Henry’s Constant, Darcy’s Law, and Contaminant Loss

A six-channel system was used to determine how differences of Henry’s law constant and vertical water velocity affect the diffusive loss of contaminants from a sandy soil. Two channels (10 cm wide x 110 cm long x 65 cm deep) were each fed with methyl tert-butyl ether, dichloromethane, or trichloroethylene in groundwater. After steady state was attained, vertical concentration gradients were measured in unplanted (inlet end) and planted (outlet end) halves of each channel. An analytical model was used to describe contaminant movement in soil of different porosity and fractional water saturation. Changing the two parameters for Henry’s law constant (different compounds) and vertical Darcy water velocity (planted vs unplanted end) provided a good qualitative fit to the different observed distributions. Calculated profiles were also sensitive to soil porosity and fractional water saturation. The chosen compounds span much of the observed range of Henry’s law constants for volatile compounds (0.02 to 0.4) and the measured Darcy velocities represent most of those likely to be encountered in natural settings for unsaturated soils. Thus we can use the model to predict volatilization losses for a wide range of compounds, if they are being released from soil water.