Miloslava Prok š ová, Marianna Cíchová and Lívia Tóthová
3.3 SAMPLING PLAN
Sampling plan is one of the most important part of monitoring proposal. Before any sampling programme is devised, it is very important that the objectives of the programme are carefully established since they are the major factors in determining the position of sampling sites, frequency of sampling, duration of sampling, sampling procedures, subsequent treatment of samples, and analytical requirements. The degree of accuracy and precision necessary for the estimation of water quality concentrations sought should also be taken into account. The sampling programme should be designed to be capable of estimating the error in such values as affected by statistical sampling error and errors in analysis.
It is important to take into account all relevant data from previous programmes at the same or similar locations and other information on local conditions. Previous personal experience of similar programmes or situations can also be very valuable when setting up a new programme for the first time or in the cases of crisis situation.
Sampling plan is depend on analytical methods as well. Another plan is built if common methods are used in laboratory or if used on line monitoring system.
If a biohazard sensor is supported by a surrogate parameters monitoring, types of instruments and their locations are determined by parameters to be monitored, detection limits and accuracy of the data needed. Other important issues that have to be considered are: installation and operational requirements Table 3.2 Minimum frequency of sampling and analyses for water intended for human consumption supplied from a distribution network or from a tanker or used in a food-production undertaking (Anon, 1998) (Continued).
Note 1:A supply zone is a geographically defined area within which water intended for human consumption comes from one or more sources and within which water quality may be considered as being approximately uniform.
Note 2:The volumes are calculated as averages taken over a calendar year. A Member State may use the number of inhabitants in a supply zone instead of the volume of water to determine the minimum frequency, assuming a water consumption of 200 l/day/capita.
Note 3:In the event of intermittent short-term supply the monitoring frequency of water distributed by tankers is to be decided by the Member State concerned.
Note 4:For the different parameters in Annex I, a Member State may reduce the number of samples specified in the table if:
(a) The values of the results obtained from samples taken during a period of at least two successive years are constant and significantly better than the limits laid down in Annex I, and
(b) No factor is likely to cause a deterioration of the quality of the water.
The lowest frequency applied must not be less than 50% of the number of samples specified in the table except in the particular case of note 6.
Note 5:As far as possible, the number of samples should be distributed equally in time and location.
Note 6:The frequency is to be decided by the Member State concerned.
(periodicversuscontinuous sampling, data collection, communications, maintenance requirements, etc.), integration of the instruments with existing water quality monitoring systems, number of instruments that may be installed, and so on.
Local site conditions and supply system considerations that should be considered as well. They include easy access to the device site by authorized personnel because all instruments require periodic maintenance.
This factor is directly related to the operations, maintenance and upgrading of the system. At the same time, the site should be secure against access of unauthorized persons. There should be available space for the instruments and auxiliary equipment. There are some other requirements to be considered, for example suitability of candidate device or sample collection method for the sampling site, including the discharge of waste stream, access to electricity power, data transfer and telecommunication equipment, physical security of the site to guard against unauthorized access or tampering. Hydraulic conditions at sampling sites are among the most important factors for the installation of instruments because turbulence in the pipe might affect sample collection or measurement. Good candidates for installing additional sampling instruments may be existing sampling sites for a basic or compliance monitoring.
3.3.1 What should the sampling plan include?
Generally for both approaches, the plan should include the following elements.
• Brief description of the water system that includes source, treatment, storage, distribution system maintenance, pressure zones, number of connections, population, and so on.
• Map of the distribution system with the routine and repeat sampling sites identified, distribution piping locations, entry points, and so on.
• Sample siting plan that includes sample site addresses, the minimum number of samples collected, rotation schedule of sample sites, chlorine residual monitoring, contact person and phone number, sampling procedure or protocol, and so on. In the event that a routine site sample tests positive for some pathogens, the plan should list repeat sites for each routine site and should include a written procedure of what steps the water system will follow to investigate a positive sample.
There are several system-wide and topological factors important for sensor locations selection, including:
• Localities with the highest potential of contamination entry (such as reservoirs, blow off valves, pump stations) due to the lack of physical security and ease insertion of contaminants;
• Probable types of contamination (according the risk analysis or educated guess);
• Contaminant transport time and concentration that also influence where and how many sensors need to be installed. Likely contaminant transport rates in the network (due to flow, dilution and decay), changes in contaminant properties due to bulk water properties, wall effects (pipe material, turbulence, biofilm) and mixing all affect the time required for the contaminant to reach consumers at a certain concentration;
• Instrument accuracy and detection limits;
• Vulnerable populations (such as the elderly, ill or children) at different parts of the network;
• Relative water demand and associated flow characteristics because of the temporal and physical characteristics of the network. Temporal factors include diurnal (e.g. morningvs. noon), daily (e.g.
weekdayvs. weekend) and seasonal (e.g. summervs. winter) variations. Physical factors include pipe length, size, condition, material, accessories, bends, T-s, and so on.
• Frequency of sampling (periodicvs. continuous) and the amount of data collected and analysed.
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Programmes for investigation of causes of contamination should be designed to determine the characterization of polluting discharges of unknown origin. They are generally based on knowledge of the nature or natures of the contaminants, and the coincidence of the periodicity of the appearance of contamination and of sampling. These criteria necessitate that the sampling, in contrast with that carried out for water quality management and quality characterization, should be carried out with a fairly high frequency in relation to the frequency of appearance of contamination. Inventory sampling from a large number of locations is often found to be useful in locating undocumented sources of contaminants.
The knowledge about whole distribution network is needed before preparation of sampling plan.
There are information such as water treatment condition, source of drinking water, water tretment construction. Under the terms these information the important part belong to flow conditions in view of bioterroristic attack.
Flow measurements for water quality purposes
Contamination loads cannot be assessed without flow measurements. This subclause indicates the flow principles that should be taken into account when setting up a sampling programme.
There are five aspects of flow that need to be measured, namely, (a) Flow direction,
(b) Flow velocity, (c) Discharge rate, (d) Flow structure, (e) Cross-sectional area.
Flow direction
The flow direction is self-evident in most inland watercourses, but in navigation canals and drainage channels. The flow direction can vary with time and there is a possibility of reversal of direction and even counter flow situations. Rivers can also show reverse flow in eddies or in other circumstances. Knowledge of the pattern of groundwater flow within an aquifer is of primary importance for assessing the consequences of aquifer contamination and for sites selection for sampling boreholes.
The pattern of water movement in tanks affects the mixing of the contents in treatment processes and the settling of suspended matter should be taken into account to ensure that representative samples are collected.
Flow velocity
Current velocity is important for
• Calculating the discharge rate,
• Calculating the mean velocity or time of travel which, for water quality purposes, is the time required for a given body of water to move through a given distance,
• Assessing the effect of turbulence and the mixing of a water body produced by velocity.
Discharge rate
The discharge rate is the volume of liquid that passes a given point per unit time. Information on the mean and on extreme rates of discharge is essential for the design and operation of water treatment plants.
Flow structure
The structure of the flow can strongly influence the rate of mixing vertically and laterally. Care should be taken to assess whether flow is in one confined channel, in several channels (i.e. braided) and whether or not eddies are present. Ideally, samples should be collected from a single, well-mixed channel; observations of flow structure in multiple channels and eddies, for example, suggest that samples might not be representative.
Cross-sectional area
Sampling cross sections can range from being approximately rectangular to having a deep channel at one edge, from shallow and wide to narrow and deep. These features affect both mixing and erosion, and they can change over time in natural streams and man-made channels.
Sampling sites
The sampling sites shall provide representative characteristics and account for any vertical, horizontal and temporal variations and shall be identified precisely following the general recommendations of ISO 5667-1 and ISO 5667-5, taking into account additional aspects specific to microbiology. The sampling sites should provide adequate coverage of the distribution network and pressure zones. It is also important to select sampling sites that provide the least amount of negative influence on the water sample. Many water systems utilize dedicated sampling stations in the distribution system that are used exclusively for sampling purposes, eliminating many outside influences that may potentially impact water samples.
Sampling points where conditions are unstable should be avoided, and the heterogeneity of the hydraulic system shall be taken into consideration. In studies on the efficacy of disinfection, the sampling point shall be chosen to ensure that the reaction is complete. All the points of a network are not equivalent, as there may be dead ends and sections where the flow is reduced, particularly if the network is fed from two sources. The quality at the outlet of a well-mixed tank is generally the same as in the body of water, but can be quite different from the inlet.
Sampling equipment
The sample equipment should be designed to preserve the composition of the sample from losses due to adsorption and volatilization, or from contamination by foreign substances.
The sample equipment used to collect and store the sample should be chosen after considering, for example, resistance to temperature extremes, resistance to breakage, ease of good sealing and reopening, size, shape, mass, availability, cost, potential for cleaning and re-use, and so on.
In addition, the sample equipment used to collect and store the samples should be selected by taking into account the following predominant criteria:
(a) Minimalization of contamination of the water sample by the material of which the container or its stopper is made, for example, leaching of inorganic constituents from glass (especially soft glass) and organic compounds and metals from plastics and elastomers (plasticized vinyl cap liners, neoprene jackets);
(b) Ability to clean and treat the walls of the containers, to reduce surface contamination by trace constituents such as heavy metals or radionuclides;
(c) Chemical and biological inertness of the material of which the container is made, in order to prevent or minimize reaction between constituents of the sample and the container;
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(d) Sample containers which can also cause errors by adsorption of chemical determinants. Trace metals are particularly liable to this effect, but other determinants (e.g. detergents, pesticides, phosphate) can also be subject to error.
Sampling pipes are generally used in automatic sampling to supply samples to continuous analysers or monitors. During the residence time within the pipe, the sample can be considered as being stored in a container having the composition of the sampling line. Guidelines for the selection of materials for sample containers also, therefore, apply to sampling pipes.
Guidance on sample containers for microbiological examination is detailed in ISO 5667-16 and ISO 19458. Sample containers should be able to withstand the high temperatures that occur during sterilization. During sterilization or sample storage, the materials should not produce or release chemicals that could inhibit microbiological viability, release toxic chemicals or encourage growth. The samples should remain sealed until opened in the laboratory, and should be covered to prevent contamination.
Contamination prevention (Sampling contamination sources)
Contamination prevention during sampling is essential. All possible sources of contamination should be taken into account and the appropriate control applied if necessary.
Potential sources of contamination can include the following:
(a) The residue of earlier samples remaining on sampling containers, funnels, scoops, spatulas and other equipment;
(b) Contamination from the sampling site during sampling;
(c) Residual water in or on ropes, chains or extension handles;
(d) Contamination of funnels from preserved samples;
(e) Contamination of bottle caps or tops by dust or water;
(f) Contamination of the barrel of syringes and through filter medium;
(g) Contamination from hands, fingers, gloves and general handling;
(h) Contamination from internal combustion exhaust;
(i) Inappropriate sampling devices, bottles and filtration devices;
(j) Degraded reagents.