Showing papers by "Indumathi M. Nambi published in 2015"

Numerical modeling of biological clogging on transport of nitrate in an unsaturated porous media

Abstract: In order to better understand nitrogen species transport and transformation in the saturated zone, it is essential to predict the contaminant concentration in the unsaturated zone as the concentration profiles estimated from unsaturated zone forms as the input for estimation of contaminant concentration in the saturated zone. Since the nitrogen transformations occur in the presence of bacteria, there is a need to account the influence of biological clogging to develop comprehensive model of nitrogen species transport. In this study, a one-dimensional numerical model is developed to investigate the nitrogen species transport in an unsaturated porous media along with microbial clogging process. Results suggest that hydraulic conductivity is enhanced initially due to increase in the water content followed by a significant reduction at larger simulation time resulting from bioclogging. As bioclogging mitigates the hydraulic conductivity significantly, a considerable delay is observed in the ammonium nitrogen and nitrate nitrogen transport with reference to the system without bioclogging. In addition, the effect of oxygen mass transfer coefficient on biological clogging was evaluated. Numerical results suggest that as the oxygen mass transfer is decreased the ammonium nitrogen and nitrate nitrogen concentration travels a larger depth. Further numerical results strongly suggest that nitrogen species transport in the unsaturated zone is significantly affected by the presence of biological clogging.

Numerical modelling on fate and transport of petroleum hydrocarbons in an unsaturated subsurface system for varying source scenario

Abstract: The vertical transport of petroleum hydrocarbons from a surface spill through an unsaturated subsurface system is of major concern in assessing the vulnerability of groundwater contamination. A realistic representation on fate and transport of volatile organic compounds at different periods after spill is quite challenging due to the variation in the source behaviour at the surface of spill as well as the variation in the hydrodynamic parameters and the associated inter-phase partitioning coefficients within the subsurface. In the present study, a one dimensional numerical model is developed to simulate the transport of benzene in an unsaturated subsurface system considering the effect of volatilization, dissolution, adsorption and microbial degradation of benzene for (i) constant continuous source, (ii) continuous decaying source, and (iii) residual source. The numerical results suggest that volatilization is the important sink for contaminant removal considering the soil air migration within the unsaturated zone. It is also observed that the coupled effect of dissolution and volatilization is important for the decaying source at the surface immediately after the spill, whereas rate-limited dissolution from residually entrapped source is responsible for the extended contamination towards later period.

Numerical studies on kinetics of sorption and dissolution and their interactions for estimating mass removal of toluene from entrapped soil pores

Abstract: When the non-aqueous phase liquid (NAPL) contaminant is entrapped in soil pores, their release to the subsurface environment is mainly limited by interphase mass transfer such as dissolution and sorption. Sorption onto the aquifer material plays an important role as a retardation mechanism in subsurface contaminant transport processes. Considering the heterogeneity associated with aquifer properties as well as contaminant source distribution, long-term contamination is an inevitable consequence of mass transfer limitation of residual mass from soil micropores. A comprehensive numerical model is presented in this study for understanding the kinetic nature of dissolution and sorption of hydrophobic hydrocarbons and their interactions in order to estimate the rate and extent of mass removal from different phases. The results showed that sorption by soils and sediments having different physicochemical properties generally follow non-linear behavior. It is observed from the study that sorption non-linearity can be effectively incorporated by the two-site kinetic model rather than combination of linear and non-linear sorption isotherms. Even though initial phase of mass transfer is dissolution-dominated, the extended tailing of concentration at higher pore volumes is controlled by non-equilibrium sorption, which is better explained by the two-site kinetic model.

Experimental and numerical investigations on nitrogen species transport in unsaturated soil during various irrigation patterns

Abstract: The transport of nitrogen coming from wastewater applied agricultural field is a major problem in assessing the vulnerability of groundwater contamination. In this study, laboratory column experiments are conducted in order to simulate the paddy, groundnut and wheat irrigation with wastewater. The experiments are carried out with high clay content (≈35%) soil from Kancheepuram, Tamilnadu and low clay (≈9%) soil from Ludhiana, Punjab, India. Furthermore, a numerical model and HYDRUS-1D model are developed to simulate the experimental results. The experimental results show that there is no effluent collected at the bottom of the column during groundnut irrigation in Kancheepuram soil and effluent collected except during first irrigation in the case of wheat irrigation in Ludhiana soil. The experimental and numerical results illustrate that when 50 mg/l of ammonium and 20 mg/l of nitrate nitrogen applied during paddy irrigation, the peak nitrate nitrogen concentration of 50 mg/l is arrived after 10 days in Kancheepuram soil due to low permeability and relatively less background soil nitrogen. But in the case of Ludhiana soil with 94 mg/l of total nitrogen applied during paddy irrigation, the peak nitrate nitrogen concentration of 1,620 mg/l is observed at first day due to high permeability and high soil background nitrogen concentration. Additionally, the model results show that the application of high nitrogen content wastewater for irrigation in Ludhiana soil will affect the groundwater quality even when the groundwater table is deep as compared with Kancheepuram soil.