DOE OSTI2016
The Shiprock, New Mexico, Disposal Site is the location of a former uranium and vanadium ore-processing mill that operated from 1954 to 1968. During milling years, tailings leachate and other mill-related liquids migrated into underlying sediments, resulting in the contamination of groundwater beneath the river terrace containing the mill and a nearby flood plain alluvial aquifer adjacent to the San Juan River. The US DOE Office of Legacy Management manages the site and is implementing the current groundwater remedy for the flood plain area, consisting of natural flushing supplemented by extraction of alluvial groundwater to accelerate reduction in contaminant concentrations. The alluvial aquifer contains elevated concentrations of sulfate, uranium, and nitrate derived largely from the historical eastward migration of groundwater through terrace alluvium and underlying bedrock to the flood plain. The bedrock formation beneath the entire Shiprock site is Mancos shale, which forms an escarpment that separates the terrace from the flood plain. Contamination in the flood plain alluvial aquifer is influenced by a dynamic flow system that changes seasonally and from year to year. Simulations with a flood plain flow model based on monitored groundwater levels and coincident river-surface elevations show that background groundwater flow, unaffected by remediation pumping, is characterized by distinct hydraulic conditions for three separate periods each year. During months of low river flow in winter and early spring, river losses to the aquifer create a large hypo-rheic zone in the south half of the flood plain. Simultaneously, recharge of continually flowing surface water emptying onto the north half of the flood plain from Bob Lee Wash creates divergent, radial flow from the wash outlet and diverts northwestward-migrating contaminant plumes to the northeast where they discharge to the river. Subsequent bank storage processes in May and June from high snow melt runoff change background flow directions, increasing the spread of contamination in the alluvium. From mid-summer through fall, river flows are typically similar in value to those observed prior to the onset of snow melt, but groundwater elevations in the aquifer are noticeably lower than water levels observed in winter and early spring and may induce river losses to the aquifer. Several different contaminant sources have impacted the alluvial aquifer. Much of the contamination originated as tailings leachate and raffinate wastewater that infiltrated the terrace subsurface and eventually reached the flood plain at the escarpment. Additional sources of contamination in the aquifer included discharge of mill effluent to Bob Lee Wash and a pond at the base of the escarpment, and aqueous mobilization of solid-phase contamination in the form of windblown deposits and contaminants adsorbed to alluvial sediments. The degree to which remnant, secondary-source processes, such as leaching of solid-phase deposits and discharge of terrace groundwater across the escarpment, contribute to existing contaminant plumes is uncertain. The flood plain remediation system consists of two groundwater extraction wells, pumping from two horizontal wells placed in trenches near the base of the escarpment, a sump collecting discharges from seeps in the escarpment wall, and discharge of collected water to an evaporation pond on the terrace. Flow-model simulations indicate that most contaminated areas are captured by remediation pumping, resulting in several zones of freshwater between the river and the escarpment. Despite the highly transient nature of groundwater flow beneath the flood plain and the varied contaminant sources, initiation of remediation pumping in 2003 and coordinated operation of the system's various components have successfully removed large amounts of contaminant mass and helped prevent discharge of contaminated groundwater to the river. The percentage reductions of dissolved sulfate, uranium, and nitrate mass in the aquifer estimated for the first 9 years of remediation range from 40% to 72%. (authors)
12 MANAGEMENT OF RADIOACTIVE AND NON-RADIOACTIVE W↗