MONITORING DRAINAGE SOLUTION CONCENTRATIONS AND SOLUTE FLUX IN UNSATURATED SOIL WITH A POROUS CUP SAMPLER AND SOIL-MOISTURE SENSORS

Soil solution samples are often collected and analyzed without conside ration for either the drainage period or changes in solution or solute flux. A sampling method to assess the vertical flux of solution and s olute in the unsaturated zone was evaluated. Potassium bromide (KBr) s olution and distilled water were applied to the surface of a soil. Est imated and measured solution and solute discharge through the soil col umn were compared. The column consisted of Plainfield sand (mixed, mes ic, Typic Udipsamments) packed in a 208-liter cylinder. Following solu tion application, discrete samples were collected at one- to two-hour intervals with a ceramic soil solution sampler centrally located in th e column. In each test, the Br concentrations of discrete samples were measured, and the Br concentration of a time-integrated composite sam ple was calculated. Solution flux was estimated by monitoring vertical soil-water tension and content gradients with tensiometers and time d omain reflectometry probes, respectively, and applying the unsaturated form of Darcy's equation. In three of four tests, differences between measured and estimated solution discharge were less-than-or-equal-to 20%. Differences in measured and estimated Br mass discharge ranged fr om 1 to 37%. Correlation coefficients between estimated and measured B r mass discharge based on discrete and composite sample concentrations were 0.905 (p = 0.095) and 0.70 (p = 0.30), respectively.