Free-phase NAPL-trapping features in intermontane basins

Just as free-phase oil and gas are trapped in the subsurface, spilled nonaq ueous phase liquids (NAPLs) can be trapped or pooled by decimeter- to hecto meter-scale features at contaminated sites. Free-phase NAPL is that which i s sufficiently saturated to flow as a body in the subsurface. In order to t rap free-phase NAPLs, the height of the trapping feature (trap-closure heig ht) must be greater than the capillary intrusion of water into the NAPL pha se, and the trap boundary must be sufficiently fine-grained to prevent the NAPL from entering its pores. Capillary intrusion of water into a free-phas e: 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 Ho bson's formula. Calculations suggest that necessary trap-closure heights ar e on the order of one to several centimeters for coarse-grained material an d 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 tr aps. These include alluvial-fan, fluvial, lacustrine, eolian, and spring-re lated (krenegenic) deposits and the contacts between them as well as biogen ic, pedogenic, volcanic, and tectonic features. Examples of possible free-p hase light nonaqueous phase liquid (LNAPL) traps include buried channels, n atural levees, gravel bars; gilgai (swelling clay hummocks), spring mounds, eolian and base-surge dunes, and eolian wedges adjacent to fault scarps. E xamples of possible free-phase dense nonaqueous phase liquid (DNAPL) traps include scoured channel bases, incised and backfilled arroyos, deformed sof t sediments, larger root traces and animal burrows, buried interdune areas, volcanic collapse features, and rotated wedges of sediments adjacent to fa ults. High-resolution noninvasive geophysical techniques promise to delinea te subsurface strata at adequate scales to show features capable of trappin g free-phase NAPE at contaminated sites. However, knowledge of geosystem pe rmeability, migration pathways, and free-phase NAPL trapping processes may be needed to guide remediation efforts.