Page 2: Guidelines for Canadian Drinking Water Quality: Guideline Technical Document – Chlorpyrifos

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Guideline

The maximum acceptable concentration (MAC) for chlorpyrifos in drinking water is 0.09 mg/L (90 µg/L).

Identity, Use and Sources in the Environment

Chlorpyrifos (C₉H₁₁Cl₃NO₃PS) is an organophosphorus insecticide used for the control of mosquitoes, flies, various crop pests in soil and on foliage, household pests and aquatic larvae. It is also used on sheep and cattle for the control of ectoparasites. Annual use in Canada is in the range of 100 000 to 500 000 kg.^{Footnote 1}

The vapour pressure of chlorpyrifos is 2.49 x 10^-3 Pa at 25°C, and its solubility in water is 2 mg/L at 25°C.^{Footnote 2} Reported log octanol-water partition coefficients are high, ranging from 4.82 to 5.11.^{Footnote 3}

Chlorpyrifos is tightly absorbed by soil and is not expected to leach significantly.^{Footnote 4} It persists in soil for 60 to 120 days,^{Footnote 5} with degradation being primarily due to microbial action.^{Footnote 6} Products of degradation include 3,5,6-trichloro-2-pyridinol, which is subsequently broken down to organochlorine compounds and carbon dioxide.^{Footnote 7} The rate of hydrolysis of chlorpyrifos in water increases with pH and temperature and is enhanced by the presence of copper.⁵ Between 30 and 60% of the total amount of chlorpyrifos in the aqueous phase may disappear within 24 hours through adsorption, degradation and vaporization.^{Footnote 8}

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