Phosphorus and Fine Particle Retention in Agricultural Headwater Streams

In many poorly drained agricultural regions, humans have introduced expansive networks of subsurface tile drains and straightened headwater streams to improve drainage. These networks serve as a direct link between cropland and larger streams and rivers, but the transport and retention of nutrients like phosphorus (P) in these networks is not well understood. Here we evaluate transport and retention of dissolved P and fine particles (which sorb dissolved P) within an agricultural drainage ditch in the Maumee River Basin in northeastern Ohio, USA. We conducted three constant rate injections of conservative salt (Cl as NaCl), dissolved P (KH₂PO₄), and a fluorescent fine particle (Dayglo AX-11-5 Aurora Pink®) following precipitation events in the spring (May), summer (July), and autumn (December). We model the breakthrough curves using the Continuous Time Random Walk (CTRW) approach to quantify solute and particle transport behavior. Preliminary analysis of Cl breakthrough curves indicates that in-stream velocities were slightly greater in spring (0.079 m/s compared with 0.039 m/s in summer and 0.060 m/s in fall), and conservative solute retention was also greatest in spring, as indicated by residence time behavior (tail power-law slope of -1.73 compared with -1.23 in summer and -1.59 in fall). Preliminary analysis of dissolved P breakthrough curves indicates that the nutrient spiraling length was longer in the spring (4070 m) and decreased in the summer (1560 m). Vegetation stands throughout the stream were denser in the summer and autumn and likely influenced P transport through both physical and biological processes. With the increasing frequency and severity of harmful algal blooms in major waterbodies that receive P from agricultural lands, it is crucial to understand how P moves through highly modified agricultural drainage networks. Tentatively, this study indicates that aquatic vegetation drives biophysical processes in drainage ditches that dictate seasonal nutrient export to larger waterbodies.