Colloid suspension stability and transport through unsaturated porous media

Abstract

Executive SummaryContaminant transport is traditionally modeled in a two-phase system: a mobile aqueous phase and an immobile solid phase. Over the last 15 years, there has been an increasing awareness of a third, mobile solid phase. This mobile solid phase, or mobile colloids, are organic or inorganic submicron-sized particles that move with groundwater flow. When colloids are present, the net effect on radionuclide transport is that radionuclides can move faster through the system. It is not known whether mobile colloids exist in the subsurface environment of the Hanford Site. Furthermore, it is not known if mobile colloids would likely exist in a plume emanating from a Low Level Waste (LLW) disposal site. No attempt was made in this study to ascertain whether colloids would form. Instead, experiments and calculations were conducted to evaluate the likelihood that colloids, if formed, would remain in suspension and move through saturated and unsaturated sediments.The objectives of this study were to evaluate three aspects of colloid-facilitated transport of radionuclides as they specifically relate to the LLW Performance Assessment. These objectives were: 1) determine if the chemical conditions likely to exist in the near and far field of the proposed disposal site are prone to induce flocculation (settling of colloids from suspension) or dispersion of naturally occurring Hanford colloids, 2) identify the important mechanisms likely involved in the removal of colloids from a Hanford sediment, and 3) determine if colloids can move through unsaturated porous media.Mobile colloid formation is commonly described as a three-step process: genesis, stabilization, and transport. Colloid genesis describes how the micron-sized particles are formed in groundwater. Stabilization describes how the colloids are brought into suspension, which is a function of the colloid and groundwater chemical composition and water flow (kinetic) forces. Transport describes how the suspended colloid move through the porous media or are retained by the porous media by physical forces (such as diffusion, straining, or gravitation settling) or physicochemical attraction of the colloid to the matrix.As the second step in mobile colloid formation indicates, colloids must remain in suspension, i.e., not flocculate, in order to be transported with groundwater flow. A series of batch experiments was conducted to identify the chemical conditions where dispersion and flocculation (settling out of suspension) of Hanford sediment colloids (<lo00 nm) would occur. These experiments determined the critical flocculation concentration (CFC), the minimum electrolyte concentration to induce colloids to flocculat from suspension. The CFC values were calculated for a range of sodium concentrations and pH values that may exist in the near and far fields. These two groundwater parameters are considered to be among the most important parameters affecting colloid suspension stability. The experiments suggest that natural Hanford groundwaters are not likely to main...

Radionuclide Distribution Coefficients of Sediments Collected from Borehole 299-E17-21: Final Report for Subtask 1a

“…These experiments have investigated the effect of the degree of moisture saturation (Kaplan et al 1996, Gamerdinger et ai. 1998), pH (Kaplan et al l996,1998a, 1998b, mobile colloids (McGraw andKaplan 1997, Lee et al 1998), ionic strength (Kaplan et al 1995(Kaplan et al , 1998b, radionuclide concentration (Kaplan et al 1996), and sediment properties on the geochemical behavior of key radionuclides (Parker 1997). All these parameters are known to affect sorption of solutes, but the degree to which they do so in Hanford Site sediments, or more specifically in ILAW-DC sediments, is not known.…”

DNA Profile Analysis and Interpretation

“…Application of polyphosphate technology to vadose zone and capillary fringe environments for remediation requires understanding the effects of water content and pore water velocity on polyphosphate mobility, both of which have been previously shown to be significant influences on the mobility of reactive species within the subsurface Gamerdinger et al 1998;Gamerdinger et al 2001b;a;Lindenmeier et al 1995;McGraw 1996;McGraw and Kaplan 1997;Wellman et al 2008a). The only identified investigation of phosphate mobility under hydraulically unsaturated conditions indicated that phosphate application to sediments increased the water retention properties of the sediments as a result of the increase in the negative charge of the soil particles (Lutz et al 1966).…”

Geochemical Processes Data Package for the Vadose Zone in the Single-Shell Tank Waste Management Areas at the Hanford Site

“…In a study by McGraw and Kaplan (1997), it was demonstrated that colloids are unlikely to remain suspended in glass leachate, cement leachate or Hanford groundwater because the ionic strength of these solutions greatly exceeded the critical flocculation concentration (the minimum electrolyte concentration to induce colloid flocculation) of most particles.…”