Tailings and Mine Waste 2001

A. A. Balkema Publishers

• Publisher: CRC Press
• Year: 2022
• Print ISBN:
  9789058091826
• eBook ISBN:
  9781000151138
• Topic: Technology & Engineering
• Subtopic: Civil Engineering
• Index: Technology & Engineering

These papers focus on mine and mill tillings and mine waste. The work also contains information on subjects related to: regulations, technical capacities and developments. This guide identifies the current and future issues facing the mining and enviromental concerns.

Hydrology and geochemistry

Groundwater characterization and alternative evaluation for the Split Rock uranium mill tailings project

DOI:10.1201/9781003078708-25

L.(Toby) Wright

Shepherd Miller Incorporated, Fort Collins, Colo., USA

ABSTRACT: This paper presents a case study of the groundwater characterization and development of ground water corrective action alternatives for the Western Nuclear, Inc. Split Rock Site in central Wyoming. A comprehensive multi-year site characterization was performed that included geologic, hydrologic and geochemical studies. These data were integrated with historical site data into a revised site conceptual model that provided the basis for groundwater flow and transport models. A broad range of technologies were screened and combined to develop four potential site-wide alternatives to provide the requisite protection of public health and safety and the environment. A preferred plan was selected based on the evaluation of these alternatives on their respective benefits and costs and using the flow and transport models.

1 INTRODUCTION

This paper presents a brief case study of the groundwater characterization and evaluation of groundwater corrective action alternatives for the Western Nuclear Inc. (WNI) Split Rock Site in Central Wyoming.

Western Nuclear Inc. processed uranium ores at the Split Rock Site from 1957 through 1981. During this period, the facility processed a total of approximately 7.7 million tons of ore with mill tailings from the process deposited into several contiguous areas that ultimately covered a total of approximately 180 acres. Following a decline in the world uranium market, the mill went on standby status in 1981 and moved to reclamation status in 1986.

The Nuclear Regulatory Commission (NRC) regulates uranium mill tailings in the United States. Reclamation of tailing sites is based on site specific conditions with general requirements to provide protection of human health and the environment for 1,000 years. There are three elements of reclaiming at uranium mill tailing sites:

1. Mill decommissioning and remediation of contaminated soils,
2. Surface reclamation, and
3. Groundwater

Mill decommissioning at the Split Rock site occurred in 1989 and consisted of dismantling the mill and burying the contaminated materials in the tailings impoundment. The mill decommissioning completion was approved by the NRC in 1990. Surface reclamation and remediation of contaminated soils consisted of identifying soils that were contaminated in the mill area and the soils near the tailings impoundment that were impacted by the wind-blown tailings. The contaminated soils were excavated and placed in the tailings impoundment. The NRC approved the soil clean up in 1999.

2 GROUNDWATER

Historically, the NRC considered groundwater reclamation activities independently of the mill decommissioning and surface reclamation activities. Groundwater monitoring has occurred on site since the 1960s. Changing regulations caused the monitoring program to expand from a few wells and a few constituents to the current monitoring program, which consists of over 30 wells that are monitored for 16 constituents.

In 1987, WNI reported to the NRC that monitoring wells showed groundwater quality concentrations near the tailings had increased and had become statistically greater than background concentrations. This initiated a process that resulted in the construction of a groundwater Corrective Action Program (CAP). This groundwater CAP, which began operation in 1990, called for pumping groundwater from wells in the Northwest and Southwest Valley to lined ponds constructed in the Southwest Valley (see Figure 1). These waters were subsequently spray-evaporated over the un-reclaimed tailings.

As surface reclamation of the tailings progressed, the surface area available for spray evaporation decreased. As a result of the decreasing area available for spray evaporation, the total volume of ground water pumped annually decreased. Subsequent pumping was reduced to the point where evaporation could be achieved using just the lined CAP ponds. The decreased volume being pumped was approved by the NRC and authorized by subsequent amendments to WNF’s license. At the present time, the NRC mandated that CAP continue to operate with 6 to 15 million gallons of ground water being pumped and evaporated annually.

3 GROUNDWATER CHARACTERIZATION

In order to complete site reclamation, groundwater corrective action efforts had to be complete. However, the existing site groundwater conceptual model was not complete and existing corrective action efforts had not proved to be effective in returning groundwater concentrations to background or drinking water standard levels. Therefore, in 1995 WNI initiated a comprehensive plan to characterize the hydrologic and geochemical aspects of the site groundwater system. The first step of this characterization consisted of the development of a comprehensive scoping and quality assurance document. This document was developed to provide comprehensive strategic direction and quality objectives for the project to maximize the efficiency and effectiveness of the characterization effort. The characterization consisted of more than 100 borings, installation of more than 100 wells and more than 100 drive point piezometers, and analysis of 4,700 water samples and 245 solids samples.

Figure 1. Water quality sampling locations

4 CONCEPTUAL SITE MODEL

The results of the characterization studies, along with a review of the historic conditions, led to the development of a conceptual site model that incorporated both the hydrologic and geochemical conditions at the site.

5 HYDROLOGY

The groundwater and transport of site-derived constituents occurs primarily in the Split Rock Formation aquifer, and the Sweetwater River Alluvium, also called the floodplain alluvial aquifer. The floodplain alluvial aquifer is a shallow aquifer (15 to 30 feet thick) of river sediments tha...

Table of contents

• Cover Page
• Half Title Page
• Title Page
• Copyright Page
• Table of Contents
• Preface
• Site characterization
• Design, operation, and disposal
• Geotechnical considerations
• Liners, covers and barriers
• Hydrology and geochemistry
• Remediation and reclamation
• Reprocessing, utilization, and treatment
• New technologies and approaches
• Case histories
• Author index