Introduction

The Shatt Al-Basrah Channel in southern Iraq, a place where the Tigris and Euphrates Rivers circulate is a sensitive ecological spot. There are various anthropogenic activities including agricultural runoff, untreated wastewater from households, effluents from oil refineries hence leading to this fragility [1,2]. However, the environment has suffered greatly in the area over the past decades, particularly during the Second Gulf War (1990-2003) when the Shatt Al-Arab system was left exposed to environmental pollutants such as heavy metals and crude oil [3]. These pollutants have been accumulated in bottom sediments over time through trophic transfer and bioaccumulation processes which are toxic for water column species and benthic ecosystems [4].Although a number of studies have been done to study the hydrochemical characteristics of the Shatt Al-Arab River, few studies have been done for comprehensive review of sediment pollution, especially with regard to selenium [2].

Selenium has complicated environmental behaviour; in oxidising circumstances, it prefers to produce selenate (SeO₄²⁻) compounds, which are very stationary in sediments, by combining with small particles and colloidal materials [5]. Because of its geochemical stability, selenium is a crucial indicator of ongoing pollution. But there are serious ecological and public health issues because of its capacity to bioaccumulate and biomagnify in aquatic food webs [6]. The significance of concurrently monitoring selenium and other potentially harmful elements is highlighted by recent studies that reveal high levels of trace elements such As, Cr, Fe, Mn, Ni, and Pb in southern Iraq [7]. With the aid of contamination indices such as the Contamination Factor (CF) and Geoaccumulation Index (I_geo) the present study aims at: (1) identifying the concentrations of residual selenium (Se) in sediment cores from the Shatt Al-Basrah Channel; (2) assessing its vertical i.e., depth-dependent distribution, in order to infer time accumulation trends; (3) discriminating the contribution of lithogenic i.e., natural and anthropogenic sources to identify the potential sources of Se contamination; and (4) assessing the ecological and health associated risks. This study enhances our understanding of the dynamics of man-made selenium in river systems by addressing these objectives, providing important data for monitoring and innovation throughout the Arabian Gulf.

Materials and Methods

Study Area

Figure 1. Figure 1 :Sample collection sites in the Shatt al-Basra canal

Methods of Sampling Gathering of SamplesDuring March and April of 2022, 0-30 cm deep sediment cores with a depth profile were collected from all the sample sites. To allow for extensive stratigraphic investigation, each core was divided into 6 depth intervals: those being 0-5, 5-10, 10-15, 15-20, 20-25, and finally 25-30 cm. To increase analysis accuracy and repeatability, each sample was collected in a tripling manner.Methods of Analysis Selenium Determination: Inductively coupled plasma optical emission spectrophotometer (ICP-OES) . This was used to measure the concentration of selenium (Se) Residual Fraction Extraction: Sequential extraction process by Tessier [8] It used to extract residual (inert) fraction of selenium. In order to separate Se embedded in crystalline mineral formations, sediment residues have to be digested using HF and HClO₄ acids.

Quality Assurance (procedural blanks, certified reference materials, CRMs) To ensure the precision and accuracy of analysis, Three geochemical indicators were computed in order to evaluate the degree of Se contamination:

1-The enrichment factor (EF) was measured according to Sutherland(2000) [9]: EF= (Se/Fe) sediment /(Se/Fe) background

Where the typical crustal value, or background Se/Fe ratio, is 0.2.

Where : Enrichment Factor (EF) < 1: No enrichment , 1–3: Minor (natural sources) and EF= 3–10: Moderate (anthropogenic influence),

2. The Contamination factor (CF) is calculated by means of the equation of Hakanson (1980) [10]:

CF = Se concentration in sample / Se concentration in background

The background value of selenium in crust erths is 0.4 ppm . Contamination Factor (CF) can be described less than < 1 which refer to Low contamination , while if their value was located between 1 to 3 that refer to Moderate contamination , and 3 to 6 refer to high contamination and very high contamination if its value up than 6.

3- Geo accumulation index (Igeo) values were calculated of different elements, (Igeo) was calculated by the equation that introduced by (Muller, 1969) [11] as described below:

I-geo = log2 (Cn / 1.5 Bn)

Where: Cn: The measured concentration of element n in the sediment .

Bn: Background for the element in the sediment.

The Geo accumulation index was originally proposed by Muller (1969) and categorized into six classes, these classes are described as follows:

Results and Discussion

The other levels of the concentration of selenium in sediments in the Shatt Al-Basrah Channel were determined in five stations and six levels of depths (0-5 cm to 25-30 cm) adding up to Table 1. The determined concentrations of Se had a range of 3.69 to 18.50 ppb, and they were also characterized by the apparent spatial and vertical heterogeneity.

The heterogeneity of Se concentrations showed the spatial and vertical heterogeneity with effective diffusion coefficients varying from 3.69 to 18.50 ppb.

Overall, the highest Se concentrations in Station 2 ranged from 18.14 ppb in surface sediments (0-5 cm) to 9.22 ppb at the deepest layer (25-30 cm), suggesting local sources or sediment properties being favourable for Se accumulation. Similarly, Station 3 had high Se contents close to the surface (mean 18.08 ppb at 0-5 cm) then a sharp decline in Se content 9with depth to 4.31 ppb at a depth of 25-30 cm. Station 1 was the lowest at all depths with Se decreasing from 12.14 ppb at the surface to 6.38 ppb in the deeper layers. Stations 4 and 5 had intermediate levels with Station 5 slightly higher than Station 4 in the intermediate depths (10-25 cm) as can be seen in Figure 2.

All sites showed a reduction in the concentration of selenium with depth. The vertical distribution shows recent input of anthropogenic selenium or an increase in the concentrations within the upper sediment layers and is likely linked to current industrial and agricultural runoff within the region.

Deeper sediments, which represent older deposits, showed reduced selenium levels, indicating historical lower selenium loading or the effects of diagenetic processes reducing Se availability in older sediments. This trend is consistent with previous studies assessing trace metal distribution in coastal and estuarine sediments, where surface sediments typically reflect recent contamination while deeper layers correspond to historical baseline levels [12,13 ,14].

Selenium is an essential trace element but can become toxic at elevated concentrations to aquatic organisms and ecosystems [15 ,14] The range of selenium concentrations found (up to ~18 ppb) in this study is relatively low compared to toxic levels reported in some contaminated sites but highlights the importance of monitoring in this region, particularly given the increasing industrial activities disrupting the Basra shoreline .Sediment selenium concentrations observed here can relate to bioavailability and potential biomagnification risks in the aquatic food web, as selenium is mobilized from sediments under changing redox conditions [16]. The spatial variability also underscores the influence of local sources and sediment characteristics on selenium distribution.

Sources and Distribution of Selenium in Sediments

Natural Sources

Selenium (Se) naturally occurs in sediments due to: Geogenic weathering of Se-rich bedrock (e.g., shales, phosphorites) .Anoxic conditions in aquatic systems, where Se binds to organic matter and Fe/Mn oxides [5]

In the Shatt al-Basrah channel, background Se levels (6–12 ppb in deeper layers) align with uncontaminated estuarine sediments globally (5–15 ppb) [17]. However, surface enrichment (up to 18.5 ppb) suggests additional anthropogenic inputs.

Anthropogenic Sources

Industrial Discharges such as Oil refineries and petrochemical plants in Basrah release Se via wastewater (Se is a byproduct of crude oil processing) [18]. Phosphate fertilizers from agricultural runoff contribute Se (average 5–100 mg/kg in fertilizers).Urban and Agricultural Inputs like Sewage effluents (Se concentrations up to 50 ppb in untreated wastewater) [19]. Irrigation drainage from Se-rich soils (common in arid regions like southern Iraq) [20].Atmospheric Deposition such as Coal combustion from regional power plants emits particulate Se [21].

Spatial and Vertical Distribution

Surface sediments (0–5 cm): Highest Se levels (Stations 2 and 3: 17–18.5 ppb) correlate with industrial proximity. Subsurface (15-30cm): Decreasing concentrations. At Station 3, a recorded 4.3 ppb at (25-30cm) reflects historical deposition trends. At Station 5, may suggest episodic contamination events, such as dredging activities or accidental discharges, as possible origins where anomalous mid-depth peaks of Se were recorded (12.22) ppb at (10-15 cm). The calculated values of the enrichment factor (EF) exceeding 3 (Table 3) are further evidence for the occurrence of anthropogenic enrichment especially in areas affected by urban or industrial activities.

Selenium Distribution in Sediments

Residual concentrations of selenium (Se) at all the sampling stations and sediment depth intervals are shown in Table 1. Here are some obvious patterns:

Station 1: Selenium concentrations varied between 6.20 and 12.64 ppb, and high selenium concentrations were found in the surface layer (0-5 cm; mean = 12.14 ppb).

Station 2: Surface sediments had significantly higher concentrations of Se (17.46 to 18.50 ppb), representing possible anthropogenic inputs in the area.

Station 5: Showed a strong fluctuating behavior of Se concentrations (7.43-13.63 ppb), which might have been a mixture of natural geochemical effects and localized anthropogenic inputs._.

Human Health Implications of Selenium Contamination

Exposure Pathways

Dietary Intake (Primary Risk)

1-Fish and shellfish consumption: Se gets bioaccumulated in food chains of the aquatic environments. Local species (e.g., Liza abu, Metapenaeusaffinis) may have 0.5-2.0 ug/g Se [22], and this exceeds WHO's safe limit (0.3 ug/g wet weight) [23].

2- Crop irrigation: Se laden water contaminate Rice and Vegetables [20].

Direct Exposure

1-Dermal contact (e.g. fishermen handling sediments).

2- Suspended Se particles can be inhaled during droughts

Toxicity Mechanisms

1- Deficiency (< 40 µg/day): Linked to Keshan disease (cardiomyopathy) [24].

2- Excess (> 400 µg/day): Causes selenosis, with symptoms:

3- Acute: Hair loss, nail brittleness, neurological disorders [25].

4-Chronic: Liver cirrhosis, teratogenicity [26].

Assumes daily fish intake of 100–200 g [22].

Case Studies and Regional Comparisons

Arabian Gulf: Se levels in fish from Kuwait Bay (2.1 µg/g) caused selenosis cases .suggesting similar risks in Shatt al-Basrah. (Tables 3 ) and (Figure 2)

Interpretation:

EF > 1.5 suggests anthropogenic enrichment (e.g., industrial discharges).

CF > 1 indicates moderate contamination, particularly at Stations 2 and 3.

Igeo values (0–1) classify sediments as "uncontaminated to moderately contaminated."

Comparison with Regional Studies

Previous studies in the Arabian Gulf report Se concentrations of 5–15 ppb in uncontaminated sediments [17]. Our findings exceed these levels, aligning with trends of increasing Se pollution due to urbanization and industrial growth [18]

Human Health Implications

The presence of high concentrations of selenium in sediments can result in a variety of pathways that are a potential health risk to the local population. One of the most important pathways is the bioaccumulation of selenium in aquatic organisms (fish, shellfish, etc.) which are important sources of nutrients for the local population. Hence, selenium consumption in humans may be elevated as a result of contaminated seafood consumption. Furthermore, selenium above the need for the organism can trigger toxic effects, which is known as selenosis and that causes neurological, gastrointestinal and dermatological disorders [25].

Figure 2. Figure 2: Mean concentration of Se in different core stations

Observations:

•Highest Se: Station 2 (0–5 cm; 18.14 ppb) and Station 3 (0–5 cm; 18.08 ppb).

•Anthropogenic Signal: EF > 3 in surface layers (0–15 cm) at all stations.

•Health Concern: Stations 2 and 3 show the highest risk (CF > 1.5)

Geogenic Sources and Anthropogenic Sources

Surface Enrichment: High Se concentrations in (0-5 cm) are indicative of recent inputs from:Abused wastewater: estimated and referred to as industrial discharges, oil refineries, and Se-rich effluents, such as Basra Petrochemical Complex [17]. Agricultural effluents: Runoff of Se-containing fertilizers/pesticides used in date palm and cereal farms [18] Residual Fraction Dominance: Se in crystalline phases suggests dominance of geogenic origins (e.g., weathering of Cretaceous-Pliocene formations). However, surface spikes imply adsorption of anthropogenic Se onto mineral lattices [14].

Basra Shore Channel’s Se levels align with moderately contaminated zones in the Gulf. Higher concentrations than Mesopotamian marshes reflect proximity to industrial hubs. Declining trends with depth mirror contamination timelines linked to Iraq’s post-2000 industrial expansion [5].

•Sediment Quality Guidelines (SQGs):

While there are no universally agreed sediment Se standards, some countries and research groups propose guideline values. For instance, the Canadian Council of Ministers of the Environment (CCME) [29] sets a Canadian Interim Sediment Quality Guideline (ISQG) for selenium at 0.80 mg/kg (800 ppb) dry weight, which is much higher than Basra sediments’ Se levels. However, the comparison requires caution since the Basra data are in parts per billion (ppb), which is equivalent to µg/kg, so 18.5 ppb is18.5 µg/kg, well below the Canadian ISQG[4,7] :

•Water Quality and Biota Intake Guidelines:

Selenium toxicity risk is most often assessed through water concentrations and biota tissue levels because selenium bioaccumulates. The U.S. EPA [30] recommends a chronic aquatic life criterion for Se in water of 5 µg/L to protect fish and wildlife [30]. Sediment-bound selenium can be a source releasing Se into water, especially under changing redox conditions.

Environmental Implications

Bioaccumulation Risk: Residual Se is less bioavailable but may transform to soluble forms under redox changes [31]. Ecological Threats: Benthic organisms and fish (e.g., Tenualosailisha) face teratogenic risks from chronic Se exposure . Baseline Assessment: Residual Se provides a "background" benchmark for future pollution studies.

Sources include geogenic weathering (dominant in deep layers) and anthropogenic inputs (surface enrichment from oil, agriculture, sewage). Concentrations are moderate in relation to Gulf region but need to be monitored in relation to increasing industrial activity.

Implications for Human Health

•Dietary Intake and Exposure:

The major mode of human exposure to selenium is through the consumption of fish and shellfish that can bioaccumulate selenium from sediments and water [6]. the typically range of Health-based maximum levels for selenium in food differ, but the range is between 0.1 to 0.5 mg/kg. The selenium levels in sediments from Basra Shore Channel are low compared to these levels, so this is less of an immediate health risk in terms of over-consumption of selenium through diet -- provided that the bioaccumulation does not concentrate selenium to a high degree in aquatic organisms.

•Toxicity Thresholds:

Chronic Selenium Toxicity and Risk Classification Selenium (Se) overexposure (selenosis) in human beings can be seen with an everyday dose of over 400 ug, or the corresponding recommended dietary allowance is around 55 ug/day [23]. Sediment sites below 100 ppb Se are typically considered low risk sites as the potential for biomagnification to reach toxic exposure in people is low [14].

Environmental and Health Risk Summary Current Risk Level: Selenium concentrations in Basra sediments are 3.69 - 18.50 ppb, and are well below known sediment quality benchmarks. These concentrations are unlikely to cause acute toxicity to benthic organisms or pose acute dietary health risks to humans[14,4]

Precautionary Considerations: Some sites exhibit slightly higher Se values (e.g. Station 2 ~18 ppb), suggesting the importance of a continued monitoring program. Anthropogenic disturbances, like redox alterations or increased release of industrial effluents, may mobilize Se in the sediment into more bioavailable forms increasing ecological risk and potential human exposure.

Conclusions

_{Selenium pollution of the Shatt Al-Basrah Channel is primarily caused by industrial pollution and agricultural runoff. ecological and human health risks were moderately high, as Surface sediment analyses reveal. Prompt mitigation measures such as improving effluent treatment systems and providing consumption advisories on diet are important to reduce future exposure.}

_{Based on the measured residual concentrations of selenium in the sediments of Shatt Al-Basrah (3.69-18.50 ppb), potential human health concerns are mostly associated with the bioaccumulation and dietary intake pathways. Although the residual fraction of selenium is relatively immobile and less bioavailable, potential geochemical transformations may take place in the aquatic environment that may increase the mobility of selenium, posing long-term ecological as well as human health concerns .}

Recommendations:

The results suggest that a more comprehensive assessment of bioavailable Se fractions (i.e. exchangeable and organic-bound pools) in future studies is required to more accurately estimate ecological and human exposure risk. Therefore, further studies of the interactions between Se and co-occurring contaminants such as heavy metals and polycyclic aromatic hydrocarbons (PAHs) may help to enhance our understanding of mechanism of synergistic toxicity and identification of sources of pollutant. In addition, there should be an intensification of regulatory mechanisms and more rigorous enforcement of limits of industrial effluent discharge in order to protect humans and aquatic environments.