**AWARDS ISSUED PRIOR TO JANUARY 20, 2025, WERE FUNDED UNDER PREVIOUS ADMINISTRATIONS AND MAY NOT REFLECT THE PRIORITIES AND POLICIES OF THE CURRENT ADMINISTRATION.** PHOSPHORUS (P) HAS LONG BEEN KNOWN TO DEGRADE WATER QUALITY AND ECOSYSTEM HEALTH, YET IT IS ONE OF THE MOST COMMON AND WIDESPREAD POLLUTANTS IN THE ENVIRONMENT. P HAS MULTIPLE NATURAL AND ANTHROPOGENIC SOURCES, THUS MAKING IT DIFFICULT IDENTIFY SOURCES WITH THE GREATEST CONTRIBUTION TO WATER QUALITY DECLINES AND ECOSYSTEM HEALTH DECLINES. P IS A MAJOR CONTRIBUTOR TO HARMFUL ALGAL BLOOMS, WHICH CAUSE HUMAN AND WILDLIFE ILLNESSES AND MAKES IT DIFFICULT TO TREAT DRINKING WATER. P POLLUTION IS PARTICULARLY PROBLEMATIC AT UPPER KLAMATH LAKE, IN CENTRAL OREGON. FOLLOWING EUROPEAN SETTLEMENT, ACTIVITIES INCLUDING FARMING, RANCHING, LOGGING, AND RESERVOIR MANAGEMENT HAVE TRIGGERED ADDITIONAL P RELEASES INTO THE SYSTEM. DESPITE SIGNIFICANT RESTORATION EFFORTS OVER THE PAST 30+ YEARS, REDUCTIONS IN P LOADINGS AND IMPROVEMENTS IN ECOSYSTEM HEALTH HAVE NOT BEEN OBSERVED, SUGGESTING THAT THE CURRENT APPROACH TO MONITORING OR MANAGEMENT IS INAPPROPRIATE. HEREIN, WE PROPOSE USING A STATE-OF-THE-ART CHEMICAL-FORENSICS WORKFLOW TO PINPOINT THE LANDSCAPE SOURCES THAT CONTRIBUTE MOST TO THE CHEMICAL COMPOSITION OF UPPER KLAMATH LAKE. SAMPLES WILL BE COLLECTED FROM DIFFERENT SOURCES, INCLUDING AGRICULTURAL FIELDS, IRRIGATION WATER RETURNS, NATURAL WATERS, FOREST SOILS, AND OTHERS. THE HOLISTIC CHEMICAL COMPOSITION OF THESE SAMPLES WILL BE MEASURED USING MASS SPECTROMETRY INSTRUMENTS, AND THE DIAGNOSTIC CHEMICAL FINGERPRINTS ASSOCIATED WITH EACH POLLUTION SOURCE AND LAND USE/COVER TYPE WITHIN THE BASIN WILL BE IDENTIFIED USING MACHINE-LEARNING TOOLS. PRELIMINARY DATA INDICATE THAT SOURCES ARE DISTINGUISHABLE WITH NEAR PERFECT ACCURACY BASED ON THE CHEMICAL COMPOSITION OF A SAMPLE. THE FINGERPRINTING PROCESS WE DEVELOP WILL ALLOW MANAGERS TO PINPOINT THE POLLUTION SOURCES THAT ARE DISPROPORTIONATELY HIGH COMPARED TO OTHER SOURCES. THIS INFORMATION IS NECESSARY FOR MAKING DECISIONS THAT REDUCE NUTRIENT LOADS TO SURFACE OR GROUND WATERS WITHIN THE KLAMATH BASIN AND OTHER WATERSHEDS. WE EXPECT THIS TOOL WILL BE USEFUL FOR MANAGING NUTRIENT RUNOFF WORLDWIDE.