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GROUNDWATER RESTORATION LONG BEYOND CLOSURE AT THE
HOMESTAKE-MILAN AND UNITED NUCLEAR-CHURCH ROCK
URANIUM MILL TAILINGS PILES, NEW MEXICO, USA:
FULL-SCALE PROGRAMS REQUIRING MORE THAN
20 YEARS OF ACTIVE TREATMENT

In: "Proceedings of Conference on Uranium Mining and Hydrogeology - II", Claudia Helling, et. al. editors, Technical University -Freiberg, Saxony, Germany, September, 1998

Wm. Paul Robinson(1)

(1) Wm. Paul Robinson is Research Director, Southwest Research and Information Center, P. O. Box 4524, Albuquerque, NM 87106

Abstract

Since as early as 1975, groundwater contamination from New Mexico uranium mill tailings has been investigated with two sites - Homestake-Milan and United Nuclear- Church Rock - showing severe enough groundwater damage to merit listing on the US Environmental Protection Agency's (EPA) Superfund National Priority List - a nationwide list based on severity of pollution and water resource usefulness. These two sites provide valuable case studies for the first - 1950s - and second - 1970s - generations of uranium mill tailings facilities demonstrating the severity of contamination which ineffective control can allow and the challenge of full scale groundwater restoration. While the groundwater restoration at these sites began in the 1970s and 1980s, active treatment is anticipated into the 21st century.

This paper summarizes the groundwater restoration programs at two of these sites - Homestake Mining Company's (HMC) Milan Mill (now called the "Grants Project") and United Nuclear Corporation's (UNC) Church Rock Mill. The two sites are summarized with respect to operations, groundwater impact, tailings disposal systems, hydrogeological characteristics of the site and affected areas, applicable standards, and remedial technology applied. This review provides a basis for initial comparisons with uranium mill tailings groundwater restoration challenges outside the USA.

These sites provide an important benchmark the complexity of restoration at for large-scale uranium mill tailings sites. The longevity of the restoration efforts demonstrate the results of low-intensity responses to contamination upon detection and delayed enforcement actions. These "witnesses" to the value of effective pollution prevention in tailings design and full review and monitoring of tailings operations, have potential to be models of effective groundwater restoration.

Introduction

Home to 45% of USA uranium production, New Mexico and its resident uranium producers established seven uranium mills and tailings piles, each with a extensive pattern of ground water contamination. Two of the site, HMC's Milan Mill - now called the "Grants Project" - and UNC's - Church Rock Mill, have been included on the USEPA's Superfund National Priority List - a nationwide list of sites ranked on the basis of severity of pollution and usefulness of affected water resources - are the focus of this paper.

Both sites are subject to groundwater restoration requirements of New Mexico Water Quality Control Commission (NMWQCC) groundwater regulations, (USNRC) uranium mill tailings reclamation criteria - 10CFR20 Appendix A - and USEPA's "Superfund" Program. These groundwater restoration programs are in addition to site-wide surface remediation, including tailings pile regrading, stabilization and capping programs required by the USNRC under authority of the 1978 US Uranium Mill Tailings Radiation Control Act (UMTRCA) as active sites - Title II. As "active sites" in operation when UMTRCA became law, the Grants Project and Church Rock facility's costs of compliance USNRC, EPA and NMWQCC requirements are borne by the site owner-operator, rather than USA and State-government funded reclamation as with "inactive - Title I - sites".

Groundwater investigations at both sites detected groundwater contamination in the late 1970's. EPA investigations near the Homestake-Grants site demonstrated dramatically increased levels of uranium, radium, chloride, molybdenum, nitrate and selenium. These 1975 analyses of ground water in residential drinking water wells downgradient of the mill showed selenium concentrations up to 3.42 mg/l - more than 300 times the maximum recommended for drinking water. Extensive groundwater contamination at the UNC-Church Rock tailings was first detected in 1979, soon after the mill reopened following repair of 10 meter breach in the tailings dam. Significant levels of chloride, sulfate, nitrate, radium and thorium were among the many elevated constituents detected in either alluvial or bedrock aquifer systems.

Both sites continue to undergo groundwater restoration long after demolition of the mill facilities and closure of surface reclamation of the tailings piles. Progress to date indicates a requirements for further groundwater treatment into the 21st century.

Site Overviews

The HMC-Grants operation processed 3,500 ton of uranium ore per day in an alkaline leach mill from 1958 - 1990, producing two tailings piles, a 1,200,000 ton "inactive pile" and a 21,000,000 ton active pile, both which were unlined. The large tailings pile was constructed using a "ring-dike" system, with a small starter "ring" raised with cyclone-separated tailings continuously over the 32 year life of the operation to create a 30 meter tall, 100 hectare pile. The tailings liquids were confined within the rectangular "ring dike" of tailings on top of the pile, substantially increasing the hydrostatic force carrying seepage through the unlined pile into the underlying alluvial aquifer. Restoration at this third largest uranium mill tailing sites in the USA has included extraction of contaminated fluid at the downgradient end of the plume and injection of clean water between the pile and the plume margin to create an artificial ground water mound. Alternative water supplies have been provided to residents of the affected downgradient residential communities for more than 15 years.

The UNC-Church Rock site contains approximately 3,600,000 tons of tailings in a 40 hectare site, produced by a 3,000 ton per day acid leach mill operated from 1977-1982. While extensive off-site contamination from a 400 million liter spill through a 1979 breach in the tailings dam, seepage through the unlined tailings containment cells into underlying alluvial and bedrock groundwater systems was identified by NMED investigations following restart of the mill the same year.

An extraction system has been in place at Church Rock since 1989, with produced liquids initially returned to the unlined tailings site, maintaining hydrostatic force driving seepage into underlying strata as was the case the Grants Project until 1992. Extraction systems are continuing in three areas and USNRC is considering a UNC application for "Alternative Concentration Limits" - less stringent that the 10CFR20 Appendix A criteria - based on concentrations of contaminants in background groundwater samples and reduced extraction volumes - in lieu of full restoration.

The Groundwater Restoration Program at HMC-Grants

HMC-Grants operates its groundwater restoration program under authority of the NMWQCC groundwater regulations and USNRC's 10CFR20 Appendix A criteria. The permits under these parallel programs, are NMED Discharge Plan DP-200, and NRC License SUA-1471. HMC began its restoration activities in 1977 and anticipates their completion in 2010. HMC describes its long-term goals as restoration of water quality in underlying aquifers as close as possible to up-stream background concentrations.

The restoration program includes an extensive array of collection ("extraction") and injection well lines, collections drains and trenches in and around the largely reclaimed tailings piles and lined evaporation ponds as shown on Figure 1 to this paper. The collection wells and trenches are designed to remove contaminated ground water by drainage or active pumping and the injection wells are designed to flush the alluvial aquifer with clean water from deeper wells. Collection wells have also been installed north - up-gradient - of the tailings ponds to remove alluvial ground water before it flows into the contaminated portions of the aquifer underneath the tailings.

Contaminants have affected two aquifer systems at the site: 1) an alluvial valley fill aquifer and 2) a bedrock aquifer system in the Chinle formation of Permian age composed of three separate aquifer units. The alluvial system extends under the tailings piles at the site and extends north of the tailings as well as south of the tailings and under a residential area composed of several housing subdivisions. The alluvial valley fill encompasses the floodplains of the San Mateo and Lobo Creek which are tributary to the Rio San Jose and eventually the Rio Grande. The Chinle aquifers, which sub-crops beneath the alluvial aquifer shows tailings-derived contamination in the Upper and Middle Chinle units, but are not addressed in this paper.

Figure 1 shows the array of collection ("o") and injection ("x") wells at the site as of 1997, as well as a system of shallow collection trenches - "toe drains" - around the tailings, drainage wells bored into the tailings -"tailings wells", lined evaporation ponds and other features at the site.

The injection well systems - designated as the "g-line", "m-line", "x-line" and "j-line" of wells on Figure 1 - were begun in 1977. Water injections into these wells have produced a groundwater "mound", reversing the piezometric surface in the alluvial aquifer back towards the collection wells. Injection rates for the "j-line" averaged 171 gallons per minute (gpm) in 1997 and injection rates for the "m-line" and associated wells averaged 295 gpm in 1997.

The collection well systems include a series of well lines completed in the alluvial aquifer - designated as "s-line", "k-line", "l-line", "c-line" and "d-line" - the construction of which began in 1978. Collection rates averaging 200-300 gpm have been attained from these wells during 1981-1997. Total groundwater collections from these systems totaled 2,654,890,324 gallons for the period 1978-1997. Table 1 provides a summary of the quantity and quality of ground water extracted by collection wells, tailings toe drains and tailings wells removed from by the HMC Grants site groundwater restoration program.

TABLE 1 - Quality and Quantity of Constituents Collected 1978-1997 by Homestake Grants Project Groundwater Restoration Program

Water
Source
Total
Volume
(hund
-reds)
Sulfate Uranium Molybdenum Selenium
mg/l lb. (mill-
ions)
mg/l lb. (thous-
ands)
mg/l lb. (thous-
ands)
mg/l lb. (hun-
dreds)
*G'water
in
1978
only
2.76 5200 1.2 35.0 8.1 40.0 9.2 2.0 4.6
*G'water
in
1997
only
9.4 4955 3.8 27.0 21.0 33.4 25.2 3.2 24.6
*Total 90.4 11094 8.8 41.8 34.5 100.0 77.7 0.8 12.3
Toe
Drains
  (1997) (1997) (1997) (1997)
*Total 36.4 10284 2.9 45.8 12.9 92.4 29.0 0.1 0.5
Tailing
Wells
  (1997) (1997) (1997) (1997)
*Total 2,654.9 4955 110.7 27 714.5 33.4 838.5 3.2 434.7
G'water   (1997) (1997) (1997) (1997)

The HMC tailings were place in two piles, the larger of which contained approximately 21,000,000 tons of mill waste and the smaller piles contains approximately 1,200,000 tons. Each of these tailings piles have been subject to surface reclamation activities authorized USNRC which have been substantially completed. Groundwater restoration activities are continuing at these sites following the recontouring, surface stabilization and armoring associated with the surface reclamation programs. As the tailings were the constructed using a ring dike methods without geologic or synthetic liner materials, large ponds of tailings liquids were contained on the unlined surface of the tailings piles providing a substantial hydrostatic force to drive the seepage plume at the site. During the first phase of groundwater collection, contaminated groundwater was returned to unlined ponds on top of the tailings, maintaining the hydrostatic force pushing seepage into the alluvial and Upper Chinle aquifers. Lined ponds were finally built in 1992 on the smaller tailings pile, and supplemented by lined ponds south of the larger pile.

Tailings-contaminated drainage has been addressed through two systems, "toe drains" around the perimeter of the larger pile and "tailings wells" installed in the larger pile. The toe drains were installed in 1992 to intercept perched groundwater seeping through the tailings into the alluvium. The toe drains were constructed using slotted drainage pipe in 3-3.5 meters deep trenches reporting to a series of sumps arrayed around the tailings perimeter. As shown in Table 1, more than 90,000,000 gallons of contaminated water have been collected by the toe drains during 1992-1997. The average collection rate for these trenches was 23 gpm in 1997, 5.5 gpm less than the 1996 average collection rate.

A series of wells have been installed within the tailings pile during 1994-1997, with additional wells installed in 1998, to first characterize and then collect the water within the tailings. Through 1997, 36 million gallons have been removed from the tailings through these wells, at an average collection rate of 41 gpm; HMC estimated the volume of "drainable" water within the tailings at approximately 277,000,000 gallons in 1994.

Groundwater restoration standards at HMC-Grants as of 1998 include: sulfate 1750 mg/l; TDS 3500 mg/l; chloride 200 mg/l; uranium 0.44 mg/l; selenium 0.25 mg/l. Homestake has proposed alternative concentration levels in its 1997 groundwater monitoring review, based on its interpretation of background groundwater quality.

Future operations will include continuation of the collection, injection and evaporation activities and maintenance of the piezometric gradient reversal between the residential areas affected by tailings seepage and the collection lines at the base of the tailings piles.

The Groundwater Restoration Program at UNC-Church Rock

The UNC-Church Rock uranium mill and tailings site operated from 1977-1982 under USNRC source material license SUA-1475. Corrective action for tailings seepage remediation begun after installation of the first extraction well systems in 1989, following extensive administrative challenges initiated by UNC.

The geologic formations impacted by seepage include: 1) alluvial soils beneath the tailings - the "Southwest Alluvium"; and 2) two areas of the underlying Upper Gallup formation in direct contact with the tailings deposits - "Zone 1" and "Zone 3". Figure 2 shows the UNC site and affected areas. The Church Rock facility was constructed in the alluvial valley of an intermittent tributary of the Rio Puerco of the West, the Pipeline Arroyo, which has been heavily impacted by discharges of mine water released from two upstream uranium mines operated by UNC and Kerr-McGee Corporation. This mine water discharge began in 1968, before mill construction, and continued until 1986, after mill closure in 1982. The mine water discharge, approximately 3,000 gpm for 18 years, creating saturated conditions alluvium and Upper Gallup zones prior to the onset of mill operations in 1977.

UNC asserts that prior to mine water discharge the alluvium was largely unsaturated, holding only minor amounts of perched groundwater replenished by surface flows in the Pipeline Arroyo are associated with major precipitation events, as is the nature of ephemeral streams. The 1968-1986 mine water discharges, which totaled more than 680,000,000,000 gallons, created a perennial stream in Pipeline Arroyo for the life of the discharge.

UNC asserts that the discharge was treated and good quality, however mine water flows began before the passage of the US Clean Water Act, UNC challenged the application of the application of the Clean Water Act permit system to its mines and EPA records document at least 37 violations of discharge standards under those permits.

Since NMED and UNC failed to identify the alluvial aquifer conditions or insure the installation of an effective liners at the tailings site at the time of initial operations, seepage from the tailings disposal area entered the alluvium and Upper Gallup zones as soon as the mill started operations. As the mill used a sulfuric acid leach, tailings liquid - and associated seepage - had a low between 1.5-2.0 and contained elevated levels of sulfate - 57,000 mg/l, chloride - 550 mg/l, total dissolved solids (TDS) - 67,000 mg/l, radionuclides - notably uranium decay series isotopes of uranium - 12 mg/l, lead-210 - 11,500 pCi/l and radium-226 - 8,000 pCi/l - and metals or metalloids - arsenic - 0.25 mg/l, iron - 5,540 mg/l, molybdenum - 1.05 mg/l, and zinc - 18 mg/l.

Groundwater contamination at the sites is associated with both tailings seepage and mine water, as well as dissolution of chemical constituents in the alluvium and underlying Upper Gallup due to the saturated conditions created by the mine water discharges into those formations. Tailings site contamination is not a result of the major breach in the Church Rock tailings dam in 1979 - though the differential settlement in the foundation of the tailings embankment which lead to the release 400,000,000 liters of tailings liquid with pH 1.5 - 2.0 and radium-226 concentration of approximately 1,000 pCi/l - was associated with the saturated conditions in the underlying alluvium.

Contamination in the Southwest Alluvium extends approximately 0.5 kilometer beyond the southwest edge of the tailings area. Since 1989, corrective action in the Southwest alluvium has focused on the operation of four extraction wells. The wells have collected more than 105,100,000 gallons during the period October 1989 - September 1997, including 9,200,000 gallons at an average rate of 20.3 gpm in 1997. Contamination in Zone 3 extends more than 1.0 kilometer beyond the northeast edge of the tailings area. Groundwater restoration in Zone 3 is focused on 15 extraction wells and 6 monitoring wells. The volume of groundwater collected from Zone 3 during the 1989-1997 period is 136,900,000 gallons, including more than 10,400,000 gallons at an average rate of 21.00 gpm in 1997. Contamination in Zone 1 extends approximately 200 meters east of the tailings area. The groundwater restoration system at Zone 1 consists of four extraction wells which have been in operation since 1989. The volume of groundwater extracted from Zone 1 is approximately 9,680,000 gallons. Low permeability, and reduced groundwater volume in Zone 1 have reduced groundwater collection rates in 1997 to 0.39 gpm, producing approximately 202,000 gallons. UNC has also installed extraction wells into the reclaimed tailings area to removed groundwater and contaminants from an area UNC calls the "Center of Seepage Impacted Area".

Table 2 - Quality and Quantity of Selected Constituents Collected 1989-1997 by UNC-Church Rock Groundwater Restoration Program

Water
Source
Total
Volume
(million
gallons)
Sulfate Manganese Radium
(Comb.)
Cobalt
mg/l lb. (thou-
sands)
mg/l lb. (thou-
sands)
pCi/l micro-
curies
mg/l lb.
*G'water
Zone 3
(1989)
106.2 2813 2490 5.67 5.0 14.6 3331.2 0.25 218.1
*G'water
Zone 3
(1997)
45.3 3316 1254 7.64 2.8 16.2 1176.4 0.41 155.4
*G'water
Zone 1
(1989-
1997)
9.6 4589 371 13.7 1.1 9.70 356.0 0.15 12.1
*G'water SW
Alluvium
(1990-
1997)
105.1 2970 2607 1.1 1.0 (N/A) (N/A)

Standards applicable to the UNC site as of 1997, by agency identified, include: sulfate 2,160 mg/l (EPA); chloride 250 mg/l (EPA); TDS 3,170 mg/l (EPA); manganese 2.6 mg/l (EPA); cobalt 0.50 mg/l (EPA); radium - combined 226+228 - 5.0 pCi/l (NRC and EPA). As of June 1998, USEPA and USNRC was evaluating proposed alternative concentration levels which would "change or eliminate the background and cleanup levels for nitrate, sulfate and TDS" as well as shutdown and decommission Southwest Alluvium, Zone 1 and Zone 3 seepage collection systems., as proposed by UNC in January 1998 letter.

Literature

Collins, J. D., (1996): "Reclamation and Groundwater Restoration in the Uranium Milling Industry: An Assessment of UMTRCA, Title II", in Journal of Natural Resources and Environmental Law, Mineral Law Center, University of Kentucky, Lexington, Kentucky, USA.

Homestake Mining Company, (1996): "Ground-Water Monitoring for Homestake's Grants Project - NRC License SUA-1471 and Discharge Plan DP-200, 1995", Homestake Mining Company, San Francisco, CA, USA.

Homestake Mining Company,(1998): "Ground-water monitoring and Performance Review for Homestake's Grants Project: NRC License SUA-1471 and Discharge Plan DP-200, 1997", Homestake Mining Company, San Francisco, CA, USA.

United Nuclear Corporation, (1998): "Re: SUA-1475 1997 Ground Water Corrective Action Report" - letter to USEPA, United Nuclear Corporation, Albuquerque, New Mexico, USA

United Nuclear Corporation, (1998): "1997 Ground Water Corrective Action Annual Review - United Nuclear Corporation's Church Rock Mill and Tailings Facility, Gallup, New Mexico", United Nuclear Corporation, Albuquerque, New Mexico, USA



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