Environmental effect assessment of Magnacide H herbicide at Río Colorado irrigation channels (Argentina). Tier 4: in situ survey on benthic invertebrates.

ABSTRACT

The objective of this study was to assess the effects of Magnacide H (Baker Petrolite, TX, USA) usage on the aquatic ecosystem at the Cooperativa de Riego y Fomento (CORFO)-Rio Colorado irrigation channels, analyzing the effects on benthic invertebrates. At recommended treatment concentrations, Magnacide H kills most of the aquatic organisms; however, its presence in the channels is transient. Magnacide H caused a significant reduction in the number of taxa (58%), abundance (57%), and community diversity (67%) in the benthic assemblage with respect to untreated channels during the application period in the first year of the study. The herbicide reduced the abundance of the most abundant taxa oligochaetes, chironomids, ostracods, and the snails Biomphalaria peregrina and Heleobia parchappii. In contrast, the herbicide only reduced invertebrate diversity during the application period at the second year of study. Two months after the applications had ceased, the benthic community in treated channels recovered their biotic attributes, reaching values similar to controls. Recovery could have occurred from upstream unimpaired reaches supplying migrating invertebrates that recolonized affected areas. Since recovery occurred within a reasonable period of time, we conclude that the impact of Magnacide H at CORFO-Río Colorado channels is acceptable from a regulatory point of view in the present practice protocol. Nevertheless, a strict control on the application conditions must be ensured to minimize the risk on the ecological receptors. We emphasize the recommendation that water release outside the channels is prevented until the product has dissipated.