Guidelines for Canadian drinking water quality – Malathion: Management strategies, international considerations

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• Management strategies
• Monitoring
• International considerations

Management strategies

All water utilities should implement a risk management approach, such as the source-to-tap or water safety plan approach, to ensure water safety (CCME, 2004; WHO, 2017b, 2012). These approaches require a system assessment to characterize the source water, describe the treatment barriers that prevent or reduce contamination, identify the conditions that can result in contamination and implement control measures. Operational monitoring is then established and operational/management protocols are instituted (for example, standard operating procedures, corrective actions and incident responses). Compliance monitoring is determined and other protocols to validate the water safety plan are implemented (for example, record keeping, consumer satisfaction). Operator training is also required to ensure the effectiveness of the water safety plan at all times (Smeets et al., 2009).

Monitoring

Malathion can be present in groundwater and surface water in areas where it is being used depending on the type and extent of its application, environmental factors (for example, amount of precipitation, soil type, hydrogeological setting) and environmental fate (for example, mobility, leaching potential, degradation) in the surrounding area. Water utilities should consider the potential for malathion to enter source water (for example, raw water supply to the drinking water system) based on site-specific considerations.

When it is determined that malathion may be present and monitoring is necessary, then surface and groundwater sources should be characterized to determine the concentration of malathion. This should include monitoring of surface water sources during periods of peak use and rainfall events and/or monitoring of groundwater annually. Where baseline data indicate that malathion is not present in source water, monitoring may be reduced.

Where treatment is required to remove malathion, operational monitoring should be implemented to confirm whether the treatment process is functioning as required. The frequency of operational monitoring will depend on the water quality, fluctuations of the raw water concentrations and the treatment process. Responsible authorities should be aware of the impact of natural organic matter on activated carbon systems, as it may impact water quality objectives for malathion removal.

Where treatment is in place for malathion removal, compliance monitoring (i.e., paired samples of source and treated water to confirm the efficacy of treatment) should be conducted at a minimum on an annual basis. When routine operational monitoring indicates the potential for contaminant breakthrough, such as with GAC, monitoring should be conducted at least quarterly to plan for change-out of media. When a degradation process, like oxidation, is utilized, monitoring of by-product formation should also be considered.

International considerations

Other national and international organizations have drinking water guidelines, standards and/or guidance values for malathion in drinking water. Variations in these values can be attributed to the age of the assessments or to differing policies and approaches, including the choice of key study and the use of different consumption rates, body weights and source allocation factors (Table 13).

The Australian National Health and Medical Research Council (NHMRC) has set a guideline value of 0.07 mg/L for malathion in drinking water based on erythrocyte ChE inhibition in rats (NHMRC and NRMMC, 2011). The U.S. EPA does not have a maximum contaminant level (MCL) for malathion (U.S. EPA, 2009). The World Health Organization (WHO) concluded that malathion occurs in drinking water at levels well below those of health concern and therefore has not established a formal guideline value for malathion (WHO, 2004, 2017b).

The European Union (EU) does not have a specific chemical parametric value for individual pesticides. Instead, the EU has a value of 0.1 µg/L for any individual (single) pesticide and a value of 0.5 µg/L for total pesticides found in drinking water. In establishing these values, the EU did not consider the science related to each pesticide, such as health effects. Instead, the values are based on a policy decision to keep pesticides out of drinking water (EU, 1998).