Impact of Land Use on Concentrations of Potentially Toxic Elements in Urban Soils of Lagos, Nigeria

Background. Among soil contaminants, potentially toxic elements (PTE) are of major significance because they are ubiquitous, toxic and persistent. Chronic exposure of humans to these elements has been linked with developmental delay, cancer, atherosclerosis and kidney damage, stomach ailments, respiratory problems, heart disease and cancer. Objectives. The present study aims to investigate current PTE concentrations in urban soils of Lagos, an example of a rapidly urbanizing megacity in a developing country. The variation in PTE (chromium (Cr), copper (Cu), iron (Fe), magnesium (Mn), nickel (Ni), lead (Pb) and zinc (Zn)) levels across different land use types was examined. Information from this study will be useful in the ranking of contaminated sites, environmental quality management, guidance for remediation, redevelopment of contaminated sites and will provide crucial information for general urban planning decisions. Methods. Five areas spread across four local government areas were selected, representing different socio-economic areas of Lagos (Victoria Island, Lagos mainland, Ikeja, Ifako-Ijaiye and Makoko). Sampling locations within the study areas were comprised of school playgrounds, roadsides, ornamental gardens, open spaces, train stations, industrial estates and dump sites. A total of 126 samples were collected. Results. The overall mean levels of PTE concentrations in this study were comparable to those found in large European cities where main pollution sources include traffic and current or former heavy manufacturing industries. Conclusions. Regulation and legislation on environmental issues, including effective solid waste management strategies and enforcement of emission standards should be emphasized in order to reduce the impact of PTE pollution on the inhabitants of urban areas in developing countries. Competing Interests. The authors declare no competing financial interests


Introduction
Soil functions as a pollution sink, but soil is also a source of pollution with the capacity to transfer pollutants to ground water, the food chain and eventually to the human body. 1,2 Over the last few decades, high levels of urban soil pollution have become a major issue. Urbanization and industrialization have resulted in the release and discharge of pollutants and other persistent toxic substances, leading to the degradation of environmental conditions. Among soil contaminants, potentially toxic elements (PTE) are of major significance because they are ubiquitous, toxic and persistent. The prolonged presence of metals in urban soils and their close proximity to human populations can greatly amplify the exposure of these populations to metal contamination via inhalation, ingestion and dermal contact. [3][4][5] Some of the elements found in soil are essential to living organisms, but can be toxic and dangerous to human health if present in soil above critical levels. The fate and transport of these elements in soils largely depend on the chemical form in which they exist. These elements associate with soil particles by different mechanisms and changes in conditions can bring about their release into the environment. The reaction mechanisms involve both physical and chemical processes including cation exchange, adsorption and desorption, co-precipitation, and organic complexation. 6,7 Chronic exposure of humans to these elements has been linked with developmental retardation, cancer, atherosclerosis and kidney damage, stomach ailments, respiratory problems, heart disease and cancer. Urban soil quality has been investigated for many different parameters and in several different ways, including studies of PTE concentrations in roadside soils, parks, school playgrounds, sports grounds, different particle sizes of urban soil, comparative studies between rural and urban soils, the influence of large cities on PTE distribution, PTE concentration in urban soil at increasing depths, and effect of Research the present study aims to conduct a comprehensive investigation of current PTE concentrations in urban soils of Lagos, an example of a rapidly urbanizing megacity in a developing country. The variation in PTE (chromium (Cr), copper (Cu), iron (Fe), magnesium (Mn), nickel (Ni), lead (Pb) and zinc (Zn)) levels across different land use types was investigated. Information from this study will be useful in the ranking of contaminated sites, environmental quality management, guidance for remediation, redevelopment of contaminated sites and will provide crucial information for general urban planning decisions.

Methods
Lagos is Nigeria's most populous city and the ninth fastest growing city in the world. 23 A recent report estimated its population at 21 million, making Lagos the largest city in Africa. 24 Lagos State is made up of 20 local government areas. For the purpose of this study, five areas spread across four local government areas were selected, representing different socio-economic areas of Lagos (Victoria Island, Lagos mainland, Ikeja, Ifako-Ijaiye and Makoko). The choice of sampling areas was based on income earnings (high, medium and low) and population densities specific to each of the areas. Victoria Island is the high income area, Lagos mainland, Ikeja and Ifako are medium income areas while Makoko is the low income area.
Sampling locations within the study areas were comprised of school playgrounds, roadsides, ornamental gardens, open spaces, train stations, industrial estates and dump sites (Figure 1). A total of 126 samples were collected in two months -February and August of 2014.
compost application on PTE load. [8][9][10][11][12][13][14][15][16][17][18][19][20][21] These studies have demonstrated that trace metal contamination of the urban environment can have long term and far reaching environmental and health implications. Lagos, Nigeria has experienced an unprecedented sporadic increase in urban migration which may be due to the fact that it serves as the commercial capital of Nigeria. 22 Over sixty percent of Nigeria's total industrial activities take place in Lagos.
In the long term, this results in environmental degradation and ultimately has the potential to endanger human health. The few studies available have demonstrated PTE contamination of Lagos soils, requiring further comprehensive investigation. However, there has been no general assessment of the PTE status of urban soils across Lagos. Previous studies have focused on PTE concentration measurements, but little effort has been devoted to determining sources and distribution across various land uses. Therefore,

Research
The sampling points in each of the locations were selected giving preference to places where people frequently relax or spend time. In the case of the school playgrounds, samples were collected at locations where children normally play. Soils were also collected from pedestrian walkways along the sampled roadsides. The industrial estates were co-located with residential layouts. The distance separating the two land-use types was about 20 to 30 m. Residential areas typically occupy about 30% of the total land space within the estates. Waste management of the estates is generally poor and most of the factories do not have installed emission control monitoring systems. Most of the sampled dumpsites are open dumps with heaps of waste littered around with no proper demarcation, less than 50 m from pathways alongside roads. Sampled railway terminals are major train terminals located in the metropolis and are accessible to users because of their closeness to residential settlements, workplaces, markets and schools.
At each sampling point, representative samples consisting of 5 -10 subsamples were obtained using a hand auger (at a depth of 0 -10 cm). Polythene bags, nylon materials, grass and leaves were removed from the soil samples. Approximately 500 g of bulked sample was taken at each location. The coordinates of sampling locations were recorded using a Garmin GP Snap 60CSx. Soil samples were kept in sealed air tight polythene bags and labelled accordingly. Soil samples were then air dried in the laboratory for 20 days, ground and sieved through a 1 mm mesh. Samples were subjected to repeated reduction by coning and quartering processes prior to digestion.

Sample digestion and analysis
Reagents used in this study were of analytical grade (Sigma Aldrich, Gillingham, UK) and a commercially prepared stock solution of multielement standard supplied by Agilent UK was used to prepare the calibrants. Soil samples were subjected to closed microwave, aqua regia digestion to determine pseudototal PTE contents. 25, 26 Replicate samples (n = 3) were digested along with procedural blanks. Inductively coupled plasma mass spectrometry (ICP-MS model 7700x instrument, Agilent Technologies, UK) was used to determine PTE concentration in digests and extracts.
Instrument consistency was assessed intermittently by ensuring the calibration standards were checked after every tenth sample measured. A clear linear fit for the calibration curve for elements was obtained with regression coefficient (R 2 ) of at least 0.999. The accuracy and precision of the analytical method employed was estimated by analyzing a certified reference material (BCR 143R; a sewage sludge amended soil) and the results were consistent with certified values.

Assessment of heavy metal pollution
A common approach in assessing anthropogenic influence in soils is to calculate the enrichment factor (EF) for metal concentrations above background levels. 27, 28 Calculation of EF reduces PTE variability and is valuable in comparing the extent of pollution in absolute terms. 29 The EF value is calculated by comparing measured PTE content with respect to a reference element. 30, 31 The EF is calculated using Equation 1 below: X a is the concentration of PTE examined in the sample, x b is the concentration of reference element in the sample, y a is the background concentration of PTE examined and y b is the background concentration of reference element.
Reference elements are normally chosen due to their lower variability and presence in trace amount in soils. 32 In this study, Fe was chosen as the reference element. The geochemistry of Fe is similar to many PTEs and its standard deviation of successive measurements in the soil samples was generally good.

Results
The quality of the pseudototal PTE analysis was assessed using BCR 143R, which is sewage sludge amended soil which was digested alongside the soil samples. The relative standard deviation for pseudo-total PTE analysis was generally less than   It should be noted that school playgrounds consistently showed the lowest PTE concentrations for all of the land uses, and therefore was the least polluted, while dump sites was the most polluted. Enrichment factor and Igeo were computed to assess the possible anthropogenic contributions of PTE to studied soils. Figure 3      Regulation and legislation on environmental issues, including effective solid waste management strategies and enforcement of emission standards should be emphasized in order to reduce the impact of PTE pollution on the inhabitants of urban areas in developing countries.

Copyright Policy
This is an Open Access article distributed in accordance with Creative Commons Attribution License (http://creativecommons.org/ licenses/by/3.0/).