Human health risk assessment of ground water contaminated with petroleum PAHs using Monte Carlo simulations: A case study of an Indian metropolitan city.

Underground pipelines are frequently used to transport petroleum fuels, through industrial as well as residential zones. Chennai is one of the four largest metropolitan cities of India. The region of interest in this study is located in the northern part of the Chennai. Ground water of this area was contaminated with polyaromatic hydrocarbons (PAHs) from the leaking oil storage tanks and pipe lines. Health risk assessment was conducted for exposure to PAHs in the ground water using incremental life time cancer risk (ILCR) models coupled with benzo[a]pyrene toxic equivalent method. The exposure pathways considered in this study were direct water ingestion and dermal contact under residential scenario. Exposure input parameters were transformed to statistical parameters using lognormal/uniform distributions and resultant probabilities of cancer risk were estimated by performing Monte Carlo simulations. Preliminary remediation goals were predicted using the combination of the cancer risk models of all the exposure routes with the consideration of high-safety risk of 1-in-1 million. Results showed that the cancer risk is predominantly contributed (greater than 98%) by dermal exposure than the oral in both adults and children. The total ILCR is found to be greater than a low safety risk of 1-in-10,000 with higher probability percentages (>90%). The 95th percentile values of the risk were presented in order to address the need for remediation. Appropriate remedial and treatment methods for the subject site were proposed. The results of the study will be useful for the regulatory boards and policy makers in India in understanding the actual impact of the contamination on receptors, setting up final remediation goals and deciding on a specific remedial method.

Fractured Geothermal Reservoir Using CO2 as Geofluid: Numerical Analysis and Machine Learning Modeling.

The effect of natural fractures, their orientation, and their interaction with hydraulic fractures on the extraction of heat and the extension of injection fluid are fully examined. A fully coupled and dynamic thermo-hydro-mechanical (THM) model is utilized to examine the behavior of a fractured geothermal reservoir with supercritical CO2 as a geofluid. The interaction between natural fracture and hydraulic fracture, as well as the type and location of geofluids, influences the production temperature, thermal strain, mechanical strains, and effective stress in rock/fractures in the reservoir. A mathematical model is developed by using the fully connected neural network (FCN) model to establish a mathematical relationship between the reservoir parameters and the temperature. The response surface methodology is applied for qualitative numerical experimentation. It is found that the developed FCN model can be utilized to forecast the temporal variation of temperature in the production well to a desired level using FCN. Therefore, the numerical simulations developed with the FCN method can be useful tools to investigate the temperature evolution with higher accuracy.

Nitrate transport in a fracture-skin-matrix system under non-isothermal conditions.

Subsurface leaching of agricultural runoff has been identified to pose a serious hazard to the soil-water ecosystem and human health, mostly due to the associated contamination with nitrate. Our understanding of the nature of contaminant spread in the vadose and aquifer zones has been improved from recent mechanistic models on the flow and transport of contaminants through fractured porous media. The present study aims to explore the impacts of skin formation in a fracture-matrix aquifer system onto the nitrogen species transport under non-isothermal settings using numerical modeling. A finite-difference scheme was employed to capture the nitrogen concentration profile and kinetics of transformation by solving the derived partial differential equations. The results show evidence of an additional mass transfer from fracture to skin so as to reduce the migration of nitrogen species (NO3-N and N2) at the fracture-matrix interface thereby reducing the peak concentration of N2 by nearly 1.5 times in fracture after denitrification. Although the thermal conductivity of the rock matrix has a direct impact on the temperature distribution in fracture-skin-matrix profiles, the presence of skin has a cooling effect for a high-temperature influent (45 °C), which also deteriorates the propagation of organic N2 and NO3-N, within the fracture. An increase in the temperature coefficient of skin has resulted in an apparent reduction in nitrogen species migration, indicating the thermo-chemical feasibility of an intermediate skin favoring the mass transfer processes. The findings of this study can be extended toward realistic estimation of groundwater contamination risks and for the design of biological filters for in situ remediation.

Numerical modelling of nitrate transport in fractured porous media under non-isothermal conditions.

Subsurface contamination is a frequent occurrence in fractured porous systems, posing a potential threat for the groundwater contamination. Tracking the movement of these contaminants is an inherent aspect of effective remediation strategy. The non-isothermal conditions prevailing in the subsurface environment further add to the complexity of the existing scenario. The current study focuses on simulating the concentration profiles of nitrogen species in a fracture-matrix system under non-isothermal conditions. The kinetics and biochemical thermodynamics of nitrogen transformation reactions were explicitly modelled in this study by adopting a finite differential numerical scheme. The numerical results clearly depicted the spatial-temporal profiles of the concentration of all the species in response to the observed peak values. Considering the sensitivity of the model parameters, an increase in flow velocity triggered the migration of all nitrogen species in the fracture, while an increase in matrix porosity reduced the concentration by enhancing the chemical reactions. An increase in fracture aperture also could trigger the denitrification process in the fracture to reduce the nitrate-nitrogen contamination in the fracture. The temperature variation between 25 °C and 45 °C in the fracture and the matrix essentially reduced the availability of nitrate-nitrogen and nitrogen gas in the fracture under non-isothermal conditions. Hence, an increase in the temperature coefficient can reduce the spike of nitrate-nitrogen and nitrogen gas in fracture by minimizing such transformation rates.

Human health risk assessment for exposure to BTEXN in an urban aquifer using deterministic and probabilistic methods: A case study of Chennai city, India.

The aquifer in Tondiarpet, Chennai, had been severely contaminated with petroleum fuels due to an underground pipeline leakage. Groundwater samples were analyzed quarterly for priority pollutants such as benzene, toluene, ethylbenzene, xylenes, and naphthalene (BTEXN) using purge and trap gas chromatography and mass spectrometer from 2016 to 2018. The maximum concentrations of BTEXN in groundwater at the site were found to be greater than the permissible limits significantly. Among the five sampling locations (MW1, MW2, MW3, MW4, and MW5), mean BTEXN levels were found to be higher near MW2, confirming the source location of petroleum leakage. Human health risk assessment was carried out using deterministic and probabilistic methods for exposure to BTEXN by oral and dermal exposure pathways. Risk analysis indicated that mean cancer and non-cancer risks were many times higher than the allowable limits of 1E-06 and 1 respectively in all age groups (children, teens, and adults), implying the adverse health effects. Oral exposure is predominately contributing (60-80%) to the total health risk in comparison to the dermal exposure route. Variability and uncertainty were addressed using the Monte Carlo simulations and the resultant minimum, maximum, 5th, 95th, and mean percentile risks were predicted. Under the random exposure conditions to BTEXN, it was estimated that the risk would become unacceptable for >98.7% of the exposed population. Based on the sensitivity analysis, exposure duration, and ingestion rate are the crucial variables contributing significantly to the health risk. As part of the risk management, preliminary remediation goals for the study site were estimated, which require >99% removal of the BTEXN contamination for risk-free exposures. It is suggested that the residents of Tondiarpet shouldn't utilize the contaminated groundwater mainly for oral ingestion to lower the cancer incidence related to exposure to BTEXN.