Biodegradation of Toluene (an LNAPL) under varying temperature and fluctuating water conditions
Rajbhandari Shrestha, S.
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Abstract Groundwater contamination occurs when there is spill of substances which are harmful to humans and the environment. One of the most common groundwater contaminant is Non-aqueous phase liquids (NAPL). These substances are notorious as they do not mix with water and are able to persist in nature causing long term harm. They are broadly classified as Light (LNAPL) or dense (DNAPL) based on their relative density with water. BTEX (Benzene, Toluene, Ethylene benzene and Xylene) is an example of a LNAPL which is commonly found in gasoline and diesel fuels. Microbes present in the environment are able to utilize contaminants as food to degrade them to non harmful daughter components. This technique is termed as biodegradation and proves to be a promising remediation method. There are various environmental factors which affect the degradability of microbes such as soil temperature and water table fluctuations. A study of these factors was done in batch and column experiments. They were replicated for field conditions in a semi-arid coastal region. Toluene was used as a representative of the LNAPL in the experiments. Several batch experiments were run to investigate the effect of temperature on degradation rates. Results from the constant temperature experiments indicated that degradation was highest in summer. The increase in degradation rate was two times for every 10°C increase in temperature. In addition, fluctuating temperature experiments were done to account for the diurnal change of temperature. The results showed that microbes were able to overcome the effect of temperature changes even in case of extreme weather conditions and continued thriving though taking slightly longer time. Column experiments were done to investigate the effect of fluctuating water conditions on degradation. Results showed that water table fluctuations brought residual LNAPL higher in the unsaturated zone from a LNAPL lens present on the water table. In absence of fluctuations LNAPL lens remained at the same place. The difference between LNAPL concentrations of live and sterile columns and consumption of oxygen in the live columns indicated biodegradation going on in the saturated as well as the unsaturated zone.