Metals bioavailability in the leachates from dumpsites in Zaria Metropolis , Nigeria

Landfill leachates pose a significant threat to both surface water and groundwater especially the wells adjacent to landfills. The study investigated the bioavailability of zinc (Zn), copper (Cu), lead (Pb), cadmium (Cd) and mercury (Hg) from leachates of ten huge dumpsites across the metropolitan city of Zaria. The trends in the mean concentrations of the metals (mg/L) among the fractions were; Zn: total > mobile > particulate > dissolved; Pb: total > mobile > particulate > dissolved; Cd: mobile > dissolved > total > particulate; Hg: particulate > mobile > total > dissolved, respectively. All the concentrations of the metal ions were above the world Health Organization (WHO) (2006) and United States Environmental Protection Agency (USEPA) (2000) tolerable limits across the sites, with the exception of lead at the control site which was not detected. The order of the metals bioavailability was; Cd > Hg > Zn > Pb > Cu, with more than 49% found in the bioavailable phase. Thus, the underground waters within the vicinity of the dumpsites were greatly at the risk of being polluted by these toxic metals and subsequently affecting the inhabitants who use the water for drinking and other domestic activities untreated, through the food chain transfer. The health implications associated with the toxic metals include an irreversible damage to nervous system, gastric and intestinal disorder, heart disease, liver, brain damage, mental retardation and teratogenic effects.


INTRODUCTION
Inadequate municipal and industrial waste collection and disposal creates a range of environmental problems in Zaria metropolis wHere considerable amount of waste ends up in open dumps or drainage system, threatening both surface water and groundwater quality.This provides a breeding ground for disease carrying pests, create health hazards, pollute the air, soil and sometimes groundwater and surface water, as well as deteriorating the beauty of the area (Botkin and Keller, 1998).The situation is high in Zaria metropolis where most households could not make use of garbage collection containers; lack of most solid wastes services in crowded, low-income neighbors is a major contributor to the high mobility and mortality among the urban poor.
The incoming wastes in the city originate mainly from households and industries (Benedine et al., 2011).
The open dumpsites are well known to release large amounts of hazardous and otherwise deleterious chemicals to nearby groundwater, surface water, soil and to the air via leachates and landfill gases.It is known that such releases contain a wide range variety of potential carcinogens and potentially toxic chemicals that represent a threat to public health (Fredlee et al., 2003).Leachates have been implicated as environmental pollutants such as air, soil, plants, surface and ground water pollution.Sufficient number of individuals near dumpsites would experienced an average increased cancer risk, at least 1 in 1000 (Fredlee et al., 2003).
As many studies have shown, municipal refuse may increase heavy metals concentrations in soils and underground water (Carlson et al., 1976;Albores et al., 2000;Okoronkwo et al., 2005a, b) which may have effects on the host soils crops and human health (Smith et al., 1996;Nyle and Ray 1999).Thus, the environmental impacts of leachates emanating from dumpsites are greatly influenced by their heavy metals contents.However, while total heavy metals contents is a critical measure in assessing risk of a refuse dumpsite, it does not provide a predictive insight on the bioavailability, mobility and fate of the heavy metals contaminants (Albores et al., 2000).Thus, it is the chemical form or species of the heavy metals that is an important factor in assessing their impacts on the environment as it controls their bioavailability and mobility (Norvell, 1984).
Past investigations on the metals impact of municipal refuse dumpsites leachates in Nigeria were concerned only with total heavy metals determination (Amadi et al., 2011;Aiyesanmi et al., 2011).The objective of this study therefore was to investigate the chemical fractionation of cadmium (Cd), copper (Cu), lead (Pb), mecury (Hg), and zinc (Zn) in dumpsites leachates of Zaria metropolis so as to assess their potential mobility, bioavailability and fate.Thus, the human health and ecological risks associated with the refuse dumpsites to the underground water (hand-dug wells) will be assessed.

Quality assurance
All reagents used were of analytical grade, de-ionized water was used for the preparation of the standard solutions, all the glassware and polythene sample bottles were washed with liquid soap, rinsed with water, soaked in 10% HNO3 for 24 h, cleaned thoroughly with double distilled de-ionized water and dried.The analytical results obtained were validated with spiked samples.The percentage recoveries of the metals and the analytical precision was confirmed with the triplicates throughout the study (Todorovi et al., 2008).Procedural blanks, reagents blanks and preparation of standard solutions were carried under clean laboratory environment.

Description of the study area
Zaria metropolis is located at latitude 11° 07' N and longitude 07° 42' E, and is presently one of the most important cities in Northern Nigeria (Uba et al., 2008) (Figure 1).It has problems of environmental sanitation such as improper disposal of refuse near residential areas resulting in the contamination of the underground water via leachates emanating from the dumpsites since most of the wells near the dumpsites were poorly covered and opened (Figure 2).It has a total area of 300 km 2 and constitutes four major settlements; Zaria City, Tudun Wada, Sabon Gari and Samaru (Zaria at a glance, 2013).It has a tropical continental climate with a pronounced dry season, lasting up to seven months (October to May).During the dry season, a cool period is usually experienced between November and February.This emanates from the influence of the North-eastern winds (the Harmattan) which control the tropical continental air mass coming from the Sahara.This weather prevails over most parts of the country.The North-East (NE) winds are characterized by hazy to dusty conditions and low temperatures, as low as 10°C at night.In the afternoon, up to 40°C is sometimes recorded.The humidity also drops to less than 15% in December/January.
Zaria experiences a brief period of hot but dry weather in March and April, followed by a progressive incursion of tropical maritime air mass from the Atlantic Ocean which displaces the NE (Harmattan) winds.During this short period, the mean daily maximum temperatures are fairly stable, and they range from 38 to 42°C.After that, the South Westerly Monsoon winds laden with moisture bring the rain in thunderstorms and squalls with heavy fall of high intensities.The rainy season lasts from May to September/ October with long-term annual rainfall of 1,040 mm in about 90 rain days (Zaria at a glance, 2013).The relatively deep tropical ferruginous soils and climate conditions of Zaria are suitable and sustain a good cover of savanna woodland (Northern Guinea Savanna), with a variety of grasses woody shrubs and short trees.Ten huge dumpsites were selected which covered the metropolitan city, and a control/uncontaminated site was selected 300 m away from the Kusfa dumpsite which is a new settlement without any dumping activities as summarized in Table 1.

Samples collections
Leachate samples were collected from ten dumpsites and a control site from June to August, 2011 in the rainy season from randomly selected leachate drains at the sites.The samples were collected in the well labeled clean polythene bottles that were rinsed with the leachates prior to sample collection.The samples for elemental analysis were collected in 1 L polyethylene bottles while those for mercury analysis were collected in glass bottles (American Public Health Association (APHA), 2005).

Samples pre-treatment
Samples for mercury analysis were preserved in 1 ml concentrated H2SO4 and 1 ml 5% K2Cr2O7 solution for every 100 ml samples.The samples for elemental analysis were preserved in 2 ml concentrated HNO3 (Aiyesanmi et al., 2011;APHA, 2005).

Chemical fractionation of heavy metals in the dumpsiteleachates
The chemical fractionations of the samples were carried out on the principle proposed by Bäckström et al., 2003;Wakawa et al., 2008).Basically, there are three steps that were involved, while fraction (IV) was taken as the difference between fractions (III) and (I) and the fractionation was carried out as follows; Fraction I (dissolve phase): 50 ml of the leachate samples were decanted from the sampling vessel and filtered through 0.5 µm teflon filters and the solution was then acidified with 5 ml 2% HNO3 and made up to 25 ml with de-ionized water (Bäckström et al., 2003).
Fraction II (mobile phase): 50 ml of the samples were decanted and acidified with 2% HNO3 (5 ml).These were followed by filtration through 0.5 µm teflon filters after 24 h.The solutions were then made up to 25 ml with de-ionized water (BäckstrÖm et al., 2003).
Fraction III (total fraction): 5 ml of 2% HNO3 were added to the 50 ml of the samples and the solution were stirred vigorously to suspend all the particulate matter and then filtered through 0.5 µm teflon filter after 24 h (Bäckström et al., 2003).
Fraction IV (particulate fraction): Particulate fraction was the difference between total residual fraction and dissolve phase (BäckstrÖm et al;2003).

Samples analysis
The digests of the samples were analysed for Zn, Pb, Cu, Cd and Hg using AAS-650 (varian double beam) and the validation of the procedure for metal determination was conducted by spiking samples with multi-element standard solutions containing 5 mg/L of the metals with the exception of Cd where 4 mg/L of the spiked sample was used.Spiked samples were used under the same experimental conditions used for the procedural blanks as samples the acceptable recoveries (>98.4%)from the spiking experiment had validated the experimental procedure (Table 2).

Statistical analysis
The data were expressed as means ± standard deviation.To show whether there is significant difference between the mean concentrations of the metals across the sites, one way analysis of variance (ANOVA) was used and the Pearsons moment correlation (r) was used to establish the degree of relationship among the fractions of the analysed metal ions across the sites using a statistical software package for social science (SPSS version 16).

Chemical fractionation of metals in the leachates (dumpsite-leachates)
Tables 3 to 7 showed the percentages of the bioavailable of zinc, lead, copper cadmium and mercury in the fractionated leachates across the sites in addition to the concentrations of various fractions (mg/L).The highest total extractable fraction of zinc (Table 3) was recorded at the control site, with 98.674% in the bio-available phase.The order of the bioavailable fractions of zinc across the sites followed the trend: CTR > SA > AJ > JK > PR > SH > BG > DD > KU > RA > NTC.Table 9 showed the results for the analysis of variance (ANOVA), and a significant difference of the mean concentrations of the metal were recorded across the sites at p < 0.05, with the exception of cadmium which was not significant at 95% confidence.
Furthermore, Table 4 showed the elevated levels of lead recorded across the sites, with the extractable fractions predominantly found in the bio-available phase (>65%) except the control site which was below the detection limit (BDL).The highest bio-available lead was recorded at BG-dumpsite leachates.The order of mobility Table 7 showed the concentrations of mercury in the fractionated leachate samples in which 29.23 (CTR) to 100% (SH, SA, PR and DD) were in the bioavailable The distribution trend among the fraction was: particulate > mobile > total > dissolved.Similarly, one way ANOVA showed a significant difference among the fractions at p < 0.05 as shown in Table 9.The metal was positively correlated with the lead however, negative correlation of the metal ion was recorded with copper, cadmium and zinc, revealing an inverse relationship.The high concentrations recorded at the sites may not be unconnected with dumpsites constituents where cadmium containing waste formed part of the constituents and the total fraction was significantly not different at p < 0.05.Table 8 showed the degree of association of the metal ions, the variables showed significant positive correlations with each other and with different metal ions with  As shown in Table 8, zinc was strongly positively correlated to dissolved, mobile, total and particulate fractions while the mobile phase of zinc was positively correlated to the particulate fraction (r = 0.312).Furthermore, all the variables (frac-tions) were positively correlated, the highest rvalue was observed on correlating mobile and total lead (r = 0.948) at p < 0.05 as shown in Table 7.
Similarly, cadmium fractions were positively correlated with the exception of mobile and particulate fractions which showed negative correlation.Copper fractions, were strongly positively correlated with the dissolved and particulate, particulate and total fractions (r = 0.869 and 0.748), respectively.However, negative correlations were recorded among the fractions of mercury across the sites with the exception of particulate and total fractions which were strongly positively correlated (0.974) at p < 0.05 (Table 8).

DISCUSSION
On comparing the results obtained for zinc with the standard limits (USEPA, 2000;WHO, 2006), sites KU, SA, SH and PR were contaminated (concentration > 5 mg/L).Thus, the zinc in the analysed leachate samples was readily bio-available to the environment contaminating especially, the underground water due to leachates percolation.Zinc pollution is known to induce vomiting, dehydration, abdominal pain, dizziness and lack of muscular co-ordination (WHO, 1999).Overall, the mobile fractions had the highest concentrations of the total extractable Zinc across the sites.The concentrations recorded were higher than the values of 0.37 to 0.65 mg/L reported by Aiyesanmi et al. (2011) in Benin City for the total elemental analysis of leachates.The difference might be attributed to the different composition of the analysed dumpsites.
The lead concentrations recorded suggests that there was a common source of pollution by the metal ions as significant difference among the fractions was observed at p < 0.05.When the concentrations (total extractable) across the sites were compared with those of the international standard (USEPA, 2000; WHO, 1999) they all exceeded the tolerable limit of 0.05 mg/L with the exception of the fractions at the control site.The total extractable fractions were higher than the range of 0.05  ANOVA run at p < 0.05, if p is < 0.05, there was a significant difference among the fractions with the variable at 95% confidence otherwise, there was not.
to 0.12 mg/L reported by Manpanda et al. (2007) in Zimbabwe and lower than 0.35 to 0.97 mg/L reported by Ahlberg et al. (2006) in Sweden, respectively.It was also noted that if significant quantity of lead was leached into the groundwater, cytogenetic alteration such as kidney and brain damage or birth defects results especially when ingested through the food chain or drinking water (Ademoroti et al., 1996;Aiyesanmi et al., 2011).The extractable fractions of cadmium were compared with the WHO (2006) standard limits of 0.003 and 0.001 mg/L (WHO, 2006;USEPA, 2003) respectively, overall, the results showed higher values with few exceptions.The recorded concentrations in this study were below the ranges of 0.02 ± 0.01 to 0.24 ± 0.31 mg/L and 3. 62 ± 0.01 to 8.15 mg/L reported by Aiyesanmi et al (2011) in Benin City and Ahlberg et al. (2006) in Sweden, respectively.Analysis of variance (ANOVA) showed a significant difference (at p < 0.05) both among the fractions and across the sites.In addition, there was a positive correlation between the lead and Zinc (Pb to Zn) across the sites suggesting a common source of pollution.Cadmium is toxic when inhaled even in trace amount in dust/particulates during incineration/burning at dumpsite because of its carcinogenicity (Aiyesanmi et al., 2011).It is also known that it is very hazardous and of no use to biological processes (Watanabe et al., 2008).
The levels of copper recorded in this study were lower than > 1.5 mg/L reported by Ikem et al. (2002) in Lagos.The distribution pattern among the fractions was Cu: total > particulate > mobile > dissolved.Copper in the blood exist in two forms: bound to ceruplasmin (85 to 95%) and the rest 'freely' loosely bound to albumin.The free copper is toxic as it generates reactive oxygen species such as superoxide, hydrogen peroxide and the hydroxyl radical.These damages proteins and DNA (Brew et al., 2010).The levels of mercury recorded were significantly high, quiet above the WHO tolerable limit across the sites, and significant amount was found in the bioavailable fraction, thus the metal was readily leachable into the nearby open wells resulting to serious health problems such as chromosomal segregation, disruption and inhibition of cell division.

CONCLUSION
The leachates samples were heavily polluted by zinc, copper, cadmium and mercury including those at the control site.However, lead was not detected at the control in all the fractions of the samples.Furthermore, significant amounts of the fractionated metals were found in the mobile phase showing a threat to the open wells within the vicinity of the dumpsites.Overall, more than 49% of the analysed toxic metals were found in the bioavailable fractions (dissolved + mobile fractions) resulting to serious health problems such as typhoid fever, cholera and other water borne related diseases to the residents who relied heavily on the untreated well waters for drinking and other domestic activities due to erratic and inadequate water supply in the city.

Figure 2 .
Figure 2. One of the dumpsites in Zaria metropolis showing one of the sampling point (RA).

Table 1 .
Description of the sampling points (dumpsites) and various activities being discharged at each dumpsite.

Table 2 .
Fractions descriptions and WHO limits.

Table 3 .
Concentrations (mean ± SD) mg/L of zinc in the fractionated dumpsites leachates.

Table 4 .
Concentrations (mean ± SD) mg/L of lead in the fractionated dumpsites leachates.

Table 5 .
Concentrations (mean ± SD) mg/L of cadmium in the fractionated dumpsites leachates.

Table 6 .
Concentrations (mean ± SD) mg/L of copper in the fractionated dumpsites leachates.

Table 7 .
Concentrations (mean ± SD) mg/L of mercury in the fractionated dumpsites leachates

Table 8 .
Correlation matrices for the fractionated metals in the leachates.

Table 9 .
ANOVA for leachates across the sites.