THE EFFECT OF SOIL-WATER TENSION ON COLLOID GENERATION WITHIN AN UNSATURATED, INTACT SOIL CORE

Colloids may facilitate the transport of some toxic chemicals through the unsaturated zone into the groundwater. It is impossible, however, to predict the rates and extent of colloid-facilitated transport, because the physical and chemical factors governing colloid mobilization in the shallow subsurface have yet to be identified. We aim to better understand the mechanisms regulating colloid generation within an intact soil core by quantifying colloid concentrations eluted as soil-water tensions are varied. Changing the tension permits different size pores to remain open. We collected a core (20 cm diameter, 23 cm length) from a silt-loam soil in Virginia. In the laboratory, we sealed the base of the core to both a 10 µm porous plate and a vacuum chamber regulated by a manometer. We distributed a 5 mM NaCl influent solution over the top of the core at a rate of 0.30 mL/minute and used a fraction collector to sample effluent from the base of the core at 30-minute intervals. We characterized transport through the core using 1-pore-volume (~2770 mL), input pulses of 5mM NaBr at soil-water tensions of -18.5 cm and -9.5 cm. Approximately 95% of the total mass was recovered at a tension of -18.5 cm, while only 73% of the total mass was recovered at a tension of -9.5 cm. At both tensions, relative bromide concentrations declined to 4.0% after the core was flushed with 6 pore volumes of 5 mM NaCl. We estimated transport parameters for each tracer experiment using the CXTFIT2 code. A physical nonequilibrium model, that accounts for both mobile and stagnant flow regions, describes flow through the core. At tensions of -18.5 cm and -9.5 cm, Peclet numbers are 22.4 and 9.0; partitioning coefficients, describing the ratio of mobile to immobile water, are 0.83 and 0.66; and, mass transfer coefficients, characterizing the movement of mass between mobile and immobile zones, are 0.26 and 1.19. Colloid concentrations were analyzed with a spectrophotometer at 750 nm. Colloid concentrations range from 2 mg/L to 19 mg/L, and from 6 mg/L to 27 mg/L at tensions of -18.5 cm and -9.5 cm, respectively. Elevated, average colloid concentrations of 10 mg/L are eluted at a tension of -9.5 cm, compared to those of 5 mg/L eluted at a tension of -18.5 cm. The magnitude of the soil-water tension may contribute to colloid mobilization and transport through the core.