AQUIFER REMEDIATION - A METHOD FOR ESTIMATING MASS-TRANSFER RATE COEFFICIENTS AND AN EVALUATION OF PULSED PUMPING

When pumping contaminated water from an aquifer, contaminant concentra tions often decline rapidly, only to rebound when the pump is shut off . One reason for this behavior is that contaminants in the mobile phas e are readily removed, but mass transfer from the immobile to the mobi le phase is rate limited. Pulsed pumping, in which the pump is periodi cally turned off, has been suggested as a means to enhance remediation . We conducted a comprehensive comparison of pulsed pumping and contin uous pumping for the case of a Gaussian plume, subject to first-order mass transfer during transport, in an aquifer with no regional gradien t and constant mass transfer rate coefficients. We developed a Laplace domain Green's function solution for concentrations during pumping pe riods and coupled it with an analytic solution for concentrations duri ng resting periods. First, we used this model to provide a simple type curve that can be used to estimate the mobile-immobile phase mass tra nsfer coefficients from field data. Second, the mass and concentration removal histories were determined during pulsed pumping and during co ntinuous pumping. The continuous pumping rate removed the same volume of water over the duration of remediation. We investigated the effects of physical parameters such as the mass transfer rate coefficients, a nd engineering design parameters such as the length of resting periods . Under the conditions considered, our evaluation shows that (1) for e qual volumes of water removed, pulsed pumping does not remove more con taminant mass than pumping continuously at an average rate; (2) if the duration of the resting period is too large, then pulsed pumping remo ves much less mass than continuous pumping at the average rate, and (3 ) if the pulsed and continuous pumping rates are the same, pulsed pump ing will take longer than continuous pumping to clean up the aquifer, but will require significantly less time during which the pump operate s.