Drilling operations of petroleum generate oily wastes. The disposal of a significant amount of oil-based drill muds has caused soil contamination and critical environmental impacts in the last decades.The current study aimed to investigate the potential of microbial remediation for an aged oil-based drilling waste and to monitor the fluctuation in microbial population throughout a ۶۰-day microcosm experiment. A representative aged oil-based drilling waste sample was obtained randomly from a contaminated mud pit in the Khangiran district, Iran. Respiration measurement was performed according to the method described by standard ISO ۱۷۱۵۵. Total petroleum hydrocarbon (TPH) was measured by the gravimetric method. Microbial counts were measured at ۱۰-day intervals during ۶۰ days of incubation. Total heterotrophic bacteria were enumerated by standard plate count using R۲A agar. Dominant heterotrophic and hydrocarbon-utilizing bacteria were selected for phylogenetic analysis. Statistical analyses of the experimental data, using one-way ANOVA were performed using Minitab ۱۶. Following the biostimulation of the contaminated soil, both heterotrophic and hydrocarbon-utilizing bacterial counts increased to above three orders of magnitude in less than ۲۰ days. The highest respiration level and hydrocarbon degradation efficiency were correlated and measured between the ۱۰th and ۲۰th days of the experiment to be ۷۰.۷ µg/g.soil.hand ۲۳.۱۳% respectively. Phylogenetic analyses indicated that the members of Actinobacteria (Georgenia, Brevibacterium, Micromonospora, and Streptomyces) were the major hydrocarbon-utilizing bacteria in the microcosm, among which the species of genus
Georgenia were dominant throughout the experiments. Furthermore, the population of
Alcanivorax species increased promptly and thrived in the microcosm during the active bioremediation phase which indicated their vital role for remediation of diesel range hydrocarbons in saline environments. In an overall view, elegant diversity of hydrocarbon utilizing bacteria along with the accomplished TPH removal efficiency of ۴۵.۴% (w/w) in the microcosms, confirmed the potential of indigenous microorganisms for bioremediation of the aged oil-based drilling waste.