NUTRIENT TRANSPORT IN SMALL EPHEMERAL AGRICULTURAL WATERSHEDS IN KENTUCKY

Small ephemeral agricultural watersheds in Kentucky have thin impermeable soils, which funnel most farm runoff through ephemeral channels bypassing riparian zones, which makes these channels nutrient contamination hotspots. We used major dissolved anions, cations, and nutrients to characterize and understand how surface water-groundwater interactions control nutrient transport during baseflow and stormflow in a watershed containing fertilized cropland and grazing pasture in Madison County Kentucky. During 2017-2018, water samples collected from eight summer storms and from baseflow surface and groundwater in the main channel and subtributaries were analyzed for major cations, anions, nutrients, and other chemical parameters. A V-notch weir coupled with a pressure transducer measured watershed discharge. Initial baseflow is Ca-Mg-HCO₃-rich groundwater sourced from dolomite bedrock. NO₃ sourced from groundwater is the dominant baseflow nutrient under normal conditions. High NO₃ concentrations are sourced from an upstream cornfield and various subtributaries. Anomalously high and temporally controlled Ca, Mg, Na, Cl, SO₄, PO₄, and NH₄ concentrations emanate from a waste outlet sourced from the central farm complex. pH increases downchannel from the upwatershed cornfield and the waste outlet suggesting buffering by groundwater. Groundwater diluted with surface water in subtributaries simultaneously dilutes and buffers waste pipe and agricultural contaminants and enriches main channel baseflow with SO₄. Furthermore, subtributary chemistry evolves over the growing season, controlling and generating complex, temporally dependent, baseflow chemistry in the main channel. During storms, high-volume overland flow dilutes groundwater, preventing buffering of runoff. Storms export high NO₃, NH₄, PO₄, and K concentrations as a mixture of NO₃-rich groundwater and PO₄, NO₄, NH₄, and K-rich fertilizer and soil sourced surface runoff.