Contrasting NO3-N concentration patterns at two karst springs in Iowa (USA): insights on aquifer nitrogen storage and delivery

Abstract

Evaluating the patterns of NO3-N concentrations at karst springs can be used to infer hydrologic processes and nutrient dynamics in karst aquifers. In this study, NO3-N concentrations observed at two karst springs in northeast Iowa (USA) were evaluated for a 2-year period using high-frequency sensors. Despite similar watershed land use dominated by intense row cropping of corn and soybean production (>70%), NO3-N concentrations and temporal patterns were very different between the two springs. At the Manchester spring, NO3-N stored in overburden materials above the karst-enhanced Silurian-age bedrock provides a continuing source of NO3-N to the spring. Rainfall events mobilize the stored NO3-N and concentrations increase. At Big Spring, the karst system is overlain by a thin layer of sediments and the bedrock is dominated by sinkholes and losing streams. Rainfall events dilute the spring NO3-N concentrations which rapidly decreased during events before rebounding to previous levels. Spectral analyses revealed that concentrations at both springs were a fractal process, with the scaling exponent at Manchester (2.0) considerably larger than that measured at Big Spring (1.4), indicating a higher degree of autocorrelation in NO3-N concentrations at Manchester, consistent with the conceptual model. Overall, results argue for greater use of high-frequency NO3-N monitoring at karst springs to better assess short- and long-term variations in NO3-N concentrations and to unravel karst processes.