Content uploaded by Nafiu Abdu
Author content
All content in this area was uploaded by Nafiu Abdu
Content may be subject to copyright.
Content uploaded by Nafiu Abdu
Author content
All content in this area was uploaded by Nafiu Abdu
Content may be subject to copyright.
ORIGINAL ARTICLE
Vertical distribution of heavy metals in wastewater-
irrigated vegetable garden soils of three West African cities
Nafiu Abdu •Aisha Abdulkadir •
John O. Agbenin •Andreas Buerkert
Received: 25 April 2010 / Accepted: 17 September 2010 / Published online: 2 October 2010
ÓThe Author(s) 2010. This article is published with open access at Springerlink.com
Abstract Application of untreated wastewater to
irrigate urban vegetable gardens is raising serious
concern about possible health risks associated with
the consumption of these vegetables particularly with
regard to the concentrations of heavy metals (HM) in
their edible portions. The soil concentrations of
cadmium (Cd), chromium (Cr), copper (Cu), nickel
(Ni), lead (Pb) and zinc (Zn), were investigated in
seven vegetable gardens from the three West African
cities of Kano (Nigeria), Bobo Dioulasso (Burkina
Faso) and Sikasso (Mali). Also determined were
input–output balances of Cd and Zn from five
vegetable gardens under 30 years of wastewater
irrigation in Kano. In these gardens Cd (2.3–4.8
mg kg
-1
) and Zn (13–285 mg kg
-1
) concentra-
tions throughout the profile attained unsafe levels.
The concentrations of Cu (0.8–18 mg kg
-1
), Cr
(1.8–72 mg kg
-1
), Ni (0–17 mg kg
-1
) and Pb
(0.6–46 mg kg
-1
) were below the safety thresholds
for arable soils. Overall, concentrations of Zn, Cd, Pb
and Ni were higher in Kano than in Bobo-Dioulasso
and Sikasso. Input–output analyses in Kano indicated
that irrigation wastewater contributed annually
400–3,700 g Cd ha
-1
and 7,200–22,300 g Zn ha
-1
,
fertilizer 30–2,100 g Cd ha
-1
50–17,600 g Zn ha
-1
,
harmattan dust 0.02–0.4 g Cd ha
-1
and 40–200 g Zn
ha
-1
while 300–500 g Cd ha
-1
and 2,700–4,700 g Zn
ha
-1
came from rainwater inputs. Input–output
calculations subtracting the amounts of HM taken
out in vegetable biomass and that lost to leaching
from total inputs yielded an annual net positive
balance of 700–4,160 g Cd ha
-1
and 9,350–39,700 g
Zn ha
-1
. If such balances remain unchanged for another
10–20 years vegetables raised in these garden fields are
likely to be unsuitable for human consumption.
Keywords Soil contamination Urban agriculture
Heavy metal balance
Introduction
During the last decade the unique agroecological
conditions of urban vegetable soils resulting from
their input intensity, year-round vegetative cover, soil
and hydrological characteristics and their spatial
variability (Landsberg 1981; Witting 1991; Hollis
1992; Renger 1993; Businelli et al. 2009) have raised
considerable research interests. This is especially so
in developing countries where treatment of wastewa-
ters is limited, thus raising concern about HM
N. Abdu A. Abdulkadir J. O. Agbenin
Department of Soil Science, Faculty of Agriculture,
Ahmadu Bello University, PMB 1044 Zaria, Nigeria
N. Abdu A. Buerkert (&)
Organic Plant Production and Agroecosystems Research
in the Tropics and Subtropics, University of Kassel,
Steinstr. 19, 37213 Witzenhausen, Germany
e-mail: tropcrops@uni-kassel.de
123
Nutr Cycl Agroecosyst (2011) 89:387–397
DOI 10.1007/s10705-010-9403-3
contamination of these soils when irrigated with
untreated wastewater and subsequent accumulation in
the food chain through plant uptake. Natural soils of
the Nigerian savannah are characterized by low HM
concentrations due to their high weathering intensity
and long period of pedogenesis (Jones and Wild
1975; Agbenin and Latifatu 2004). However, with
intensification of urbanization, agricultural activities
and industrialization, the extent of HM accumulation
has grown. Among such metals cadmium (Cd) and
Zinc (Zn) are notoriously mobile and likely to move
down through the soil profile to contaminate ground
water (Citeau et al. 2003), even though they are easily
intercepted by clay particles in subsurface horizons
(Fernandez et al. 2007). Citeau et al. (2003) reported
lead (Pb) to be largely stable given its solubility rate.
Semlali et al. (2004) observed downward migration
of Pb to be negligible. Agbenin (2002) reported lead
to accumulate in insoluble forms in a Nigerian
savannah soil after 50 years of continuous application
of organic manure.
The West African cities of Kano (Nigeria), Bobo-
Dioulasso (Burkina Faso) and Sikasso (Mali) are
characterized by intensive urban production of veg-
etables. Even though the cities of Bobo Dioulasso and
Sikasso are less populated and industrialized com-
pared to Kano, HM may as well be accumulated in
the profiles of these urban garden soils owing to the
quality of irrigation water, high inputs of mineral
fertilizers (Diogo et al. 2010; Predotova et al. 2010a)
and pesticide application. While recent reports
stressed the importance of HM loads in irrigation
water and soils used for urban vegetable production
in Kano (Binns et al. 2003; Abdu et al. 2010), to our
knowledge comparative studies of heavy metal
pollution of urban vegetable gardens across West
African cities are missing.
Balance approaches have been used to assess HM
flows in agricultural systems whereby nutrient bal-
ances of the surface soil and farm-gate balances are
used most often (Bassanino et al. 2007). While the
former addresses nutrient flows at the crop scale, the
latter quantifies farm-inputs and outputs at the farm
level (Velthof et al. 1996; Moolenaar and Lexmond
1998) or at the regional and national scales (Keller
et al. 2001; Keller and Schulin 2003; Dach and
Starmans 2005).
In view of the above this study aimed at
determining the vertical distribution of Cd, Cr, Cu,
Ni, Pb, and Zn in urban vegetable gardens of three
West African cities. Given the elevated concentration
of Cd and Zn in the Kano urban gardens input–output
balances of these two metals (Cd and Zn) were
monitored at the garden level.
Materials and methods
Description of the study areas and management
practices
Seven urban vegetable gardens were selected for this
study. They comprised five farmers’ gardens in Kano
(Nigeria) and one each in Bobo Dioulasso (Burkina
Faso) and Sikasso. Kano is located at 12°000N
latitude and 8°310E longitude at 480 m above sea
level in the Sudan savannah zone of Nigeria. Rainfall
is seasonal and highly variable with an annual mean
of 705 mm recorded during 2008 when this study
was conducted. In Kano irrigation of vegetable
gardens is intensively practiced along the major
rivers which serve as effluent discharge routes for the
municipal and industrial wastes produced by local
industries.
In Kano the gardens of Koki, Zungeru, Kwakwaci,
Gada and Katsina road were evenly distributed across
the city and used different wastewater sources. A
natural uncultivated reference/control site was
selected for comparison of HM contamination in soil
profiles. Another well water irrigated garden
(‘Legal’) was also selected as a control in Kano for
comparison of vegetable contamination. Management
practices and garden sizes were similar across
locations. Field plots were mostly between \0.1 ha
to 0.4 ha and planted to fast growing vegetables such
as (Amaranthus caudatus L.), lettuce (Lactuca sativa
L.), parsley (Petroselinum crispum) and carrot (Dau-
cus carota ssp. sativus) which is often intercropped
with the former ones. Fertilizer use among farmers
was only occasional, whereby urea and compound
soluble fertilizer in the form of NPK (15-15-15) were
the predominant fertilizer types used during the rainy
season. However, one farmer (Zungeru) did not use
fertilizer throughout the period of the study (Novem-
ber 2007 to April 2009) as he considered wastewater
to contain sufficient nutrients for his crops.
In Kano two of the three major industrial estates,
Challawa and Sharada, comprise 115 industries
388 Nutr Cycl Agroecosyst (2011) 89:387–397
123
ranging from tanneries, rubber and plastic factories,
textile industries and units dedicated to food pro-
cessing, metal and wood processing of which most
discharge their untreated effluents into the city
streams. Egboka et al. (1989) reported a severe
contamination of boreholes in Kano with residues
from food processing industries and Binns et al.
(2003) reported Cd concentration as high as
30 mg l
-1
in urban irrigation water.
Bobo-Dioulasso (11°100N, 4°190W, 432 m asl)
has a typical southern Sudanian climate with the
rainy season starting in April/May and lasting until
October. Annual rainfall for the study year 2008 was
728 mm.
Sikasso (11°190N, 5°400W, 375 m asl) has a
typical Guinean climate comprising two main peri-
ods: a 5–6 months rainy season from May/June to
October and a subsequent 6–7 months long dry
season. Annual rainfall during the study year 2008
was 1,271 mm.
Selection of gardens was based on an in-depth
survey comprising a GIS-based mapping and farmer
interviews about the various sources of irrigation
water and management practices across the cities.
According to the farmers in Kano and Bobo-Diou-
lasso, the gardens selected have been under waste-
water irrigation for 30 years while gardeners in
Sikasso use mostly well water for irrigation. In
Bobo-Dioulasso and Sikasso, all gardens suspected to
be affected by high HM loads were located in the
same area and irrigated with the same water source;
therefore only one garden was selected from each
city.
Sampling strategy and analytical procedures
Soil and crop sampling
In Kano at each location duplicate soil samples were
taken in 2007 from 0–15, 15–25, 25–35, 35–45,
45–75, 75–100 and 100–150 cm depth. In Bobo-
Dioulasso and Sikasso, however, sampling occurred
only to a depth of 0.8 and 0.7 m given occurrence of
a hard pan and a high water table, respectively. All
samples were air-dried, crushed, and passed through a
2-mm mesh sieve prior to analysis.
In Kano, where monitoring of farmers comprised
activities such as planting date, frequency, date and
duration of irrigation, type and date of fertilizer applied
and date of harvest, the year was divided into three
seasons: the cold dry season (CDS), the hot dry season
(HDS) and the wet season (WS). The CDS lasted from
November to January, the HDS from February to May
and the WS from June to October. Vegetable cropping
was monitored throughout each season and crop and
soil samples consisting of 15–20 auger points at
0–20 cm depth from each farmer’s garden were
collected at each harvest independently of whether a
single or more crops were planted throughout the
season. For the vegetable sampling, 20–25 sub-
samples were taken in each of the five gardens just
prior to farmers’ harvest. The plant samples were
washed with clean tap water to remove adhering soil
particles. Vegetable samples were oven-dried to con-
stant weight at 65°C. At Bobo-Dioulasso and Sikasso
soil and vegetable samples were only collected once, in
November 2007.
Input sources of HM
Irrigation water sampling
At Kano, about 100 ml of wastewater samples used
for irrigation were collected fortnightly from January
2008 to March 2009 (the CDS and HDS) into pre-
washed 250 ml plastic containers. At each site
monthly irrigation water samples were pooled per
season yielding a total of three irrigation wastewater
samples per year. Two drops of concentrated HCl
were added to each sample to suppress microbial
activity followed by refrigerator storage until
analysis.
Atmospheric deposition and rain water collection
In Kano rain and dust samples were collected during
the wet and dry seasons. To this end bulk deposition
collectors were made of a 0.1 m
3
plastic container
and mounted in each of the five gardens at 2 m above
ground level as described by Drees et al. (1993)to
minimize potential effects of human activities. The
plastic containers were covered with a white cotton
mesh to reduce contamination from bird droppings,
trees and other unwanted materials. The dust in the
trap was collected into a clean plastic bag using a
clean brush after removing the cotton mesh. Dust
samples were collected every week from December
2007 to April 2008 and monthly samples were pooled
Nutr Cycl Agroecosyst (2011) 89:387–397 389
123
by season yielding three samples per year and
location. Rain samples were collected immediately
after each rainfall event during three rainy days in
August 2008 yielding a total of three rainfall samples
per year and mixed with two drops of concentrated
HCl to suppress microbial activity prior to refriger-
ation until analysis.
Fertilizer sampling
Representative samples of NPK and urea fertilizers
were collected at each application event in the
specific gardens in triplicate, ground and stored until
analysis.
Output of HM
Estimation of crop yield
Harvested amaranthus, lettuce and parsley were
typically packed in bundles and carrots in sacks prior
to being taken to local markets. Total harvested
biomass per unit area was estimated by measuring the
dry weight of five bundles of amaranthus, lettuce and
parsley and of five sacks of carrot harvested from
approximately 7.5 m
2
. The average weight obtained
was then multiplied by the total number of bundles or
sacks harvested.
Leaching losses of metals
Following the description of Bischoff (2007), Lang
and Kaupenjohann (2004) and Predotova et al.
(2010b), in all five gardens of Kano HM leaching
was estimated using PVC cartridges of 0.1 m height
and 0.103 m diameter with a nylon net at the bottom.
The PVC cartridges were filled with an anion cation
exchange resin–sand mixture and installed according
to the guidelines of TerrAquat Consultancy (Stutt-
gart, Germany; www.terraquat.com), the patent
holder of this method. The tubes were left buried for
10 months (December 2007–October 2008).
Chemical analyses
Particle size distribution was determined by the
hydrometer method following dispersion of the soil
with calgon solution (Gee and Bauder 1986) and
cation exchange capacity (CEC) was determined by
extracting the soil with silver-thiourea solution as
described by van Reeuwijk (1993). Soil pH was
measured in 1:2.5 soil:water suspension using a glass
electrode pH meter and organic carbon (OC) content
of the soil was determined by dichromate oxidation
method (Nelson and Sommers, 1986). For total soil N
and P, 1 g soil sample was digested with a mixture of
H
2
SO
4
-salicylic acid-H
2
O
2
using selenium as a
catalyst. Total N was measured colorimetrically in
the digest using the Bertholet reaction (Chaney and
Marbach 1962) with an N-autoanalyzer (TECHNI-
CON AAII, TechniCon Systems, Emeryville, CA,
USA) and total P was determined by the molybdate-
blue method of Lowry and Lopez using ascorbic acid
as a reductant (van Reeuwijk 1993).
For heavy metal analysis, 1 g each of soil, plant,
and dust samples was digested with concentrated
HNO
3
(puriss, p.a. 65%; Sigma–Aldrich Corp., St.
Louis, MO, USA) and HCl (37%, Sigma–Aldrich
Corp.) at 80°C following the procedure described by
Lim and Jackson (1986). Fertilizer samples were
digested in a beaker with concentrated HNO
3
according to the procedure of the Association of
Official Analytical Chemists (Williams 2000). The
beakers were swirled gently until white fumes
indicated full digestion. The digest was cooled to
ambient temperature, filtered and adjusted to 50 ml
with distilled water prior to analysis. The water
samples were swirled with concentrated HNO
3
at
80°C until the solution turned white. Finally the
digest was filtered through Whatman No. 42 and
diluted to 50 ml with distilled water prior to
analysis. Concentrations of Cd, Cr, Cu, Ni, Pb and
Zn were determined by atomic absorption spectro-
photometry (AAS; Model AA 6680, Shimadzu,
Kyoto, Japan).
Resin extraction
After removal from the soil the resin-sand mixture in
the cartridge was separated into four layers to obtain
an element concentration profile for each cartridge
(Bischoff et al. 1999). A sub sample of exactly 30 g
was weighed from each layer into an extraction bottle
and extracted four times by mechanical shaking with
100 ml of 2 M HCl for16 h. The solution was
decanted into a beaker and the supernatant solution
filtered into a plastic vial. Cadmium and Zn were
determined by AAS.
390 Nutr Cycl Agroecosyst (2011) 89:387–397
123
Quantification of HM balance
Heavy metal balances were established for each
garden based on inputs and outputs. Major sources of
these inputs and outputs were irrigation water,
fertilizer, rainwater, atmospheric dust, crop removal
and leaching losses.
The quantity of irrigation water used throughout
the life cycle of each crop was estimated based on the
discharge rate of the irrigation pumps, length of
irrigation per day and frequency of irrigation for the
individual crops. The amount of water was multiplied
by the concentration of Cd and Zn. Cadmium and Zn
concentrations in rain water (mg l
-1
) were converted
to kg ha
-1
by multiplication with the amount of
annual rainfall. The Cd and Zn input through dust
deposition were calculated by multiplying the con-
centration of dust in g by a yearly estimated dust
input of 936 kg ha
-1
dust from an average weekly
dust deposition of 0.18 g collected per 0.1 m
2
. The
total metal content in the harvested biomass was
estimated from the metal concentrations of the crops
and the harvested dry matter.
Soil metal balance calculation
In this study, measured input variables were irrigation
water and atmospheric deposition in the form of
rainfall and dust (Eq. 1):
DSSHM ¼IWHM þRWHM þDHM
½
CRHM þLHM
½ ð1Þ
where DSS
HM
is the balance of heavy metal (HM),
IW
HM
,RW
HM
and D
HM
are measured inputs of HM
through irrigation water, rainwater and dust deposi-
tion, respectively; CR
HM
is the measured crop
removal of HM and L
HM
denotes losses of HM
through leaching. All data are in g ha
-1
year
-1
.
Statistical analysis
Computed metal balances for each crop were expre-
sed on a hectare basis. Analysis of variance
(ANOVA) was conducted using the General Linear
Procedure (PROC GLM) and seasonal differences in
HM concentration of soil and crop was determined
using Least Significant Difference (LSD
0.05
) tests.
Independent variables were season and location,
while dependent variables comprised HM concentra-
tion in soil and crop samples. Simple correlation
analysis was used to relate the change in HM content
at each soil depth to the calculated HM balance. All
analyses were conducted using SAS 9.2 (SAS 2007)
and Microsoft Excel (2003).
Results and discussion
Distribution of HM in the soil profiles
Heavy metal concentrations ranged from 2.3 to
4.0 mg kg
-1
for Cd, from 2.6 to 50.1 mg kg
-1
for
Cr, from 4.9 to 13.6 mg kg
-1
for Cu, from 3.3 to
12.4 mg kg
-1
for Ni, from 1.7 to 16.5 mg kg
-1
for
Pb and from17.4 to 233 mg kg
-1
for Zn (Figs. 1and
2) whereby concentrations of Cr, Cu, Ni, and Pb were
below the threshold levels for agricultural soils as
given by the EU and CCME (2001; Table 1). Across
sites most HM concentrations decreased with profile
depth, reflecting enrichment at the surface. Higher
concentrations of some HM at lower profile depth
likely reflected the effects of leaching-related trans-
location. Cadmium concentrations were with some-
times [3mgkg
-1
at the bottom of the profile well
above safety limits but lower than the 5.3 mg kg
-1
reported by Awode et al. (2008) and far below the
mean value of 10.3 reported by Mashi and Alhassan
(2007) from Kano.
At the Gada site Cd concentration ranged from 3.7 to
4.8 mg kg
-1
and Zn levels were with 233 mg kg
-1
in
the surface horizon surprisingly high (Fig. 2). Affinity
of metals to organic matter (Agbenin 2002) could be
responsible for this surface enrichment because of the
relatively high OC concentration in the topsoil.
In Bobo-Dioulasso and Sikasso the concentrations
of HMs were often several times lower than in Kano.
In the former city, Cd concentrations ranged from
0.35 to 0.46 and 0.46 to 0.59 mg kg
-1
, respectively,
as compared to the lowest concentration value of
2.3 mg Cd kg
-1
in Kano (Fig. 1). Zinc concentra-
tions were up to 20 times higher in Kano than in
Bobo-Dioulasso and Sikasso (Fig. 2). However, Cr
concentration in the surface layer of the vegetable
garden in Sikasso was with 67 mg kg
-1
almost 25%
higher than in Kano. As the few industries of Sikasso
are dedicated to the processing and packaging of
agricultural crops, oil and soap, and unlike tanning
Nutr Cycl Agroecosyst (2011) 89:387–397 391
123
and ginning enterprises do not produce Cr-rich waste,
the high values in this Malian city are surprising. In
Bobo-Dioulasso, the concentration of Cd and Zn
decreased with profile depth (Figs. 1and 2). As this
was similar for the other HMs it may again reflect an
anthropogenic origin of the studied HMs in the
surface soil. The high concentration of Cr observed in
the control soil as well as in the soils from Bobo-
Dioulasso and Sikasso likely reflects its association
with the soil parent granite and ultramafic rocks.
Changes in redox conditions and other chemical
properties of the soil as a result of flooding during
irrigation and modification of soil pH might have
helped to change the oxidation state of Cr in the
cultivated soil from the stable Cr(III) to highly
mobile Cr(VI) leading to a low residual concentration
of this metal. Binding to dissolved organic matter
could also be responsible for low Cr concentration in
the cultivated soil.
Seasonal variation of heavy metals in Kano
Mean Cd concentration was with 1.4 mg kg
-1
high-
est at the Zungeru site. Mean Zn concentration in the
wastewater irrigated gardens ranged from 90 to
Fig. 1 Distribution of cadmium (Cd) in soil profiles of
wastewater irrigated vegetables gardens in three cities of West
Africa (Kano, Bobo Dioulasso and Sikasso)
Fig. 2 Distribution of zinc (Zn) in soil profiles of wastewater
irrigated vegetables gardens in three cities of West Africa
(Kano, Bobo Dioulasso and Sikasso)
Table 1 International threshold values for heavy metals con-
centrations in soils (mg kg
-1
)
Heavy metal Regulatory system
EU UK USA Canada
Cd 3.0 3.0 19.5 1.4
Cr 180.0 400.0 1500.0 6.4
Cu 140.0 80.0–200.0 170.0 63.0
Ni 75.0 50.0–110.0 210.0 50.0
Pb 300.0 300.0 150.0 70.0
Zn 300.0 200.0–300.0 1400.0 200.0
Source: CCME (2001)
392 Nutr Cycl Agroecosyst (2011) 89:387–397
123
167 mg kg
-1
with the highest and lowest concentra-
tions observed in Gada and Kwakwaci, respectively
(Table 2). Concentration of Cd and Zn in the well
water irrigated garden soil (Legal) were 0.1 and
22.7 mg kg
-1
Cd and Zn, respectively (Table 2).
This indicates the effects of wastewater irrigation on
Cd and Zn accumulation in the soil. The highest
concentration of metals was observed in the CDS
followed by HDS (Table 3). Irrigation was most
intensive during these seasons given high evaporation
rates and crop water demands. During the period
from January/February to March vegetable produc-
tion largely depends on wastewater, therefore it is not
surprising that the concentration of both metals in the
irrigation water was highest during this season and
that the intensity of wastewater use for irrigation
largely determines soil accumulation of HMs.
Input sources of Cd and Zn to soils
Heavy metal concentration in irrigation water,
fertilizer, rainfall and atmospheric dust
Gardening in Kano is characterized by very low inputs
of mineral fertilizers. Irrigation water and
atmospheric depositions were therefore, the major
sources of Cd and Zn at all sites. Large seasonal
concentration changes of HM in irrigation water, rain
and dust and site specific differences were observed
(Table 4). In the irrigation water average Cd concen-
tration at the five sites in Kano was higher than the
0.01 mg l
-1
threshold level set by the FAO. Zn
concentrations were highest in Gada and lowest in
Kwakwaci. Even though the mean concentration of
0.87 mg Zn l
-1
in the irrigation water was less than
half of the 2.0 mg l
-1
threshold, high annual irriga-
tion rates may have caused Zn enrichment as indicated
by the significant correlation between irrigation water
and soil Zn concentrations (r=0.45, P\0.05). In
rain water Zn ranged from 0.4 to 0.6 mg l
-1
which is
2-3 fold lower than what was measured in the dry
season dust. At the Gada location Zn concentration
was highest (Table 4) and may reflect the effects of
the high concentration of Zn 44–235 mg kg
-1
and Cd
(0.02–0.45 mg kg
-1
) in the dust. Their concentration
in NPK fertilizers was 16.5–17.7 mg Zn kg
-1
and
Table 2 Concentration (mg kg
-1
) of cadmium (Cd) and zinc
(Zn) in five wastewater- and one well water-irrigated vegetable
garden soils in Kano, Nigeria
Location Cd Zn
Koki (n=5) 0.7 ±0.2 136 ±21
Zungeru (n=5) 1.4 ±0.4 118 ±15
Kwakwaci (n=5) 0.5 ±0.2 90 ±32
Gada (n=5) 1.0 ±0.3 167 ±31
Katsina road (n=4) 1.2 ±0.5 110 ±21
Legal (n=4) 0.1 ±0.01 23 ±5
Data show means ±one standard error
Table 3 Seasonal change of cadmium (Cd) and zinc (Zn)
concentration (mg kg
-1
) in wastewater irrigated garden soils in
Kano, Nigeria
Season Cd Zn
Cold dry season 1.1 ±0.2a
118 ±17a
Hot dry season 0.9 ±0.2a 127 ±21a
Wet season 0.3 ±0.09b 68 ±21b
Data show means ±one standard error
Values followed by a different letters are statistically
different at PB0.05
Table 4 Concentration of cadmium (Cd) and zinc (Zn) in
irrigation wastewater (mg l
-1
), rain water (mg l
-1
) and
atmospheric dust (mg kg
-1
) in Kano, Nigeria
Location/Source Cd Zn
Koki
Irrigation wastewater 0.05 ±0.01 0.7 ±0.02
Rain water 0.07 ±0.00 0.5 ±0.02
Atmospheric dust 0.28 ±0.02 266.8 ±18.4
Zungeru
Irrigation wastewater 0.06 ±0.01 0.7 ±0.05
Rain water 0.04 ±0.01 0.6 ±0.01
Atmospheric dust 0.38 ±0.01 63.8 ±3.4
Kwakwaci
Irrigation wastewater 0.07 ±0.02 0.4 ±0.02
Rain water 0.05 ±0.00 0.4 ±0.02
Atmospheric dust 0.47 ±0.01 87.6 ±6.80
Gada
Irrigation wastewater 0.05 ±0.00 1.0 ±0.34
Rain water 0.05 ±0.00 0.6 ±0.04
Atmospheric dust 0.21 ±0.10 125.2 ±47.2
Katsina road
Irrigation wastewater 0.07 ±0.01 0.7 ±0.14
Rain water 0.04 ±0.01 0.4 ±0.01
Atmospheric dust 0.31 ±0.16 132.7 ±42.5
Data show means ±one standard error
Nutr Cycl Agroecosyst (2011) 89:387–397 393
123
0.16–0.22 mg Cd kg
-1
while urea contained
0.03–0.1 mg Cd kg
-1
and 0.13–0.85 mg Zn kg
-1
.
The rather high concentrations of HMs in NPK
fertilizers may be reflect use of Cd-rich phosphate
fertilizers.
Wastewater irrigation led to annual additions of
400–3,700 g Cd ha
-1
and 7,200–22,300 g Zn ha
-1
equivalent to 68 and 78% of the total respective
inputs (Table 5). Fertilizer application contributed
annually 30–2,100 g Cd ha
-1
and 50–17,600 g Zn
ha
-1
. Atmospheric wet deposition from rainfall
accounted annually for 250–500 g Cd ha
-1
and
2,700–4,700 g Zn ha
-1
. Cadmium and Zn inputs
from the estimated 934 kg dust ha
-1
, instead, were
negligible (Table 5). In Kano total atmospheric
deposition of Cd and Zn were thus 750- and 23-fold
higher than the 0.5 g Cd ha
-1
and the 160 g Zn ha
-1
reported by Azimi et al. (2004) for Versailles, France.
While Cd concentrations in irrigation and rainwa-
ter were above the 0.01 mg l
-1
FAO threshold level,
those of Zn were below the 2 mg l
-1
threshold but
still higher than values reported by Akoto et al.
(2008) for five polluted streams in Ghana. At the
Gada site, construction activities at about 100 m from
the sampling garden might have contributed to the
high Zn concentration of 1.59 mg l
-1
in the irrigation
water (Table 4). Dust at this site also had a Zn
concentration of 207 mg kg
-1
. The highest concen-
tration of Cd (0.07 mg l
-1
) and Zn (0.96 mg l
-1
) was
measured in irrigation water at Katsina road
(Table 4) and might be caused by discharge from a
seed and pesticide industry in the vicinity of the
sampling site. Although the concentration of HM in
the irrigation water may be low, continued use of this
wastewater as practiced in Kano can lead to the build-
up of HM in soils (Rattan et al. 2002).
Export of metals
Mean Cd concentration in vegetables ranged from 0.1
to 0.3 mg kg
-1
while Zn export was between 11.5
and 163 mg kg
-1
. Highest concentration of Cd and
Zn was recorded in amaranthus and lettuce, respec-
tively (Table 6). Amaranthus with its total dry matter
(TDM) of 3.5–28 t ha
-1
led to annual exports of
0.7–2.0 g Cd ha
-1
and 100–1,700 g Zn ha
-1
while
such values were regligible for carrots with TDM
Table 5 Annual balance of horizontal Cd and Zn fluxes for
five vegetable gardens in Kano, Nigeria
Location/Source Input–output (g ha
-1
year
-1
)
Cd Zn
Koki
Irrigation water 1,400 15,700
Fertilizer (NPK) 160 17,600
Dust 0.3 220
Rainfall 300 4,700
Leaching losses -300 -1,500
Amaranthus -1.0 -600
Carrot -0.5 -300
Total balance 1,560 35,800
Zungeru
Irrigation water 400 7,200
Dust 0.3 54
Rainfall 500 3,900
Leaching losses -200 -1,800
Amaranthus -0.7 -100
Parsley -0.3 -100
Total balance 700 9,354
Kwakwaci
Irrigation water 3,700 22,300
Fertilizer (NPK) 210 15,470
Fertilizer (Urea) 50 770
Dust 0.4 70
Rainfall 500 3,900
Leaching losses -300 21,100
Amaranthus -2.0 -1,700
Carrot -0.2 -50
Total balance 4,158 39,660
Gada
Irrigation water 700 20,000
Fertilizer (Urea) 80 210
Dust 0.03 41
Rainfall 400 4,000
Leaching losses -200 -2,100
Amaranthus -3.0 -1,700
Lettuce -1.0 -500
Total balance 976 19,950
Katsina road
Irrigation water 840 13,500
Fertilizer (Urea) 30 50
Dust 0.02 190
Rainfall 500 2,700
Leaching losses -200 -2,800
Parsley -1.0 -900
Lettuce -1.0 -600
Total balance 1,168 12,140
394 Nutr Cycl Agroecosyst (2011) 89:387–397
123
yields of 1.8 and 3.0 t ha
-1
in Kwakwaci and Koki,
respectively (Table 6). The corresponding Cd and Zn
export was equally low. Leaching of Cd estimated
from resin extraction was 200–300 g ha
-1
and of Zn
1,100–2,800 g ha
-1
.
Input–output balances
High inputs of nutrients through wastewater irrigation
have been reported for several studies from West
African cities. Diogo et al. (2010) showed large
nitrogen (N) surpluses as a result of N rich wastewa-
ter irrigation in Niamey, Niger. Similarly, Khai et al.
(2007) reported large Zn inputs into vegetable
gardens of Hanoi, Vietnam through wastewater
irrigation. The net surplus of 9,200–39,700 g Zn
ha
-1
year
-1
(Table 5) observed in our amaranthus
gardens by far exceeded the 650–7,700 g ha
-1
reported by Khai et al. (2007). This largely reflects
the level of contamination of the wastewater used for
irrigation in Kano. With 9,200 g ha
-1
Zn surplus was
lowest in the garden under parsley and amaranthus.
Gardens under amaranthus and carrot production,
however, had highest values with 39,700 g Zn ha
-1
and 4,200 g Cd ha
-1
(Table 5). In the present study,
leafy vegetables seemed to accumulate more HM
than fleshy vegetables despite the fact that the fleshy
vegetable (carrot) had a longer growing cycle
(Table 6). The significance of correlations between
the concentration of HMs and HM balances
decreased with soil depth (Table 7) reflecting the
effects of anthropogenic activities. The close and
Table 6 Crop dry matter yield, cadmium (Cd) and zinc (Zn) concentration and annual removal of Cd and Zn in crops from five
vegetable gardens in Kano, Nigeria
Location Crop Yield (t ha
-1
) Concentration (mg kg
-1
) Crop removal (g ha
-1
)
Cd Zn Cd Zn
Koki Amaranthus (n=8) 7.1 0.1 ±0.02 76 ±19 1.0 600.0
Carrot (n=2) 3.0 0.2 ±0.02 110 ±10 0.5 300.0
Zungeru Amaranthus (n=6) 3.5 0.3 ±0.13 47 ±11 0.7 100.0
Parsley (n=3) 1.7 0.2 ±0.08 79 ±5 0.3 100.0
Kwakwaci Amaranthus (n=10) 28.1 0.1 ±0.03 87 ±12 2.0 1,700.0
Carrot (n=2) 1.8 0.1 ±0.02 12 ±2 0.2 50.0
Gada Amaranthus (n=6) 25.8 0.2 ±0.03 67 ±25 3.0 1,700.0
Lettuce (n=4) 6.4 0.2 ±0.07 61 ±27 1.0 500.0
Katsina road Parsley (n=4) 8.1 0.1 ±0.01 112 ±27 1.0 900.0
Lettuce (n=2) 4.3 0.2 ±0.01 164 ±91 1.0 600.0
Data show means ±one standard error
Table 7 Relationship between profile distribution of Cd and Zn with HM balances in wastewater irrigated vegetable gardens in
Kano, Nigeria
Soil depth (cm) Cd balance Zn balance
Equation r
2
value Equation r
2
value
0–15 y =8E-10x
2
–1E-05x ?4.02 0.99 Y =-6E-07x
2
?0.03x ?3.36 0.98
15–25 y =2E-09x
2
–5E-05x ?3.89 0.99 0.51
25–35 y =-7E-10x
2
–5E-05x ?3.45 0.93 0.32
35–45 y =-6E-10x
2
–6E-05x ?3.03 0.80 0.26
45–75 y =-2E-09x
2
–5E-05x ?3.72 0.99 0.48
75–100 0.33 0.56
100–150 0.43 0.17
Equations were only presented for significant relationship (PB0.05)
Nutr Cycl Agroecosyst (2011) 89:387–397 395
123
significant relationships observed between HM bal-
ance and soil depth largely indicated superfial
enrichment of Cd and Zn. HM accumulation and
balance can thus be predicted by changes in HM
concentration over a defined length of time. Further-
more, the higher relationship observed for Cd can be
attributed to higher Cd loading and ease of Cd
movement and translocation in the soil profile as
compared with Zn. Such accumulations may in the
long term cause environmental and human health
risks even if at present those risks seem to be limited
except for Cd and Zn.
Conclusions
Our study shows Cd and Zn accumulation in surface
soils of vegetable gardens in Kano (Nigeria). Soil HM
concentrations followed the sequence Kano [[ Sik-
asso [Bobo-Dioulasso. Garden-based flux measure-
ments of HM revealed that the major sources of Cd
and Zn in Kano are wastewater irrigation, atmospheric
deposition and NPK fertilizer wherever it was applied.
Cadmium and Zn loads may lead to elevated concen-
trations in marketed vegetables and leach to the
groundwater with negative consequences for human
health. Legislation is needed to restrict HM loads to
soils by limiting HM discharges from industries into
wastewater that will continue to be used to irrigate
urban vegetable gardens.
Acknowledgments The authors are grateful to the
Volkswagen Stiftung, Hannover, Germany for supporting this
research financially under the UrbanFood project within the
collaborative programme ‘‘Resources, their Dynamics, and
Sustainability-Capacity-Development in Comparative and
Integrated Approaches’’ (No. I/82 189). The first author also
gratefully acknowledges Ahmadu Bello University, Zaria,
Nigeria for supporting his studies at the University of Kassel-
Witzenhausen, Germany.
Open Access This article is distributed under the terms of the
Creative Commons Attribution Noncommercial License which
permits any noncommercial use, distribution, and reproduction
in any medium, provided the original author(s) and source are
credited.
References
Abdu N, Agbenin JO, Buerkert A (2010) Fractionation,
mobility and bioavailability of cadmium and zinc in urban
vegetable gardens of Kano, Northern Nigeria. Environ
Monit Assess (submitted)
Agbenin JO (2002) Lead in a Nigerian savanna soil under long-
term cultivation. Sci Total Environ 286:1–14
Agbenin JO, Latifatu AO (2004) Competitive adsorption of
copper and zinc by a Bt horizon of a savanna Alfisol as
influenced by pH and selective removal of hydrous oxides
and organic matter. Geoderma 119:85–95
Akoto O, Bruce TN, Darko G (2008) Heavy metals pollution
profiles in streams serving the Owabi reservoir. Afr J
Environ Sci Technol 11:354–359
Awode UA, Uzairu A, Balarabe ML, Harrisson GFS, Okunola
OJ (2008) Assessment of peppers and soils for some
heavy metals from irrigated farmlands on the Bank of
river Challawa, Northern Nigeria. Pak J Nutri 2:244–248
Azimi S, Cambier P, Lecuyer I, Thevenot D (2004) Heavy
metal determination in atmospheric deposition and other
fluxes in northern France agrosystems. Water Air Soil
Pollut 157:295–313
Bassanino M, Grignani C, Sacco D, Allisiardi E (2007)
Nitrogen balance at the crop and farm-gate scale in live-
stock farms in Italy. Agric Ecosyst Environ 122:282–294
Binns JA, Maconachie RA, Tanko AI (2003) Water, land and
health in urban and peri-urban food production: the case
of Kano, Nigeria. Land Degrad Develop 14:413–444
Bischoff W-A (2007) Entwicklung und Anwendung der selbst-
integrierenden Akkumulatoren: Eine Methode zur Erfas-
sung der Sickerfrachten umweltrelevanter Stoffe. PhD
Thesis, Technical University Berlin, Berlin, Germany
Bischoff W-A, Siemens J, Kaupenjohann M (1999) Solute
leaching into groundwater—a comparison of field meth-
ods considering preferential flow. Wasser Boden
51:37–42
Businelli D, Massaccesi L, Onofri A (2009) Evaluation of Pb
and Ni mobility to groundwater in calcareous urban soils
of Ancona, Italy. Water Air Soil Pollut 201:185–193
Canadian Council of Ministers for the Environment (CCME)
(2001) Canadian water quality guidelines for the pro-
tection of aquatic life: summary table. Winnipeg,
Canada
Chaney AL, Marbach EP (1962) Modified reagents for deter-
mination of urea and ammonia. Clin Chem 8:130–132
Citeau L, Lamy I, van Oort F, Elsass F (2003) Colloidal
facilitated transfer of metals in soils under different land
use. Colloids Surf A Physicochem Eng Asp 217:11–19
Dach J, Starmans D (2005) Heavy metals balance in Polish
agronomy: actual state and previsions for the future. Agric
Ecosyst Environ 107:309–316
Diogo RVC, Buerkert A, Schlecht E (2010) Horizontal nutrient
fluxes and food safety in urban and peri-urban vegetable
and millet cultivation of Niamey, Niger. Nutr Cycl Ag-
roecosyst 87:81–102. doi:10.1007/s10705-009-9315-2
Drees LR, Manu A, Wilding LP (1993) Characteristics of
aeolian dust in Niger, West Africa. Geoderma 59:213–233
Egboka BCE, Nwankwor GI, Orajaka IP, Ejiofor AO (1989)
Principles and problems of environmental pollution of
groundwater resources with case examples from devel-
oping countries. Environmental Health Perspectives,
Nigeria
Fernandez C, Labanowski J, Cambier P, Jongmans AG, van
Oort F (2007) Fate of airborne metal pollution in soils as
related to agricultural management. 1. Zn and Pb distri-
butions in soil profiles. Eur J Soil Sci 58:547–559
396 Nutr Cycl Agroecosyst (2011) 89:387–397
123
Gee GW, Bauder JW (1986) Particle-size analysis. In: Klute A
(ed) Methods of soil analysis, Part 1. 2nd ed. Agronomy
Monogram. 9. ASA and SSSA, Madison, WI. pp 383–411
Hollis JM (1992) Proposal for the classification, description
and mapping of soils in urban areas. English Nature,
Peterborough
Jones MJ, Wild A (1975) Soils of the West African Savanna,
Commonwealth Bureau of Soils, Technical Communica-
tion, No. 55, Harpenden
Keller A, Schulin R (2003) Modelling heavy metal and phos-
phorus balances for farming systems. Nutr Cycl Agro-
ecosyst 66:271–284
Keller A, Steiger B, Van der Zee SEATM, Schulin R (2001) A
stochastic empirical model for regional heavy metal bal-
ances in agroecosystems. J Environ Qual 30:1976–1989
Khai NM, Ha PQ, O
¨born I (2007) Nutrient flows in small-scale
peri-urban vegetable farming systems in South Asia-a
case study in Hanoi. Agric Ecosyst Environ 122:192–202
Landsberg HE (1981) The urban climate. International geo-
physics series, vol 28. Academic Press, London
Lang F, Kaupenjohann M (2004) Trace element release from
forest floor can be monitored by ion exchange resin tubes.
J Plant Nutr Soil Sci 167:177–183
Lim CH, Jackson ML (1986) Expandable phyllosilicate reac-
tions with lithium on heating. Clays Clay Miner 34:
346–352
Mashi SA, Alhassan MM (2007) Effects of wastewater dis-
charge on heavy metals pollution in Fadama soils in Kano
city, Nigeria. Bio Environ Sci 20:70–77
Moolenaar SW, Lexmond TM (1998) Heavy metal balances of
agro-ecosystems in the Netherlands. Netherlands J Agric
Sci 46:171–192
Nelson DW, Sommers LM (1986) Total carbon, organic carbon
and organic matter. In: Sparks DL (ed) Methods of soil
analysis part 2, chemical methods. ASA, Madison,
pp 961–1010
Predotova M, Schlecht E, Buerkert A (2010a) Nitrogen and
carbon losses from dung storage in urban gardens of
Niamey, Niger. Nutr Cycl Agrocosyst 87:103–114. doi:
10.1007/s10705-009-9316-1
Predotova M, Bischoff W-A, Buerkert A (2010b) Mineral
nitrogen and phosphorus leaching in vegetable gardens of
Niamey, Niger. J Plant Nutr Soil Sci (in press). doi:
10.1002/jpln.200900255
Rattan RK, Datta SP, Chandra S, Saharan N (2002) Heavy
metals and environmental quality: Indian scenario. Fer-
tilizer News 47:21–40
Renger M (1993) In: Puska
´s I, Farsang A (2009) Diagnostic
indicators for characterizing urban soils of Szeged, Hun-
gary. Geoderma 148:267–281
Semlali RM, Dessogne JB, Monna F, Bolte J, Azimi S, Nav-
arro N, Denaix L, Loubet M, Chateau C, van Oort F
(2004) Modelling lead input and output in soils by using
lead isotopic geochemistry. Environ Sci Technol
38:1513–1521
Van Reeuwijk LP (1993) Procedures for soil analyses; tech-
nical paper No. 9, 4th edn. International soil reference and
information centre (ISRIC), The Netherlands
Velthof GL, van Erp PJ, Moolenaar SW (1996) Optimizing
fertilizer plans for arable farming systems, II. Effects of
fertilizer choice on inputs of heavy metals. Meststoffen
1996:74–80
Williams H (2000) In: Kane PF (Ed). Official methods of
analysis of AOAC International (17th edn). Gaithersburg,
Maryland
Witting R (1991) In: Puska
´s I, Farsang A (2009) Diagnostic
indicators for characterizing urban soils of Szeged, Hun-
gary. Geoderma 148:267–281
Nutr Cycl Agroecosyst (2011) 89:387–397 397
123
