Gowanus Canal Superfund Site. VII: Rapid Verification of Organoclay–Sand Capping Blends

Abstract

At the Gowanus Canal Superfund site in Brooklyn, New York, a multilayer reactive capping system is required as part of the approved remedy. The base reactive layer is an organoclay (OC)–sand layer containing a minimum of 25% by dry weight OC that is intended to block any upwelling mobile nonaqueous phase liquids (NAPLs). The construction quality assurance (CQA) method for verifying the OC content of the OC–sand stockpiles prior to placement in the canal consists of Loss on Ignition (LOI) testing performed at an offsite laboratory with a turnaround time of 3 days. To mitigate construction delays, rapid field testing methods to verify the OC content of the OC–sand blends were sought. Interestingly, electrical conductivity (EC) measurements on the as-is (not sieved or pulverized) OC, sand, and their blends provided several correlations with goodness of fit (R2) values >0.95 for a variety of sand-sized materials including: a commercial playground sand, beach sand, a limestone quarry sourced sand, and the sediment capping sand from the first Remedial Target Area (RTA-1) at the Gowanus Canal. The correlation curves and R2 values were relatively insensitive to change in the sand borrow source (silicate or karst geology or salinity). EC measurements were available in as little as 30 min and up to approximately 24 h using a tabletop pH/EC probe, distilled water, beaker, and magnetic stirrer. The EC approach and preliminary correlations presented here can greatly reduce CQA testing turnaround times to within hours (not days) in field settings. Lastly, similar findings were made when another commercially modified bentonite clay known as Fluorosorb-400™ was blended with playground sand.