PHOSPHORUS TRANSPORT ACROSS THE GROUNDWATER-SURFACE WATER INTERFACE IN INTENSIVELY MANAGED AGRICULTURAL STREAMS AND RELEVANCE TO GREAT LAKES WATER QUALITY (Invited Presentation)

In agricultural areas with poorly drained soils, subsurface tile drains are commonly installed to improve drainage but also serve as transport pathways for excess nutrients to streams. Here, I quantify the transport of phosphorus (P) across interfaces—from soils to tile drains to streams, and through surface water, aquatic plants, and sediments. We added a novel mixture of tracers (conservative chloride (Cl), potassium phosphate, and fluorescent micrometer-sized particles) to a farm field and sampled their breakthrough at the tile drain outlet to the stream. We also used time-lapse electrical resistivity imaging to monitor saline tracer migration towards the tile drain. A small but sizable fraction of the added tracer arrived within minutes at the tile drain outlet 30 meters away—the pulse contained 7% of added Cl, 3% of dissolved P, and 2% of fine particles. ERT images showed fast downward movement of Cl to the tile drain depth but also prolonged retention in the soils after the tracer test. Successive storm events remobilized solutes and particles over days to weeks. Tracer experiments were also conducted in the stream to understand mobility and cycling of dissolved and particulate P. Transient storage was greatest during the spring, when thicker vegetation stands caused more pooling and flow stagnation, and decreased through fall, as vegetation stands thinned. Soluble P uptake lengths were 8.7 times longer in fall than spring, and particle capture lengths were 4.3 times longer. Sediment P extractions performed on core samples over multiple seasons show that aquatic vegetation plays a major role in retaining internal P in streambed sediments. A site with dense aquatic vegetation had chronically greater internal P concentrations by 25−75%. Additionally, mobile P binding fractions nearly doubled in summer, possibly due to accelerated rates of organic matter mineralization or iron reduction beneath suboxic, stagnant surface waters. Preliminary observations suggest that woody buffers create the biophysical conditions needed to shade out aquatic vegetation, mitigate internal P concentrations in sediments, and increase oxygen concentrations in surface water. These insights can help manage dissolved P concentrations in agricultural streams in the Lake Erie Basin and other agricultural areas where rising P loads are exacerbating harmful algal blooms.