RECONCILIATION OF EXPRESSIONS FOR THE MODIFIED RETARDATION FACTOR ANDINCORPORATION OF NONLINEAR CONTAMINANT SORPTION TO THE STATIONARY-PHASE

The retardation factor describes the average velocity of a sorbing con taminant in groundwater relative to the average velocity of groundwate r. The modified retardation factor (R' or R) describes the average ve locity of a sorbing contaminant relative to groundwater in the presenc e of a carrier that binds the contaminant and facilitates its transpor t. Dissolved organic matter and particulate colloids are examples of p otential facilitating carriers. The modified retardation factor R' con cerns the case when sorption of the carrier itself to the stationary p hase is negligible, whereas R accounts for significant sorption of a facilitating carrier to the stationary phase, and so represents a magn itude less than R, but greater than R'. A well-cited expression for R is shown to differ in form and behavior relative to a lesser known ex pression for R, and the bounds of applicability of these expressions are examined. The lesser-known expression for R is shown to be more b roadly applicable in its ability to describe situations where carrier sorption to the stationary phase is significant, and even rivals or ex ceeds that of the contaminant. The well-known expression for R is sho wn to be not applicable to situations involving significant sorption o f the carrier. The derivation of the expression for R presented in th is paper is given by two separate methods used to develop the previous ly published expressions, in order to relate the presented and previou sly published expression to each other. The expression is extended to include non-linear contaminant sorption to the stationary phase, and p redictions from this expression are compared to a set of experimental data. (C) 1998 Elsevier Science B.V. All rights reserved.