عنوان مقاله [English]
Assessment of heavy metal pollution in the soil of Ahwaz oilfield by using the five-stage process of chemical dissociation and Dendrogram interpretation and pollution severity index
The aim of this study is to evaluate heavy metal pollution in the soil of Ahwaz oil field which is the biggest oil field in Iran, by using chemical dissociation process and Dendrogram interpretation and pollution severity index. The 20 stations were selected on the soil surface. Parameters of nickel, vanadium, cobalt, iron, chromium, barium, arsenic and total petroleum hydrocarbons concentrations were determined. Five-step process of chemical dissociation method was used to identify and determine human-made and natural heavy metals concentration, and also MVSP software was used to draw the cluster analysis dendrogram. The results of chemical dissociation show that the origin of the nickel, vanadium and cobalt elements and also of iron, chromium and arsenic in new wells is higher in the human-made and the natural phase, respectively. The results of pollution severity index for nickel showed that 25% of the stations in clean class, 50% in low pollution class and 25% are in middle pollution class. 40% of the stations in terms of vanadium pollution in clean class, 40% in low pollution class and 20% are in middle pollution class. In terms of cobalt pollution, 35% of the stations in the clean class, 40% in low pollution class, 20% in the middle pollution class and 5% are in the high pollution class. In terms of chromium pollution, 75% of the stations in the clean class, 25% are in low pollution class and also for iron and arsenic all the stations are in the clean class.
Key Words: Ahvaz oilfield, five-stage chemical dissociation, heavy metals
After discharging drilling wastes into pits, they left without any care, and any maintenance or advantage does not take happened.
The main objective of this research is evaluating heavy metals in the Ahvaz oilfield soil by using five-stage process of chemical dissociation and also assessment of soil contamination by pollution severity index. The present study was conducted in Iran in 2016. The researches on the challenge of heavy metals in drilling fluid in the oil industry have often exploited geochemical indicators. [15,16,17], Nevertheless, in the present study and geochemical indicators, we will also apply a five-phase chemical dissociation methodology.
2. Materials and Methods
Khuzestan province is located in southwestern of Iran . Large number of oil reserves causes particular attention to pollution studies in addition to geologic studies.The study area is the Folded Zagros with spectral range from the Cambrian to Recent, and major outcropping formations in oil fields and related to the groundwater can be classified in calcareous formations (Asmari), formations of gypsum - marl (Gachsaran), Bakhtiari conglomerate formation and recent sediments.
20 wells were selected to determine sampling sites in the soil surface of Ahvaz oilfield due to limitations, the UGC map, and data related to type of wells, construction time of wells, drilling date and duration in the Ahvaz oilfield. The five-stage chemical dissociation method was used to determine the phase’s share of man-made and natural elements and the percentage of each element in the chemical bonds. A large percentage of the study area is barren lands, agricultural lands and residential areas so the depth of 0 to 30 cm were considered for soil sampling because roots of plants are shallow and on the other hand the depth of achieving groundwater static surface is low, and also according to the similar conducted studies.
3. Results and Discussion
The results of chemical dissociation of heavy metals bonds in Station 3 shows that the percentage of nickel in each of the five phases was 65%, 15%, 9%, 10% and 1% for loosely bond, sulphide, organic-metallic, resistant and within lattice respectively. The percentage of vanadium in five phases was 71%, 9%, 9%, 10%, % 1 respectively. The percentage of cobalt in each of, sulphide, organic - metallic, resistant and within lattice bonds was determined 50%, 17%, 24%, 8% and 1% respectively. The percentage of barium in each bond or phase was 40%, 25%, 22%, 11% and 2% respectively; the highest amount of nickel, vanadium, cobalt and barium was in loosely bond. Loosely bond is the most dangerous elements bonds with the soil particles and also serious danger to the environment.
The highest concentration percentage for all elements was determined in the resistant phase.
The highest percentage of elements was reported in the resistance phase.
Finally it can be said that total petroleum hydrocarbons, barium, mercury, salt, chromium, cobalt, nickel, iron and vanadium have the natural origin, are under the influence of oil pollution, and also have petroleum origin. (Figure 15).
Normal and standard level of metals is achieved by chemical dissociation process and there is no longer need to use the correction factor in the formula of geoaccumulation index. Given that the 5-stage chemical dissociation process was conducted, the pollution estimation is more accurate and closer to reality. Due to the weather conditions and pollution in recent years at the southern regions, Inter-basin water transfer projects from Karun River to Zayande Roud, severe shortage of drinking water, reducing rainfall and frequent droughts, big dilemma of dust, sand storms and also the establishment of more than 75% of the country's industry in Khuzestan province, make the attention necessity of soil and ground water pollution as reliable source of drinking water much more higher. Since the Ministry of Petroleum has privacy for the all oil and gas wells in which has oil and gas or without, other utilities will not be allowed around the oil wells.
Usually oil wells adjacent to each other and often other industry does not establish in oil fields except in special cases. The soil generated pollution is spotty and located around the oil wells and drilling activities space which affected by the drilling mud type, user type wells (oil or gas), type of oil and gas, drilling waste management system and the time of drilling that is the most important variable. Since the heavy metals concentrations were different in the wells by various ages and the concentrations of most elements were as a function of well age, it can be concluded that several processes can be the cause of this incident. Including the phenomenon of dust from the old age (oldest wells) until recent years has increased, and on the other hand the processes of leaching and increased drought and wind erosion of soil is also can be responsible for the emergence of this phenomenon.
In the reference , the drilling wastes in one of the wells of oil field Ahvaz have been studied. The results of this study represented that the concentration of lead, copper, arsenic, zinc, molybdenum, antimony, mercury, and strontium in wastes is higher than the region's standard and soils. Also, calculating the correlation coefficient between the elements in the waste represented that the mineral phases in the form of impurities with drilling barite are quite evident.
The results represented that the average values of the ecological risk factor of these elements are in the range of very high ecological potential risk. Also, enrichment and pollution coefficients represented that the source of pollution of this metal is human-centered activities such as oil drilling activities in the region. Therefore, continuous monitoring of the concentration of metal contaminants in the drilling fluid and the performing of heavy metal stabilization methods in the drilled areas is critical.