Spatial ecological and health risk assessment of heavy metal contamination in surface soils from Lagos lagoon wetlands, Nigeria

Wetlands quality and spatial distribution are being threatened by anthropogenic drivers in addition to the emerging threats of climate change. In this study, selected heavy metals (Cd, Cr, Cu, Ni, Pb and Zn) in soils from Lagos lagoon wetlands were investigated to assess spatial distribution, ecological and health risks. The data obtained were subjected to statistical analysis using GraphPad 7.0 and SPSS 22.0. Spatial distribution mapping of heavy metals was performed using ArcGIS10.0 (ESRI, Redlands, CA, USA) with Kriging interpolation. Results showed that heavy metals in the soil varied significantly (p<0.05). The contamination factors (CF) were generally low with the values for Cr, Ni, Cu, and Zn very low (<1). The CF values for some of the sampling points showed that the soils are generally moderately contaminated by Pb, Cd and Cu. The modified degree of contamination of Pb (2.35) indicates a moderate degree of contamination while that of Cd (12.60) indicates a high degree of contamination of these wetland soils. The potential ecological risk index (RI) of Pb, Cd, Cr, Ni, Cu and Zn were 70.40, 2264.40, 0.68, 1.55, 13.65, and 2.29, respectively. The RI for Pb, Cr, Ni, Cu and Zn were less than 100, hence, low, while the RI value of Cd was a very high risk (RI ≥ 400). Soils from this wetland’s areas show serious significant potential ecological risk due to Cd. Additionally, children were more susceptible to the potential health risk irrespective of the carcinogenic or non – carcinogenic risk. There were no significant carcinogenic and non – carcinogenic risks for adults and children. This wetland assessment provided important information for policymaking to reduce the potential effects of soil contamination on humans and the eco-environment.


Introduction
Wetlands are distinct ecosystems that are inundated by water, either permanently or seasonally, with their characteristic vegetation of aquatic plants, adapted to the unique hydric soil [1]. They are among the most productive ecosystems on earth with unique aquatic and terrestrial communities of high biodiversity [2,3]. Degradation of the environment due to rising rapid urbanization and industrialization is now a serious concern globally with significant effects in the estuarine and wetlands acting as a sink for receiving leachates, effluents, emissions, fossil fuels, fertilizer erosion, herbicides and pesticides from agricultural run-off, and sewage and municipal wastes [4,5].
Extensive agriculture, drainage and urban development have greatly altered the natural hydrology and wetland ecosystem of Lagos State, Nigeria. Although these changes in the landscape have brought great economic prosperity to the state and her citizens, they have degraded and eliminated more than 90% of the wetlands in the state [6]. The remaining wetlands in the landscape often have their functions and biological quality impaired by natural and anthropogenic stressors, such as invasion by non-native plants, hydrologic changes, nutrient enrichment, sedimentation and pollutants [7]. The lack of strict adherence to the waste management regulations, poor urban planning and inadequate effort by governmental agencies had also contributed to improper waste disposal in them [8].
Wetland soils pollution by heavy metals has been regarded as a critical problem because of their toxicity, persistence, and bioaccumulation [9,10]. They adversely affect ecosystems, and they can be associated with direct and indirect human health risks when they pollute soils. Human health risk assessment, including non-carcinogenic and carcinogenic risk assessments, is a method that estimates the different possible heavy metal pathways that may cause harmful effects to people living close by or inhabiting them [11,12]. Three pathways that could lead to heavy metals in soils affecting humans have been suggested by several researchers to be oral, dermal absorptions and inhalation [13,14].
Metals without known biological functions (such as lead, cadmium and arsenic) and even some essential metals for human beings (copper, zinc and chromium) can cause health risks when present in excess levels. Some metals, such as Pb, As, Cd and Cr have been classified as carcinogenic elements by the International Agency for Research on Cancer [15]. Thus, it is necessary to understand the contaminant status and potential risks of these sites, closely related to human health, require serious consideration and effective measures to protect them.
The analysis and evaluation of heavy metal pollution in wetlands has become an important area of research within the field of wetland environmental pollution [16]. The use of contamination factor (Cf), degree of contamination, index of geoaccumulation, (Igeo), pollution load index (PLI), etc. are some of the conventional methods which had been developed and employed to evaluate the pollution status of heavy metals in the soil. The ecological risks assessment is part of the contemporary research in soil pollution studies and environmental management, it indicates the tendency of the adverse effects of heavy metals on the ecological health [8].
Studies have been conducted to evaluate the possible effect on human health due to exposure to metallic contaminants in soils from mining areas [14], industrial areas [17,18], agricultural areas [19,20] and wetlands [13,21]. The few available published studies on the pollution of wetland in Nigeria have reported elevated concentrations of metals as well as the associated high ecological and/or health risks [22,23]. Till date, no previous work had assessed the potential ecological and health risk of heavy metals of the wetland soil for the study area. Hence, our primary aim was to determine the distribution of heavy metals, evaluate the degree of heavy metal pollution based on contamination factor, pollution index, ecological risk indices and the health risks associated with the wetland soils.

Study Area
The Lagos lagoon wetlands form part of an intricate system of water ways made up of lagoons and creeks ( Figure 1).

Sampling and Field Data Collections
The samples were collected in these wetlands monthly for 1 year. The coordinates for all sampling points was obtained using Garmin GPS and recorded. Soils samples were kept in foil papers and polythene bags and labelled properly as was described by Jha et al. [24].

Soil sample preparation and physicochemical analyses
All of soil samples were air-dried after collection and sieved through a 2-mm nylon sieve to remove coarse debris. Heavy metals concentration was determined by inductively coupled plasmaatomic emission spectrometry (ICP-AES) as described by Zhang et al. [25].

Pollution load index (PLI), contamination factor (CF) and modified contamination degree (mCd)
Pollution load index is calculated as geometric mean of concentration factor (equation 1) value of n number of studied metals [26]. The index is based on the CF of each metal present in the soil which is expressed as equation 2.
where n is the number of metals and CF is the contamination factor.
The modified degree of contamination was estimated according to Eq. 3 [27] Where n= number of analysed elements, i=ith element and CF = contamination factor.

Assessment of potential ecological risk
Potential ecological risk assessment was to assess the degree of heavy metal pollution in soil, according to the toxicity of heavy metals and the response of the environment. is the monomial potential ecological risk factor calculated using Eq 4, where is the toxicity response factor and CF is the contamination factor:

Health Risk Assessment
Exposure assessment Non carcinogenic risk assessment The HQ is the quotient for chronic daily intake, and the HQ for a single chemical is determined by Eq 13.

= ( 13)
Where RfD is the chronic dose for the chemical [mg/kg (day)]. Hazard Index (HI) approached was used to assess the overall potential for non-carcinogenic effects posed by more than one chemical (Eq 13).

14)
Carcinogenic risk assessment The slope factor (SF) converts estimated daily intake of a toxin averaged over a lifetime of exposure directly to the incremental risk of an individual developing cancer, and it is calculated using Eq (14).

CR=ADD ×SF (Eqn 15)
Where CR id the unit-less probability of an individual developing cancer over a lifetime, and SF is the carcinogenicity slope factor (kg day/mg). Risk surpassing 1 x 10 -4 is viewed as unacceptable, risk below 1 x 10 -6 is not considered to pose significant. The values of the parameters used for health assessment are in table below.

Distribution of heavy metal and their extent in the wetland soils
The descriptive analysis of Pb, Cd, Cr, Ni, Cu and Zn are presented in The kurtosis (a measure used to describe the normality of a distribution) and skewness for all the heavy metals were generally high and significantly very high for Pb (July, Sept, and May), Ni (Aug, Oct, and Jan), Cu (Jan) and Zn (Jul, Feb, Apr and May) which indicates a non-normality of the distributions for these heavy metals and greater variation among the soils in the study area. The spatial distribution maps of the mean metal concentration in the wetlands soil are presented in figure 2 -7.
The ecological maps reveal variability in the distribution patterns of heavy metals in the water and soil in the study area. There were significant spatial variations in the soil heavy metals (p<0.05).
The mean concentrations of Cr, Cu, Zn and Ni in soil were below both the optimal and action values of the Dutch and Canadian soil quality and guidance values (SQGV) and the NESREA standard. Cd was above the optimal Dutch SQGV (1 mg/kg) while Pb was also above the optimal Dutch SQGV (85 mg/kg), and this was reflective in the metal enrichment assessment. The Insert * Table 2 Heavy metal level in the wetland soils*

Contamination (mCD) of soils 16
The Contamination factors (CF) show that generally the values for Cr, Ni, Cu, and 17 Zn in the wetlands were less than 1 (low) ( Table 3) The modified degree of contamination (mCD) in the present study is based on 36 integrating and averaging all the available analytical data for a set of soil samples. This 37 modified method therefore provided an integrated assessment of the overall enrichment 38 and contamination impact of groups of pollutants in the soil. For the classification and 39 description of the modified degree of contamination (mCD) in the sediment, the 40 following gradations are proposed: mCD <1.5 is nil to a very low degree of 41 contamination; 1.5 ≤ mCD < 2 is a low degree of contamination; 2 ≤ mCD <4 is a 42 moderate degree of contamination; 4 ≤ mCD <8 is a high degree of contamination; 8 ≤ 43 mCD <16 is a very high degree of contamination; 16 ≤ mCD <32 is an extremely high 44 degree of contamination; mCD ≤ 32 is an ultra-high degree of contamination. In this 45 study, the modified degree of contamination (mCD) indicates moderate degree of  With parts of the wetlands having very serious potential ecological risk for Cd been 77 used for vegetable production, and fishing of aquatic species in the nearby surface water, 78 these constitute current and future health risks. Of particular concern are the 79 significantly high-risk levels in the farm areas, which could negatively impact plants as  Table 4: Ecological risk factors (E i r) and the potential ecological risk index 93 (RI) of the pollutants in the wetlands soil 94

Non-carcinogenic risk assessment 96
The results of non-carcinogenic risk of heavy metal (HM) exposure in soils through 97 ingestion, inhalation, and dermal contact on adults and children are presented in Table  98 5 and 6. The contribution of ingestion route to HI was the highest at more than 75% for 99 children and 60% for adults for the daily intake. This suggests that ingestion was the 100 main exposure pathway to threaten human health. These results agreed with previous 101 reports [14,40]. 102 The HQ of ingestion, inhalation and dermal peaked for Cr and their minimal level 103 for Ni for the adult population, while the children population showed different trends. 104 In risk assessment, when HQ and HI values are below 1, there is no obvious risk to the 105 population, but if these values exceed 1, there may be some concern for potential non-106 carcinogenic effects [41]. The heavy metal HI values for children and adults were in 107 the following order: Cr > Pb > Ni > Cd > Zn > Cu and Cd > Pb > Ni > Cu > Cr > Zn, 108 respectively. The HQ and HI values for all the metals were lower than 1, which 109 indicated that there was no non-carcinogenic risk to children and adults. The health risk 110 to children due to heavy metal exposure from the soils was higher than for adults. The 111 high non-carcinogenic risk to children is mostly due to their behaviour and hand or 112 finger sucking [42]. 113 The health risk assessment demonstrates capacity to distinguish the toxic chemical 140 and various exposure pathways. However, this assessment has several inherent 141 uncertainties in quantitative risk evaluation. First of all, bioavailable concentration 142 rather than the total amounts of heavy metals can obtain more reliable risk assessments 143 for eco-environment and human health, which suggests that total concentration of 144 heavy metals potentially results in overestimation of the ADI and the resulting HI. 145 Furthermore, the widely used exposure parameters were majorly from the USEPA, 146 which may not be applicable in Nigeria. Also, there is no exposure assessment guideline 147 for human health risk assessment posed by heavy metals in soils in Nigeria. However, 148

INSERT*
our study scored the eco-environmental and human health effects based on a temporal-spatial sampling assessment, particularly, three exposure pathways and variable heavy 150 metals leading to potential ecological and human health risks in a typical wetlands area 151 in Lagos lagoon are highlighted.

Availability of data and materials 167
All data generated or analyzed during this study are… 168

Competing interests 169
All generated or analysed data during this study are included in this published article 170 and are also available from the corresponding authors upon request.