An Analysis of Migration of Bio-Amended Water for In-Situ Remediation of Hexavalent Chromium Through the Vadose Zone

A method is presented to analyze migration of carbohydrate-amended water for in-situ remediation of hexavalent chromium, applied at the top of the vadose zone.  The analyses have been developed for specific application at a proposed treatability test site within the River Corridor of the Hanford site, Washington State, to support the design of bio-infiltration treatability tests.  The analyses are conducted to constrain the probable water saturation and vertical transport rates achieved in the vadose zone under a range of application rates of bio-amended groundwater.  The following questions are addressed as part of the analysis:

 

1. Assuming that water is applied at a rate of 80 gpm over an area of 0.5 acres:

a)      When will the amended water initially reach the water table?

b)      When will a specified number of pore volumes be flushed across the vadose zone?

      2. What infiltration rate can maintain partial saturation in the vadose zone while flushing three pore volumes in   approximately 90 days?

To address these questions, numerical simulations of variably‑saturated flow across the vadose zone were conducted with the VS2DT code (2004).  The van Genuchten (1980) constitutive relations are used, which relate the water content water content,q, to the capillary pressure, y, and the relative permeability with respect to water, k_rw, through empirical parameters n and a.

The simulation results are checked with a convergence analysis and with simplified analytical calculations. Sensitivity analyses suggest that the results are relatively insensitive to the van Genuchten parameters.  They also suggest that flushing time is relatively insensitive to saturated hydraulic conductivity, but is directly proportional to saturated porosity. 

The method described can provide guidance on the potential performance of a treatability test: however, monitoring during the start of operations will be particularly important for assessing the reliability of the underlying assumptions, and validating the modeling approach.