Virus survival in sewage sludge amended desert soil

Currently Pima County, Arizona, disposes all of its anaerobically digested sewage sludge in liquid form (1.5% solids) on agricultural land used for non-food crop production by subsurface injection or surface spreading. Present in these sludges are human enteric viruses in concentrations as high as 1,000 per liter of sludge. These viruses could potentially contaminate surface and groundwater sources during periods of irrigation or extended rainfall. This study was designed to assess the survival of viruses under field conditions typical of the arid Southwestern United States during the winter and summer months. This study was also conducted in the laboratory to simulate field conditions. Soil samples taken from freshly amended fields were seeded with poliovirus type 1 (stock titer = 10⁶/ml) and bacteriophage MS2 (stock titer = 10¹⁰/ml) and thoroughly mixed with the sludged soil. The seeded samples were put into containers and buried 10 cm below the soil surface, and samples were taken at pre-determined time intervals. Average soil temperature (measured at the 10 cm depth) ranged from 15°C in the winter to 33°C in the summer. Soil moisture decreased from 25% to 15% in the winter and from 40% to less than 5% in the summer. During the winter study, no inactivation of poliovirus was observed after 7 days, while greater than a 90% reduction was observed for MS-2. During the summer study, no poliovirus was recovered after 7 days, and no MS-2 was recovered after 3 days. The results of this study suggest that high soil temperature and rapid loss of moisture limit the survival of viruses in desert soils.