Repeated Synoptic Sampling and Phosphorus Fractionation: A Strategy for Sourcing Phosphorus at the Watershed Scale.

Limiting phosphorus (P) mobilization from nonpoint source areas is a central goal of water quality protection plans. Locating nonpoint sources of P in mixed-use watersheds is challenged by the spatial, temporal, and chemical variability of critical source areas (CSAs) of P. Synoptic sampling is a proven method for capturing spatial variation in water quality, though it is not often used to track temporal dynamics across the same study area. Phosphorus fractionation is an analytical method that divides the total P (TP) in water into fractions, which for this study included total dissolved P (TDP), particulate P (PP), dissolved reactive P (DRP), and dissolved organic P (DOP). The objective of our study was to combine repeated synoptic sampling with P fractionation into a unique strategy for locating and characterizing CSAs of P. We evaluated this strategy by conducting synoptic sampling campaigns over a two-year period in a rural, montane watershed in north central Utah, USA. In each campaign, we sampled 18 sites across three tributaries during three distinct, annual hydrologic periods. By using this combined strategy, we identified critical source areas associated with PP and erosion, DOP and snowmelt pathways, and groundwater-driven DRP. Additionally, this study showed that the characterization of CSAs could not have been made using only a traditional synoptic sampling approach, but required a strategy that incorporated temporal and chemical variability. The combination of repeated synoptic sampling and P fractionation was essential to locating and characterizing critical P source areas in this mixed-use watershed. The assessment of P source areas and associated pathways and processes will guide ongoing development of best management practices that limit P loads from these respective critical source areas.