Transport of photosystem II (PS II)–inhibiting herbicides through the vadose zone under sugarcane in the Wet Tropics, Australia

Abstract

The usage of Photosystem II (PS II)–inhibiting herbicides, identified as a risk to ecosystem health and resilience, has been reduced in agricultural activities adjacent to the Great Barrier Reef, Australia over the past 10 years. Two PS II herbicides (diuron and hexazinone) and three metabolites (desethyl atrazine, hydroxyatrazine and DCPMU) were recently detected at low concentrations in the regional aquifer of the South Johnstone River sub-basin, in the Wet Tropics, Australia. This study presents results from monitoring 12 PS II herbicides in soil profile, porewater sampled through the vadose zone monitoring system, and groundwater from bores. Among the 12 PS II herbicides, only hexazinone and metribuzin were applied relatively recently, in November 2014. Hexazinone exhibited substantial transport through the vadose zone, with decreasing concentration with depth, and eventually disappeared over-time. Metribuzin was not detected in the groundwater, in soil profile or through the vadose zone. Diuron was detected in the soil profile, through the vadose zone and in bores at the site. Its two metabolites—1-(3,4-dichlorophenyl)-3-methylurea (DCPMU) and 3,4-dichloroaniline—were detected in the deepest port at 4 m, indicating the transformation of diuron into these metabolites and potential leaching. The other nine PS II herbicides were not detected in the soil profile, through the vadose zone or in bores at the site. Some of the PS II herbicides were detected in the soil profile and beyond the root zone, depending on their application record and persistence in the environment. Therefore, the data shows that the herbicides are long lived at low concentrations. However, herbicide concentrations in subsoil, vadose zone and groundwater are low (less than environmental guideline values) and are detected at much lower concentrations than in surface runoff.