According to the statistics, Iran has the second largest natural gas reserves in the world. Although the population of Iran is less than 19% of EU countries and 6% of China, but natural gas consumption in Iran is equivalent to gas consumption in China and the total consumption of EU countries as well. About 214 billion cubic meters of natural gas are consumed in Iran annually. Almost 651 billion cubic meters of natural gas are consumed by Americans every year and this is expected to increase up to 974 billion cubic meters by 2020 . Annual US gas consumption has increased up to 30% from 481.39 billion cubic meter to 628.63 billion cubic meter during 15 years from 1987 to 2001. The average global consumption of natural gas has been continuously increased by 1.8% annually in the last 8 years which is considered as the fastest-growing energy source .
Gas consumption amount has significantly augmented by considering some parameters such as prominent population growth, industrial development in various domains, and efforts to fulfill the existing needs. On the other hand, concerning the evaluation of gas development indicators in different parts of the country, the expansion speed of the main and secondary gas networks has been increasing rapidly in recent years in Iran. Thus, assuring and removing the existing defects in each stage of gas transmission in the gas distribution network to prevent gas leaks and the environmental and economic hazards caused by it, is necessary and undeniable [3, 4]. The expansion of urban residential areas towards refineries, the development of these industries, and the increase in the number of working people in these industries also lead to an increase in the number and severity of the accidents and leave drastic and irrecoverable damages .
Some factors involved in gas loss (gas imported into gas distribution network and gas consumed by consumers) are as follows: gas leak from pipes and fittings (less than 10%), gas theft, measurement errors, third-party damages, and illegal exploitation [6, 7]. Permeation and emission of flammable and toxic hazardous substances in chemical and process industries are always one of the hazards which threaten the lives of people working there, of the residents around these industries, and also the environment . Natural gas leakage and diffusion from transmission and distribution networks, in addition to the environmental consequences like increase in the process of global warming, also leads to the safety hazards in the centers of population as well as the health outcomes for the residents of these areas. Overall, some consequences of natural gas leakage are environmental pollution, a decline in health index in humans and other organisms, explosion hazards, natural gas loss, environmental recovery fees, repairing and replacing costs of buildings equipment, waste of time, and possible statutory offenses: therefore, the sufficient motivation to solve such a problem is provided by the three factors of economy, safety and environmental considerations .
In recent years, one of the most important challenges of sustainable development is the wide set of quick processes like changes in demographic profile, economic and social dynamics changes, technological advancement, and declining environmental trends. Therefore, it is necessary to have a better understanding of the relationship between these trends and the related changes in the environmental, social, and economic conditions. Due to the position and necessity of the sustainability of development and the different economic systems experiences in this field that tend to neglect the role of the environment, the importance of political guidance to consider the interactions between the economy and the environment along the sustainable development is evident .
Today, one of the most pivotal issues in protecting the environment is to find some ways to remove various types of pollutants from the environment. Some of these pollutants include heavy metals, chemical pollutants, and particularly gases and household gases: gases release into the environmental resources through different ways such as industrial, household, commercial, and a small percentage of agricultural ways and unbalance the material cycles in natural ecosystems and adversely affect the environment . The main reason for global warming in the last 50 years has been due to the carbon dioxide and methane emissions caused by human activities; however, the per capita greenhouse gas emissions rate in developed countries is 2 to 4 times higher than the required amount for the stability of greenhouse gases in the atmosphere. The ecological footprint in these countries is also 4 to 9 times more than the biological capacity of the earth . The global ranking of Iran in terms of the Environmental Performance Index (EPI) among 132 countries in 2012 with a 61-step decline just during 6 years has been announced 114. Although the ranking of Iran improved slightly in 2015 and was ranked 105th, even it showed an unfavorable performance compared to the countries of the region [12,13].
Leakage through pipelines, especially in areas where are environmentally sensitive can cause many threats to the creatures that live on earth. Methane gas has a relatively stronger effect on global warming than carbon dioxide thus the identification and prevention of long-term leaks is also of particular importance. Furthermore, the leak in the town gas network, due to the loss of some part of valuable materials which are among the products or raw materials, also leads to economic losses [8,14]. Therefore, the current study was designed to estimate methane gas leaks from Mashhad town gas risers and investigate the environmental and economic impacts to satisfy the following purposes:
Selecting the statistical population of Mashhad town gas risers;
Identifying the risers having leakage in the statistical population;
Evaluating the relationship between the ground variables and their leak rate;
Examining the environmental – economic aspects.