In this study, an integrated approach for environmental risk assessment of subsurface contamination is proposed. This integration is based on a Monte Carlo method for simulating pollutant transport in subsurface and the consideration of several scenarios for risk assessment guidelines. The method can reflect uncertainties associated with the simulation results and the environmental guidelines, as well as the resulting risks of human health injury. In detail, this research considers: (1) the fate and transport of the pollutant in heterogeneous porous media under uncertainty, (2) distribution of pollutant concentrations under natural attenuation, (3) relationships between drinking water standards and health risk guidelines, and (4) probabilistic quantification of health injury risks. This method is applied to a site contaminated by leaking underground storage tanks. The results indicate that reasonable outputs have been generated. They are useful for clarifying potential health effects when the groundwater is withdrawn for domestic uses, as well as providing support for the related risk-management and site-remediation decisions.