Seasonal Variation of Heavy Metals in Playgrounds of Public Schools within Owerri Metropolis, Imo State, Nigeria

Knowledge of variation in metal concentrations of playground soil in dry and rainy season could assist in controlling metal accumulation in children if playtime is regulated according to seasons. The present investigation was designed to find out the existence and trend of such variation. Thirty six surface soil samples from nine playgrounds of Owerri metropolis were collected during the dry season and rainy seasons of 2012 and 2013 and were digested in Pyrex glassware using nitric acid. Total heavy metal concentration was determined by AANALYST 400 Perkin Elmer absorption spectrophotometer and coefficient of variation was used to categorize variability. Results demonstrated significant seasonality for all five heavy metals with major variability observed between dry and the rainy season. Metal concentrations showed zinc as highest metal in both seasons whereas cobalt was lowest metal with Ni occasionally being lowest in the rainy season. The variation of metals concentrations in playground soil especially in the rainy season may add to the body burden of the children, and therefore reduced hours of play during this season is recommended.


INTRODUCTION
Pollution of surface soil with toxic heavy metals has become a major global concern due to growing health risks to the public [1,2]. Child health has been linked to playgrounds and type of play. According to the National Program for Playground Safety (NPPS), based at the University of Northern Iowa, children need 60 minutes of play with moderate to vigorous activity every day to grow up to a healthy weight. NPPS developed a four-step national action plan for playgrounds: provide proper playground supervision; design age-appropriate playgrounds; provide proper surfacing under and around equipment; and keep equipment properly maintained. Urbanization, industrialization and other aspect of modernization have caused the generation of huge quantity of solid, liquid and gaseous waste resulting in soil contamination. Many industrial activities such as surface treatment, galvanizing, electroplating, pickling, paper, chloro-alkali, batteries, and textiles make use of heavy metals. Due to the improper management of the waste generated from these types of industries, playground soils are being contaminated, more so when located in and around industrial areas. [3] Elevated heavy metals in playgrounds soil is due to natural processes such as atmospheric deposition, erosion, mineral weathering as well as anthropogenic sources in urban areas such as industrial and agricultural activities amongst the major source of these metals in the environment. However studies have reveal that traffic emissions are unanimously considered the major contributor of metal concentrations on playground soil and dust [4]. Exposure to heavy metal contamination has been found to cause such problems to children as kidney damage, liver damage, carcinogenic, paralysis, convulsions depression and pneumonia and etc [4]. These exposure problems are exacerbated by children vulnerability. Researchers have been focusing on quantifying these heavy metals and their assessments on children's environment and facilities [5,6,7]. Past studies have reported ingestion of playground dust with significant amounts of metals particularly arsenic. Thus numerous researchers were conducted on child health risk assessment relating to metals [8,9,10]. By way of culture, Nigerian children have direct contact with soil. Aliyu et al., [12] investigated, lead contamination of soil in children's playgrounds of some selected schools in Kaduna State with emphasis on the premises of nursery/primary schools. Results showed some variations from the background lead levels.
Seasonal variations in agricultural activity, storm water runoff, interflow and atmospheric deposition has strong effects on concentration of heavy metals [13] in the environment including playground soil. Thus, characterization of seasonal variability in surface soil quality is imperative for evaluating temporal variations of playground soil pollution from natural or anthropogenic contributions. Recently there has been growing awareness of the environmental impact of contaminated soil on play grounds [14]. However, in Owerri metropolis publications covering the subject of heavy metal contaminations especially on children playgrounds are lacking.
It is clear that such information aids our understanding of heavy metal behavior in the environment and is therefore of widespread interest. The hypothesis tested was that there is a seasonal variation in heavy metal concentrations during the year.

Site description
Owerri municipal is one of the three local government areas (LGAs) that make up Owerri city, the capital of Imo state of Nigeria set in the heart of the Igboland. Its population density ranks fourth as of 2006 census. It lies within coordinates: 5°29′ N 7°2′E / 5.483°N 7.033° with tropical climate. This area is under heavy traffic all year round since and is home to the biggest and modern market and the seat of state government. Even though there are no industries, connecting roads to neighboring states pass through the metropolis thereby increasing traffic volume.

Sampling of soil
Nine different sampling sites were taken from playgrounds along major roads connecting Owerri municipality in Imo state as follows: Surface soil or dust samples at 0-5 cm depth were collected in the months of January (dry season) and June (rainy season) of 2012. At each sampling site, a "W" shaped line was drawn on a 2 x 2m surface along which samples were collected from five points into previously treated polythene containers using a perforated container to allow water to drain.

Digestion of soil samples
1 gram of the soil samples were digested in a Pyrex glassware using 20mls of 4N HNO 3 acid at 90 o C for 6hrs. The mixture were then filtered into a 25mls standard volumetric flask and made up to the mark with deionized water. All laboratory glassware and plastic wares were first washed with high grade laboratory soap, rinsed with deionized water and soaked in 10% nitric acid (overnight), then rinsed again with double deionized water. All reagents used in this work were of analytical grade.

Determination Heavy Metal in Soil Samples
The total heavy metal concentration in the soil samples was determined by the use of The AANALYST 400 Perkin Elmer absorption spectrophotometer. Quantification was carried out using appropriate calibration curves prepared in the same acid matrix with standard metal solutions for atomic absorption spectrophotometer (Zhu et al., 2008).

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ILCPA Volume 50  2.6. Quality control/Quality assurance Nitric acid (HNO 3 ) (suprapur), HCl (suprapur), sodium sulphate and potassium hydrogen carbonate were purchased from Merck (Fin lab. Owerri), while distilled water was used for heavy metals analyses; standard metal solutions for atomic absorption spectrophotometer were purchased from Fluka (Buchs, Switzerland). Appropriate calibration curves prepared in the same acid matrix with standard metal solutions for atomic absorption spectrophotometry were used for quantification. The detection limits (3x standard deviation of the baseline noise/sensitivity) of the analytical technique for each element were: 0.001 μg/l for Mn, 0.003 μg/l for Co, 0.002 mg/l for Ni, 0.001 mg/l for Zn and 0.02 μg/l for Cu. Values obtained were blank corrected and reported as mean value of the triplicates.

Statistical Analysis
Data analysis was done using SPSS version 17.0. Mean of the three results were recorded and the mean for each metal concentration in the nine playgrounds calculated. The standard deviation (SD) was determined and co-efficient of variation (CV %) was used to determine the variation in heavy metal content between seasons. Variability was categorized as little variation (CV % < 20), moderate variation (CV % = 20 -50) and high variation (CV % > 50).   The content of Ni was highest at WBP (69.35mg/Kg) but this was only 11.60% compared to WHO threshold value of (600 mg/Kg). Playgrounds CSO, TSO and WBP had nickel contents higher than background value while the mean value was lower than background value.

Results
Ni  The trends in seasonal variability observed for metals in the years 2012 and 2013 are shown on table 2. Annually variability was observed to follow the trends Mn>Ni>Co>Cu>Zn and Co>Mn>Ni>Zn>Cu in which Mn, Ni and copper showed anthropogenic influence. High variation is caused by anthropogenic rather than geogenic enrichment of the soil. Owerri municipality is a bee hive of commercial activities ranging from heavy construction of roads bridges and hotels to government owned estates. These could lead to heavy metals released in the environment.

5.
CONCLUSIONSIn conclusion all five metals showed significant variation between dry and rainy season. Despite being highest in concentration, Zinc showed lowest variability. Overall variability could be ranked as follows: Mn>Co>Ni>Cu>Zn. The metal variability is likely linked to metal concentrations and thus further research to find out this like is recommended.

ACKNOWLEDGMENTS
This work is completed as part of PhD thesis in the department of Pure and industrial chemistry, University of Port Harcourt. We thank members of the Research in Analytical Chemistry and Environmental Pollution Studies (RACEPS) and Doctoral students of Professor M. Horsfall Jnr and Professor Mrs. A. I Spiff. We are grateful for the financial assistance of Engr. Dr. Ernest Fonyuy of Slumberger, U.S.A. We express deep gratitude to the Director; Centre for Energy Research University of Nigeria Nsukka, for the use of certain scientific instruments.

AUTHOR CONTRIBUTIONS
Verla, Evelyn Ngozi and designed the study and Verla Andrew Wirnkor assisted her with sampling and data collection in the experiments. Ugwulor Louise advised on ethical issues and contacted the school head teachers and finally performed the statistical analysis of the data. All authors contributed in writing and critically discussed the results and implications for the manuscript and approved the final version.

CONFLICTS OF INTEREST
We declare no conflict of interest now and in the future.