CHEMICAL CONTROLS ON COLLOID GENERATION AND TRANSPORT IN A SANDY AQUIFER

Colloid generation and transport in a highly weathered subsurface mate rial from the Upper Coastal Plain (Aiken, SC) was controlled by factor s other than the Na+ concentration of the leaching solution. Repacked columns were leached with solutions of various Nai to Ca2+ and Mg2+ ra tios. Following injection of the treatment solution, the columns were leached with deionized water (DIW). For the mixed cation solutions pre pared from Cl- salts, colloid generation occurred only when the treatm ent solution was replaced with DIW, but the level of effluent turbidit y decreased with increasing Naf concentration and increasing duration of exposure to the high Na+ solution. CaCl2 solutions produced substan tial mobile colloids during injection that coincided with a decrease i n effluent pH. The mobile colloids possessed a positive electrophoreti c mobility, suggesting that the drop in pH during salt injection, thou ght 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 res ults suggest that even minor changes in groundwater composition can in fluence surface charge and colloid generation in an iron oxide-dominat ed system.