The first step in selecting water quality monitoring sites is to divide the area into logical irrigated zones that are served by a common water source. The only consideration should be hydrology; the size of the area served should not be considered at this time.
The next step is to consider the contamination sources that would influence bacterial quality of the irrigation water. The two most important contamination sources are:
contamination that is external to the irrigated (canal) area or occurs prior to the supply water being diverted into the main supply canal (primary contamination); and
contamination occurring within the irrigation system (secondary contamination).
In establishing monitoring points, the programme needs to consider the external influences (primary contamination) as a first priority. The first monitoring points are set based upon preliminary data that are available to the project which show how extensively discharges to the rivers impact or influence irrigation water quality. Monitoring points are then set to assess the actual influence of these discharges. In instances where data are not available for this preliminary assessment, water quality sampling will be necessary to establish the extent of supply or river water contamination (see discussion section Selection of study areas to determine safe production zones).
Based upon a preliminary assessment of primary contamination, a decision tree needs to be developed to determine if further sampling should be conducted to evaluate the extent of secondary contamination. A recommended decision tree is shown in Figure 7. The overall goal of the decision tree is to limit expenditure of resources and focus on monitoring those areas that the Agricultural Ministry could promote as clean production areas.
The decision tree was developed to conform with the WHO bacterial Guidelines (WHO, 1989). The decision tree in Figure 7 shows how these guidelines were slightly modified to reflect the variability that occurs under field conditions (see discussion in section Bacterial indicator). The decision tree also reflects the need to identify both the lowest and highest risk areas without strict adherence to any established guidelines. This preliminary phase of monitoring is still far removed from the actual irrigation use and is only attempting to define areas with the greatest potential for being used as a safe production area.
The decision tree (Figure 7) is the initial step of an area-wide water quality
evaluation and starts with the water source not the actual point of use. During this initial assessment, it is recommended that 10 000 faecal coliforms per 100 ml (104) be used as a decision point rather than 1000 faecal coliforms per 100 ml (103) as specified in the WHO Guidelines. One of the primary reasons for this
recommendation is that areas with water supplies < 104 have the greatest potential to reduce the contamination to a level that would make this a safe production area in the future. It must be remembered that this is an initial area-wide assessment and may still be several kilometres from the actual point of use and some natural
treatment and coliform die-off may occur.
This approach evaluates a broader area. The selection of the 104 rather than 103 is not based upon the intent to certify these areas or imply that they meet health guidelines; this is used only as a point of decision making for choosing additional sampling sites. The decision tree (Figure 7) was built upon the following three goals:
develops a legal basis for decision making; i.e., it allows Agricultural and Health Ministry to comply with established health guidelines;
limits sampling of heavily contaminated areas where little or no chance of developing safe production areas exists; and
emphasizes further sampling in areas that have a high potential in the present or future to become safe production areas.
Using the decision tree allows a determination of which areas are potentially clean and would be worth spending additional resources to promote safe production. The decision to stop further sampling after samples showed >104 faecal coliforms per 100 ml was based upon experience gained in an irrigation water evaluation in Chile which showed these highly contaminated areas remain highly contaminated throughout the entire irrigation system (Figure 8) (FAO, 1993). These high levels also pose a significant danger to public health and worker safety in the rural areas. Once it has been determined that an area has a source water that is <104 faecal coliforms/100 ml, the extent of secondary contamination or contamination that occurs within the irrigation system should be determined. Secondary
contamination will be an important factor in the quality of water used in vegetable production. The principal reason is that vegetable production in most developing countries occurs on small farms in densely populated rural areas. With a lack of adequate rural sanitation, the canal water supply is frequently used as a disposal point. Because rural villages often congregate near supply canals, the secondary discharges can be significant.
FIGURE 7: Criteria used for taking initial water quality samples for faecal coliform (FC) in irrigation water
FIGURE 8: Extent of faecal contamination in the irrigation water of the
Metropolitan Region of Chile within the irrigation system as compared to the initial level of contamination in the source of water
(Source: FAO, 1993)
Figure 8 shows that in the Metropolitan Region surrounding Santiago, Chile, almost 40 percent of the irrigated area was served by river or groundwater supply that met the WHO Guidelines of less than 1000 faecal coliforms per 100 ml, yet after sampling within the irrigation system less than 10 percent of the irrigated area still remained safe for vegetable production. The reason for the loss of safe areas was the result of secondary discharges into the irrigation supply system after the water was diverted from the river.
Sampling for secondary contamination within an irrigation system can be time- consuming and costly. The approach recommended is to assume that an irrigated zone is the focus for developing a safe production area and not individual channels or the individual fields. To assess secondary contamination, sampling points need to be chosen within the system. The geometric mean can be determined from multiple samples from an individual site or from single samples from multiple sites within the irrigation network. The criterion for selecting how to determine the geometric mean is based on the assumption that the individual channel or group of channels represents similar channels in the irrigation network. The goal is to identify whether secondary contamination is a significant factor in the irrigated zone but without monitoring the quality of water used in each individual field. The choice of sampling points is extremely important. The following factors need to be considered:
select points that represent as closely as possible water being used for irrigation at the farm;
select points below potential sources of secondary contamination (villages, towns, factories); and
select points where, if contamination occurred below this point, it would be insignificant on a national or regional scale.
The final factor listed above implies that a minimum size area be established. This is extremely difficult to determine and needs to be a judgement factor based on cropping patterns, water use, contamination potential as well as previous experience. As a guideline, the minimum area for monitoring secondary
contamination should not exceed 1000 -2000 hectares until sufficient experience is gained in sampling irrigation water quality for secondary contamination.