Impermeable 'Hard' Cap Evaluation – Thea Foss Waterway, Tacoma, Washington

摘要

This paper presents a review of the design, installation and effectivenessof a hybrid cap consisting of an impermeable "hard" plastic sheet and sand designed tocontain DNAPL seeps in the Thea Foss Waterway, Tacoma, Washington. Coal tarderived materials were deposited in the waterway during the early 1900s that weresubsequently covered by fine grain sediment. In the mid-1990s, a subtidal DNAPL seepwas observed in the waterway during low tides beneath a recently constructed bridge.Pulling of timber piles and other construction activities appeared to have createdpreferred pathways for the upward migration of DNAPL. The DNAPL seepage had adensity greater than water. Site observations indicated that the upward movement of theDNAPL was facilitated by degassing (gas bubbles) that occurred during low tides. Thehybrid cap consisted of a 3/8-inch (0.15cm) thick HDPE plastic sheet overlain by three tosix feet (1m to 2m) of sand. The goal of the cap was to physically, chemically andhydraulically contain the underlying contaminated sediments and seep to preventrecontamination of surface sediments. The hybrid cap was installed in November 2003and three monitoring rounds were completed between April 2004 and May 2006. Visualobservations and sediment sampling indicate the hybrid cap is functioning as intended.