**AWARDS ISSUED PRIOR TO JANUARY 20, 2025, WERE FUNDED UNDER PREVIOUS ADMINISTRATIONS AND MAY NOT REFLECT THE PRIORITIES AND POLICIES OF THE CURRENT ADMINISTRATION.** PESTICIDES ARE A UNIQUE CLASS OF CHEMICALS BECAUSE THEIR USE NECESSITATES THEIR RELEASE DIRECTLY INTO THE ENVIRONMENT. AS A CONSEQUENCE, THERE ARE RELATIVELY STRICT REQUIREMENTS FOR THE EVALUATION OF THEIR MOVEMENT AND DEGRADATION IN ENVIRONMENTAL SYSTEMS. HOWEVER, SOME OF THE TESTING GUIDELINES USED TO EVALUATE THESE PROCESSES HAVE BECOME OUTDATED AS OUR KNOWLEDGE HAS IMPROVED. ONE EXAMPLE IS THE CURRENTLY USED EPA GUIDELINES TO TEST THE ABILITY FOR SUNLIGHT-DRIVEN REACTION TO DEGRADE PESTICIDES IN AQUATIC ECOSYSTEMS. IN PARTICULAR, SEVERAL POTENTIAL SUNLIGHT-DRIVEN DEGRADATION PATHWAYS HAVE BEEN OVERLOOKED THAT ARE HYPOTHESIZED TO BE IMPORTANT FOR PESTICIDES. THIS RESEARCH WILL USE MODEL SYSTEMS TO EVALUATE INDIVIDUAL DEGRADATION PATHWAYS TO DETERMINE (1) THEIR RELATIVE IMPORTANCE IN THE ENVIRONMENTAL FATE OF PESTICIDES, (2) THE EFFICACY OF RESULTS FROM SIMPLIFIED MODEL SYSTEMS TO PREDICT THE DEGRADATION OBSERVED IN COMPLEX ENVIRONMENTAL SYSTEMS, AND (3) HOW TESTING GUIDELINES CAN BE UPDATED TO OBTAIN MORE ACCURATE DATA THAT CAN BE USED TO PREDICT SUNLIGHT-DRIVEN DEGRADATION IN DIVERSE ENVIRONMENTS AND UNDER CHANGING CONDITIONS.