Free-phase NAPL-trapping features in intermontane basins
Love, D. W., Whitworth, T. M., Davis, J. M., and Seager,W. R., 1999, Environmental and Engineering Geoscience, v. 5, p. 87-102.
Just as free-phase oil and gas are trapped in the subsurface, spilled nonaqueous phase liquids (NAPLs) can be trapped or pooled by decimeter- to hectometer-scale features at contaminated sites. Free-phase NAPL is that which is sufficiently saturated to flow as a body in the subsurface. In order to trap free-phase NAPLs, the height of the trapping feature (trap-closure height) must be greater than the capillary intrusion of water into the NAPL phase, and the trap boundary must be sufficiently fine-grained to prevent the NAPL from entering its pores. Capillary intrusion of water into a free-phase NAPL body is a function of the physical properties of the liquid phases as well as the grain size within the trap, and it can be estimated using Hobson's formula. Calculations suggest that necessary trap-closure heights are on the order of one to several centimeters for coarse-grained material and range from one to more than five meters for fine-grained sands. Features in intermontane basins with centimeter-to meter-scale positive or negative topographic relief may form environmentally significant free-phase NAPL traps. These include alluvial-fan, fluvial, lacustrine, eolian, and spring-related (krenegenic) deposits and the contacts between them as well as biogenic, pedogenic, volcanic, and tectonic features. Examples of possible free-phase light nonaqueous phase liquid (LNAPL) traps include buried channels, natural levees, gravel bars, gilgai (swelling clay hummocks), spring mounds, eolian and base-surge dunes, and eolian wedges adjacent to fault scarps. Examples of possible free-phase dense nonaqueous phase liquid (DNAPL) traps include scoured channel bases, incised and backfilled arroyos, deformed soft sediments, larger root traces and animal burrows, buried interdune areas, volcanic collapse features, and rotated wedges of sediments adjacent to faults. High-resolution noninvasive geophysical techniques promise to delineate subsurface strata at adequate scales to show features capable of trapping free-phase NAPL at contaminated sites. However, knowledge of geosystem permeability, migration pathways, and free-phase NAPL trapping processes may be needed to guide remediation efforts.