Olivia L. Miller Scott Ator Matthew P. Miller Gregory E. Schwarz Dale M. Robertson Andrew J. Sekellick Kenneth D. Skinner David A. Saad Noah M. Schmadel 2024 <div id="ab0005" class="abstract author" lang="en"><div id="as0005"><div id="sp0035" class="u-margin-s-bottom">Quantifying nutrient sources in streams, their temporal and spatial variability, and drivers of that variability can support effective water resources management. Yet a lack of data and modeling capabilities has previously prevented comprehensive quantification across both space and time. Here a dynamic SPARROW (Spatially Referenced Regressions on Watershed attributes) model that accounts for a lagged delivery of nutrients to streams was developed and applied to simulate seasonal and source-specific total nitrogen (TN) and total phosphorus (TP) loads in streams across the Illinois River basin (IRB). Dynamic load predictions from 2000 through 2020 revealed that a third of the TN and a quarter of the TP instream load originated from non-point sources that were lagged in their delivery from land-application to streams by more than a season. This lagged mass was the largest overall TN source—which was estimated as a lagged expression of previous seasonal non-point sources including fertilizer, manure, atmospheric deposition and fixation, and urban land use. Treated wastewater effluent was the largest TP source exported from the basin, contributing 39&nbsp;% of the TP load and 15&nbsp;% of the TN load, and dominated the load in the upper Illinois River near Chicago. Loads in the lower river during this period, conversely, were attributed primarily to a mix of agricultural sources and their lagged fractions from headwater tributaries. Instream processes removed 10&nbsp;% of the TN load while only 4&nbsp;% of the TP load was removed during instream transport. With appropriate datasets, the models could be extended to other basins or time periods and used to forecast future seasonal nutrient loads.</div></div></div><div id="ab0010" class="abstract graphical" lang="en"><br></div> application/pdf 10.1016/j.scitotenv.2024.176816 en Elsevier Seasonally varying contributions of contemporaneous and lagged sources of instream total nitrogen and phosphorus load across the Illinois River basin article