Describing the Transport of Ion‐Exchanging Contaminants Using an Effective K_d Approach

Hydrogeologists have traditionally used distribution coefficients K_d to describe the rate of migration of sorbing contaminants. Recent investigations have questioned the validity of the K_d approach for contaminants governed by multicomponent ion exchange and have noted that computed K_d values in such cases are not constant across an advancing solute front. This report demonstrates that mass balance arguments can nevertheless be utilized to define a constant “effective” K_d which may then be used to calculate the contaminant front velocity. For selected cases of one‐dimensional binary and ternary exchange, front locations calculated using the effective K_d approach agree closely with concentration profiles calculated using a general numerical simulation model of ion‐exchanging solute transport. The effective K_d approach is also applied to a field problem involving the injection of advanced‐treated municipal effluent into a shallow confined aquifer. Although the effective K_d approach is sometimes a practically useful tool, it is shown to be limited to situations where dispersion can be neglected.