NON-EXPONENTIAL COLLOID DISTRIBUTION PROFILES AS A RESULT OF RADIAL FLOW FIELD

Many laboratory column transport experiments show non-exponential decrease of colloid concentration with travel distance, in contrast to the exponential decay profiles predicted by colloid filtration theory. The non-exponential distribution profiles are often attributed to heterogeneity in colloid population and in collector grain surface, and have been modeled using a distribution in colloid deposition rate coefficients (i.e., log-normal, bimodal, and power-law distributions). Straining has also been proposed as the cause of non-exponential decay profiles. Such non-exponential distribution profiles, however, may be the result of very high local pore velocity and subsequent high deposition of colloids near the injection point, since pore velocity rapidly increases as radius decreases in a radial flow system, and colloid deposition increases as the pore velocity increases (to the power of 1/3) according to the colloid filtration theory. Our direct observation of microsphere transport through packed sand with an epi-fluorescence imaging system showed very high deposition of microspheres near the radial injection point. A particle model incorporating the radial flow field and velocity-dependence of deposition rate coefficient (K_f = av^1/3 where a is a constant and v is the velocity) was used to simulate colloid transport. The retained microsphere concentration from the particle model is approximately log-linear at longer distances, but increases sharply near the injection point, consistent with the profiles obtained via direct imaging.