www.elsevier.com/locate/scitotenv

Science of the Total Environ

Spatial distribution of total Hg in urban soils from an Atlantic coastal

city (Aveiro, Portugal)

Sonia Rodrigues *, M. Eduarda Pereira, Luciana Sarabando, Lıdia Lopes,

Anabela Cachada, Armando Duarte

Departamento de Quımica, Universidade de Aveiro, Campus Universitario de Santiago, 3810-193 Aveiro, Portugal

Received 19 October 2004; received in revised form 11 May 2005; accepted 29 September 2005

Available online 2 November 2005

Abstract

The aim of this study was to investigate the levels and the spatial distribution of total Hg concentrations in soils from the urban

area of Aveiro (Portugal) in order to assess the impact of industrial activities and identified Hg emission sources in these urban

soils. For this purpose, soils were collected in 25 sampling points (at two depths) within the urban perimeter and in places

considered representative of the main green areas of the city. A median concentration of 0.091 mg kg�1 (dry weight) was obtained,

regardless the depth. Aveiro was considered a low polluted city in terms of total Hg and no direct effects of emissions of Hg from

industrial activities nearby could be detected in these urban soils. Despite of the low values obtained, high variability (range of

60.5 mg kg�1) was observed in the results. Such was considered to be related to characteristic features of soils in urban settings

and to the behaviour of Hg in the urban environment.

D 2005 Elsevier B.V. All rights reserved.

Keywords: Total mercury; Urban soils; Spatial distribution

1. Introduction

Mercury (Hg) pollution is regarded as a major con-

cern due to its potential impacts on the environment and

on public health (Horvat et al., 2003; Fang et al., 2004).

Soils are very important pools in the global biogeo-

chemical cycle of Hg, acting both as source and sink

(Gillis and Miller, 2000). Although this metal may

naturally occur in soil (in levels generally not exceeding

0.1 mg kg�1, Adriano, 1986), human activities may

contribute to Hg concentrations’ enrichment in certain

0048-9697/$ - see front matter D 2005 Elsevier B.V. All rights reserved.

doi:10.1016/j.scitotenv.2005.09.088

* Corresponding author. Tel.: +351 234370737; fax: +351

234370084.

E-mail address: smorais@dq.ua.pt (S. Rodrigues).

areas. Coal burning, municipal solid waste incineration

(Biester et al., 2002; Sznopek and Goonan, 2000),

electronic, paper, and pharmaceutical industries were

identified as the major sources of anthropogenic emis-

sion of Hg (Sznopek and Goonan, 2000; Tack et al.,

2005). Emissions of Hg from chlor-alkali plants have

been reported to have a significant impact on the envi-

ronment (Biester et al., 2002). The literature shows that

once emitted to the atmosphere Hg may be transported

over long distances (Jernelov and Wallin, 1973; Johans-

son et al., 1995; Biester et al., 2002; Wang et al., 2003)

and that due to the combination of its volatility and its

chemical persistence, Hg tends to participate in global

air–soil exchange processes (Shroeder and Munthe,

1998). Wang et al. (2003) found significant positive

correlations between atmospheric and soil Hg concen-

ment 368 (2006) 40–46

S. Rodrigues et al. / Science of the Total Environment 368 (2006) 40–46 41

trations in urban and suburban districts in China and

measured soil Hg decreasing trends with increasing

distance from emission sources.

There is a need for a better understanding of Hg

levels in urban soils and its variability. Some limited

work has been undertaken (McGrath, 1995; Chatter-

jee and Banerjee, 1999; Manta et al., 2002; Fang et

al., 2004) and increasingly Hg contamination status

has been appraised as an indicator of soil quality in

urban areas. Due to anthropogenic influence, soils in

urban areas tend to be very disturbed and a highly

heterogeneous media (particularly in what concerns

the upper, surface layers), largely differing from

their agricultural and forest counterparts (Thornton,

1991).

Aveiro (Portugal) is an urban area located in a

coastal lagoon, around 15 km south from a well

known chemical complex, which includes several

chemical industries, particularly a chlor-alkali plant.

Hg contamination of the Aveiro lagoon sediments

and biota has been subject of research throughout

the years (Pereira et al., 1997; Abreu et al., 1998,

2000) but in what concerns soil pollution, only the

vicinity of the complex has been assessed (Inacio et

al., 1998).

The present study is focused on the evaluation of

total Hg contents in the urban soils from Aveiro. Soil

total Hg levels were compared to concentrations

occurring in the vicinity of a chemical complex nearby

and with values reported in literature for other urban

areas. The main objectives were to gain insight into

the gradient of Hg concentration along this coastal

Fig. 1. Maps showing geographical details ab

area, address aspects of Hg pollution in urban areas

and to evaluate potential effects of nearby identified

sources. Spatial distribution of Hg concentrations was

also taken into consideration to provide an assessment

of its variability.

2. Methods

2.1. The study area (Aveiro, Portugal)

Aveiro is a coastal town, located in the Centre of

Portugal (latitude 40.38 N and longitude 8.40 W). The

urban centre of Aveiro has an area of 14 km2, and a

population of 35,948. The ceramic industry, the pro-

duction and processing of metals and pulp and paper

industry are the main industrial activities of the munic-

ipality of Aveiro (Fig. 1).

The annual average temperature is 14.6 8C, with

annual thermal amplitude of 11 8C. The wind direc-

tion is predominantly from Northwest (24% of the

days), and North to a less extent. The average pre-

cipitation is 914 mm per year. The number of private

vehicles per day in Aveiro is estimated to be around

50,000.

The chemical complex named bComplexo Quımico

de EstarrejaQ (CQE), operating since the 1950s, is 1 km

north from Estarreja. This village is located around 15

km, North East from Aveiro. The CQE is formed by 5

main chemical industries, including a chlor-alkali plant.

According to Pio et al. (1987), the annual consumption

of Hg as raw material of this plant increased from 15

ton/year in 1981 to 28 ton/year in 1986 and the total

out the study area of Aveiro (Portugal).

S. Rodrigues et al. / Science of the Total Environment 368 (2006) 40–4642

emissions of Hg to the atmosphere were estimated to be

around 12% of the annual consumption of Hg.

2.2. Soil sampling and analysis

Samples were collected from 25 sampling sites in

Aveiro urban area for including different types of land

use. Six samples were collected in ornamental gardens

(OG), 5 samples were collected in parks (PA) and 12

samples were collected in roadsides (RS). A play-

ground (PG) and a riverbank (RB) were also sampled.

Sites selected were considered representative of the

main public urban green areas in Aveiro. Two sampling

depths were defined: surface (0–10 cm, SF) and sub-

surface (10–20 cm, SB). Sampling was conducted using

a plastic spade and samples were immediately trans-

ported to the laboratory in plastic bags. Once in the

laboratory soils were dried at room temperature until

constant weight, sieved to b2 mm and ground to b150

Am (according to ISO 11464:1994 method). Parameters

such as soil pH (according to ISO 10390:1994 method),

total carbon percentage (Elemental Analysis, LECO,

CNHS-932), and Fe percentage (Aqua Regia digestion

according to ISO 11466:1995 method and determina-

tion by ICP-OES) were determined for general charac-

terization of these soil samples. The determination of

total Hg concentrations was performed by pyrolysis

atomic absorption spectrometry with gold amalgam-

ation (LECO, model AMA-254). No sample digestion

was involved on this process. Analyses were performed

directly on soil samples. At least three soil replicate

measurements were carried out for each sample. Certi-

fied reference materials (BCR 141R, calcareous loam

soil, Hg total content 0.25F0.02 mg kg�1 (mean-

Fhalf-width 95% confidence interval of the mean)

and BCR 142R, light sandy soil, Hg total content

0.067F0.011 mg kg�1) were used for QC/QA pur-

poses. Recovery percentages were always above 95%

(n =15). All determinations were expressed on a dry

weight basis.

0.7

0.6

0.5

0.4

0.3

0.2

0.1

0

OG

.1

PA

.2

PG

.3

PA

.5

RS

.4

PA

.6

PA

.7

RS

.8

OG

.9

OG

.10

OG

.11

RS

.12

sampl

To

tal H

g (

mg

kg

-1)

Fig. 2. Distribution of total Hg (mg kg�1) concentrations in the 25 s

3. Results

The pH of all samples varies from slightly alkaline

to acidic (5.2–7.5) regardless of the depth. A median

pH of 6.3 and 6.4 was obtained for SF and SB depths,

respectively. A median of 0.89% and 0.80% was

obtained for Fe in SF and SB samples, respectively,

which fits well with the lowest end of the common

range of Fe in soils, from 0.5 to 5%, according to

Kabata-Pendias (2001). These soils may also be con-

sidered poor in organic matter since the median per-

centage of total carbon was 1.7%, regardless the depth.

The highest value of total C was found in sampling

point 22, a riverbank (4.9% and 3.8% for SF and SB,

respectively).

Fig. 2 shows the distribution of median values of

total Hg contents for the several sampling points, both

for SF and SB. The total Hg concentration obtained for

the 25 surface soil samples (SF) ranged from 0.015 to

0.50 mg kg�1, in subsurface samples (SB), total Hg

contents varied from 0.013 to 0.59 mg kg�1. The

median concentration of total Hg is 0.091 mg kg�1

(regardless the depth) and no differences in results due

to sampling in different land uses (ornamental gardens,

parks, roadsides, playground and riverbank) could be

inferred from these 25 sampling points. Only 3 of the

25 samples exceeded the Dutch Target Value of 0.3 mg

kg�1 (VROM, 2000), for Hg in soils. All values were

below the Dutch Intervention Value of 10 mg kg�1.

Values obtained for total C and total Hg are

significantly correlated. Nevertheless, the r value is

low (r =0.58 and r=0.52 for SF and SB, respectively;

linear correlation is significant at p b0.01 level), reveal-

ing that total C contents only partially explain the total

variance of Hg concentrations. Fig. 3 shows relation-

ship between total C and total Hg concentrations, for SF

samples. It becomes clear that sampling points RS.08

and OG.21 (Fig. 3) show higher total Hg values (0.47

and 0.50 mg kg�1, respectively) than one would expect

attending to their carbon content (2.4 and 3.1%, respec-

RS

.13

PA

.14

RS

.15

OG

.17

RS

.18

RS

.19

RS

.20

OG

.21

RB

.22

RS

.23

RS

.24

RS

.25

RS

.16

SF

SB

ing point

ampling points for surface (SF) and subsurface samples (SB).

RS.08

OG.21

RS.18

RB.22

0

0,1

0,2

0,3

0,4

0,5

0,6

0,0 1,0 2,0 3,0 4,0 5,0 6,0

Total C (%)

To

tal H

g (

mg

kg

-1)

Fig. 3. Relationship between total C (%) and total Hg concentrations (mg kg�1) for SF samples.

S. Rodrigues et al. / Science of the Total Environment 368 (2006) 40–46 43

tively) and that points RS.18 and RB.22 (given their

high total carbon contents of 3.2 and 4.9%, respective-

ly) would be likely to present higher total Hg concen-

trations than those obtained (0.068 and 0.22 mg kg�1,

respectively).

Total Hg concentrations were displayed in maps,

using GIS tools, to assess spatial distribution of the

metal contents in the study area. No difference in

terms of spatial distribution for surface and subsurface

samples could be inferred. Two values were found to be

even higher than three times the median concentration

(sampling points 8 and 21), regardless the depth. For

these two points, contents of 0.47 mg kg�1 (SF) /0.44

mg kg�1 (SB) and 0.50 mg kg�1 (SF) /0.59 mg kg�1

(SB) were obtained, respectively. Sampling point 8 is

located in a roadside site, on the vicinity of Aveiro’s

biggest recreational park. Sampling point 21 is located

in an ornamental garden in the Eastern area of Aveiro

urban centre.

4. Discussion

The levels of Hg in soils from the vicinity of the

above mentioned chemical complex are reported to

have a range of 0.12 to 49 mg kg�1, a geometric

mean of 0.82 mg kg�1 and a median value of 0.59

mg kg�1 (Inacio et al., 1998). The results reported by

these authors refer to 103 samples collected from the

vertices of a grid with an interval of 1000 m (500 m,

near the chemical complex), within a sampling area of

10�6 km. An area of heavy contamination of 8 km2

was defined around the chemical complex, considering

the value of 1.5 mg kg�1 as a threshold (Inacio et al.,

1998).

The median value of Hg concentrations found in

Aveiro urban soils (0.091 mg kg�1) is 6.5 times

lower than median Hg soil content in the vicinity of

the chemical complex of Estarreja. In fact, the highest

value found in Aveiro soils is almost 84 times lower

then the maximum concentration reported in the study

from Inacio et al. (1998). There is a prominent decrease

of Hg contents in soils of Aveiro comparing to those

from the vicinity of Estarreja.

The chemical complex is located North East, in

relation to Aveiro. As North West and North wind

direction are predominant one could expect that Hg

levels in Aveiro could be at least at some extent in-

creased due to these industrial activities. As mentioned,

besides this chemical complex other anthropogenic ac-

tivities such as the production and processing of metals

and pulp and paper industry occur in the Aveiro mu-

nicipality, and even to a shorter distance than the Estar-

reja chemical complex. These could also have impact in

terms of Hg concentrations in soils. Hg has a very long

residence time in soils, which could lead to metal

accumulation. As urban soils tend to be highly dis-

turbed environments, this study focused not only in

the immediate soil surface. Samples were collected

down to 20 cm to assess vertical distribution of Hg

and accumulation of this metal in deeper layers. The

results obtained showed no apparent differences be-

tween SF and SB layers, and therefore no accumulation

of total Hg in soil subsurface layers due to years of

potential atmospheric deposition was found. Although

the impact of the Estarreja chemical complex on the

aquatic environment of the Aveiro lagoon has been

reported (Pereira et al., 1997; Abreu et al., 1998,

2000), no impact on Aveiro soils due to atmospheric

deposition was concluded. In spite of the close distance

of relevant industrial activities and identified Hg

sources, according to the values reported, Aveiro soils

may be considered unpolluted in what concerns total

Hg. The results of Hg concentrations reported in liter-

ature are shown on Table 1, together with their soil

type/land use, location and references.

Data on soils Hg concentration, particularly in urban

areas are rather scarce in literature. The median of the

Hg concentration values found in Aveiro urban soils is

Table 1

Hg concentrations reported in literature

Soil type/land use Location Median/mean Range Reference

Unpolluted soils Korea Median: 0.045 mg kg�1 – Kim and Kim (1999)

Background Worldwide b0.4 mg kg�1 – Kabata-Pendias and Pendias (1984)

Soil world median Worldwide Median: 0.05 mg kg�1 – Reimann and Caritat, 1998

Agricultural soil Canada Median: 0.04 mg kg�1 0.005–0.13 mg kg�1 Reimann and Caritat, 1998

Typical normal range in soil UK 0.008–0.19 mg kg�1 Thornton, 1991

Urban soil, town gardens Wexford, Ireland Mean: 0.68 mg kg�1 0.09–2.97 mg kg�1 McGrath, 1995

Urban soil Palermo, Italy Median: 0.68 mg kg�1 0.04–6.96 mg kg�1 Manta et al., 2002

Urban soil Changchun, China 0.139–0.479 mg kg�1 Fang et al., 2004

Urban soil Trondheim, Norway Median: 0.13 mg kg�1 b0.2–4.49 mg kg�1 Reimann and Caritat, 1998

Urban soil Pittsburg, USA Mean: 0.51 mg kg� 1 – Carey et al., 1980

Urban soil Cornwall, Canada Mean: 0.698 mg kg�1

(n =33)

0.04–5.1 mg kg�1 Sherbin, 1979

Urban soils, industrial centre Khabarovsk, Russia 0.080 mg kg�1 (n =122) 0.011–0.950 mg kg�1 Kot and Matyushkina, 2002

Urban soils, industrial centre Amursk, Russia Median: 0.175 mg kg�1

(n =30)

0.004–0.464 mg kg�1 Kot and Matyushkina, 2002

Urban soils, industrial centre,

heavily contaminated

Amursk, Russia 0.712–16.65 mg kg�1

(n =6)

Kot and Matyushkina, 2002

S. Rodrigues et al. / Science of the Total Environment 368 (2006) 40–4644

just slightly higher than the median content found for

Khabarovsk, Russia (Kot and Matyushkina, 2002), but

in general is lower than reported concentrations, espe-

cially in what concerns maximum values. Nevertheless,

the median concentration found in the studied urban

area is higher than concentrations found in agricultural

soils (Reimann and Caritat, 1998) and higher than

values reported for unpolluted soils in Korea (Kim

and Kim, 1999) and worldwide soil values (Kabata-

Pendias and Pendias, 1984; Reimann and Caritat,

1998). The range of Hg concentrations obtained in

the urban soils of Aveiro is broader than the typical

normal range in soil from the UK (Thornton, 1991).

Although Aveiro may be considered an unpolluted

urban area, differences of over 30 times were obtained

between minimum and maximum concentrations.

Values extend far beyond the median, showing a long

tail of values to the right if posted in a histogram. This

confirms that total Hg concentrations are highly vari-

able and that urban soils are very heterogeneous media.

There is no Portuguese legislation in what relates to

limits of Hg concentrations in soils. In what concerns to

soil quality guidelines from The Netherlands, Aveiro

shows concentrations which are in general below

legislated accepted values. In The Netherlands both

the background and the target value for Hg concentra-

tion in soils are defined as 0.3 mg kg�1, and the so-

called bintervention valueQ was set 10 mg kg�1

(VROM, 2000).

Total C explained only part of the total variability in

the Hg contents. If the inputs of total Hg were uniform

across the city area, differences in Hg contents would

be explained on the basis of soil sorptive ability derived

from its carbon content. Therefore one may assume that

even in a small city (such as Aveiro) there are other

sources of variability associated to factors contributing

to Hg soil pollution and affecting total Hg concentra-

tions’ spatial distribution. Further investigation, using a

larger set of samples and considering different urban

environments could confirm whether this is due to the

particular conditions occurring in urban environments.

As in other cities, most of Aveiro green areas are

subject to active planting and frequent disturbance of

the soil matrix (wastes and residues of construction

works were often found mixed with soil) which may

contribute to the variability observed.

Within this set of data and in such a small city, it is

difficult to infer clear relationships between soil Hg

concentrations and human influence. Results suggest

that there is no significant contamination arisen from

atmospheric inputs originated at nearby potential

sources. Despite of this, the highly variable and random

distribution of values obtained seems to prove that

spatial variability is a highly significant feature for

the definition of soil quality in urban environments.

5. Conclusion

Total Hg content in Aveiro urban soils may be

considered low, when compared to other soil quality

studies. No vertical variability between 10 and 20 cm

layers was found, and therefore no accumulation of Hg

in soils from Aveiro urban area was observed. Despite

of the fact that several studies have reported Hg con-

tamination in the aquatic environment of the Aveiro

Lagoon, and that high contents by Hg were found in

S. Rodrigues et al. / Science of the Total Environment 368 (2006) 40–46 45

soils from the vicinity of a nearby chemical complex,

no contamination was found in the studied urban soils.

Apparently, no influence from this or other industrial

activities around this urban area was observed. When

looking to the results of this investigation and those

reported in literature both for soils from the Estarreja

chemical complex and those related to the Aveiro La-

goon, a decreasing North–South relation in terms of Hg

contamination is suggested. Further investigation of

total Hg contents in soils from other areas in the Aveiro

and Estarreja municipalities, particularly areas located

South and Southeast the above mentioned chemical

complex would allow to conclude whether Hg soil

contamination is limited to the immediate vicinity of

the industrial activities.

Carbon content only partially explains the variabil-

ity of total Hg concentrations’ spatial distribution in

this urban area. Despite of the low Hg pollution

levels, the variability associated with the results

obtained reveals that metal distribution in urban soils

has peculiar characteristics, which differentiates them

from their agriculture and forest counterparts. Due to

the highly significant impacts of urban soil quality in

human health, it is vital to understand the behaviour of

potentially toxic chemical elements and even other

substances such as organic compounds within the

urban ecosystem. Further investigation, particularly

the comparison of this total Hg contents distribution

with results for other urban contexts, is needed to

assess and draw conclusions on total Hg enrichment

of soils and its variability within urban areas and on

the relative significance of various anthropogenic ac-

tivities. The identification of different scales of vari-

ability of metallic contamination in urban areas should

be considered in future investigations. A similar study

considering both Hg and other metals contents in other

cities in Europe is being conducted, in order to assess

relationships between levels of pollution and different

urban contexts (variable in terms of climate, popula-

tion, geology and pollution sources). It is also very

important to verify to what extent total Hg may be

considered a robust soil quality indicator for urban

environments.

Acknowledgements

This work was part of EU funded FP5 Project

EVK4-CT-2001-00053: URBSOIL (bUrban Soils as

Source and Sink for Pollution: Towards a common

European methodology for the evaluations of their

environmental quality as a tool for sustainable resource

managementQ).

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