As discussed earlier, it is recommended to use faecal coliforms as the indicator of faecal contamination of water used for irrigation (see section Bacterial indicator). This conforms with the WHO Guidelines and US Environmental Protection Agency Standards which describe the maximum limits of faecal coliforms for unrestricted irrigation use. The maximum limit established in these standards is 1000 faecal coliforms/100 ml of water.
To determine compliance or to evaluate the monitoring results, it is recommended to use the geometric mean of five samples taken within a 30-90 day period. The decision to take only five samples per site is to minimize cost, but still account for variability under field conditions. Larger numbers of samples should be collected if resources permit. Collection of at least five samples also allows for establishing a legal basis for decision making, since most regulations set this minimum number (see section Bacterial indicator). Established regulations often specify that all samples must be collected in a 30 -day period. These established health
regulations should be complied with. In the absence of established time periods, a 30-90 day period represents a typical vegetable growing cycle. The selection of an alternative time period for completing the minimum number of samples should be based on the period between irrigation applications or a series of irrigation events. In no case should sampling outside the irrigation season be considered as
acceptable evidence of no contamination.
An additional consideration in setting the sampling frequency is the hydrology of the river that supplies irrigation water. Most river flows are cyclic, often
uniform throughout the year. This results in periods of low natural stream flow when wastewater makes up a great portion of the total flow and vice versa (Figure 9).
As discussed earlier, quality control samples should be collected as part of any routine sampling programme. At least 10 percent of the samples collected should be for quality control. Quality control samples should be a mixture of both duplicate samples and blank samples. Duplicate samples should comprise at least 80 percent of the quality control samples. Duplicate samples are samples taken from the same source at the same time. This assumes that the water source is well mixed which may or may not be true under field conditions. True duplicate samples are normally a split sample of a larger sample.
Taking duplicate samples for microbial analysis will never produce identical results. Sampling for a live organism which must live and thrive during the collection and analytical process may cause differences. In reviewing data from duplicate samples, it is important to remember that faecal coliform results are presented as the most probable number (MPN). This means that a range of values could be found. In judging duplicate samples, small differences between samples should not be judged as important as differences in orders of magnitude. For example, if the first sample is 500 faecal coliforms per 100 ml and the duplicate sample is 1000 faecal coliforms per 100 ml, this type of difference can be expected and should not be of concern because both samples are below the WHO
Guideline level. Concern, however, should be expressed if the duplicate sample is >5000 faecal coliforms. In this case, the data for both samples should be
discarded and new samples taken. In field testing irrigation water supplies for faecal coliforms, if any sample exceeds the WHO Guideline and if the duplicate sample shows variability greater than 5-10X (times) the first sample, all the
analyses conducted by that laboratory, on that date, should be considered suspect and new samples taken. Such a magnitude of difference would imply a laboratory error or the samples were contaminated. In instances where the difference
between duplicate samples is from 2 - 5X, strong consideration should be given to additional sampling in the same area to assess the reasons for the differences in duplicate samples.
Approximately 20 percent of the quality control samples should be blank samples. These samples are from a known source of low bacteria water such as a drinking water supply. The sample is then handled in a manner similar to all field samples. The purpose of this sample is to detect any contamination that may occur in any step of the process from sample bottle sterilization to completed analytical work. When blank samples show faecal coliform concentrations greater than 2X the expected value, a review should be conducted for potential sources of
contamination.
The irrigation season for an entire area is well defined even though irrigation frequency may vary. Collecting water quality data and quality control samples within a time period that corresponds to the frequency of an irrigation season or cycle permits evaluation of the data to detect erratic values. Determining erratic values for faecal coliforms is extremely difficult. The faecal coliform procedure attempts to measure live organisms that are growing and reproducing and erratic or unexplained values do occur. Resampling is needed when a value is 10X the geometric mean of the other samples. Because of the long time period needed to obtain laboratory results and analyse the data, resampling is frequently not done within a 30-day period. Being outside the 30-day period specified in public health regulations should not limit the resampling effort. The goal is to define conditions
within the irrigation season and this typically extends beyond a 30-day period.
Ranges should be established to evaluate the extent of contamination. Ranges should be based upon preliminary studies done prior to the project, initial results obtained through project sampling, and the WHO Guidelines (WHO, 1989). In the absence of preliminary information, it is recommended that four ranges be
established to conduct a preliminary evaluation of the area-wide extent of contamination. These ranges can be modified as experience is gained. The first range refers to areas of bacterial contamination that are below the 1000 faecal coliforms/100 ml of water standard. These areas would be unrestricted for irrigation of vegetable crops or other high health-risk crops. The second range is between 1 000 and 10 000 faecal coliforms/100 ml (1-10X the WHO Guideline). These are generally areas found to be affected by local discharges that occur along the entire length of the canal. These areas are potentially safe production areas if the sources of the contamination can be eliminated from the irrigation system. The third range is between 10 000 and 100 000 which shows levels of heavy contamination which will require treatment systems on the primary sources of contamination. The fourth range is values over 100 000 which show extensive heavy contamination with the majority typically being from major discharges of
THE IRRIGATION SEASON AND DISEASE INCIDENCE
Shuval (1993) shows that the peak of the typhoid fever incidence occurs in the Santiago area of Chile following the initiation of large-scale irrigation of salad crops during the dry summer and early autumn months (November through June in the Southern Hemisphere (Figure 10)). The impact of changes in streamflow can be seen even more dramatically during the 1992 outbreak of cholera in Chile. The largest outbreaks occurred in late summer and early autumn when vegetable production was still at a maximum but natural runoff in the streams from the Andes was at a minimum.
Wastewater discharges to the river were relatively constant and therefore made up a very high percentage of the water diverted for irrigation during this period.
Figure 10: Seasonal variations in typhoid fever rates in Chile (Source: Shuval et al ., 1986a; Shuval, 1993)
untreated urban wastewater.
One difficulty in interpreting data from faecal coliform measurements is that they do not differentiate whether the contamination is of human or animal origin. In large streams and rivers that show high faecal coliform counts, the contamination is most likely from human sources and in particular from wastewater discharges from urban areas. A preliminary study on the main surface water supply being diverted for an irrigated area may assume that any faecal coliform detections are from human sources.
The source of the contamination is less clear when the water supply being diverted from the river is below the WHO Guideline of 1000 faecal coliforms per 100 ml and secondary contamination into the irrigation system within rural areas increases the level above the Guideline. This is not an unfamiliar occurrence. In these instances, faecal contamination from rural areas is normally below 10X the WHO Guideline value. These areas have a high potential to lower contamination levels which would allow unrestricted crop irrigation. Determining the contamination source is crucial so that clean-up can be focused on the most important sources.
In many rural situations where human pollution is suspected on the basis of initial faecal coliform test results, the actual pollution may be caused by animal waste discharges (Geldreich, 1976). Establishing the source of pollution can be very important. Use of the faecal coliform/faecal streptococci (FC/FS) ratio is helpful in establishing the source of pollution in rural areas (see section Bacterial indicator for a discussion of faecal streptococci).
Research has shown that the quantities of faecal coliforms and faecal streptococci that are discharged by humans are significantly different from the quantities
discharged by animals. Research suggests that the ratio of the faecal coliform (FC) count to the faecal streptococci (FS) count in a sample can be used to show whether the suspected contamination derives from human or from animal wastes. Typical data on the ratio FC/FS for human and various animals are shown in Table 13. The FC/FS ratio for domestic animals is typically less than 1.0, whereas the ratio for humans is more than 4.0.
A major difficulty occurs if ratios are obtained in the range of 1 to 2; interpretation is uncertain. If the sample was collected near the suspected source of pollution, the most likely interpretation is that the pollution derives equally from human and animal sources. It is difficult to see the ratio when mixed pollution sources are present or if the stream or canal is sampled too far downstream from the pollution source because of differences in die-off rates. To minimize interpretation
problems, researchers suggest two important steps (Geldreich, 1976): samples should not be taken farther downstream than 24 hours of flow time from the suspected source of pollution; and
only the faecal coliform counts obtained at 44°C should be used to compute the ratio.
TABLE 13: Estimated per caput contribution of indicator micro -organisms from human beings and some animals
Source: Geldreich, 1976
Because of the concerns for obtaining a correct ratio and the vast number of sources that can discharge into an irrigation canal or river, the following are recommended guidelines:
the FC/FS ratio should not be used for evaluation of the primary surface water supply diverted from rivers because of the vast network of streams and water sources that can supply an irrigation system;
if the canal flow network is well understood, the FC/FS ratio could be an effective tool to develop information on secondary sources of contamination within an irrigation system;
because there are no data linking disease transmission on high- risk crops with the faecal streptococci (FS) levels, the ratio FC/FS or actual FS levels should not be used to evaluate cropping patterns or cropping restrictions; and
because faecal streptococcus is not a routine analysis conducted for wastewater and the analytical techniques, although well defined, are not widely used outside public health laboratories; all analytical work should be done by specially trained staff or the public health authorities.