a Thermo-Jerroll Ash 61E

Induc-tively Coupled Plasma Atomic Emission

Spectropho-tometer(ICP-AES, Spectro, Fitchburg, MA)

2.4 Data analyses

All element concentrations are presented on a dry

matter basis Both AM and GM were used to describe

the central tendency of the data Baseline concentrations

of arsenic were calculated using GMyGSD2 and

GM=GSD2 (upper baseline limit (UBL)) of the

sam-ples, which include 97.5% of the sample population

(Dudka et al., 1995) Chen et al (1999) provide details

on definition and calculation of baseline concentrations

All statistical analyses were performed using SAS䉸

(SAS Institute, 2000) The generalized linear model

was used in preference to the analysis of variance

procedure to account for the unequal number of samples

within each class and quantile–quantile(QQ) plots were

used to eliminate outliers from our dataset These

outliers represented samples with abnormally high

arsenic concentrations that could not be attributed to the

background levels However, outliers were not

eliminat-ed when distribution graphs were plotteliminat-ed The

Shapiro-Wilks test was used to test for normality Because the

distribution of arsenic concentrations was not normal

(data not shown), the data were log-transformed before

analysis to meet the assumption of normality required

for the regression model

Spatial analyses were done using Spatial Analyst tools in Arcview䉸 Geographical Information Systems software(ESRI, Redlands, CA) Pathfinder (Trimble,䉸

Sunnyvale, CA) was used to geoprocess the Global

Positioning System unit-logged positions and transform them into forms that could be read by Arcview These䉸 images were used to assess spatial distribution, and graphically display the analytical results from the study

on a digital map(not shown)

3 Results and discussion

It is important to note that most Florida soils are very sandy This leads to low retention of trace elements in general, with important implications on regulatory con-centrations for many trace elements Furthermore, the populations in this study only approached the normal distribution after log-transformation Therefore, the 95% upper confidence limit(UCL) of the mean was

calcu-lated using the H-statistic from Eq.(2):

UCL1yasexp(x q0.5s qs =Hy 1yay ny1 ) (2)

where x is the AM of the log-transformed data, s isy

the standard deviation of the log-transformed data,n is

the number of samples, H1yaand H are the H-statistica from tables provided by Land(1975) for the UCL The

UCL depends onx , n and the chosen confidence limity (Gilbert, 1987) Therefore, the calculated UBL,

dis-cussed previously, was also based on the GM

3.1 Comparison of soil arsenic concentrations between urban and non-urban areas

Table 1 summarizes the mean concentrations and other relevant descriptive statistics for soil arsenic

con-Fig 1 Soil arsenic concentration (raw data) distribution in (a)

Gainesville (ns200), (b) Miami (ns240), and non-urban

areas (ns448) in Florida.

Table 2

The UCL, 95th percentile and percentage of soil samples with arsenic concentrations exceeding the SCTL (residential and com-mercial ) in different areas in Florida

UCL : upper confidence limit of the mean at as0.05.

a

as0.05

UBL : upper baseline limit at as0.05.

b

as0.05

0.8 mg kg : the Florida SCTL for residential areas.

3.7 mg kg : the Florida SCTL for commercial areas.

centrations in non-urban areas surrounding the two cities

and land-use categories analyzed within the two urban

areas The distributions of arsenic concentrations in the

three separate classes, with the exception of values

greater than 60 mg kgy 1, are shown in Fig 1 For the

non-urban soils, samples from South Florida (ns65)

and North Florida(ns158) were used to compare with

Miami and Gainesville samples, respectively Arsenic concentrations from the urban areas of Miami and Gainesville were significantly greater than those from non-urban soils (as0.05) in the same regions (GMSouth Floridas0.44 vs GMMiamis2.80 and

GM s0.21 vs GM s0.40 mg kgy 1;

North Florida Gainesville

Table 1) As discussed earlier, non-urban soils have

lesser anthropogenic disturbances than urban areas as they are not exposed to the same activities that often lead to increases in concentrations of trace elements in urban soils In general, the differences in the distribution

of arsenic in urban areas can be attributed to land-use, while those in non-urban areas can be attributed to soil forming factors

Based on the GM, the upper baseline limit

(UBLas0.05, 95% of all data fall below this value) and

the 95% upper confidence level (UCL) of the GM for

both urban and non-urban soils were calculated(Table

2) The combined UBLas0.05 for all the land-use cate-gories for Miami (14.3 mg kg ) was more than 6y 1

times greater than for Gainesville(2.3 mg kg ; Tabley 1

2) Both the UCL and UBL are dependent on the

variation of the data set, hence these results demonstrate the greater variation in urban areas than non-urban areas The UCL is not a very reliable measure of the confidence level of the mean for background studies because it is highly dependent on the number of sam-ples, approaching the mean as the number of samples increases Table 1 demonstrates this point for both Gainesville and Miami The UCL is generally useful for site-specific measurements of arsenic concentrations Comparison of properties of soils from Gainesville with soils collected from non-urban areas close to the city and on the same parent material did not show any significant difference, except for pH This is discussed

in more detail in a later section There was a significant difference between urban soils from Miami and non-urban soils from the surrounding areas The non-non-urban areas surrounding Miami had significantly greater

arsen-Table 3 Comparison of pH, OC and siltqclay content between urban and non-urban soils

Non-urban Gainesville Miami

Siltqclay: represents the sum of silt and clay as a

a

percentage.

Organic carbon.

b

ic background concentrations than both the non-urban

and urban areas in Gainesville (Fig 1; Table 1) These

differences can be attributed to the different soils, which

are a manifestation of different parent materials in these

two regions The comparison between public parks in

each city and non-urban soils in areas surrounding the

same city provided the best results because public parks

in most urban areas have very little human disturbance

However, it must be noted that most of the parks in

Miami had significant fill in them, unlike parks in

Gainesville

Another comparison was also made between

‘turbed’ and ‘undis‘turbed’ non-urban soils, where

dis-turbed soils represented areas that had significant

anthropogenic influence e.g farmland, managed

plan-tations etc There was no significant difference between

undisturbed and disturbed non-urban soils (GMs0.25

and 0.29 mg kgy 1, respectively) This can be explained

by the fact that, although disturbed non-urban soils have

some anthropogenic influence, these activities do not

directly lead to contamination by point sources, as is

the case in most urban areas

There were significant interactions between cities and

land-use categories, hence comparisons of the combined

land-use categories from the two cities from non-urban

areas were not possible Nonetheless, all four land-use

classes in Miami had significantly greater arsenic

con-centrations than the corresponding land-use classes in

Gainesville (Table 1) In fact, the land-use category

with the lowest arsenic concentration in Miami(public

parks) had significantly greater arsenic concentrations

than the land-use category with the highest arsenic

concentration in Gainesville (commercial areas)

Approximately a third of all samples collected in Miami

had arsenic concentrations greater than the Florida soil

clean-up target level (SCTL) for commercial areas, 3.7

mg kgy 1 Gainesville, on the other hand, had

approxi-mately 29% samples above the Florida SCTL for

resi-dential areas and only 4% were above the SCTL of 3.7

mg kgy 1 for commercial areas Corresponding

propor-tions of samples falling above the SCTL for residential

and commercial areas in both North and South Florida

non-urban areas were lower than those of Gainesville

and Miami, respectively(with the notable exception of

North Florida for the commercial SCTL, Table 2)

The differences in the arsenic concentrations between

Gainesville and Miami soils can be explained by several

factors First, the depth of sampling for the top layer of

soil was different between the two cities The sampling

depth for the analyzed samples in Gainesville was 0–

20 cm while that in Miami was 0–10 cm This has

important implications on the observed concentrations

because arsenic concentrations generally decrease with

depth in the top 30 cm of soil However, we can still

compare results from these two cities because a smaller

subsample (ns30) that was reanalyzed in Miami

showed a difference in arsenic concentration of less than 30% between 0–10 and 10–20 cm depths This difference is relatively smaller in magnitude than the difference between the two cities (Gainesville and

Miami) Comparisons of arsenic concentrations between

the depths of 0–10 and 10–20 cm in Daytona Beach

(ns64) also showed very small differences, possibly

due the to the extensive mixing in the top 50 cm in urban soils (data not shown) Secondly, Gainesville

soils have greater sand (quartz) content than Miami

soils (91 vs 72%, Table 3) which is expected to

facilitate greater arsenic leaching or loss with runoff The presence of significant amounts of carbonate in South Florida soils, 30–94% CaCO3 (Li, 2001) would

also help retain trace elements and hence such soils are expected to show greater accumulation of anthropogen-ically-added trace elements such as arsenic

The high background concentrations of soil arsenic observed in the urban areas in Florida are supported by observations in studies from other parts of the USand

in other countries (Murphy and Aucott, 1998; Tiller,

1992; Tripathi et al., 1997) For example, Folkes and

Kuehster(2001) observed very high baseline

concentra-tions of arsenic in the suburban areas of Denver, Colorado(residential areas had GM ;6 mg kgy 1while urban areas in general had GM ;7 mg kgy 1) However,

the rural background concentrations of arsenic in Colo-rado were also significantly greater than those of Florida soils (GMs3.7 vs 0.4 mg kg , respectively) Thisy 1

difference may be attributed to geologic factors, e.g Colorado soils are derived from parent materials with higher concentrations of arsenic than parent materials from which Florida soils are derived

In New Jersey, Murphy and Aucott(1998) attributed

the high arsenic concentrations in residential areas to historical land-use and former heavily sprayed orchards The importance of historical land-use was also demon-strated by Tiller(1992) in a similar background study

in Australian urban areas Tiller (1992) avoided areas

whose historical land-uses increase their probability of being contaminated In spite of these efforts, arsenic concentration ranges of -1–8 mg kgy 1were observed The relative contributions of both natural and

anthro-Fig 2 QQ plots for (a) Gainesville (ns200), (b) Miami (ns

240 ), and (c) non-urban areas (ns448), for transformed data.

pogenic activities in the distributions of soil arsenic

concentrations were investigated in detail by Bak et al

(1997) Not surprisingly, they concluded that sludge

application contributed the greatest amount of arsenic

to the soil annually This has important ramifications

because land spreading of sludge is a common practice

in many urban areas worldwide, and the regulations

governing these applications often have loopholes that

can be exploited by many unscrupulous waste managers

3.2 Soil arsenic distribution characteristics

The complexity of urban soils often leads to distinct

patterns in arsenic distribution The distinction between

natural background, anthropogenic background, and

contaminated arsenic concentrations is more discernible

in urban areas than in non-urban areas(Fig 2) There

is a greater possibility of finding contaminated areas in

urban environments due to greater human disturbance

than in non-urban areas (Fig 2a) On the other hand,

non-urban areas are likely to exhibit mostly natural

background concentrations of trace elements

The most critical shortcoming of these distribution plots is that not all soils that have high concentrations

of arsenic have been exposed to contamination Some soils naturally have high arsenic concentrations from their parent material The determination of pollution can only be done if the parent material is known or if the historical land-use of the sites in question suggests contamination Furthermore, some sites with sandy soils

(e.g most Gainesville sites) may be exposed to

contam-ination, but the arsenic is not retained in the soil long enough to be picked up in studies like the current one

In such cases, the low concentration observed is not necessarily the natural background Such a determina-tion can only be made if the groundwater at all sites is analyzed However, analyzing groundwater may not provide the clues needed if enough time elapses between the pollution and sampling events

Censoring data on both ends(non-detects and

outli-ers) can also have a significant impact on the shape and

nature of the distributions The plots of the Gainesville data demonstrate this point Lower end censoring

(non-detects) may yield a set of ‘equal’ concentrations

leading to clumping on the lower(left tail) end of the

curve Furthermore, if the data are also censored at the high end before plotting the distributions, the ‘contam-inated sites’ disappear from the distribution Nonethe-less, the slope of the curves gives us a clear indication

of the variation in each sample stratum

3.3 Correlation between soil arsenic concentrations with soil properties

Correlation is widely used in trace element analyses

(Bradford et al., 1996; Dudka et al., 1995; Lee et al.,

1997) because of its ability to quantify how one factor

changes in response to the other Correlation analyses between elemental concentrations and soil properties

(total Fe, total Al, pH, clay, OC, and cation exchange

capacity(CEC)) of both the urban and non-urban soils

were conducted in this study The correlation between

pH and arsenic concentrations in urban areas was both very low statistically insignificant

There was higher correlation between clay content and arsenic concentration in the non-urban soils than urban soils (Table 4, significant at as0.05) This is

consistent with previously published data by Ma et al

(1997) They reported that arsenic concentrations were

strongly correlated with clay content in 40 Florida surface soils Higher correlation was also reported between clay content and concentrations of arsenic in Canadian(Mermut et al., 1996), Polish (Dudka, 1993)

and Dutch soils(Forstner, 1995; Edelman and de Bruin,

1986) suggesting that clay content is important in

controlling the level and distribution of trace metal concentrations in soils A study conducted in both urban and non-urban areas in Denmark and Holland showed

Table 4

Correlation coefficients of arsenic concentrations with soil a

properties in urban and non-urban areas

Element pH Clay OC Total Fe Total Al

Non-urban 0.14 0.33* 0.58* 0.66* 0.60*

Gainesville 0.10 0.01 y 0.05 y 0.04 0.02

Miami 0.09 0.04 y 0.08 -0.09 y 0.06

Correlations coefficients denoted with ‘*’ are significant at

a

as0.05.

low correlation coefficients for soil texture although

clay soils consistently had higher arsenic concentration

than sandy soils in the non-urban areas (Bak et al.,

1997) The investigation concluded that arsenic

concen-trations in studied areas were more sensitive to soil

factors(e.g clay content) than anthropogenic activities

Anthropogenic activities in urban areas, especially the

use of fill, tend to interfere with the relationship between

soil forming factors and trace element concentrations

In contrast to urban soils, there was significant

cor-relation between OC and arsenic concentrations in

non-urban areas (Table 4) Humic substances in organic

soils(peat) can serve as strong reducing and complexing

agents and influence the processes controlling

mobili-zation of many toxic elements including arsenic(Gough

et al., 1996) Similar to the results from the non-urban

soils, other researchers have reported strong positive

correlation between trace element concentrations and

OC and the siltqclay content of the soil (Aloupi and

Angelidis, 2001; Chirenje, 2000; Wilcke et al., 1998)

There was significant(as0.05) correlation between

arsenic concentrations and total Fe and Al

concentra-tions in non-urban soils(Table 4) Both Fe and Al react

with the arsenate to form stable, immobile compounds

in the soil, and oxides and hydroxides of both elements

also provide reactive surfaces on which arsenic can be

adsorbed However, the same trend was not observed in

urban soils, possibly due to the increased use of fill

Dudka(1993) found good correlation between

concen-trations of arsenic and concenconcen-trations of Al and Fe in

surface soils of Poland He concluded that levels of

most elements were mainly controlled by the minerals

(Fe and Al oxides) present in the soils (Dudka, 1992)

Total Fe and Al concentrations (2300 and 2200

mg kgy 1) in Florida soil are 16–32 times lower than

the average concentrations reported for other soils

(38 000 and 71 000 mg kg ; Lindsay, 1979) Nonethe-y 1

less, total Fe and Al, even at such low concentrations,

are significant in controlling metal concentrations in

Florida soils

Multiple regression of concentrations of trace

elemen-ts against clay, OC, pH, CEC, and total concentrations

of Al and Fe supported the relationships of trace

elements with important soil properties (data not

shown) Regressions of log-transformed concentrations

of arsenic against six soil variables explained between

9 and 65% of the total variance However, no such correlation was observed in urban areas In the non-urban soils, partial correlation analyses confirmed that total Fe and total Al were the two major variables controlling concentrations and distributions of arsenic

in Florida surface soils as demonstrated previously using simple correlation analysis

4 Conclusions

This study compared the distribution of arsenic in soils from urban and non-urban areas In general, arsenic concentrations in urban areas were higher than those in non-urban areas Arsenic concentrations varied signifi-cantly with land-use in Miami but only parks had lower arsenic concentration than the other land-uses in Gaines-ville Soil arsenic concentrations in non-urban areas showed significant correlation with natural soil proper-ties (clay content, OC, and total Fe and Al) because

they are exposed to relatively lower disturbance than urban soils Knowledge of classical pedology can easily

be employed to predict arsenic distribution in these areas On the other hand, land-use categories can serve

as good indicators of arsenic distribution in urban areas More research is needed to better understand the tem-poral variation of arsenic in different compartments in both urban and non-urban areas so that better decisions can be made about land application of waste and remediation of possibly contaminated soils

Acknowledgments

This research was sponsored in part by the Florida Center for Solid and Hazardous Waste Management

(Contract No 96011017) and Florida Power and Light

Helpful discussions and consultations with Dr John Thomas of the Soil and Water Science Department at the University of Florida, and Drs Patricia Cline(Golder

Associates) and Thomas Potter (USDA) are gratefully

acknowledged The authors would also like to thank Dr Peter Hooda for his help in improving the manuscript after initial review

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