IN-SITU ALKALINITY STABILIZATION PILOT STUDY, UNC CHURCH ROCK SITE, GALLUP, NEW MEXICO

An in-situ alkalinity stabilization pilot study was conducted at the former United Nuclear Corporation (UNC) Church Rock Site in Gallup, NM. This site consists of a former uranium mill and tailings impoundments that are regulated under a Source Materials License with the USNRC. Groundwater corrective actions are being taken under the license and CERCLA. Alkalinity enriched groundwater was injected through 4 wells into the tailings seepage-impacted arkosic sandstone (Zone 3) of the Gallup Formation. The tailings seepage is a low pH (~2) solution with sulfate concentrations around 50,000 mg/ L. A set of 5 recovery wells was added to 4 hydraulic-control extraction wells on the perimeter of the well field and a central recovery well to form a 9-spot pattern, which built upon the 5-spot layout traditionally used by the in-situ leach (ISL) industries. The fixiviant was intended to neutralize the acidic water along a mixing front and ultimately displace the seepage-impacted groundwater. The increased pH associated with the injected alkaline groundwater was hypothesized to reduce migration and/or immobilize COCs via chemical precipitation and surface adsorption reactions. The injection and recovery rates were based on the hydraulic parameters determined in earlier studies. Chemical transformations in aqueous chemistry were inconclusive because the injection and extraction rates observed during the pilot study were lower the expected, and the total amount of cumulative extraction was considerably lower than calculated. As a result, the estimated travel time between the injection and extraction wells became prohibitive. SEM, XRF, and EDX data, and geochemical sampling and petrographic analysis support the interpretation that kaolinite and other amorphous alumino-silicates, generated by feldspar alteration, have altered the original sandstone properties and reduced the hydraulic conductivity of Zone 3. Based on these results, the use of alkalinity rich solutions to remediate the Zone 3 impacted groundwater in-situ is not feasible.