The Impact of War on Heavy Metal Concentrations and the Seasonal Variation of Pollutants in Soils of the Conflict Zone and Adjacent Areas in Mosul City

Abstract

The present study addresses the war-related soil contamination with heavy metals in the urban area of Mosul/Iraq as a result of the war of liberation from ISIS (2014–2017). In order to cover seasonal influences, a total of eight sample sets from soils in the conflict area and adjacent areas were collected over the course of the year in two three-month test series, and the parameters pH, E.C., salinity and the heavy metals Cd, Pb, Zn, Cr and Ni were taken as indicators for contamination. Results showed average heavy metal levels in the conflict areas above the global average limits, with some limits also being exceeded in the adjacent areas. All sampling sites were highly contaminated with Cd and moderately contaminated with Pb. The Igeo contamination factors indicated that the sampling sites in the conflict area were moderately to heavily contaminated with Cd, Pb, Zn, Cr and Ni, while the pollution load index indicated that all sites in the conflict zone were extremely to heavily contaminated with heavy metals. The study data give cause for concern that heavy metals may be released into other ecosystems.

1. Introduction

The soil in and around Mosul was considered as one of the best soil types available in Iraq for agricultural use [1]. However, during the occupation and liberation of Mosul from 2013 to 2017, the soil was exposed to a major environmental disaster. On the one hand, it was contaminated by large quantities of toxic chemicals released during the explosion of industrial facilities built by ISIS in the urban area for the production of explosives and chemical weapons; on the other hand, oil leaks, the use of ammunition and the explosions of armaments further exacerbated the situation [2].

In addition, air pollution from burning oil wells and sulphur fields had a strong negative impact on ground pollution in Mosul, as 18 oil wells ignited by ISIS in Qarrayah burned continuously for 9 months during the occupation, exposing the ground to dense black smoke. ISIS also ignited a 50,000 tonne sulphur storage facility in the Mishraq complex, creating a dense, yellowish cloud of toxic sulphur oxides that reached into the neighbourhood [2,3,4]. The results were significant emissions and exposure of the soil to carbon monoxide, nitrogen oxides, sulphur dioxide and tar particles, the latter of which can accumulate hazardous organic pollutants and heavy metals (Pb, Cd, Zn, Cr, Ni) due to incomplete combustion and adsorption. Further unburnt hydrocarbons and toxic aldehydes can also be emitted through the combustion of oil and fuels, often in simple low-tech cookers. These emissions can be washed out of the atmosphere by rainfall and further pollute the soil [5,6,7].

The accumulation of high concentrations of heavy metals and organic pollutants in soil over a long period of time can be harmful to agricultural land and crops and ultimately lead to food contamination, which is potentially harmful to human health [8,9,10].

Military activities—whether military training sites or real conflict zones—have a drastic impact on physical and chemical soil properties [11], which is why areas with intensive combat activity, but also military training areas, shooting ranges and sites for the production/disposal of explosives and ammunition, are among the most important sources of contamination for terrestrial ecosystems [12,13,14].

Excessive concentrations of heavy metals such as lead, cadmium, mercury, arsenic, chromium and nickel in the soil may have a significant impact on soil quality and adjacent environmental compartments and may even have a toxic effect on these compartments and humans in even more elevated concentrations.

In general, increased concentrations of heavy metals can negatively affect soil quality in such a way that they can lead to a nutrient imbalance. Cadmium and lead, for example, can mimic the essential nutrients zinc and calcium, which can unbalance nutrient uptake and negatively affect plant growth. Not only may plant growth and development be inhibited in high concentrations, but acute toxicity may also occur [15]. Similarly to salinity, increased heavy metal concentrations can also have an inhibitory or toxic effect on the existing mixed microbial culture, which can negatively affect its composition and activity. This effect has been described in particular for cadmium and copper [16].

The resulting accumulation of heavy metals in the soil is a cause for concern in agricultural production, as it has a negative impact on plant growth due to plant poisoning and the negative impact on soil organisms and thus negatively affects both food safety and the marketability of agricultural products [17,18].

Besides the acute toxicity for plants [19], a long-term problem is the uptake of toxic metals from the soil by the plant and thus an increased exposure of crops to heavy metals, the consumption of which can lead to reduced growth, reduced intelligence, behavioural abnormalities, damage to the cardiovascular system, kidneys, reproductive organs and nervous system and, in extreme cases, the death of humans and animals if permissible limits are exceeded [20].

Abdulsattar [21] investigated the concentrations of some heavy metals occurring in the Danfeli stream valley (Mosul city) and their effects on soil, plants and water pollution. Compared to the natural geological composition of the soil layer (Igeo), results showed that the soil samples examined were heavily contaminated with lead (Pb), very heavily contaminated with cadmium (Cd), and moderately contaminated with nickel (Ni), resulting in the order Cd > Pb > Ni.

Hamad [17] analysed the contamination with the heavy metals Cu, Pb, Zn, Ni, Co, and Mn in the soils of Halgurd-Sakran National Park (Kurdistan, Iraq) after clearance and removal of mines by controlled explosions in dugout pits during de-mining in two hotspot areas. The geo-accumulation assessment index (Igeo) showed contamination levels ranging from very high to moderate for all five heavy metals. It can therefore be concluded that the detonations of these mines left heavy metal residues in the soil, which can be harmful to vegetation, animals and ultimately also to humans.

The distribution of selected heavy metals in the urban soil of Al-Anbar City in western Iraq after the ISIS war (As, Cu, Sb, Cd, Ag, Ni, Cr, V, Cs, Te, Zn, Mn, Ti, Fe and Ca) showed exceptionally high concentrations, especially of trace elements, indicating the increased input of these elements due to the war [22].

The first comparable studies on soil pollution and the resulting environmental impact of heavy metals in the urban area of Mosul have already been published. For example, Ref. [23] described the effects of soil pollution in the Al-Karama industrial area in the west of Mosul. Samples were taken from the soil there during two different seasons, and their content of the metals Se, Mn, Hg, Co and Sb was determined using X-ray fluorescence. The study shows that the elements Mn and Hg occur in concentrations above the global average in the local soil layers due to the industries located there. In a follow-up study focusing on the industrial areas of Al-Karama and Okab and the analysis of the heavy metals Cr, Ni, Cd, As and Pb, contamination levels of Ni, Cr and As above the WHO limits and of Pb and Cd within the limits were detected. For Pb, it was concluded that the increase in contamination levels was due to the war-related bombardment of vehicles.

A heavy metal pollution study of the old city of Mosul immediately after the war was already published in 2020 [24]. Soil samples were also taken at two different times of the year from depths of 0–15 cm and 15–30 cm and analyzed for their heavy metal content using X-ray fluorescence. The results showed that the elements Cd, Ni, Cr, Zn and Cu were present in concentrations significantly higher than the global average. A comparative t-test between the two soil layers showed that the heavy metal contamination differed significantly, underlining the war-related input of these heavy metals. The study by Al-Sheraefy [25] comes to a similar conclusion.

The post-war study by Aswad [26] focusing on mercury in the urban area of Mosul showed that the lower the vegetation density of the respective soil body, the higher the mercury contamination of the soil, while it decreased with increasing vegetation. The absolute highest contamination was detected near the Al-Nouri Mosque, which was located directly in the conflict area. This finding is understandable, as mercury is particularly present in its inorganic forms in soils poor in organic matter, for example as cinnabar, and therefore has very low mobility [27]. If the organic content in the soil increases, mobile Hg forms such as humic and fulvic complexes increase, which increases the bioavailable mercury content and thus also plant uptake [27,28]. Based on a mean organic matter content of the soil zones in Mosul of 4.57%, Refs. [25,28] describes increased Hg mobilization, plant uptake and accumulation in the leaf at a soil organic matter content of 3.0% or higher. Overall, mercury showed the strongest plant uptake in this study compared to other heavy metals.

Since it has been proven that the use of military sites in particular and military combat operations in general can be regarded as potential sources of high concentrations of trace elements and toxic heavy metals, the focus of this study is on quantifying the environmental impact of past military operations in the context of heavy metal emissions on the soils in the immediate conflict zone and adjacent zones in the urban area of Mosul/Iraq, focusing on residential and light-commercial zones with non-expectable input of heavy metals by their use. The assessment of the pollution rates during the present study period will be based on known factors and indices, namely the pollution factor (CF), the geo-accumulation index (Igeo) and the pollution load index (PLI), taking particular account of seasonal factors.

2. Materials and Methods

2.1. Study Area

The research area was located in the area of the city of Mosul (north-western Iraq) at the coordinates 35°–37° latitude and 41°–44° longitude. The World Reference Base for Soil Resources [29] provides a comprehensive framework for classifying soils globally. In Mosul City, the soil types and their distribution can be categorized based on the WRB guidelines within soil layer (0–15 cm) as follows:

• Calcisols: These soils are rich in calcium carbonate and are typically found in arid and semi-arid regions.
• Gypsisols: Characterised by a significant amount of gypsum, these soils are also common in arid areas.
• Vertisols: These are clay-rich soils that expand and contract significantly with moisture changes.
• Fluvisols: Found in floodplains, these soils are influenced by river sediments.

The particle size distribution in Mosul’s soils can vary significantly depending on the specific location and environmental conditions. The type of soil also varies from one site to another. The exact sampling points for the soil samples are shown in Figure 1 [30].

For each sampling, the respective sampling location was provided with a 20 × 20 m grid, and the soil samples were taken from a profile depth of 0–15 cm at intervals of 10 m along the grid axis. This resulted in 9 individual samples per measuring point and measuring day, which were combined to form a qualified composite sample. A total of 8 locations in the urban area were sampled over a total of two measurement series (see

Table 1. Location of soil sampling sites in Mosul city.

Site No.	Location	Latitude N	Longitude E	Site Description	Soil Description	Soil Types	No. of Samples
S1	Alrashidia	36.40005	43.103287	Residential Area	Clayey and sandy soil with some organic matter and minerals such as quartz and feldspar.	Calcisols, Vertisols Fluvisols	48
S2	Mosul Forest	36.383410	43.117947	Recreation Area			48
S3	Old city	36.34022	43.128143	Commer + Resi Area	Mixture of clay and sand, with a higher percentage of organic matter with intensive mixing by explosions.	Calcisols, Vertisols Gypsisols	48
S4	AL-Faysalia	36.34651	43.142455	Residential Area		Fluvisols	48
S5	Alyarmuk flats	36.321480	43.076216	Residential Area	Mixture of clay and sand, with a higher percentage of organic matter. Contains minerals such as illite and kaolin.	Calcisols, Vertisols Gypsisols	48
S6	Al-Karama Ind.	36.34201	43.205897	Industrial Area		Fluvisols	48
S7	Al-Danadan	36.32333	43.161559	Residential Area	Heavy clay, with a high percentage of organic matter. Mixing by explosions. Contains minerals such as vermiculite and micas.	Calcisols, Vertisols Fluvisols	48
S8	Al Busaf	36.27324	43.163325	Agricultural Area			48

and Figure 1). In the study, conducted after 5 years of the war, series 1 covered the months of January–March (2022), and the summer series the months of July–September (2022). A total of 24 different composite samples were taken in duplicate by weekly sampling from each measuring point, i.e., 48 samples per site were available, covering changes of heavy metal charge over both rainy and dry seasons. The sample material was sealed gas-tight in 250 mL polyethylene plastic bags and stored in a refrigerated container at −4 °C until analysis. The soil temperature was 0–10 °C during the first series and 32–45 °C in the second series.

The selection of the sites was based on the distance to the conflict area and their accessibility: three sites were defined close to the conflict area (S3, S4, S7) and five additional sites further away from the conflict area (S1, S2, S5, S6, S8). Standard procedures were used to collect and analyse the samples in the environmental laboratory. The soil samples were evaluated using the standard parameters pH, E.C., salinity and the heavy metal content of Cd, Pb, Zn, Cr and Ni (atomic absorption spectroscopy).

2.2. Measurement Methods

Physical parameters: Temperature, pH, electrical conductivity (E.C.) and salinity (%) of the soil samples were measured in the laboratory by mixing sample aliquots of 10–15 g with 25 mL of distilled water. After 2–3 min of continuous stirring, the soil samples were uniformly suspended, and the parameters pH, E.C. and salinity of the solution were measured in the liquid phase using an Oumefar 5 in 1, Digital Water Quality Monitor Tester UPC 886108495111. The tests were then conducted again in laboratory equipment inside the laboratory. The temperature was determined immediately at sampling sites.

Heavy metals: Sample aliquots were dried in the laboratory at 104 °C for 48 h, cooled down, ground to a fine powder and sieved through a 106 μm mesh stainless steel sieve. The samples were then stored in polyethylene containers until sample digestion. Sample digestion was performed according to a previously described method (Jackson, 1958). Here, 0.5 g of dry soil was weighed into a glass flask and mixed with 5 mL of digestion solution. This consisted of concentrated sulphuric acid (H₂SO₄, 96 w%), concentrated nitric acid (HNO₃, 68 w%) and perchloric acid (HClO₄, 70 w%) in a volume ratio of 3:1:1. The samples were then digested for 2 h at 90 °C under a fume hood and then filled to a volume of 25 mL by adding deionized water. The actual analysis of the acidic heavy metal solutions was carried out using atomic absorption spectrometry [31,32]. The concentration-dependent specific absorptions measured here were converted into concentrations (in ppm) using standards carried along by creating a regression equation based on them [33].

2.3. The Main Use of Heavy Metals in Military Operations

Cadmium (Cd): Cadmium is used in military operations primarily in the production of batteries, particularly in rechargeable nickel-cadmium (Ni-Cd) batteries, which are used in various types of military equipment and vehicles.

Lead (Pb): Lead is extensively used in military applications, especially in the production of bullets and other ammunition. It is also used in shielding materials to protect against radiation and in the construction of military vehicles and aircraft.

Zinc (Zn): Zinc is used in military operations for galvanizing metals to prevent corrosion, which is crucial for maintaining the integrity of military equipment and infrastructure. It is also used in the production of brass and other alloys for various military components.

Chromium (Cr): Chromium is used in military operations for its corrosion-resistant properties. It is often used in the production of stainless steel and other alloys for military vehicles, aircraft, and naval vessels. Chromium compounds are also used in paints and coatings to provide a durable finish.

Nickel (Ni): Nickel is used in military operations for its high strength and resistance to corrosion. It is used in the production of stainless steel and other alloys for military equipment, as well as in batteries, such as nickel-metal hydride (Ni-MH) batteries, which are used in various military devices.

These metals play critical roles in ensuring the functionality, durability, and effectiveness of military equipment and infrastructure. Their unique properties make them indispensable in various military applications [11].

2.4. Assessment of Metal Contamination

The contamination factor (CF), the pollutant load index (PLI) and the geo-accumulation index (Igeo) were used as the indices and parameters for assessing the degree of pollution at the various sites. Their respective definitions are given below.

2.4.1. Contamination Factor (CF)

The level of contamination of soil by metal is expressed by the contamination factor (CF), which is calculated as follows:

CF = C_m,Sample/C_m,Background

(1)

where, C_m,Sample is the concentration of a particular metal in the soil, and C_m,Background is the concentration value of the metal equal to the world surface rock average given by Martin [34]. The reference CF values for describing the contamination level are shown in

Table 2. Contamination factor (CF) value classification [34].

Contamination Factor	Contamination Level (CF)
CF < 1	Low
1 ≤ CF < 3	Moderate
3 ≤ CF < 6	Considerable
CF > 6	Very high

.

2.4.2. Igeo Index

The enrichment of a metal concentration above the baseline concentration was calculated using the method proposed by Mueller [35], termed as geo-accumulation index (Igeo). The geo-accumulation index is expressed as follows:

I_geo =log₂ [C_m,Sample/(1.5 × B_m,Background)]

(2)

where C_m,Sample is the measured concentration of element n in the soil sample and Bm_,Background is the geochemical background value of world surface rock average given by Martin [34].

The factor 1.5 was later introduced by Mueller [35] to account for possible variations in the background contamination levels of the rocks. He also proposed the seven classification levels of the geo-accumulation index as shown (

Table 3. Igeo factor value classification [35].

Igeo Value	Class	Sediment Quality
≤0	0	Unpolluted
0–1	1	Moderate to low polluted
1–2	2	Moderately polluted
2–3	3	Moderately to strongly polluted
3–4	4	Strongly polluted
4–5	5	Strongly to extremely polluted
5–6	6	Extremely polluted

).

2.4.3. Pollution Load Index (PLI)

The pollution load index (PLI) for a particular site was evaluated following the method proposed by Tomlinson [36]. This parameter is expressed as follows:

PLI = (CF1 × CF2 × CF3 …CF n) 1/n

(3)

where the respective indexed CF value describes the concentration factor of the respective heavy metal for n considered elements. PLI values for the contamination level are shown in

Table 4. Pollution load index (PLI) value classification.

PLI	Class	Sediment Quality
<1	0	Unpolluted
1–2	2	Moderately polluted
2–3	3	Heavily polluted
>3	4	Extremely polluted

.

3. Results and Discussion

3.1. pH Value

As expected, there was a correlation between the detected heavy metal concentrations and the corresponding pH values in the soil, as heavy metal mobility and thus leaching are strongly dependent on the pH value [37,38]. Accordingly, the highest concentrations of the analyzed elements [39] and heavy metals in general [40] were observed under acidic conditions.

Comparing the sites and the two study phases with each other, the measured pH values ranged between 6.67 and 8.2 with a clear impact of seasonal factors, reflected in average values between 7.03 and 7.68 during series 1 and 6.97 to 8.12 in series 2 (see

Table 5. Characterisation of heavy metal levels in soil samples at all sites during series 1 (winter and spring).

Series 1		pH	E.C. mS/cm	Salinity%	Cd ppm	Pb ppm	Zn ppm	Cr ppm	Ni ppm
WHO Std. limits		6.5−9.0	0.5–2	3%	0.06	10	50	100	40
World surface rock average		-	-	-	0.2	16	127	71	49
S1	Mean	7.2	0.5	0.9	2.4	18.0	137.0	31.0	32.0
	±Sd	0.2	0.1	0.6	0.8	6.1	46.6	12.2	11.8
S2	Mean	7.3	0.7	1.2	3.0	22.0	141.0	47.0	41.0
	±Sd	0.2	0.1	0.6	1.0	7.5	47.9	17.7	14.8
S3	Mean	7.6	1.7	2.7	4.3	42.0	323.0	101.0	92.0
	±Sd	0.1	0.1	0.5	1.5	15.0	109.8	46.6	32.3
S4	Mean	7.3	1.1	2.2	4.2	37.0	317.0	97.0	84.0
	±Sd	0.2	0.1	0.8	1.3	13.9	107.8	43.5	29.6
S5	Mean	7.3	1.1	2.1	2.1	17.0	147.0	37.0	32.0
	±Sd	0.1	0.1	0.2	0.7	5.8	50.0	14.3	10.9
S6	Mean	7.6	1.0	0.4	3.6	22.0	244.0	92.0	53.0
	±Sd	0.4	0.1	0.4	1.2	9.2	97.2	33.0	19.0
S7	Mean	7.0	1.2	2.6	3.9	42.0	302.0	93.0	65.0
	±Sd	0.4	0.1	0.3	1.4	14.3	102.7	35.0	20.7
S8	Mean	7.7	0.9	1.6	3.3	33.0	287.0	98.0	58.0
	±Sd	0.4	0.1	0.1	1.1	13.3	83.0	39.1	23.1

,

Table 6. Characterisation of heavy metal levels in soil samples at all sites during series 2 (summer and fall).

Series 2		pH	E.c mS/cm	Salinity%	Cd ppm	Pb ppm,	Zn ppm	Cr ppm	Ni ppm
WHO std. limits		6.5–9	0.5–2	3%	0.06	10	50	100	40
World surface rock		-	-	-	0.2	16	127	71	49
S1	Mean	7.34	0.60	1.73	3.26	24.7	157.6	42.3	48.9
	±Sd	0.14	0.08	0.40	0.50	3.9	28.4	7.3	8.4
S2	Mean	7.70	0.75	1.87	3.95	29.5	162.2	60.2	61.0
	±Sd	0.20	0.05	0.21	0.62	4.8	29.2	10.5	10.6
S3	Mean	6.97	2.07	2.80	5.45	55.9	371.5	155.4	130.0
	±Sd	0.17	0.06	0.32	0.89	9.5	66.9	27.6	23.0
S4	Mean	7.05	1.22	2.33	4.87	52.3	364.6	145.4	119.2
	±Sd	0.20	0.03	0.25	0.79	8.9	65.6	25.8	21.1
S5	Mean	7.15	1.17	2.33	2.80	23.5	169.1	49.0	45.3
	±Sd	0.29	0.10	0.56	0.41	3.7	30.4	8.5	7.7
S6	Mean	7.26	1.24	2.80	4.64	35.5	328.9	110.6	77.6
	±Sd	0.31	0.07	0.56	0.75	5.8	59.2	19.6	13.6
S7	Mean	7.03	1.50	2.53	5.33	53.5	347.3	130.8	93.7
	±Sd	0.30	0.03	0.62	0.87	9.1	62.5	23.2	16.5
S8	Mean	8.00	1.03	2.82	4.30	49.9	280.6	117.4	84.3
	±Sd	0.16	0.08	0.55	0.68	8.4	50.5	20.8	14.8

and

Table 7. Heavy metal levels at all sites as mean, minimum and maximum values.

2022		pH	E.C. mS/cm	Salinity%	Cd ppm	Pb ppm	Zn ppm	Cr ppm	Ni ppm
	WHO limits	6.5−9.0	0.5–2.0	3%	0.06	10	50	100	40
	World surface rock	-	-	-	0.2	16	127	71	49
S1	Mean	7.29	0.90	1.31	2.83	21.4	148.5	34.2	40.5
	Max	7.50	0.64	2.10	3.26	25.5	185.9	48.2	55.0
	Min	7.00	0.46	0.20	1.58	11.9	90.4	23.8	22.9
S2	Mean	7.52	0.92	1.55	3.48	25.8	152.8	49.6	51.0
	Max	7.90	0.77	2.10	4.07	31.2	191.3	69.7	69.3
	Min	7.20	0.49	0.50	1.98	14.5	93.1	34.3	28.8
S3	Mean	7.50	1.42	2.63	4.87	50.0	348.5	128.2	111.0
	Max	8.20	2.30	4.00	5.84	62.3	438.3	183.6	150.7
	Min	6.86	1.55	1.38	2.84	29.0	213.2	90.4	62.7
S4	Mean	7.20	1.25	2.55	4.34	46.7	342.0	121.2	101.6
	Max	7.45	1.99	3.40	5.16	58.1	430.2	171.5	138.0
	Min	6.93	0.88	1.90	2.51	27.1	209.2	84.5	57.4
S5	Mean	7.24	1.45	2.35	2.45	20.3	159.3	43.0	41.2
	Max	7.50	2.03	3.60	2.81	24.1	52.0	56.3	51.7
	Min	6.93	1.04	1.99	1.39	11.2	31.2	27.7	28.1
S6	Mean	7.41	1.09	2.10	4.12	28.8	308.7	88.8	65.3
	Max	7.89	1.09	3.40	4.89	38.2	388.1	130.0	88.8
	Min	6.98	0.79	0.90	2.38	17.8	188.8	64.0	37.0
S7	Mean	6.96	1.63	2.08	4.77	47.8	325.9	111.9	79.4
	Max	7.40	2.07	3.30	5.63	56.3	404.7	154.1	107.8
	Min	6.67	1.15	1.38	2.77	27.7	199.3	75.9	44.9
S8	Mean	7.41	1.93	2.18	3.80	44.5	263.6	102.2	71.1
	Max	8.12	0.89	2.90	4.48	55.2	331.1	138.0	82.0
	Min	6.99	0.67	1.80	2.18	25.7	161.0	68.0	40.3

), with all measured values complying with the WHO limits. Most of the soil samples showed a slightly alkaline tendency with pH values of 7.0–7.5 and a tendency towards lower pH values in the heavily polluted areas S3, S4 and S7. In view of the fact that the soil order of Aridisols, a CaCO₃₋rich soil type in arid zones, is dominant in Iraq and is characterised by alkaline pH values, the pH values actually measured appear to be too low, particularly in the heavily polluted areas. As a result of the burning of oil and sulphur fields during the war, acidic nitrogen and sulphur oxides were released [41,42], which lowered the pH value of the soil through wet deposition. In addition to potential negative effects on vegetation and general soil acidification [2,4], a greater drop in pH indicates a high content of mobile heavy metals in the soil. Comparing the seasonal impacts, the pH drop is less pronounced in the winter and spring phases (series 1) than in the summer and autumn phases (series 2), as the acidic combustion products are gradually washed out by further rainfall, increasing the pH value of the soil while the pH value of the groundwater drops [42].

3.2. Electrical Conductivity (E.C.)

The electrical conductivity of the soil is a sum parameter for the content of salts and thus an important quality indicator in agricultural terms with regard to the need for fertilization, but also as an indicator of existing environmental contamination. In addition to the moisture content of the soil [43], conductivity is naturally influenced in particular by the salts dissolved in the soil or near-surface groundwater. The conductivity determined in soil samples is primarily influenced by the degree of erosion of the soil rock and secondarily by anthropogenic interventions (fertilization, wastewater, landfill leachate, road salt, salt formation through the burning of deposits, deposition through direct armed action), which primarily influence the near-surface soil and groundwater layers [44]. Thus, water samples from sediments or limestones formed during the Quaternary period naturally have a higher conductivity than soil water samples from erosion-resistant rocks such as granites or quartzites. References [45,46] give a range of 100–300 µS/cm for the conductivity of mineral-poor groundwater. Clayey soils generally have a higher conductivity than sandy soils, as they can retain more moisture and dissolved ions due to their existing ion exchange capacity. Other factors influencing the electrical conductivity are the soil temperature, as the ion mobility increases with increasing temperature, and the organic matter content, as an increase in cations can be expected [47].

Although electrical conductivity gives no indication of individual dissolved substances, it is very well suited to record temporal changes in the mixing ratio of water or soil constituents and can therefore be used as an indicator of anthropogenic soil contamination. The aforementioned change can be caused by groundwater salinization following excessive groundwater extraction and infiltration of seawater or by mixing with another aquifer, as well as by increased deposition of heavy metals, salts and acidic gaseous combustion products.

The average values of the electrical conductivity EC were between 0.46 and 2.07 mS/cm during the investigation period (see Table 7). The highest average value was reached at sampling point S3 (1.7 mS/cm in series 1, 2.07 mS/cm in series 2; see Table 5 and Table 6). In general, the values increased during the hot and dry season (summer, autumn; corresponds to series 2) compared to the season with more rainfall (winter, spring; corresponds to series 1) due to soil infiltration of rainfall and the associated dilution of the salt concentration. The dilution effect was counteracted by the increased mobilisation of pollutant deposits close to the soil, which were dissolved, dissociated and carried into deeper soil layers by percolating water [35,48]. Accordingly, the highest conductivity values are to be expected at contaminated sites. In addition to chloride (Cl⁻) as an indicator of increased seawater input and nitrate ($N{O}_3^{-}$) as an indicator of agricultural sources of contamination, sulphate ($S{O}_4^{2-}$; combustion product of the sulphur fields) [41,42] and certain heavy metal cations, the exposure of which is attributable to the fighting, play a particularly important role in the total ion concentration in the soils of the study area of Mosul. However, all sites passed WHO limit values.

3.3. %Salinity

Salinity is directly related to electrical conductivity, as the electrical conductivity of the soil increases with increasing salt concentration. High salt concentrations in the soil may have a negative impact on agricultural yields, as high salt concentrations lead to an osmotic imbalance and thus to more difficult water uptake by the plants, resulting in reduced growth [49]. Increased salinity can also have a negative impact on the activity and composition of microbial flora in the soil and aqueous systems [50,51]. This in turn has a direct impact on the catabolism of pollutants and the cycling of nutrients [50].

The salinity levels measured during the two study periods were between 0.2 and 4% (see Table 7); the average values for both series were below the WHO guideline limits at all sites. Once again, the highest average value of 2.6% in series 1 and 2.8% in series 2 was achieved at sampling site S3. The higher salt content in the soils during the summer period is primarily due to the strong evaporation of water during this arid season, which increases the concentration of salt in the remaining soil solution [52,53].

3.4. Heavy Metals

In comparison with the global average concentrations of specific heavy metals in soils and the WHO guideline values (see Table 5, Table 6 and Table 7), the concentration levels determined in this study show that cadmium concentration limits are exceeded at all sites. The highest concentrations occur at sites S3, S4 and S7, which are located in the historic city centre or in the close vicinity and therefore within the former conflict zone. The average values generated on the basis of the daily measurements from nine individual samples per measurement site were 1.39–5.84 ppm over the whole year according to Table 7. Mean values of up to 4.3 ppm Cd were detected at measuring point S3 during series 1 and 5.45 ppm during series 2 (see Table 5 and Table 6), Figure 2.

Similarly, and therefore as expected, exceedances also occurred for lead at measuring points S3, S4, S6 and S8, whereby the mean value of the qualified mixed samples for Pb at site S3 was 42.0 ppm (during series 1) and 55.9 ppm (during series 2). The mean values for Zn were between 148.53 and 438.3 mg/kg (see Table 7); again, site S3 stood out ingloriously with average values of 323 ppm in series 1 and 371.5 ppm in series 2. The highest concentrations of zinc (438.3 ppm) were detected at site S3.

With individual concentrations of 23.7–183.5 ppm during the overall investigation period (see Table 7), an increased concentration of chromium was found at all sites, which even exceeded the WHO limits at sites S3 and S4 (during series 1) and S3, S4, S6, S7 and S8 (during series 2). The highest mixed sample concentration value of 101 ppm in series 1 and 155.4 ppm in series 2 (see Table 5 and Table 6) was achieved at measurement location S3. In contrast, the chromium limit values were complied with in both series of measurements at measuring points S1, S2 and S5, see Figure 2.

As with chromium, the concentrations of nickel in series 2 were also consistently higher than in series 1. The concentrations detected according to Table 7 ranged from 22.9 to 150.6 ppm Ni and exceeded the WHO limits at all locations except S1 and S5. The highest average values were again recorded at sites S3, S4 and S7, with S3 taking the leading position with 92.0 ppm in series 1 and 130.0 ppm in series 2. At sites S1 and S5, the limit values (40 ppm) were observed during test series 1, but were exceeded in the second series due to the generally increased concentration levels in series 2 (see Table 5 and Table 6), Figure 2.

Overall, in observations of heavy metal levels at all sites in both series, soil samples taken at sites S3, S4 and S7 revealed the highest heavy metal concentrations, which—to put it very simply—were a factor of 3 higher than at measuring points S1 and S5, which had comparable land use (residential). These three sites were located within the former conflict zone or in its close vicinity. As significant variations in the rock morphology within the urban area can be ruled out, and at the same time, there is comparable land use at the named sites, the increase in heavy metal concentrations by a factor of 3 (with the exception of cadmium at S5 in series 1) can therefore be attributed to the effects of the military operations. The relevance of the named heavy metals in the manufacture of weapons and ammunition has already been described previously [41,42,44]. The contamination of areas used for military purposes with the highly toxic heavy metals Cd, Pb, Ni and Cr has also been described previously [11,21,35].

Site S6 is exceptional, as it is an industrial area in terms of land use, and higher heavy metal emissions might occur according to industrial production processes; an aspect that has already been described in advance [23].

In general, the following ranking can be made with regard to soil contamination: S3 > S4 > S7 > S6 > S8 > S5 > S2 > S1.

The comparison of the two test series and thus the seasonal impact shows that the concentration levels in series 2 (summer, autumn) and thus the dry season are generally higher than in the rainy season (series 1; winter and spring). This difference is due to percolation of the upper soil layers by rainwater and thus leaching of heavy metal precipitates near the surface during the rainy season. This effect is supported on the one hand by the porous nature of the soil in the urban area [54], and on the other hand by the mineral soil matrix, which is poor in organics and therefore causes poor adsorptive binding of heavy metals to the soil rather than absorptive dissolution in soil water [55], or their occurrence as precipitates when the soil dries out. Accordingly, the rainy season results in increased leaching of heavy metals into deeper soil layers or aquifers [41,42,56,57,58].

These metals can be adsorbed onto soil particles or precipitated as insoluble compounds, meaning that they can persist in the environment for long periods. The problem of elevated metal concentrations is unlikely to resolve on its own and may be exacerbated over time by ongoing inputs from sources such as industrial runoff, military waste, and agricultural practices.

3.5. Statistical Analysis

3.5.1. t-Test

The p-value of the statistical t-test represents a benchmark for the significance of the statistical deviation in the comparison of two test series. p-values < 0.05 are considered significant, i.e., this means that there was a significant seasonal difference in the present study. In fact, a significant seasonal impact was found for the heavy metals Cd, Pb, Cr and Ni with p < 0.05, while no relevant seasonal influence was found for zinc with p > 0.05. The generally high concentration level of zinc, which is independent of location, indicates non-primary input by the military conflict for this heavy metal [56]. The contamination with zinc originates from the discharge or leaching of zinc-containing slag and waste, from mine tailings, fly ash from coal combustion and large-scale soil contamination through the use of zinc-containing commercial fertilizer products and wood preservatives [59], (see

Table 8. t-Test value between Series 1 and Series 2 of all heavy metals in soil samples.

Heavy Metal	t-Test	DF	p *	Status **
pH	−0.27	14	0.395	0
EC	−1.72	14	0.024	1
%Sal.	−1.39	14	0.023	1
Cd	−2.25	14	0.020	1
Pb	−1.87	14	0.041	1
Zn	−0.86	14	0.203	0
Cr	−1.76	14	0.045	1
Ni	−1.89	14	0.040	1

* p < 0.05: significant, p > 0.05: non-significant; ** 0 = not significant, 1 = significant, DF: degree of freedom.

). The military impact path is therefore partially masked by other entry paths, which means that the t-test proves to be indifferent, in contrast to the subsequent Pearson test.

3.5.2. Seasonal Percentage Deviation of Soil Parameters

The highest percentage seasonal difference between the two test series was observed for salinity. In detail, the highest difference occurred at sites S1 and S8 with 96.23% and 72.65%, respectively. The sites’ deviation, which can be considered significant, is due to the partial agricultural use of these areas, where an increase in salt concentration can be observed in summer due to artificial irrigation and increased evaporation.

The highest difference in the EC values was recorded at S3 with 24.8%. As the salinity was not the highest one and S3 is located within the conflict zone, this constellation of salinity and EC value indicates contamination with heavy metals [60,61,62].

The percentage seasonal variation in heavy metal contamination was subject to greater variation depending on location, season and element. The seasonal variations were in the ranges of 26.6–35.8% (Cd), 27.0–61.4% (Pb), 15.8–16.8% (Zn), 24.6–39.4% (Cr) and 29.2–36% (Ni). In addition to contamination from the conflict phase, these differences are caused by current heavy metal inputs into the environment as well as differences in the distribution of rainwater and soil permeability (see

Table 9. Percentage variations in heavy metal concentrations at all sites comparing series 1 vs. series 2.

Sites	EC%	%Sal	Cd%	Pb%	Zn%	Cr%	Ni%
S1	13.6	96.2	35.8	37.2	16.8	38.6	34.5
S2	6.8	51.4	31.7	34.1	16.8	35.3	32.8
S3	24.9	68.7	26.6	27	15.8	35	29.2
S4	12.3	12.9	28.2	27.6	15.8	33.3	29.5
S5	2.9	23.7	33.3	38.2	16.7	24.6	36
S6	18.7	55.7	28.9	61.4	15.9	39.4	31.7
S7	8.0	29.6	26.9	27.4	15.8	28.9	30.6
S8	16.0	72.7	30.2	27.9	16	25.9	31.2

).

3.5.3. Pearson’s Correlation Coefficient for Heavy Metals

The Pearson correlation coefficient generally describes a negative or positive correlation between two parameter groups and their intensity (so-called correlation coefficient r). According to [63], a weak correlation occurs at 0.1 < r < 0.3, a moderate correlation at 0.3 < r < 0.5, and a strong correlation at r > 0.5. With values of 0.867–0.967 between the heavy metal couples, there was a strong correlation between the heavy metals Pb, Cd, Zn, Cr and Ni at all sites (see

Table 10. Pearson correlation between heavy metal couples in soil samples of all sites.

Pearson Correlation
	Cd	Pb	Zn	Cr	Ni
Cd	1
Pb	0.8806662	1
Zn	0.9174506	0.8665371	1
Cr	0.9198652	0.95539986	0.96654252	1
Ni	0.8952122	0.91603492	0.90773979	0.95010035	1

), indicating a common source of soil pollution.

3.6. Assessment of Heavy Metal Pollution in Soil Samples

3.6.1. Contamination Factor (CF) in Soil Samples

When the world surface rock average levels of heavy metals according to Table 7 is used as the reference for the respective site-specific contamination factor (CF) according to Table 2, it can be seen that in both test series, all sites were highly contaminated with Cd (CF > 6). The highest cadmium value was achieved at sampling site S3 with a CF value of 24.4. In contrast, the lowest contamination was measured at site S1 with a value of 14.2, which is still more than a factor of 2 above the next best assessment class. Furthermore, with the exception of site S3, all sites were moderately contaminated with Pb. According to the CF classification, all sites were moderately contaminated with Zn, while this was valid only for S3, S4, and S6–S8 in the case of Cr. The excluded sites S1, S2 and S5 showed only low levels of contamination with this heavy metal. With the exception of sites S1 and S5 (low contamination), the remaining sites showed moderate levels of nickel contamination (Ni), ( see

Table 11. Contamination factor of soil samples at all sites.

CF	Cd	Pb	Zn	Cr	Ni
S1	14.2	1.33	1.16	0.5	0.8
S2	17.4	1.6	1.2	0.7	1.0
S3	24.4	3.12	2.74	1.8	2.3
S4	21.7	2.9	2.7	1.7	2.1
S5	12.3	1.3	1.3	0.6	0.8
S6	20.6	1.8	2.4	1.3	1.3
S7	23.8	3.0	2.6	1.6	1.6
S8	19.0	2.8	2.1	1.4	1.5

).

3.6.2. Igeo Factor in Soil Samples

In a similar way, the Igeo factor specified in more detail in Table 3 can be used as a benchmark for the heavy metal contamination of the sites. It can be seen here that site S3, which is directly adjacent to the combat zone, exhibited strong to extreme contamination with Cd and thus corresponded to class 5, while the contamination levels of the other sites were slightly lower for this heavy metal and could thus be assigned to class 4 (strong contamination). However, for lead, the sampling sites S3, S4, S7 and S8 were located in the transitional area between classes 1 and 2 and thus with moderate contamination, the other four sites showed no or very low contamination. The same applied to zinc, although site S6 was more heavily contaminated, and site S8 less so. Nevertheless, all five contaminated sites (S3, S4, S6, S7, S8) were low to moderately contaminated, while the remaining sites showed no detectable contamination.

For the two heavy metals Cr and Ni, low levels of contamination were identically detectable at sites S3, S4 and S7, while the remaining sites could be regarded as uncontaminated (see

Table 12. Igeo factor of soil samples in all sites.

Igeo	Cd	Pb	Zn	Cr	Ni
S1	3.24	−0.17	−0.36	−1.64	−0.86
S2	3.53	0.10	−0.32	−1.10	−0.53
S3	4.02	1.06	0.87	0.27	0.59
S4	3.85	0.96	0.84	0.19	0.47
S5	3.03	−0.25	−0.26	−1.31	−0.98
S6	3.78	0.26	0.70	−0.26	−0.17
S7	3.99	0.99	0.77	0.07	0.11
S8	3.66	0.89	0.47	−0.06	−0.05

).

3.6.3. Pollution Load Index (PLI) in Soil Samples

Classifying the soil samples according to the PLI classification for heavy metals (see Table 4) showed that all soil samples were generally contaminated with heavy metals, with only the contamination levels differing depending on the sampling site. As expected, sites S3, S4 and S7 revealed the highest heavy metal contamination due to their proximity to the conflict zone (classification level 4, extremely polluted). In terms of pollution, sites S6 and S8 showed heavy pollution, and sites S1, S2 and S5 showed moderate to heavy metal contamination in both test series (see

Table 13. Pollution load index values of soil samples in all sites.

Sites	PLI	Classification
S1	1.54	Moderately
S2	1.90	Moderately
S3	3.86	Extremely
S4	3.60	Extremely
S5	1.55	Moderately
S6	2.72	Heavily
S7	3.42	Extremely
S8	2.96	Heavily

).

Deduced from the measured values, it can be pointed out that sites S3, S4 and S7 have the highest levels of pollution due to their proximity to the conflict area, while the pollution at site S6 is primarily due to its industrial use. The extraordinarily high deflection of the PLI indicator with respect to S8 arises from the fact that this site is located in the immediate vicinity of the Algayyarah oil fields south of the city, which were set on fire during the occupation and where the uncontrolled burning of crude oil caused contamination of the surrounding area.

Accordingly, sites S1, S2 and S5 have a comparatively low level of contamination due to their form of utilization and the greater distance from the conflict areas and can therefore be assigned to class 2 according to the PLI methodology (moderately polluted, see Table 13).

4. Conclusions

The results of this study show that the eight sampling points in the urban area of Mosul were consistently characterised by heavy metal concentrations above the global average values for at least one of the heavy metals investigated (Cd, Pb, Zn, Cr, Ni). While measuring points S3, S4, S6, S7 and S8 were characterised by consistently elevated measured values for all heavy metals, only the concentration levels of Cd, Pb and Zn exceeded the valid limit values at sites S1 and S5. At S2, the limit values were exceeded for all heavy metals except Cr.

Statistical analyses using a t-test show that the loads between the test series (series 1: winter, spring; series 2: summer, autumn) were subject to significant differences for the heavy metals Cd, Pb, Cr and Ni. The measured values from the winter/spring period showed lower concentrations than in the summer phase due to the leaching of heavy metals through soil passage into deeper zones during the rainy phase. Due to zinc sources in the soil, a corresponding effect could not be observed for Zn.

The evaluation based on the contamination factor (CF) shows that all sites are strongly contaminated with Cd and moderately contaminated with Pb (with the exception of the considerable contamination at S3). All sites were moderately contaminated with Zn, Cr and Ni, whereas sites S1, S2 and S5 were only slightly contaminated with Ni and Cr.

The evaluation using the Igeo factor shows that sites S2, S3, S4, S6, S7 and S8 were moderately to heavily contaminated with Cd, and the two remaining sites were moderately contaminated with Cd. For lead (Pb), sites S2, S3, S4, S6, S7 and S8 showed moderate to low contamination, while no significant contamination was detected at S1 and S5. Similarly, sites S3, S4, S6, S7 and S8 were moderately to not at all contaminated with Zn, and sites S1, S2 and S5 were not contaminated with Zn. For chromium (Cr) and nickel (Ni), low to moderate contamination occurred in sites S3, S4 and S7 close to the conflict zone, while no contamination was detected in the remaining sites.

The PLI-based characterisation shows that sites S3, S4 and S7 exhibited extreme heavy metal pollution in accordance with their location, while sites S6 and S8, which are further away from the conflict area, were heavily polluted according to other aspects (industrial use, incineration fall-out), and sites S1, S2 and S5 were only moderately polluted with heavy metals.

The study indicated the high pollution risk with heavy metals, especially with Cd and Pb within the conflict area and adjacent areas due to the effect of war and military activities within the city. By focusing on test areas with residential and light-commercial use and therefore negligible impact of the land use on heavy metal input, military activities were identified as the main pollution source.

There is also a fundamental concern about the possibility of releases to other ecosystems. Therefore, the study recommends the complete removal of debris in the conflict area to prevent the leakage of heavy metals.