Retardation of Colloidal Actinides Through Filtration in Intrusion Borehole Backfill at the Waste Isolation Pilot Plant (WIPP)

Abstract

A depth filtration model was used to evaluate filtration of four types of colloids (mineral fragments, humics, microbes, and mature actinide intrinsic colloids) and their agglomerates by borehole backfill material in the event of inadvertent human intrusion to the Waste Isolation Pilot Plant (WIPP). The WIPP is a proposed repository sited in bedded salt for transuranic wastes generated under our nation’s defense programs. Under a human intrusion scenario involving two or more boreholes, flow from an underlying brine reservoir could potentially result in the migration of colloids from the repository up a borehole and then outward to the overlying Culebra Dolomite aquifer. However, in the performance assessment of the WIPP it is assumed that any intrusive borehole would be backfilled immediately after the infringement. The borehole filler material is assumed to have the hydraulic properties of either degraded concrete or grout, or silty sand with a maximum permeability of 10−11 m2 (worse case scenario). Mechanisms of filtration were modeled by trajectory analysis with borehole particles regarded as collectors. The dominant filtration mechanisms were diffusion and interception. The collision efficiency (α) of the colloid particles to the collector grains presented the greatest uncertainty for the filtration modeling. Extensive review of the literature indicated that the colloidal particles displaying the lowest collision efficiencies are microbes and particles stabilized by humic substances (0.1 to 0.01 and approximately 0.001, respectively). These collision efficiency values are based upon low ionic strength water; collision efficiencies have been shown to increase, by orders of magnitude in some cases, upon increasing ionic strength. Conservative α values ranging from 10−2 to 10−3 were used in our model calculations. The model predicts that most of the entrained colloids and colloid agglomerates will be filtered out by the borehole filling within a few meters (or fractions of meters) of brine flow distance. Particles displaying the least efficient filtration (e.g., particles between 1 and 5 μm) would have their concentrations reduced by about an order of magnitude over the travel distance (395 m) between the repository and borehole interface with the Culebra.