Investigation of NAPL transport through a model sand cap during ebullition

Abstract

AbstractNonaqueous‐phase liquid (NAPL) migration from sediments to the surface of water bodies has been reported frequently at sites with sediments contaminated with NAPLs, such as coal tar and creosote. Commonly, transport of NAPL from sediment is facilitated by gas ebullition caused by anaerobic biodegradation of organic matter in the sediment. A remedy often specified for these sites is a sand cap, and sand caps amended with sorbent materials (such as organoclays) are being pilot‐tested. This article discusses a laboratory study to assess the effectiveness of a sand layer for controlling NAPL migration. The study used a test column composed of a Plexiglas tube containing a tar source that was buried beneath a 30‐cm‐thick layer of fine sand. Water was added to the column until 5 cm of standing water covered the sand layer. To simulate ebullition, air was injected into the base of the sand column at approximately 200 mL/min. It was observed that the gas and NAPL migrated primarily through channels and fractures in the sand, and was not filtered through a network of stable pores. Tar migrated through the sand layer in 12 hours and accumulated on the water surface for several hours before losing its buoyancy and settling back down to the sand surface. After ending the tar migration experiment, the test column was frozen to preserve structures in the sand. The study showed that the tar migrated through the simulated sand cap in small (2‐mm) channels only a few sand grains thick. The results of this laboratory work call into question the effectiveness of sand caps for controlling NAPL migration from sediment in the presence of ebullition. © 2009 Wiley Periodicals, Inc.