Random daytime sampling

NOTE A sample taken by the consumer cannot be considered a true random daytime sample, as the consumer is likely to take a sample with extreme stagnation time (either a first draw sample or a fully flushed sample). 

Random daytime sampling (RDT) Figure 3.9 shows the results of the random daytime samples in all properties, compared to the composite proportional sample. Figure 3.9 indicates that the relationship between RDT and COMP showed considerable variation. In general, RDT seems to overestimate COMP (slope >1), though for some individual properties RDT underestimates COMP. As a result of the variations the correlation coefficient, R, is 0.61. 

Random daytime sampling (RDT) is defined as a sample taken at the consumers tap at a random, unannoimced, time during office hours. At a composite COMP lead level of 10 ug/l, the 90% prediction range of RDT is 9 8 ug/l. Nevertheless, in the European study, RDT enables detection of 83 % of properties where the lead concentration of the proportional sample exceeds 10 ug/l (problem properties under the Drinking Water Directive). The number of falsely detected properties amounts to 10%. The reproducibility of RDT sampling is poor (the median relative range is 0.6 at lead levels around 10 ug/l). In terms of costs, practicality and consumer acceptance, RDT is the most favourable protocol. 

Zonal assessment monitoring is based on random daytime sampling. 

Guidelines for benchmarking problems with lead in drinking water have been prepared by COST Action 637 in liaison with the EC Joint Research Centre and the World Health Organization (Hoekstra et ai, 2009). These guidelines are based on risk assessment to identify a representative number of water supply areas that are then surveyed by random daytime sampling to determine priorities for corrective action (see Chapters 3 and 6). 

In Europe, there are proposals to normalise compliance monitoring by the use of random daytime sampling. 

The definition of inventory monitoring is the monitoring to estimate the lead problem in a water supply zone or country. To support the implementation of the Protocol on Water and Health, the guidelines for the sampling and monitoring of lead in drinking water (Hoekstra et al., 2009) recommend surveys based on random daytime sampling with the sample numbers in each water supply system surveyed as shown in Table 3.3. 

Ortho-phosphate when dosed for corrosion control (i) at the water treatment plant to check process control, and (ii) through the distribution network to check variation caused by mixing of source waters and to check water stability (phosphate attenuation should not normally exceed 10%) - random daytime sampling from consumers taps will provide the most representative check of variation across the water supply system. 

The definition of a random daytime sample is a sample that is taken at a random time of a working day directly from the tap in a property without previous flushing. When the sample is taken the tap should be fully opened or as open as possible without losing sample. The stagnation of water in the domestic distribution system influences the concentration of lead in a random manner. It is common practice to select the sampling points at random and for 1 litre sample volumes to be taken. 

Random daytime sampling, of metals at consumers taps, has been a regulatory requirement in the UK since 1989 and the Netherlands since 2004. It is common practice in the UK for such sampling to be undertaken by trained water company personnel, a reflection of stringent regulations that apply. 

Variation in results front random daytime sampling... 

The problem of reproducibility in random daytime sampling is illustrated by Table 3.5. Before orthophosphate dosing commenced, the zonal failure rates varied fairly substantially, year by year. However, when compared to the UK Government s benchmark for optimising corrective action, that is 5% or more RDT samples exceeding 10 pg/1, the variation in results has no consequence as each year clearly exceeded the benchmark. 

As an alternative to random daytime sampling, but preferably as a supplementary monitoring method, stagnation sampling at selected properties can provide useful information on the effectiveness of dosing ortho-phosphate for reducing plumbosolvency. This approach is not relevant if total lead pipe replacement is undertaken as the sole corrective measure. 

The consequence of these observed lags is that zonal compliance monitoring by random daytime sampling needs to extend to several years to be able to demonstrate a particular ortho-phosphate dose, making dose optimisation very difficult by zonal compliance monitoring alone. 

Taking repeat random daytime samples, to attempt to conhrm the hrst sample result, is not recommended, because lead emissions from taps vary too much. 

At the zonal level, risk from lead in drinking water can be defined by compliance with the WHO guideline value of 10 p,g on the basis of random daytime sampling. 

The success of this treatment is demonstrated by the 2007 and 2008 summary statistics for England and Wales (Table 8.2), based on random daytime sampling. 

After the sampling period, the ortho-phosphate dose can be adjusted (up or down) depending on the optimisation criteria that have been set. One of the UK s optimisation criteria was that no more than 2% of random daytime samples from consumers taps should exceed a lead concentration of 10 pg/1. 

Inventory monitoring by random daytime sampling (Section 3.2). 

The ortho-phosphate doses needed to achieve the UK criterion, that no more than 2% of random daytime samples exceed 10 pg/1, varied from 0.5 to 2.0 mg/1 (P) in the great majority of cases. Exceptionally, with poorly treated, highly coloured, low alkalinity waters, a dose of 2.8 mg/1 (P) was found to be necessary. Table 8.3 illustrates the dose requirements for low and high alkalinity waters, based on extensive laboratory plumbosolvency testing. Such doses of ortho-phosphate are considered to be entirely safe, given that ortho-phosphate concentrations are many thousands of times higher in many carbonated soft drinks. 

The success of the integrated approach to plumbosolvency control in Wales is illustrated (Figure 8.3) by the random daytime sampling results before and after the introduction of ortho-phosphate to the 29 water supply systems subject to regulatory control. 

From 2004 to 2008,1280 random daytime samples were taken from houses where the lead connection pipe had been replaced ... 

Random daytime sampling from across Brussels gave the following results ... 

From 1999 to 2002, 17.5% of random daytime samples had lead >10 pg/1 and 7.9% >25 pg/1. Laboratory plumbosolvency testing gave an average lead concentration after 30 minutes contact of around 50 pg/1 Zonal modelling indicated that an average ortho-phosphate dose in the range 0.6 to 0.9 mg/1 (P) should achieve >98% compliance with the fiiture lead standard of 10 pg/1. 

Cardew, P. T. 2003. A Method for Assessing the Effect of Water Quality Changes on Plumbosolvency Using Random Daytime Sampling. Water Research 37 2821-2832. 

On the basis that the model was adequately calibrated, it was possible to predict the results of random daytime sampling for the water supply system and to predict the percentage of samples that exceeded the WHO guideline value of 10 pg/l. The percentage of failing RDT samples equates to the percentage of population at risk on this basis. 

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