1.0 INTRODUCTION
Human
exposure to heavy metals has been identified to be through food, water and air [1,2].Water
being a major constituent of the body which accounts for about 70-80% of the
weight of most tissues like the muscles, brain, liver, among others; alone has
been estimated on average to be about 61% of the whole body weight in an
adult[3].Despite the reality of how common and the importance of water to life,
its quality and accessibility have posed a great problem to mankind in many
parts of the world especially the developing countries due to its pollution [4]
and that is why, lots of sources of water are being explored to ascertain good
ones of which bore-hole is not an exception. Though some of these sources are
bedeviled by lots of contaminants occasioned by both natural and human
activities such as chemical pollution which has been increased with the
increase in anthropogenic activities such as new farming techniques,
industrialization, mining, fossil fuel application among others; natural
processes such as volcanic activities and weathering of rocks also contributes
significantly [4-10]. Heavy metals are individual metals and metal compounds
that may influence human health. They are metals and metalloids with high
atomic weight and specific gravity five times the specific gravity of water [5,7]. Most of these metals such as Hg, Pb,
Cd, Ni, As and Sn are toxic in nature and can cause
health problem to humans when exposed above the minimum standard concentrations
and may consequently lead to diseases such as cancer, reproductive problem
among others and even death [7,11,12].Heavy metal
contamination of water has posed a serious threat to human life because of
their toxicity, bio-accumulative nature and persistence in the environment
[13]. Several of these metals and their compounds are suspected to have
carcinogenic potentials in humans [14] and their accumulation in selective
tissues of the living organisms have overall potential to be toxic even at
relatively low exposure or in take [15] and as such may lead to health risk of
the populace [16].
Bore-hole water being a major source of water
supply at least, 50% here in Borno south and the
worldwide at large [17] needed surveillance to ascertain its suitability for
consumption, that was why it became necessary to embark on a research to assess
the extent of heavy metals contamination in the area and infer the probable
health implications on the populace.
2.0 METHODOLOGY
This
research was carried out in Borno state in five local
government areas in southern Borno viz. Biu, Kwaya-Kusar, Bayo, Shani and Huwul respectively.
Samples of Bore-hole water were collected
randomly from the study areas vis;
Biu, Kwaya-Kusar, Bayo, Shani and Huwul in thoroughly cleaned plastic container which were
tightly covered, labeled and moved to Desert Research Monitoring and Control
Centre located at Yobe State University, Damaturu for analysis.
2.1 Sample
Preparation
100
ml of each of the water samples were pipetted into a separate container with a
particular identification number and transported immediately to the laboratory for
analysis at Desert Research Monitoring and Control Centre (DRMCC) located at Yobe State University, Damaturu.
Energy Dispersive X-ray Fluorescence (EDXRF) Spectrometer (Model: Epsilon 5, PANalytical, The Netherlands) was the analytical technique
used to determine the concentration of elements in various water samples.
2.2 Sample Analysis
1ml
of each water sample was pipetted into a pellet-like container of 25 mm
diameter, and a transparent X-ray foil cover (Polypropylene with a thickness of
6μm) was used to cover the pellet-like cup with a pellet maker (Automatic
Hydraulic Presses, model: 3889-4NEI). The samples pellets like cups were loaded
into the X-ray excitation chamber for irradiation with the help of an automatic
sample changer system. A time-based program, controlled by a software package (PANalytica) provided with the systems was used to irradiate
the real samples and the standard materials as well for the construction of the
calibration curves for quantitative elemental analysis for the respective
samples and afterward the generated X-ray spectra of the materials were stored
into the computer.
Risk assessment is defined as the process of
estimating the probability of occurrence of any probable adverse health effect
over a specified period which is a function of the hazard and exposure [18,19]. Human exposure to heavy metals occurs through several
pathways including direct ingestion, dermal absorption through skin and
inhalation through mouth and nose. The US-EPA pointed out that the human body
absorbed pollutant dose is calculated from chronic daily intake (CDI), which
means the pollutant dose per kilogram of body weight per day that is absorbed
through direct ingestion, dermal absorption or inhalation. Direct ingestion and
skin absorption were used as the main exposure pathways and therefore we
examined the risk of heavy metals in Bore-hole water in this research by
computing the CDI of water through ingestion and dermal absorption using
equations (1) and (2) [18-22].
(1)
(2)
The 
and 
are the chronic daily intake of water through ingestion and dermal absorption (mgKg-1day-1)
respectively, 
is the concentration of the ith heavy metal (mgL-1),Di is
the daily intake of the ith heavy
metal (Lday-1), ABS is the absorption factor, EF is the exposure frequency
(Daysyear-1), EP is the exposure duration (Years), BW is the body weight(Kg),
AT is the average time(Days), SA is the exposed skin area(Cm2), KP is the dermal permeability coefficient of
metals(Cmh-1),
CF is the conversion factor(LCm-3) , ET is the exposure
time(Hoursday-1). While that HQ
was estimated by comparing chronic daily intakes of contaminants from each
exposure route (ingestion and dermal) with the corresponding reference dose (RfD) for the same heavy metal
using equations (3) and (4) respectively [18-22].
(3)
(4)
On the other hand, the hazard index,
HI is computed according to US- EPA guidelines for ingestion and dermal
absorption of water using equations (5)
and (6)
(5)
(6)
Carcinogenic
risk, CR was estimated using equation
(7) [20].
CR = CDI × SF CDI × SF < 0.01(7)
Where SF is the cancer slope factor
(mgKg-1day-1)
3.0
RESULTS/DISCUSSION
The mean concentration of heavy
metals (Fe, Cu, Hg, Cd, As,Pb,
Zn, Cr) in water samples obtained from Borno South
are presented in Table1. The values of chronic daily intake for adults through
ingestion and dermal exposure pathways are presented in Tables 5 and 6 while,
the values of total chronic daily intake are presented in Table7. The values of
hazard quotient for ingestion and dermal pathways with the corresponding hazard
index for adults are presented in Tables8 and 9. The estimated total hazard
quotient and total hazard index for adults are presented in Table 10. The
carcinogenic risk assessment for adults via ingestion and dermal pathways are
presented in Tables 11 and 12while, the total estimated carcinogenic risk in
the samples is given in Table13.
Since
heavy metal contamination in water has the potentials to increase human health risks
through various exposure routes, this research explores the carcinogenic and
non-carcinogenic health risks caused by oral ingestion and dermal exposure to
water. In table4, a wide variation in the mean values of heavy metals was
observed with a maximum concentration for Zn whose mean value for the five
locations was 6.5mgL-1and minimum concentration for Hg whose mean
value for the five locations was 0.0035mgL-1 respectively. The order
of toxicity of heavy metals measured from the study area was: Zn>Cu>Fe>Cr>As>Pb>Cd>Hg. Also, contrast of the mean values of heavy
metals and the WHO (2008) standard values showed that; Biu
had low values for Fe, As and Cu. Kwaya-Kusar had low
values for As and Pb. Shani
had low value for Cu. All locations had high values for Cd, Zn and Cr.
Conversely, all locations had low values for Hg.
Non-Carcinogenic Risk
Analysis
Human
health risk assessment encompasses the determination of the nature and
magnitude of the adverse health effects in humans who may be exposed to toxic
substances in a contaminated environment. This research uses US-EPA methodology
to assess the exposure and resulting health risks of heavy metals on humans.
Since the degree of toxicity of heavy metals to human health is directly
related to their daily intake, ingestion through drinking and dermal absorption
were adopted for this research. The non- carcinogenic
risk analysis was done by calculating the chronic daily intake, hazard quotient
and hazard index respectively. The results of chronic daily intake through
ingestion and dermal pathways are presented in Tables 5 and 6. The values of
total chronic daily intake, CDItotal(mgKg-1day-1) are presented in Table7.
The CDItotal of the heavy metal
concentration for adults were found in the order of: Zn>Cu>Fe>Cr>Pb>As>Cd>Hg. The results of hazard quotient and
hazard index for ingestion and dermal pathways as presented in Tables 8 and
9shows that, there is no noticeable harmful health risk in all the samples
since their values were all below the threshold value of 1. In order to
estimate the total potential non-carcinogenic impact induced by more than one
metal, the values of HQ computed were summed and expressed as a hazard index,
HI [19] as presented in Table10. The total HI values: 8.26E-03, 1.51E-02,
1.70E-02, 1.71E-02 and 2.32E-02 for Biu, Kwayakusar, Bayo, Shani and Huwul respectively
implied a negligible risk on residents.
Carcinogenic Risk, CR
Analysis
The
heavy metals Cd, As, Pb and Cr can enhance the risk
of cancer in humans [13,18,19,21-23]. Long term exposure
to low amount of toxic metals could result in many types of cancers. The
results of carcinogenic risk assessment through ingestion and dermal absorption
for adults are presented tables11 and 12 while, the total cancer risk for
adults is presented in Table13. A value of CR less than 1.0E-06 is considered
insignificant and the cancer risk is negligible while a value of CR above 1E-04
is considered harmful and the cancer risk is troublesome. Among all the studied
heavy metals, none has a CR value greater than 1.0E-04 which implied negligible
cancer risk.
4.0 CONCLUSION
From
the result, the order of heavy metal toxicity in the borehole water in the
study area was as presented: Zn>Cu>Fe>Cr>As>Pb>Cd>Hg
and based on the evaluation of the health risks exposure on heavy metals from
borehole water on the people of the area, coupled with the analysis of
the risk assessment which was carried
out by computing carcinogenic and
non-carcinogenic risk of the water through ingestion and dermal pathways using
a defined formula, both the computed
total values of chronic daily intake, CDItotal
of heavy metals and the hazard quotient and hazard index as obtained from the exposure routes which is
by dermal and ingestion, have potentials of harmful cancer risk ,but
however, it may be clearly stated that
there is a risk factor for continual
intake of some of these metals no matter
how little over a long period because of their toxic nature as such
something is still needed to be done in
order to reduce some these metal
contaminants in water.
ACKNOWLEDGEMENT
We
are very grateful to the management of Desert Research Monitoring and Control
Centre (DRMCC) Damaturu for their assistance during
laboratory work.
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