Oil exploration is the basis for Nigeria's economy and sources of development and power [1]. This intense exploration is often associated with an environmental consequence[2,3], as the Oil and Gas Industry is the most profitable industry contributing significantly to the Nigerian economy. The Niger Delta Basin is the reservoir for crude oil deposits, recognized for its accumulation of liquid hydrocarbon for millions of decades [4,5] suggested that heavy metals have been found in crude oil, and oil spills contaminate the environment and exposed humans to heavy metals toxins. Petroleum industries added significantly to Nigeria's Gross Domestic Product (GDP); an increase in oil exploration increased the rate of exposure of these metallic toxin s to humans. Despite the expected environmental consequences, the government and stakeholders in the oil industry neglect the impacts of heavy metals on the environment. 

Evaluating the health impacts is necessary because the human system's toxicity level is directly proportional to heavy metal consumed [5] These involved direct and indirect contacts. For instance, this finding was supported by [6] that heavy metals toxins are trapped by plants and consumed directly by humans, leading to organ and tissues disruption as well as environmental pollutants.  Apart from oil exploration, anthropogenic activities, such as mining, smelting, industrial activities, and some agricultural pursuits bring humans into contact with these metals. It also documented its exist naturally in nature. Also, conducted haematological and histopathological tests. It was observed that carp (Catla Catla) exposed to different concentrations of copper (Cu) and cadmium (Cd); therefore, red blood cells, haemoglobin (Hb), hematocrit (Hct), lymphocyte, liver, and monocyte were affected. 

[7]  Relates that all living things experience the negative impacts of heavy metals. Hence, the harmful effect manifests when the intake amount is higher than the amount released from the body. Thus, humans become susceptible through absorption, inhalation, and ingestion—this metallic toxicity triggered, which results in an overload beyond the human body's natural detoxification capacity. [8] Stated high solubility potentials are characterized by heavy metals, which prompted its high toxicity rate; it oxidizes and traps human tissues and the physical environment, affecting all living and non-living things. 

 Despite the relativeness in the environmental discharge of the metallic toxin,  oil spills account for a substantial release of heavy metals into the environment [9-11]; this depicts why the evaluation of heavy metal contamination within the Niger Delta should not be overruled.   

Review on Heavy Metals

Scholars have defined heavy metals in different research fields  classified heavy metals as elements with relatively high densities, atomic weights, or atomic numbers.  [12] emphasized heavy metals environmental sustainability and categorized heavy metals as a group of elements known as metalloids with high toxicity and high environmental degradation [12,13] Although heavy metals occurred naturally, their numerous industrial, residential, agricultural, medical, and technical applications have resulted in their widespread diffusion in the environment, raising worries about their possible health and environmental repercussions [14]. Heavy metals have been recognized as a source of pollution in the soil and continue to be an active research area among environmental researchers investigating the best approaches to enhance the life of living things. [13] examined the heavy metal content of crude oil from Bonny Light, Nigeria. The examination was carried out using Atomic Absorption Spectrophotometric analysis. Its purpose was to figure out how much heavy metal existed in the Niger Delta. Metals such as Cd, Cu, Ni, V, and Pb were detected. Despite the lack of Cd, the percentage increase in metallic concentration was Pb > Cu > Ni > V. Furthermore, the significant rise in Pb and Cu percentages in the area validated anthropogenic occurrences. Secondly, it was established that the heavy discovery depended on the geological formation in the area. In assessing the health risk of heavy metals in humans and fish populations [15], the health concerns associated with six different heavy metals were assessed: Cr, Cd, Fe, Mn, Cu, and Pb. The non-carcinogenic human health concerns were shown to be strongly influenced by Pb and Mn. On the other hand, Cd was thought to have the most tremendous potential for both carcinogenic and non-carcinogenic health impacts. Cd was also essential in the bioaccumulation factor of the fish samples, with the liver having the highest bioaccumulation capacity. Additionally, children were found to have significantly more significant effects (both carcinogenic and non-carcinogenic) than adults, implying that aquatic habitats should be monitored and restored. More to [6] experimented on the industrial effluence discharge of heavy metals in the aquatic ecosystem resulting in bioaccumulation in plants and animal tissues, causing environmental toxicant, food contamination and reduction in the food quality. [15] suggested the heavy metal assessment and the social status of the communities. Therefore stipulated that carcinogenicity of heavy metal is more severe in a poor neighbourhood and children are more susceptible. Thus added that the level of the susceptibility depended on the soil type and land used. This vulnerability rate is applicable in the Niger Delta Region, where oil exploration pre-exposed the host community to heavy metal contamination, increasing the susceptibility of the oil-producing communities [17]

Environment Impact of Heavy Metals

Everything in our environment and the activities that take place within it are referred to as the environment. Heavy metal contamination impacts all environmental components and damages ecological values, according to Because they are all interconnected, they are recognized as a significant source of soil contamination, water bodies, plants, and humans. These contaminants influenced the soil microflora, resulting in microbial population reduction and changes in the microscopic soil structure . Pb in the soil, in particular, obstructs the photosynthesis of green plants [18,19]. It also affects water absorption, causing the colour of the leaves to change [20]. In addition, it produces stunted leaf growth and brown roots that are short. These factors contribute to low crop yields and farm produce production [21]

The availability of enzyme assays is a microbial interaction product and serves as an indicator of soil fertility [22].  It has been observed that these elements do not only result in an adverse effect on plants. Still, it causes a significant alteration in the soil's size, composition, and microbial setup [23] . For example, [21]  declared that lead and silver in the soil inhibits microbial growth, whereas zinc facilitates microbial growth.  Heavy metals are therefore considered a key source of soil pollution.  Cu, Ni, Cd, Zn, Cr, and Pb have been identified as primary pollutants among other heavy metals.  The soil composition and properties influence and determine the extent to which metals affect the soil's biological and biochemical properties [24,25] Discovered that heavy metals indirectly affect the enzymatic activities of the soil microbes by causing a shift in the microbial setup that promotes enzymes' synthesis.  [26]  posited those heavy metals brought about a decrease in the bacterial diversity with a visible increase in soil actinomycetes and possibly a reduction in bacterial biomass and diversity in contaminated soils.

[24] The enzymes' activities are affected by metals' interaction due to the chemical affinities of enzymes present in the soil.  It was revealed that Cd remains the more toxic element because of its high mobility and low relationship with soil colloids. Likewise, Cu inhibits b-glucosidase activity more than cellulose activity. Generally, an increase in heavy metals concentration negatively affects soil microbial activities such as respiration rate and chemical activities, valuable soil pollution indicators [27]

Although heavy metals are toxic to the plant, [28,29] described that some plant species could withstand high metallic toxicity. These are known as "hyperaccumulators." The metalliferous are stored in the leaves, stem and areal region of the plants and detoxified, thereby becoming harmless to the adverse soil condition.  It was noted on the importance of caution and concern at the risk of transference from most vegetable species to humans. Also, compost is used mainly to improve agricultural yield and contain heavy metals [30,31] hypothesized on the used wastewater or contaminated water prone to heavy metals as a source for plant irrigation.  As mentioned, there was a significant risk and threat to animal and human lives when plants' uptake of heavy metals accumulated along the food chain. The plant absorbs heavy metal through its roots, and the plant's roots remain the main route of the entrance of heavy metal to the food chain [32,33]

It is established that the absorption and subsequent accumulation depend on multiple factors such as organic matter, temperature, pH and nutrient availability. For example, [34] realized that the absorption and accumulation of Cd, Zn, Cr and Mn in Spinach was higher during summer periods. Low Pb levels can hinder essential plant processes like photosynthesis, mitosis, and water absorption. In contrast, high Pb levels can reduce soil productivity Bhattacharyya et al., Heavy metal's broad categorization stems from its extreme toxicity and photo-toxicity. These often result in low plant growth, poor yields, chlorosis, insufficient nutrient intake, disrupted metabolism and a decreased ability to fixate molecular nitrogen in legumes. Hence, an increased heavy metals content induces a delay in seed germination [21]

The aquatic organism's insufficient oxidative mechanism is equated to heavy metals prevalent in the marine environment, even in small amounts. These metallic reactions have been substantially linked to ecotoxicology. Significantly, metals are not affected by bacterial breakdown; hence, they remain persistent pollutants indefinitely [35,36] investigated industrial waste in the maritime environment. The presence of Cd, Mn, and Pb in industrial effluent released into aquatic bodies has been determined. According to the Environmental Protection Agency, these were at high concentrations that exceeded the permissible level.

Aquatic plants also have higher Zn, Cu, and Mn concentrations. When released into the marine environment, heavy metals are mostly bounded to particulate matter, which later settles and becomes mixed up with sediments. This additive makes the surface sediments a storehouse for heavy metals pollutants taken up by rooted aquatic macrophytes [37,38]

According to[17,39], heavy metal pollution has also affected the diatom community structure. Once heavy metal gets integrated into the aquatic system, the natural aquatic food chain becomes affected. Humans are not left off the bio-magnification process[15,40] . In line with [35] heavy metals can produce reactive oxygen species (ROS), harmful to fishes and other aquatic species. There are five established ways heavy metals enter into a fish through food, non-food particles, gills, water intake and respiratory activities of the gills. Once absorbed into the fish, usually bound with proteins which are consequently accumulated to a varying extent in different organs within the fish [36]

Humans make part of the food chain as a secondary consumer. Due to heavy metals from plants are susceptible to health risks—it points to the carcinogenic properties and the increased cancer rate within populations.  Food crops remain the most excellent exposure for humans. Planting plants in areas contaminated by heavy metals represent potential dangers [41,42] reviewed heavy metals toxicity and the affected organs.  These include the liver, placenta, kidneys, lungs, bones and brains. The affected humans often displayed symptoms such as vomiting, nausea, muscular weakness, and abdominal cramps. 

[43] The reactive stage of heavy metals that is harmful to human health is specified. The metallic concentration and oxidation stage, the leaching process, chemical transformations, and deposition ways to harm the environment are examples of these approaches. They pointed out that when heavy metal is absorbed through food, it mixes with the body's biomolecules, such as proteins and fats. As a result, stable biotoxic chemicals, body mutilation, and loss of body function and bioreactions are produced. Severe exposure can cause pulmonary oedema, which can lead to mortality.

[44] Humans are mostly exposed to copper intake through food and water.  Lead (Pb) is very toxic to humans, found in crude oil. It affects humans, both physiologically and neurologically.  Lead poisoning affects the kidney, reproductive health, liver and brain [45] It was found that lead poisoning also results in the inhibition of haemoglobin and a possible chronic impairment to the central nervous system and the peripheral nervous system fatigue, gastrointestinal disorders, high blood pressure, difficulty in pregnancy and muscle and joint pains [46,47,43] stated that lead poisoning affects intellectual development in children. Chromium (Cr) remains one of the most abundant elements globally. The toxic capacity of chromium is due to its ability to diffuse through membranes and oxidize molecules  [48]. Shaffer et al., this oxidation effects increase its toxicity potentials in both plants and animals and increase the rate of carcinogens [49]

For a comparative review of the eight existing heavy metals data from chosen oil fields, allowable limits were retrieved from the Department of Petroleum Resources (DPR) Standard, EGASPIN. The selected Niger Delta oil fields include 10" Ekulama 1 Pipeline in Freetown, the 24" Nembe Creek Pipeline in Kalibiama, the 16" Egbema-Assa Pipeline in Umudike, the 8" Isimiri Pipeline in Obiakpu, the Alakiri 6" pipeline in Amabakiri, the Cawthorne channel Akaso well 13L, and the OB 20/64 OML 58 locations including their Geo Position Coordinates. The Oil Spill and Detection Response Agency (NOSDRA) provided this secondary data. Lead, Arsenic, Barium, Cobalt, Cadmium, Nickel, Zinc, Copper, Chromium, Mercury, and Vanadium are among the heavy metals discovered. The distribution of heavy metals across selected oil spill impacted areas were mapped using Quantum Geographic Information System(QGIS 3.18.3) using NOSDRA preexisting data. NOSDRA also provided data on heavy metal levels collected after prior crude oil-damaged locations were cleaned up, which formed the baseline data. Sorting and computing the mean concentration of each heavy metal for each area as part of the data pre-processing, on the other hand, yielded Nigerian administrative boundary data. These were also saved as CSV files that could be imported into QGIS. Using the Data specified override tied to the size input box under Symbology in QGIS, the loaded heavy metal distribution data reflected proportionate size. The Niger delta map, on the other hand, was extracted from the Nigeria administrative boundaries using the Vector Selection group's Extract by Expression tool. The resultant layers were then overlaid to produce the heavy metal concentration distribution map. The amount of                                   each heavy metal present was quantified and magnified in the GIS tools in terms of sizes. The heavy metal concentration maps are shown in figure 3. 

To further access the standard of mitigation exercise regarding the applied remediation methods, the OB 20/64 OML 58 pollution data before remediation shown in Table 1 and after remediation in Table 2 were examined. 

Table 1: Heavy Metal Distribution before Remediation

Table 2: Heavy Metal Distribution after Remediation

Figure 1: Flowchart for Heavy Metal Concentration Maps

concentrations are below the DPR Intervention levels, according to the findings. However, in line with the heavy metals tables in Tables 1 and 2 and the heavy metals quantification in Figure 3, the quantities of two heavy metals, Cu and Ni, were marginally more significant after remediation than before the intervention. It implies that the remediation technique failed to reduce the heavy metal content or concentration to the point where further action was unlikely to reduce it. As a result, under the context of EGASPIN, the lands are free of heavy metals. Nonetheless, depending on the sensitivity of various locations, a site-specific risk study may be required to identify tolerable levels. Furthermore, heavy metal pollution has an environmental impact due to the gradual accumulation over time, which is harmful to human health and has negative environmental consequences. Figure 2 shows the GIS result for the Heavy Metal known as the Niger Delta Heavy Metal Maps. Figure 3 is the GIS Heavy Metal Distribution Map called the Niger Delta Heavy Metal Concentration Maps. Based on the 11 different heavy metals discovered, the GIS result assesses the magnitude of the Niger Delta Oil Spilt Sites. All of the heavy metal.

Figure 2: The Heavy Metal Map of the Niger Delta

Figure 3: The Heavy Metal Concentrations of the Niger Delta after and before Remediation

Residents living next to the oil spillage points nor the immediate surroundings are likely affected by the heavy metals pollution due to continuous exposure; as such have a shorter life expectancy and are less healthy. Therefore, the Nigerian regulators' level of permissibility for heavy metal pollutants in assessing the environmental nor health consequences should not be utilized as the basis for determining the impacts. The gradual addition of its dosages, the accumulation of contaminants over time is essential, also known as the residual effects. As a result, it's critical to think about effective management or eradicate these pollutants as part of the Environmental Management System (EMS). The importance of abstinence rather than indulging, public health and environmental health education for residents are necessary to understand the importance of self-restraint rather than ignorant —the resultant effect results from the gradual accumulation and frequency of occurrence. Subsequently, future environmental sustainability requires remediation, restoration, and proper planning. Therefore, the consequences of heavy metals within the oil-producing communities should be part of the preparedness in oil spill management. 

The authors declare that they have no conflict of interest

No funding sources

The study was approved by the Glasgow Caledonian University.

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