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Heavy metal and trace element distributions ingroundwater in natural slopes and highly urbanized spacesin Mid-Levels area, Hong Kong

Chi-Man Leung�, Jiu Jimmy Jiao

Department of Earth Sciences, The University of Hong Kong, Pokfulam Road, Hong Kong, China

a r t i c l e i n f o

Article history:

Received 30 May 2005

Received in revised form

24 November 2005

Accepted 2 December 2005

Keywords:

Groundwater

Heavy metals

Trace elements

Urbanization

Leakage from service pipes

Pollution

nt matter & 2006 Elsevie.2005.12.016

uthor. Rm 206, James Lefax: +852 2517 6912.iman46@graduate.hku.h

A B S T R A C T

The lower slope of the Mid-Levels area, Hong Kong, is one the most heavily urbanized

coastal areas in the world. A comprehensive groundwater heavy metal and trace element

study was conducted in the Mid-Levels area aiming to investigate the impacts of

urbanization on the aqueous distributions of these chemicals. Groundwater samples were

collected in the upper natural slopes and the lower highly urbanized spaces in the area in

different seasons, and analyzed for heavy metal and trace element contents.

Compared to the results from natural slopes, groundwater samples in the developed

spaces did not exhibit significant elevated levels in Zn, Cr, Cu, Cd, Pb and Fe, which are

commonly found in stormwater. On the other hand, the samples were found to have

elevated contents in Mn, V, Co and Mo, minor stormwater-related heavy metals, suggesting

that stormwater drains may be leaking to some extent. However, the results suggested that

the vadose zone could remove many of the heavy metals, protecting groundwater from

being contaminated seriously. Statistical analysis suggested that a certain amount of Mn

and Co was likely to be re-mobilized from natural soils due to the changes in local redox

conditions, while Mn, V, Co and Mo may also be derived from steel corrosion as a result of

prolonged submergence. Besides, the average B concentration in the developed spaces was

about eight times higher than that in the natural slopes, indicating the presence of sewage.

The mean Se concentration in the developed spaces was about 100 times higher than that

in the natural slopes. About 40% of samples in the developed spaces contained Se level

higher than the drinking water guideline value proposed by the World Health Organization.

Se was found to be positively correlated with B and SO42� (R ¼ 0.534 and 0.631, respectively),

suggesting that Se may also be related to leakage from sewage pipes. Part of the Sr may

come from leakage of flushing water and/or sewage as Sr was strongly correlated with Cl�

(R ¼ 0.929). According to the measured results, deep groundwater samples collected from

piezometers (410 m in depth) in the urbanized spaces appeared to be virtually free from

any anthropogenic contaminations.

This study may shed important light on the identification and evaluation of leakage

from service pipes in a particular area based on aqueous distributions of heavy metals and

trace elements. Moreover, the above findings may be instructional for other coastal cities

with a similar level of urban development to understand the potential threats to their

groundwater resources.

& 2006 Elsevier Ltd. All rights reserved.

r Ltd. All rights reserved.

e Science Building, The University of Hong Kong, Pokfulam Road, Hong Kong SAR, China.

k (C.-M. Leung).

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WA T E R R E S E A R C H 4 0 ( 2 0 0 6 ) 7 5 3 – 7 6 7754

1. Introduction

In Hong Kong, thousands of cut slopes have been created

in the process of urbanization because of the rugged

topography. Groundwater samples from the weepholes or

drains installed in these slopes, which are otherwise

extremely difficult or expensive to collect, provide a unique

and economical chance to understand the physical and

chemical natures of the subsurface flow system in

the intensively urbanized hillslopes and the impacts of

urbanization on groundwater chemical systems. Previous

studies were mainly concerned about the impacts of

urbanization on the groundwater flow and major element

chemical systems in the highly urbanized coastal

areas in Hong Kong (Jiao et al., 2006; Leung and Jiao, 2005;

Leung et al., 2005). In this paper, heavy metal and trace

element distributions in groundwater samples collected

from natural slopes and urbanized spaces in the Mid-Levels

area are discussed. The data collected from the natural

slopes could be regarded as the background trace element

concentrations in the natural environment of this area. These

data are employed to evaluate the extent of heavy metal and

trace element contamination in groundwater in the urba-

nized spaces. In our knowledge, this is the first comprehen-

sive study of heavy metals and trace elements in groundwater

in Hong Kong. The results may be instructional for other

coastal cities with similar levels of urban development

to understand the potential threats to their groundwater

resources.

Fig. 1 – Overview of the study area. The shaded area represen

bounded by the gray line is the Mid-Levels area. Dotted lines re

rock. Black lines represent the locations of normal faults.

2. Geology and hydrogeology of the study area

The Mid-Levels area, approximately 1.5 km2 in size, is situated

on the northern slope of the Victoria Peak [550 m above

Principle Datum (mPD)] on Hong Kong Island (Fig. 1). The

study area can be divided into two parts with significantly

different modes of development. The upper part of the area

(4170 mPD) is essentially a natural slope with minimum

development. In contrast, the lower part of the area has been

extensively urbanized and is regarded as one of the most

heavily urbanized areas in the world.

The geology and hydrogeology of the area has been

described elsewhere in Geotechnical Control Office (GCO)

(1982). The geology is dominated by two rock types, acidic

volcanic rocks and a granitic intrusion. The volcanic rocks

have been subject to low grade regional metamorphism and

deformation and affected by contact metamorphism where

close to the granite. Both lithologies have been subsequently

intruded by basaltic dykes. The irregular contact between the

granite and volcanic rocks crosses the area and is disrupted

by normal faults in several locations (Fig. 1).

In general terms, colluvium overlies several meters of

decomposed rock above the bedrock. The granite underlies

most of the developed spaces, composed of quartz (23–42%),

potassium feldspar (31–42%), plagioclases (16–35%) and biotite

(�5%) according to Allen and Stephens (1971). Volcanic rock

underlies the upper undeveloped slopes.

Although it is likely that the lithologies in the subsurface

are very heterogeneous and anisotropic, GCO (1982) have

ts the natural slope with minimum development. The area

present the contact between granite and volcanic

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Table 1 – Summary of geochemistry of rock samples collected within and near the Mid-Levels area, Hong Kong (GEO, 2001)

Sample 1 2 3 4 5 6 7 8 9

Ref. no. 2189 2228 2457 2857 4258 11027 2230 4237 11028

Rock type (a) (a) (a) (a) (a) (a) (b) (c) (d)

SiO2 67.75 71.61 72.90 73.67 72.47 74.40 75.77 77.01 80.33

TiO2 0.49 0.56 0.49 0.26 0.26 0.23 0.13 0.13 0.10

Al2O3 14.92 15.39 15.30 13.05 13.43 13.23 12.74 12.09 10.52

Fe2O3 4.16 4.80 4.18 2.26 2.83 2.17 1.61 1.52 1.01

MnO 0.10 0.03 0.05 0.07 0.06 0.07 0.09 0.07 0.02

MgO 0.99 0.84 0.75 0.34 0.23 0.27 0.13 0.11 0.08

CaO 2.85 0.06 0.06 1.67 1.57 1.29 0.74 0.90 0.53

Na2O 3.07 0.09 0.04 3.01 2.88 3.80 3.72 2.77 1.58

K2O 4.92 5.27 5.51 4.87 5.61 4.09 5.14 4.76 5.80

P2O5 0.17 0.03 0.04 0.06 0.07 0.06 0.01 0.02 0.01

LOI 0.35 1.19 0.30 0.77 0.23 0.23 0.29 0.72 0.10

Total 99.77 99.87 99.62 100.00 99.64 99.84 100.40 100.10 100.10

Cr 21 28 20 12 23 3 15 30 6

Ni 9 7 4 1 10 4 14 o1 5

Co 3 4 4 5 5 3 o1 na 3

V na na o1 13 na na na na na

Cu o1 o1 o1 o1 o1 1 4 o1 1

Pb 16 102 23 31 44 14 33 31 21

Zn 19 116 23 56 33 36 103 30 12

Sn na na o1 2 o1 na 10 2 Na

W 10 5 7 2 8 6 7 4 2

Rb 244 238 204 193 263 241 320 241 343

Ba 548 640 801 464 228 483 83 140 555

Sr 148 158 205 189 100 133 32 86 83

Ga 16 16 17 15 17 15 16 13 15

Nb 21 19 16 19 25 18 29 19 13

Zr 147 150 165 174 194 148 193 152 136

Y 54 41 32 37 49 43 57 46 29

Th 33 29 30 29 42 34 39 33 35

U 5 8 7 7 6 5 5 8 5

La 53 43 52 49 90 25 40 30 17

Ce 111 107 111 70 200 58 97 70 36

F 1881 1686 1686 na 1813 1130 1585 na 905

Remarks: 1. Rock type (a) represents coarse ash crystal tuff; (b): eutaxitic fine ash vitric tuff; (c): medium-grained granite; (d): microgranite.

2. Major element in %wt; Trace element in ppm; na: not analyzed.

WAT ER R ES E A R C H 40 (2006) 753– 767 755

grouped them into three aquifer units corresponding to (a)

colluvium, (b) decomposed volcanic and granite rocks, and (c)

volcanic and granite bedrocks. The colluvium contains

transient and permanent perched water tables, whereas, as

recently demonstrated by Jiao et al. (2003, 2005), and the

highly decomposed rock or saprolite below the colluvium is

relatively impermeable due to its clay-rich content. The

bedrock zone along the uppermost part of parent rocks may

be fairly permeable with confined groundwater contained

within a well-developed fracture network.

Geotechnical Engineering Office (GEO) conducted a geo-

chemical survey for some major rock types in Hong Kong

(GEO, 2001). In the Mid-Levels area, seven volcanic rock

samples (coarse ash crystal tuff and eutaxitic fine ash vitric

tuff) and two granitic rock samples (medium-grained granite

and microgranite) were collected and analyzed. Table 1 shows

their whole rock composition results. From Table 1, it seems

that no significant difference in the compositions could be

observed among those samples.

3. Field and analytical methods

Groundwater samples were collected in natural and cut

slopes, and piezometers in the study area in September

2002 (wet season) and January 2003 (dry season). The

sampling locations are showed in Figs. 2 and 3, respectively.

At each site, the pH, temperature, electrical conductivity

(EC) and dissolved oxygen (DO) of water samples were

measured. Sampling and analytical techniques followed

the suggestions by APHA (1998) and are described in

detail in Leung (2004). In brief, water samples for chemical

analysis were filtered through a hand-held Hanna filter

system using a 0.45mm cellulose filter paper (Advantec MFS

Inc.) and collected in a 500 mL clean HDPE bottle. Two 125 mL

aliquots were collected at each site, one unacidified and the

other acidified to pHo2 using ultrapure nitric acid. The

samples were then refrigerated at 4 1C before chemical

analysis.

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Fig. 2 – Sampling location in the wet season in 2002 in and around the Mid-Levels area. Squares represent water samples

collected from seepage or a spring from natural or cut slopes. Circles represent water samples collected from

piezometers. Shaded area represents the natural slopes.

Fig. 3 – Sampling location in the dry season in 2003 in and around the Mid-Levels area. Triangles represent water samples

collected from seepage or a spring from natural or cut slope. Circles represent water samples collected from

piezometers. Shaded area represents the natural slopes.

WA T E R R E S E A R C H 4 0 ( 2 0 0 6 ) 7 5 3 – 7 6 7756

Acidified aliquots were analyzed for heavy metals and trace

elements by inductively coupled plasma mass spectrometry

(ICPMS) except for boron (B) which was analyzed by induc-

tively coupled plasma atomic emission spectrometry (IC-

PAES). Unacidified aliquot was analyzed for major anions by

Ion Chromatography. Detailed analytical results are listed in

Leung (2004). The pH values, DO content, heavy metal and

trace element concentrations of groundwater samples are

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Table 2 – Heavy metals that are found in variouscomponents of a motor vehicle

Component Heavy metal

Exhaust Al, Sb, Ba, B, Cr, Co, Cu, Fe, Pb, Li, Ni, Se, Sr, V, Zn

Engine Cd, Cr, Cu, Fe, Ni, Zn

Brakes Al, Sb, Ba, Cr, Cu, Fe, Mo, Sn, Zn

Tires Cd, Pb, Ti, W, Zn

References: Kiem (2002), Valcav and Valcav (1992), Ball et al. (1998)

WAT ER R ES E A R C H 40 (2006) 753– 767 757

listed in Appendices A and B in this paper. Sample batches

were regularly interspersed with standards and blanks, and

all data were corrected for instrument drift. A three-point

calibration curve was constructed for each element. National

Institute of Standards and Technology (NIST) Standard

Reference Material (SRM) 1640 was employed to check the

reliability of the analysis. The measured values of SRM 1640

were all within 5% of the certified values. The results of three

replicate analyses indicated that the precision of cation

measurements was generally better than 5%.

and Friedlander (1973).

Table 3 – Typical concentration of some heavy metal (inppb) in stormwaters from literatures

Element Typical literature values

Al 17089–17760e

Fe 18368–19742e

V 7–2200a

Cr 1–2300b, 51.1–58.73e

Mn 7–3800b

Co 1.3–5.4a, 6.7d

Ni 53c, 93d, 248–266e

Cu 52c, 274d, 1–54.3f, 267–283e

Zn 498c, 208d, 50–1462f, 2548–2792e

Mo 20d

Cd 5c, 14.1d, 3.02–4.42e

Sb 3.5–23b

Pb 1558c, 81d, 1–199.5f, 2548–2792e

a Dannecker et al. (1990).b Cole et al. (1984).c Yousef et al. (1984).d Hares and Ward (1999).e Kiem (2002).f Barbosa and Hvitved-Jacobsen (1999).

4. Sources of heavy metals and trace elementsin the study area

Heavy metals and trace elements in subsurface environments

come from natural and anthropogenic sources. The weath-

ering of minerals is one of the major natural sources. As

shown in Table 1, no significant difference in heavy metal and

trace element concentrations between the bedrocks is

observed in the study area. Anthropogenic sources include

fertilizers, industrial effluent and leakage from service pipes.

In the area, industrial and agricultural activities are unlikely

to be the major contamination sources because of the

absence of these activities. The main features of urbanization

in the area are (1) high density of residential and commercial

buildings, (2) extensive areas of impervious surfaces and (3)

extensive construction of subsurface drainage systems such

as flushing water pipes, sewers and stormwater drains.

High traffic rate is common in the study area. Table 2 lists

the heavy metals present in various car components. Thus,

motor vehicles are likely to be the contributor to the

pollutants on roads. Pollutants accumulated on road surfaces

could be washed by storm or street cleansing as road runoff

and finally collected into the nearby stormwater drainage.

These pollutants include heavy metals (Ball et al., 1998;

Dannecker et al., 1990), organic chemicals (Cole et al., 1984;

Krein and Schorer, 2000) and organic compounds (Blumberg

and Bell, 1984). In this paper, focus is placed on heavy metals

and trace elements. Many studies have investigated the

concentrations of heavy metals in stormwater (Dannecker

et al., 1990; Cole et al., 1984; Yousef et al., 1984; Hares and

Ward, 1999; Kiem, 2002; Barbosa and Hvitved-Jacobsen, 1999).

Table 3 shows the typical concentrations of heavy metals in

stormwater from the literature. It should be noted that the

heavy metal concentrations in stormwater are controlled by

various factors such as average daily traffic, the type of

vehicles, the nature of fuels used and climate, etc.

It is commonly accepted that stormwater contains high

levels of heavy metals, especially aluminum, iron, zinc,

copper, chromium, cadmium, nickel and lead (if leaded petrol

is still in use). In Hong Kong, although the actual heavy metal

contents in road runoff have not been studied, some studies

measured heavy metals in dusts collected in urban areas

including streets (Yim and Niu, 1987; Tong and Lam, 2000;

Poon et al., 1999; Ng et al., 2003). These data may help to

assess the heavy metal compositions of road runoff in Hong

Kong. In this paper, the results from Ng et al. (2003) are

employed mainly because their results reflect the most recent

heavy metal compositions of dusts in urban environments in

Hong Kong. In their study, a total of 89 dust samples were

obtained from 62 urban playgrounds and parks of various

sizes and types from November 2000 to February 2001. The

dust samples were then analyzed with heavy metals (Zn. Cu,

Cd, Cr, Pb, Fe, Mn) and organic carbon (OC) contents. The

analytical results are listed in Table 4. Although the dust

samples were collected in playgrounds and parks located in

urban areas, Ng et al. (2003) successfully demonstrated the

significant traffic related inputs of heavy metals by the

presence of tire fragments, metallic debris and sand grains

with a tar coating. The results presented by them could be

used as a reference for the heavy metal contents of road dusts

in Hong Kong. It is believed that the heavy metal concentra-

tions in dust samples collected from roadsides may be even

higher than that from urban parks because the former are

closer to the source (motor vehicles).

Ng et al. (2003) concluded that roadside dusts in Hong Kong

are characterized by high concentrations of Zn, Cu and Cr.

This finding agrees with Yim and Niu (1987), Poon et al. (1999)

and Tong and Lam (2000). Since road runoff is collected by

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Table 4 – Chemical composition of playground dust (n ¼ 89) (Ng et al., 2003)

Chemical variables Mean Maximum Minimum Std. dev.

Zn (mg/g) 1883 6658 159 1309

Cu (mg/g) 143 859 17.0 109

Cd (mg/g) 7.0 13.7 4.6 1.9

Cr (mg/g) 263 2681 11.8 408

Pb (mg/g) 77.3 263 1.8 39.4

Fe (mg/g) 22,991 159,596 4826 19,554

Mn (mg/g) 518 1359 216 179

OC (%) 11.6 25.8 2.1 4.8

Table 5 – Trace element (in ppb) statistics of groundwater in the natural and developed areas in the wet season

Element Natural slope (n ¼ 18) Developed area (n ¼ 20)

Min Median Mean Max SD CV Min Median Mean Max SD CV

Al 0.78 3.20 8.19 43.72 11.15 136.14 0.01 0.26 1.02 6.17 1.58 154.90

Fe 2.23 3.68 5.57 19.13 4.60 82.59 1.73 3.87 4.89 16.18 3.54 72.39

Mn 0.85 2.20 2.72 7.34 1.92 70.59 0.37 5.55 89.87 789.36 188.90 210.19

Cu 0.00 0.00 0.14 1.42 0.35 250.00 0.00 0.10 1.33 14.95 3.35 251.88

Zn 8.11 46.18 40.83 73.75 26.23 64.24 7.39 26.91 33.69 92.39 23.34 69.28

Sr 14.72 28.16 28.18 47.47 9.32 33.07 75.82 325.90 412.01 1477.00 323.24 78.45

Se 0.00 0.01 0.07 0.37 0.10 142.86 0.56 5.62 8.60 23.25 7.22 83.95

Li 0.20 1.34 2.04 6.79 1.80 88.24 0.00 1.00 3.31 15.83 4.69 141.69

Be 0.04 0.39 0.45 1.12 0.35 77.78 0.00 0.02 0.18 1.71 0.41 227.78

V 0.27 0.55 0.55 1.06 0.21 38.18 1.21 3.65 4.77 14.61 3.21 67.30

Cr 0.00 0.10 0.54 5.12 1.19 220.37 0.60 1.45 1.58 4.31 0.83 52.53

Co 0.00 0.02 0.02 0.07 0.03 150.00 0.00 0.18 0.34 2.54 0.58 170.59

As 0.12 0.33 0.42 1.78 0.37 88.10 0.00 1.81 2.65 17.49 3.65 137.74

Rb 2.61 6.15 6.39 11.38 2.79 43.66 2.22 33.21 35.70 81.92 20.03 56.11

Mo 0.00 0.06 0.12 0.88 0.21 175.00 0.02 1.03 1.60 5.72 1.66 103.75

Ag 0.68 2.90 3.16 7.10 1.39 43.99 0.52 1.48 1.90 6.10 1.46 76.84

Cd 0.01 0.06 0.07 0.18 0.04 57.14 0.01 0.09 0.14 1.18 0.25 178.57

Sb 0.00 0.02 0.03 0.10 0.03 100.00 0.00 0.18 0.28 0.81 0.27 96.43

Ba 8.07 32.52 31.51 55.79 13.07 41.48 11.18 39.75 57.15 195.99 47.22 82.62

Pb 0.03 0.15 0.54 3.34 0.89 164.81 0.06 0.35 0.95 6.67 1.59 167.37

B 12.54 18.41 18.84 31.22 4.81 25.53 46.70 123.05 146.07 433.54 99.20 67.91

Remarks: ‘‘SD’’ represents standard deviation; ‘‘CV’’ (in %) represents coefficient of variation.

WA T E R R E S E A R C H 4 0 ( 2 0 0 6 ) 7 5 3 – 7 6 7758

nearby stormwater drainage, any leakage from the drains

could bring heavy metals into the surrounding environments,

which may in turn pollute groundwater. Besides, leakage

from other service pipes may also bring additional trace

elements into the environment.

5. Results and discussions

5.1. Heavy metal and trace element distributions ingroundwater samples in the Mid-Levels area

The main objective of this study is to evaluate the impacts of

urbanization on groundwater in terms of heavy metal and

trace element contaminations. Groundwater samples in the

natural slopes are assumed to be free from any anthropogenic

contaminations and thus they could be used to evaluate the

degree of heavy metal and trace element contaminations in

the urbanized spaces. Tables 5 and 6 present the statistical

summary of heavy metal and trace element concentrations in

groundwater samples collected in wet and dry seasons.

According to Appendices A and B, in general groundwater

samples collected in the natural slopes were slightly more

acidic and of higher DO content than that in the developed

spaces for both seasons. Most of the groundwater samples in

the study area are weakly acidic (pH ranged from 5 to 7) and

oxic (DO above 5 mg/L) in nature.

Since the developed spaces are located downhill to the

natural slopes, the distribution of heavy metals and trace

elements may be modified by natural processes such as

water–rock interactions. For most of the heavy metals and

trace elements measured, their average concentrations in

the developed spaces were higher than that in the natural

slopes. This may be the results of natural processes and/or

ARTICLE IN PRESS

Table 6 – Trace element (in ppb) statistics of groundwater in the natural and developed areas in the dry season

Element Natural slope (n ¼ 12) Developed area (n ¼ 14)

Min Median Mean Max SD CV Min Median Mean Max SD CV

Al 2.31 7.02 10.06 27.74 8.27 82.21 0.00 0.92 2.41 16.21 4.28 177.59

Fe 3.63 8.43 12.52 63.25 16.27 129.95 1.61 5.72 7.84 26.86 6.55 83.55

Mn 0.36 1.28 1.76 6.07 1.57 89.20 0.20 3.42 138.62 1192.00 335.79 242.24

Cu 0.28 0.76 1.16 4.53 1.19 102.59 0.23 0.88 1.05 2.45 0.67 63.81

Zn 1.50 2.10 4.62 12.65 3.96 85.71 1.56 5.23 9.08 39.60 11.09 122.14

Sr 5.82 18.81 20.03 35.13 7.80 38.94 46.17 280.15 308.71 742.20 211.18 68.41

Se 0.00 0.36 0.38 0.70 0.21 55.26 0.52 5.64 7.92 18.96 6.63 83.71

Li 0.61 1.46 1.79 4.71 1.27 70.95 0.06 0.53 1.19 5.01 1.49 125.21

Be 0.03 0.32 0.34 0.82 0.26 76.47 0.00 0.01 0.18 1.36 0.37 205.56

V 0.00 0.20 0.021 0.49 0.15 714.29 0.11 1.91 2.24 4.69 1.58 70.54

Cr 0.25 0.47 0.71 2.34 0.61 85.92 0.35 1.34 1.44 2.90 0.798 55.42

Co 0.00 0.00 0.0008 0.01 0.0029 362.50 0.00 0.05 0.28 1.95 0.53 189.29

As 0.00 0.18 0.24 0.97 0.27 112.50 0.00 0.45 0.75 3.27 0.91 121.33

Rb 2.39 5.07 5.05 9.09 2.00 39.60 1.45 25.39 30.30 78.24 23.50 77.56

Mo 0.04 0.19 0.29 0.86 0.28 96.55 0.13 0.58 1.03 4.45 1.30 126.21

Ag 0.00 0.00 0.0033 0.01 0.0049 148.48 0.00 0.01 0.05 0.30 0.09 180.00

Cd 0.01 0.03 0.05 0.16 0.0465 93.00 0.00 0.03 0.0743 0.31 0.087 117.09

Sb 0.01 0.04 0.05 0.20 0.05 100.00 0.03 0.09 0.14 0.63 0.17 121.43

Ba 11.57 27.27 27.63 39.54 8.38 30.33 6.18 36.26 39.38 95.60 22.47 57.06

Pb 0.01 0.24 1.84 12.10 3.59 195.11 0.01 0.35 1.09 6.85 1.81 166.06

Ni 0.00 0.67 1.78 7.66 2.44 137.08 0.00 1.36 3.81 35.53 9.20 241.47

Remarks: ‘‘SD’’ represents standard deviation; ‘‘CV’’ (in %) represents coefficient of variation.

WAT ER R ES E A R C H 40 (2006) 753– 767 759

anthropogenic pollution. In the following parts, the degree of

heavy metal contamination in groundwater in the urbanized

spaces are discussed. As stated, one of the major heavy metal

and trace element sources in the area may be leakage from

service pipes.

5.1.1. Leakage from stormwater drainsIn Hong Kong, road dusts are characterized by high concen-

trations of heavy metals such as Zn, Cr and Cu. It is thus

reasonable to assume that stormwater may also contain

high concentrations of these heavy metals. According to

Tables 5 and 6, no significant difference in the concentrations

of Zn, Cr, Cu, Cd, Pb and Fe in groundwater can be

observed between the developed spaces and the natural

slopes in both seasons. This suggests that groundwater in the

developed spaces was almost unaffected by the major heavy

metals found in stormwater. Two possible reasons may

account for this: (1) there is no significant leakage from

stormwater drains; (2) Heavy metals in leaked stormwater

cannot reach groundwater. Owing to the fact that the storm-

water drainage in the study area was built decades ago, zero

leakage is hardly possible. In this sense, it may be more

reasonable to suspect that heavy metals in the leaked

stormwater cannot reach groundwater. This possibility will

be discussed in detail.

The impacts of leakage from stormwater drains on ground-

water quality were controlled by factors such as pollutant

abundance in stormwater and pollutant mobility in the

vadose zone (Pitt et al., 1999). In the study area, the amount

of pollutants in stormwater would possibly be large because

of the dense traffic. Heavy metal concentrations in ground-

water would, therefore, be mainly controlled by pollutant

mobility in the vadose zone.

Many studies demonstrated that most of the heavy metals

are removed, degraded or accumulated with little downward

movement in the vadose zone (Hathhorn and Yonge, 1995; Ku

and Simmons, 1986; Hampson, 1986; Nightingale, 1987; Legret

et al., 1999; Dierkes and Geiger, 1999; Mikkelsen et al., 1997).

Crites (1985) suggested five metal removal processes by soil.

They are (1) soil surface association, (2) precipitation, (3)

occlusion with other precipitates, (4) solid-state diffusion into

soil minerals and (5) biological system or residue incorpora-

tion. Table 7 lists the possible removal mechanisms of some

of the common heavy metals in soil.

Dissolved heavy metal ions are removed from stormwater

during infiltration mostly by adsorption onto the near-surface

particles in the vadose zone (Ku and Simmons, 1986). The

particulate heavy metals are readily filtered out at soil

surfaces or as water infiltrates into the soil (Ku and Simmons,

1986; Pitt et al., 1995). In some cases, the direct physical

removal mechanisms are more important than chemical

removal mechanisms for most heavy metals (Pitt et al., 1995).

It appears that most of the heavy metals from the leakage of

stormwater drains are generally not important groundwater

contaminants because of their affinity for soils. However, it

may be possible that the contaminants accumulate in the

subsurface materials in the built-up areas to environmentally

critical levels.

According to Tables 5 and 6, some heavy metals, including

Mn, V, Co and Mo, in groundwater in the developed spaces

were at significantly higher concentrations than that in the

natural slopes. A few sites were even found to have Mn levels

ARTICLE IN PRESS

Table 7 – Metal removal mechanisms in soil

Element Principal removal mechanisms

Arsenic Strong associations with clay fractions of soil

Barium Precipitations and sorption onto metal oxides and hydroxides

Cadmium Ion exchange, sorption, and precipitation

Chromium Sorption, precipitation, and ion exchange

Cobalt Surface sorption, surface complex ion formation, lattice penetration, ion exchange, chelation, and precipitation

Copper Surface sorption, surface complex ion formation, ion exchange, and chelation

Iron Surface sorption and surface complex ion

Lead Surface sorption, ion exchange, chelation, and precipitation

Manganese Surface sorption, surface complex ion formation, ion exchange, and chelation, precipitation

Mercury Volatilization, sorption, and chemical and microbial degradation

Nickel Surface sorption, ion exchange, and chelation

Selenium Ferric-oxide selenite complexation

Silver Precipitation

Zinc Surface sorption, surface complex ion formation, lattice penetration, ion exchange, chelation, and precipitation

Source: modified from Crites (1985).

WA T E R R E S E A R C H 4 0 ( 2 0 0 6 ) 7 5 3 – 7 6 7760

higher than the drinking water guideline value of 500 ppb

recommended by WHO (1993). Some of these heavy metals

could be derived naturally. For example, Edmunds and

Smedley (2000) and Edmunds et al. (2002) suggested that Mn

could be released by incongruent or disproportionation

reactions from silicate or oxide minerals and emerge as

potential residence-time indicators. Besides, some elements

are redox-sensitive and local chemical conditions could affect

their availability and mobility in groundwater (Zachara et al.,

1995; Abrams et al., 1998; Kedziorek et al., 1998; Davis et al.,

2000). Statistical analysis indicated that Mn and Co are

negatively correlated with DO in both seasons (with R40.7).

Certain groundwater samples from the lower part of the

developed spaces (e.g. sites ‘‘27’’ and ‘‘LLR’’) contained

relatively high Mn and Co concentrations, which may

possibly be related to their low DO contents. This suggests

that the concentrations of some heavy metals could be

affected by local redox conditions. On the other hand, the

concentration of Mo is found to be positively correlated with

pH (R40.65 for both seasons). For most of the other heavy

metals, only weak statistical correlations with pH were

observed in both seasons.

However, natural processes alone may not be able to

account for the observed concentrations. Mn, V, Co and Mo

may be related to vehicles (Table 2) and present in stormwater

(Table 3). Loranger et al. (1994) found that the ambient

manganese concentrations are significantly correlated with

traffic density. Moreover, it is shown that vadose zones may

be less effective at removing certain heavy metals under

certain circumstances. For example, Wilson et al. (1990) found

that manganese was mobile in the vadose zone and showed

up in the groundwater at elevated concentrations at a

residential site in Arizona. Therefore, it appears that the

occurrence of Mn, V, Co and Mo in groundwater in the

developed spaces was partially related to the leakage from

stormwater drains.

Besides, Mn, V, Co, Mo could also be derived from steel

corrosion. Leung et al. (2005) demonstrated that groundwater

in the study area was highly aggressive. These heavy metals

are used as additives on steel production. Manganese

improves the rolling and forging qualities, strength, tough-

ness, stiffness, wear resistance and hardness of steel.

Vanadium foil is used as a bonding agent in binding titanium

to steel. Molybdenum contributes to the hardenability and

toughness of quenched and tempered steels. Almost all ultra-

high strength steels contain molybdenum in amounts from

0.25% to 8%. Much of the in situ materials in the urbanized

spaces have been replaced by subsurface engineering struc-

tures which consist of steels. Heavy metals may be leached

out from steels as a result of the prolonged submergence

under highly aggressive groundwater. If this is the case, the

structures of some of the buildings may be adversely affected.

5.1.2. Leakage from sewage pipesBoron is a good indicator of the presence of sewage. In the

developed spaces, the concentration of boron in groundwater

ranged from 46.70 to 433.54 ppb with average of 146.07 ppb. In

the natural slopes, it ranged from 12.54 to 31.22 ppb with an

average of 18.84 ppb. The average boron concentration in the

developed spaces was about eight times higher than that in

the natural slopes, indicating that groundwater in the

developed spaces was likely to be contaminated by the

leakage from sewage pipes to some degree. Boron is also

found to be positively correlated with sulfate (R ¼ 0.921)

(Leung, 2004), suggesting that boron and sulfate may be

derived from the same source, possibly from sewage.

Groundwater samples in the developed spaces appeared to

be commonly contaminated by selenium (Se). The average Se

concentration in the developed spaces was about 100 times

higher than that in the natural slopes. Moreover, about 40% of

the samples collected in the developed spaces contained Se

levels higher than the drinking water guideline value of

10 ppb recommended by WHO (1993). Se is used in the

production of photocells, semiconductor, stainless steel and

glass. However, none of the above industrial activities exists

in the Mid-Levels and the surrounding areas. Instead, the area

is crowded by residential buildings with a high population

density. Selenium sulfide is one of the additives in anti-

ARTICLE IN PRESS

WAT ER R ES E A R C H 40 (2006) 753– 767 761

dandruff shampoo with concentrations of 1% (non-prescrip-

tion) and 2.5% (prescription) (Pierard et al., 1997; Greer, 2000;

McKenzie, 2000). Being one of the shampoo ingredients,

selenium could be found in the sewage pipes after bathing

of local residents. Statistical analysis indicated that selenium

is correlated with boron and sulfate (with R ¼ 0.534 and 0.631,

respectively) (Leung, 2004). This further supports the

speculation that selenium may be related to leakage from

sewage pipes. Given that anti-dandruff shampoos generally

contain at most 2.5% of selenium sulfide, the highest

selenium concentration measured in the developed spaces

was relatively low (�20 ppb). This may imply that only small

scale leakage from sewage pipes occurred in the developed

spaces.

5.1.3. Leakage from flushing water pipesGroundwater in the developed spaces may also be affected by

leakage from flushing water pipes (Leung et al., 2005).

Flushing water samples were collected and analyzed for trace

element contents. It was found that flushing water contained

an exceptionally high strontium (Sr) concentration of about

9334 ppb. The leaked flushing water may contribute trace

elements, especially Sr, to groundwater in the urbanized

spaces. Groundwater samples in the developed spaces con-

tained Sr concentrations ranging from 75.82 to 1477 ppb with

an average of 412.01 ppb, which is significantly higher than

that in the natural slope. Sr is also found to be statistically

correlated with chloride (R ¼ 0:929) (Leung, 2004), indicating

that Sr may be related to the flushing water. Besides

contribution from natural sources such as the weathering of

plagioclase feldspar, it seems that a large part of the

Table 8 – Seasonal differences of trace element concentrations

Element Dry season

Min Mean Max SD

Al 2.31 9.417 27.74 8.346

Fe 4.39 13.328 63.25 16.809

Mn 0.36 1.366 2.71 0.820

Cu 0.28 1.118 4.53 1.238

Zn 1.50 4.906 12.65 4.023

Sr 13.58 21.325 35.13 6.695 1

Se 0.00 0.375 0.70 0.218

Li 0.61 1.851 4.71 1.319

Be 0.03 0.352 0.82 0.267

V 0.00 0.206 0.49 0.159

Cr 0.25 0.736 2.34 0.634

Co 0.00 0.001 0.01 0.003

As 0.00 0.260 0.97 0.276

Rb 2.39 4.995 9.09 2.086

Mo 0.04 0.3091 0.86 0.285

Ag 0.00 0.003 0.01 0.005

Cd 0.01 0.052 0.16 0.047

Sb 0.01 0.056 0.20 0.053

Ba 11.57 28.525 39.54 8.168

Pb 0.01 1.986 12.10 3.727

Remarks: % difference is calculated by the mean trace element level by (E

measured Sr in groundwater was contributed from leakage

of flushing water. As sewage pipes contain flushing water, a

certain amount of Sr may also come from leakage from

sewage pipes.

5.2. Seasonal differences of trace element contents in thestudy area

Seepage samples collected both in wet and dry seasons are

compared to examine the seasonal effects on groundwater

heavy metal and trace element contents. Samples collected

only in one season are omitted. A total of 11 and 12 sites in

the natural slopes and urbanized spaces, respectively, are

selected for comparison. According to Appendices A and B, in

general, for both the natural slopes and the developed spaces,

groundwater samples collected in wet season were slightly

less acidic and of lower DO contents than that in dry season.

The seasonal comparisons of trace element contents in the

natural slopes and the developed spaces are presented in

Tables 8 and 9, respectively.

From Tables 8 and 9, for both the natural slopes and the

developed spaces, the average concentrations of most of the

heavy metals and trace elements are higher in wet

season than in dry season. It is suggested by Vaze et al.

(2002) that the concentrations of heavy metals in road runoff

are particularly high when short duration, intense summer

storms follow a long dry period during which pollutants have

accumulated on the road surface. However, as discussed, the

impacts of leakage from stormwater drains on the heavy

metals and trace elements in groundwater appear to be

insignificant. Another explanation is that more heavy metals

(in ppb) in the natural slope in the Mid-Levels area (n ¼ 11)

Wet season % difference

Min Mean Max SD

0.78 5.356 17.94 5.653 �75.82

2.23 4.758 10.06 2.375 �180.12

0.85 2.258 7.34 2.031 39.50

0.00 0.060 0.51 0.153 �1763.33

8.11 48.564 73.75 25.074 89.90

5.31 29.867 47.47 10.302 28.60

0.00 0.064 0.37 0.116 �485.94

0.20 2.369 6.79 2.083 21.87

0.10 0.476 1.03 0.361 26.05

0.27 0.483 0.77 0.166 57.35

0.00 0.772 5.12 1.488 4.66

0.00 0.031 0.07 0.027 96.77

0.12 0.466 1.78 0.467 44.21

2.61 5.396 9.85 2.554 7.43

0.00 0.154 0.88 0.260 �100.71

0.68 2.662 4.20 1.063 99.89

0.01 0.073 0.18 0.047 28.77

0.00 0.036 0.08 0.028 �55.56

8.07 29.298 47.89 11.127 2.64

0.03 0.503 3.34 0.978 �294.91

lementwet�Elementdry)/Elementwet� 100%.

ARTICLE IN PRESS

Table 9 – Seasonal differences of trace element concentrations (in ppb) in the developed space in the Mid-Levels area(n ¼ 12)

Element Dry season Wet season % difference

Min Mean Max SD Min Mean Max SD

Al 0.00 1.294 4.98 1.721 0.05 0.917 6.17 1.758 �41.11

Fe 1.61 6.622 13.65 3.796 2.02 3.796 7.48 1.725 �74.45

Mn 0.20 62.38 536.7 155.9 0.37 83.11 789.36 227.1 24.94

Cu 0.23 0.990 1.82 0.553 0.00 0.373 2.91 0.850 �165.42

Zn 1.56 7.125 27.76 7.213 7.39 29.87 92.39 25.39 76.15

Sr 61.40 323.2 742.2 213.3 75.82 361.6 722.9 184.0 10.62

Se 0.52 7.589 17.79 6.015 0.56 9.862 23.25 7.076 23.05

Li 0.06 1.176 5.01 1.591 0.00 1.369 4.17 1.609 14.10

Be 0.00 0.091 0.41 0.152 0.00 0.221 1.71 0.493 58.82

V 0.43 2.363 4.69 1.576 1.21 4.087 6.64 1.742 42.18

Cr 0.35 1.428 2.90 0.743 0.60 1.542 4.31 1.006 7.39

Co 0.00 0.158 0.84 0.233 0.00 0.295 2.54 0.714 46.44

As 0.00 0.829 3.27 0.960 0.00 3.045 17.49 4.669 72.78

Rb 1.45 28.243 72.05 19.74 2.22 36.72 81.92 23.96 23.09

Mo 0.13 1.148 4.45 1.368 0.02 1.395 5.72 2.003 17.71

Ag 0.00 0.047 0.30 0.091 0.96 1.991 5.55 1.267 97.64

Cd 0.00 0.069 0.31 0.088 0.01 0.083 0.24 0.076 16.87

Sb 0.03 0.149 0.63 0.177 0.00 0.193 0.72 0.225 22.80

Ba 6.18 35.31 60.4 16.91 11.18 46.19 95.16 24.09 23.55

Pb 0.01 0.686 2.19 0.766 0.07 0.665 3.19 0.880 �3.16

Remarks: % difference is calculated by the mean trace element level by (Elementwet�Elementdry)/Elementwet�100%.

Table 10 – Depths of piezometers with water samplescollected

Piezometer Depth (m)

HW1 15.00

HW2 20.00

B3 Unknown

AB1 11.00

NPZ 17.84

MLS66 19.50

MLS66H 32.00

WA T E R R E S E A R C H 4 0 ( 2 0 0 6 ) 7 5 3 – 7 6 7762

and trace elements could be leached out in wet season

because of the generally higher water table during the season.

In addition, more chemicals may be washed out directly from

the vadose zone by infiltrated rainwater during the wet

season.

5.3. Trace element concentrations of groundwater collectedfrom deep piezometers

A few deep groundwater samples were collected from piezo-

meters installed in the urbanized spaces in the study area

(Figs. 2 and 3). The depths of piezometers are shown in Table

10. Many of the heavy metals and trace elements in these

samples showed similar concentrations as that in the natural

slopes. However, certain trace elements showed obviously

higher concentrations than that in the natural slopes or even

the developed spaces.

The total iron (Fe) concentration of HW2 was about

2755 ppb, which was significantly higher than the other

samples in the area. The sources from anthropogenic

activities appear to be unlikely because of the low concentra-

tions of other heavy metals. Instead, the high Fe content may

be related to the weathering of mineral grains (such as biotite

and feldspar). The primary reaction through which Fe3+

oxides are formed is the hydrolytic and oxidative decomposi-

tion of Fe2+ containing primary minerals (mainly Fe2+

silicates) through the reaction (Awoleye, 1991):

Fe2þFOFSiþH2Oþ ½O� !FFe3þOHþFSiOHþ e�;

ðFe2þ silicateÞ ðFe3þ oxideÞ(1)

Once formed, the Fe3+ oxides can be dissolved either through

(microbial) reduction to Fe2+ or through complexation by

organic ligands (Awoleye, 1991). Ruxton (1987) collected over

80 soil samples from the Mid-Levels area and found that 20 of

them showed significant quantities of iron cemented grains

(45% of iron cemented grains in fine sand). A few of the

samples even show 50% of iron cemented grains of the whole

sample. This may explain the high Fe content observed in the

groundwater sample. Ruxton (1987) further suggested that

deeper materials usually contain higher percentage iron

cemented grains.

Some water samples (for examples, AB1, B3, MLS66 H and

MLS66) contained relatively high lead (Pb) concentration.

Owing to the low concentrations of other heavy metals (such

as Zn, Cr and Fe, etc.) and the usage of unleaded petrol in

Hong Kong several years ago, it appears that contamination

ARTICLE IN PRESS

WAT ER R ES E A R C H 40 (2006) 753– 767 763

from stormwater is unlikely. Lead could be found naturally in

galena (lead sulfide, PbS), anglesite (lead sulfate, PbSO4),

minim (a form of lead oxide with formula Pb3O4), cerussite

(lead carbonate, PbCO3) and other minerals. Galena may be

the most important natural source. Water samples HW1 and

HW2 contained high barium (Ba) concentrations. Barium

could be derived from the weathering of minerals such as

barite and witherite. The host rocks also contained a

relatively high concentration of barium. Some water samples

showed elevated Mn contents, which may be related to the

corrosion of steel, as discussed. The boron concentration of

water samples from deep piezometers ranged from 13.47 to

34.24 ppb, which were comparable to that in the natural

slopes. This may indicate the absence of sewage contamina-

tion.

As limited by equipment, manpower and the scope

of study, only few groundwater samples from piezometers

were collected. Better evaluation of the heavy metal

contamination of deeper groundwater environment would

be achieved if more samples from piezometers can be

collected.

6. Conclusion

This paper presents the heavy metals and trace element

concentrations in groundwater samples in the natural slopes

and the highly urbanized spaces in the Mid-Levels area, Hong

Kong. The results from the natural slopes were used to

evaluate the impacts of urbanization on the heavy metal and

trace elements contents of groundwater in the developed

spaces.

The extents of leakage from service pipes and their effects

on heavy metal and trace element contents in groundwater

were investigated. Although it is believed that leakage from

stormwater drains would be common in the study area,

groundwater samples in the developed spaces did not contain

elevated levels of major heavy metals found in stormwater

(including Zn, Cr, Cu, Cd, Pb and Fe). This shows that the

vadose zone could effectively remove many of the heavy

metals and thus protect the underlying groundwater from

contaminations. On the other hand, groundwater samples in

the developed spaces were found to contain elevated levels of

minor, stormwater-related heavy metals including Mn, V, Co

and Mo which may support the occurrence of leakage from

stormwater drain. These heavy metals may also be derived

from remobilization from natural soils due to the changes in

local redox conditions and the corrosion of subsurface

engineering structures due to prolonged submergence

under acidic groundwater. More attention should therefore

be paid to investigate this corrosion process that could

possibly affect the stability of high-rise structures in the

study area. The average B and Se concentrations in the

developed spaces was about 8 times and 100 times higher

than that in the natural slopes, respectively, indicating the

presence of sewage in the groundwater in the developed

spaces. B and Se were statistically correlated to each other,

which further suggested that they may be derived from the

same source (leaked sewage). Besides, it is found that a large

part of the strontium (Sr) in groundwater may come from the

leakage of flushing water and/or sewage. These results

demonstrated that leakage from service pipes was

common in the developed spaces and they affected the heavy

metal and trace element distributions in groundwater to

different extents.

Although limited in number, the deep groundwater samples

collected from piezometers showed similar heavy metal and

trace element levels as that of the samples in the natural

slopes. Some trace elements with elevated levels were

considered to be mainly the result of natural processes such

as the weathering of minerals.

This study suggested that the vadose zone could filter many

of the heavy metals in leaked stormwater. Further heavy

metal studies are suggested to investigate the subsurface

materials (especially sediments and soils) in order to have a

more comprehensive picture about the impacts of urbaniza-

tion on the subsurface environment. The proposed studies

would be crucial for further modeling studies in the Mid-

Levels area or other areas with a similar level of urbanization.

The reason is that if soils collected in the developed spaces

are found to be insignificantly affected by heavy metals, this

may suggest that leakage from stormwater drains is minimal.

Then it is reasonable to assume that there is minimum

rainwater infiltration in the urbanized spaces in the Mid-

Levels area or other areas with a similar degree of urbaniza-

tion.

Acknowledgement

This study is partially supported by the Hong Kong Research

Grants Council (RGC) (HKU 7013/03) of the Hong Kong Special

Administration Region, China, the Seed Funding within the

Faculty of Science in The University of Hong Kong, and the

Development Budget for Area of Excellence in Water Environ-

ment Engineering, the University of Hong Kong.

Appendix A

Summary of chemical results (in ppb) of samples in the Mid-

Levels area in the wet season (DO is expressed as mg/L; ‘‘—’’

represents not measured; ‘‘nd’’ represents not determined

(Table A1).

Appendix B

Summary of chemical results (in ppb) of samples in the Mid-

Levels area in the dry season (DO is expressed as mg/L; ‘‘—’’

represents not measured; ‘‘nd’’ represents not determined

(Table B1).

ARTICLE IN PRESS

Ta

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50.0

12.9

51.7

30.1

91.7

819.5

72.9

16.1

00.1

30.5

111.3

90.1

746.7

0

30

—5.9

71.2

85.8

90.7

0n

d24.5

475.8

20.5

64.1

70.3

91.8

60.6

0n

d1.1

02.2

21.0

80.9

80.0

40.0

455.3

80.0

747.1

2

35

7.0

55.6

60.3

22.5

5220.2

7n

d26.4

289.7

82.0

40.8

00.0

62.1

10.7

90.2

70.6

834.0

41.4

61.0

50.1

00.1

040.9

60.3

2115.7

8

36

5.6

34.0

30.5

04.8

0352.6

7n

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8312.0

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03.8

20.7

52.8

11.4

61.1

40.8

759.8

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31.1

51.1

80.0

3161.7

10.1

075.3

1

37

6.8

05.5

11.1

49.0

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30.6

826.8

5134.1

02.0

32.3

80.0

82.7

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90.0

81.9

118.4

52.6

21.0

20.0

60.7

038.5

30.3

957.1

0

39

6.8

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90.6

59.6

945.2

53.3

144.4

5329.7

01.7

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25.9

22.3

60.4

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129.8

02.1

60.5

20.1

00.8

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80.6

593.8

2

43

6.5

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90.1

33.7

62.5

6n

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0322.1

020.4

00.6

80.0

53.5

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30.0

32.9

728.9

70.7

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80.1

753.6

40.3

6144.9

3

44

6.7

05.6

30.1

44.1

00.7

4n

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6340.0

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40.6

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23.3

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51.1

132.3

80.9

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50.1

70.2

164.2

50.3

4129.4

4

No

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45.3

10.1

55.3

60.7

1n

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9192.7

01.5

3n

dn

d1.2

11.7

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3n

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40.0

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80.3

548.6

8

87.5

25.6

70.6

37.4

81.9

20.9

863.9

9518.1

06.7

13.0

00.0

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82.3

50.3

12.6

245.9

45.7

25.5

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30.5

864.8

90.2

980.7

9

27

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62.9

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9391.9

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60.6

12.5

417.4

981.9

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40.0

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63.1

960.0

3

42

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54.9

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52.0

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20.2

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60.4

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12.6

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80.4

81.9

80.0

80.1

938.0

31.4

9150.8

6

45

6.9

65.5

60.1

43.9

70.3

7n

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8443.2

05.4

00.2

1n

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91.4

30.0

50.8

917.9

50.6

91.5

80.0

20.1

421.2

60.1

0116.6

6

46

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44.8

30.0

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223.6

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2179.2

2

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8230.9

5

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1

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4276.6

1

WA T E R R E S E A R C H 4 0 ( 2 0 0 6 ) 7 5 3 – 7 6 7764

ARTICLE IN PRESS

Ta

ble

B1

Sa

mp

lep

HD

OA

lFe

Mn

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21.5

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0n

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5

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0

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5.9

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6n

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9278.6

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60.0

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5n

dn

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97

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8

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7

MM

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7n

dn

d7.0

00.2

6n

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99

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71.0

3

No

.26.6

6—

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131.6

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40.0

9n

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60.6

2n

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37.1

60.4

70.6

4

WAT ER R ES E A R C H 40 (2006) 753– 767 765

ARTICLE IN PRESS

WA T E R R E S E A R C H 4 0 ( 2 0 0 6 ) 7 5 3 – 7 6 7766

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