THE EFFECT OF EQUILIBRATION TIME ON DESORPTION RATE MEASUREMENTS WITHCHLORINATED ALKENES AND AQUIFER PARTICLES

Sorption and desorption phenomena affect the uptake and release of con taminants from aquifer solids, and can play an important role in deter mining the duration of an aquifer remediation effort. Independent esti mates of desorption rates are useful in evaluating various cleanup str ategies (e.g., pump-and-treat, in situ biorestoration). In this study, two type of aquifer solids, a weakly sorbing sand (Borden) and a rela tively strongly sorbing solids fraction (Moffett), were equilibrated f or periods of one day and greater than 190 days with tetrachloroethane (PCE) and trichloroethene (TCE), respectively. The samples were subje cted to an intermittent purging technique, and the resulting contamina nt release curves were analyzed using a radial pore diffusion model wh ich incorporated the effects of equilibrium partitioning. Failure to a ccount for the insufficient equilibrium conditions led to a moderate u nderestimation of the effective pore diffusion coefficient value (D(p) ) for the Borden under-equilibrated sample (one day), and a substantia l overestimation of the D(p) value for the Moffett under-equilibrated system. A second set of simulations, was used to find the D(p) value t hat best fit the observed release data while accounting for the nonequ ilibrium initial conditions. Given these adjustments, the Borden/PCE u nder-equilibrated system provided approximately the same desorption ra te value (effective pore diffusion coefficient, D(p)) as its long-term sample. For the Moffett/TCE under-equilibrated sample, accounting for the insufficient equilibration time led to a significantly greater D( p) value than was obtained for the long-term sample. The results for t he two solute/sorbent systems are discussed with respect to the pore d iffusion mechanism, and the prospect of short-term parameter estimatio n methods.