From Local Experience

M. I. Kuchmi and N. A. Koblova

On the Selection of Sites for Irrigation Fields

From the Uzbek Scientific Research Institute of Sanitation and Hygiene

Extensive sanitary engineering construction is currently under way in the republics of Central Asia and other southern regions of the USSR: a new irrigation system, new water supply systems, and sewerage facilities. This construction will expand still further in the coming years. The particular local climatic and soil conditions and patterns of water use have attracted the attention of hygienists, who are called upon to resolve a number of public health problems arising from this construction. These problems have become especially pressing in connection with the construction of one of the great projects of communism—the Main Turkmen Canal—which will necessitate solutions not only for water supply but also for wastewater treatment in newly established and rapidly expanding settlements. In this connection, the experience of local workers is of undoubted practical and scientific interest, particularly when it has been accumulated through prolonged observation. This article presents certain findings obtained in Tashkent that may be useful under conditions similar to those of the Uzbek SSR.

The study aimed to determine the effect of wastewater on soil and groundwater contamination, a matter of considerable sanitary importance under Central Asian conditions.

When the technical design for Tashkent’s sewerage system was approved, it was proposed that municipal wastewater be treated on irrigation fields.

Land in the valley of the Chirchik River, 1.5 km from the city limits, was to be allocated for the irrigation fields. During 1945, 1950, and 1951, observations were made of the processes by which municipal wastewater was rendered harmless on a specially designated experimental plot at the proposed site of the fields.

The soil’s self-purification capacity was studied at various concentrations of organic matter in the wastewater.

The soils of the experimental plot and the adjoining collective-farm land are irrigated and consist of loams and clays underlain by gravel. The groundwater level varies from 0.5 to 1.5 m below the ground surface. The soils have low water permeability.

In 1945 and 1950, the processes of mineralization in soil irrigated with unsettled wastewater were studied.

The soil was investigated during three seasons: spring, when unsettled wastewater was applied infrequently; summer, when irrigation was intensive; and autumn, when irrigation was again infrequent. The total irrigation rate was 5,400 m³/ha, with 25–30 m³/ha applied per day. Soil samples were taken from five plots planted with different crops. To obtain a composite sample, material was collected at five points in each plot. Samples were taken from five horizons down to the groundwater level. A total of 140 samples were examined.

In spring, the soil of the upper horizon (0–25 cm) had coli and anaerobic titers ranging from 0.1 to 0.0001 g; in the lower horizon (100–150 cm), the coli titer ranged from 1 to 0.01 g and the anaerobic titer from 0.1 to 0.01 g. In summer, both the coli and anaerobic titers of the soil fell sharply in the upper and lower horizons alike. In the upper horizon, the coli titer ranged from 0.0001 to 0.00001 g, and the anaerobic titer was 0.0001 g; in the lower horizon, the coli titer ranged from 0.01 to 0.0001 g and the anaerobic titer from 0.1 to 0.0001 g.

In autumn, both the coli and anaerobic titers rose somewhat compared with summer. Nevertheless, they did not reach the levels observed in spring.

The presence of large numbers of coliform and anaerobic bacteria in the soil of the upper and lower horizons, especially in summer, indicates heavy bacterial contamination by wastewater throughout the soil profile down to the groundwater level.

Chemical analyses indicate elevated chloride levels in the soil of the upper and lower horizons, ranging from 73.1 to 149.1 mg/kg. Ammonia is detected in the soil of the upper horizon in

ranged from 18.5 to 70.2 mg/kg, and in the lower horizon from 14.1 to 47.3 mg/kg. Ammonifying bacteria are present in equally large numbers in both the upper horizon (2.5 to 40.9 million) and the lower horizon (2.2 to 43.8 million). Particularly large accumulations of ammonia and ammonifying bacteria are observed in autumn.

The large quantities of ammonia and ammonifying bacteria in the lower soil layers indicate that contaminants may enter the groundwater flow. They also show that ammonification proceeds intensively throughout the entire soil profile.

Unlike ammonification, nitrification processes, with small quantities of nitrite nitrogen present, develop intensively only in the upper half-meter of soil. This is indicated by a nitrifying-bacteria titer ranging from 0.001 to 0.000001 g and a high nitrate content, from 22.3 to 25.15 mg/kg, despite the uptake of nitrates by plants. In the lower soil horizons, from a depth of 0.5 m down to the groundwater, nitrifying bacteria are either absent or present in small numbers, and nitrate nitrogen is not detected, except in autumn, when its presence may be explained by leaching from the upper horizons by atmospheric precipitation.

Thus, our observations showed that, even with a low concentration of organic matter in unsettled wastewater and at the minimum loading rates recommended for loamy soils (25–30 m³/ha per day), the soils of the experimental plot did not provide complete mineralization of organic matter throughout the soil layer down to the groundwater table. As a result, the soil became contaminated, creating a possibility that this contamination would penetrate into the groundwater flow.

For these reasons, the land at the experimental irrigation-field site must be considered unsuitable for the treatment of unsettled wastewater.

Accordingly, in 1951 we studied the self-purification capacity of the soil when irrigated with wastewater previously clarified for 15–60 minutes in a specially constructed experimental horizontal settling tank.

The treatment of settled wastewater in the soil of the experimental site was studied on three specially designated plots by chemical and bacteriological analysis of the soil. Analyses were also made of the wastewater before and after settling, as well as of groundwater in the vicinity of the experimental irrigation fields.

The three experimental plots were irrigated with settled wastewater at an application rate of 5,400 m³/ha, or 30 m³/ha per day.

According to S. N. Stroganov’s classification, the wastewater entering the settling tank could be characterized as weak wastewater.

Settling reduced the wastewater’s oxidizability by 23.1%, its BOD₅ by 9.9%, and its suspended-solids content by 48.2%.

Irrigation of the experimental plots with clarified wastewater caused heavy bacterial contamination throughout the entire soil profile, down to the groundwater flow, similar to the soil contamination we had previously observed during irrigation with unsettled wastewater.

Thus, during six months of observation in summer and autumn, the coliform titer of the soil in the upper horizon ranged from 0.01 to 0.000001 g. In the lower horizon, adjacent to the groundwater flow, it likewise reached low values, ranging from 0.1 to 0.000001 g, and increased slightly in both horizons in autumn.

The titer of intestinal anaerobic bacteria ranged from 0.01 to 0.00001 g in the upper horizon and from 0.1 to 0.0001 g in the lower horizon; it too increased slightly in autumn.

An increase in chloride content was also observed in the soil of both horizons, to levels ranging from 87.14 to 127.87 mg/kg.

The mineralization of organic matter in the soil proceeded similarly to that observed when the experimental site was irrigated with unsettled wastewater. Ammonification also proceeded intensively throughout the soil layer down to the groundwater flow, as evidenced by the presence of substantial quantities of ammonia nitrogen—12.46 to 40.17 mg/kg—and ammonifying bacteria, from 1.1 to 374 million.

Studies of the groundwater flow showed that well water in the vicinity of the irrigation fields was contaminated. This contamination was characterized by a low coliform titer (0.1–0.0001) and high oxidizability (5.8 mg/l of oxygen). The presence of 0.1 mg/l of ammonia nitrogen, together with 0.11 mg/l of nitrite nitrogen and 3.08 mg/l of nitrate nitrogen, indicates that mineralization is proceeding rapidly in the groundwater.

Thus, our observations show that, when irrigated with weak wastewater, whether clarified or unsettled, and at minimum loading rates of 25–30 m³/ha per day, the soil of the experimental irrigation-field site cannot adequately treat the wastewater and allows contamination to pass into the groundwater flow. Consequently, the soils of the experimental site and adjacent lands, where the groundwater table is high (0.5–1.5 m), cannot be used for irrigation fields. Another tract of soil that meets the sanitary requirements for irrigation fields must be selected for this purpose.