TRACE ELEMENTS’ POLLUTION ASSESSMENT IN VEGETABLES FROM IRRIGATION AREAS OF DADIN KOWA DAM, GOMBE STATE, NIGERIA

Vegetables are part of a healthy and important diets to human being, but they could be contaminated in many ways: through irrigation, atmospheric deposit and some human activities. This work is aimed at assessing heavy metal concentrations in some vegetables in the irrigation area of Dadin Kowa Dam, Gombe State Nigeria. Energy Dispersive X-ray Fluorescence Spectroscopy (EDXRF) was used for the detection for spectral analysis. The vegetable samples were cultivated and harvested locally between October, 2019 to January, 2020. After which the edible vegetables (onion bulb and cabbage leaf) samples were harvested at maturity and collected along transect lines within the farms. The vegetable samples were prepared and taken to the laboratory for the analysis. The following metals mean concentrations were obtained in onion and cabbage samples respectively (Values in part per millions (ppm)): Mo (9.582 and 9.880), Zr(5.523 and 4.778), Sr(21.780 and 30.732), Rb(9.139 and 17.228), Zn(26.866 and 14.576), Cu(24.466 and NA), Fe(83.265 and 61.925), Ti(30.604 and 24.075), Ca(15752.460 and 2736.190), K(15076.784 and 29039.254), Ba(969067.123 and 961907.100) and Nb(10.512 and 9.929). The obtained results were interpreted using Transfer factor, Daily intake of metals and Geo-accumulation index. It was found out that in vegetables, the element with highest transfer factor (TF) observed in onion is Ca while Zr, Sr, Rb, Cu, Ti, Fe, and Nb had virtually less than one transfer factor. In the case of cabbage Zn happened to have the highest TF. Daily intake in onion, almost all the elements were below the tolerable limits of 77ppm except Nb which is within the limit 0.2 ppm as stated by FAO/WHO. In Geo-accumulation index, only Mo was extremely polluted because is greater than five Muller, (1969). Mo, Zr, Sr, Rb, Zn, Fe, Ti, and Cu were found to be below the reported standard limits. Ca, K and Ba in both vegetables were found to be higher than recommended limits but they are reported to be relatively harmless. Significant correlation was found for most of the elements determined. However, in vegetables, at α = 0.05 some of the metals Fe with Cu, Ca with Zn have shown positive correlation while Zn with Rb and Ca with Rb, showed negative correlation. At α = 0.01, K with Rb, Ti with Cu and Ti with Fe showed positive correlation while Ba with K are negatively correlated. The vegetables under study in Dadin Kowa Dam irrigation area showed relatively law state of pollution for most of the elements determined.


INTRODUCTION
There is a growing concern about heavy metals contamination of food crops in world today.This is as results of toxicity and diseases in human and animals.Among the six classes of food with vitamins, vegetables are one of its examples.Vegetables are part of healthy and important diets to human being but, they could be contaminated in many ways, through irrigation, atmospheric deposits and some human activities.However, from contamination and toxicity points of view, that is contaminations of onion and cabbage with heavy metals are of major concerned in this work.Heavy metals are naturally occurring elements, and are present in varying concentrations in all ecosystems.They are found in elemental form and in chemical compounds.Each form or compound has a different property which also affects what happen to it in food web, and how toxic it may be.
Heavy metals have atomic densities higher than 4g/cm 3 , and these include lead (Pb), cadmium (Cd), zinc (Zn), mercury (Hg), arsenic (As), silver (Ag), chromium (Cr), copper (Cu), iron (Fe), and platinum (Pt).The high level of environmental contamination by these metals is dangerous because their uptake by plants and subsequent accumulation in food crops consumed by humans and animals is deleterious to health.Evidence shows that vegetables and other food crops consumed in Nigeria are contaminated by heavy metals, and this is associated with adverse health issues, such as cancer, which is currently on the rise in Nigeria (Onakpa et al.,2018).
However, in Dadin Kowa, Yamaltu-Deba Local Government Area Gombe State of Nigeria, the contamination of metals in vegetables has not been givenmore consideration.So, the assessment of heavy metals in soils and vegetables will not just give the health risk but also the assessment of environmental pollution.As many cases of high blood pressure, miscarriage, hepatitis and other related diseases could be as a result of taking large quantity of the heavy metals.
Consequently, the continuous consumption of such vegetables samples contaminated with toxic heavy metals exceeding the safe permissive limits may result in serious health problems.It is therefore vital to educate communities around the study area to avoid or monitor eating large quantities of these vegetables items with high levels of heavy metal pollution.
This work is aimed at assessing heavy metals concentrations in vegetables in the irrigation areas of Dadin Kowa Dam Gombe Sate, Nigeria.

Description of the Study Area
Dadin Kowa Dam is located in Dadinkowa.A village in Yamaltu Deba Local Government Area of Gombe State in the North East of Nigeria, which is about 35 kilometres to the east of Gombe town.It provides drinking water for the neighboring villages and Gombe metropolis.The Dam was completed by the Federal Government of Nigeria in 1984, with the goal of providing irrigation and electricity for the planned Gongola sugar plantation project.The coordinates for the study area are 10 o 19' 19'' N, 11 o 28' 54'' E and has the total capacity of eight hundred million metre cube (800,000,000 m 3 ).Tables 1 and 2 give the farms location with the referencing coordinates and sample codes with description respectively.

Study Design/Procedure
The procedure adopted in this work involves Sample Collection, Preparation and Laboratory Analysis that is, assessment of heavy metal concentrations in edible vegetables samples from the irrigation area were carried out.

Vegetables Samples Collection and Preparation
Vegetables were collected along transect (Melville and Welsh,2001).This was done for the vegetables planted in the irrigation area.The following edible vegetables (onion and cabbage), five samples of the onion bulb were collected / harvested at maturity (Two months after planting seedlings) and five samples of the cabbage leaf were also harvested (Two months after planting seedlings), packed into clean polythene bags for laboratory analysis.In preparation, soil particles were washed off from the samples.Then cut it in to small pieces and air dried at the room temperature for two weeks and then pounded into powder.Then the samples were collected in a labeled polythene bags and were taken to the laboratory for the analysis (Ebong et al., 2008;Faithfull, 2002)

Assessment of Heavy Metals Concentrations in Soils and Vegetables Samples
Heavy metals in soils and vegetables samples were determined using energy dispersive x-ray Fluorescence Spectroscopy (EDXRF) system in the laboratory.
Samples were collected from the site taking into consideration the possibility of future changes in the samples, example washing the sample properly and putting it into good container before taking it to the laboratory for preparation.

Data Analysis and Evaluation
The trend distribution of the elements was assessed by various statistical analysis tools after determining the heavy metals in the vegetables samples collected from the study sites.The pollution indices and statistical analysis includes: Transfer Factor, Daily Intake of Metals (DIM) and Geo-Accumulation Factor.
The statistical software SPSS (Statistical Package for Social Science) was used to obtain a descriptive interpretation of the Pearson correlation of the elements found in the vegetables.

Transfer Factor for vegetables
Transfer factor was calculated in other to found out the heavy metals accumulation by the onion and cabbage samples from the soil.The risk and associated hazard due to ingestion, consequent upon the heavy metals accumulation in vegetables was also assessed.The computed transfer factor for each heavy metal was based on the Harrison and Chirgawi, (1989) method.The heavy metal transfer from soil is expressed as:

Daily Intake of Metals (DIM)
Daily intake of metal (DIM) as reported by Cui et al., 2004 is given by: In equation above, Cmetal, and Dvegetableintake, stand for the heavy metal concentration in vegetables and daily intake of vegetables respectively.The required amount of vegetables for human is 77g/day(GEMS/Food,2000) The Geo Accumulation Index The Geo Accumulation Index characterized surface soil from unpolluted to extremely polluted.Geo Accumulation Index (Igeo) was originally defined by Muller, (1969) as: where, Cs is the measured concentration of the examined heavy metal in sample, 1.5 is the background matrix correction due to terrigenous effect or is introduced tominimize the effect possible variations in the background and Cb is the geochemical background concentration or reference value of the metal or background value of the heavy metal in the uncontaminated sample.The classification of the as reported by Hankouraou and Jibrin, 2020 is given in Table 3 below.

Correlation Analysis
The measure of similarity between paired data is termed correlation analysis.The degree of inter relation between variables can be estimated with the help of correlation coefficient (r) without any influence by measurement units.Correlation is the ratio of covariance (joint variation of two variables about their common mean) of two variables to the product of the standard deviation (Davis, 1973;Hankouraou, 1998).
Correlation Coefficient being a ratio is a dimensionless number and covariance may equal but can never exceed the product of the standard deviation of its variables.
Correlation ranges from +1 to -1.A correlation of +1 is an indication of a perfect direct relationship between two variables.While that of -1 indicates that one variable changes inversely in relationship to the other.A spectrum of less than perfect relationship lies between the two extremes including zero which indicates the lack of any linear relationship.

RESULTS AND DISCUSSION
The following heavy metals (Mo, Zr, Sr, Rb, Zn, Cu, Fe, Mn, Ti, Sc, Ca, K, S, Ba, and Nb) and their concentrations were obtained from the comprehensive analysis of vegetables using EDXRF.But for the purpose of this work only the following elements were considered.Those elements include: Mo, Zr, Sr, Rb, Zn, Fe, Cu, Ti, Ca, K, Ba and Nb.
The concentrations of all trace elements in the vegetables are as shown in Tables 4 and The metals concentrations transfer factor from the farms soils to their individual harvested vegetables were calculated and the results were as shows in

NA = not available
In Table 7, the elements with highest transfer factor observed in Onion were Ba, Ca and K while Zr, Sr, Rb, Cu, Ti, Fe, and Nb had virtually less than one transfer factor.
In the case of Cabbage transfer factor, Zn happened to have the highest TF.This could be as a result of the contamination from the external sources of pollutants such as fertilizers and pesticides which are important inputs in agricultural production.Nevertheless, the long-term excessive application has resulted in the heavy metal contamination of soils.The vast majority of pesticides are organic compounds, and a few are organic -inorganic compound or pure mineral, and some pesticides contain Hg, As, Cu, Zn and other heavy metals (Adamu et al.,2010).
The required daily intake for vegetables as stated is 77g/day (GEMS/Food, 2000).The metals daily intakes were calculated for Mo, Zr, Sr, Rb, Zn, Cu, Fe, Nb, Ti, Ca, K, and Ba concentrations in onion and cabbage separately.The results are as shown in Table 8 below.Geo accumulation Index (Igeo) for vegetables samples was calculated as defined earlier.
The results were shown in Table 9 below.The geo accumulation index of the vegetables (onion and cabbage) and almost all the elements in both onion and cabbage were found to be greater than zero but less than one.So we can conclude that, they were Unpolluted/Moderately.Except K in cabbage which is greater than one but less than two, so it was moderately polluted.Mo was extremely polluted because is greater than five so, is more terrifying Muller, (1969).

NA = not available
The Pearson's Correlation matrices among the studied metals (Mo, Zr, Sr, Rb, Zn, Cu, Fe, Ti, Ca, K, Ba, and Nb) in vegetables samples were determined and the values are shown in Table 10.At α = 0.05, some of the metals such as Fe with Cu and Ca with Zn showed significant positive correlation between each other, while, Zn with Rb and Ca with Rb, has significant negative correlation between them.At α = 0.01, K with Rb, Ti with Cu and Ti with Fe showed the existence of positive correlation for each pair of metals.It is important to note that Ba with K was negatively correlated.The obtained results here show that the complexes of some elements within the soil strata could be influenced by the presence of other given elements (Seydou ,1998) In Table 11 above it can observed that the concentrations of Mo, Sr, and Rb in onion samples were found to be lower than the corresponding obtained values in cabbage, while Zr, Zn, Fe and Nb had higher concentrations in cabbage, Cu was only available in onion.
Molybdenum concentration in onion and cabbage samples at different sampling points range from 8.750 to10.855ppm and 7.642 to 11.447ppm respectively which are below the required level.Permissive limit for Molybdenum in vegetables (onions and cabbage) is 40ppm as given by by FAO/WHO.High concentration of this element can cause headache, fatigue, loss of appetite, muscle and join pain.It may also raise the uric acid in the body which can lead to gout and may damage kidneys and liver (Ugonna, 2020).

CONCLUSION
The results obtained show that both onion and cabbage grown from the study area have heavy metals of environmental concern.The element with highest transfer factor observed in Onion was Ca, while, Zr, Sr, Rb, Cu, Ti, Fe, and Nb had virtually less than one transfer factor.In the case of Cabbage transfer factor, Zn happened to have the highest Transfer Factor, though it was not detected in its farm soil somewhere else.This could be as a result of the contamination from the external sources of pollutants.
Daily Metal Intake clearly shows that the contamination level of the analyzed vegetables samples was discovered to be within the tolerable limits.Except Ca, K, Ba and Nb which are found to be above maximum tolerable limits.This might be as the result of the longtime application of pollutants such as fertilizer, pesticides, herbicides etc. during the irrigation farming which may lead to the contamination of soil.Though relatively harmless, but long-term metal exposure by regular consumption of such locally grown vegetables with such elements in high concentration may pose potential health problems to animal and humans.Therefore, close monitoring of the irrigation area is necessary based on our findings even though most the concentrations obtained are within the tolerable limits sat by international reported organization such us WHO and FAO.

Table 1 :
Farms locations with their geo-referencing coordinates

Table 4 :
Trace elements in onion samples and their concentrations in ppm

Table 5 :
Trace element in cabbage samples and their Concentrations in ppm

Table 6 :
Mean concentrations ± SD of trace elements in vegetables samples (Values in ppm)

Table 7 Table 7 :
Transfer Factor (TF) for vegetables

Table 8 :
Metal Daily Intake(MDI)for vegetables (Values in mg/day) It is clearly shown that Ba has the highest daily intake values are 74618.2mg in onion sample and 74066.9mg in cabbage sample, although K and Ca also have high concentrations but are reported to be relatively harmless(Seydou,1998)

Table 10 :
Person's correlation matrices between variable parameters in vegetables

Table 11 :
Comparison of this work values for vegetables with FAO/WHO (Values in ppm or as stated)In onion and cabbage, the concentration of Niobium at different sampling points ranges from 8.048 to 12.529 and 7.918 to 12.405ppm respectively.Which were all above the required limits of 0.2ppm stated byFAO/WHO (2003).The concentration of Titanium in onion ranges from 18.032 to 43.176 ppm and 000 to 21.075 ppm in cabbage.These values are above the recommended level set byWHO  (2001)for vegetables.The Calcium concentration in onion ranges from 10189.387 to 21897.883ppm while in cabbage it is from 1159.452 to 4492.805ppm which are above the FAO/WHO standard level(FAO/WHO,2003)