Comparative Toxicity of Glyphosate-Based Herbicides: Aqueous and Sediment Porewater Exposures

Abstract

Glyphosate-based herbicides are widely used for aquatic weed control. However, their aquatic toxicity data, especially those on sediment, are relatively scarce. In this study, the water-only acute toxicity of three formulations based on glyphosate (Rodeo, Roundup Biactive, and Roundup) were compared using a water-column organism (cladoceran: Ceriodaphnia dubia) and a benthic organism (amphipod: Hyalella azteca). In addition, Roundup Biactive and Roundup were spiked into a clean sediment which was amended with appropriate amounts of peat moss to study the effect of different organic carbon levels (0, 0.4, 1.2, and 2.1%) on their sediment toxicity, with C. dubia exposed to overlying water or porewater prepared from the contaminated sediments. Results showed that the toxicity based on 48-h LC50s for the three herbicides in the water-only tests was Roundup (1.5–5.7 mg L^-1) > Roundup Biactive (82–120 mg L^-1) > Rodeo (225–415 mg L^-1), and H. azteca was generally more sensitive than C. dubia to these herbicides. Toxicity differences between formulations were due to the different surfactant components in these herbicides. From the porewater toxicity tests, Roundup Biactive (340 mg kg^-1) and Roundup (244 mg kg^-1) were similarly toxic in the sediment tests at 0% organic carbon, indicating that the surfactants in Roundup were considerably more adsorptive than those in Roundup Biactive to the sediment of the same organic carbon. Also, an increase in organic carbon significantly decreased the toxicity of Roundup in sediment, but not for Roundup Biactive. Sediment–porewater partitioning of glyphosate was found to be influenced by sediment organic carbon (i.e., glyphosate adsorption increased with sediment organic carbon).