Isotopic and Hydrochemical Investigation of Surface-Spring Water Dynamics in Urbanized Catchment, Portland, Oregon, USA
On this page:
Watersheds dominated by urban areas hold most of the world’s population and often have heavily polluted stream systems. To complicate matters, urban dominated watersheds import a significant amount of water beyond the input precipitation. Due to the input of water from other sources and the high land use intensity, understanding the seasonal dynamics of source waters in an urban setting is pertinent to mitigate the quality of urban waters. Within west Johnson Creek Watershed in Portland, Oregon, nitrate concentration in spring waters and creek waters exceeds drinking water standards. We applied geochemical and isotopic tracers on this spring-fed creek to understand the source of spring waters, and the seasonal groundwater contributions to creek waters relative to imported sources of residential drinking water from the Willamette River and Bull Run watershed. The chloride and sulfate data showed linkages between creek waters, spring waters, and local groundwater and contained an elevated concentration of sulfate compared to local streams. Isotopic data of local groundwater represent signatures of long-term precipitation with average value of -9.5 ‰ δ18O and -66 ‰ δD. Seasonal shifts of imported sources have been distinguished where in summer months imported waters consist of relatively light signatures averaging at -10.75 ‰ δ18O and -75 ‰ δD similar to the Willamette River and in winter months were relatively heavier averaging at -9 ‰ δ18O and -63 ‰ δD similar water from the Bull Run Watershed. The isotopic data data reveals that local springs and groundwater are major sources of creek waters in spring, summer, and fall months. In contrast, imported waters provide the bulk of creek water in winter months. Our findings also demonstrate that the high nitrate level in creek water is related to septic waste, local alders, or legacy fertilizer. This project adds knowledge about the controls in hydrologic systems by applying tracers to understand seasonal changes in urban source waters and their potential links towards pollutants.