Dynamics of Escherichia coli Numbers in the Soil of Irrigation Fields

A. S. Gorodetsky and S. S. Lokshina

From the Ukrainian Scientific Research Institute of Communal Hygiene

In 1958–1959, we studied the dynamics of E. coli numbers in the soil of irrigation fields. The work was conducted at the Kurenevka irrigation fields in Kiev. These fields receive wastewater with an E. coli titer of \(10^{-5}\)–\(10^{-7}\) and a microbial count in the first two tens of millions.

The study was conducted directly on irrigation-field plots, on specially established experimental plots at the site, and under laboratory conditions.

Under operating conditions, soil was examined from five plots on which cabbage, tomatoes, zucchini, cucumbers, and eggplants were grown. The plots were irrigated with wastewater according to agricultural requirements. In autumn and winter, all plots were irrigated intensively with wastewater. Investigations began in spring after the soil had been plowed and continued throughout the growing season until late autumn. Cultures were made to detect pathogenic enteric microorganisms and to determine the dynamics of E. coli die-off and the microbial count.

Fig. 1. E. coli titer in the soil of operating plots of the irrigation fields.
1—cabbage plot; 2—zucchini plot; 3—eggplant plot; 4—tomato plot; 5—cucumber plot.

During the study, no pathogenic enteric microorganisms were detected in the soil of the irrigation-field plots. However, paracolon organisms were isolated, including strains that agglutinated positively with typhoid and dysentery antisera at dilutions no greater than 1:80.

Observations showed that in spring, before the fields were put into use and before the vegetables were planted, the E. coli titer in the soil of all plots was \(10^{-2}\). Following the flooding of the plots with wastewater, the soil E. coli titer fell to \(10^{-5}\)–\(10^{-7}\). Throughout the growing season, E. coli numbers in the soil of the operating plots fluctuated widely, decreasing as a result of soil self-purification and increasing again when wastewater introduced renewed contamination. By autumn, however, after plowing, the soil was substantially freed of the introduced contamination (coli titer \(10^{-1}\)–\(10^{-2}\)) (Fig. 1).

The number of microorganisms growing on meat-peptone agar (MPA) at \(37^\circ\) in the soil of all plots ranged from hundreds of thousands to tens of millions, indicating substantial contamination of the soil of the operating plots with microorganisms of this group. Alongside the observations of soil in the operating irrigation-field plots, the dynamics of E. coli numbers were also studied on 11 experimental plots using specially prepared wastewater with a defined E. coli content.

Because the literature has paid little attention to differences in the degree of soil contamination when soil is fertilized with clarified and unclarified wastewater, the experimental plots were irrigated with heavily and lightly contaminated wastewater. This was intended to reproduce the soil contamination resulting from the application of clarified and unclarified wastewater. To achieve uniform soil contamination, a layer of soil was removed from the surface of each experimental plot with a shovel and placed in a large metal vessel. The soil in the vessel was gradually flooded with the prepared wastewater…

…with wastewater and mixed thoroughly. The resulting mass was applied to the study plot and distributed evenly over its entire surface. During the subsequent observation period, the experimental plots received no further wastewater irrigation but were regularly moistened with tap water.

Soil from all plots was examined periodically and simultaneously. Some of the heavily and lightly contaminated experimental plots were sown with beets, carrots, and parsley. The others were left unsown. The initial E. coli titer of the soil was \(10^{-8}\)–\(10^{-11}\) in the heavily contaminated plots and \(10^{-5}\) in the lightly contaminated plots.

Observations showed that approximately 1–2 months after irrigation, the number of E. coli in the soils studied decreased sharply in all experimental plots, indicating that the introduced contamination was rapidly being eliminated. By the end of the observation period, the E. coli titer had reached \(10^{-1}\) and even 10 g of soil, regardless of the initial degree of contamination (Fig. 2). No substantial difference in the elimination of E. coli from the soil was observed between plots sown with vegetables and those left unsown.

In the laboratory, the investigations were conducted by the following method. A typical, previously tested culture of B. coli commune was added to soil from irrigation-field plots with an E. coli titer of 1.0. After inoculation, the soil was placed in vegetation vessels with a capacity of 1,000–1,200 g and an opening in the bottom through which a gauze wick was threaded. Through this wick, the experimental vessel was moistened as needed from an adjacent vessel filled with tap water. Soil samples were periodically taken from the vegetation vessels for examination.

Fig. 2. Changes in the E. coli titer in experimental plots of the irrigation fields.

Vertical axis: Coli titer.
Horizontal axis: Month (IV–XI).

1 — plot no. 2, heavily contaminated; 2 — plot no. 3, heavily contaminated; 3 — plot no. 4, heavily contaminated; 4 — plot no. 6, lightly contaminated; 5 — plot no. 7, lightly contaminated; 6 — plot no. 8, lightly contaminated.

The data obtained in the laboratory experiment agreed with the field observations.

After the B. coli commune culture was introduced into the soil, the E. coli titer was low: \(10^{-10}\)–\(10^{-11}\). It gradually increased, indicating a fairly rapid die-off of E. coli (1.0).

Conclusions

1. Wastewater introduces enormous numbers of E. coli into irrigation fields, and by the beginning of the growing season the soil is heavily contaminated with them.

2. Between irrigation events, self-purification processes occur in the soil of operational irrigation-field plots, as reflected in a decrease in the number of E. coli. When the fields are irrigated again with wastewater, E. coli contamination of the soil increases once more. By autumn, the soil of the irrigation fields is largely freed of the introduced contamination.

3. A decrease in the number of E. coli was observed in the soil of operational irrigation-field plots, in experimental plots, and under laboratory conditions, indicating the capacity of irrigation-field soil for self-purification.

Received January 13, 1961.