CHARACTERIZING PREFERENTIAL FLOW OF NITRATE AND PHOSPHATE IN SOIL USING TIME DOMAIN REFLECTOMETRY

Abstract

Significant quantities of agricultural chemicals can be transported rapidly through preferential flow pathways in soil. Time domain reflectometry (TDR) has been used to characterize solute transport in soil. However, previous TDR studies have not fully addressed the preferential flow of reactive solutes. A TDR method was tested for its ability to measure the preferential flow of nitrate and phosphate in soil. Saturated miscible displacement experiments were conducted using three undisturbed soil cores and a tracer solution containing chloride, phosphate, and nitrate. An inverse curve fitting method (CXTFIT) was used to estimate mobile-immobile model (MIM) parameters using the TDR and observed effluent data. The parameters fitted to the time-varying TDR-determined relative resident concentration were similar to the estimates from measured effluent chloride breakthrough curves (BTCs). Predicted BTCs were obtained from the parameters fitted to the TDR data. The predicted BTCs were comparable to the measured effluent nitrate BTCs, with a root mean square error (RMSE) of 0.0054. The peak times were 0.18, 0.16, and 0.12 pore volumes for the predicted BTCs and 0.26, 0.19, and 0.21 for the effluent nitrate BTCs. Phosphate BTCs differed distinctly from chloride and nitrate BTCs in our study; thus, the TDR method did not work for phosphate. However, the vertical TDR probe technique proved to be a practical method for a first approximation of nitrate preferential flow in soil.