ABSTRACT
The need to assess the heavy metals uptake by vegetables
cultivated and irrigated with wastewater along the watercourse of a stream,
have led to a controlled field experiments in Zaria Metropolis. Five heavy
metals; Cadmium (Cd), Chromium (Cr), Lead (Pb), Manganese (Mn) and Zinc (Zn)
were determined using the Atomic Absorption Spectrophotometer (AAS).The
pollution load index (PLI), plant transfer factor (TF), daily intake of metals
(DIM) and health risk index (HRI) were used to study the pollution status and
the health risks the consumers may be exposed to by the consumption of the
heavy metal contaminated vegetables in the metropolis. In this study, the
quality of the stream (Kubanni River) contaminated with municipal wastewater
and used for irrigation of carrot, spinach and cabbage was investigated at
three sampling points (A, B, and C) for its heavy metal buildup. The
contaminated water used for irrigation at each irrigation date, and the
irrigated vegetables harvested after maturity were collected and analyzed for
their metal concentrations. The results show that the stream water used for
irrigation was contaminated with Cr, 0.4214mg/l, 0.291mg/l and 0.3774mg/l, and
Mn, 0.245mg/l, 0.2127mg/l and 0.1618mg/l at point A, B and C respectively,
which is above Food and Agriculture Organisation (FAO) threshold value. Pb and
Zn were below the threshold and Cd was not detected in the wastewater samples
collected during the study. The maximum Health Risk Index (HRI) was found in Daucus
carota and Brassica oleracea which coincidently share the same value
0.2225. HRI of Cd, Cr, Pb, Mn, and Zn ranged between 0.17-0.19, 0.0033-0.0041,
0.1850-0.2225, 0.0236-0.0385 and 0.0168-0.0300 respectively. It was therefore
concluded that irrigated crops harvested from these areas should be carefully
washed and decanted after boiling before eating in order to prevent accumulation of
these metals in man’s body metabolism.
CHAPTER ONE
INTRODUCTION
1.1 BACKGROUND TO
THE STUDY
Water is a fundamental and an irreplaceable resource to all
forms of life on earth. The growing dearth of water has threatened the
profitable development, quality of the environment, sustainable human
livelihood, and a multitude of other public goals in many developing Asian and
African regions (Nawaz et al., 2012). In order to solve the problem of water
shortage, Nigeria especially northern areas has begun to use wastewater for
irrigation. It contains plant nutrients and organic matter, so this approach
may increase soil fertility. However, wastewater composition is variable and
contains high levels of toxic metals; therefore heavy metal contamination of
agricultural soils has also become increasingly serious in sewage irrigation
area (Mustafa, 2003; Jintao, 2011). Studies have shown that heavy metals are
potentially toxic to crops, animals and humans when contaminated soils were
used for crop production, because heavy metals are easily accumulated in vital
organs to threaten crop growing and human health (Sharma et al., 2007; Jintao,
2011).
Globally, around 20 million hectare of land is irrigated with
municipal wastewater (raw, diluted or treated) (Qaisaret al., 2012). This
figure is most likely to increase noticeably over the next few decades in
responses to the growing levels of water stress in inhibited catchments. In
Nigeria, wastewater is increasingly being used for agricultural irrigation in
urban and peri-urban areas especially during dry season farming. It drives
significant economic activity, supports countless livelihoods particularly
those
of poor farmers, and substantially changes the water quality
of natural water bodies (Marshall et al., 2007). There are serious drawbacks of
using wastewater without ample safeguards for human health and the environment
(Scott et al., 2004; Nawaz et al., 2012).
Heavy metal contamination of environment is a worldwide
phenomenon that has attracted a great deal of attention (Qingjie, 2008;
Ogunkunle and Fatoba, 2012). This is because, heavy metals are ubiquitous in
the environment, as a result of both natural and anthropogenic activities and
humans are exposed to them through various ways (Wilson and Pyatt, 2007; Khan et
al., 2008). Wastewater irrigation, solid waste disposal, sludge applications,
vehicular exhaust, and industrial activities are the major sources of soil
contamination with heavy metals and an increased uptake by food crop grown on
such contaminated soils is often observed (Khanet al., 2008). Compared with
other pathways such as inhalation and dermal contact, dietary intake is the
main route of exposure to heavy metals for most people (Tripathiet al., 1997;
Qianet al.,2010; Yeganeh et al., 2012; Chen et al., 2013).
Runoff from heavy downpour may lead to horizontal leaching
from dump sites causing metal uptake by roots of plants; the rest may find
their way into open water bodies and the entire aquatic ecosystem. Extreme
accumulation of heavy metals in agricultural soils through wastewater
irrigation may not only result in soil contamination but also lead to elevated
heavy metal up-take by crops, threatening food quality and safety (Muchuwetiet
al., 2006). The entry into the food chain of these metals lead to increased
susceptibility and exposure to metal poisoning of local population (Orishet
al., 2012).
Heavy metals can be very harmful to the human body even in
low concentrations as there is no effective excretion mechanism (Ghoshet al.,
2012; Chen et al., 2013). A number of drastic health challenges can arise as a
result of excessive up-take of dietary heavy metals. Furthermore, the
consumption of heavy metal contaminated food can seriously deplete some
essential nutrients in causing a decrease in immunological defenses,
intrauterine growth retardation, impaired psycho- social behaviors,
disabilities associated with malnutrition and a high prevalence of upper
gastrointestinal cancer (Arora et al., 2008; Orisakwe et al., 2012).
Although, some heavy metals have nutritional functions and
are essential to maintain the metabolism of the human body. Metals like copper
(Cu), manganese (Mn) and zinc (Zn) are essential for physiological processes;
certain enzymes need metal ions for their catalytic activity. On the other
hand, Cadmium (Cd) and lead (Pb) are toxic metals. All of them may be harmful
if excessive amounts are consumed (Donadiniet al., 2008; Pizzolet al., 2011;
Tadele and Ariaya, 2014).
The present study aims to determine human health risks
associated with food chain contamination of heavy metals routing from
irrigation using urban and peri-urban wastewater. Irrigating water, soil and
vegetables were analyzed for Cr2+, Pb2+, Cd2+, Mn2+ and Zn2+; Transfer Factor
(TF), Daily Intake of Metals (DIM) and Health Risk Index (HRI) were also
calculated.
1.2 STATEMENT OF
RESEARCH PROBLEM
It is essential to study the relationship between human
health risks and food chain (vegetables) contamination by heavy metals. The
establishment of a clear
relationship between human health risks-food chain
contaminations of heavy metal is difficult due to the large number of variables
which are associated with the process. It is more challenging to quantify the
impact of heavy metals, as heavy metals are potentially toxic to crops, animals
and human when contaminated soils are used for crop production, because heavy
metals are easily accumulated in vital organs of crops grown on these
contaminated soils. Humans and animals that consume such crops are also prone
to this potential toxicity (Liang et al., 2011; Ogunkunle and Fatoba 2012).
This gave impetus to the study on environmental problems of soil pollution by
heavy metals in the last few decades (Zhang et al., 2007; Ogunkunle and Fatoba,
2012) with the development of an ecological geochemistry survey to aid in
determining levels of heavy metal pollution and its potential risk.
Furthermore, the Kubanni River passes through settlement areas and places of
commercial activities towards its mouth, which are sources of pollutants. This
is the source of water for irrigation of crops.
1.3 AIM AND OBJECTIVES
1.3.1 Aim
The aim of the study is to investigate the uptake of heavy
metals by vegetables irrigated with wastewater contaminated with heavy metals
and the perceived health risk to the metropolitan.
1.3.2 Objectives of
the Study
i. To determine some physiochemical parameters (such as pH, EC,
SAR, % organic matter of soil and soil texture) of the wastewater and soil of
the plots used.
ii. To determine the concentration of
some selected heavy metals (Cr2+, Pb2+, Cd2+, Mn2+ and Zn2+) in the irrigation
water along Kubanni watercourse in Zaria, Metropolis.
iii. To determine the concentration of some selected heavy metals
(Cr2+, Pb2+, Cd2+, Mn2+ and Zn2+) in the soil of the irrigated farms.
iv. To determine the concentration of the selected heavy metals
in the selected vegetables cultivated on the soil and irrigated with the water
from Kubanni watercourse in Zaria, Metropolis.
v. To determine the Daily Intake of Metals (DIM) and the Health
Risk Index (HRI) for the locals through the consumption of contaminated
vegetables.
1.4 JUSTIFICATION
OF THE STUDY
World Health Organization (WHO) estimates that about a
quarter of the diseases facing mankind today occur due to prolonged exposure to
environmental pollution (Pruss-Ustun and Corvalan, 2006; Kimani, 2007; Njagi.
2013). The usual belief that wastes are sometimes hazardous to health cannot be
overemphasized. The improper discharge of untreated wastes (both solids and
liquids) into drainage channels which empties itself into watercourse, and planting
of dumpsites along watercourse by small scale industries, domestic and
agricultural activities are a common site in Zaria and other urban and
peri-urban areas in Africa. These discharges are not only eyesores, but a
health hazard. This results into a substantial buildup of heavy metals in
wastewaters. Excessive accumulation of heavy metals in agricultural soils
through wastewater irrigation, may not only result in soil contamination, but
also lead to elevated heavy metal uptake by crops, and thus affect food quality
and safety (Muchuwetiet al., 2006). Heavy metal
accumulation in soils and plants is of increasing concern
because of the potential human health risks. Till date, there is relatively
scanty information on environmental dynamics and health implications of heavy
metals in plants irrigated with contaminated wastewater. This study aims to
fill in this knowledge gap by assessing the occurrence of heavy metals in
vegetable plants growing along Kubanni watercourse area, in Zaria metropolis.
1.5 SCOPE AND
LIMITATION OF THE STUDY
This research work covered the determination of human risks
associated with food chain contamination of heavy metals routing from
irrigation water of Kubanni watercourse in Zaria metropolis. The study area is
a flood plain as such wet season farming of vegetables in that farm site is
difficult because the entire area is flooded during the wet season, hence, wet
season vegetables farming is not obtainable.
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