ANÁLISIS DE LA INFORMACIÓN RECOLECTADA EN RELACIÓN CON LOS

A full description of the field sampling techniques is given in Standard Methods for Examination of Waters and Wastewaters (APHA, 1992). As described in Standard Methods, field sampling is subject to many sources of contamination and must be done with care. Because even the slightest contamination can affect sample results, it is recommended that all samples be collected by the laboratory or other professional staff that are trained in bacterial sample collection.

A few of the key points to remember in field sampling are:

sample bottles should be at least 125 ml volume and be resistant to sterilization temperatures;

plastic bottles are preferred to avoid sample breakage; all bottles must be sterilized;

gloves or other protection should always be worn during sampling or handling of samples;

samples are often collected by hand or with a sampling device where the sampling site has difficult access;

sample bottles should always be pointed upstream and after capping should have at least an airspace of 2-5 cm;

in no case should a composite bacteriological sample be collected; sample bottles should be properly labelled and a field record

completed on each sample. Marking of sample should be done with non-smear permanent ink;

bacteriological samples should be stored at a temperature of 1-4°C during transit to the laboratory;

bacteriological samples should be examined as soon as possible after collection but they should not be held longer than six hours between collection and initiation of analysis; and

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 and blank samples. A more detailed

discussion of quality control samples is given in the section Sampling frequency and criteria used to evaluate the data.

During development of a bacteriological sampling programme there are several points that should be considered because they limit the number of samples collected and raise the cost of sample collection:

all bottles need to be prepared under sterile conditions, therefore the number of samples is limited by the laboratory ability to supply bottles and the programme's ability to keep them in a sterile condition until sampling;

each laboratory is limited in the number of samples it can process and examine each day thus limiting the number of samples that can be collected each day by the sampler;

an extensive sampling effort is conducted for only the 4 -6 month irrigation season thus requiring extensive staffing for a short period of time;

because sample results must be read 24-48 hours after processing in the laboratory, some laboratories will not accept samples late in the week which would require readings over weekends or holidays; and

many health regulations require multiple samples be taken at each site within a specified period to be legally based. This minimizes the number of sites that can be sampled during a given time period. For example, if a regulation calls for a monthly mean of a minimum of five samples, the sampling schedule has to be based upon the monthly mean. Figure 6 presents a typical sampling schedule. This schedule is based upon meeting the monthly mean requirement and also the need to have a sampling cycle that represents a typical irrigation cycle. The main constraint in this example schedule is not, however, the

requirement for five samples, rather it is the availability of the laboratory.

FIGURE 6: Typical sampling schedules to accomplish five samples within 30 days

SAMPLING TECHNIQUES CAN AVOID CONTAMINATION

For example, sampling from a bridge may require a sampling device. A sampling pole with the bottle attached is the preferred approach. Often the sample is collected in a bucket and then a sample taken from the bucket. In instances where a bucket is needed, it must be remembered that the bucket represents a serious source of contamination if it has not been thoroughly cleaned after each sampling site. A thorough cleaning would mean use of a disinfectant such as soap and rinsing at least 3 times with the water to be sampled prior to its use for sampling.

a) Laboratory Available 5 Days Per Week

b) Laboratory Available Only 4 Days Per Week

It is not recommended to carry more than three irrigated zones during any sampling cycle as samplers loose track of established sites.

The constraints listed above mean that sample collectors are often utilized for only partial days and often not on Fridays. As a result, the total cost per sample

collected is extremely high unless these people are trained for other work. These constraints may also limit the ability to sample extensively over large areas or far from established laboratories.

The constraint of not having access to laboratories or attempting to sample in remote areas will seriously hamper the widespread appraisal of safe production areas. This constraint could be overcome if the membrane filter (MF) technique could be adopted for use on irrigation water. This would enable the samplers to use portable equipment which would allow greater flexibility in scheduling and locating sites.

If a programme to develop safe production areas is being proposed, it is recommended that the Agricultural Ministry, through its research branch,

cooperate jointly with the Health Ministry and the university to evaluate the use of the MF technique for irrigation water and whether portable equipment could be developed for use in that country. The use of the MF technique could greatly expand the ability of the Agricultural and Health Ministries to promote safe production areas and eliminate production areas that have a significant health threat. The analytical cost per sample may be more for the MF technique but this would be offset by reduced staff time for sample collection and transport.

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