Elemental Identification and Radiological Characterization of Core and Surface Samples for Environmental Assessment in Gas Field Regions of Bangladesh

Abstract

In the oil and gas production industries all over the world, produced water contain the largest portion of wastes and the second largest portion of wastes is drilling waste, both of which are dumped into the evaporation pond and so the pond is assumed to be the main deposit of higher level of toxic element wastes and naturally occurring radioactive materials (NORMs). In Bangladesh, this contaminated water mostly in rainy season, over-flooded and spread to the nearby environment and resulting soil contamination. The yearly produced water deposition of Shabazpur gas field, Saldanadi gas field and Fenchuganj gas field are 27,53,244 L, 2,26,499 L and 28,13,944 L respectively, along with producing more scale and sludge. Metal concentrations above threshold levels affect the microbiological balance of soils and can reduce their fertility where most vital soil pollutants are heavy metals owing to carcinogenic, hazardous, and non-degradable properties The hydrocarbon extraction industries are the source of heavy metals like arsenic, barium, cadmium, chromium, iron, lead, mercury, nickel, vanadium, and zinc which are introduced chiefly through the disposal of produced water and drilling muds where arsenic, cadmium, chromium, lead, and mercury have comparatively higher degree of toxicity. Again, radiation can produce fetus abnormality, sterility, erythema, leukemia, epilation, genetic diseases and cancer in the blood-forming tissue, skin, bone, lung, thyroid and so on. So environmental assessment both for radioactivity and toxic elements concentration have been analyzed for Shahbazpur gas field, Saldanadi gas field and Fenchuganj gas field of Bangladesh in this study. Newly developed integrated in-site hydrocarbon and uranium recovery technology with very low grades of uranium ore deposits and also Th and many other valuable minerals recovery technology is economically feasible. So, core samples from gas reservoir wells of different depth from 1279m to 4091m of these three gas fields have been analyzed for elemental identification with concentrations and radioactivity levels, which can reflect information of the sedimentary formation at greater depth of gas reservoir zone and could be used as baseline data for mentioned technology in near future. Neutron Activation Analysis (NAA) method has been used to represent a set of high-quality elemental data for the core sediments of gas reservoir zone along with rare earth elements (REE) and trace elements more precisely for baseline data and to study the possible elemental contamination from gas reservoir wells to the surface environment due to gas abstraction activity. Elemental abundance analysis of gas field environmental samples for toxic waste elements identification have been done using energy dispersive X-ray fluorescence (EDXRF) method with inter-comparison among NAA and EDXRF methods. In the Shahbazpur gas field, Saldanadi gas field and Fenchuganj gas field environmental soil and sediment samples, the mean concentration of Pb are found to be 70.5 ppm (range in ppm: 7.27 – 113), 493 ppm (range in ppm: 85.5 – 1675) and 107 ppm (range in ppm: 101 - 114) respectively in this study, where the concentration of Pb of upper continental crust (UCC) and world soil median values are 17 ppm and 35 ppm respectively. In the Saldanadi gas field, highest concentration detected for the Pb in the sample is from north side of the waste pumped evaporation pond where, used up batteries of car and other heavy vehicles of this gas field also have been dumped from which very high Pb and Sr concentration can be occurred compared to other environmental soil samples of these gas field areas. The average concentration of As are 26.6 ppm (range in ppm: 9.83 - 43.9), 33.8 ppm (range in ppm: 29.2 - 37.6) and 32.4 ppm (range in ppm: <MDL - 42.6) respectively in the Shahbazpur gas field, Saldanadi gas field and Fenchuganj gas field environmental soil and sediment samples, where the concentration of As of upper continental crust (UCC) and world soil median values are 4.8 ppm and 6 ppm respectively. So, the average concentration of both Pb and As in soil samples of these three gas fields are significantly higher than corresponding UCC and world soil median values. According to all three environmental indicators, namely geo-accumulation index (I geo ), contamination factor (CF) and enrichment factors (EF) of average elements abundances values of the three gas fields, environmental samples are contaminated mostly by As, Pb and Ti which can be due to anthropogenic incorporation for gas abstraction activities. Core samples data reveal moderate enrichment of the values of As and Cs relative to UCC of all three gas fields. So, there is a possibility of environmental soil and sediment samples to be contaminated by As and Cs from gas-abstraction activities of these three gas fields. The X-Ray diffraction (XRD) analytical technique is used for chemical composition information of all core samples and some core samples have been analyzed using energy dispersive X-ray (EDX) analysis to get an idea about the elemental abundance concentration. Arsenic exposure affects all organ systems including the cardiovascular, dermatologic, nervous, hepatobiliary, renal, gastrointestinal, and respiratory systems with significantly higher mortality rates for cancers of the bladder, kidney, skin, and liver. In the human body, the greatest percentage of lead is taken into the kidney, then liver and the other soft tissues such as heart and brain and skeleton, again lead absorbed by the pregnant mother is readily transferred to the developing fetus. 232 Th and 40 K radioactivity concentration ranges of this study are significantly higher than other soil samples of the oil and gas production facilities in different countries of the world which may be due to geological formation variation as explained in one of this thesis research papers “Geochemical characterization of Miocene core sediments from Shahbazpur gas‑wells (Bangladesh) in terms of elemental abundances by instrumental neutron activation analysis”. All seven radiological hazard indices namely, radium equivalent activity, gamma representative level, external absorbed dose rate, annual effective dose rate, external hazard index, internal hazard index and excess lifetime cancer risk of core samples of all three gas fields are significantly higher than the corresponding recommended values. On the other hand, environmental surface soil and sediment samples in and around the gas-fields have significantly lower radioactivity levels compare to those in core samples of these gas fields but hazard indices, like gamma representative level index, external absorbed dose rate and excess lifetime cancer risk of most of the surface samples are around the corresponding recommended values. Day by day the waste increases in gas production field. So, this is high time to be aware of preventing more contamination and taking necessary action against contamination from both radiological and chemically toxic elements. Furthermore, there are many fruit trees and leafy vegetables with in the boundary of the gas field areas where comparatively higher toxic elemental concentration and radioactivity concentration levels along with excess values of some radiological hazard indices and environmental contamination indices than corresponding recommended values have been found in soil samples. Moreover, abundant grass grows in the areas of high radioactivity and toxic elemental abundance, which are collected by the local inhabitants for their domestic animals. Thus, by consuming fruits, vegetables, milk and meats can indirectly be exposed to the radiation and toxic metals/metalloids, that may develop into cancer and other diseases. Environmental assessment for the elemental contents and radiological hazard indices suggesst considerable risks and invoke that environmental samples in and around the gas-field should be monitored routinely as the soil and sediment samples can be contaminated more by toxic elements like, Pb, As etc. and NORMs originated from gas-abstraction activities. Outcomes of this study can help the policy-makers to take necessary actions and adopt required regulations to avoid toxic elements and radiation hazards from the studied site as well as the other sites around the globe having identical category.