Control of viral contamination of reclaimed irrigated vegetables by drip irrigation

Abstract

A number of factors have contributed to the interest in reclaimed wastewater irrigation of vegetables. However safety of water as related to fresh cut vegetables has been a paramount concern of responsible agencies, growers and consumers. The objective of this study was to evaluate the risk mitigation potential of subsurface drip irrigation during reclaimed wastewater irrigation. Virus detection methodologies on produce were first optimized. Beef extract (3%) +0.04M sodium pyrophosphate was found to be the most efficient eluent to recover viruses from soil and plant material. The recovery efficiency of poliovirus type l and adenovirus type 40 from Pima clay loam soil ranged from 12--16%, and recoveries from Brazito sandy loam ranged from 58--81%. The recovery efficiencies of poliovirus type 1, adenovirus type 40, MS2 and PRD1 from lettuce ranged from 45 to 70%. The recovery efficiencies of MS2 and PRD1 from tomato fruit were in the range of 90%. Poliovirus type 1, adenovirus type 40, MS2 and PRD1 survived longer in Pima clay loam than in Brazito sandy loam. All enteric viruses remained stable at 4°C suggesting that little virus inactivation occurs during transportation and marketing. Poliovirus type 1 and adenovirus type 40 lost 1 log₁₀ in 11 and 17 days respectively on lettuce at room temperature. Hepatitis A virus lost 2.5 log₁₀ in 50 days on a lettuce head in a greenhouse during the winter season. Tomato, lettuce and cucumber crops were irrigated with virus-seeded water by subsurface and surface drippers. Subsurface drip irrigation resulted in 99% less viral contamination of vegetable leaves as compared to surface drip irrigation. The greatest risk of infection occurs from the outer leaves of lettuce. The risk of infection from consumption of reclaimed wastewater irrigated tomatoes and cucumber was 32% and 72% less than lettuce. The risk of infection from rotavirus by ingestion of vegetables is greater than poliovirus type 1. The risk of infection from subsurface drip irrigated vegetables did not approach the United States Environmental Protection Agency's acceptable annual risk (1:10,000) until the concentration of viruses in the irrigation water reached 100/l. No internal contamination by viruses of the vegetables was observed during their growth. The coliphages PRD1 survived longer than poliovirus type I, adenovirus type 40, and MS2. It would thus appear to be a good model for studying the persistence of those viruses on produce and in irrigated agricultural systems.