Groundwater Chemistry of the Grover Uranium In-Situ Recovery Pilot Test Site, Weld County, Colorado

The Grover pilot project is the only in-situ recovery (ISR) uranium operation completed in Colorado. Discovered in 1970, uraniferous sandstones within the Cretaceous Laramie Formation are capped and underlain by black carbonaceous claystones, creating a confined aquifer favorable to ISR. A pilot mining, restoration, and monitoring project operated from June 1977 to June 1980 by Wyoming Minerals Corporation (1982). Local water wells were sampled for one month prior to mining to provide baseline groundwater chemistry. Two wellfields were constructed, with 21 production holes in five-spot patterns along with 12 monitoring wells. Uranium was then leached for 16 months using an alkali leach lixivant ((NH₄, Na, Ca)HCO₃ + H₂O₂). After mining, a program aimed at restoring groundwater was conducted in three stages: 1) treatment with reverse osmosis (RO) to reduce total dissolved solids for seven weeks, 2) enrichment with CaCl₂ to reduce ammonium for nine weeks , and 3) treatment with RO for six months. Post-restoration groundwater chemistry was monitored for 15 months. Pre-mining baseline groundwater at Grover exceeded six maximum contaminant levels for EPA primary and secondary drinking water standards (Al, Pb, gross alpha, Fe, ²²⁶Ra, and U), precluding it as a domestic water supply. Post-restoration groundwater exceeded pre-mining baseline samples for 17 of 41 measured variables and exceeded EPA drinking-water standards for gross alpha and ²²⁶Ra. Ten of 14 analytes sampled quarterly over 15 months (alkalinity, ammonia, Ca, conductivity, gross beta, Mg, ²²⁶Ra, total dissolved solids, total hardness and U) increased in concentration above post-restoration levels. This trend indicates that chemical equilibrium had not been reached between groundwater and aquifer solid phases by the end of the monitoring period.

Reference: Wyoming Mineral Corporation, 1982, Grover Uranium Solution Mining Test Site, Technical Support for Subsurface Disposal System Permit Application: Colorado Dept. of Public Health and Environment, 302 p.