A FIELD-STUDY OF ENHANCED RECOVERY OF DNAPL POOLED BELOW THE WATER-TABLE

A large subsurface pool of waste solvent product, consisting primarily of 1,1,1-trichloroethane and carbon tetrachloride, was encountered du ring investigations at an industrial site in northern New Jersey In th e 1950s the product was discharged through a settling chamber directly below the shallow water table. Eventually, the product accumulated wi thin elongated depressions of erosional surface of varved clays at dep ths 10 to 15 feet below grade. The host sediment, fine to medium sand, was overlain by fine sand and silt. The delineated area of pooled DNA PLs covered 2750 feet(2), and the maximum pool thickness exceeded 3 fe et. The primary recovery involved pumping product from nine wells. Eac h recovery well was equipped with a sump extending into the clay, whic h enabled the system to keep the product pumping level below the botto m of the pool. A total of 3495 gallons of solvent product was recovere d over two years. Nearly half of this volume was produced by two wells placed at the lowest points of the pools. Postpumping sampling of the former pools indicated that 93 to 94 percent of the pooled solvent ma ss was removed during the primary recovery. Average initial product sa turation within the pool was estimated at 53.2 percent of the total po rosity measured at 31 percent. Average residual saturation after the p rimary product recovery was 3.7 percent of the total pore volume. To t est the feasibility of residual product recovery, an experimental seco ndary recovery was undertaken. Using sheet piling, a 506 feet(2) test cell was constructed inside the former DNAPL pool. The cell featured a central recovery well, six peripheral wells, and monitoring probes. T he selected sequence of secondary operations included partial dewateri ng, hot water injection, final dewatering, and thermally enhanced vapo r extraction (TVE). During six weeks of the secondary recovery operati ons, 87.9 gallons of product were removed, of which 72 percent was fro m TVE, 25 percent from hydraulic mobilization effects, and 3 percent f rom dissolution of residuals. Confirmatory soil sampling showed an ave rage reduction of residual contamination by 93.4 percent in comparison to the concentration of residuals prior to the secondary recovery For the test cell, a combined total solvent recovery of 99.6 percent was achieved. This high recovery exceeded DNAPL recoveries expected or ach ieved in other field-scale attempts.