Simulating tracer and resident contaminant transport to investigate the reduced efficiency of a pump-and-treat operation

Influent trichloroethene (TCE) concentrations have levelled off at approximately 100 ppb during the 10 years of pump-and-treat conducted at a superfund site in Tucson, Arizona. A dual-well, forced-gradient tracer experiment was conducted to help evaluate the factors responsible for the extreme tailing. Tracer (bromide) breakthrough and resident TCE elution were monitored at an extraction well and at discrete levels of a centreline monitoring well. The vertical variability of hydraulic conductivity was analysed by layered numerical and analytical models. Parameters to be used in a TCE transport model were determined based upon laboratory experiments conducted using aquifer material from the site and from previous modelling results for flow and bromide transport. Some of the processes considered include rate-limited diffusion from low conductivity zones, rate-limited desorption, and rate-limited dissolution from immiscible liquid. These factors are important processes at many sites contaminated with organic pollutants.