RESIDUAL CONTAMINATION OF CROPS IRRIGATED WITH EFFLUENT OF DIFFERENT QUALITIES - A FIELD-STUDY

Unrestricted crop irrigation with wastewater effluents is a major conc ern for public health. The aim of this investigation was to compare tw o wastewater effluent reservoirs used for irrigation of a variety of v egetables. The study covers the period from January to the end of July , with sprinkle irrigation starting in mid-April. Lettuce, parsley, ca bbage, onion, carrots, fennel, radish and tomato irrigated with highly polluted effluents, displayed elevated numbers of indicator microorga nisms. In contrast, vegetables irrigated with slightly polluted efflue nts, showed significantly lower numbers of indicator microorganisms. A lthough a low background of microbial indicators was detected in both cases before irrigation started, a significant increase of residual co ntamination was observed on vegetables irrigated with the polluted eff luents, following the irrigation onset in mid-April. Independently of the need to correlate quality standards for unrestricted crop irrigati on with epidemiological data, irrigation with poorly managed wastewate r, based on microbiological and chemical parameters, results in highly contaminated vegetables. Vegetables often eaten raw or with rich dres sings (which may result in regrowth of some pathogenic bacteria) may t hreaten the public health, without clear outbreaks. It must be borne i n mind that drinking water that does not differ much from these vegeta bles in the way of consumption, has very restrictive quality standards . Similar restrictive standards should be applied for vegetables irrig ated with reused water. The results of this study strongly suggest the necessity to treat wastewater effluents to an extent in which no resi dual contaminants will be detected on irrigated crops.