Comparison of Performance of Recirculation Wells with Conventional Pump-and-Treat Systems

摘要

A major difference between CPT and RWT is that CPT does not produce vertical gradients for advective flushing of low-K layers. By inducing vertical gradients, RWT shortens the flow path in low-K zones, flushing contaminants vertically into higher-K material, where rapid lateral transport can take place. In contrast, CPT induces minimal vertical gradients so that contaminants must move by the relatively slow process of diffusion out of the clays or horizontal advection through the clay. In any implementation of RWT, the effectiveness of vertical advection across low-K zones should be investigated because this will have a significant effect on the time to reach cleanup goals where silt layers have become contaminated with VOCs. The presence of continuous low-K layers presents a challenge to adequate RWT performance, but there are ways to design around this problem. Often, screen placements need to be adjusted, or a dual recirculation cell should be implemented so that capture could be effected on either side of the low-K zone. In summary, most field investigations for installation of either CPT or RWT should include detailed lithologic logs and geophysical logs of the well borings. As long as low-K zones lie outside the radius of recirculation, RWT should perform as effectively as CPT. For the lithologies, screen placements, and pumping rates used in this study, small-scale heterogeneities more than 30 feet from an RWT well will not significantly degrade its performance. Thus, given adequate characterization, RWT can be implemented under a wide variety of conditions, allowing remedial designs to take advantage of its more rapid flushing within the zone of induced vertical gradients.