Correlations of surface and groundwater flow with nitrate-N concentration in agricultural drainage ditches in two illinois watersheds

Ditch water nitrate-N (NO₃-N) concentrations in the watersheds in Illinois routinely exceed the maximum contaminant level of 10 mg/L set by the Unites States Environmental Protection Agency. Prediction of accurate NO₃-N concentration is critical in making correct and timely management decisions regarding water quality in these watersheds. A study was conducted in two watersheds, namely the Big Ditch and the Upper Embarras River watersheds in east-central Illinois to investigate the correlations between flow and NO₃-N concentration in the agricultural drainage ditches. A ditch section of 200 m was selected in each watershed and surface flow at upstream and downstream ends of the ditch section was monitored. Mass balance of surface water flow was carried out between upstream and downstream ends of the ditch sections to estimate the total groundwater contributions of flow to the ditches. Nitrate-N concentrations in the ditch water were monitored at upstream and downstream ends of the ditch sections. Periods after spring fertilizer application in the corn-soybean fields were selected to study the correlations between flow (surface water and groundwater) and NO₃-N concentration. Kendall’s tau test was conducted to determine any correlation between flow and NO₃-N concentration in the ditch water. Regression equations and coefficients of determination (R²) were determined for flow and ditch water NO₃-N concentration to evaluate the correlation between them. It was observed that there were good correlations between ditch water flow and ditch water NO₃-N concentration in both watersheds. It may be possible to estimate ditch water NO₃-N concentrations based on ditch flow using regression equations. Weak correlations were observed between groundwater flow contributed by the 200 m ditch sections and ditch water NO₃-N concentrations at the two sites for the study period.