A TOOL FOR EVALUATING A NEED FOR VARIABLE SELECTIVITIES IN CATION-TRANSPORT IN SOIL

The empirical Rothmund-Kornfeld equation was used to describe a collec tion of cation exchange isotherms for K and Ca in Brucedale soil over a range of total solution concentrations (C(T)). A comparison of varia ble binary exchange selectivity coefficients (K(KCa)) which vary with C(T) and solution phase ion concentrations (C(K), C(Ca)) With constant K(KCa) values was thus possible. Percent differences between sorbed e quilibrium concentrations calculated using variable and constant K(KCa ) over a range of C(T) and C(K) were arranged into a contour map. Zone s of potentially observable difference were limited, implying that sim ulations of cation transport using variable and constant K(KCa) values would be essentially identical. The predicted effect was examined usi ng a numerical model for cation transport. The model successfully desc ribed observed spatial distributions for cation concentrations followi ng constant-flux infiltration of electrolyte solution into soil column s. Simulations using variable K(KCa) values provided only a slight imp rovement over simulations using constant K(KCa) values, thus validatin g the use of the contour map as a convenient method to evaluate the ne ed to use variable selectivities in simulations of cation transport.