PARENT SESSION
MP4a The use of ecological models and concepts in aquatic higher tier risk assessment
3:00 PM to 4:00 PM, Monday, 07 May 2001
Session Chair: P. van der Brink
Room 4

(100) Aquatic ecological risk assessment of atrazine - A tiered, probabilistic approach following ECOFRAM recommendations.

Giddings, Jeffrey¹, Anderson, Todd², Hall, Lenwood³, Hosmer, Alan⁴, Kendall, Ronald², Richards, Peter⁵, Solomon, Keith⁶, Williams, Martin⁷, ^{1 2 3 4 5 6 7}

ABSTRACT- An aquatic risk assessment of atrazine followed the tiered probabilistic framework recommended by the Ecological Committee on FIFRA Risk Assessment Methods (ECOFRAM). The extensive environmental fate, monitoring, and toxicological data base for atrazine was integrated into the risk assessment process incrementally as if it were generated according to the ECOFRAM model. Through the tiers, the exposure analysis was progressively refined: point estimates (Tier 1) were replaced by distributions from simulations incorporating spatial and temporal variability (Tier 2, refined in Tier 3), and finally by distributions reflecting variability in soil, weather, and landscape using Monte Carlo analysis (Tier 4). While the upper extremes of these distributions approached the Tier 1 point estimate, much lower concentrations were shown to be far more likely. Surface water monitoring data confirmed this conclusion. The effects analysis was also refined through the tiers. Tier 1 used point estimates of toxicity to the most sensitive species in a base set. Tier 2 used full concentration-effect curves for the most sensitive species. Effects analysis in Tier 3 was based on sensitivity distributions for 45 plant species and 52 animal species. In Tier 4, results of more than 30 mesocosm and microcosm studies indicated that whole aquatic communities are less sensitive to atrazine than the most sensitive species of aquatic plants and animals. Integration of an unusually comprehensive data set including bioassays, field studies, simulation modeling, and environmental monitoring reveals that atrazine poses no ecologically significant risk to the majority of aquatic environments. Some inhibition of aquatic plants may occur in certain habitats especially vulnerable to agricultural runoff, but effects should be transient and quick recovery is expected.

Key words: ecological risk assessment, probabilistic risk assessment, atrazine, pesticide