Formatted Title

Quantifying Contaminant Release Rates from Secondary Sources at a Uranium Disposal Cell

Background/Objectives

The Shiprock, New Mexico, Disposal Site is a former uranium- and vanadium-ore processing facility within the Navajo Nation managed by the U.S. Department of Energy Office of Legacy Management. The mill operated from 1954 through 1968, when underlying groundwater was contaminated with uranium, among other primary and secondary contaminants. In 1986, an engineered disposal cell was constructed on top of the existing tailings impoundment for stabilization and reclamation of surface contamination. The disposal cell is located on an alluvial terrace that is adjacent and approximately 20 meters above a floodplain of the San Juan River. Uranium contamination occurs in the water and solid phases on both the terrace and the floodplain.

Evaluating groundwater remedial design scenarios requires quantitative estimates of existing contaminant mass distribution and removal rates. Although the tailings impoundment was remediated using an engineered cover system, secondary solid-phase uranium contamination is prevalent above and below the water table, along with an ongoing uranium plume. Reactions between the uranium plume and the solid phase have created secondary sources that need to be considered when evaluated remedial designs. This study focuses on the spatial distribution of solid-phase uranium and estimating kinetic release rates for the development of a hydrogeochemical conceptual model to provide input parameters for a site reactive transport model for use in evaluated remedial designs.

Approach/Activities

The vertical and horizontal distribution of uranium bound to soil and bedrock above and below the water table was done by collecting representative 2-ft intervals adjacent to the disposal cell, along proposed contaminant groundwater pathways, and in the floodplain. Approximately 550 intervals were collected and measured from 36 borings to develop a three-dimensional visualization of solid-phase uranium across the site. These results, supported by current understanding of the contaminant transport pathways, were used to select discrete depth intervals for column tests. Column studies were designed to simulate kinetically-controlled uranium release and transport under representative site conditions. Influent water included site groundwater, deionized water, and San Juan River water to simulate changes in hydrological conditions and water/rock interactions.

Results/Lessons Learned

In the alluvium, solid-phase uranium is generally associated with iron-rich, organic-rich, and clay-rich interbedded layers at and above the alluvium-bedrock contact. Column tests results show that uranium release from (de)sorption reactions with the greatest influence from groundwater chemistry, particularly alkalinity. The higher alkalinity in influent terrace groundwater promotes quick uranium release, suggesting uranium transport to the downgradient floodplain, while floodplain columns show slower, steady desorption release rates. The variability in uranium release rates between the terrace and floodplain will have direct implications in reactive transport modeling and remedial design solutions.