Estimating Groundwater and 1,4-dioxane Contaminant Mass Flux in a Karst Aquifer using the Discrete Fracture Network - Matrix Field Approach
1,4-dioxane was detected in a municipal supply well in the karstic Upper Floridan Aquifer. High aquifer transmissivity and site dimensions create uncertainty in hydraulic gradients, reducing confidence in Darcy’s law-based mass flux calculations. Depth-discrete, high-resolution rock core contaminant profiles and borehole geophysical logs were collected to assess contaminant mass storage in the matrix and proximity to fractures. Hydraulically active features were identified under natural hydraulic conditions using active distributed temperature sensing in FLUTe™ lined boreholes. Modified passive flux meters and pressure/temperature transducers were deployed in depth-discrete zones (1-2 m long) external to the FLUTe™ liner to quantify water flux, contaminant flux, and characterize site hydraulics. PFMs deployed behind FLUTe™ liners are effective in this field setting enabling quantification of groundwater specific discharge and mass flux despite low hydraulic gradients. Rock core matrix concentrations range higher and lower than groundwater concentrations suggesting 1,4-dioxane matrix diffusion is a key process onsite.