THE ROLE OF TERNARY MIXING OF RAINWATER, GROUNDWATER, AND FARM DEVELOPMENT OUTFALL ONNITROGEN AND PHOSPHORUS EXPORT FROM A SMALL, EPHEMERAL, AGRICULTURAL WATERSHED IN KENTUCKY

Small ephemeral agricultural watersheds in Kentucky have thin impermeable soils which bypass riparian zones by funneling most farm storm derived runoff through ephemeral channels, making these channels nutrient contamination hotspots. We examine a multi-use watershed containing fertilized cropland, grazing pasture, and a farm development complex in Madison County, Kentucky. A ternary mixing model was developed to quantify the proportions of rainwater, groundwater subtributary derived baseflow, and farm industrial complex outfall drainage in storm event water. This model will be used to help understand how total nutrients are exported to larger rivers. Also, mass fluxes of N and P and end-member waters were calculated for each storm event to determine how inter-storm mixing of end-member waters impacted nutrient export.

We used major dissolved anions, cations, and nutrients in storm waters and baseflow to generate a ternary mixing model and to investigate how surface water-groundwater interactions control nutrient transport during stormflow. During 2017-2018, water samples collected from eight summer storms, from main channel baseflow, and from subtributaries were analyzed for major cations, anions, nutrients, and other chemical parameters. A V-notch weir, coupled with a pressure transducer, measured watershed discharge.

The ternary mixing model was calculated using homogenous barycentric coordinates for three end member waters (baseflow tributary water, outfall drainage, and rainwater.) using dissolved K⁺ and SO₄^2-. Watershed baseflow was controlled by a mixture of outfall and tributary waters where K⁺ was predominantly sourced from Na⁺, Cl^-, NH₄⁺, and PO₄^3- rich outfall drainage. SO₄^2- was predominantly sourced from tributaries weathering sulfide rich fractured shales. Storm hydrograph rising limbs often experienced abrupt K⁺, NO₃^-, NH₄⁺, Na⁺, and Cl^- spikes that preceded storm peaks, suggesting outfall drainage origins; however, this signal decreased over the storm duration. Dissolved SO₄^2- concentrations decreased inversely to discharge, indicating dilution of tributaries by rainfall sourced overland flow. The total P and N masses exported during each storm will be calculated by multiplying linear interpolation of measured P and N concentrations against hydrograph discharge.