Impact of the Capillary Fringe on Local Flow, Chemical Migration, and Microbiology

Abstract

are less than atmospheric. Studies of flow in the vadose zone often focus on issues of fingering of water fronts We critically examine historical and recent studies of flow and and characterization of the relationships among moistransport at the local scale within the capillary fringe (CF). The characterization of subsurface pathways traveled by water-and the impact ture content, matric potential, and relative permeability. of these pathways on the movement of chemicals and the viability of In contrast, flow in the groundwater zone is generally subsurface microbial populations-has been the subject of intensive characterized by pores that are essentially saturated investigation in the last 50 yr in the fields of soil science and hydrology.with water. The liquid within the pores is typically at However, consideration of the complexity of local pathways within pressures greater than atmospheric. Flow is no longer the CF has been largely ignored. Recent studies, as well as some predominantly vertical, but reacts to local or regional historical ones, have reported observations of fluid flow and chemical recharge and discharge fluxes, including natural fluxes transport within the CF, emphasizing the impact of physical heterogesuch as recharge from precipitation or discharge to surneity and exchange of water and chemicals between the CF and the face water. region below the water table. These observations lead to the conclu-At the contact between these two zones is a relatively sion that the CF may affect, at the local scale, the natural geochemical and microbial conditions present in the region of transition from narrow region that serves as a transition between the unsaturated to saturated groundwater flow far more significantly than vadose zone and the groundwater zone. Sediments imis usually assumed. Here, we examine underlying physical factors that mediately below the water table, for example, are charcontrol local flow and transport behaviors in and across the CF and acterized by pore water pressures greater than atmoillustrate these factors through examination of images collected in the spheric pressure, and nearly full saturation by the liquid laboratory. We also provide a brief survey of the literature on the phase in most portions of the medium. Also present are CF. It is argued that existing definitions of the CF are inadequate to local zones of entrapped air in the form of bubbles or describe flow and transport behavior in the vicinity of the water table.small inclusions of sediments at low moisture content. We suggest replacing the concept of the CF with the concept of aThe region immediately above the water table contains partially saturated fringe, which involves multiphase transport in the water that essentially saturates the pores, although the immediate vicinity of (both above and below) the water table.

Studies of Water Velocity in the Capillary Fringe: The Point Velocity Probe

“…From laboratory experiments, lateral flow in the capillary fringe was reported by Wyckoff et al (1932) and later by Luthin and Day (1955). More recently, also based on laboratory experiments, flow and solute transport in the capillary fringe were demonstrated by Silliman et al (2002) and Berkowitz et al (2004). Xie (1994), using a saturated‐unsaturated flow and transport model, demonstrated the development of contaminant plumes within the capillary fringe and further showed the effect that contaminants within the capillary fringe could have on the performance of pump‐and‐treat remediation schemes.…”

Patterns of Entrapped Air Dissolution in a Two‐Dimensional Pilot‐Scale Synthetic Aquifer

“…Gas trapped within a porous (or fractured) medium tends to fill the largest pores, effectively reducing the saturated hydraulic conductivity by a factor of 2 to 20 (Marinas et al and references therein). Such reductions can decrease natural and artificial groundwater recharge (Christiansen ; Constantz et al ; Faybishenko ; Heilweil et al ) and/or create stagnant flow zones (Ryan et al ), which can impede solute, contaminant, and microbial transport within the vicinity of the groundwater table (Orlob and Radhakrishna ; Berkowitz et al ; Amos et al ). Interactions with entrapped air can also affect groundwater biogeochemistry and contaminant transport.…”

Monitoring Lateral Transport of Ethanol and Dissolved Gasoline Compounds in the Capillary Fringe

“…This has been shown in laboratory tests using 2D boxes (Silliman et al 2002;Henry and Smith 2003;Berkowitz et al 2004). This has been shown in laboratory tests using 2D boxes (Silliman et al 2002;Henry and Smith 2003;Berkowitz et al 2004).…”