Five sediment core samples were obtained at different locations that represent specific sites within the Khor Al-Zubair area with regards to both assessment and estimation of hydrocarbon variances. Subsequently, the dried sample was initially mixed together using a mechanical grinder to achieve fine powder followed by sieving through a 63 µm mesh to achieve a more constant grain size.

Measurement of Total Petroleum Hydrocarbons (TPHs)

The extraction and analysis of hydrocarbons followed the methodology of Goutx and Saliot 1980. An exact amount of 50 g of dried, ground, and sieved sample sediment was used, and placed in a thimble for the Soxhlet apparatus for intermittent extraction. The extract was made free of elemental sulfur by the addition of activated copper. After, 150 mL of solvent, containing a mixture of methylene chloride and methanol (3:1, v/v), was added to the sediment sample. Before the extract was passed through a column which was packed in this order on the bottom of the column, glass wool, 5 g of silica gel, 2 g of alumina to remove residual fatty acids, and then 2 g of anhydrous sodium sulfate to capture remaining moisture. A standard for crude was used to create standard solutions. Near the 0.003 g weight of crude oil was solubilized for stock in a 10 mL tube with analytical n-hexane. This solution was used as the calibration standard for the determination of TPHs in the extracted sediment samples. TPH concentrations were measured using a Shimadzu RF-530 spectrofluorometer at an excitation wavelength of 310 nm and an emission wavelength of 360 nm and with monochromator slit widths of 10 nm [12].

Measurement of Total Organic Carbon (TOC%)

For the determination of total organic carbon (TOC%), the combustion method described by Ball 1964 was employed. Prior to analysis, all sediment samples were finely ground and passed through a 63 µm sieve to ensure homogeneity and consistency in particle size. A clean crucible was oven-dried and weighed accurately. Subsequently, approximately two grams of the prepared sediment were placed into the crucible and subjected to ignition at 550 °C for 48 hours. After combustion, the crucibles were transferred to a desiccator and allowed to cool to ambient temperature to achieve equilibrium with the surrounding environment. They were reweighed several times until a constant mass was obtained [13]. The TOC content was calculated based on the difference in weight before and after combustion using the following formula:

TOC % = (W2-W3/W2-W1) *100%

where:

W₁ represents the weight of the empty crucible,

W₂ is the combined weight of the crucible and sediment sample before ignition, and

W₃ is the weight of the crucible with the residue after combustion.

Figure 3

Figure 4

Figure 5

Most of the values of TOC are decreased gradually as depth increased. This means that lower concentrations of TOC are typically found in the deeper depths. The primary reason for this trend is that the upper layers of sediment contain a higher concentration of organic materials [14]. Excess TOC in the marine environment may be a result of a combination of an excess of plant debris combined with anthropogenic input. Elevated organic carbon concentrations are often associated with recurring algal blooms in the upper water column largely induced by nutrient enrichment of nitrogen and phosphorus. In heavily populated areas along the coast, anthropogenic inputs from pathways such as stormwater can cause serious and at times, ecologically disruptive events [15].

Khor Al-Zubair has a well-documented record of anthropogenic historical activities contributing to Total Petroleum Hydrocarbon (TPH) contamination. This pollution originates primarily from land-based sources and additional human intervention [8]. The southern region of Iraq, south of the Al-Faw Peninsula and into the eastern Arabian Peninsula, possesses a major oil refinery; one of the primary discharge channels of the oil refinery is the Al-Zubair channel—this channel carries treated waste to the Arabian Gulf. The area has intense marine traffic including oil tankers and crew transfer activity that provide the transport of petroleum from refineries to oil fields and offshore and marine industries. The extent of human activities in the marine environment is an important contributor of hydrocarbon residues to the environment. Added to the marine activities, industrialization and urbanization in connected urban areas has created yet another layer of nuisance light pollution, with potential impacts on sedimentation as well as the adverse impacts on aquatic organisms [16].

The study showed that the pollution levels increased as sampling sites approached 4 and 5, which is likely due to the fact that the port an area historically known to introduce higher pollutant levels from vessels and vessel activity, was located nearby. Table 2 shows the change in concentration for different pollutants and their general trend, specifically noting that the 5th sampling station had a higher presence of pollution compared to the 3rd sampling station.

The relationship between Total Organic Carbon (TOC) and Total Petroleum Hydrocarbons (TPH) indicated a strong positive slope at most sampling stations, meaning sites that have higher concentration of organic carbon would also have higher sedimentary hydrocarbons. The response is often interpreted to be attributed to relatively strong bonding of organic matter with hydrophobic compounds to allow for the adsorption or bonding of hydrocarbons in sediments. In addition, organic matter and contaminants relating to organic matter are likely to become fixed in areas of fine sediment accumulation and when low energy conditions of hydrodynamic processes are present. Station 2 showed a negative correlation with TOCs and TPHs, which could imply that the local environmental or sedimentary processes (i.e, more current activity, less clay) would result in less sedimentary hydrocarbons being retained in the sediment, even if organic matter was present.. Another plausible explanation is that the organic matter in this station is primarily of natural origin (e.g., derived from terrestrial or marine productivity rather than being associated with petroleum pollution sources.

Overall, the predominance of positive correlations in the study area supports the notion that organic carbon plays a significant role in controlling the distribution and accumulation of petroleum hydrocarbons in surface sediments. The differences in relation between stations indicate that not only is the amount of hydrocarbons in sediments dependent on organic matter concentration at all stations, but that the concentration of hydrocarbons in sediments may also depend on some combination of hydrodynamic regimes, grain size distribution, and the level of pre-existing environmental contamination. Most studies report a positive TPHs-TOC relationship, suggesting that organic-rich sediments accumulate more hydrocarbons.

Tables 3 and 4 show the comparison of our results and the previous studies results