The Grants uranium district in northwestern New Mexico was a prolific producer of uranium

from the 1950s to the early 1980s. Most of the uranium mining and milling activities occurred prior to the development of environmental laws and regulations aimed at protecting human health and the environment. As a result, conventional industry practices employed during this period caused extensive groundwater contamination throughout the area. Adequately addressing groundwater contamination has been complicated by the extensive mining operations in the area and the large-scale dewatering that was required to remove the ore.

HistoryThe Grants district (see map) is a large area within the San Juan Basin, extending from east of Laguna to west of Gallup. It comprises eight subdistricts that contained hundreds of mines, including 112 that produced at least 200,000 pounds of uranium oxide, U3O8. From 1948-1980, the Grants district yielded more uranium than any other in the United States. More than 340 million pounds of U3O8 were produced there from 1948-2002, accounting for 97 percent of the total production in New Mexico and more than 40 percent of the total U.S. production (McLemore, 2002).

Most uranium production in New Mexico has come from the Morrison Formation, mainly from the Westwater Canyon Member, a significant aquifer in the area. Therefore, the formation had to be

partially dewatered in order to remove ore. Prior to the mid-1970s, water generated during dewatering activities was discharged to the surface and allowed to flow into natural water courses without any treatment. These mine water flows were a significant source of contamination of sediments, alluvial aquifers, and even deeper aquifers in areas of faulting.

Groundwater in the Grants district was also contaminated by other mine-related activities, including seepage from evaporation ponds and mill-tailing ponds, mine-stope leaching, leaching of waste materials stored on the surface, and underground-mine disturbance related to removal of ore, which introduced oxygen into the system, causing geochemical reactions that dissolved contaminants into the groundwater. Contaminants present in the groundwater system include molybdenum, selenium, 226+228radium, sulfate, total dissolved solids, and uranium.

Regulatory FrameworkAll conventional underground and open-pit uranium mines in New Mexico closed by 1989, due mainly to economic factors. The Federal Clean Water Act was passed in 1972 and New Mexico followed suit by enacting the New Mexico Water Quality Act in 1974 and

accompanying regulations in 1977. These regulations addressed, among other things, discharges from uranium mines and mills. However, by this time the uranium market was already deteriorating and most mines and mills ceased operations shortly thereafter. Therefore, the majority of uranium mining and milling operations in New Mexico predated environmental laws and regulations.

When New Mexico’s regulations were promulgated in 1977, a groundwater-protection standard for uranium of 5 milligrams per liter (mg/l) was established; at that time, neither the World Health Organization (WHO) nor the U.S. Environmental Protection Agency (EPA) had established a standard. In 1993,

Groundwater RemediationJerry Schoeppner – New Mexico Environment Department

New Mexico

Churchrock—Crownpoint

Smith Lake

Grants

Laguna

Chaco Canyon

Nose Rock

Ambrosia Lake

Marquez

BarnabeMontaño

Morrison Formation (Jurassic)sandstone uranium deposits

Other sandstone uraniumdeposits

Limestone uraniumdeposits

Other sedimentaryrocks with uranium

Albuquerque

Grants uranium district, San Juan Basin, New Mexico. Polygons outline approximate areas of uranium mine subdistricts (from McLemore, 2007).

from Uranium Mining in New Mexico

St. Anthony’s Pit, New Mexico. See water quality analysis of pit lake water in table, opposite page.

22 • November/December 2008 • Southwest Hydrology

WHO recommended that the limits for radiological characteristics for uranium be used until adequate short- and long-term studies on the chemical toxicity of uranium could be completed. Based on these limits, the equivalent for natural uranium is approximately 0.14 mg/l. In an addendum to the WHO Guidelines, published in 1998, a health-based guideline value of 0.002 mg/l was established (WHO, 2004). In 2000, EPA issued its first uranium drinking water standard of 0.03 mg/l; this standard is higher than the WHO standard because it was derived using both health data and economic considerations. Finally, in 2005 New Mexico revised its uranium standard to 0.03 mg/l to be consistent with EPA.

Challenges of Groundwater

RemediationAddressing groundwater contamination in the Grants uranium district is complicated by several factors: 1) extensive mining and related dewatering activities have created regional, rather than localized groundwater contamination; 2) underground workings are so extensive that they connect one mining operation to another, making it difficult to determine responsibilities; 3) many different companies operated mines throughout the Grants uranium district, and many no longer exist; 4) much of the groundwater quality data are dated and large data gaps exist; 5) premining groundwater data are insufficient to establish cleanup criteria; 6) the uranium standard has only recently been established and has been revised over time; and 7) proposed new operations could further cloud the issue of background and cleanup criteria.

Because groundwater is contaminated throughout the district, a regional solution is required. This may take the form of allowing the groundwater system to recover to premining levels to restore geochemical conditions (which could take hundreds to thousands of years), implementing an engineering solution that may involve extraction and treatment to drinking water standards (and possible sale of the resource to offset reclamation costs), or some other solution that lies between these two extremes. Any engineered

solution would require participation from many different parties, which would likely slow down the process with litigation.

Obviously, addressing groundwater contamination in the Grants uranium district is very complicated. It will take a great

deal of assessment work, coordination, and funding that is currently lacking. It also will inevitably take a long time, which raises the issue of how to protect the public from potential health impacts of drinking contaminated water. Very few residents live in the most contaminated portions of the district, but those that do rely on domestic wells for their primary drinking water supply. A combination of public education and individual water treatment systems may be the only short- and long-term solutions.

One of the few encouraging aspects is that, based on groundwater modeling, recovery of groundwater levels will take upwards of

see Remediation, page 34

Pit lake contaminantsRange* of samples collected

(2004-2005)Surface-water

standard*Groundwater

standard*

molybdenum <0.0055 – 0.020 none 1.0

selenium <0.015 – 0.035 0.05 0.05

uranium 4.2 – 5.3 none 0.03

total dissolved solids 23,000 – 32,000 none 1,000

sulfate 16,000 – 25,000 none 600

gross alpha 3,050 – 4,590 pCi/l 15 pCi/l none

226+228radium 11.59 – 24.83 pCi/l 30 pCi/l 30 pCi/l

* All measures in mg/l unless otherwise noted. pCi/l = picoCuries per liter

Water quality in the St. Anthony’s uranium mine pit lake compared to standards. Comparison for compliance is against surface-water standards for the designated use of livestock watering, wildlife habitat, and aquatic life. Concentrations of some contaminants are high, but lacking surface-water standards, are not out of compliance. If pit lake water were to flow into the groundwater system, it would have to meet groundwater standards outside the lake.

One of the few encouraging aspects is that recovery of groundwater levels will take upwards of several hundred years.

November/December 2008 • Southwest Hydrology • 23

Aquifer Impacts?Before ISR even begins, the uranium ore-bearing aquifer contains naturally occurring 226radium, 222radon, and other uranium-decay products at concentrations exceeding EPA drinking water standards (see table below). Nonpotable water such as this can be exempted as an underground source of drinking water under EPA’s Safe Drinking Water Act, and the field of injection and extraction wells can be permitted for Class III underground injection control (UIC) activity. UIC regulations require ISR operations to be designed to produce only from the exempted area, and monitoring must demonstrate that the leach solution is contained within the ore zone. Monitoring parameters are typically chosen that are high in concentration compared to surrounding ambient groundwater, are robust, and may be rapidly analyzed at site laboratories. Parameters such as conductivity, chloride, bicarbonate, sulfate, and uranium are common. Restoration must be completed before monitoring ceases, to prevent regional contamination.

Construction, operation, monitoring, and reporting at ISR sites in the United States have been highly successful in ensuring that leach solution remains confined to the exempted ore zone, as required by UIC regulations. As a result of these practices and the fact that the ore bodies are not in drinking-water-quality aquifers, ISR uranium operations have caused no adverse impact to underground sources of drinking water in the United States.

Contact Mark Pelizza at mspelizza@uraniumresources.com.

several hundred years. This isn’t good news as far as restoring geochemical conditions to premining conditions, but it provides assurance that contaminated groundwater will not migrate and contaminate new areas.

Future OperationsThe Grants uranium district still contains several hundred million pounds of uranium, now worth $60 per pound of U3O8. This elevated price will only raise interest in renewed mining and milling in the area. Conventional, open pit, and stope leach mining have historically been conducted in the Grants uranium district; all these methods, along with in-situ leaching, may be proposed in the future.

Environmental regulations that were absent during most of the past mining activities are now in place, along with more stringent mining regulations that will protect human health and the environment to a much greater degree. If water produced during new dewatering activities will be discharged to the surface, it will have to be treated to groundwater and possibly drinking-water standards prior to discharge. This will help prevent additional contamination, but water discharged to the surface could remobilize any contamination still present in the soil from the previous operational period if not addressed before new operations begin. This and continued exposure and oxidation of the ore body above the water table will continue to present challenges in managing potential contamination. However, the current regulations include flexibility to require protective engineering controls during operations and adequate financial assurance to address closure requirements.

Contact Jerry Schoeppner at jerry.schoeppner@state.nm.us.

ReferencesMcLemore, V.T., 2002. Database of Uranium Mines,

Prospects, Occurrences, and Mills in New Mexico, New Mexico Bur. Geology and Mineral Resources, Open-file Report 461, 11 pp.

McLemore, V.T., 2007. Uranium resources in New Mexico, Society for Mining, Metallurgy, and Exploration 2007 Annual Meeting, SME preprint 07-111.

World Health Organization, 2004. Guidelines for Drinking-water Quality: Summary Statement, 3rd ed., Geneva, WHO.

Solution-collapse breccia pipe uranium deposits occur in the CPUP, particularly in the Grand Canyon region.

The surface disturbance that results from mining this type of deposit historically has been remarkably small because of the high-grade, compact nature of the mineralization and use of underground waste rock backfill techniques during mine development. A 1,000- to 1,600-foot-deep shaft is usually required to access the deposits unless the pipe occurs near a deep canyon.

Breccia pipe ore grades are at least as high as any other global uranium-deposit type, at 0.4 to 1 percent, because the limited size of the pipe concentrates the uranium. Average ore reserves for an individual mineralized pipe are about 3.5 million pounds U308, with an average grade of about 0.6 percent uranium.

Volcanic uranium deposits are found in volcanic and volcaniclastic rocks. Volcanic deposits and hydrothermal veins occur in rhyolitic flows and tuffaceous ash flows, formed by hydrothermal, hot springs, or meteoric waters. Tabular lacustrine sandstone deposits occur in carbonaceous tuffaceous sandstone and mudstones, deposited by cooler groundwaters.

Several major uranium deposits in the RMIBUP occur as veins in metamorphic and sedimentary rocks, primarily within the Front Range and central Rocky Mountains of Colorado. Here, hydrothermal fluids directly deposited the uranium in fracture systems. Most of the BRUP deposits are volcanic, occurring as vein deposits and tabular ore bodies in paleolake sediments associated with volcanic activity. Volcanic deposits generally are developed by conventional mining methods.

Contact Clyde Yancey at clyde_l_yancey@msn.com.

ReferenceFinch, W.I., 1996. Uranium Provinces of North

America, Their Definition, Distribution, and Models, U.S. Geological Survey Bulletin 2141, U.S. Department of the Interior.

Geology, continued from page 21 In Situ, continued from page 29Remediation, continued from page 23

Water quality data from 89 baseline wells, collected prior to initiation of ISR operations, in the mineralized portion of the Oakville aquifer at the URI Inc. Vasquez ISR project in Duval County, Texas. EPA’s maximum contaminant levels (MCLs) are shown for comparison.

Parameter Average EPA MCL

uranium (ppb) 488 30

226radium (pCi/l) 215 5.0

222radon (pCi/l) 207,133 300

gross alpha (pCi/l) 865 15

34 • November/December 2008 • Southwest Hydrology