Carbon Tetrachloride Flow Behavior in Unsaturated Hanford Caliche Material: An Investigation of Residual Nonaqueous Phase Liquids

cally, water is assumed to be immobile at the residualwater saturation. In the petroleum industry, the residAt many contaminated sites, nonaqueous phase liquids (NAPLs) ual-water saturation is also called the irreducible-water persist in the vadose zone for long periods of time. This occurs because the permeability of the NAPL becomes negligible at some saturation saturation. This terminology has also been adapted in and downward movement ceases, resulting in residual NAPL. To obhydrology (e.g., Lenhard, 1992; Oostrom et al., 1999). tain data that can be used to study the development of a residual When applied to NAPL in two-phase water NAPL sysNAPL saturation and to test corresponding models, a detailed trantems in strongly water-wet porous media, a common defsient experiment was conducted in a 170 cm long by 90 cm high by inition of residual NAPL is that which becomes dis5.5 cm wide flow cell. Fluid saturation measurements were obtained continuous (entrapped) in the pore spaces when the with a dual-energy radiation system. The experimental conditions apparent water saturation is on an imbibition path. reflected those at the Hanford Site in Washington State, where an However, for three phase water–NAPL–air systems, the estimated 363 to 580 m3 of carbon tetrachloride (CCl4) was disposed term has also been used to indicate NAPL, not trapped to the subsurface. A key subsurface feature at the Hanford Site is a by water, which is negligibly mobile. To avoid confusloped Plio-Pleistocene caliche layer, which was reproduced in the experiment as a sloped lens in a medium-grained, uniform, sand masion with the term residual NAPL, we define NAPL that trix. The caliche contains considerable amounts of CaCO3 and may are discontinuous (i.e., blobs) and occluded by water have fluid wettability properties other than strongly water wet. A as trapped NAPL and NAPL that are negligibly mobile total of 800 mL of CCl4 was injected into the experimental domain and not trapped by water as residual NAPL. Residual at a rate of 0.5 mL min 1 from a small source area located at the surNAPL can be either continuous throughout the pore face. After apparent static conditions were obtained with respect to spaces or discontinuous as a result of isolation in pore CCl4 redistribution, saturation measurements indicated that all of the wedges or formation of lenses on water surfaces. For dense nonaqueous phase liquids (DNAPL) that had initially moved three-phase water–NAPL–air systems in strongly waterinto the caliche remained in this layer. Water was subsequently applied wet porous media, NAPL can be present in free, residual, to the surface at a constant rate over the full length of the caliche and trapped form. However, for the same conditions in layer to study CCl4 displacement as a result of changing water saturations. Water saturation in the caliche layer rose to as high as 0.91 during two-phase water–NAPL systems, only free and trapped water infiltration. Results show that 25% of the DNAPL present in the NAPL might exist. caliche migrated from this layer as a consequence of water infiltration, Although residual NAPL is known to occur, there while 75% remained in the caliche layer. The experimental results are no numerical simulators that account for residual could not be reproduced with numerical multifluid flow simulations NAPL as a function of saturation-path history. The bebased on common constitutive theory. This indicates that improvehavior of residual NAPL in three-phase air–NAPL– ments in constitutive theory may be needed to accurately model air water systems has not been studied in great detail for DNAPL water flow behavior. environmental purposes, although its presence has broad implications for groundwater contamination. Dense vapors originating from volatile organic compounds may N phase liquids, like fuel hydrocarbons sink and spread rapidly in the vadose zone followed by and chlorinated solvents, have entered the subsurpartitioning into the groundwater. In addition, recharge face via chemical spills, leaks, and direct disposal. At water may come into contact with the NAPLs and transmany contaminated sites, NAPLs persist in the vadose port dissolved components downward to aquifers. zone for very long periods. This occurs because the perOne of the most ubiquitous NAPLs at U.S. Departmeability of the NAPL becomes negligible at some satument of Energy sites is the DNAPL CCl4. At the Hanration and downward movement ceases, resulting in reford Site in Washington State, CCl4 mixtures were dissidual NAPL. posed to waste sites (Last and Rohay, 1993). Plutonium The term residual saturation has been applied to reprecovery operations at the Plutonium Finishing Plant resent several processes or phenomena. When applied have resulted in organic and aqueous wastes that have to the wetting fluid, it characteristically refers to a water been disposed at several cribs, tile fields, and drains. saturation at which the effective permeability (i.e., the The major disposal facilities received an estimated permeability at a prescribed fluid saturation) of water 13 400 000 L of liquid waste containing 363 000 to approaches a value of zero during water drainage. Typi580 000 L of CCl4. The liquid waste included aqueous waste and DNAPL. Calculations indicated that the maM. Oostrom, Environmental Technology Division, Pacific Northwest jority of the disposed CCl4 entered the subsurface as National Laboratory, P.O. Box 999, MS K9-33, Richland, WA 99352; DNAPL. Last and Rohay (1993) conducted a rough R.J. Lenhard, Idaho National Engineering and Environmental Laboorder-of-magnitude estimate of the discharge inventory, ratory, P.O. Box 1625, Idaho Falls, ID 83415-2214. Received 15 May 2002. Original Research Papers. V02-0020. *Corresponding author (mart. [email protected]). Abbreviations: DNAPL, dense nonaqueous phase liquids; NAPL, nonaqueous phase liquids; PCE, perchloroethylene. Published in Vadose Zone Journal 2:25–33 (2003).