Influence of point-of-use dispensers on lead level assessment in drinking water of a lead pipe-free campus

Lead levels in a lead pipe-free campus

Figure 1 (number of dispenser and faucet samples not stacked but overlapped) shows the distribution of Pb levels in the 558 water samples collected from POU dispensers (n = 204) and faucets (n = 354) during the survey. The distribution of total number of samples collected using different protocols is shown in Supplementary Table 1. Table 1 shows the water quality parameters of the sample water. Among the total 558 samples collected, regardless of sampling protocols used, 89 samples (16%) had Pb levels greater than the Taiwan EPA standard value (or the WHO guideline value) of 10 μg/L.

Samples with Pb levels above 10 μg/L are considered “unsafe” in this study. Since the number of samples collected from dispensers and faucets varied, a percentage was used to represent the proportion of unsafe samples. In this regard, 66 out of 354 (19%) samples from faucets and 23 out of 204 (11%) samples from dispensers were not safe for consumption. Hence, faucet samples were approximately twice as likely to be contaminated as dispenser samples. As expected, the use of POU dispensers could effectively reduce Pb levels, but not always below the regulatory standard of 10 μg/L. Possible reasons include inadequate removal efficiency of dispenser filters and Pb-containing components in the filter system. The extent of Pb reduction (or unlikely addition) through a dispenser was, however, not determined in this study. Although POU dispensers have become necessary in delivering safe drinking water, the occurrence of unsafe samples from such dispensers showed that water from dispensers does not always meet the regulatory standard. The results also indicated that Pb contamination issues could be prevalent even if no aged Pb pipes were present. For faucet samples, the Pb sources are most likely Pb-containing plumbing materials such as brass fittings and Pb solders^5,6. Although regulations of Pb in plumbing materials have evolved with time, legacy plumbing materials may still be present in the buildings. Harvey et al.⁵ collected water samples from kitchen tap fittings in Australia and demonstrated that Pb-containing fittings could significantly contribute to Pb in drinking water. Similarly, Ng and Lin⁶ concluded that brass fittings were the main source of Pb in drinking water in a simulated copper pipe premise plumbing.

All buildings except Building VIII (Supplementary Table 1) had at least one sample from the faucet and dispenser exceeding 10 μg/L Pb. Building VIII is the only building without any unsafe samples from the dispensers. Buildings VII had the highest percentage of samples that were unsafe (24%), followed by buildings VI (23%), IV (17%), and III (16%) (Supplementary Table 2). Although the percentage of unsafe samples from faucets was approximately twice that from dispenser samples (Fig. 1), a higher proportion of faucet samples compared to dispenser samples collected in a building did not always correspond with an increase in the proportion of unsafe samples among the buildings (Supplementary Table 2). For example, Building III had more samples collected from faucets (70%) than Building VII (63%). Still, the proportion of unsafe samples in Building III (16% of samples) was less than in Building VII (24% of samples).

Figure 2 shows the median total Pb concentration for dispensers and faucets in the eight buildings surveyed. The median Pb level ranged from 1.3 to 5.7 µg/L and 2.2 to 5.7 µg/L for dispenser and faucet samples, respectively. The median Pb level for dispenser samples was lower than faucet samples in six of the eight buildings. The difference in medians between dispenser (filtered) and faucet (unfiltered) samples were significantly different using t test (p value < 0.05) (Supplementary Table 3). This suggests that filter systems in POU dispensers contribute to reducing Pb levels in drinking water. However, the anomaly observed in the remaining two buildings may be due to higher water usage, as those buildings are mainly catered for routine class activities, causing the filters to reach their treatment capacity prematurely. Figure 2 also shows the sporadic Pb levels in the dispenser and faucet samples among the buildings surveyed, implying that Pb-containing materials may be used in common practice. Similar findings regarding the sporadic Pb release in plumbing systems with no Pb pipes have been reported^51,52. Both studies attributed Pb release to the use of brass fittings, fixtures, and water meters.

The distribution of Pb in the samples was further investigated in Building VII, which had the highest proportion of unsafe samples, using the modified FD method, which involved collecting 10 × 100 mL samples from dispensers and faucets (Fig. 3). Thirty of the 90 sequential samples collected from the dispenser exceeded the regulatory standard. Six samples within the first 100 mL indicated that Pb was released from the dispenser component near the outlet. The rest showed a more uniform distribution (Fig. 3a). In contrast, 43 of the 100 sequential samples collected from the faucets exceeded the standard, with 27 samples within the first three 100 mL, including 10 first 100 mL samples (Fig. 3b). This finding for faucets reveals that the fittings and components near the faucets were the most likely source of Pb in the samples. In many circumstances, sequential sampling can help identify Pb leaching from plumbing materials at various sampling locations^6,28,35 because a Pb source is expected to be present when at least one sequential sample exceeds the regulatory standard.

Impact of POU dispenser on different sampling protocols

Maximum Pb levels measured using FD, RDT, and FL sampling protocols, irrespective of dispenser and faucet samples, were 88.6, 54.1, and 37.8 μg/L, respectively (Supplementary Table 1). This finding on the trends in Pb level among protocols was consistent with the previous studies^23,31,36,53, which have shown that FL sampling exhibited the lowest total Pb compared to other protocols. However, as water is filtered in the dispenser before being discharged, the extent of impact due to dispensers on the overall Pb assessment may vary depending on the types of sampling protocol employed to collect the samples. Hence, to elucidate the impact of POU dispensers on the survey results of three sampling protocols, a comparison between dispenser and faucet samples exceeding the Taiwan EPA standard for Pb among the protocols is shown in Fig. 4. In FD and RDT samples, respective percentages exceeding the standard were the highest for faucet samples considered alone (26%, 14%), followed by a combination of faucets and dispensers (23%, 12%), and the lowest for dispenser samples considered alone (14%, 9%). These findings indicate that POU dispensers can differentially affect survey outcomes. In contrast, exceedance in FL samples was higher in dispensers than faucets. This result contradicts the findings of previous sampling surveys^23,31,53, which primarily collected samples from faucets.

Furthermore, the cumulative Pb levels in dispenser and faucet samples collected using three sampling protocols are shown in Fig. 5, to illustrate the lead levels at each quartile and 90th percentile, which would reflect the extent and severity of contamination. In addition, a comprehensive cumulative graph is provided in Supplementary Figure 2, which shows the maximum Pb levels measured using each protocol for dispenser and faucet samples. Such graphs can provide crucial information regarding the type of remediation measures based on the extent of the contamination. FL sampling yielded lower Pb levels in faucet samples (Fig. 5), for each percentile, consistent with results in the literature^{31,34,36,37,54}. Lower Pb levels in FL samples for faucets might be due to higher flow rates (compared to dispenser samples) involved during flushing. Several previous studies^35,36,55,56 have demonstrated that different flow rates during pre-flushing may produce different Pb levels in drinking water. Flushing is generally known to reduce the exposure to Pb at the consumer end and its efficiency is governed by variables including premise plumbing configuration, water use, duration, and extent of flushing prior to sampling, which is usually difficult to control^35,56. FL samples for faucets had a 90th percentile value of 8.8 μg/L, which was much lower than those for FD (21.5 μg/L) and RDT (12.3 μg/L) samples. The difference between the highest and lowest 90th percentile Pb levels yielded respectively by FD and FL sampling was 12.7 μg/L indicating that FL sampling should not be used for faucets as it tends to produce lower Pb levels than other sampling methods unless a more stringent guideline for FL sampling can be proposed and implemented. The guideline may have to consider the effects of flushing time and flushing flow rate on the Pb assessment.

Interestingly, unlike in faucet samples, all three sampling protocols produced similar Pb levels in dispenser samples, as evident by 90th percentile values of 10.7, 10.4, 8.2 μg/L for FD, FL, and RDT sampling, respectively. The difference between the highest and lowest 90th percentile Pb levels yielded by FD and RDT sampling was only 2.5 μg/L, compared with 12.7 μg/L in faucet samples. This phenomenon is likely due to storage tanks in dispensers which provided a buffering effect against changes in water quality and sampling methods. The buffering effect was illustrated by the distribution of the unsafe samples collected using a follow-up sequential FD from the building with the highest contamination. A more even distribution was observed in samples collected from dispensers than in faucets (Fig. 3), where contamination tends to occur near the faucet. There are no reported studies that considered collecting water samples only from the POU dispensers, and hence, the effects of dispensers on sampling surveys have not been documented yet. As water from the faucet is generally not consumed directly, water samples collected from dispensers are more representative of drinking water quality in schools in Taiwan. Although some previous studies suggest that the choice and guideline for a particular sampling protocol varies among countries^23,32,57, which depend on factors such as the minimum number and volume of samples collected, and stagnation period⁵⁸, the findings of this study illustrate that any of the three sampling protocols can be recommended for sampling drinking water. The limitation, however, is that only dispenser samples should be considered during sampling.

Unlike several previous studies that have deemed FL sampling undesirable, this study demonstrated that FL sampling does not necessarily produce lower Pb levels, especially samples from dispensers. Hence, FL sampling being recommended in Taiwan can reflect Pb levels in drinking water only if the water samples are collected from dispensers. Since the public generally regards filtered water as their drinking water, sampling only from dispensers can provide a more accurate Pb assessment for human exposure risk.

Effects of water usage

Water usage varies among the seasons and during weekdays which may result in different Pb levels in the water samples. The seasonal variation in Pb levels in drinking water obtained using three sampling protocols and differences between weekday and weekend samples are illustrated in Fig. 6. The proportion of unsafe samples was higher in summer (June–August) than in winter (December–February) for FL and FD sampling, whereas RDT showed otherwise. The maximum Pb level recorded in summer (49.0 µg/L) was nearly twice as high as the Pb level in winter (26.0 µg/L) (Data not shown). Source of water, temperature, configuration of premise plumbing, and water chemistry are among some factors causing variations in Pb level in drinking water^58,59. The effects of these factors were not individually addressed in this study. Nevertheless, the findings regarding higher Pb levels in summer samples were consistent with the previous studies^1,60,61. The higher temperature in summer is expected to increase the dissolution rate of the Pb-containing scale and Pb release from plumbing materials³⁴.

Similarly, all three sampling protocols showed higher proportions of unsafe samples during weekdays than weekends. The extent of difference was the most pronounced in FD sampling (29% on weekdays compared to 4% on weekend samples). This finding contradicts previous literature reporting an increasing Pb contamination during the weekends, possibly due to lower water usage and longer stagnation of water^1,62,63,64. Prior studies are based on premises with Pb service lines where the stagnation can affect Pb scales and their dissolution in the supply water. However, this study was conducted on a Pb pipe-free campus. The effects of stagnation on Pb levels in distribution systems with Pb-containing plumbing materials may be less significant than a system containing Pb pipes. In addition, site-specific factors such as local water quality and plumbing configuration can also influence Pb levels⁶⁵. However, both factors can be challenging to determine in a large-scale survey. Thus, conducting pilot studies to elucidate the impact of each of these factors on Pb level assessment might be imperative.

Implications for decision-makers

This study uncovers the scenarios of possible Pb contamination in a lead pipe-free campus using a systematic sampling survey method. Findings are expected to provide crucial information for decision-makers (government) in selecting appropriate sampling protocols. The incidence of Pb levels exceeding the Taiwan EPA standard for dispenser samples warrants a comprehensive and large-scale sampling survey for Pb level assessment studies in drinking water supplies. Sampling guidelines should also include important factors, such as the number of samples required, the frequency of sampling, the location of sample collection, and proposed corrective measures. Information on such factors can substantially contribute to updating the existing local EPA guideline³⁸, which provides information on the sampling protocols allowed for collecting drinking water samples. Very low Pb concentrations below 10 µg/L have also been shown to be harmful to human health^13,14,15. Thus, the existing standard of 10 µg/L for Pb in drinking water can be recommended to be gradually reduced to below 10 µg/L (such as 5 µg/L) for drinking water samples, especially samples collected from dispensers.

The harmful consequences of Pb contamination in drinking water have been well reported^8,9,10,11. This study shows that even Pb pipe-free plumbing could not eliminate exposure risks. Hence, sampling surveys should be performed on a regular basis as part of a monitoring program, especially in places such as nurseries, elementary schools, and hospitals catered for vulnerable groups.

Discussion of the key findings

This study, for the first time, investigated the influence of POU dispensers on the assessment of Pb levels in the drinking water of a lead pipe-free campus. Some of the important findings of this study are:

• Owing to the extensive use of POU dispensers for drinking water in Taiwan, the study concludes that such dispensers can effectively reduce the Pb levels in water but do not always guarantee meeting the regulatory standard and, hence, monitoring the filters used in the dispensers and their periodic maintenance are recommended.

• Pb concentration in water samples varied among three sampling protocols employed for the faucets, whereas all the protocols produced similar Pb levels for dispenser samples. Hence any of the three protocols can be used for collecting water samples from dispensers.

• The FL sampling method, incorporating at least 20 s of flushing, does not necessarily demonstrate lower Pb concentrations than other sampling methods. In particular, this protocol can be as effective as other sampling protocols when collecting water samples from dispensers.

• Considering water consumption habits, sampling only from dispensers is recommended while assessing Pb levels in drinking water in public utilities.

Source: Resources - nature.com