Synergistic effects of temporal variations in flow and chemistry on colloid retention and remobilization in saturated porous media

Rapid infiltration due to rainfall events, groundwater pumping, and artificial recharge of groundwater result in temporal variations in both flow and chemistry in the subsurface and may lead to remobilization of retained colloids, thereby re-contaminating the groundwater resources. Many researchers have studied the effects of temporal variations in water velocity and in water chemistry separately. These variations, however, almost always occur simultaneously. To the best knowledge of the authors, there are no studies reported in the literature where the effect of simultaneous temporal variations in flow velocity and ionic strength on colloid release is investigated. One cannot assume that those effects can be simply superimposed. In fact, in this study, we show that these effects are not simply additive. We have performed eight sets of saturated column experiments on the effects of temporal variations in flow velocity alone and simultaneous temporal variations in flow velocity and ionic strength on retention and remobilization of colloids. Temporal increases in average water velocity caused an instantaneous spike release of colloids in the measured breakthrough curve, with the time to peak coinciding with the instant of velocity increase. Further, the effect of temporal variations in average water velocity on remobilization of deposited colloids is ionic-strength dependent. The simultaneous temporal increases in flow and decreases in ionic strength caused a spike release of colloids followed by long tails. The release curve for every step-change in velocity and ionic strength was unimodal for deposition ionic strengths of 100 and 10 mM, with the peak coinciding with the instant of arrival of ionic strength front at the column outlet. However, the release curve was bimodal for deposition ionic strengths of 50 and 25 mM. There, the first instantaneous spike coincided with the instant of increase in flow velocity, and the second delayed one coincided with the passage of the ionic strength front. Temporal variations in both flow and ionic strength released more colloids in the effluent than when only ionic-strength or only flow velocity was varied. This is due to the combined effects of greater hydrodynamic torque exerted on attached particles together with smaller adhesive torque.