Sodium Chlorite Treatments in Irrigation Water Are a Significant Source of Chlorate and Perchlorate in Hop Cones

Chlorine-based disinfectants used for cleaning and maintaining hop ( Humulus lupulus ) irrigation systems may contribute to chlorate (ClO 3 − ) and perchlorate (ClO 4 − ) residues in hop cones, thus raising regulatory concerns in various export markets. Because of their associated risk of disrupting iodine uptake in the thyroid gland, ClO 3 − and ClO 4 − are oxyanions designated as contaminants in vulnerable populations. This study evaluated their impact on hop cone contamination. A 2-year field study (2023 and 2024) at Washington State University examined the following four drip irrigation cleaner treatments: 1) continuous hydrogen peroxide + peroxyacetic acid; 2) continuous 2 ppm sodium chlorite; 3) continuous 2 ppm sodium chlorite with two late-season 25-ppm shock treatments; and 4) two late-season 25-ppm sodium chlorite shock treatments. A residue analysis was performed using liquid chromatography-tandem mass spectrometry. Continuous sodium chlorite application led to ClO 3 − residues up to 47 mg·kg −1 , which far exceeded the European Union limit (0.05 mg·kg −1 ). The ClO 4 − levels peaked at 0.25 mg·kg −1 , which was below the 0.75 mg·kg −1 limit. Hydrogen peroxide-treated hops still contained ClO 3 − , suggesting other contamination sources. A soil analysis showed ClO 4 − accumulation but no detectable ClO 3 − . Interestingly, the use of sodium chlorite resulted in higher hop cone yield. Chlorine-based cleaners significantly increase ClO 3 − and ClO 4 − residues in hops. Alternative treatments reduced but did not eliminate ClO 3 − and ClO 4 − contamination in hop cones.