Abstract. The effective specific air-water interfacial area (•i) in a sand-packed column was measured at several water saturations (Sw) using a surface-reactive tracer (sodium dodecylbenzene sulfonate (SDBS)) and a nonreactive tracer (bromide). Miscible displacement experiments were conducted under steady water flow conditions to quantify the retardation of SDBS resulting from its adsorption onto the air-water interface in a sand-packed column. A consistent trend of increased retardation of SDBS compared with the nonreactive tracer, bromide, was observed with decreasing S w. The data for air-water surface tension measured at various SDBS concentrations were interpreted using the Gibbs model to estimate the required adsorption parameters. The retardation factors (Rt) for SDBS breakthrough curves were then used in combination with the estimated SDBS adsorption coefficient to calculate the •i values at different Sw. For the range of experimental conditions employed in this study, the retardation factor for SDBS ranged from R t = 1.07 at Sw = 1.00 (R t > 1 due to SDBS sorption on sand) to R t = 3.44 at Sw = 0.29 (which corresponds to •i = 46 cm2/cm3). These values are in agreement with theoretical predictions and recently published data. Improvements needed to overcome the experimental limitations of the presented method are also discussed.

No abstract

No abstract

No abstract

No abstract

A new method, passive flux meter (PFM), has been developed and field-tested for simultaneously measuring contaminant and groundwater fluxes in the saturated zone at hazardous waste sites. The PFM approach uses a sorptive permeable medium placed in either a borehole or monitoring well to intercept contaminated groundwater and release "resident" tracers. The sorbent pack is placed in a groundwater flow field for a specified exposure time and then recovered for extraction and analysis. By quantifying the mass fraction of resident tracers lost and the mass of contaminant sorbed, groundwater and contaminant fluxes are calculated. Here, we assessed the performance of PFMs at the Canadian Forces Base Borden field site in Ontario, Canada. Two field tests were conducted under imposed groundwater flow fields: (1) radial flow to a well and (2) linear flow in a test channel confined by sheet pile walls on three sides. Both tests demonstrate that the local fluxes measured by PFM and averaged overthe screen interval were within 15% of imposed groundwaterflow and within 30% of measured contaminant mass flux. Patterns in depth variations in groundwater and contaminant fluxes, determined by the PFM approach, allow for site characterization at a higher spatial resolution. These results support the PMF method as a potential innovative alternative for measuring groundwater and contaminant fluxes in screened wells.

Two types of gas‐phase flow patterns have been discussed and observed in the in situ air sparging (ISAS) literature: bubble flow and air channels. A critical factor affecting the flow pattern at a given location is the grain size of the porous medium. Visualization experiments reported in the literature indicate that a change in the flow pattern occurs around 1 to 2 mm grain diameters, with air channels occurring below the transition size and bubbles above. Analysis of capillary and buoyancy forces suggests that for a given gas‐liquid‐solid system, there is a critical size that dictates the dominant force, and the dominant force will in turn dictate the flow pattern. The dominant forces, and consequently the two‐phase flow patterns, were characterized using a Bond number modified with the porous media aspect ratio (pore throat to pore body ratio). Laboratory experiments were conducted to observe flow patterns as a function of porous media size and air flow rate. The experimental results and the modified Bond number analysis support the relationship of flow patterns to grain size reported in the literature.

A series of gaseous miscible displacement experiments were conducted to estimate specific air–water interfacial areas (ai) and water contents in an unsaturated sand column. A straight‐chain hydrocarbon (n‐decane) was used as the gaseous interfacial tracer and methylene chloride and chloroform were used as the water‐partitioning gaseous tracers. A gas chromatographic technique was employed for the tracer experiments conducted at room temperature using nitrogen as the mobile phase and water as the immobile liquid. Tracer experiments covered a water saturation (Sw) range of 1.5 to 56%. The largest ai value (∼1500 cm2 cm−3), measured at the lowest Sw (1.5%), was somewhat smaller than the solid surface area (∼2000 cm2 cm−3) determined using the nitrogen‐sorption technique. As Sw increased, ai values decreased exponentially to ∼80 cm2 cm−3 at Sw of 56%. Within a limited Sw range (0.29 < Sw < 0.55), where both aqueous and gaseous interfacial tracer data were measured, the ai values measured using a gaseous tracer (n‐decane) were 2 to 3 times larger than those measured in a previous study using an aqueous interfacial tracer (sodium dodecylbenzene sulfonate [SDBS]). The velocity of the air–water interface was estimated to be between 23 and 36% of the bulk pore‐water velocity. The water contents measured using water‐partitioning tracers were within ±5% of those based on gravimetric measurements.