Toxicological reconnaissance of Arsenal? use along the Rio Grande River through Big Bend National Park: Final report

Abstract

This study examined the biological response of aquatic organisms, collected in situ and from laboratory toxicity testing of water and soil to the herbicide Arsenal? primarily used to decrease the invasive species Arundo donax in the Rio Grande River through Big Bend National Park in Texas (USA). Biological, toxicological, physicochemical, and flow data were collected before, during, and after an Arsenal? treatment from 4 reference and 3 treatment sites. Significant differences existed in ecoregion 24 B-IBI scoring, and in toxicity testing of water and sediment, between treatment and reference sites. Greater toxicity, and a higher inverse relationship to B-IBI scoring, was observed in sediment samples compared to water samples. This could be due to the surfactant nonylphenol ethoxylate (NPE), added to the Arsenal? formulation rather than the active ingredient imazapyr. Care must be implemented with the results of this research due to many confounding variables that occurred during the study period including flash flooding. Although flashy hydrology is relatively common to the region, it can influence aquatic macroinvertebrate assemblage structure, function, and species richness measures with these dropping due to scour and downstream displacement within a reach during, and for some period following, flooding. Therefore, flash flooding would be expected to have some effect on aquatic macroinvertebrate assemblages and subsequent B-IBI scoring. Additionally, flooding will cause some displacement, and deposition, of sediment throughout the study area. This flooding could impact the sediment toxicity results as sediment-bound contaminants could be spread some distance from their origin. Arsenal? treatment occurred during this study near sites Treatment 1 and Treatment 2 on 5/29 and 5/31 2022 respectively. This represented an extreme condition as far as concentration of potential toxicants in the water are concerned and these relatively high concentrations would be expected to have decreased within a relatively short period of time, especially in the water. It was outside of the scope and budget of this project to obtain concentrations of imazapyr or surfactants from sediment or water. Therefore, exact doses of imazapyr or surfactant for response curves could not be obtained. No sample site exists in isolation rather, influences from one reach can significantly impact downstream areas. This is especially true for aquatic ecosystems in arid regions that often exhibit flashy hydrology. However, we believe the findings from this study provide indication of potential toxicity of some magnitude as well as provide direction for management. Decisions of whether the toxic effects of herbicide use to aquatic macroinvertebrates outweigh the beneficial effects of Arundo donax control should be carefully considered.