Modeling of Microbially Mediated Ureolytic Calcite Precipitation for the Remediation of Sr-90 Using a Variable Velocity Streamtube Ensemble

Biogeochemical modeling using PHREEQC2 and a streamtube ensemble approach is utilized to understand a well-to-well subsurface treatment system at the Vadose Zone Research Park near Idaho Falls, Idaho.  Treatment involves in situ microbially-mediated ureolysis to induce calcite precipitation for the immobilization of strontium-90.  PHREEQC2 is utilized to model kinetically-controlled ureolysis and consequent calcite precipitation. Reaction kinetics, equilibrium phases, and cation exchange are used within PHREEQC2 to track pH, calcium, ammonium, urea, and calcite precipitation over time within a series of one-dimensional advective-dispersive transport paths which represent well-to-well transport.  Understanding the impact of physical heterogeneities within radial flowfields is critical for remediation design; we address this via the streamtube approach: instead of depicting spatial extents of solutes, we focus on their arrival distribution at the control well(s).  Traditionally, each streamtube maintains uniform velocity; however, in radial flow in homogeneous media the velocity within a streamtube is spatially-variable: highest at the wells and a minimum at the midpoint between the wells.  Streamtube velocity patterns for any particular configuration of injection and withdrawal wells are available as explicit calculations from potential theory and from particle tracking programs.  This velocity variability is of significance in the case of ureolytically driven calcite precipitation; the actual spatial distribution of velocity along streamtubes is approximated by assuming idealized radial non-uniform velocity associated with homogeneous media.  This is implemented in PHREEQC2 via a non-uniform spatial discretization within each streamtube that honors the streamtube’s travel time and the “fast-slow-fast” velocity along the streamline.  Breakthrough curves from each simulation are weighted by path-respective flux fractions (obtained by tracer test deconvolution) to obtain the flux-average of flow contributions to the observation well.  This multiple streamtube approach is cast in alternative computational platforms for cross-validation and is also part of an interdisciplinary effort to evaluate soil matrix strengthening via calcite precipitation.