LOADING AND FATE OF PESTICIDES, PHARMACEUTICALS, AND OTHER ORGANIC WASTEWATER COMPOUNDS ALONG A HYDROGEOCHEMICAL/LAND-USE GRADIENT

A hydrogeological/geochemical/land-use gradient was used to evaluate sources and processes controlling the fate of pesticides, active pharmaceutical ingredients (API), and organic wastewater compounds (OWC) in the Boulder Creek Watershed, Colorado. Two samplings were conducted along a 70 km reach of the snowmelt-driven hydrological system under high-flow (7.2 m³/s) and low-flow (1.6 m³/s) conditions. Boulder Creek originates at the Continental Divide (4120 m), flows through sparsely-populated mountains, passes through the city of Boulder (1600 m), and then flows through rural agricultural lands before emptying into St Vrain Creek (1480 m). As the stream reaches Boulder, the underlying geology changes from granitic bedrock to shale and sandstone, and the stream gradient abruptly changes from 34 m/km to 4 m/km. Samples were analyzed for 22 API, 47 OWC including ethylenediaminetetraacetic acid (EDTA) and nonylphenolethoxycarboxylic acids (NPEC), and 84 pesticides. Water composition ranged from pristine snowmelt to agriculture-impacted wastewater treatment plant (WWTP) effluent. Eleven pesticides (atrazine, carbaryl, desethylatrazine, diazinon, dichlobenil, lindane, malathion, metolachlor, parathion-methyl, picloram, and prometon) were detected, with the highest concentrations and greatest abundance occurring in the agricultural portion of the watershed. API including prescription (cimetidine, codeine, diltiazem, sulfamethoxazole, trimethoprim) and non-prescription (acetaminophen, caffeine, dimethylxanthine, diphenhydramine, cotinine, ibuprofen) compounds were detected with the greatest frequency and highest concentrations (up to 0.33 ìg/L) downstream of WWTP discharges. The dominant OWC were EDTA (210 ìg/L maximum) and NPEC (300 ìg/L maximum) which occurred at concentrations 3 orders of magnitude greater that the API. Although present at similar concentrations in WWTP effluent, EDTA undergoes preferential in-stream degradation relative to NPEC, primarily due to photolytic degradation. The dynamic nature of the metal complexing agent EDTA in the stream environment has implications for the environmental fate of trace inorganic WWTP contaminants derived from human health care, such as the rare earth element gadolinium, by forming complexes that may modify their solution chemistry and transport behavior.