Chemical Controls on Colloid Generation and Transport in a Sandy Aquifer

Abstract

Colloid generation and transport in a highly weathered subsurface material from the Upper Coastal Plain (Aiken, SC) was controlled by factors other than the Na{sup +} concentration of the leaching solution. Re-packed columns were leached with solutions of various Na{sup +} to Ca{sup 2+} and Mg{sup 2+} ratios. Following injection of the treatment solution, the columns were leached with deionized water (DIW). For the mixed cation solutions prepared from Cl{sup -} salts, colloid generation occurred only when the treatment solution was replaced with DIW, but the level of effluent turbidity decreased with increasing Na{sup +} concentration and increasing duration of exposure to the high Na{sup +} solution. CaCl{sub 2} solutions produced substantial mobile colloids during injection that coincided with a decrease in effluent pH. The mobile colloids possessed a positive electrophoretic mobility, suggesting that the drop in pH during salt injection, thought to be the result of specific cation adsorption and Al exchange and hydrolysis, may enhance dispersion by increasing the positive surface charge on both the mobile colloids and the immobile matrix. These results suggest that even minor changes in groundwater composition can influence surface charge and colloid generation in an iron oxide-dominated system. 44 refs., 8 figs., 2more » tabs.« less