Pore-Size-Dependent Apparent Viscosity for Organic Solutes in Saturated Porous Media

The value of hydraulic conductivity under saturated conditions, K, depends on the nature of the porous medium and the physical properties of the perfusing fluid. The equation K = kρg/η, where k is the permeability, η is the fluid viscosity, ρ is the fluid density, and g is the gravitational constant, has been used to account for porous medium and fluid properties on K. A laboratory study was conducted to determine whether the equation properly accounts for viscosity effects of organic solutes on K. Hydraulic conductivity of 0, 25, 50, 100, 200, 300, and 400 mg L⁻¹ solutions of large molecular weight polyacrylamides (PAM); 0, 10, 20, 30, 40 and 50% (v/v) of glycerol in distilled water; and undiluted n-propanol was measured in two size fractions of quartz sand. The apparent viscosities of PAM and glycerol solutions were higher in the fine sand than in the coarse sand. The apparent viscosity of n-propanol was about the same in the two sand-size fractions. The above-stated equation did not properly account for the effects of fluid properties of PAM and glycerol on K. Use of viscosity values as measured by a viscometer in the equation could result in a several-fold error in prediction of K. Quantitatively characterizing those conditions under which the equation is valid may be important in analyzing the transport of environmental contaminants through soil, particularly when the contaminants may be organic chemicals of various molecular sizes and conformations. This research was supported by the Univ. of California Kearney Foundation of Soil Science.