Determination of heavy elements in agricultural regions, Saudi Arabia

2.4.1 Physicochemical properties of soil samples

To identify the presence and levels of heavy metals in the soil samples, diverse analytical techniques were utilized. These methods encompassed atomic absorption spectroscopy employing both flame and furnace procedures, along with ICP analysis. Over time, these methodologies have been developed and perfected to specifically address the detection and quantification of metals in environmental samples. The efficiency of the suggested approach in evaluating soil acidity and contamination levels was evaluated by analyzing various essential parameters. This thorough assessment involved scrutiny of several factors.

2.4.2 Statistical analyses

To determine the importance of and notable distinctions within the soil samples, statistical analyses were conducted using an ANOVA test.

2.4.3 Heavy metal pollution assessment

The PLI was calculated [25] using the following formula:

where PLI represents the pollution level, P _i denotes the contamination index for element i, and n denotes the total number of elements under consideration.

The individual contamination index (P _i ) for each element was calculated as follows: P i = C i / S i , where C _i is the concentration of the sample element, and Si represents the background concentration of the element in the sample.

Interpreting the PLI values, a PLI ≤ 1 indicates an uncontaminated site, PLI ≤ 1 signifies a slightly polluted site, 2 < PLI ≤ 3 suggests moderate pollution, 3 < PLI ≤ 5 indicates substantial pollution, and PLI > 5 signifies a very high level of pollution [26,27].

2.4.4 Extent of metal pollution by quantitative measurements

The extent of metal pollution is evaluated based on the geoaccumulation index (I _geo) [28] as follows:

In this equation, C _n is the total concentration of the element “n” in the soil, and B _n is the average concentration of element “n”. The constant “1.5” is used to mitigate the potential impact of variations in soil background values.

Interpreting the I _geo values, the following categories apply: I _geo ≤ 0 indicates an unpolluted condition, 0 < I _geo ≤ 1 suggests a condition ranging from unpolluted to moderately polluted, 1 < I _geo ≤ 2 refers to moderately polluted, 2 < I _geo ≤ 3 implies a condition ranging from moderately to high polluted, 3 < I _geo ≤ 4 signifies a high polluted environment, 4 < I _geo ≤ 5 denotes a condition that can vary from strongly to extremely polluted, and I _geo > 5 indicates an extremely highly polluted state [26,27].

2.4.5 Soil enrichment evaluation

An indicator known as the EF is used to evaluate the contaminant deposition on the soil. EF is calculated by normalizing the specific element concentration in the topsoil, typically iron [25,26,27], in relation to the reference element concentration. The EF is calculated as follows [28]:

where EF is the enrichment factor, C _x represents the element of interest concentration in the sample, and C _reference is the reference element concentration; in most cases, Fe is used for normalization purposes.

EF < 2 indicates a low deficiency. When 2 < EF < 5, it suggests a moderate enrichment range. A significant level of enrichment is indicated when EF falls within the range of 5 < EF < 20. Strong enrichment is represented by EF values in the range of 20 < EF < 40. EF values exceeding 40 are indicative of extremely high enrichment.

3.1.1 Physicochemical evaluation

As detailed in Table 1, the pH levels exhibited a range from 5.1 to 6.4, with an average value of 5.8. These pH measurements collectively indicate the acidic nature of all soil samples analyzed. Notably, these pH conditions are unfavorable for agricultural purposes [29]. Lowering the soil pH could potentially enhance the essential nutrients within it. It is worth mentioning that pH plays a pivotal role in the transport of metals within the soil matrix [30].

In terms of EC values, the recorded measurements spanned from 2.14 to 7.89 µS/m. A similar trend was observed when assessing TDS, where an increase in TDS values correlated with higher EC values in the soil. The data in Table 1 further illustrate the significant variability in TDS values across the three sampled sites. Notably, the great TDS value is estimated in the Qassim region’s soil, measuring at 1,710 mg/L. In contrast, the TDS value markedly decreased in soil sampled farther from the Medina region.

Additionally, the analysis revealed Cl^– ions in the soil, with the highest percentages recorded in the Qassim region, amounting to 283.9 mg/L. This trend in Cl^– concentrations remained consistent across various soil types, including sandy, clay, and yellow soils, as well as both topsoil and soil samples obtained at a depth of 50 cm. It is worth noting that irrigation with groundwater containing elevated chlorine levels can adversely affect plant growth. This is because soil does not readily absorb chlorine, causing it to be transported along with soil water. As a result, plants absorb and accumulate chlorine in their leaves, potentially leading to adverse effects on overall health and vigor [29] (Table 2).

3.2.1 Analysis of the Qassim agricultural soil

Table 3 presents the heavy metal concentrations observed in the Qassim agricultural soil. Iron (Fe) exhibited the highest concentration at 700 mg/kg, while beryllium (Be) had the lowest concentration at 0.15 mg/kg. Notably, elevated concentrations were observed for copper (Cu), manganese (Mn), and titanium (Ti). Selenium (Se) had a significantly higher average concentration of 3.9 mg/kg, which is approximately 13 times higher than the background level of 0.3 mg/kg. It is worth mentioning that calcium (Ca) exceeded the permissible soil limit, possibly linked to the use of phosphate fertilizers, a significant factor contributing to the increase in cadmium concentration [31,32].

In general, most mineral levels in the soil remain within permissible limits [33], but mercury (Hg) exhibits a higher concentration of 0.325 mg/kg, surpassing the global average metal concentration by a factor of 11 [33]. The relatively higher Hg concentrations can be attributed to anthropogenic factors such as fossil fuel combustion, incineration of metal waste, and mining activities [34].

Therefore, when considering the order of increasing mineral concentrations, it follows the sequence: iron > copper > titanium > manganese > selenium > nickel > chromium > molybdenum > lead > cobalt > zinc > arsenic > mercury > cadmium > vanadium > beryllium.

3.2.2 Metal analysis of the Medina agricultural soil

Table 3 provides insights into the concentrations of heavy metals in agricultural soil in the Medina region. The mineral concentrations within the soil of the Medina region exhibited 0.16 and 2.75 mg/kg. Notably, the average iron (Fe) concentration stood out, reaching 1,400 mg/kg, signifying its prevalence among the studied minerals. It is important to note that several reports have highlighted that the elevated presence of Fe in soil is associated with living organisms [35].

Another noteworthy finding was the substantial concentration of titanium (Ti) at 19.5 mg/kg in the soil. Mn emerged as the third most prominent mineral in Medina (13 mg/kg), which matches with other work in agricultural practices [36].

In addition to iron, elevated copper (Cu) and nickel (Ni) percentages were recorded in the soil of the Medina region, related to the estimated 10 and 4.45 mg/kg, correspondingly. However, it is crucial to highlight that all of these elements in the soil remained within permissible limits, except for mercury (Hg), selenium (Se), cadmium (Cd), and molybdenum (Mo). Their concentrations in the soil were evaluated at 0.36, 0.5, 0.25, and 2.5 mg/kg, respectively, indicating that they exceeded the permissible limits.

In terms of the order of increasing mineral concentrations, it followed this sequence: iron > titanium > manganese > copper > nickel > lead > molybdenum > chromium > copper > zinc > arsenic > selenium > mercury > cadmium > vanadium > beryllium. These findings provide valuable insights into the metal percentages in the soil of the Medina region and their adherence to permissible limits, except for a few exceptions, highlighting the importance of monitoring and managing soil quality to mitigate potential environmental risks.

3.2.3 Metal analysis in the Al-Kharj agricultural soil

For the metals in the Al-Kharj, the evaluated percentages are 800, 10, 9.5, 4.55, and 4.05 mg/kg for Fe, Mn, Cu, Ti, and Ni, respectively. The metal order is iron > manganese > copper > selenium > titanium > nickel > molybdenum > lead > chromium > zinc > copper > arsenic > mercury > cadmium > beryllium > vanadium.

All elements were within permissible limits except for mercury (Hg), selenium (Se), cadmium (Cd), and molybdenum (Mo). The concentrations of Cd, Se, and Hg exceeded the normal levels, with cadmium, selenium, and mercury ratios being 14, 20, and 4 times higher than the permissible concentrations, respectively.

Table 3 provides an overview of metal types and their concentrations in Medina, Al-Kharj, and Qassim.

3.2.4 Comparison of mineral concentrations in different regions

The mineral concentrations in the three regions are presented in Table 3 and Figure 1. According to this analysis, the sequence of minerals is as follows: beryllium < vanadium < cadmium < mercury < selenium < arsenic < zinc < cobalt < chromium < molybdenum < lead < nickel < titanium < copper < manganese < iron. This order is attributed to land irrigation involving fertilizers containing minerals and other agricultural practices.

Figure 1

The Qassim soil, Medina, and Al-Kharj regions relative to the mineral concentration (column standard error 1.1%).

Iron (Fe) emerged as the most abundant mineral among those studied, with concentrations ranging from 700 to 1,400 mg/kg. The medina has the highest concentration of iron, which varies significantly depending on soil type and local factors.

Manganese (Mn) had a concentration of 13 mg/kg in Medina soil, followed by Al-Kharj and Qassim. Mn is an essential nutrient, and a daily intake of a small amount is crucial for maintaining health.

Copper (Cu) was identified as the third most abundant metal in the studied soils (9–10 mg/kg). These percentages were below the permissible limit, indicating compliance with the safety standards. Overall, these findings highlight the role of natural factors and agricultural practices in determining these concentrations.

The presence of titanium in the soil of Medina was particularly noteworthy, with a concentration of 19.5 mg/kg. This was a surprising finding, considering that the concentrations of titanium in the soils of the Qassim and Al-Kharj regions were significantly lower, measuring 6 and 4.55 mg/kg, respectively. Importantly, titanium in the Medina soil remained within permissible limits. This discrepancy in titanium concentrations among the regions highlights the variability in soil composition and natural factors influencing mineral distribution.

Both nickel (Ni) and chromium (Cr) were found to be within safe and permissible limits. The main concentrations of nickel and chromium are 3.8–4.45 mg/kg and 1.9–2.25 mg/kg, respectively. Medina soil exhibited the highest concentrations of both Ni and Cr, while the Qassim soil had the lowest concentrations. Exposure to chromium can occur from both natural and industrial sources, emphasizing the need for continued monitoring and assessment of soil quality to ensure the safety of agricultural practices and environmental health [37,38,39].

The concentrations of nickel and chromium in human inputs, such as manure and fertilizers, were relatively low compared to the normal ratios. The percentage of Zn in Medina is within safe and standard levels (1.4 mg/kg) [40], while V has a lower ratio than the normal percentage. Additionally, the estimated value for Pb is 2.75 mg/kg, which is considered an acceptable ratio [41]. The mineral content is 0.55–0.7 mg/kg, representing a low ratio relative to standard percentages [42]. The cobalt ratio is 1.85 mg/kg, indicating a low ratio compared to standard percentages [43].

The presence of cadmium in the soil is notably elevated compared to the average concentration of metals estimated by the WHO. The typical range for metal concentration in soil, according to WHO standards, is between 0.2 and 0.25 mg/kg. In contrast, the Cd levels in this soil are four times higher than the safe limit. This heightened cadmium content can be attributed to a combination of natural processes and human activities that introduce cadmium into the environment.

Cadmium is an element known for its extreme toxicity, and exposure to it can have severe health implications for humans. Even relatively low exposure levels can lead to disease symptoms. Cadmium can cause anemia, a diminished sense of smell, and cardiovascular issues. These health risks underscore the importance of addressing and mitigating cadmium contamination in soil to safeguard the environment [44].

Among the minerals examined, Mo exhibited the highest concentration in the soils of both Medina and Al-Kharj. It reached levels of 2.5 and 2.3 mg/kg, respectively. Notably, in these two regions, the Mo concentration surpassed the limit set for soil, while in the Qassim region, its content remained below the safe limit. Molybdenum has nutritional effects on both animals and plants. Imbalances, whether in deficiencies or excessive levels, can lead to health problems. Therefore, there is a significant concern regarding elevated molybdenum levels, particularly in terms of its impact on copper absorption in the human body, which can result in skeletal malformations [45].

Selenium, another mineral, has the potential to be toxic to the environment in recent decades. In the studied soil samples, Se concentrations ranged from 0.5 to 6 mg/kg. Notably, the levels of selenium exceeded the recommended limits for soil, with the Al-Kharj area exhibiting levels 20 times the permissible limit, while Medina had the lowest selenium concentration. These Se levels may be attributed to the ratio of selenium in fertilizers, and heavy rain can also influence the selenium content of the topsoil [46].

The mercury concentration in the soil across the three studied regions is within the range of 0.32–0.41 mg/kg. All three regions exceeded both the permitted and recommended levels of mercury in the soil. A high concentration of mercury was observed in the Al-Kharj area, surpassing the recommended level by a factor of 14. When comparing the concentrations of these elements across the three regions, it is apparent that Medina had the highest mineral content in its soil, even though it remained within permissible limits. Conversely, among the three regions, the Qassim region exhibited the lowest mineral content. The highest mineral concentrations are typically found in urban, suburban, and rural areas [47].

Statistical analysis indicated that the one-way analysis of variance did not yield differences (P > 0.05) in the element averages across different agricultural areas. This suggests that there are no significant disparities in the mean concentrations of these elements based on the various agricultural zones studied.

3.2.5 Statistical analysis

Table 4 provides data on the statistical analysis of various metals in nine agricultural soils from three regions of Saudi Arabia. A careful examination of Table 4 reveals that when we compare the range of concentrations of these metal ions to their respective means, most of the metals do not exhibit significant variability. This lack of substantial variability suggests that these metals are present in the soil samples at levels that do not raise concerns regarding pollution or excessive accumulation.

However, it is crucial to note that there are exceptions to this general trend. Specifically, in the case of cadmium, molybdenum, selenium, and mercury, the range of concentrations is notably wider when compared to their respective means. This disparity implies that these particular metals display a higher degree of variation in their distribution within the soil samples. Such variations could be indicative of localized pollution sources or natural factors that lead to the uneven distribution of these metals in the soil. Consequently, this observation highlights the need for further investigation and monitoring in areas where cadmium, molybdenum, selenium, and mercury concentrations are elevated, as this could signify potential environmental issues or health hazards.

3.2.6 Comparison of the sublayers and the surface of the soil

The limited minerals in the soil result in low mobility of heavy elements within it. This low mobility contributes to a high degree of uniformity in the concentration of these elements in the soil, particularly in arid and semi-arid regions. Figure 2 shows a comparison of heavy metal concentrations in agricultural soil (surface minerals and other minerals from a depth of 50 cm). These metals can be categorized into three classes based on their behavior with depth. The first class comprises elements whose concentrations remain relatively consistent regardless of depth. This group includes mercury, nickel, vanadium, cadmium, and molybdenum. The second class consists of minerals that exhibit a decrease in concentration as they move deeper into the soil. Selenium and nickel belong to this category. In the third category, we found metals whose concentrations increase as one goes deeper into the soil. This group includes lead, arsenic, zinc, copper, iron, manganese, chromium, and titanium. It is important to note that iron has the highest concentration in all the samples collected in this study. Arsenic, for instance, is primarily introduced into the soil through pesticides and fertilizers. It is noteworthy that statistical analysis of the data reveals that the significance level (P > 0.05) suggests no variation in mineral concentrations from the surface soil samples and those obtained at a depth of 50 cm. This indicates that the uniformity of the metal distribution in the soil is true regardless of the depth at which the samples are collected.

Figure 2

Mineral (mg/kg) from the surface and subsurface (depth 50 cm) (column standard error 1.2%).

3.2.7 Comparison of soil types (clay–sandy–loam) relative to mineral concentrations

Figure 3 illustrates the mineral concentrations relative to the types of soil (clay–sandy–loam), in which all the observed minerals are present in these various soil types except arsenic, which was absent in sandy soils.

Figure 3

Metals (mg/kg) for (sandy-clay-loam) soil (column standard error 1.5%).

In sandy soil, certain minerals were detected at low concentrations, including Mo (molybdenum), V (vanadium), and Be (beryllium). Some minerals were found at medium ratios, such as Mn (manganese), Ni (nickel), and Se (selenium), while others were present at high ratios; titanium, iron, and copper represent these high ratios. Se, Cd, and Hg were observed at 11, 9, and 3.

Clayey soil exhibited higher mineral concentrations than sandy soil, reaching levels of 17.5, 11.5, 1,250, 11, and 8mg/kg; manganese, copper, iron, titanium, and nickel were observed, respectively. However, it is worth mentioning that all minerals in clayey soil remained below the ratio specified by the WHO for soil, considering other metals that exceeded these ratios by 12- and 16-fold, such as Hg and Se, respectively. Yellow soil displayed mineral concentrations that closely resembled those in sandy soil, with the primary distinction being higher levels of Fe, Mn, Ni, and Cu. In yellow soil, Fe was notably elevated at 700 mg/kg, while Mn, Ni, and Cu were present at concentrations of 6, 3.8, and 9 mg/kg, respectively. Similar to sandy soil, all elements in yellow soil remained within permissible limits, except for Hg, Se, and Cd, which exceeded these limits at 0.32, 3.9, and 0.2 mg/kg, respectively.

Upon comparing these soil types, it became evident that the yellow soil closely resembled sandy soil in terms of mineral content, with the main difference being the higher levels of certain metals. In contrast, the clayey soil exhibited the highest mineral content among the soil types studied.

Statistical analysis of the data revealed no differences in the concentrations of the studied minerals among sandy, clayey, and yellow soils. This suggests that mineral distribution remained relatively consistent across these soil types.

3.2.8 Comparison of agricultural, sewage-contaminated, and industrial soils relative to metal concentrations

The variation in the soil will affect the mineral ratio, so sewage-contaminated soil was used approximately 50 m away from a sewage stream in the Medina region and industrial soil (third industrial city, Riyadh).

Additionally, agricultural soil samples were collected from the Qassim region for comparison. Figure 4 presents a comparative analysis of mineral concentrations in three types of soil: agricultural, industrial, and polluted.

Figure 4

Comparison of heavy metal ratios in agricultural, industrial, and sewage-contaminated soils (column standard error 1.2%).

Notably, there was a significant disparity in mineral concentrations based on the type of soil (agricultural, industrial, or polluted). The study revealed a substantial increase in mineral content in soils with sewage water pollution compared to both industrial and agricultural soils. When arranging the soil types in terms of their mineral content, they can be ranked as follows: polluted soil < industrial soil < agricultural soil.

Furthermore, when assessing the concentrations of minerals across these soil types, a significant accumulation of iron was observed in all three categories (agricultural, industrial, and polluted). This iron accumulation was particularly pronounced in soil contaminated with sewage water, where iron concentrations reached as high as 5,000 mg/kg. In industrial soil, iron levels were slightly lower, measuring at 1,100 mg/kg, while in agricultural soil, they were at 700 mg/kg.

These findings highlight the substantial differences in mineral composition among these soil types, with polluted soil displaying the highest mineral content and a particularly notable buildup of iron. This information underscores the importance of monitoring and managing soil quality, especially in areas exposed to contamination from industrial or sewage sources, as it can have significant implications for environmental health and agricultural practices.

It is important to note that some studies have indicated exceptionally high levels of iron in soil, raising concerns about its potential carcinogenic and mutagenic effects on the health of organisms.

Manganese, on the other hand, ranks second in terms of its presence in all soil types studied. It is particularly abundant in soils near sewage water sources, with concentrations reaching up to 100 mg/kg. In comparison, industrial and agricultural soils contain lower manganese levels, measuring at 9.5 and 6 mg/kg, respectively (safe ratio). Some agricultural applications emphasize the importance of manganese in soil management and nutrient balance.

Copper, while essential in trace amounts, can be highly toxic if its concentration exceeds the permissible limits. After manganese, copper exhibits the highest concentration in all the soil types under investigation. Notably, the Cu concentration increased suddenly to 28.5 mg/kg in the pollutant regions, but still safe (<30 mg/kg), relative to Lindsay in 1979.

Titanium concentrations in all soil types were relatively high, ranging from 6 to 19 mg/kg. Importantly, these levels were within safe limits for the soil. Titanium is a notable component of soil and does not raise concerns about exceeding acceptable concentrations.

The concentration of nickel in both industrial and agricultural soils exhibited similar levels, measuring at 3.8 and 3.25 mg/kg, respectively. In contrast, there was a notable disparity in polluted soils, where the concentration of nickel reached 10.5 mg/kg. It is worth noting that this higher concentration of nickel in polluted soil remained within the safe ratio for soil.

Within agricultural soil, chromium concentrations range from 1 to 9 mg/kg, reaching 7 mg/kg in specific localized areas. Importantly, all these concentrations remained below the permissible limit for soil.

In contaminated soils, the highest recorded zinc concentration is 9 mg/kg, which is within the safe limit for soil. Cobalt levels in agricultural soil range from 1.55 to 1.05 mg/kg, extending to 5.5 mg/kg in other polluted regions, still maintaining a safe ratio. Vanadium and beryllium exhibit concentrations of 0.15–1 mg/kg, indicating a very limited ratio within the safe range. Overall, the soil samples in this study contained all types of minerals at concentrations lower than the average general concentration of minerals typically found in soil.

Figure 4 shows an array of data for various mineral concentrations. However, five minerals require special attention because their levels exceed the recommended limits.

Arsenic: In industrial soil, arsenic levels reach 60 mg/kg, which is a high risk relative to the WHO, which is typically around 5 mg/kg. In sewage-contaminated soil, arsenic concentration also exceeded permissible limits, reaching 13 mg/kg, whereas in agricultural soil, it remained below the permissible limit of 0.6 mg/kg.

Molybdenum: In both sewage-contaminated and industrial soils, molybdenum concentrations surpassed permissible limits. However, in agricultural soil, molybdenum levels are within safe limits.

Selenium, mercury, and cadmium: These three minerals also exceed permissible limits in different soil types. Their concentrations were notably higher than the recommended levels, with selenium, mercury, and cadmium being up to 20, 17, and 4 times higher, respectively, in industrial soil than the permissible limits for soil. These percentages decreased in the agricultural soil.

3.2.9 Comparison of current results for metal concentrations in Saudi with previous studies

This comparative analysis is encouraging as it highlights the present metal concentrations relative to other regions in Saudi Arabia (Table 5). Nickel (Ni), zinc (Zn), lead (Pb), cobalt (Co), and chromium (Cr) exhibit lower levels compared to previous regions such as Riyadh and Al-Kharj. Copper (Cu) maintains a safe ratio in the current estimation and is lower than that in other regions. Selenium (Se) shows a higher ratio than the Gulf region, exceeding the safe limits. Additionally, cadmium (Cd) surpasses the permissible limit in this current study and is relatively higher than in other regions studied previously.

3.3.1 EF evaluation of soils

Table 6 and Figure 5 present data regarding the statistical analysis of heavy metal concentrations in soils from agricultural areas in three regions of Saudi Arabia: Qassim, Medina, and Al-Kharj. Additionally, the EF is evaluated to determine the extent of mineral enrichment in the soil.

Figure 5

EF in agricultural soils of the selected regions.

For certain elements like Zn, Mn, Cr, and V, the EF values were <2, indicating that their presence in the soil is primarily linked to natural sources and geological processes. These elements are considered to provide light fertilization to the soil. Co, Ni, As, and Be showed EF values ranging from 2 to 5 (moderate ratio). As and Be have a high ratio in Medina. With EF = 5–20, copper and lead have a high ratio, in which lead decreases to 4.5 in Medina (moderate ratio).

In contrast, elements like Mo, Cd, Hg, and Se had EF values greater than 40 (EF > 40) in the soil of the Qassim and Al-Kharj regions, signifying strong fertilization. This high degree of enrichment of these elements could potentially be related to pesticides in agricultural practices or the application of chemical and organic fertilizers. Notably, the soil in Medina showed lower EF values for these elements than the other regions. Overall, the EF values for the soil in the Medina region were lower. EFs are valuable tools for distinguishing between naturally occurring mineral pollutants and those originating from human activities, aiding the assessment of environmental and agricultural impacts [48,49,50].

3.3.2 Pollution index (PI)

The PI values in Medina, Qassim, and Al-Kharj agriculture regions are shown in Table 7 and Figure 6. Across all studied sites, the PI values for all heavy elements were greater than 1, with the exception of Hg and Se minerals.

Figure 6

PI values for metals in selected regions (column standard error 1.2%).

In the case of selenium (Se), the PI values are notably high, reaching 10 (in Qassim) and 6.5 (in Al-Kharj). These elevated PI values suggest that the soil in these regions experiences severe pollution due to these metals. However, the PI values for Se were lower in the Medina soil, indicating that it was relatively uncontaminated.

For mercury (Hg), the PI value in the Al-Kharj region is 1.0375, which is slightly above 1. This suggests that the soil in the Al-Kharj region experiences minor Hg pollution.

Overall, the PI values revealed some variations in metal pollution levels in the soils of these regions.

Table 8 provides information about the range and mean values of the PI for metals in the agricultural areas of Medina, Qassim, and Al-Kharj.

3.3.3 Quantitative evaluation of metal pollution

Table 9 and Figure 7 present a quantitative evaluation of the metal contamination. This measure is represented by the geographical accumulation index (I _geo) for various heavy elements in the agricultural soils of the Medina, Al-Kharj, and Qassim regions.

Figure 7

I _geo evaluation for metals (column standard error 0.7–1.05%).

For selenium (Se), the I _geo values were found to be 2.1 (Qassim) and 2.7 (Al-Kharj) soil, both of which are moderately to strongly polluted concerning selenium. In contrast, selenium did not contaminate Medina soil, as the I _geo for this element was below the pollution threshold.

For all other studied minerals in the soil of the three agricultural areas, the I _geo values were (I _geo ≤ 0), indicating that these soils are unpolluted by these elements. This aligns with the low PI values for these minerals, suggesting minimal pollution in the soil. The mean I _geo values show a consistent trend in pollution levels, mirroring the PI findings. Notably, the highest pollution level was observed for mercury which exhibited a very strong pollution environment with I _geo values exceeding 2. For selenium (Se), the I _geo values of 2 in Al-Kharj and Qassim soil indicate moderate to strong pollution. Overall, the findings suggest that the accumulation of these metals in agricultural soils is generally low, except for selenium and mercury in specific areas, which exhibit varying degrees of pollution.

Table 10 provides an assessment of the geographical accumulation index (I _geo). It includes ratings, presented ranges, and average I _geo values for these metals.