Investigation of the hydrocarbon contamination of the Dibdibba aquifer in Al-Zubair area, southern Iraq

The importance of Al-Zubair area in Basra governorate is highlighted by the presence of agricultural activities. The Dibdibba aquifer is the main supplier of water which used for irrigation, livestock and some different industries are scattered around the region. This research aims to investigate the extent and impact of hydrocarbon contamination of the Dibdibba aquifer in Al-Zubair region, southern Iraq, due to its proximity to oil fields sites. 18 groundwater samples from different locations within the aquifer were collected in dry and wet season to determine their chemical and physical properties and total petroleum hydrocarbon (TPH) including petroleum-based compounds and their derivatives. The analytical results revealed that the Dibdibba groundwater in this area is polluted with TPH and have high levels of electrical conductivity (EC) and total dissolved salts (TDS) due to the high content of salts along with the high concentrations of calcium and magnesium ions. The overall outcomes of the main elements of water showed that it is unfit for human use due to the high levels of the above parameters. All the water tests carried out indicate the presence of contamination with petroleum hydrocarbons, surpassing the allowable limits set by the World Health Organization (2008) and Iraqi Quality Standard (2009). This observed particularly in areas adjacent oil spills, where the TPH concentrations exceeding 35 μg/l in the northwest of Al-Zubair area near the Rumaila oil field. Significantly, this investigation indicated a clear evidence of hydrocarbon contamination in the study area.


Introduction
Groundwater is widely used in areas where the surface water is limited, particularly in dry and desert areas.It is used for various purposes, depending on its quality, including drinking, cooking, cleaning, agricultural irrigation and industry.Groundwater is used for drinking water in most places of the world.
Contamination of groundwater by contaminants like sewage and industrial waste has resulted in significant declines in its quality and contamination, endangering public health and necessitating immediate monitoring of groundwater's physical, chemical, and biological properties, in order to determine its suitability for a wide range of potential uses [1].Groundwater is subjected to several contaminants arising from a number of sources, including urban and agricultural pollution, which led to be a global concern nowadays [2].
Groundwater quality is influenced by several stages of the hydrological cycle, for example rainfall, snow and runoff, which contain water that has lost its purity throughout its passage through the upper atmosphere prior to reaching the Earth.Rainwater is affected by pollutants in the atmosphere such as gases, dust, Solid microparticles that are transported by wind to the atmosphere and Furthermore, industry emissions of harmful and dangerous contaminating gases.When rainfall infiltrate into the soil, elements and pollutants will transfer from the atmosphere and enter by infiltration process to the soil and then by percolation transfer to groundwater.Permeable water then interacts with the soil's minerals and rocks and dissolve them [3].
Contamination from hydrocarbons is a common hazard in industrial areas where oil industries are prevalent.The contamination of groundwater with hydrocarbons considers as a serious issue that facing the Dibdibba aquifer in Al-Zubair area due to its proximity to the areas of crude oil production.The locations of the investigated area and the oil field sites are shown in (figure 1).It is well known that the oil contamination causes direct and indirect harm to humans by altering the physiochemical characteristics of water and causes harm to aquatic organisms [4].The Dibdibba aquifer may have been tainted by oil spills and gaseous emissions due to the combustion of hydrocarbon materials in wells at neighboring oil fields.
Agricultural and domestic use of groundwater in the southern and southeastern areas of Basrah governorate, is the primary source of water.This is due to the limit of fresh surface water and location of Al-Zubair area within a dry desert environment [5].
A number of authors have studied the Dibdibba aquifer on different aspects including the quantity and quality of groundwater in addition to its contamination (for example, [6]; [5]; [4]; [7]; [8].
The present research is designed to evaluate the extent of hydrocarbon contamination and the purity of the groundwater in the Dibdibba aquifer in the Al-Zubair region, which is close to the oil fields.It also aims to measure the physical and chemical properties in addition to the amount of total petroleum hydrocarbons to determine its suitability for the potential uses.

The study area
The area under study is located in Al-Zubair-Safwan region, southern Iraq, precisely between (30°00 -30°30 ' ) latitude and (47°00 -47°53 ' ) longitude, (figure 2) shows the locations of the wells in the study area, as well as the location of the North Rumaila oil fields and the impact of oil emissions in the area.The area has a hot, dry summers and cold, humid winters.The temperature in last years has reached more than 50 °C in July and August, while it is dropped to 8 °C in January.Exposed to the lack of rainy seasons, the highest rainfall rates over the last ten years in January was 20 mm [7].Which reflects the highest rainfall rates during this month in the last decade.
The area of study is covered mainly by the Dibdibba Formation which it considered the main aquifer in this area and composed of friable alluvial sand gravel [9].In general, the area is flat with a gradual decrease in the elevation from Jabal Sanam in the southwest which is highest surface level, to the northeast the lowest levels in study area as displayed in Figure 3 [5]; [10].It is important to note that Jabal Sanam, shallow wadies and sand dunes are the most recognizable geomorphological features in the Dibdibba plain [11]; [12].The Dibdibba Formation considered as the most important aquifer in the study area.It comprises of two types of aquifers; the upper unconfined aquifer and the semi-confined to confined one, the two aquifers are separated by a hard clayey layer of 2 to 4 m thick [11].According to the appropriate static water levels, the direction of groundwater is from the west and southwest towards the drainage area in the Shatt Al-Basrah canal and Khor Al-Zubair at the east and north-east directions in the Al-Zubair-Safwan area characterizes the flow system in the region.[11] (figure 4).While the flow direction in the northwest areas is predominantly directed towards the extreme northwest, aligning with the path of the Shatt Al-Basrah canal to the north and northwest.Notably, this flow pattern is evident in the surrounding areas near the Rumaila and Luhais oil fields.This direction of flow corresponds to the flow with [13] study where he stated that "The water table (groundwater flow direction) map has a radial flow from southern area towards west, north, east and northeastern directions" as illustrated in (figure 5).
The pattern of groundwater flow depends on the structural and geological setting.Tectonically, the area is located on the unstable shelf within the Zubair subzone of the Mesopotamian zone [14]; [15].The principal subsurface features in the area are the Al-Zubair, Rumaila, and Luhais anticlines, which are typically long and narrow, producing subsurface anticlines separated by synclines-oriented NW-SE [16]

Groundwater samples collection
18 wells distributed in the region were selected to collect groundwater samples in order to measure the physiochemical properties and the total petroleum hydrocarbons (TPH).The samples were collected, transported, and stored for subsequent analysis according to the protocol of the American Public Health Association [17].The samples of groundwater were taken in five-liter opaque glass bottles and tested in the field with carbon tetrachloride to measure the content of hydrocarbons.Portable Hanna Instruments meters were used to measure electrical conductivity (EC), total dissolved salts (TDS), pH, and temperature on-site in real time.All of the physiochemical properties were analysed at the laboratories of the University of Basrah according to the standard procedures outlined by [18].The total petroleum hydrocarbons was extracted from groundwater samples based on the United Nations Environment Program (1989) technique.Ten milliliters of chloroform (CHCl3) were applied to each liter of water sample.A spectrofluorometer was used to determine the total petroleum hydrocarbon concentrations in groundwater samples.Data of 47 wells acquired from the General Commission for Groundwater in Basrah were used to draw a flow map in the study area.ArcGIS (ArcMap 10.8.1) software was utilized to depict the spatial distribution of groundwater quality in the area under investigation.

The systematic error for groundwater samples
Prior to conduct the tests of the well water samples, a systematic error for these samples should be applied in order to ensure that the data obtained from the laboratories are reliable for the subsequent interpretation of the measured parameters for any area.
The analytical accuracy for the main ions in the water samples is carried out by calculating the absolute difference between the total cation and anion concentrations in (epm) [19]: (1) The absolute difference or relative difference (R.D) for the groundwater is within the standard levels (R.D ≤ 5%), the results are accurate and accepted for interpretation, (5% ≤ R.D ≤ 10%) the results are acceptable and probable certain, and (R.D > 10%) the results cannot depend in hydrochemical interpretations with uncertain of analysis [20].The average R.D% values ranges between (0.63-8.5%) as listed in Table 1.

Results and discussion
The physical properties of groundwater samples including temperature, EC, TDS, pH, and total hardness (TH) were examined in the collected groundwater samples.The chemical, physical, and microbiological properties of groundwater are essential to determine the water nature and the most appropriate applications for domestic, agricultural, livestock, and industrial purposes.Chemical analysis of groundwater includes determining the concentrations of inorganic constituents, including the cations like Ca 2+ , Mg 2+ , Na + , and K + , and the anions like Cl -, SO4 2-, and NO3 -and those contribute to the alkalinity, which are typically expressed as an equivalent amount of carbonate and bicarbonate [21]; [7].

Physical properties
The pH scale measures how acidic or alkaline of water is, and it ranges between 0 and 14.The pH level at 7 refers to the neutrality, while less than 7 indicates acidity and greater than 7 implies alkalinity.The pH is a crucial indicator of the purity of water as presence the contaminants can change the pH value of water.Thus, it will affect and harm animals and plants when using it.
It can be indicated from Table 2 that the pH of the groundwater samples during the dry season varies from 7.10 to 8.67 with an average of 7.85, while in the wet season it varies from 7.03 to 8.44 with an average of 7.63, suggesting that the groundwater tends to be slightly alkaline in the research area.The alkaline properties might be due to the dominance of the bicarbonate contents in this aquifer.The capacity of water to conduct electrical flow is termed as electrical conductivity (EC).
Measuring the EC of a solution gives a relative indication of the amount of dissolved salts [22].
The EC of groundwater samples during the dry season ranges between 1440-18830 μs/cm with an average of 6688.89μs/cm, while it is 1100-17530 μs/cm with an average of 6159.16 μs/cm during the wet season (Table 2).
TDS refers to the total amount of dissolved solids present in groundwater.In general, groundwater tends to have higher TDS levels than surface water because its contact with more dissolved solids as it travels through underground rock formations and soil.It measures the suitability of water for human use.The TDS value varied between 3900-20100 mg/l with an average of 10522.67 mg/l during the dry season, while it is varied between 3260-18800 mg/l with an average of 8970.75 mg/l during the wet season (Table 2).
Total hardness (TH) of groundwater refers to the concentration of calcium and magnesium ions dissolved in the water.These elements i.e.Ca 2+ and Mg 2+ are the frequent cations that contribute of water hardness, which are indicate of the water ability to form insoluble compounds.It is essential to measure the TH parameter for assessing the suitability of groundwater for different purposes, for example drinking water, irrigation, and industrial processes.In general, water with a TH value is less than 60 mg/l is considered soft, while it considers hard when the TH value is greater than 180 mg/l (WHO, 2008).The average TH value of groundwater samples during the dry season is 3629.77mg/l, while its average is 3326.55mg/l during wet season the measured.

Chemical properties
The chemistry of groundwater is crucial for understanding the parameters that influence groundwater suitability for various applications [6].Groundwater chemical composition is split into main and minor ions, trace components, and dissolved gases [22].
Table 2 illustrated that all cation and anion concentrations in the groundwater samples during the wet and dry periods are exceeded the permissible limits which the World Health Organization [23] was recommended it, as well as, the Iraqi Quality Standard [24], except the nitrate NO3 concentration which is below the permissible limits.The results, however, showed that the groundwater in the area under investigation is unsuitable for human consumption, it could be used for irrigation, livestock, and some industrial purposes after treatment.Further explanation about the hydrochemistry of the Dibdibba aquifer are found in [6]and [5].

Total petroleum hydrocarbon (TPH)
The total petroleum hydrocarbon concentrations (μg/l) in groundwater samples during dry and wet seasons are illustrated in Table 3.The investigation revealed that all groundwater samples from the study wells are contaminated with petroleum hydrocarbons and their concentrations exceeded the allowable limits set by [25].The region is diversely impacted by the release of different pollutants from the nearby oil fields.This is due to the approximty next to the oil fields reservoir as shown in figures 1 and 2. Among the pollutants are those generated by the combustion of associated gas from oil wells in addition to those resulted from oil spills on the earth's surface or untreated solid oil waste stored in landfill disposal site.
The spatial distribution of the concentrations of total petroleum hydrocarbon in the Dibdibba aquifer during the dry and wet periods is shown in Figures 6 and 7.The highest TPH values of 35 μg/l is recorded in the northwest of the study area during the dry period (Figure 6), which is close to the North Rumaila oil field (figure 6).This could be due to the presence of oil residue catchments which settled on the land's surface for a long time (more than 15 years) although it has recently begun to be treated.It is important to note that from the field observations, a 3 oil residual waste catchments have been monitored with a storage area of up to 1 km 2 and depth of 1 to 2 m.
In the central and eastern parts of the Dibdibba aquifer, relatively lower concentrations of TPH substances are noted, comparing with the west side.These parts are relatively far from oil well sites where petroleum waste is collected.Conversely, the concentrations of TPH substances in the groundwater samples are increased in the areas near the Shuaiba refinery.Obviously, the concentrations of the TPH pollutants in the groundwater samples during the wet period are different from those during the dry period due to the effect of raining seasons on the TPH levels (figure 7).Overall, the highest TPH concentrations are observed in the northwestern and southern parts of the Dibdibba aquifer.This increase, as mentioned earlier, is due to the presence of oil waste accumulations from the Northern Rumaila oil field.
For the far southern region, near Safwan-Umm Qasr areas, the concentrations of elements and pollutants tend to increase.This could be attributed to various industrial activities carried out by humans in this area including power generation stations and scattered private factories.

Conclusions
The results of the physiochemical analyses revealed that the groundwater in Al-Zubair area exhibit higher levels of TH, EC and TDS reflecting the high salinity of groundwater in the area under investigation.Also, the increase of water hardness resulted from the presence of Ca 2+ and Mg 2+ cations.The analysis of the major elements in groundwater indicates that all cation and anion concentrations are exceeded the permissible limits set by the World Health Organization [25] and Iraqi Quality Standards [24], except for nitrate concentrations, which are below the permissible limits.The water quality analysis indicated that the water is unsuitable for human consumption but can potentially be used for the irrigation, livestock, and certain industrial purposes after appropriate treatment.
Significantly, the analysis of TPH revealed that all groundwater samples from the study wells are contaminated by hydrocarbon pollutants which exceed the allowable limits specified by WHO.This contamination is likely due to the proximity of the study area to nearby oil sites, where various pollutants are released, including those from associated gas combustion, oil spills, and untreated solid oil waste.The spatial distribution of TPH concentrations indicated higher contamination (35 μg/l) in the northwest part of the study area, near the North Rumaila oil field, indicated by the presence of oil residue catchments that have remained untreated for a long time.
These pollutants are expected to be carried out by groundwater movement and may reach agricultural areas that rely on water for irrigation and grazing.As a result, increased concentrations of oil waste transfer from high-concentration sites in the northwest to groundwater drainage areas in the southeast of the research area will have effects.
The findings highlight the significant impact of oil industry-related contamination on Dibdibba aquifer in Al-Zubair area.The contamination of groundwater with TPH poses a potential risk to human health.Thus, this study recommends for immediate remediation measures and proper waste management practices in the region.Also, further research and monitoring are necessary to assess the long-term effects of hydrocarbon contamination on the aquifer and develop effective strategies for its protection.Collaborative efforts between relevant stakeholders, including government agencies, oil industry operators, and environmental organizations, are crucial to mitigate the environmental and health risks associated with hydrocarbon contamination in the Al-Zubair area.

Figure 1 .
Figure 1.The oil fields near the study area after (Jaffer, 2018).

Figure 3 .
Figure 3. Digital Elevation Model for study area.

Figure 4 .
Figure 4. Static water of groundwater in the study area.

Figure 5 .
Figure 5. flow net direction of groundwater in the study area.

Figure 6 .
Figure 6.The spatial distribution of TPH in groundwater during the dry period.

Figure 7 .
Figure 7.The spatial distribution of TPH in groundwater during the wet period.

Table 1 .
The relative difference (R.D) for all the groundwater samples.

Table 2 .
The minimum, maximum, averages of the physical and chemical parameters of groundwater samples along with the worldwide standard limits.

Table 3 .
The total petroleum hydrocarbons (TPH) in groundwater samples.