Risk Assessment of Potentially Toxic Elements in Agricultural Soils of Al-Ahsa Oasis, Saudi Arabia

Abstract

Contamination of soil with potentially toxic elements (PTEs) is receiving great attention worldwide due to its apparent toxicity and hazards to local residents. The assessments of soil PTE distribution, sources, and environmental risks are, therefore, the first steps of high-efficiency pollutant degradation and sustainable utilization. The current study used a variety of contamination indicators and multivariate methods to evaluate the environmental risk of As, Cd, Cr, Cu, Hg, Ni, Pb, and Zn in Al-Ahsa soils in eastern Saudi Arabia. For analysis, 30 surface soil samples were collected from palm fields irrigated with groundwater and treated sewage water. Landsat images of Al-Ahsa indicated an increase in the total vegetative area and the residential area, and a decrease in the bare land area from 1985 to 2021. The average concentrations of PTEs (mg/kg⁻¹) were lower than the maximum admissible concentrations and had the following decreasing order: Zn (54.43) > Cr (28.67) > Ni (14.53) > Cu (10.83) > Pb (5.23) > As (2.27) > Hg (0.35) > Cd (0.26). The enrichment factor (EF) findings confirmed that the Al-Ahsa soil is significantly enriched with Hg, moderately to severely enriched with As, and moderately enriched with Cd. The potential ecological risk index (RI) demonstrates a moderate ecological risk, with only certain parts presenting a high risk. The different PTE levels in agricultural soils may be caused partly by the various qualities of groundwater that originate from various aquifers and sewage-treated water. The results of a multivariate analysis showed that most of the anthropogenic sources of Hg, As, and Cd may come from using a lot of fertilizers and insecticides. Levels of the remaining PTEs indicated natural sources from earth crust materials.

1. Introduction

Soil contamination refers to the excessive deposition of toxic elements, such as Hg, Cd, Pb, Cr, As, Zn, Cu, Ni, Sn, and V, into soil and water due to natural material or anthropogenic activities, causing severe environmental degradation [1,2]. Excessive PTEs in soil occur because of many factors including natural sedimentation and precipitation, irrigation using contaminated water from urban and industrial waste, and the usage of various fertilizers and pesticides in agriculture practices [1,3,4]. Potentially toxic elements from the soil can enter the human body through skin absorption and dust inhalation, potentially causing significant health problems, particularly for children [5].

Soil toxicity with PTEs kills microorganisms, which enhance fertility and the nutrition levels of soils. For human beings, arsenic toxicity causes liver, skin, blood, and prostate gland cancers [5,6,7,8]. Hg toxicity causes Minamata disease, several physiological effects, and carcinogenic effects on the brain, skin, and lung, while Pb is the reason for central nervous system, lung, and intestinal cancers [8,9]. Cadmium accumulates in soils due to the widespread intensive use of phosphate fertilizers and then enters food chains, causing Alzheimer’s disease and lung, gastrointestinal, breast, and renal cancer in humans [10,11].

The Kingdom of Saudi Arabia (KSA) is one of the Gulf region’s fastest-growing economies in a variety of sectors, including petrochemicals, oil and gas, agriculture, pharmaceuticals, and a variety of other industries. This development led to the growth of urbanization and population, with a direct impact on the country’s natural resources [12]. Most soils in Saudi Arabia could be considered immature or young soils due to the dearth of moisture [13]. Based on mineralogical properties and the parent rocks, Saudi Arabia is characterized by Torrifluvents and Gypsiorthids soil in the eastern region, Haplargids soil in the western region, and Torriorthents, Torrifluvents, and Torripsamments in the central region [14].

On the Arabian Peninsula, Al-Ahsa is distinguished as the most significant and oldest oasis. The distance of Al-Ahsa from Riyadh is roughly 320 km, and from the Arabian Gulf coast, it is about 75 km. Al-Ahsa includes Al-Hofuf and Al-Mubaraz cities, which are densely populated and industrialized, with more than fifty villages. Al-Ahsa is located on a sedimentary succession comprising carbonates, evaporates, and subordinate marl and shale, with a total thickness of 800–2500 m, which increases and slopes toward the Arabian Gulf. Three partially interconnected aquifers compose the groundwater system in this area [15]: the Neogene aquifer complex with a total thickness of 180 m, the Khobar aquifer beneath the Neogene with a depth between 180 and 250 m, and the Umm-er Radhuma at the base with a depth between 280 and 240 m.

Most of the soil in Al-Ahsa is sandy or sandy loam that has a lot of calcium carbonate and very little clay or organic matter [16,17]. Al-Ahsa Oasis is dominated by date palms with more than three million trees, as well as leafy green plants, vegetables, and fruits [17,18]. Furthermore, the Al-Ahsa region is well known for rice cultivation (Hassawi rice), which is resistant to salinity and drought [19,20]. Groundwater levels in Al-Ahsa have decreased significantly over time due to excessive irrigation of vast farmlands. Consequently, portions of the sewage water from the Hofuf, Al-Ayoun, Al-Omran, and Saudi Aramco wastewater treatment facilities are used to meet the water requirements of agriculture and industry. These plants treat residential and industrial sewage as well as agricultural drainage water.

There are very few studies in the literature, particularly those examining the risk assessment of PTEs and their associated human health hazards. Mohammed et al. [21] discovered a high enrichment in Cu, Zn, Pb, and Ni in the cultivated soil of Al-Ahsa Oasis and attributed the pollution to agriculture production and using varying irrigation water quality. AlMulla et al. [22] investigated the health concerns related to As, Cd, Ni, Pb, Cr, and Sb in Hassawi Brown Rice and reported that the As and Pb concentrations in all samples exceeded the FAO and WHO maximum allowed limits. The contamination of soil with PTEs and their hazardous effects to the soil and human beings need a periodic and continuous evaluation. Thus, the primary objectives of the current work are to document the agricultural soil land use patterns in the Al-Ahsa, investigate the hazardous levels of As, Cd, Cr, Cu, Hg, Ni, Pb, and Zn in Al-Ahsa soil, and explore the possible sources of PTEs in the examined area utilizing various contamination metrics and multivariate analytical techniques.

2. Materials and Methods

In 2022, 30 surface soil samples were taken from palm farms in Al-Ahsa Oasis between 25°21′00″ and 25°37′00″ North and 49°33′00″ and 49°46′00″ East (Figure 1) to evaluate the soil contamination with PTEs. The samples were collected at a depth less than 10 cm using a hard plastic hand trowel. At each station, a composite sample consisting of four subsamples was mixed to have a representative sample, and then placed in plastic bags and stored in an ice box. Then, the samples were air-dried and sieved. In the laboratory, the samples were air-dried and manually disaggregated by an agate mortar after removing large rocks and organic debris. The samples were size-fractionated using a nest of sieves of >500 μm, 500–250 μm, 250–125 μm, 125–63 μm, and <63 μm (Table S1). Inductively coupled plasma-atomic emission spectrometry (ICP-AES) was used at the ALS Geochemistry Laboratory in Jeddah, Saudi Arabia, to analyze As, Cd, Cr, Cu, Hg, Ni, Pb, and Zn. An amount of 0.50 g of each sample with a fraction of <63 μm was incubated with HNO₃–HCl aqua regia for 45 min in a graphite heating block. After cooling, the resultant solution was diluted to 12.5 mL using deionized water, mixed, and analyzed. To avoid the adsorption of Hg on the Teflon surface of the digestion vessels, specific concentrations of HNO₃ and HCl were used. However, a high chloride concentration was used in As analysis. The ICP-AES method was tested for linearity, limits of detection (LODs), limits of quantification (LOQs), and accuracy and precision. Three samples were analyzed in duplicate to verify the precision of analysis. ALS Geochemistry Laboratory used a standard analytical batch including a reagent blank to measure the background and Certified Reference Material (CRM) to assure the accuracy of data prior to release. Table S2 summarizes the land use pattern of the study area, coordinates of sampling sites, and results of PTE analysis.

The land use pattern in Al-Ahsa was determined by a comparison between the Landsat-5 TM image (Path 164, Row 042) obtained on 6 January 1985 and the Landsat-8 image (Path 164, Row 042) acquired on 9 November 2021. The overall accuracy of Landsat-5 and Landsat-8 image classifications was 86% and 88%, respectively. Cloud-free Landsat images that covered the entire study area were selected for the highest-quality appearance. The concentration of a given PTE in the soil was calculated using the enrichment factor, EF; contamination factor, CF; pollution load index, PLI; potential ecological risk index, RI [23]. The aforementioned indices are classified in

Table 1. Pollution indices and their classifications.

EF	EF < 1	No enrichment	CF	Cf < 1	Low contamination
	EF < 3	Minor enrichment		1 ≤ Cf <3	Moderate contamination
	EF = 3–5	Moderate enrichment		3 ≤ Cf < 6	Considerable contamination
	EF = 5–10	Moderately severe enrichment		Cf ≥ 6	Very high contamination
	EF = 10–25	Severe enrichment	PLI	PLI < 1	Unpolluted
	EF = 25–50	Very severe enrichment		PLI > 1	Polluted
	EF > 50	Extremely severe enrichment	Eri and RI	Eri < 40 and Ri < 150	low ecological risk
Igeo	I-geo < 0	Unpolluted		40 ≤ Eri < 80 and 150 ≤ RI < 300	Moderate ecological risk
	0 < Igeo < 1	Unpolluted to moderately polluted		80 ≤ Eri < 160 and 300 ≤ Ri < 600	Considerable ecological risk
	1 < Igeo < 2	Moderately polluted		160 ≤ Eri < 320	High ecological risk
	2 < Igeo < 3	Moderately to strongly polluted		Eri > 320 and RI ≥ 600	Very high ecological risk
	3 < Igeo > 4	Strongly polluted
	4 < Igeo < 5	Strongly to very strongly polluted
	Igeo > 5	Very strongly polluted conditions

and were calculated based on the following equations [24,25,26,27,28]:

EF = (M/X) _sample/(M/X) _background,

I_geo = Log2 (Cm _sample/1.5 × Cm _background),

CF = C_o/C_b,

PLI = (CF₁ × CF₂ × CF₃ × CF₄….. × CF_n)^1/n,

Eri = Tri × Cfⁱ,

RI = Ʃ (Trⁱ × Cfⁱ),

where M, C_o, and Cm _sample represent the analyzed metal; X, C_b, and Cm _background denote the level of the normalizing element (Fe in this study); CF_n denotes the CF for metal n; Eri is each element’s potential ecological risk; Tri is the biological toxic response factor. The origin of PTEs in the soil was traced using multivariate statistical methods such as principal component analysis (PCA), the correlation coefficient (Pearson’s r), and hierarchical cluster analysis (HCA).

3. Results and Discussion

3.1. Land Use Pattern

Al-Ahsa is the largest agricultural area in Saudi Arabia’s Eastern Province, covering ~12,000 hectares. Karrar et al. [29] mentioned deep, non-saline, homogenized loamy soils (Eutrochrepts) in a low plain watered by many fresh-water springs in Al-Ahsa plain, some patches have a high water-table, where the soils are wet (Haplaquepts). The Al-Ahsa oasis is an arid area and shows diverse variations in temperatures, humidity, evaporation, and precipitation throughout the year, with long and extremely hot summers. The average temperature and evaporation rates are extremely high at 43 °C and 12 mm, respectively, and the humidity decreases to a minimum of 20%. Alternatively, winters are very cold with daytime and nighttime temperatures of 20–28 °C and 8–10 °C, respectively. The average annual rainfall amount, average humidity, and evaporation rate are 147 mm, 71%, and 5 mm, respectively. Landsat data have been available for a long time and are archived more frequently than other types of remote sensing data [30]. The comparison between the Landsat-8 image acquired on 09 November 2021 (Figure 1) and the Landsat-5 TM image obtained on 06 January 1985 (Figure 2) clarifies that vegetation, residential, and barren lands are the primary land use pattern in Al-Ahsa (Figure 2A,B). During 1985–2021, the evolution of the study area for these main classes was examined. In 1985, the main LULC classes in Al-AHsa were 79 km² of vegetation, 75 km² of residential, and 279 km² of barren land. The total vegetative area in 2021 was 116 km² (a 32% increase), the residential area was 82 km² (a 9% increase), and the bare land area was 235 km² (a 16% decrease).

3.2. Distribution and Contamination of Potentially Toxic Elements

Soil samples S3, S7, S9, S14, and S27 were collected from farms irrigated using treated sewage water, while the remaining samples were collected from farms irrigated with groundwater. The average PTE concentrations (mg kg⁻¹) are listed in the following descending order: Zn (54.43) > Cr (28.67) > Ni (14.53) > Cu (10.83) > Pb (5.23) > As (2.27) > Hg (0.35) > Cd (0.26). S6 reported the highest concentrations of Pb, Cd, Hg, and Ni (11.0, 0.49, 0.50, and 38.0 mg/kg, respectively). S15 and S26 reported the highest concentrations of Zn and As, respectively (447 and 5.00 mg/kg⁻¹, respectively). The highest recorded Cr and Cu levels were seen in S19 and S25 (77.0 and 25.0 mg/kg, respectively). S14, S16, and S23 reported the lowest PTE concentrations (Table S2). The PTEs in the soil samples were examined using the Q-mode HCA, a classification method for identifying clusters of variables with similar features [31]. Q-mode HCA grouped the soil samples into three clusters (Figure 3). The first cluster included S6 and S29, which reported the highest values of Cu, Pb, Cd, Hg, and Ni. The second cluster comprised S15, which reported the highest values of Zn. The third cluster comprised the remaining soil samples. Samples of the third cluster reported the lower values of the PTEs, except As in S26 and Cr in S19.

Figure 4 shows the spatial distribution of the PTEs in the study area. The lowest PTEs were recorded in farms located in the central and northern parts of the area (S14, S16, and S23). By contrast, farms in the southeast part showed higher concentrations of Pb, Cd, Hg, and Ni, while As and Cu increased in the northwestern part of the study area.

Table 2. Average PTE concentration (mg/kg) in the study area and other local and national sediment quality guidelines.

Location and References	Ni	Zn	Cu	As	Cd	Hg	Pb	Cr
Al-Ahsa, Eastern Saudi Arabia (present study)	14.53	54.43	10.83	2.27	0.26	0.35	5.23	28.67
Maximum allowable concentrations [32]	60	300	150	20	5	5	300	200
Al Uyaynah soil, Saudi Arabia [33]	19.25	64.33	10.56	13.8	0.38	0.11	28.48	30.18
Wadi Jazan, Saudi Arabia [34]	48.66	75.80	72.85	14.13	20.97		19.41	77.22
Al-Ammariah area, northwest Riyadh [35]	26.94	52.16	11.36	3.78	0.25	0.50	5.08	19.97

displays the average PTE concentrations found in the soil samples, along with a comparison to other soil types in Saudi Arabia and the maximum admissible concentrations. The average levels of PTEs were below the maximum admissible concentrations [32]. Furthermore, the average values of Ni, Zn, As, Cu, and Cr in our samples were lower than those found in the case of palm farm soils from Al Uyaynah-Al Jubailah [33] and Wadi Jazan, Saudi Arabia [34]. Overall, the average Hg value in the study area was higher than in Al Uyaynah soil [33], but lower than in Al-Ammariah soil [35].

3.3. Risk Assessment of Potentially Toxic Elements

Pollution indices can provide a better geochemical assessment of environmental soil conditions [36,37,38]. Results of the EF (

Table 3. Minimum, maximum, and average values of EF, Igeo, CF, and Erⁱ.

	EF			Igeo			CF			Eri
	Min.	Max.	Avg.	Min.	Max.	Aver.	Min.	Max.	Aver.	Min.	Max.	Aver.
Pb	0.45	2.24	1.05	−2.01	−0.31	−1.16	0.10	0.55	0.26	0.50	2.75	1.33
Cd	1.89	6.38	3.56	-0.55	0.80	0.10	0.43	1.67	0.88	13.00	50.00	26.38
Zn	0.63	20.01	2.19	−1.78	1.84	−0.69	0.13	4.71	0.55	0.13	4.71	0.69
Ni	0.39	1.71	1.00	−2.16	−0.14	−1.21	0.09	0.66	0.25	0.52	3.93	1.55
Cu	0.40	1.70	0.93	−4.08	−1.43	−2.84	0.09	0.56	0.23	0.44	2.78	1.23
As	1.78	12.29	5.41	−1.35	0.97	−0.33	0.59	2.94	1.32	5.88	29.41	13.60
Hg	8.41	26.50	17.27	1.12	2.08	1.68	2.29	6.02	4.24	91.57	240.96	169.58
Cr	0.55	3.77	1.37	−1.73	0.21	−0.95	0.13	0.93	0.33	0.27	1.86	0.71

) indicated that the Al-Ahsa soils are significantly enriched with Hg (average EF = 17.27), moderately severe enriched with As (average EF = 5.41), moderately enriched with Cd (average EF = 3.56), and minorly to negligibly enriched with the remaining PTEs (average EF less than 3). Some individual samples exhibited high enrichment; for example, S10 showed very high Hg enrichment (EF = 26.50) and S15 had significant Zn enrichment (EF = 20.01). The high EF values indicated anthropogenic activities in the case of these PTEs, mostly from the extensive use of fertilizers and insecticides [32,39,40].

The I_geo values were used to assess PTEs in the Al-Ahsa soil. The soil was moderately polluted with Hg (average I_geo = 1.68), and unpolluted to moderately polluted with Cd (average I_geo = 0.10). The soil samples were unpolluted with the remaining PTEs (average I_geo ˂ zero). The Al-Ahsa soil’s quality and the existence of PTEs were described using the CF. The CF results in the study area indicated a considerable contamination with Hg (average CF = 4.24), moderate contamination with As (average CF = 1.32), and low contamination with the rest PTEs (average Cf < 1). However, S15 and S25 yielded CF values of 4.71 for Zn, suggesting considerable contamination of the two PTEs. The PLI was used to assess PTEs at a particular site [41,42]. The mean PLI results suggested that the investigated area was not contaminated with PTEs (PLI < 1).

The potential ecological risk index is applied to assess the impact of PTE contamination on a site’s environment [42,43]. The RI ranged from 125.53 in S14 to 325.42 in S6, with an average of 215.13, indicating moderate ecological risk for the Al-Ahsa soil (Table S2). S6, S7, and S15 showed a considerable ecological risk (RI greater than 300). The average values of the risk factor (Erⁱ) suggest that Hg presents a considerable ecological concern (Erⁱ = 169.58) and low potential ecological risk for the other PTEs (Erⁱ = 40). Hg showed a high ecological risk in S1–S3, S6–S8, S10, S12, S15, S19, S21, S25, S26, and S29.

Pearson’s correlation coefficient is most frequently used for determining a linear relation between two variables. This study measured the correlation between PTEs using Pearson’s coefficient so that their possible sources might be determined [44,45].

Table 4. Correlation matrix for PTEs of the soil samples.

	As	Cr	Cu	Hg	Cd	Ni	Pb	Zn
As	1
Cr	0.182	1
Cu	−0.105	0.344	1
Hg	0.383 *	0.550 **	0.463 *	1
Cd	0.275	0.594 **	0.410 *	0.704 **	1
Ni	0.396 *	0.718 **	0.293	0.556 **	0.514 **	1
Pb	0.302	0.768 **	0.272	0.634 **	0.650 **	0.813 **	1
Zn	−0.221	0.403 *	0.531 **	0.359	0.467 **	0.160	0.188	1

* Correlation is significant at the 0.05 level (2-tailed). ** Correlation is significant at the 0.01 level (2-tailed).

lists the correlation coefficient matrix. The positive correlations between several pairs of the PTEs, such as Pb–Cr, Pb –Hg, Pb –Cd, Pb –Ni, and Cu-Zn, indicated similar sources for these PTEs, mostly of natural sources owing to the weathering of clay minerals (hydrous aluminum silicates) in the soil [46,47]. Alternatively, Cu and Zn showed weak and negative correlations with the remaining HMs, indicating different sources [48].

PCA enables researchers to summarize large datasets with many variables into fewer principal components that can be easily visualized and analyzed. This method contributes to our understanding of the main processes involved in soil contamination and its possible sources [49]. Herein, PCA revealed two principal components that cumulatively explained 71.32% of the total data variance (

Table 5. Principal components for the investigated PTEs.

	Component
	1	2
As	0.355	−0.724
Cr	0.840	−0.007
Cu	0.542	0.588
Hg	0.829	−0.023
Cd	0.829	0.084
Ni	0.816	−0.305
Pb	0.864	−0.242
Zn	0.484	0.727
% of Variance	51.846	19.475
Cumulative %	51.846	71.321

, Figure 5). The first component accounted for 51.85% of the total variance. It is strongly associated with Cr, Cu, Hg, Cd, Ni, and Pb. In comparison, the second component accounted for 19.48% of the total variance and was strongly associated with Zn and Cu. Results of PCA indicated mixed natural and anthropogenic sources for the investigated PTEs. Agricultural chemicals and overuse of fertilizers might be the potential anthropogenic sources of Hg, As, Cd, and Zn (average EF values >2) [50,51,52]. Moreover, the use of groundwater from different aquifers for soil irrigation, with their differences in the composition, might cause a difference in PTE concentrations among studied soil samples.

4. Conclusions

The land use pattern and hazardous concentrations of PTEs in agriculture soil of the Al-Ahsa oasis were examined using Landsat images and several pollution indices and multivariate tools. The assessment revealed that the investigated soil was severely enriched, highly contaminated, and had a high environmental risk with Hg. The findings of the RI indicated that the soil samples suggested a moderate PTE risk. Cu, As, Cd, and Pb enrichment ranged from moderate to negligible in the study area. Multivariate statistical methods indicated combined natural and human effects for the PTE sources, primarily originating from the weathering of earth materials and atmospheric deposition, as well as sewage and agricultural activities. No fundamental difference was observed between soil samples collected from farms irrigated using treated sewage water and those irrigated using groundwater with respect to PTE contamination. The concentration levels of PTEs in the soil of Al-Ahsa need to be checked regularly to measure the accumulation of HM and prevent their increase, specifically Hg, As, and Sr. Moreover, farmers should use biofertilizers and manure and reduce their dependency on chemical fertilizers and pesticides.