Geochemical effects on colloid-facilitated metal transport through zeolitized tuffs from the Nevada Test Site

Natural colloids were generated from zeolitized tuffs from the Nevada Test Site (NTS) and the effects of colloids on the transport of a strongly sorbing metal, lead [Pb(II)], were investigated in column experiments under different geochemical conditions. Because of the high sorption affinity of Pb(II) for zeolitized tuffs, the migration of Pb(II) without colloids was strongly retarded. The presence of mobile colloids, however, enhanced the mobility of Pb(II). Approximately 75-90% of the eluted Pb(II) was transported as a colloid-associated phase. The migration of colloids was closely related to the geochemical conditions in the background solution. Immobilization of colloids increased as the ionic strength of the background electrolyte solution increased, because of double layer compaction. Remobilization of initially deposited colloids occurred when the ionic strength of the background electrolyte solution was reduced. The mobility of colloids increased with increasing pH and increasing flow rates. These results have significant implications for the migration of strongly sorbing radionuclides and other metals at nuclear testing facilities and metal-contaminated sites and are consistent with the hypothesis that inorganic contaminant migration in the subsurface is a function of geochemical conditions.