Virus Transport and Survival in Saturated and Unsaturated Flow through Soil Columns

Abstract

Water with entrained disease-causing virus entering soil normally passes through water saturated and unsaturated regions before reaching the groundwater. Twelve experiments were conducted to evaluate the effect of saturated versus unsaturated flow, and the effect of organic matter in unsaturated flow on the survival and transport of a virus, MS-2 bacteriophage, in soil columns. Additional experiments were conducted to characterize the soil, to assure that the experimental equipment did not remove virus, and to determine the extent of reversible adsorption of virus to soil. The virus were added to well water and applied to soil columns 0.052 m in diameter and 1.05 m long. KBr was used as a chemical tracer. In two experiments organic matter in the soil water was increased by using soil humic material or extract from sewage sludge. The soil material was Vint loamy fine sand (a sandy, mixed, hyperthermic Typic Torrifluvent) mixed with recent alluvium. Water samples were extracted from 0.10, 0.20, 0.40, and 0.80 m depths through porous stainless steel samplers, and from the 1.05 m depth through the percolate tube. Four different eluants were tested to remove virus from soil, and one, tryptic soy broth, was used to elute virus after three transport experiments. For saturated flow the virus concentrations reached the influent concentration in less than 2 pore volumes (T), with a retardation coefficient R at 1.05 m = 0.80. For unsaturated flow with low organic matter the relative concentrations reached steady-state values (C/C₀)(s) ranging from a mean of 27% of inflow at 0.20 m (5 to 18 T) to a mean of 5% at 1.05 m (1 to 3.3 T). Under unsaturated conditions with increased organic matter, virus (C/C₀)(s) at 1.05 m was from 41% to 49% of influent, 8 to 10 times greater than with low organic matter. Elution permitted calculation of a partition coefficient k(p) essentially equal to 0 (saturated average k(p) = -0.07 mL/g, SD = 0.15 mL/g; unsaturated average k(p) = 0.28 mL/g, SD = 0.40 mL/g), indicating little or no adsorption of virus to soil solids. Under unsaturated flow conditions enhanced removal of this virus occurs, and the removed virus are apparently inactivated. Organic matter reduced the removal of virus during transport by unsaturated flow. Virus concentrations reached and maintained a steady-state, exponentially-declining profile with depth.