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

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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).


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).

Table 13. Comparison of international drinking water values for malathion
Agency(year) Value (mg/L) Key endpoint (reference) NO(A)EL (mg/kg bw per day) UF ADI (mg/kg bw/day) bw (kg) DW
AF (%) Comments
Health Canada - MAC
0.29 Increase in severity of chronic kidney disease in a 2-year toxicity and carcinogeni
city study in rats
(Daly, 1996)
100 0.030 74 1.53 20 -
  • (2009; 2018)
  • (non-regulatory lifetime health advisory)
Erythrocyte ChE inhibition in offspring from the comparative ChE multiple dose oral study in rats (U.S. EPA, 2009) 7.1 (BMDL10)
  • 100
70 2 20 U.S. EPA has set a non-regulatory lifetime health advisory rather than a MCL for malathion in drinking water, which is calculated from its associated Drinking Water Equivalent Level (DWEL) of 2 mg/L, obtained from its RfD (U.S. EPA, 2018)
  • WHO
  • (2004; 2017b)
  • 0.9 (non-regulatory HBV)
Decreased survival, reduced body weight and decreased ChE activity in a 2-year toxicity and carcinogenicity study in rats (Daly, 1996)
  • 29
  • 100
0.3 60 2 10
  • WHO has set a non-regulatory HBV rather than a formal guideline for malathion in drinking water (WHO, 2017b)
  • Australia
(NHMRC and NRMMC, 2011)
  • 0.07
Erythrocyte ChE inhibition in 2-year rat study
(Daly, 1996)
2 (NOEL)
  • 100
0.02 70 2 10 No reference for the 2-year rat study is provided in NHMRC and NRMMC (2011) although description is consistent with Daly (1996)
  • EU
0.1 µg/L 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, including health effects. Instead, the values are based on a policy decision to keep pesticides out of drinking water.

AF: allocation factor; BMDL10: benchmark dose lower confidence limit associated with a 10% response; bw: body weight; ChE: cholinesterase; DW: drinking water; DWEL: drinking water equivalent level; EU: European Union; HBV: health-based value; MAC: maximum acceptable concentration; MCL: maximum contaminant level; NO(A)EL: no observed (adverse) effect level; RfD: reference dose; UF: uncertainty factor

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