Phosphorus transport across the groundwater-surface water interface in intensively managed agricultural streams and relevance to Great Lakes water quality

A Warren Distinguished Lecture with
Audrey Sawyer 
Earth Sciences, The Ohio State University

ABSTRACT
In agricultural areas with poorly drained soils, subsurface tile drains are commonly installed to improve drainage, but they also serve as transport pathways for excess nutrients to enter adjacent streams. Audrey Sawyer quantifies the transport of phosphorus (P) across interfaces—from soils to tile drains to streams, and through surface water, aquatic vegetation, and sediments. Her research team added a novel mixture of tracers (including 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. The researchers 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 almost immediately at the tile drain outlet 30 meters away—the pulse contained 7% of added Cl, 3% of dissolved P, and 2% of fine particles and had a mean arrival time of 21 minutes. 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 across additional sites 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 (USA and Canada) and other agricultural basins where rising P loads are exacerbating harmful algal blooms. 

SPEAKER
Audrey Sawyer is a Professor of Earth Sciences at The Ohio State University with a courtesy appointment in Civil, Environmental, and Geodetic Engineering. Her research focus is surface water-groundwater interaction in streams and coastal waters. Her overall goal is to understand how fluid flow near the earth’s surface influences water quality and ecology. Sawyer and her research group pursue this problem through a combination of numerical and physical modeling and field observations. She has authored over fifty papers in leading journals, including Science. Sawyer is the recipient of the Kohout Early Career Award in Hydrogeology from the Geological Society of America. Her recent projects include nitrogen cycling in tidally influenced rivers, phosphorus transport in small agricultural streams, algal toxin fate in shallow lakebed sediments, saltwater intrusion, and submarine groundwater discharge at continental scales. Sawyer teaches courses in hydrogeology, water issues, and introductory geology. 
     Sawyer received a B.S. in geology and environmental engineering from Rice University in 2004, a M.Sc. in geoscience from the Pennsylvania State University in 2007, and a Ph.D. in geological sciences at the University of Texas-Austin in 2011. She conducted postdoctoral research at the University of Delaware. She was a visiting scientist at Instituto de Diagnóstico Ambiental y Estudios del Agua (IDAEA-CSIC) in Barcelona, Spain, 2021-2022 and in the Department of Civil and Environmental Engineering at the Universitat Politècnica de Catalunya in 2024.