10.3.2 Long-term evaluation of health-based chemical performance
The long-term evaluation of health-based chemical performance is a system-wide assessment that includes evaluation, against guideline values, of chemical monitoring data for the distribution system, including the point of supply to consumers over a period, supplemented by all available operational, investigation and validation monitoring data from catchment to consumer.
The objective of the long-term evaluation is to understand system performance and confirm the robustness of the system to deliver drinking water reliably with concentrations of chemicals below the recommended maximum concentrations. The long-term evaluation of chemical performance should inform any short- and long-term actions required to improve the management of risk.
Review of health-based chemical monitoring within the distribution system
In assessing and reporting long-term chemical performance against health-based guideline values, the number of monitoring results available for the reporting period needs to be considered.
In most cases, monitoring of chemical characteristics is monthly, although in some cases it may only be quarterly. Therefore, there may only be between 4 and 12 data points available to assess system performance.
Additionally, there is strong evidence that water quality data is not normally distributed, therefore it is not appropriate to use statistics that are based on the assumption of normality. More information on the statistical issues can be found in Information Sheet 3.3 - Statistics – Statistical Principles.
In assessing and reporting long-term chemical performance against health-based guideline values, monitoring results for each chemical characteristic should be assessed and reported per water quality monitoring zone, listing the number of samples collected per zone, the minimum, maximum and mean result, and the number of results above the health-based guideline value.
Given that generally few data points will be available for evaluation, performance against the guideline value is determined based on the maximum result.
If a longer data set is being evaluated, or there are sufficient sample results from the 12 month review period to support a statistically valid evaluation of the results, then the 95ᵗʰ percentile statistic should also be listed and used to determine performance against the guideline value. Information Sheet 3.5 – Number of Samples Required provides advice on the number of samples required for a statistically valid evaluation. Advice from a statistician could also be sought.
The reporting statistics should not include results derived from repeat samples, or from emergency or investigative samples undertaken in response to an elevated result.
Monitoring data that are to be averaged or otherwise numerically adjusted should not be rounded prior to averaging or other manipulations¹. Comparing monitoring data with a guideline value is described in Table 10.3, Section 10.2.2.
Any investigations and corrective actions arising from individual results that exceed a health-based guideline value should be incorporated into the long-term review and reported, to ensure that the cause of the exceedances is understood and appropriate actions have been taken or planned to prevent a recurrence. Where these investigations indicate that the system lacks robustness to provide confidence that exceedances will not recur, actions to improve system performance should be evaluated.
If a health-based guideline value is exceeded or regularly close to being exceeded, this may indicate a system deficiency, and the drinking water supplier needs to determine what is required to meet the guideline values and agreed levels of service consistently. Operational corrective actions to reduce the risk of health-based chemical exceedance should be implemented as quickly as practicable. Where corrective actions include major capital works, an acceptable schedule should be agreed in consultation with the relevant health authority or drinking water regulator and/or other stakeholders, based on the risk to public health.
Review of operational monitoring – catchment to consumer
In Australian drinking water systems, many of the chemicals listed in these Guidelines are either not present or present at concentrations that are stable and sufficiently low that the risk of exceeding the health-based guideline value is very low. In these cases, the long-term evaluation of chemical performance consists primarily of confirming and reporting concentrations against the relevant guideline value.
There are likely to be a small number of chemicals that are consistently close to the guideline value, that exhibit high variability, or for which a control, such as treatment, has been implemented to manage the hazard. For these chemical characteristics, the long-term evaluation should incorporate review of any associated operational monitoring data throughout the system, to enhance system understanding and confirm the reliability of delivering drinking water that meets the specified requirements.
Depending on the chemical characteristic, relevant operational monitoring could include, for example:
observational catchment monitoring, such as observation of pesticide and fertiliser application;
events such as bushfires, which may increase organic load in source water, and subsequently, trihalomethanes (THMs) in the distribution system;
operational characteristics related to the performance of any treatment process used to remove a specific chemical (e.g. nitrate or arsenic removal); or
operational characteristics relevant to the management of microbial quality that may, indirectly, have an impact on chemical quality (e.g. increasing chlorine dose to deal with higher source water turbidity resulting in elevated THMs).
For any chemical characteristics where there is risk of exceeding specified guideline values, it may be useful to establish target criteria or an operational limit at some value below the guideline value to provide warning and trigger prompt investigation before a health-based chemical exceedance occurs. Box 10.6 provides an example illustrating how these principles may be used to manage THMs concentrations in the distribution system.
The long-term evaluation of chemical performance, with inputs from drinking water quality monitoring data and any associated operational data, should increase understanding of overall system performance and provide input to any short- and long-term improvements to improve the management of any risk of exceeding health-based guideline values.
Box 10.6 Assessing system performance for management of Trihalomethanes (THMs)
Assessing system performance for management of Trihalomethanes (THMs)
Setting target criteria
The health-based guideline value for trihalomethanes (THMs) at the point of supply to consumers is 0.250 mg/L. To ensure that the guideline value is not exceeded, an operational target for THM concentration in the distribution system can be set below this level, for example at 0.175 mg/L. If a drinking water quality result greater than 0.175 mg/L is obtained, then this can trigger a more detailed evaluation of the sample result and possible corrective action.
Short-term evaluation and response
If, for example, a drinking water quality monitoring result of 0.200 mg/L for THMs is obtained, the operational target is exceeded, and this will trigger a response. The first action may be to increase sampling frequency for a period to obtain a more comprehensive understanding of THM concentrations in the distribution system. If results are consistently above the target criterion, then further investigation is warranted to determine the THM formation potential of the source and treated water, and the relationship between chlorine dose and THM formation.
Operators can then set a maximum chlorine dose rate to avoid THM exceedances, as long as the dose rate does not compromise effective disinfection. This chlorine dose rate is then the operating target for disinfection. If the source water quality is highly variable, however, it may be necessary also to establish a surrogate for the organic content of the source water (e.g. UV absorbance), with target criteria and routine monitoring of this characteristic in the source water.
Long-term evaluation of THM performance
The objective of the long-term health-based chemical evaluation is to determine whether the water supply system can consistently deliver water that meets specified requirements. As one aspect of this long-term review, the long-term evaluation of THM monitoring data used in this example would involve reviewing:
drinking water quality monitoring for guideline and target criterion exceedances;
THM formation potential of source water and the likelihood of this changing in the future;
whether the recommended maximum chlorine dose was adversely affecting disinfection or chlorine residuals in the distribution system, creating other health risks.
If the long-term evaluation concludes that the risk of THM exceedances is unacceptably high, then improvements should be considered. Considered first are operational options such as source water blending, lowering tank levels to reduce water age, or not using the source water when risk of THM formation is at its greatest. Alternatively, capital solutions may be required such as tank aeration or treatment to remove organics.
¹ Water suppliers may wish to retain monitoring data in its original state in order to permit future calculations.
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