Gowanus Canal Superfund Site. VI: Rapid Field Screening Techniques to Verify Sediment Capping Blends

Abstract

At the Gowanus Canal Superfund site in Brooklyn, New York, layers comprising different granular activated carbon (GAC)/sand blends are being placed as part of a multilayer reactive capping system required for the approved remedy. Depending on the capping zone, the targeted GAC contents can vary from 6% to 60% by dry weight (%). The construction quality assurance (CQA) method for verifying the GAC content of each GAC/sand stockpile consists of loss on ignition (LOI) testing performed at an offsite laboratory with a turnaround time of 3 days. To mitigate construction delays and site congestion at the upland blending site, rapid field (onsite) testing methods to verify the GAC content of the GAC/sand blends were sought. Portable X-ray fluorescence (pXRF) was successfully used on oven dried and crushed GAC/sand materials to develop three linear elemental [calcium (Ca), silicon (Si), and sulfur (S)] correlations with goodness of fit (R2) values greater than 0.95 for Ca and S. Sulfur was not detected in the sand and appeared linked to the bituminous coal source materials for manufacturing the GAC and thus became the primary element to track the GAC content. The measured GAC contents of the CQA samples by pXRF were within 2 to 3 percentage points of the values determined by LOI testing and were obtained onsite within hours. In addition, the pXRF measurements more closely matched the initially batched materials on site that were intentionally overdosed by up to 25% of the GAC blending target to avoid failure. The preliminary correlation presented here provides strong promise for the use of pXRF as the CQA method to furnish accurate GAC content results for the GAC/sand blends onsite within hours (not days) during the implementation of the Gowanus Canal capping remedy.