REPRESA ESTATICA PICO DE PATO

Representative sampling in the distribution system is an indication of system water quality.

Results of sampling should show if there are quality changes in the entire system or parts of it, and they may point to the source of a problem (such as tastes and/or odors). Sampling points should be selected, in part, to trace the course from finished-water source (at the well or plant) through the transmission mains, and then through the major and minor piping of the system. A sampling point on a major transmission main, or on an active main directly connected to it, would be representative of the plant effluent water quality.

FIGURE 2-8 Suggested in-plant sample points (indicated by numbered circles)

8

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Sample points in the distribution system are used to determine the quality of water delivered to consumers. In some cases, the distribution system samples may be of signifi-cantly different quality than samples of finished water at the point of entry to the distribu-tion system. For example, corrosion in distribudistribu-tion system pipelines can cause increases in water color, turbidity, taste and odor, and physical constituents such as lead and copper.

Microbiological growth may also be taking place in the water mains, which degrades water quality. In addition, a cross-connection between the distribution system and a source of contamination can result in chemical or microbiological contamination of the water in the system.

Most of the samples collected from the distribution system will be used to test for coliform bacteria and chlorine residual. Others may be used to determine water quality changes. Still others will be used to test for maximum contaminant levels (MCLs) of inor-ganic and orinor-ganic contaminants and for compliance with the Lead and Copper Rule, as required by the applicable drinking water standards. Distribution system sampling should always be performed at locations representative of conditions within the system. Radio-logical samples must be from the source water supply under current regulation.

The two major considerations in determining the number and location of sampling points, other than those required by regulation, are that they should be

• representative of each different source of water entering the system

• representative of conditions within the system, such as dead ends, loops, storage facil-ities, and pressure zones.

The precise location of sampling points depends on the configuration of the distribu-tion system. The following examples provide some general guidance for sample point selection.

Example 1. Figure 2-9 provides an example of how sample points may be selected for a small surface water distribution system serving a population of 4,000. This is a typical small branch system having one main water line and several branch or dead-end water lines. For this system, a single point, A, is sufficient for turbidity monitoring. This point is representative of all treated water entering the distribution system.

For a community of 4,000, the NPDWRs require a minimum of four bacteriological samples per month to be taken at four different points in the system. Point B represents water in the main line, and point C represents water quality in the main-line dead end.

Points D and E were selected to produce samples representative of a branch line and a branch-line dead end, respectively.

Consideration of how often and at what times these points are sampled is also neces-sary to ensure that the samples accurately represent conditions in the distribution system.

Although the minimum requirement of four samples per month could be met by collecting samples from all points on one day, this sampling frequency would not produce samples that represented bacteriological conditions within the system throughout the month. A

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better program would be to sample points B and E at the beginning of the month and points C and D at mid-month. Sampling should be representative both in location and in time.

Although this type of program is adequate to meet the minimum monitoring require-ments of the NPDWRs, good operating practices would include periodic sampling at each dead end and several additional sampling points within the distribution system, with sam-ples taken each week. The exact number and location of these operational sampling points depend on the characteristics of the specific system and on state requirements.

Chlorine residual samples should be taken from each sample point when bacteriolog-ical samples are collected and should be analyzed within 15 minutes of sampling, prefera-bly at the sample location. Sampling for routine water chemistry, along with the required sampling for inorganic and organic chemicals, also can be conducted at one of the coli-form sampling points.

Sampling for a similar system using a groundwater source would be the same, except that turbidity sampling generally is not required and samples for organic chemical analy-sis must be collected at each well.

Example 2. Figure 2-10 illustrates a typical small-loop distribution system having one main loop and several branch loops, serving a population of 4,000. One turbidity sample point, A, is sufficient because that point is representative of all treated water entering the distribution system.

For bacteriological sampling, two sampling points, B and C, are adequate. Point B is representative of water in the main-line loop, and point C is representative of water in one of the branch-line loops. To produce the required minimum of four samples per month, points B and C can be sampled on alternate weeks, or additional similar sampling points can be selected. However, good operating practice would include two to three times this number of samples, depending on the characteristics of the particular system. As with the FIGURE 2-9 Sampling points (indicated by x) in a typical small-branch distribution system

Main Water Line

Branch Water Lines Population Served = 4,000 Treatment Plant

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system in the previous example, chlorine residual samples should be taken whenever bac-teriological sampling is performed.

Example 3. Figure 2-11 illustrates a system serving a population of 17,440 that obtains water from both a creek and a well. The distribution system has the features of both the branch and the loop systems shown in Figures 2-9 and 2-10.

To determine sample-point locations, the following four questions should be considered:

1. What tests must be run?

2. From what locations will the samples be collected?

3. How often must the samples be taken?

4. How many sampling points will be needed?

The answers to the first and third questions—what tests must be run and how often—may vary from state to state, and they are also likely to change periodically in response to changes in federal requirements.

Additional samples may also be required for the system’s own quality control (QC) program. Examples include taste and odor, color, pH, TDS, iron, manganese, and het-erotrophic plate count.

Once the tests and test frequencies have been determined, the number and specific locations of sampling points must be selected. The NPDWRs require a turbidity sample to be taken at each point representative of the filtered surface water that enters the distri-bution system. Because waters from parallel treatment plants enter two separate clearwells in Figure 2-11, two turbidity sampling points are required (points 11 and 12). The well will not have to be sampled for turbidity, but periodic sampling directly from the well for chemical quality analysis will be required as directed by the state.

In the selection of sample points that will be representative for coliform analysis, a variety of factors must be considered:

FIGURE 2-10 Sampling points (indicated by x) in a typical small-loop distribution system

B

A

C

Treatment Plant

Branch Loop Main Loop

Population Served = 4,000

Creek

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• Uniform distribution of the sample points throughout the system;

• Location of sample points in both loops and branches;

• Adequate representation of sample points within each pressure zone;

• Location of points so that water coming from storage tanks can be sampled;

• For systems with more than one water source, location of sample points in relative proportion to the number of people served by each source.

On the basis of these fundamental considerations, bacteriological sample points can be selected. A treatment plant serving a community with a population of 17,440 must test 20 coliform bacteria samples per month, according to the NPDWRs. After a careful review of the configuration of the distribution system layout, 10 coliform bacteria sample sites were selected. The reasons for the selection of each point shown in Figure 2-11 are as follows for bacteriological purposes only:

• Point 1 is on the main loop in the high-pressure zone; it should produce representative samples for that part of the system.

• Point 2 is on the branch loop in the high-pressure zone, representative of storage flow to the system.

FIGURE 2-11 Sampling points (indicated by solid dots) in a medium-size system with surface and groundwater sources

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• Point 3 is on a dead end. Some authorities advise against dead-end sampling points because they do not produce representative samples. However, consumers do take water from branch-line dead ends. In the example, there are seven branch-line dead ends that no doubt serve significant numbers of consumers. It is representative to have one or two sample points on these branch lines at or near the end. If there are indica-tions of chlorine residual decline or bacteriological problems in water sampled at branch-line dead ends, hydrants and blowoff valves should be flushed and branch lines resampled immediately to determine if the problem has been corrected. If the problem persists, additional investigation is needed to locate the condition contribut-ing to the problem.

• Point 4 is located on the main loop of the low-pressure zone and represents water from treatment plant 2, the well water source, the storage tanks, or any combination of these (depending on system demand at sampling time).

• Point 5 allows for sampling of water flowing into the system from storage.

• Points 6 through 9 were selected by uniformly distributing points in the low-pressure zone, the zone that serves the major part of the community.

• Point 10 was selected as representative of a branch-line dead end in the high-pressure zone, just as point 3 was selected in the low-pressure zone.

• Points 11 and 12, as stated previously, are used as turbidity monitoring points.

• Point 13 was added to monitor a dead-end branch that is fairly isolated from other sampling points yet serves a large population.