Subsurface drip irrigation with wastewater and the effects of environmental factors on virus survival in soil

Abstract

Wastewater reuse for agricultural purposes has been considered as an alternative strategy for water conservation. This study compared subsurface drip irrigation with furrow irrigation in terms of water use efficiency and crop contamination when microbial-contaminated water was used for irrigation. The effects of temperature and moisture on virus survival in soil were assessed quantitatively. Escherichia coli ATCC 25922, Clostridium perfringens, and coliphage PRD-1 served as the study microorganisms. Subsurface drip irrigation used water more efficiently than furrow irrigation due to lower water requirement for similar crop yields. Subsurface drip irrigation performed better for the deep-rooted crops, cantaloupe and bell pepper, compared to the shallow-rooted crop, lettuce. The more water was held by soil and was available for the crop uptake with the deeper root crops than shallow root crops. Overall, significantly greater microbial contamination of produce and soil surface occurred in furrow irrigated plots as compared to subsurface drip irrigated plots. Direct movement of irrigated water to the soil surface appeared to cause crop contamination in subsurface drip irrigation plots. Coliphage PRD-1 demonstrated greater crop contamination and longer survival in the environment compared to E. coli. The smaller size of PRD-1 than that of E. coli seemed to facilitate its movement through soil matrices and increased the chance to contaminate crop produce. The inactivation rate of PRD-1 increased as temperature increased. A soil moisture decrease from 20.9 to 8.9% appeared to be inversely related to the inactivation rate. Further decrease of the soil moisture content to 5.1% increased the inactivation rate. An optimum or a threshold soil moisture level for PRD-1 survival may exist. Evaporation increased the inactivation rate of PRD-1 substantially in higher temperatures but minimally in lower temperatures. A developed model predicted the inactivation rates of PRD-1 in field conditions with an average error of 11.0%. Subsurface drip irrigation with a proper management can be a sustainable strategy to conserve irrigation water as well as to reduce crop contamination when wastewater is used for crop production. PRD-1 survived better at low temperatures and dry soil moisture conditions.