Role of preferential flow on water and chemical transport in a glacial till soil
T. Al Austin
Water and solutes were found to move nonuniformly through a Nicollet loam soil. Two irrigations of water containing nitrate, bromide, lithium, and rhodamine WT were applied by sprinkler to a soil, with a growing corn crop, above a tile drainage line. Within 25 min, tile drainage responded with increased concentrations of the four tracers and increased flow. Concentrations of all four tracers peaked in tile drainage 70 min after an irrigation began and then declined, even as the irrigation continued;The mass of a tracer moving in drainage water was related to tracer adsorption properties. Nitrate and bromide showed the greatest mobility, while rhodamine WT had the least. Of the total accumulated tracer loss in tile drainage water within 56 h after the start of an irrigation, 83% of the rhodamine WT loss had occurred within one hour after an irrigation ended. During this same time period, losses of lithium, bromide, nitrate, and water in tile drainage were 67%, 40%, 35%, and 25% respectively;Tile drainage flow was separated according to a combined flow model into matrix and preferential flow based on assumptions of their respective concentrations. The resulting hydrographs showed that the preferential flow component reached a maximum of 29% of total drainage during the first irrigation;A tension infiltrometer used to determine pore size distribution at the field experiment site showed 50% of total infiltration occurring at soil water tensions less than 40 mm (>0.75 mm effective pore diameter);Solute breakthrough curves were made for nitrate, bromide, lithium, and rhodamine WT in a repacked column of soil taken from the same site as the field tracer experiment. Results from two different columns show lithium reached a C/C[subscript] o value of 0.8 after 6 pore volumes were displaced, and then leveled off. Rhodamine WT was more strongly adsorbed, reaching a relative concentration of 0.46 after 45 pore volumes. Equilibrium batch tests confirmed strong soil adsorption of rhodamine WT. Adsorption was influenced by soil organic matter, clay content, co-ions in solution, and pH. The average adsorption coefficient (K[subscript] oc) for rhodamine WT, corrected to a one molar solution of BKr, was 4720 ml/g. This value suggests rhodamine WT is more strongly adsorbed than most commonly used Iowa pesticides.