QUANTIFYING NUTRIENT EXPORT FROM AN UPLAND, INTERMITTENT STREAM DRAINING A WORKING FARM, MADISON COUNTY, KENTUCKY: PRELUDE TO NUTRIENT MITIGATION

Excess nutrients emanating from agricultural areas cause eutrophication, decreases in the diversity of aquatic biota, and degradation of water quality at local to regional scales. We measure the amount of dissolved ammonium (NH₄⁺), nitrate (NO₃^-), and orthophosphate (PO₄^3-) as well as total phosphorus (orthophosphate plus complexed and adsorbed phosphorus; ΣP) exported from a small (watershed area: 0.46 km², 113 acres), intermittent stream draining a working farm (Meadowbrook Farm operated by Eastern Kentucky University). This upland watershed drains a representative portion of the Farm characterized by crops, cattle grazing, and dairy operations, then enters Muddy Creek, a tributary to the Kentucky and Ohio River system. Nutrient sources include fertilizer, pasture manure, and applied manure.

We have constructed an instrumented weir near the mouth of this intermittent stream to measure its discharge over time during both low-water and high-water conditions. We use an autosampler to collect water samples during runoff events and measure nutrients using ion chromatography and colorimetric techniques. A weather station that is part of the Kentucky Mesonet system records rainfall directly at the watershed.

Between runoff events, background or baseflow concentrations of nutrients are generally low, usually much less than 1 mg/L for all nutrient species. Nutrient levels during rain events are generally higher by 1 or 2 orders of magnitude, demonstrating that the preponderance of nutrient export occurs under rainy conditions. Storm events vary widely in total volume from ~68,000 to 8 million liters with most discharge events at 10⁵ L. Nutrient export also varies significantly with the highest values at 1 kg of N as ammonium; 5.3 kg of N as nitrate, and 3.1 kg of P as dissolved orthophosphate.

The quantitative relationship between rainfall amount and the magnitude of nutrient export is problematical, although export generally increases as rainfall increases. Confounding variables include rainfall intensity, rainfall periodicity, pasture rotation, and manure applications. Our present estimates precede nutrient sequestration efforts at the Farm. Nutrient export values before and after sequestration measures are deployed can then compared to test mitigation efficacy.