Findings Related to the Geochemistry of Groundwater at a Former in-Situ Uranium Mine in Live Oak County, Texas: Evidence of Natural Attenuation and the Potential for Accelerated Groundwater Restoration By the Use of Reductants.

Geosyntec Consultants was retained by Signal Equities, LLC to conduct a hydrogeochemical characterization of groundwater at a close in situ uranium recovery mine at Signal’s Boots/Brown property in Live Oak County, Texas.  The investigation included developing a groundwater samping and analysis program for on-site and off-site wells and evaluating the groundwater analytical data to assess the differences, if any, in the geochemistry of groundwater from on-site wells and off-site wells and to evaluate whether there were any detectable changes in groundwater chemistry indicative of natural attenuation.  The first objective was to ascertain how chemical signatures now compare with those approved by the Texas Commission on Environmental Quality – Office of Underground Injection Control (UIC) in 1998 for restoration of the site and with hydrochemical fingerprints for other Live Oak County wells that produce groundwater from the Oakville Sandstone.

A second objective was to consider uranium concentrations, along with measurements of pH and oxidation-reduction potential (ORP), as a basis for determining whether natural attenuation through re-establishment of reducing conditions might be an effective means of reducing uranium concentrations in groundwater after the cessation of mining operations.  A second part of that objective was to evaluate radium activities, compared with restoration target values, to ascertain whether sorption might account for current radium activities.

The geochemical data illustrate that the proposed original and amended restoration target values for major metals and nonmetals lie within the range of hydrochemical fingerprints representative of the project site.  The data also indicate that the signatures are consistent with Oakville wells from other areas of Live Oak County.  The conclusion to be drawn from this assessment is that in situ recovery has not caused groundwater to have a major-ion fingerprint that is abnormal with respect to Oakville groundwater in other areas of the county.

 The investigation illustrates that uranium concentrations are below the maximum contaminant level prescribed by the USEPA and far below the original and amended restoration target values approved by UIC.  ORP (recalculated as Eh) and pH data illustrate that reducing conditions have been re-established since the cessation of groundwater restoration operations.  It is highly probable, there, that the low uranium concentrations in groundwater samples are related to re-establishment of reducing conditions within that part of the Oakville Sandstone.

Only one well yielded samples of groundwater with radium activity greater than the 5 pCi/L MCL.  The restoration target value in 1998 was 150p/Ci/L.  Because radium is not a redox-sensitive element, the factor that most easily accounts for the reduction in activity is sorption of radium to surfaces of minerals that make up the matrix and the confining layer of the Oakville Sandstone.

The results of this investigation warrant consideration by UIC of the role of natural attenuation in the restoration of groundwater at in situ recovery mines and the use of reductants to counter-act oxidizing agents used during mine development.