Residual contamination of crops irrigated with effluent of different qualities: a field study

Abstract

Unrestricted crop irrigation with wastewater effluents is a major concern for public health. The aim of this investigation was to compare two wastewater effluent reservoirs used for irrigation of a variety of vegetables. The study covers the period from January to the end of July, with sprinkle irrigation starting in mid-April. Lettuce, parsley, cabbage, onion, carrots, fennel, radish and tomato irrigated with highly polluted effluents, displayed elevated numbers of indicator microorganisms. In contrast, vegetables irrigated with slightly polluted effluents, showed significantly lower numbers of indicator microorganisms. Although a low background of microbial indicators was detected in both cases before irrigation started, a significant increase of residual contamination was observed on vegetables irrigated with the polluted effluents, following the irrigation onset in mid-April. Independently of the need to correlate quality standards for unrestricted crop irrigation with epidemiological data, irrigation with poorly managed wastewater, based on microbiological and chemical parameters, results in highly contaminated vegetables. Vegetables often eaten raw or with rich dressings (which may result in regrowth of some pathogenic bacteria) may threaten the public health, without clear outbreaks. It must be borne in mind that drinking water that does not differ much from these vegetables in the way of consumption, has very restrictive quality standards. Similar restrictive standards should be applied for vegetables irrigated with reused water. The results of this study strongly suggest the necessity to treat wastewater effluents to an extent in which no residual contaminants will be detected on irrigated crops.