Inclusion of Pesticide Transformation Products Is Key to Estimating Pesticide Exposures and Effects in Small U.S. Streams

Improved
analytical methods can quantify hundreds of pesticide
transformation products (TPs), but understanding of TP occurrence
and potential toxicity in aquatic ecosystems remains limited. We quantified
108 parent pesticides and 116 TPs in more than 3 700 samples
from 442 small streams in mostly urban basins across five major regions
of the United States. TPs were detected nearly as frequently as parents
(90 and 95% of streams, respectively); 102 TPs were detected at least
once and 28 were detected in >20% samples in at least one regionTPs
of 9 herbicides, 2 fungicides (chlorothalonil and thiophanate-methyl),
and 1 insecticide (fipronil) were the most frequently detected. TPs
occurred commonly during baseflow conditions, indicating chronic environmental
TP exposures to aquatic organisms and the likely importance of groundwater
as a TP source. Hazard quotients based on acute aquatic-life benchmarks
for invertebrates and nonvascular plants and vertebrate-centric molecular
endpoints (sublethal effects) quantify the range of the potential
contribution of TPs to environmental risk and highlight several TP
exposure–response data gaps. A precautionary approach using
equimolar substitution of parent benchmarks or endpoints for missing
TP benchmarks indicates that potential aquatic effects of pesticide
TPs could be underestimated by an order of magnitude or more.