Hazardous Impacts of Open Dumpsite of Municipal Solid Wastes on Soil: Case of M’Ploussoue Park Dump at Bonoua in Ivory Coast*

In Ivory Coast, most of the uncontrolled dumpsites are open-air and is one of the most common methods adopted. To study the impact of the solid waste on soil, soil samples were collected from the dumpsite of M’Ploussoue Park in Bonoua. The physical and chemical parameters such as size particle, pH, Exchangeable cations, CEC and the contents in heavy metals, were studied using various analytical techniques. It has been found that most of all the heavy metals analyzed contents exceeds the permissible limits in accordance with CCME soil quality standards. It’s conclude that M’Ploussoue Park dumpsite is characterized as polluted site whose the contamination is due to the solid waste materials that are dumped in the area.


Introduction
In Ivory Coast, most of the dumpsites are open-air (Kouamé and al., 2006) and is one of the most common methods adopted since years in almost all the cities, due to the low budget for waste disposal and non-availability of trained manpower (Hakkou and al., 2001). These last decades, the rapid rate of industrialization, population and modernization have contributed to generate million tons of solid wastes of different categories ( hazardous wastes, nonhazardous wastes) which are generally constituted of both the degradable and non-degradable substances (Praveena and Rao , 2016). These solid wastes may contain several heavy metals or metalloids which could poses severe environmental threat (Partha and al., 2011) particularly affected groundwater resources and soil quality (OJEC, 2001). According to Mor and al., (2006) and ISWA, (2015), dumpsites have been identified as one of the major environmental risk related to unsanitary landfilling of solid waste.
Solid wastes pollutants can affect the physicochemical characteristics of soil ultimately contributing towards the contamination of plant as observed by Partha and al., (2011) on dumpsite of india. The higher intensity of the contaminants can cause adverse effects on human health, animals and sometimes endanger the survival of some species (Papageorgiou, 2006;Akpaki and al., 2014). In fact, several wastes from different sources find the way into the environment and finally end up in the dumpsites posing severe contamination of soil due to the heterogeneity. Soils serve as a natural sink for the pollutants released from both natural and man-made sources (Praveena and Rao, 2016). However, the assessment of soil pollution depends to the different sources where the contaminants belong. Therefore, it is utmost important to know the contamination degree of the open dumpsite of the park M'Ploussoue of Bonoua situated at the heart of city, to prevent possible sanitary risks for the users and the dwellers. The aim of this paper is to have an exact knowledge of the nature of the contaminant present in the dumpsite of the M'Ploussoue park in Bonoua.

Study area
The study area is situated in the South-East part of Ivory Coast at Bonoua, 60 km to Abidjan, between latitude 5°14' to 5°31' N and longitude 3°13' to 3°51' W. The Dumping site is located in the M'Ploussoue Park, Bonoua at latitude 5°16' N and longitude 3°36' W ( Fig. 1). This Dumping site has been abandoned since 2006 and covers an area of 16 ha. One kind of waste are dumped in this site: solid municipal waste.
The control site is situated at 5 km of dumping site at latitude 5°15' N and longitude 3°35' W. The study area is located in tropical climate and has mean annual rainfall of about 2000 mm (Aké, 2010) with a rainy season (May to July and September to October) and a dry season (July to August and December to April). Geology of area is dominated by the sedimentary formations specially detritics rocks. The average monthly temperature of Bonoua is above 20°C.
The soil of study area is classified as an arenic ferralsol and the dumping site is classified as a Fumic Anthroposol in the World Reference Base for Soil Resources (WRB).

Soil samples
The soil samples were collected at 18 different points on the abandoned dumping site at 0-30 cm deep, according to random sampling technique. Soil samples were sieved to 2 mm to remove various types of wastes (paper, used batteries, electronic goods, wood, plastic paper, straws, buckets, tin cans, sacks, clothes, glass bottles, cotton wool, food wastes, leaves, fruit wastes, medicine bottles, foams, ashes, water sachet, card board and human excreta), then mixed and homogenized to obtain a composite sample. The composite sample were air-dried and brought to the laboratory for various analysis.
Another soil sample from other sector was also taken as soil control to compare the quality of soil from dumping site.

Laboratory analysis
Several analyses were performed to study the parameters that evaluate soil quality. The particles size was carried out by pipette of Robinson-Köln method and soil textural classes were carried out by using the textural triangle (Robert and Frederick, 1995). The Walkley and Black method and Kjeldahl method were used to determine respectively Organic Carbon and total nitrogen (N-total). Available P was determined using Bray ascorbic acid method. Soil pH was determined in 1: 2.5 (w/v) (soil/water or potassium chloride solution) ratio using electrode pHmeter (Mathieu and Pieltain, 2003). Exchangeable cations (Ca +2 , Mg +2 , Na + and K + ) and Cation exchange Capacity (CEC) were determined using Metson (1956) method. Heavy metals (Pb, Cd, Cr, As, Cu and Zn) were then analyzed by atomic absorption spectroscopy (ICP-OES) (Spectroblue). The analysis of the pH, Exchangeable cations, CEC and the contents in heavy metals were performed in the laboratory of Institute of Ecology and the Sciences of Environment of Paris (IESE), France. But the analysis of the granulometry, C, N, P available were realized in the laboratory of soils and plants in National Polytechnic Institute Félix Houphouët Boigny (INP-HB) of Yamoussoukro, Côte Ivoire.

Statistical Analysis
The data was subjected into statistics analysis using 7.1 statisticat software at 5% probability level. Significant differences between different parameters (particle size, chemical parameters, heavy metal content) under dumping soil and control soil were soil were performed using Student-Newman-Keuls (SNK) test at α<5% probability level.

Soil Particle Size
Soil physical properties are presented in Table I. Sand is the dominant particle size fraction in both areas studied (dumping site soil and control soil, ranged 77% -79%), followed by Clay (18% -21%), with silt been the lowest (1.6% -3%). They are not significant difference in the mean value of percentage of sand and clay in both areas (control and dumping sites). Silt content is significantly highest in control soil (3%) then in waste dumping site soil (1.6%) as presented in Table I.

Chemical parameters at control and wastes dumping sites
The chemical characterization of the two soils studied (dump and control sites) indicates that the control soil is moderately to strongly acidic with a water pH = 5.8 and a potassium chloride pH KCl = 4.5 while the dumpsite soil is slightly acidic to neutral with a water pH = 6.9 and potassium chloride pH KC l = 6.1 (Table  II). The pH variation (ΔpH = water pH -pH KCl ) of the dumpsite soil is ΔpH <1 showing high acidity potential.

Contents in heavy metals of dumpsite and control site soils
Metal content of both control and dumpsite soils is presented in Table III. The mean concentration of all heavy metal (Cr, Pb, Cd, Zn, Ni, As, Cu and Se) detected in dumpsite soil is significantly higher at p <0.001 than the mean concentration in control soil sample (Table III). The mean value of Pb, Cr and Ni are significantly at p <0.001 very higher in dumpsite soil with respectively 118 mg.kg -1 soil dry, 130mg.kg -1 soil dry and 119mg.kg -1 soil dry than in control site soil where the mean value of heavy metal is respectively 5.4 mg kg -1 soil dry for Pb, 0.23 mg kg -1 soil dry for Cr and 3.2 mg kg -1 soil dry for Ni (Table III). For Cd (81 mg.kg -1 soil dry), Zn (344 mg.kg -1 soil dry), As (9 mg.kg -1 soil dry), Cu (9.5 mg.kg -1 soil dry) and Se (4.3 mg.kg -1 soil dry), their contents are significantly high in the dumpsite soil than at control site soil where their values are below the limit of detection (Table III). In the dumpsite soil, the highest value of heavy metal is observed with Zn (344 mg.kg -1 soil dry).  Table IV shows that Cr, Pb, Cd, Zn and Ni concentrations detected in dumpsite soil exceeds different standards recommended by World Health Organization and also those proposed in French and Canadian soils. But the content of As, Cu and Se are below of standards different recommended (Table  IV). ** Canadian environmental quality criteria for contaminated sites (CCME) recommendation (CCME, 1999).

Comparison between of heavy metals contents of the Bonoua Dumpsite soil and recommended concentrations of metal in soil from different countries
*** French Standards Association (AFNOR) limit recommendation (AFNOR, 1996).

DISCUSSION
Soil is a crucial component of rural and urban environments, and in both places land management is the key to soil quality. In the study area, the pH of the dumpsite soil is slightly acidic. The low acidity of dumpsite soil may be attributed to the age of waste dumping site which aging 10 years according to Praveena and Rao (2016) and El-Fadel et al., (2002). This low acidity could favored the metal complexation (Kouamé and al., 2006).
Moreover, the high organic matter content observed in open dumpsite soil than in control site soil could be attributed to the nature of various types of municipal solid wastes found in study area such as paper, used batteries, electronic goods, wood, plastic paper, straws, buckets, tin cans, sacks, clothes, glass bottles, cotton wool, food wastes, leaves, fruit wastes, medicine bottles, foams, ashes, water sachet, card board and human excreta. Other researchers attributed the high concentration of organic matter observed in the dumpsite soil to the presence of nonfermentable materials in the wastes which tend to resist to the decomposition and therefore, break down very slowly ( The concentration of Cd (81 mgkg -1 ), Cr (130 mgkg -1 ), Zn (344 mgkg -1 ), Pb (118 mgkg -1 ) and Ni (119 mgkg -1 ) in dumpsite soil are greater than limit values recommended for agricultural soil. This result stipulate that dumpsite soil is contaminated with those metals, which may constitute an environmental problem, if those metals migrates into the groundwater and plant.
The soil content of Cr (130 mgkg -1 ) in the dumpsite soil studied is far above the permissible limits and is also higher than the data from Akouedo dumpsite soil (125 mgkg -1 ) at Abidjan of Ivory Coast (Kouamé and al., 2006) and also than the data from Al AIN (19.