PORE SCALE VARIATION IN RETARDATION FACTOR AS A CAUSE OF NONIDEAL REACTIVE BREAKTHROUGH CURVES .3. COLUMN INVESTIGATIONS

A series of laboratory column experiments were conducted in a glass be ad medium with Cl-36 and Sr-85 as nonreactive and reactive tracers, re spectively. The primary purpose was to compare breakthrough curve (BTC ) properties under various conditions with those of the model which ac counts for pore scale variation in the retardation factor (R) caused b y pore scale heterogeneity. In particular, the purpose was to determin e whether pore scale heterogeneity contributes to the nonideality of r eactive Solute BTCs that cannot be described by the conventional local equilibrium assumption (LEA)-based advection-dispersion equation (ADE ) for a homogeneous medium. To eliminate other sources of nonideality, it was verified that the LEA was valid for the solute and porous medi um used in the study. The following BTC properties Were observed: (1) nonreactive BTCs dan be described by the ADE (ideal), whereas reactive BTCs are nonideal, (2) materials with larger pore size variation gave greater nonideality in the reactive BTC, (3) although nonreactive BTC s showed equally symmetric ideal shapes regardless of distance, reacti ve BTCs showed slight travel distance dependence in the degree of noni deality, and (4) the position and shape of dimensionless BTCs are inde pendent of mean fluid velocity. Calculated stochastic dispersivities w hich accounted for measured pore size variations were significantly la rger than those for; a nonreactive solute but were smaller. than the e ffective dispersivities actually observed for the reactive BTCs. From these findings, it is concluded that pore scale variation in R caused by pore scale heterogeneity in the: medium,is at least partially respo nsible for the nonideality observed for reactive solute transport in h omogeneous media. Pore scale variation in R gives larger asymptotic di spersivity and thus longer preasymptotic time and distance for reactiv e solute transport.