Movement of nitrate through regolith covered karst terrane, northwest Arkansas

A quantitative approach was used to monitor movement of nitrate through a karst spring basin. The research follows nitrate along flow paths, from application as fertilizer, through the soil matrix, into the bedrock, and ultimately, as discharge from a spring. On the basis of data from analysis of soil cores and spring water samples, it was calculated that for the period of one year, 22,000 kg of nitrate as nitrogen was discharged from a spring with a drainage basin area of 43.85 km2. After application of animal waste as fertilizer, the nitrate leaches through the soil primarily as piston flow in the matrix, unless macropores are encountered. Cores collected from fertilized fields show that the nitrate moves at a rate of approximately 65 cm/year through the silt and clay loams. The cores also revealed the presence of nitrate maximums with depth that were associated with timing of fertilizer application. Soil sampling and spring monitoring suggest that the soil serves as a reservoir for nitrate. Nitrate as nitrogen concentrations in the soil range from 0.012 to 0.036 g/kg of soil, which would be equivalent to 11–106 mg of NO3–N/l water. After reaching the bedrock surface, nitrate enters the aquifer in fractures and solution-enlarged joints, eventually discharging from the spring. Spring discharge data indicate that 74% of the nitrate exits from the basin through the spring during baseflow conditions, and the remaining 26% is discharged as a result of rapid recharge associated with precipitation storm pulses. During precipitation events, overland flow and runoff moves primarily through macropores in the soil carrying mainly surface derived nitrate, and displaces little of the soil nitrate.