Quantitative assessment of intrinsic groundwater vulnerability to contamination using numerical simulations

Intrinsic vulnerability assessment to groundwater contamination is part of groundwater management in many areas of the world. However, popular assessment methods estimate vulnerability only qualitatively. To enhance vulnerability assessment, an approach for quantitative vulnerability assessment using numerical simulation of water flow and solute transport with transient boundary conditions and new vulnerability indicators are presented in this work. Based on a conceptual model of the unsaturated underground with distinct hydrogeological layers and site specific hydrological characteristics the numerical simulations of water flow and solute transport are applied on each hydrogeological layer with standardized conditions separately. Analysis of the simulation results reveals functional relationships between layer thickness, groundwater recharge and transit time. Based on the first, second and third quartiles of solute mass breakthrough at the lower boundary of the unsaturated zone, and the solute dilution, four vulnerability indicators are extracted. The indicator transit time t₅₀ is the time were 50% of solute mass breakthrough passes the groundwater table. Dilution is referred as maximum solute concentration C_max in the percolation water when entering the groundwater table in relation to the injected mass or solute concentration C₀ at the ground surface. Duration of solute breakthrough is defined as the time period between 25% and 75% (t_25%-t_75%) of total solute mass breakthrough at the groundwater table. The temporal shape of the breakthrough curve is expressed with the quotient (t_25%-t_50%) / (t_25%-t_75%). Results from an application of this new quantitative vulnerability assessment approach, its advantages and disadvantages, and potential benefits for future groundwater management strategies are discussed.