MULTI-SCALE FATE AND TRANSPORT OF LNAPL UNDER VARYING SUBSURFACE FLOW CONDITIONS

Abstract

Pollution of soil-water resources by release of hydrocarbons such as light non-aqueous phase liquids (LNAPL) is of major concern because of their high water solubility and wide coverage under dynamic subsurface conditions. The rate of biodegradation of LNAPL is significantly governed by site prevailing environmental conditions and groundwater dynamics. Therefore, the focus of this study is to investigate LNAPL fate and transport under varying subsurface conditions using multi-scale laboratory experiments and numerical modeling. A series of microcosms experiments were first performed to see the biodegradation capability of indigenous microbiota at different initial substrate conditions. A wide range of initial dissolved toluene (10-250 ppm), the selected LANPL, was taken in microcosms and periodically analysed for toluene concentration using GC-MS. The results show that LNAPL degradation increases with increasing initial dissolved LNAPL concentration up to about 50 ppm and remain maximum till about 100 ppm before started decreasing with increment in the LNAPL concentration. Microcosm experiments were then conducted to investigate the role of soil moisture and temperature on biodegradation of LNAPL. Each set of sterile and live microcosms having a different level of soil moisture content (20, 40, 60, 80% of s  ) was kept at prevailing high room temperature (30±2°C) while the other was incubated at low soil-water temperature (10±0.5°C). It was found that biodegradation rate was high in case of high moisture contents i.e. 60-80% at 30°C than the microcosms having low moisture contents (40- 20%) at the same soil-water temperature level. Results show that temperature of soil-water system was more sensitive to temperature at high moisture content as compared to the low moisture level. Role of varying groundwater temperature on dissolution and biodegradation was also investigated in a continuous system of column setup. The column experiments were performed at four levels of groundwater temperature (40C, 200C, 280C and 360C) separately. Results of the column experiments show that accelerated dissolution rate of LNAPL at 360C temperature was observed followed by 280C, 200C and 40C cases. The biodegradation rates of LNAPL were found 0.002, 0.008, 0.012 and 0.015 mg-L/hr at groundwater temperature of 40C, 200C, 280C and 360C, respectively. Microbial number was found high in region of 140 ppm-150 ppm IV dissolved toluene concentration at 280C and 360C temperature. Thereafter, the role of plant on different LNAPL compounds was investigated using treatment wetlands planted with and without P. australis. It was found that the plant significatly remove the LNAPL componts from root zone from plated treatment wetlands. Thereafter, fate and transport of LNAPL under dynamic groundwater table was investigated using 2D sand tank setup having a dimension of 125cm-L × 90cm-H × 10cm-W integrated with an auxiliary column. Initially, tracer experiments were performed to determine soil-water flow and solute transport parameter for different groundwater fluctuation cases. The LNAPL transport experiment was then conducted under stable groundwater table condition followed by rapid, general and slow groundwater table fluctuation cases by rising/falling the groundwater table in 2, 4 and 8 hours, respectively. Numerically runs were conducted for the LNAPL transport under varying groundwater conditions for the experimental domain. The results show that a large LNAPL pool under fluctuating groundwater conditions contributes to high concentration of dissolved LNAPL plume in saturated zone. The transport of dissolved LNAPL plume was comparatively fast in case of rapid fluctuating groundwater case resulting in closely spaces concentration isolines of toluene contaminated plume. A high biodegradation rate was observed in regions having LNAPL concentration ranging from 140 - 160 ppm. The response of microbial community was found to be increasing as plume moved away from the pure phase pool. To see the role of varying groundwater flow regimes, laboratory experiments and numerical runs were conducted using 3D sand tank setup. A constant water flux was allowed to flow through homogeneously packed 3D sand tank first for maintaining a base flow velocity of 1.2 m/day in the horizontal direction. The flow velocity was then increased/decreased by changing the water flux passing through the saturated zone by keeping the water table location at the same height. The LNAPL mass transfer coefficient was found to increase linearly with velocity and was estimated for the selected groundwater flow regimes varying from 0.083 to 0.129 cm/h. The observed high rate of degradation of toluene for faster flow velocities shows the dependency of the degradation kinetics on dissolved LNAPL concentration. The breakthrough curves at different ports showed that horizontal and transverse transport of the LNAPL was more prominent as compared to its vertical movement. The concentration of V dissolved toluene compared well with the simulated curves for the three cases of groundwater flow conditions. Finally, a simulation-optimization approach was to evaluate the performance of bioremediation system with respect to its remediation time and cost. Design of in-situ bioremediation system consisted of three injection wells to provide oxygen/nutrients to polluted saturated zone and one extraction well to control toluene plume spreading. The extracted groundwater was used to recharge polluted unsaturated zone which significantly enhanced the removal of toluene by maintaining optimal soil moisture content. The biodegradation rates estimated in laboratory experiments having different combination of soil moisture and temperature were incorporated as sink of toluene in unsaturated zone. While BIOPLUME III was used to estimate the biodegradation rate in saturated zone by considering additional oxygen and nutrient supply by injection wells. These degradation rates were used in HYDRUS 3D to simulate the saturated and vadose zone soil-moisture flow and toluene transport. The data generated from simulation runs were used in Extreme Learning Machine (ELM)-Particle Swarm Optimization (PSO) tools to achieve the minimum remediation time and cost. Spreading of toluene was more through vadose zone having soil moisture level higher than 60% at 30°C than 10°C. A minimum cost of $106,570 and $107,245 was achieved when the soil moisture was 80% and 60%, respectively, at 30°C during one year. A small difference of remediation cost clearly demonstrate the effect of vadose zone recharge on accelerated biodegradation. However, for soil moisture content of 40% and 20%, the optimized cost was $120,306 and $126,905 in time frame of 1 year and 3 months and 1.5 years, respectively. The total cost estimated by the ELMPSO approach was found minimum after fulfilling all the regulatory constraints of the contaminated site. Findings of this study are of direct use in applying engineered bioremediation techniques in field having dynamic subsurface conditions.