Field Assessment and Groundwater Modeling of Pesticide Distribution in the Faga'alu Watershed on Tutuila, American Samoa

Abstract

Pesticide use is prevalent worldwide and their dispersal across water resources has been gaining increased attention. Pesticides in natural waters can be toxic not only to terrestrial and aquatic biota, but also to humans. Pesticide studies usually focus on surface waters since surface runoff and sediments are considered their primary migratory pathways. Groundwater pathways, however, have not been as thoroughly investigated since most pesticides are not commonly considered mobile and persistent enough for subsurface dispersal. Pesticide transport through aquifers may take several decades, eventually reaching drinking water sources or the marine environment. Groundwater and surface water interactions were investigated as a vector of pesticide migration on the island of Tutuila in American Samoa in August 2016. For the island-wide study that looked at pesticide spatial variation, samples were collected from groundwater wells and springs as well as streams, for selected pesticides (glyphosate, imidacloprid, azoxystrobin, DDT/DDE). A more detailed pathway-specific study was conducted in the Faga`alu watershed, where field data were integrated into a groundwater model to reconstruct flow paths and pesticide discharge from the groundwater aquifer to the stream and the coastline. In combination with land-use maps, groundwater models were used to identify potential pesticide sources. All measured pesticide concentrations on Tutuila were well below EPA regulated limits. They were found at the same frequency and in the same concentration ranges in surface and groundwater samples. DDT was uniformly distributed, but there was a variation in glyphosate and imidacloprid spatial distribution that followed population density. In Faga`alu, groundwater pathways were responsible for a majority (~70%) of glyphosate and DDT discharge (350 & 482 mg/d and 2990 & 5519 mg/d) to the stream and coastline, respectively, in comparison to surface runoff according to my model and field measurements during dry season. The timescale of the transport spans from many decades (DDT applications were banned in the 1970s) to most recent applications of glyphosate that were traced back to populated regions, where this herbicide is probably applied in backyard-scale agriculture rather than large-scale farming operations. This study shows the significance of groundwater flow for pesticide transport and discharge into the coastal ocean.