Survival of indicator organisms in sonoran desert soil amended with sewage sludge

Anaerobically digested sewage sludge is currently applied to farmland surrounding Tucson, Arizona to supply nutrients for the growth of cotton. Bacterial pathogens introduced into the environment by this practice may survive or move through the soil profile thus posing health risks to animals and humans. We have conducted both laboratory and field studies to monitor the survival and transport of bacterial pathogens added to soil via sludge. In laboratory studies, sewage sludge was added to soil and incubated at constant moisture and temperature conditions. Populations of fecal streptococci, fecal coliforms and total coliforms were monitored at weekly intervals using the Most Probable Number (MPN) method. Two soils and 3 temperatures were evaluated. Field studies determined the survival of indicator organisms in the surface horizon, and survival and transport of fecal coliforms up to a depth of 300 cm. We found that soil moisture, texture and temperature all affected the persistence of bacterial pathogens in sludge amended soil. Survival of organisms increased with increasing soil moisture and clay content, and with decreased soil temperatures. In the field, when soil moisture content increased after rainfall events, regrowth of indicator organisms occurred. Soil acted as an efficient filter negating bacterial transport, and thus fecal coliforms did not appear to migrate through the soil profile under non-irrigated conditions.