Influence of initial and boundary conditions on solute transport through undisturbed soil columns

The effect of initial soil water content (theta) and steady and transient water flow boundary conditions (Jw) on the transport of identical solute pulses was examined in four undisturbed soil columns. Solute breakthrough curves (BTCs) and (theta) were monitored nondestructively using time domain reflectometry (TDR). Pulse concentrations were predicted from TDR impedances (omega) using either a steady Jw calibration or a transient Jw calibration. The steady Jw calibration,βs, was estimated from the steady Jw BTC. The transient calibration, β(theta), was estimated from the changes in (omega)-1 (theta) between three different calibrating solutions of Cl- (1.106, 1.503, and 1.900 g L-1) in the pore space. All β(theta) curves had a very similar decreasing curvilinear shape. At one location the β(theta) curve was high and residual water was not completely replaced by the calibrating solutions. The steady Jw calibration values were generally slightly lower than the equivalent β( theta) value, but collectively they followed a similar decreasing curvilinear trend with steady theta(s). Solute applied to initially dry soil tended to lead the transient Jw wetting front and breakthrough began with the start of cumulative drainage. These BTCs also had the greatest amount of BTC tailing. Solute applied to initially wet soil had less BTC tailing. The least amount of BTC spreading occurred during the steady Jw condition. The two transient Jw experiments had very similar distributions of solute with depth, despite the contrasting initial conditions. The pores actively transporting solute appear to be determined by the Jw boundary condition.