- Email: [email protected]
Geochemical constraints on underground disposal of uranium mill tailings
Applied Geochemistry. Vol. 1. pp. 335--343, 1986.
0883-2927/86 $3A10~- .lgJ Pergamon Journals Ltd.
Printed in Great Britain
Geochemical constraints on underground disposal of uranium mill tailings BRUCE M. THOMSON Department of Civil Engineering, University of New Mexico, Albuquerque. NM 87131, U.S.A. PATRICK A . LONGMIRE Environmental Health Department, City of Albuquerque, P.O. Box 1293. Albuquerque, NM 87103. U.S.A. and DOUGLAS G. BROOKINS Department of Geology, University of New Mexico, Albuquerque. NM 87131, U.S.A.
A b s t r a c t - - A three phase investigation has been conducted on groundwater quality impacts of the
underground disposal of tailings from acid-leach milling of uranium ores. These phases included field collection and analysis of samples obtained during backfilling of mill tailings in empty underground mine stopes, collection of soil samples from mill tailings piles and previously backfilled stopes, and evaluation of thermodynamic constraints on possible geochemical transformations. Contaminants of principal concern include As, Mo, Se, U, V and Ra-226. The investigation has shown that short-term degradation of groundwater due to backfill disposal of the sand fraction of uranium tailings is negligible. Long-term effects, defined as those occurring after mining operations cease and the mine fills with water, are predicted to also be very small. This is attributed to immobilization of pollutants through chemical reduction and precipitation, as well as adsorption onto aquifer materials. This conclusion is substantiated, in part, by observation of high concentrations of most of the contaminants on the silt and clay fraction of the soil samples collected, in contrast to the concentrations found on the sand fraction.
INTRODUCTION THE SAFE disposal of uranium mill tailings is one of the major environmental obstacles currently facing the front-end of the nuclear fuel cycle (CRAWFORD, 1985). It is estimated that there are 300 million metric tons of tailings associated with U.S. uranium mills, which are either operating, on standby basis, or at inactive or abandoned sites. Though the tailings themselves constitute a low level radioactive waste, principally due to gaseous Rn, disposal problems are primarily the result of conventional air- and waterborne contaminants. The principal air-borne contaminants consist of blowing dust and Rn. Due to the nature of the milling process, which uses either acid or alkaline leaching of U precipitates, aqueous solutions associated with spent tailings are of extremely poor quality. Water quality of tailing raffinates (the aqueous solution remaining after uranium has been extracted by a solvent) from U mills, which were operating in New Mexico in 1980 and 1981, is presented in Table 1 (THOMSON et al., 1982). This paper focuses on impacts of subsurface tailings disposal on groundwater quality. The principal objective of any hazardous waste
disposal option is prevention of interaction between the waste material, the hydrosphere and the biosphere. The Nuclear Regulatory Commission has identified nine alternatives for mill railings disposal, of which all but three involve some form of subsurface disposal (U.S. NUCLEAR REGULATORY COMMISSION, 1980). Most subsurface disposal alternatives eliminate direct interaction between the biosphere and the waste material, and also provide barriers to Rn emanation. The remaining area of concern for these options is the potential for groundwater contamination. There are two basic variations on subsurface disposal options: disposal in specially prepared sites, and placement of the tailings in empty underground mine stopes. This latter alternative has seen extensive practice in the Grants Mineral Belt (GMB, Fig. 1), New Mexico. The objective of this investigation is to characterize the geochemical processes taking place that may affect groundwater quality. Though the studies to be described deal specifically with backfill disposal of tailings, the findings should be applicable to other forms of subsurface disposal such as burial in trenches. The results of this investigation are used to discuss the suitability of underground disposal options as a method for disposal of U mill tailings.
B. M. Thomson et al.
336 !
Grants Minerot "-[==.~orea" I
; k
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
Belt
I McKintey County I
I
,. ~
N
Churchrock
[
District ~,.,,.~,..,_,
AmbrosiaLake District
'
0
I. °
:g l;z. . . . . . . .
.
.
~ . ~Loguna \
I
.
.
j'll
Grants
0
,2
,m
, Cibolo County I
,, I
FIG. 1. Location of Grants Mineral Belt. BACKGROUND INFORMATION
Backfilling
Uranium ore deposits are frequently associated with aquifers. In these cases, deposition of U ore is thought to be a result of aqueous transport of oxidized, and therefore soluble, U species to organically-rich reducing zones where reduction and subsequent precipitation of U minerals occurs (DEVoTo, 1978; BROOrI~S, 1977). LAr~GMUm(1978) has summarized the oxidation-reduction and acid-base chemistry of U. Due to the location of ore deposits in aquifers, U mines must continually pump accumulated water from the mine. In New Mexico, dewatering requirements range from less than 0.006 m3/s to greater than 0.2 m3/s (TrtoMsoN et al., 1982). In the Grants Mineral Belt, the ore bearing strata, the Morrison Formation,
is overlain by two or more distinct aquifers, each separated by shale aquicludes (CHENOWETn, 1979). Collapse of open stapes causes fracture propagation towards the surface resulting in hydraulic connection with overlying aquifers and greatly increasing mine dewatering requirements. To minimize the extent of this subsidence, many of the mining companies in New Mexico have backfilled empty stapes with tailings. The tailings used in the backfill process consist of classified sands. Unclassified tailings typically contain approximately 30% slimes (<200 mesh) by weight, which inhibits their dewatering. Backfill disposal of the total tailings mix would thus present a hazard to underground workers in that failure of a retaining bulkhead would release a slurry of tailings into access drifts within the mine. Removal of the slimes allows rapid dewatering by gravity drainage to the mine sump. The sands are piped into empty mine stapes as a slurry consisting of equal weights of treated mine water and sand.
Table 1. Concentrations of selected constituents of New Mexico uranium mill tailings pond raffinates. Analyses of unfiltered samples collected by New Mexico EnvironmentalImprovement Division personnel, 1978to 1981. All concentrations in mg/l unless noted (from THOMSONet al., 1982) Four acid leach mills ( 14samples) Constituent Gross radioactivity (pCill) Ra-226 (pCi/l) As Mo Se SOdU V NH 3(mg N/I) TDS pH
Minimum 3200 15 0.18 0.20 0.006 300.0 1.1 39.0 3.3 17 900 0.3
Median
One alkaline leach mill (5 samples) Maximum
38 000
73 000
70 1.3 0.90 0.21 29 700 15 74 400 39 800 1.05
1800 5.6 29.5 6.97 56 000 69 107 3960 72 800 2.15
Minimum
Median
3400
6700
56 2.1 72 22.1 5500 4.2 1.2 1.1 17 000 9.9
58 5 98 29.5 8400 54 14 16 25 400 10.1
Maximum 10000 90 7.2 105 51.2 16 700 70 16 335 39 700 10.3
Geochemical constraints on underground disposal of U mill tailings The impacts of backfilling can be broken into two categories (THOMSON and HEGGEN, 1983). Short-term effects occur while the mine is in operation and being dewatered. Drainage from the backfilled stope flows rapidly to the mine sump where it is diluted with other minewater, and is then p u m p e d to the surface, treated and discharged. D u e to the relatively small volume, dilution with treated water, and the brief period in which contaminated water remains in the mine, the short-term impacts of backfilling are negligible. L o n g - t e r m effects are those which may occur after dewatering ceases and saturated conditions return to the formation as the original potentiometric head is reestablished. D u e to access limitations (the mine fills with water), it is not possible to sample directly such stapes; therefore, projections about long-term impacts are based on laboratory investigations and theoretical considerations.
FIELD AND LABORATORY INVESTIGATIONS
Sampling program The objective of the sampling program was to characterize the geochemistry of uranium mill tailings from acid and alkaline-leach mills in New Mexico. It should be noted that no alkaline-leach tailings have been used recently for backfill in New Mexico mines. The sampling program involved collection of tailings from surface tailings impoundments, back filled stapes of various ages, and water samples from backfill drainage. Tailings were collected from two operating mills, an acid-leach and an alkalineleach facility. Backfill samples were collected from three mines and represented tailings from two different acid-leach mills. Six different stapes were sampled, with ages ranging from approximately six months to nearly 10 y. Loose tailings samples were collected using a stainless steel spatula and placed in sealable plastic bags. Core samples 1 m in length were collected using 5 cm (2 in) galvanized steel thin walled tubes. In the laboratory these cores were extruded and sealed in plastic. Water samples were collected in 21 plastic jugs and preserved with 2 ml/l HNO3.
337
TaKings sample
t
!
Scanning electron microscopy
T
JChemi cendal mechanical dispers on
t X- roy diff roction !
Water froction INAA
!
SiLt/ctoy fraction I TNAA
Sand fraction I INAA
FIc. 2. Analytical procedures used to characterize soil samples. (NaPO3)6). The solution was shaken and allowed to settle in covered beakers for sufficient time such that no particles with a diameter >50/~m remained in suspension. The fines. which remain in solution, represent the silt and clay fractions of the samples. Due to classification of the tailings during the milling process, a sufficient amount of pure clay material (diameter < 5/zm) could not be collected from backfilled stapes. At the end of the settling period, the supernatant liquid was removed from the beaker. The remaining sand was dried and transferred to a sealed vial. The decant solution was passed through a 0.45 ~tm membrane filter that had been previously washed with distilled water, the solids were dried, and transferred to a sealed vial. The filtrate was collected and placed in a sealed container. Due to the nature of the separation process, this filtrate is representative of the water soluble constituents associated with the sample and is analogous to a desorption experiment. The sand and silt-clay fractions were crushed in an agate mortar and pestle by hand and passed through nylon sieves to less than 200 mesh. The mortar and pestle and sieve sets were washed with dilute HC1 and distilled water between samples to minimize contamination. The samples were then subject to instrumental neutron activation analysis (INAA) at the Omega West Reactor operated by Los Alamos National Laboratory. Details and complete results of the INAA procedure are contained in THOMSONet al. (1984) and LONGMIRE(1983). RESULTS
Analytical program Water samples collected in the field were analyzed by procedures published in Standard Methods LAMER. Pun. HEALTH ASSOC., 1980). Metals were measured by inductively coupled argon plasma spectroscopy and atomic absorption spectroscopy. Anions were analyzed by wet chemical procedures. Uranium-238 was determined by alpha-spectrometry. Figure 2 presents a schematic of the analytical program which was used on the soil samples. Initially, each of the samples was split, one fraction being subjected to mineralogical analysis and the other to chemical analysis. Mineralogical analysis consisted of scanning electron microscopy and X-ray diffraction studies. Prior to chemical analysis, the soils were separated by size based on settling velocity using a procedure similar to that used to determine particle size distributions (ASTM D42263. ASTM. 1981). A solution of the soil was prepared using a chemical dispersant (sodium hexametaphosphate - -
Results of underground monitoring of water quality associated with backfill placement in two different mines are summarized in Table 2. These two mines utilized the same mine water treatment system, consequently the discharge quality parameters are averaged together. Of special interest is the variable quality associated with groundwater infiltrating into the mine. Results of the analytical program applied to the soil samples are summarized in Tables 3-6. These results represent the averages of all analyses on the backfilled sands and acid-leach mill tailings samples. Tables 3 and 4 contain the elemental and selected radionuclide concentrations of the sand and silt--clay fractions, as well as the soluble concentration of each element found when 100 g of material is leached with
B. M. Thomson et al.
338
Table 2. Results of water quality monitoring program during backfill event. All units in mg/l except as noted
Constituent As Ca Fe Mn Mo NOq (mg N/I) Se SO]TDS U-238 V pH (pH units)
Mine water
Slurry
Backmix drainage
Commingled water
Surface discharge
0.056 219 0.017 <0. I 46 6 2.6 1680 3090 101 0.81 7.3
0.023 39[) 0.06 <0.1 6 24 0.91 2120 3560 20 0.27 7.3
0.022 -0.13 <0.1 8.9 28 1.8 2390 3800 74 0.49 6.6
0.015 -0.12 <(~. l 37 7 2.8 1960 3190 63 0.55 7.5
0.021 32.0 0.042 -8.0
Table 3. Average concentrations in each fraction of backfilled sands, determined by INAA (9 samples) Element AI As Ba Ca Cr Fe K Mg Mo Se Th-223 U-239 V
Sand (ppm)
Clay (ppm)
Water* (mg/l)
Enrichment Clay/Sand
35633.3 3.16 695.8 2362.2 10.4 2081.6 22366.7 0.0 10.2 8.24 1.828 29.27 82.4
66800.0 18.47 945.8 32887.8 317.9 25994.4 25855.6 3173.3 178.6 80.31 7.513 226.41 928.8
3.3 0.12 0.0 323.9 0.2 7.8 228.5 0.0 0.3 0.90 0.002 0.70 0.6
1.9 5.8 1.4 13.9 30.7 12.5 1.2 17.5 17.5 9.8 4.1 7.7 11.3
Table 4. Average concentrations in each fraction of acidleach uranium, mill tailings by INAA (2 samples)
Element A1 As Ba Ca Cr Fe K Mg Mo Se Th-223 U-239 V
Sand (ppm)
Clay (ppm)
Water* (mg/l)
Enrichment Clay/Sand
37850.0 2.51 778.5 2775.0 9.88 2387.5 23500.0 0.0 5.39 8.17 1.172 19.04 169.45
60050.0 20.44 642.0 41150.0 157.30 30370.00 25400.0 2650.0 216.00 124.10 2.488 118.35 954.00
0.0 0.17 0.0 470.1 0.0 11.5 64.9 0.0 0.0 0.13 0.0 0.10 0.69
1.6 8.2 0.8 14.8 15.9 12.7 1.1 40.1 15.2 2.1 6.2 5.6
* Concentrations in 250 ml of solution containing 100 g of backfilled sands.
* Concentrations in 250 ml of solution containing 100 g of acid-leach uranium mill tailings.
Table 5. Distribution of the total mass of each element from a 100 g sample of backfilled sands.
Table 6. Distribution of the total mass of each element from a 100 g sample of acid-leach uranium mill tailings
Element AI As Ba Ca Cr Fe K Mg Mo Se Th-223 U-239 V
Sand (g)
Silt and Clay (g)
Soluble (g)
Element
3.3753 2.99E-04 0.0659 0.2238 9.82E-04 0.1972 2.1186 0.00E-00 9.69E-04 7.80E-04 1.73E-04 2.77E-03 7.81E-03
0.3525 9.74E-05 0.0050 0.1735 1.68E-03 0.1372 0.1364 1.67E-02 9.43E-04 4.24E-04 3.96E-05 1.19E-03 4.90E-03
0.0011 2.90E-05 0.0000 0.0845 5.56E-05 0.0025 11.0672 0.00E-00 9.55E-05 3.04E-04 4.49E-07 2.12E-04 1.62E-04
AI As Ba Ca Cr Fe K Mg Mo Se Th-223 U-239 V
Sand (g)
Silt and Clay (g)
Soluble (g)
2.839 1.88E-04 0.058 0.208 7.41E-04 0.1791 1.7625 0.0000 4.04E-04 6.13E-04 8.79E-05 1.43E-03 1.27E-02
1.501 5.11E-04 0.016 1.029 3.93E-03 0.7593 0.6350 0.0663 5.40E-03 3.10E-03 6.22E-05 2.96E-03 2.39E-02
0.000 5.20E-05 0.000 0.141 0.00E+00 0.0035 0.0190 0.0000 0.00E+00 3.88E-05 0.00E+00 3.08E-05 2.03E-04
Geochemical constraints on underground disposal of U mill railings IO
-4 -2-1
/5 Gibbsite saturation
aS04 2HzO
I
6
339
log o (SO2").a (H+)Z=-12 0
z
Atunite
5 °
-5c,~
-Z-~og . . . a. (SO~-) . " a- ~ (H)z + =-~
/
I -5
I[ -I
I°
3
logo (FeS+)/a(H+) 3
)
4
6
8
0
12
I
pH FIG. 4. Activity diagram of gypsum-calcite at 1 arm pressure and 25° C using 10-I (9600 mg/1), I0-: (960 mg/l) and 10-4 (9.6 mg/l) sulfate. Tailings raffinate backfill slurry and native groundwater are denoted by C), [] and/~ respectively.
FIG. 3. Activity diagram of jarosite-alunite-potassium
feldspar-gibbsite-goethitesystem at 1 atm pressure and 25° C (modified from BLADn, 1982). Tailings raffinate and native groundwater are denoted by © and/X respectively.
250 ml of sodium hexametaphosphate solution. The right-most column presents the ratio of the elemental concentration associated with the sand fraction to that of the silt-clay fraction. It is presented as an enrichment factor. Using these data from Tables 3 and 4, together with the results of the particle size analysis, the mass of each element and nuclide associated with each fraction of a 100 g soil sample is presented in Tables 5 and 6. Thus in a 100 g sample of backfill sands there would be 3.38 g of A1 with the sand fraction, 0.35 g bound to the silt-clay fraction, and 1.0 mg would be soluble (Table 5). THEORETICAL CONSIDERATIONS
Two distinct major diagenetic processes are expected to occur in a backfilled stope which will affect the pore water chemistry; precipitation of supersaturated phases followed by chemical reduction and subsequent precipitation of originally oxidized constituents. Other processes which may occur include mixing of waters of different composition (backfill slurry water and native groundwater), and compaction of the backfilled material (LoN6MIRE, 1983). This section focuses on the precipitation reactions. In considering geochemical processes which may occur in underground disposal of uranium mill railings, there are two very useful theoretical tools available. These are equilibrium phase diagrams and numerical modeling of aqueous solutions. The backfill solution is supersaturated or nearly saturated with respect to a number of sulfate, carbonate, oxide and silicate minerals. Sulfate salts include gypsum, jarosite and thenardite (BRoorxr~s et al., 1982). The stabilities of alunite ((K,Na)A13(SO4)_,(OH)6), jarosite ((K,NaFe3)(SO4)(OH)6), gibbsite (AI(OH)3), goethite ( F e O O H ) , and K-feldspar as functions of activities of Fe s+, A! 3+, H + , and SO24- in
contiguous solution are shown in Fig. 3 (modified from BLADn, 1982). This figure illustrates that tailings raffinate is in equilibrium with jarosite, whereas groundwater is in equilibrium with gibbsite (AI(OH)3). Gibbsite occurs as surface coatings within the Westwater Canyon Member (MORRISON Formation) aquifer. Additionally, uncontaminated groundwater is nearly saturated with respect to goethite. The stabilities of gypsum and calcite, as functions of the activity of Ca "-+ and pH, are illustrated in Fig. 4. The taitings raffinate is supersaturated with respect to gypsum, whereas calcite should precipitate in groundwater. The backfill slurry is supersaturated with respect to gypsum. Formation of these minerals in the backfill may have a beneficial consequence in that precipitation may result in removal of other group II metals including Ba and Ra through coprecipitation. A more thorough discussion on the geochemical processes occurring during mixing of backfill slurry with groundwater is presented by LONGMIRE (1985). Dissolution of gypsum, following first-order kinetics, will control the release of Ca z+ and SO~- ions to solution as uncontaminated groundwater (Westwater Canyon Member) resaturates the backfill. Gypsum dissolution is diffusion-controlled, where calculated diffusion coefficients range from 4 to 8 × 10 -6 cm2/s (BARTON and WILDE, 1971; CHRISTOFFERSON and CHRISTOFVERSO~, 1976). These data suggest that a long-term impact on groundwater quality due to gypsum dissolution should be negligible. Additionally, sorbed species, including Ba and Ra, should undergo a limited release to groundwater. THOMSON et al. (1984) investigated the possibility of an increase in unconfined compressive strength of backfilled sands as a result of cementation following precipitation of these species. No statistically identifiable increase in strength was found, which was attributed to the relatively small amount of material present in solution. Mass balance calculations suggest also that there is too little soluble material for its precipitation to affect, in a measurable way, the porosity or hydraulic properties of the formation.
340
B.M. Thomson et al.
~0
-4~ 2
20
2
Setenium
Uronium
p-'/
H2SeO~ I
--)8
" ' P O ~
06
US[04
04
04
, ",. I
uoJco=)~
Se
-o2
o2~ "~
o
Ferrosetite
'=''
0
SeO§-
-02 -04
-I0
0
-IC
2
4
6
pH
8
I0
12
14
0
-06
2
4
6
8
I0
12
14
pH
FIG. 5. Eh-pH diagram for uranium species at I atm pressure and 25° C. Total activitiesof uranium, total dissolved carbon and silicic acid are 10-6 (0.24 mg/l), 10-: (120 mg/l) and 10-3.5 (30.4 mg/l), respectively. Modified from BROOr]NS (1977).
FIG. 6. Eh-pH diagram for selenium and iron species at 1 atm pressure and 25° C for 10-° (0.08 mg/l) selenium and 10-6 (0.06 mg/l) iron, respectively (LoN6MIRE, 1983).
E h - p H diagrams are useful for evaluating equilibrium oxidation-reduction processes in the backfill material. A t the time of burial, the backfill slurry is near neutral p H and under moderately high oxidizing conditions (Eh = 0.3 V). With time, the Eh of the pore water is expected to fall sharply as reducing conditions characteristic of the formation are reestablished. E h - p H diagrams for U and Se are presented in Figs 5 and 6, respectively, as two of the constituents of most concern. Under reducing conditions, coffinite (U(SiO4)I_x(OH)4x) is expected to precipitate, and indeed it is the major crystalline uranium mineral found in the Grants Mineral Belt. Upon oxidation to the hexavalent state, U becomes soluble as an anionic uranyl carbonate complex. Selenium exhibits similar chem-
istry in that under reducing conditions it is insoluble, and therefore immobile, as either a selenide (Se(-II)) or metallic selenium (S(O)). Ferroselite (FeSe2) is found in the Grants Mineral Belt (BaooKx~s, 1977). Table 7 summarizes the oxidation-reduction characteristics of a number of inorganic constituents. The important point to note is that under reducing conditions most transition metal species are insoluble and therefore not mobile. Although equilibrium phase diagrams are useful in illustrating transitions and stability regions, numerical simulation of the various solutions is more useful for addressing a specific aqueous system. The P H R E E Q E geochemical model (PARKHURST et al., 1980) was used in this investigation. The W A T E Q F C geochemical model was also used to perform additional thermodynamic calculations on different waters associated with mine stope backfilling (Rur~NELLS and LINDBERO, 1981). Of particular interest is comparison of the ion activity products to the thermodynamic solubility product for the minerals and solid compounds of interest. The relative degree of saturation is measured by the saturation index (SI) (BARNES and CLARKE, 1969):
Table 7. Typical dominant species of inorganic constituents present under oxidizing and reducing conditions in aqueous systems Metal
Oxidizingconditions
Reducing conditions
As Cd Cr Cu Pb Hg Ni N Se Ag Zn
H2AsO4Cd 2÷, CdCO3(s) CrO~Cu 2+. CuO(.) PbCO~) HgO,) Ni:+ NO~ SeO 2-, HSeO; AgCl(s) Zn 2+, ZnO,)
As.,S3(s),AsSis) CdS(~) Cr203(s)
CuS(s) PbS(~ Hg~s).HgS,) NiS(~) NH~, Nzcg) Se°~s),FeSe2,) Ag°.). Ag2S(~) ZnS.)
SI = log]0 (IAP/Ks0) where IAP = ion activity product and Ks0 = solubility product. If a solution is in equilibrium with its corresponding solid phase SI = 0. SI > 0 indicates a supersaturated solution, and SI < 0 represents an undersaturated solution.
Geochemical constraints on underground disposal of U mill tailings
341
Table 8. Saturation indices (si ~ log (IAP/Kti) for tailings raffinate and groundwater (Westwater Canyon Member), Ambrosia Lake Mining District, Grants Mineral Belt. Calculated by WATEQFC (RuNNELLSand LZNDBERG,1981)
Formula
Raffinate
Westwater Canyon Member Section 35 Mine
KFe3( SO4 )~.(OH ), Fe203 NaFea(SO4)2(OH)6 SiO2 BaSO4 CaSO4 • 2H20 FePO4.2H20
5.7775 3.9803 3.3572 1.8353 1.1316 0.3250 -0.0421 - 0.1560 -0.8001 -3.4772 -4.6603 - 8.4743 - 11.1805 -9.4185 - 12.2008 - 12.2873 - 13.6475 - 17.5814 - 18.3992 - 54.7910 -9.5664 - 18.4292 -24.1194
- 12.0887 6.4945 - 14.2089 0.0984 1.0233 -0.8778 -6.9039 - 10.4177 0.4687 -2.2629 -6.3248 1.7542 -0.7425 0.1856 3.5285 0.2682 3.3376 3.7498 4.6849 10.0837 -2.6191 11.6625 - 1.2051
Mineral phase Jarosite Hematite Natrojarosite Quartz Barite Gypsum Strengite Ilsemannite Goethite Fe(OH)3 amorphous Thenardite Microcline Albite Gibbsite Kaolinite Calcite Selenium native Coffinite Uraninite Ferroselite Alunite Magnetite UO2 amorphous
Mo308
FeO(OH) Fe(OH)3 Na2SO4 KAISi308 NaAISi308 AI(OH)3 AI2Si2Os(OH)4 CaCO 3 Se USiO4 UO2 Fe Se2 KAla(SO4)~(OH)6 FeFe204 UO2
Table 9. Comparison of predicted species in backfill solutions at equilibrium under oxidizing (Eh = 0.8 V) and reducing (Eh = -0.43 V) conditions, pH = 7.3. Calculations performed by the WATEQFC model (Rur~NELLSand LINDBEXG,1981) (a) Saturation Indices (SI) of common minerals Mineral Gypsum Jarosite UO2-amorphous Coffinite Goethite Fe(OH)j--amorphous Albite Barite Pyrite Native Se
Eh = 0.8V
Eh = -0.43V
0.06 7.84 -28.5 -23.6 6.21 3.48 -0.98 1.36 Not present -55.6
0.06 -29.9 0.61 5.52 -6.36 -9.10 -0.98 1.36 0.28
(b) Dominant complexes predicted under oxidizing and reducing conditions Species s As
Fe Mo Se U V
Eh = 0.8V
Eh = -0.43V
so~, -
HS-
H2AsO~ Fe(OH) ° MoO42SeO~UO2(CO3)~H2VO~
H3AsO ° Fe 2+ MoO~HS~ U(OH).~ V(OH)3°
Total concentrations used as input to the m o d e l are listed in Tables 1 and 2. The results are presented in Table 8 for both acid-leach tailings raffinate and groundwater representative of the Westwater Canyon M e m b e r . In the tailings raffinate, sulfate minerals are supersaturated, including various forms of jarosite, gypsum and barite. Jarosite exhibits a high degree of supersaturation. In the groundwater, aluminosilicate minerals are supersaturated, together with U and Se minerals. The S E M (scanning electron microscopy) study showed abundant Se-bearing gypsum in the backfill e n v i r o n m e n t (THOMSON et al., 1984). The S E M resolution was not sufficient to identify eoffinite, although this phase must also be present based on theoretical considerations. A n especially informative exercise with respect to underground disposal of tailings is to examine how the solution changes with falling Eh. Using the solution described in Table 2, the Eh was varied from 0.8 to - 0 . 4 3 V to simulate reestablishment of reducing conditions in the formation. Table 9 presents the results of these calculations. The first section (Table 9a) gives the Saturation Index (SI) for common mineral phases. Those minerals formed by metals which exhibit a strong d e p e n d e n c e on redox conditions, including Fe, U and Se, b e c o m e insoluble under reducing conditions. Metals which do not depend on redox conditions (e.g. Ca and AI) are not affected by solution Eh. Table 9b presents the dominant species of the solutions under oxidizing and reducing conditions. As with the mineral phase, those metals which have
342
B.M. Thomson
mq!tiple oxidation states in aqueous solution will form different complexes as the Eh changes.
DISCUSSION In this paper, three distinct pieces of information have been presented: results of water quality monitoring programs of tailings raffinates and backfill slurries; results of analyses of solid samples collected from tailings piles and backfilled stopes; and a brief discussion of the solution mineral equilibria of the U and associated systems. It is readily apparent that tailings raffinate solutions contain extremely high levels of SO4- and TDS (total dissolved solids), and relatively high concentrations of numerous inorganic contaminants including As, Ba, Pb, Mo, Se, U, V, Th and Ra-226. Backfill slurry decant solutions, on the other hand, are of fairly good quality with only marginally high concentrations of U, Se and Ra. Furthermore, T,oraso~ and HEO6Er~ (1983) have shown that most of the water associated with the backfill slurry rapidly drains to the mine sump and is pumped to the surface where it is treated and discharged. The soil sampling and analysis program has shown that the silt and clay fraction of the backfill material, though approximately 5% by weight, contains a disproportionately high percentage of the inorganic contaminants. This is attributed to the high adsorptive and exchange capacities and high surface areas of clay minerals (mainly smectites and kaolinites) and is most clearly demonstrated by the enrichment factors presented in Tables 3 and 4. Especially high enrichment factors were found for multi-valent cations (Ca, Fe, and Mg) and oxyanions (As, Cr, Mo, Se, U and V). Silts and clays associated with tailings from the acid-leach mill process were found to contain most of the inorganic contaminants by weight (Table 6). Although enrichment factors are similar to those found for backfill samples, the silt-clay fraction is much larger, approaching 30%. Finally, equilibrium thermodynamic considerations suggest that precipitation of carbonate and aluminosilicate minerals should occur in the backfill as a result of supersaturation. The resulting cementation has been observed in backfilled mine stopes; however, this cementation did not increase the compressive strength of the backfilled sands in laboratory experiments. In addition to precipitation reactions resulting from supersaturation, reestablishment of reducing conditions in the backfill should result in chemical or microbial reduction, precipitation and subsequent immobilization of several soluble contaminants including As, Se, U and V. These constituents will fall to levels well below those established for consumptive uses of the water. It is therefore apparent that, although backfill disposal of mill tailings wastes does involve a one
et al.
time introduction of contaminated material into the underground environment, both the short and longterm consequences are negligible. Though the objective of the work described in this paper was to examine backfill disposal of mill railings, it is possible to extrapolate these findings to address underground disposal of the whole mill railings, not just the sand fraction. The two principal differences are that the whole railings fraction contains slimes typically 30 wt % and the pore fluid is of much poorer quality, being highly acidic and having a very high total dissolved solids concentration. There are two methods which can be used for emplacement of the whole mill tailings in an underground disposal site: adding water and pumping the tailings as a slurry, or moving dry tailings mechanically. If the slurry alternative is utilized, alkalinity of the water used to make the solution will raise the pH to near neutral. However, due to the high fraction of slimes the slurry will dewater slowly if at all. The slurry will therefore have little or no unconfined strength which may present stability problems, depending upon the disposal method. In any event, the geochemical processes should be similar to those described for backfill disposal of the sand fraction of the tailings; these include precipitation of carbonate and silicate minerals, adsorption/ion exchange of soluble constituents onto the clay surfaces, and chemical reduction and subsequent precipitation of As, Se, U, and V. As long as reducing conditions are maintained in the underground environment, these contaminants should remain immobile, consequently prevention of groundwater infiltration into the disposed tailings is not as critical as for other options. If air dried tailings are placed underground mechanically, the geochemistry will be significantly different due to the high acidity of associated pore waters and salts. In this case, sulfate minerals are predicted to occur. These minerals have been identified in tailings from the Grants Mineral Belt (LONGMIRE,1983). The very low pH values, below pH 2 in most cases, will result in continued solubility of most contaminants at levels near those values reported for surface tailings impoundments (see Table 2). Therefore, assurance must be provided that uncontaminated groundwaters will not come in contact with the tailings for this disposal option.
SUMMARY
This paper has described geochemical investigations of subsurface disposal of uranium mill tailings in the Grants Mineral Belt, New Mexico. The present disposal method is limited to slurry placement of the sand fraction of tailings from the acid-leach milling process in empty stopes to prevent roof collapse. Three sources of information are presented: water quality monitoring of underground drainage from
Geochemical constraints on underground disposal of U mill tailings backfill events, instrumental neutron activation analysis of surface tailings and backfill railings samples, and theoretical considerations based on conditions within the tailings which are expected to occur following the termination of underground mining. Pore fluids associated with tailings from the acidleach milling process are highly acidic, contain very high concentrations of total dissolved solids, S O l and C l - , as well as high concentrations of a n u m b e r of contaminants including As, Se, U , Th, V and Ra-226. Mixing of the tailings with treated mine wastewaters results in dilution of the acidity and the soluble constituents by, typically, two orders of magnitude. Rapid drainage of the slurry decant water to the mine sump, where it is p u m p e d to the surface, treated and discharged, results in little or no short-term contamination. Supersaturation of the backfill pore solution with respect to carbonate, sulfate and silicate minerals results in precipitation of c o m m o n minerals that have been reported in the Grants Mineral Belt. Coprecipitation of barite (BaSO4) provides a likely removal mechanism for Ra-226. Reestablishment of reducing conditions within the backfill should result in chemical reduction and subsequent precipitation of some transition metal contaminants including As, Se, U and V. Iron and Mn may be r e m o v e d through adsorption onto silt and clay minerals, or may precipitate as oxides and oxyhydroxides under oxidizing conditions or sulfides under reducing conditions. Based on these findings it is, therefore, likely that contaminants of the suite including As, Ra-226, Mo, Se, U, and V will precipitate under the reducing conditions found in backfilled stopes. Acknowledgements--Funding for the water analysis was provided by the New Mexico Energy Research and Development Institute under contract no. EMD-2-69-1107 and funding for the soil collection and analysis was provided by the U.S. Bureau of Mines, Spokane Research Center, contract no. J0225002. Assistance in analysis of the water samples was provided by the Kerr-McGee Nuclear Corporation and the Scientific Laboratory Division of the Health and Environment Department, State of New Mexico. Natalie Keller of the New Mexico Environmental Improvement Division provided assistance with the computer modeling.
REFERENCES
AMER. PUB. HEALTH. ASSOC., AMER. WATER WORKS ASSOC., WATER POLL. CONT. FED. (1980) Standard Methods for the Examination of Water and Wastewater, 15th ed., Washington, D.C. AMER. SOC. FOR TESTINGAND MATERIALS(1981) Standard method for particle-size analysis of soils, ASTM D 42263, pp. 113-123.
343
BARNESi. and CLARKEF. E. (1969) Chemical properties of ground waters and their corrosion and encrustation effects on wells. U.S. Geol. Surv. Prof. Pap. 498-D. BARTONA. F. M. and WILDEN. M. (1971) Dissolution rates of polycrystalline samples of gypsum and orthorhombic forms of CaSO4 by a rotating disc method. Trans. Faraday Soc. 67, 3590-3597. BLADHK. W. (1982) The formation of goethite, jarosite and alunite during the weathering of sulfide-bearing felsic rocks. Econ. Geol. 77,176-184. BROOKINS D. G. (1977) Uranium deposits of the Grants Mineral Belt: geochemical constraints on origin. In Rocky Mountain Association of Geologists -- I977 Symposium, pp. 337-352. BROOKINS D. G., THOMSON B. M. and LONGMIREP. A. (1982) Early diagenesis of uranium mine stope backfill. In Proc. Fifth Ann. Uranium Mill Tailings Management Conf., Fort Collins, CO, pp. 27-37. CHENOWETHW. L. (1979)Grants uranium region, prepared for field trip symposium on Grants uranium region, U.S. Dept. of Energy, Grand Junction office, CO. CHRISTOFFERSONJ. and CHRISTOFFERSONM. R. (1976) The kinetics of dissolution of calcium sulfate dihydrate. J. Crystal Growth 35, 29-88. CRAWFORD M. (1985) Mill railings: a $4-billion problem. Science 229,537-538. DEVOTO R. H. (1978) Uranium Geology and Exploration. Colorado School of Mines Press, Golden, CO. LANGMUIRD. (1978) Uranium solution-mineral equilibria at low temperatures with applications to sedimentary ore deposits. Geochim. Cosmochim. Acta 42,547-569. LONGMIREP. (1985) Geochemistry and alteration processes of uranium railings in groundwater, Grants Mineral Belt, New Mexico. In Practical Applications of Groundwater Geochemistry (eds B. Hitchon and E. I. Wallick), Proc. 1st Can./Amer. Conf. on Hydrogeology, pp. 190-199. Natn. Water Well Assoc., Dublin, OH. LONGMIREP. A. (1983) Geochemistry, diagenesis and contaminant transport of uranium tailings, Grants Mineral Belt, New Mexico. M.S. thesis, Dept. of Geology, University of New Mexico, Albuquerque, NM. PARKHURSTD. L., THORSTENSOND. C. and PLUMMERL. N. (1980) PHREEOE - - a computer program for geochemical calculations. U.S. Geol. Surv. Water-Resour. Invest. 80-96. RUNNELLS D. D. and LINDRERG R. D. (1981) Hydrogeochemical exploration for uranium ore deposits: use of the computer model WATEQFC. J. Geochem. Explor. 15, 37-50. THOMSON B. M., BROOKINSD. G. and LONGMIREP. A. (1984) Investigation of geochemical and hydrological transformation in backfilled uranium mill tailings. Final report submitted to U.S. Bureau of Mines, Spokane, WA. THOMSONB. M. and HEGGENR. J. (1983) Water quality and hydrologic impacts of disposal of uranium mill tailings by backfilling. In Management of Wastes From Uranium Mining and Milling, pp. 373-384. Inter. Atomic Energy Agency, IAE-SM-262/51, Vienna. THOMSON B. M.. LONGMIREP. A. and GALLAHERB. M. (1982) The hydrologic environment and uranium production in New Mexico. In Proc. of Conf. on Water and Energy, Technical and Policy Issues, pp. 525-531. Amer. Soc. of Civil Engrs., Fort Collins, CO. U.S. NUCLEAR REGULATORY COMMISSION (1980) Final Generic Environmental Impact Statement On Uranium Milling, NUREG-0706. Vol. 1, Ch. 8, pp. 18-28. Office of Nuclear Material.
0883-2927/86 $3A10~- .lgJ Pergamon Journals Ltd.
Printed in Great Britain
Geochemical constraints on underground disposal of uranium mill tailings BRUCE M. THOMSON Department of Civil Engineering, University of New Mexico, Albuquerque. NM 87131, U.S.A. PATRICK A . LONGMIRE Environmental Health Department, City of Albuquerque, P.O. Box 1293. Albuquerque, NM 87103. U.S.A. and DOUGLAS G. BROOKINS Department of Geology, University of New Mexico, Albuquerque. NM 87131, U.S.A.
A b s t r a c t - - A three phase investigation has been conducted on groundwater quality impacts of the
underground disposal of tailings from acid-leach milling of uranium ores. These phases included field collection and analysis of samples obtained during backfilling of mill tailings in empty underground mine stopes, collection of soil samples from mill tailings piles and previously backfilled stopes, and evaluation of thermodynamic constraints on possible geochemical transformations. Contaminants of principal concern include As, Mo, Se, U, V and Ra-226. The investigation has shown that short-term degradation of groundwater due to backfill disposal of the sand fraction of uranium tailings is negligible. Long-term effects, defined as those occurring after mining operations cease and the mine fills with water, are predicted to also be very small. This is attributed to immobilization of pollutants through chemical reduction and precipitation, as well as adsorption onto aquifer materials. This conclusion is substantiated, in part, by observation of high concentrations of most of the contaminants on the silt and clay fraction of the soil samples collected, in contrast to the concentrations found on the sand fraction.
INTRODUCTION THE SAFE disposal of uranium mill tailings is one of the major environmental obstacles currently facing the front-end of the nuclear fuel cycle (CRAWFORD, 1985). It is estimated that there are 300 million metric tons of tailings associated with U.S. uranium mills, which are either operating, on standby basis, or at inactive or abandoned sites. Though the tailings themselves constitute a low level radioactive waste, principally due to gaseous Rn, disposal problems are primarily the result of conventional air- and waterborne contaminants. The principal air-borne contaminants consist of blowing dust and Rn. Due to the nature of the milling process, which uses either acid or alkaline leaching of U precipitates, aqueous solutions associated with spent tailings are of extremely poor quality. Water quality of tailing raffinates (the aqueous solution remaining after uranium has been extracted by a solvent) from U mills, which were operating in New Mexico in 1980 and 1981, is presented in Table 1 (THOMSON et al., 1982). This paper focuses on impacts of subsurface tailings disposal on groundwater quality. The principal objective of any hazardous waste
disposal option is prevention of interaction between the waste material, the hydrosphere and the biosphere. The Nuclear Regulatory Commission has identified nine alternatives for mill railings disposal, of which all but three involve some form of subsurface disposal (U.S. NUCLEAR REGULATORY COMMISSION, 1980). Most subsurface disposal alternatives eliminate direct interaction between the biosphere and the waste material, and also provide barriers to Rn emanation. The remaining area of concern for these options is the potential for groundwater contamination. There are two basic variations on subsurface disposal options: disposal in specially prepared sites, and placement of the tailings in empty underground mine stopes. This latter alternative has seen extensive practice in the Grants Mineral Belt (GMB, Fig. 1), New Mexico. The objective of this investigation is to characterize the geochemical processes taking place that may affect groundwater quality. Though the studies to be described deal specifically with backfill disposal of tailings, the findings should be applicable to other forms of subsurface disposal such as burial in trenches. The results of this investigation are used to discuss the suitability of underground disposal options as a method for disposal of U mill tailings.
B. M. Thomson et al.
336 !
Grants Minerot "-[==.~orea" I
; k
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
Belt
I McKintey County I
I
,. ~
N
Churchrock
[
District ~,.,,.~,..,_,
AmbrosiaLake District
'
0
I. °
:g l;z. . . . . . . .
.
.
~ . ~Loguna \
I
.
.
j'll
Grants
0
,2
,m
, Cibolo County I
,, I
FIG. 1. Location of Grants Mineral Belt. BACKGROUND INFORMATION
Backfilling
Uranium ore deposits are frequently associated with aquifers. In these cases, deposition of U ore is thought to be a result of aqueous transport of oxidized, and therefore soluble, U species to organically-rich reducing zones where reduction and subsequent precipitation of U minerals occurs (DEVoTo, 1978; BROOrI~S, 1977). LAr~GMUm(1978) has summarized the oxidation-reduction and acid-base chemistry of U. Due to the location of ore deposits in aquifers, U mines must continually pump accumulated water from the mine. In New Mexico, dewatering requirements range from less than 0.006 m3/s to greater than 0.2 m3/s (TrtoMsoN et al., 1982). In the Grants Mineral Belt, the ore bearing strata, the Morrison Formation,
is overlain by two or more distinct aquifers, each separated by shale aquicludes (CHENOWETn, 1979). Collapse of open stapes causes fracture propagation towards the surface resulting in hydraulic connection with overlying aquifers and greatly increasing mine dewatering requirements. To minimize the extent of this subsidence, many of the mining companies in New Mexico have backfilled empty stapes with tailings. The tailings used in the backfill process consist of classified sands. Unclassified tailings typically contain approximately 30% slimes (<200 mesh) by weight, which inhibits their dewatering. Backfill disposal of the total tailings mix would thus present a hazard to underground workers in that failure of a retaining bulkhead would release a slurry of tailings into access drifts within the mine. Removal of the slimes allows rapid dewatering by gravity drainage to the mine sump. The sands are piped into empty mine stapes as a slurry consisting of equal weights of treated mine water and sand.
Table 1. Concentrations of selected constituents of New Mexico uranium mill tailings pond raffinates. Analyses of unfiltered samples collected by New Mexico EnvironmentalImprovement Division personnel, 1978to 1981. All concentrations in mg/l unless noted (from THOMSONet al., 1982) Four acid leach mills ( 14samples) Constituent Gross radioactivity (pCill) Ra-226 (pCi/l) As Mo Se SOdU V NH 3(mg N/I) TDS pH
Minimum 3200 15 0.18 0.20 0.006 300.0 1.1 39.0 3.3 17 900 0.3
Median
One alkaline leach mill (5 samples) Maximum
38 000
73 000
70 1.3 0.90 0.21 29 700 15 74 400 39 800 1.05
1800 5.6 29.5 6.97 56 000 69 107 3960 72 800 2.15
Minimum
Median
3400
6700
56 2.1 72 22.1 5500 4.2 1.2 1.1 17 000 9.9
58 5 98 29.5 8400 54 14 16 25 400 10.1
Maximum 10000 90 7.2 105 51.2 16 700 70 16 335 39 700 10.3
Geochemical constraints on underground disposal of U mill tailings The impacts of backfilling can be broken into two categories (THOMSON and HEGGEN, 1983). Short-term effects occur while the mine is in operation and being dewatered. Drainage from the backfilled stope flows rapidly to the mine sump where it is diluted with other minewater, and is then p u m p e d to the surface, treated and discharged. D u e to the relatively small volume, dilution with treated water, and the brief period in which contaminated water remains in the mine, the short-term impacts of backfilling are negligible. L o n g - t e r m effects are those which may occur after dewatering ceases and saturated conditions return to the formation as the original potentiometric head is reestablished. D u e to access limitations (the mine fills with water), it is not possible to sample directly such stapes; therefore, projections about long-term impacts are based on laboratory investigations and theoretical considerations.
FIELD AND LABORATORY INVESTIGATIONS
Sampling program The objective of the sampling program was to characterize the geochemistry of uranium mill tailings from acid and alkaline-leach mills in New Mexico. It should be noted that no alkaline-leach tailings have been used recently for backfill in New Mexico mines. The sampling program involved collection of tailings from surface tailings impoundments, back filled stapes of various ages, and water samples from backfill drainage. Tailings were collected from two operating mills, an acid-leach and an alkalineleach facility. Backfill samples were collected from three mines and represented tailings from two different acid-leach mills. Six different stapes were sampled, with ages ranging from approximately six months to nearly 10 y. Loose tailings samples were collected using a stainless steel spatula and placed in sealable plastic bags. Core samples 1 m in length were collected using 5 cm (2 in) galvanized steel thin walled tubes. In the laboratory these cores were extruded and sealed in plastic. Water samples were collected in 21 plastic jugs and preserved with 2 ml/l HNO3.
337
TaKings sample
t
!
Scanning electron microscopy
T
JChemi cendal mechanical dispers on
t X- roy diff roction !
Water froction INAA
!
SiLt/ctoy fraction I TNAA
Sand fraction I INAA
FIc. 2. Analytical procedures used to characterize soil samples. (NaPO3)6). The solution was shaken and allowed to settle in covered beakers for sufficient time such that no particles with a diameter >50/~m remained in suspension. The fines. which remain in solution, represent the silt and clay fractions of the samples. Due to classification of the tailings during the milling process, a sufficient amount of pure clay material (diameter < 5/zm) could not be collected from backfilled stapes. At the end of the settling period, the supernatant liquid was removed from the beaker. The remaining sand was dried and transferred to a sealed vial. The decant solution was passed through a 0.45 ~tm membrane filter that had been previously washed with distilled water, the solids were dried, and transferred to a sealed vial. The filtrate was collected and placed in a sealed container. Due to the nature of the separation process, this filtrate is representative of the water soluble constituents associated with the sample and is analogous to a desorption experiment. The sand and silt-clay fractions were crushed in an agate mortar and pestle by hand and passed through nylon sieves to less than 200 mesh. The mortar and pestle and sieve sets were washed with dilute HC1 and distilled water between samples to minimize contamination. The samples were then subject to instrumental neutron activation analysis (INAA) at the Omega West Reactor operated by Los Alamos National Laboratory. Details and complete results of the INAA procedure are contained in THOMSONet al. (1984) and LONGMIRE(1983). RESULTS
Analytical program Water samples collected in the field were analyzed by procedures published in Standard Methods LAMER. Pun. HEALTH ASSOC., 1980). Metals were measured by inductively coupled argon plasma spectroscopy and atomic absorption spectroscopy. Anions were analyzed by wet chemical procedures. Uranium-238 was determined by alpha-spectrometry. Figure 2 presents a schematic of the analytical program which was used on the soil samples. Initially, each of the samples was split, one fraction being subjected to mineralogical analysis and the other to chemical analysis. Mineralogical analysis consisted of scanning electron microscopy and X-ray diffraction studies. Prior to chemical analysis, the soils were separated by size based on settling velocity using a procedure similar to that used to determine particle size distributions (ASTM D42263. ASTM. 1981). A solution of the soil was prepared using a chemical dispersant (sodium hexametaphosphate - -
Results of underground monitoring of water quality associated with backfill placement in two different mines are summarized in Table 2. These two mines utilized the same mine water treatment system, consequently the discharge quality parameters are averaged together. Of special interest is the variable quality associated with groundwater infiltrating into the mine. Results of the analytical program applied to the soil samples are summarized in Tables 3-6. These results represent the averages of all analyses on the backfilled sands and acid-leach mill tailings samples. Tables 3 and 4 contain the elemental and selected radionuclide concentrations of the sand and silt--clay fractions, as well as the soluble concentration of each element found when 100 g of material is leached with
B. M. Thomson et al.
338
Table 2. Results of water quality monitoring program during backfill event. All units in mg/l except as noted
Constituent As Ca Fe Mn Mo NOq (mg N/I) Se SO]TDS U-238 V pH (pH units)
Mine water
Slurry
Backmix drainage
Commingled water
Surface discharge
0.056 219 0.017 <0. I 46 6 2.6 1680 3090 101 0.81 7.3
0.023 39[) 0.06 <0.1 6 24 0.91 2120 3560 20 0.27 7.3
0.022 -0.13 <0.1 8.9 28 1.8 2390 3800 74 0.49 6.6
0.015 -0.12 <(~. l 37 7 2.8 1960 3190 63 0.55 7.5
0.021 32.0 0.042 -8.0
Table 3. Average concentrations in each fraction of backfilled sands, determined by INAA (9 samples) Element AI As Ba Ca Cr Fe K Mg Mo Se Th-223 U-239 V
Sand (ppm)
Clay (ppm)
Water* (mg/l)
Enrichment Clay/Sand
35633.3 3.16 695.8 2362.2 10.4 2081.6 22366.7 0.0 10.2 8.24 1.828 29.27 82.4
66800.0 18.47 945.8 32887.8 317.9 25994.4 25855.6 3173.3 178.6 80.31 7.513 226.41 928.8
3.3 0.12 0.0 323.9 0.2 7.8 228.5 0.0 0.3 0.90 0.002 0.70 0.6
1.9 5.8 1.4 13.9 30.7 12.5 1.2 17.5 17.5 9.8 4.1 7.7 11.3
Table 4. Average concentrations in each fraction of acidleach uranium, mill tailings by INAA (2 samples)
Element A1 As Ba Ca Cr Fe K Mg Mo Se Th-223 U-239 V
Sand (ppm)
Clay (ppm)
Water* (mg/l)
Enrichment Clay/Sand
37850.0 2.51 778.5 2775.0 9.88 2387.5 23500.0 0.0 5.39 8.17 1.172 19.04 169.45
60050.0 20.44 642.0 41150.0 157.30 30370.00 25400.0 2650.0 216.00 124.10 2.488 118.35 954.00
0.0 0.17 0.0 470.1 0.0 11.5 64.9 0.0 0.0 0.13 0.0 0.10 0.69
1.6 8.2 0.8 14.8 15.9 12.7 1.1 40.1 15.2 2.1 6.2 5.6
* Concentrations in 250 ml of solution containing 100 g of backfilled sands.
* Concentrations in 250 ml of solution containing 100 g of acid-leach uranium mill tailings.
Table 5. Distribution of the total mass of each element from a 100 g sample of backfilled sands.
Table 6. Distribution of the total mass of each element from a 100 g sample of acid-leach uranium mill tailings
Element AI As Ba Ca Cr Fe K Mg Mo Se Th-223 U-239 V
Sand (g)
Silt and Clay (g)
Soluble (g)
Element
3.3753 2.99E-04 0.0659 0.2238 9.82E-04 0.1972 2.1186 0.00E-00 9.69E-04 7.80E-04 1.73E-04 2.77E-03 7.81E-03
0.3525 9.74E-05 0.0050 0.1735 1.68E-03 0.1372 0.1364 1.67E-02 9.43E-04 4.24E-04 3.96E-05 1.19E-03 4.90E-03
0.0011 2.90E-05 0.0000 0.0845 5.56E-05 0.0025 11.0672 0.00E-00 9.55E-05 3.04E-04 4.49E-07 2.12E-04 1.62E-04
AI As Ba Ca Cr Fe K Mg Mo Se Th-223 U-239 V
Sand (g)
Silt and Clay (g)
Soluble (g)
2.839 1.88E-04 0.058 0.208 7.41E-04 0.1791 1.7625 0.0000 4.04E-04 6.13E-04 8.79E-05 1.43E-03 1.27E-02
1.501 5.11E-04 0.016 1.029 3.93E-03 0.7593 0.6350 0.0663 5.40E-03 3.10E-03 6.22E-05 2.96E-03 2.39E-02
0.000 5.20E-05 0.000 0.141 0.00E+00 0.0035 0.0190 0.0000 0.00E+00 3.88E-05 0.00E+00 3.08E-05 2.03E-04
Geochemical constraints on underground disposal of U mill railings IO
-4 -2-1
/5 Gibbsite saturation
aS04 2HzO
I
6
339
log o (SO2").a (H+)Z=-12 0
z
Atunite
5 °
-5c,~
-Z-~og . . . a. (SO~-) . " a- ~ (H)z + =-~
/
I -5
I[ -I
I°
3
logo (FeS+)/a(H+) 3
)
4
6
8
0
12
I
pH FIG. 4. Activity diagram of gypsum-calcite at 1 arm pressure and 25° C using 10-I (9600 mg/1), I0-: (960 mg/l) and 10-4 (9.6 mg/l) sulfate. Tailings raffinate backfill slurry and native groundwater are denoted by C), [] and/~ respectively.
FIG. 3. Activity diagram of jarosite-alunite-potassium
feldspar-gibbsite-goethitesystem at 1 atm pressure and 25° C (modified from BLADn, 1982). Tailings raffinate and native groundwater are denoted by © and/X respectively.
250 ml of sodium hexametaphosphate solution. The right-most column presents the ratio of the elemental concentration associated with the sand fraction to that of the silt-clay fraction. It is presented as an enrichment factor. Using these data from Tables 3 and 4, together with the results of the particle size analysis, the mass of each element and nuclide associated with each fraction of a 100 g soil sample is presented in Tables 5 and 6. Thus in a 100 g sample of backfill sands there would be 3.38 g of A1 with the sand fraction, 0.35 g bound to the silt-clay fraction, and 1.0 mg would be soluble (Table 5). THEORETICAL CONSIDERATIONS
Two distinct major diagenetic processes are expected to occur in a backfilled stope which will affect the pore water chemistry; precipitation of supersaturated phases followed by chemical reduction and subsequent precipitation of originally oxidized constituents. Other processes which may occur include mixing of waters of different composition (backfill slurry water and native groundwater), and compaction of the backfilled material (LoN6MIRE, 1983). This section focuses on the precipitation reactions. In considering geochemical processes which may occur in underground disposal of uranium mill railings, there are two very useful theoretical tools available. These are equilibrium phase diagrams and numerical modeling of aqueous solutions. The backfill solution is supersaturated or nearly saturated with respect to a number of sulfate, carbonate, oxide and silicate minerals. Sulfate salts include gypsum, jarosite and thenardite (BRoorxr~s et al., 1982). The stabilities of alunite ((K,Na)A13(SO4)_,(OH)6), jarosite ((K,NaFe3)(SO4)(OH)6), gibbsite (AI(OH)3), goethite ( F e O O H ) , and K-feldspar as functions of activities of Fe s+, A! 3+, H + , and SO24- in
contiguous solution are shown in Fig. 3 (modified from BLADn, 1982). This figure illustrates that tailings raffinate is in equilibrium with jarosite, whereas groundwater is in equilibrium with gibbsite (AI(OH)3). Gibbsite occurs as surface coatings within the Westwater Canyon Member (MORRISON Formation) aquifer. Additionally, uncontaminated groundwater is nearly saturated with respect to goethite. The stabilities of gypsum and calcite, as functions of the activity of Ca "-+ and pH, are illustrated in Fig. 4. The taitings raffinate is supersaturated with respect to gypsum, whereas calcite should precipitate in groundwater. The backfill slurry is supersaturated with respect to gypsum. Formation of these minerals in the backfill may have a beneficial consequence in that precipitation may result in removal of other group II metals including Ba and Ra through coprecipitation. A more thorough discussion on the geochemical processes occurring during mixing of backfill slurry with groundwater is presented by LONGMIRE (1985). Dissolution of gypsum, following first-order kinetics, will control the release of Ca z+ and SO~- ions to solution as uncontaminated groundwater (Westwater Canyon Member) resaturates the backfill. Gypsum dissolution is diffusion-controlled, where calculated diffusion coefficients range from 4 to 8 × 10 -6 cm2/s (BARTON and WILDE, 1971; CHRISTOFFERSON and CHRISTOFVERSO~, 1976). These data suggest that a long-term impact on groundwater quality due to gypsum dissolution should be negligible. Additionally, sorbed species, including Ba and Ra, should undergo a limited release to groundwater. THOMSON et al. (1984) investigated the possibility of an increase in unconfined compressive strength of backfilled sands as a result of cementation following precipitation of these species. No statistically identifiable increase in strength was found, which was attributed to the relatively small amount of material present in solution. Mass balance calculations suggest also that there is too little soluble material for its precipitation to affect, in a measurable way, the porosity or hydraulic properties of the formation.
340
B.M. Thomson et al.
~0
-4~ 2
20
2
Setenium
Uronium
p-'/
H2SeO~ I
--)8
" ' P O ~
06
US[04
04
04
, ",. I
uoJco=)~
Se
-o2
o2~ "~
o
Ferrosetite
'=''
0
SeO§-
-02 -04
-I0
0
-IC
2
4
6
pH
8
I0
12
14
0
-06
2
4
6
8
I0
12
14
pH
FIG. 5. Eh-pH diagram for uranium species at I atm pressure and 25° C. Total activitiesof uranium, total dissolved carbon and silicic acid are 10-6 (0.24 mg/l), 10-: (120 mg/l) and 10-3.5 (30.4 mg/l), respectively. Modified from BROOr]NS (1977).
FIG. 6. Eh-pH diagram for selenium and iron species at 1 atm pressure and 25° C for 10-° (0.08 mg/l) selenium and 10-6 (0.06 mg/l) iron, respectively (LoN6MIRE, 1983).
E h - p H diagrams are useful for evaluating equilibrium oxidation-reduction processes in the backfill material. A t the time of burial, the backfill slurry is near neutral p H and under moderately high oxidizing conditions (Eh = 0.3 V). With time, the Eh of the pore water is expected to fall sharply as reducing conditions characteristic of the formation are reestablished. E h - p H diagrams for U and Se are presented in Figs 5 and 6, respectively, as two of the constituents of most concern. Under reducing conditions, coffinite (U(SiO4)I_x(OH)4x) is expected to precipitate, and indeed it is the major crystalline uranium mineral found in the Grants Mineral Belt. Upon oxidation to the hexavalent state, U becomes soluble as an anionic uranyl carbonate complex. Selenium exhibits similar chem-
istry in that under reducing conditions it is insoluble, and therefore immobile, as either a selenide (Se(-II)) or metallic selenium (S(O)). Ferroselite (FeSe2) is found in the Grants Mineral Belt (BaooKx~s, 1977). Table 7 summarizes the oxidation-reduction characteristics of a number of inorganic constituents. The important point to note is that under reducing conditions most transition metal species are insoluble and therefore not mobile. Although equilibrium phase diagrams are useful in illustrating transitions and stability regions, numerical simulation of the various solutions is more useful for addressing a specific aqueous system. The P H R E E Q E geochemical model (PARKHURST et al., 1980) was used in this investigation. The W A T E Q F C geochemical model was also used to perform additional thermodynamic calculations on different waters associated with mine stope backfilling (Rur~NELLS and LINDBERO, 1981). Of particular interest is comparison of the ion activity products to the thermodynamic solubility product for the minerals and solid compounds of interest. The relative degree of saturation is measured by the saturation index (SI) (BARNES and CLARKE, 1969):
Table 7. Typical dominant species of inorganic constituents present under oxidizing and reducing conditions in aqueous systems Metal
Oxidizingconditions
Reducing conditions
As Cd Cr Cu Pb Hg Ni N Se Ag Zn
H2AsO4Cd 2÷, CdCO3(s) CrO~Cu 2+. CuO(.) PbCO~) HgO,) Ni:+ NO~ SeO 2-, HSeO; AgCl(s) Zn 2+, ZnO,)
As.,S3(s),AsSis) CdS(~) Cr203(s)
CuS(s) PbS(~ Hg~s).HgS,) NiS(~) NH~, Nzcg) Se°~s),FeSe2,) Ag°.). Ag2S(~) ZnS.)
SI = log]0 (IAP/Ks0) where IAP = ion activity product and Ks0 = solubility product. If a solution is in equilibrium with its corresponding solid phase SI = 0. SI > 0 indicates a supersaturated solution, and SI < 0 represents an undersaturated solution.
Geochemical constraints on underground disposal of U mill tailings
341
Table 8. Saturation indices (si ~ log (IAP/Kti) for tailings raffinate and groundwater (Westwater Canyon Member), Ambrosia Lake Mining District, Grants Mineral Belt. Calculated by WATEQFC (RuNNELLSand LZNDBERG,1981)
Formula
Raffinate
Westwater Canyon Member Section 35 Mine
KFe3( SO4 )~.(OH ), Fe203 NaFea(SO4)2(OH)6 SiO2 BaSO4 CaSO4 • 2H20 FePO4.2H20
5.7775 3.9803 3.3572 1.8353 1.1316 0.3250 -0.0421 - 0.1560 -0.8001 -3.4772 -4.6603 - 8.4743 - 11.1805 -9.4185 - 12.2008 - 12.2873 - 13.6475 - 17.5814 - 18.3992 - 54.7910 -9.5664 - 18.4292 -24.1194
- 12.0887 6.4945 - 14.2089 0.0984 1.0233 -0.8778 -6.9039 - 10.4177 0.4687 -2.2629 -6.3248 1.7542 -0.7425 0.1856 3.5285 0.2682 3.3376 3.7498 4.6849 10.0837 -2.6191 11.6625 - 1.2051
Mineral phase Jarosite Hematite Natrojarosite Quartz Barite Gypsum Strengite Ilsemannite Goethite Fe(OH)3 amorphous Thenardite Microcline Albite Gibbsite Kaolinite Calcite Selenium native Coffinite Uraninite Ferroselite Alunite Magnetite UO2 amorphous
Mo308
FeO(OH) Fe(OH)3 Na2SO4 KAISi308 NaAISi308 AI(OH)3 AI2Si2Os(OH)4 CaCO 3 Se USiO4 UO2 Fe Se2 KAla(SO4)~(OH)6 FeFe204 UO2
Table 9. Comparison of predicted species in backfill solutions at equilibrium under oxidizing (Eh = 0.8 V) and reducing (Eh = -0.43 V) conditions, pH = 7.3. Calculations performed by the WATEQFC model (Rur~NELLSand LINDBEXG,1981) (a) Saturation Indices (SI) of common minerals Mineral Gypsum Jarosite UO2-amorphous Coffinite Goethite Fe(OH)j--amorphous Albite Barite Pyrite Native Se
Eh = 0.8V
Eh = -0.43V
0.06 7.84 -28.5 -23.6 6.21 3.48 -0.98 1.36 Not present -55.6
0.06 -29.9 0.61 5.52 -6.36 -9.10 -0.98 1.36 0.28
(b) Dominant complexes predicted under oxidizing and reducing conditions Species s As
Fe Mo Se U V
Eh = 0.8V
Eh = -0.43V
so~, -
HS-
H2AsO~ Fe(OH) ° MoO42SeO~UO2(CO3)~H2VO~
H3AsO ° Fe 2+ MoO~HS~ U(OH).~ V(OH)3°
Total concentrations used as input to the m o d e l are listed in Tables 1 and 2. The results are presented in Table 8 for both acid-leach tailings raffinate and groundwater representative of the Westwater Canyon M e m b e r . In the tailings raffinate, sulfate minerals are supersaturated, including various forms of jarosite, gypsum and barite. Jarosite exhibits a high degree of supersaturation. In the groundwater, aluminosilicate minerals are supersaturated, together with U and Se minerals. The S E M (scanning electron microscopy) study showed abundant Se-bearing gypsum in the backfill e n v i r o n m e n t (THOMSON et al., 1984). The S E M resolution was not sufficient to identify eoffinite, although this phase must also be present based on theoretical considerations. A n especially informative exercise with respect to underground disposal of tailings is to examine how the solution changes with falling Eh. Using the solution described in Table 2, the Eh was varied from 0.8 to - 0 . 4 3 V to simulate reestablishment of reducing conditions in the formation. Table 9 presents the results of these calculations. The first section (Table 9a) gives the Saturation Index (SI) for common mineral phases. Those minerals formed by metals which exhibit a strong d e p e n d e n c e on redox conditions, including Fe, U and Se, b e c o m e insoluble under reducing conditions. Metals which do not depend on redox conditions (e.g. Ca and AI) are not affected by solution Eh. Table 9b presents the dominant species of the solutions under oxidizing and reducing conditions. As with the mineral phase, those metals which have
342
B.M. Thomson
mq!tiple oxidation states in aqueous solution will form different complexes as the Eh changes.
DISCUSSION In this paper, three distinct pieces of information have been presented: results of water quality monitoring programs of tailings raffinates and backfill slurries; results of analyses of solid samples collected from tailings piles and backfilled stopes; and a brief discussion of the solution mineral equilibria of the U and associated systems. It is readily apparent that tailings raffinate solutions contain extremely high levels of SO4- and TDS (total dissolved solids), and relatively high concentrations of numerous inorganic contaminants including As, Ba, Pb, Mo, Se, U, V, Th and Ra-226. Backfill slurry decant solutions, on the other hand, are of fairly good quality with only marginally high concentrations of U, Se and Ra. Furthermore, T,oraso~ and HEO6Er~ (1983) have shown that most of the water associated with the backfill slurry rapidly drains to the mine sump and is pumped to the surface where it is treated and discharged. The soil sampling and analysis program has shown that the silt and clay fraction of the backfill material, though approximately 5% by weight, contains a disproportionately high percentage of the inorganic contaminants. This is attributed to the high adsorptive and exchange capacities and high surface areas of clay minerals (mainly smectites and kaolinites) and is most clearly demonstrated by the enrichment factors presented in Tables 3 and 4. Especially high enrichment factors were found for multi-valent cations (Ca, Fe, and Mg) and oxyanions (As, Cr, Mo, Se, U and V). Silts and clays associated with tailings from the acid-leach mill process were found to contain most of the inorganic contaminants by weight (Table 6). Although enrichment factors are similar to those found for backfill samples, the silt-clay fraction is much larger, approaching 30%. Finally, equilibrium thermodynamic considerations suggest that precipitation of carbonate and aluminosilicate minerals should occur in the backfill as a result of supersaturation. The resulting cementation has been observed in backfilled mine stopes; however, this cementation did not increase the compressive strength of the backfilled sands in laboratory experiments. In addition to precipitation reactions resulting from supersaturation, reestablishment of reducing conditions in the backfill should result in chemical or microbial reduction, precipitation and subsequent immobilization of several soluble contaminants including As, Se, U and V. These constituents will fall to levels well below those established for consumptive uses of the water. It is therefore apparent that, although backfill disposal of mill tailings wastes does involve a one
et al.
time introduction of contaminated material into the underground environment, both the short and longterm consequences are negligible. Though the objective of the work described in this paper was to examine backfill disposal of mill railings, it is possible to extrapolate these findings to address underground disposal of the whole mill railings, not just the sand fraction. The two principal differences are that the whole railings fraction contains slimes typically 30 wt % and the pore fluid is of much poorer quality, being highly acidic and having a very high total dissolved solids concentration. There are two methods which can be used for emplacement of the whole mill tailings in an underground disposal site: adding water and pumping the tailings as a slurry, or moving dry tailings mechanically. If the slurry alternative is utilized, alkalinity of the water used to make the solution will raise the pH to near neutral. However, due to the high fraction of slimes the slurry will dewater slowly if at all. The slurry will therefore have little or no unconfined strength which may present stability problems, depending upon the disposal method. In any event, the geochemical processes should be similar to those described for backfill disposal of the sand fraction of the tailings; these include precipitation of carbonate and silicate minerals, adsorption/ion exchange of soluble constituents onto the clay surfaces, and chemical reduction and subsequent precipitation of As, Se, U, and V. As long as reducing conditions are maintained in the underground environment, these contaminants should remain immobile, consequently prevention of groundwater infiltration into the disposed tailings is not as critical as for other options. If air dried tailings are placed underground mechanically, the geochemistry will be significantly different due to the high acidity of associated pore waters and salts. In this case, sulfate minerals are predicted to occur. These minerals have been identified in tailings from the Grants Mineral Belt (LONGMIRE,1983). The very low pH values, below pH 2 in most cases, will result in continued solubility of most contaminants at levels near those values reported for surface tailings impoundments (see Table 2). Therefore, assurance must be provided that uncontaminated groundwaters will not come in contact with the tailings for this disposal option.
SUMMARY
This paper has described geochemical investigations of subsurface disposal of uranium mill tailings in the Grants Mineral Belt, New Mexico. The present disposal method is limited to slurry placement of the sand fraction of tailings from the acid-leach milling process in empty stopes to prevent roof collapse. Three sources of information are presented: water quality monitoring of underground drainage from
Geochemical constraints on underground disposal of U mill tailings backfill events, instrumental neutron activation analysis of surface tailings and backfill railings samples, and theoretical considerations based on conditions within the tailings which are expected to occur following the termination of underground mining. Pore fluids associated with tailings from the acidleach milling process are highly acidic, contain very high concentrations of total dissolved solids, S O l and C l - , as well as high concentrations of a n u m b e r of contaminants including As, Se, U , Th, V and Ra-226. Mixing of the tailings with treated mine wastewaters results in dilution of the acidity and the soluble constituents by, typically, two orders of magnitude. Rapid drainage of the slurry decant water to the mine sump, where it is p u m p e d to the surface, treated and discharged, results in little or no short-term contamination. Supersaturation of the backfill pore solution with respect to carbonate, sulfate and silicate minerals results in precipitation of c o m m o n minerals that have been reported in the Grants Mineral Belt. Coprecipitation of barite (BaSO4) provides a likely removal mechanism for Ra-226. Reestablishment of reducing conditions within the backfill should result in chemical reduction and subsequent precipitation of some transition metal contaminants including As, Se, U and V. Iron and Mn may be r e m o v e d through adsorption onto silt and clay minerals, or may precipitate as oxides and oxyhydroxides under oxidizing conditions or sulfides under reducing conditions. Based on these findings it is, therefore, likely that contaminants of the suite including As, Ra-226, Mo, Se, U, and V will precipitate under the reducing conditions found in backfilled stopes. Acknowledgements--Funding for the water analysis was provided by the New Mexico Energy Research and Development Institute under contract no. EMD-2-69-1107 and funding for the soil collection and analysis was provided by the U.S. Bureau of Mines, Spokane Research Center, contract no. J0225002. Assistance in analysis of the water samples was provided by the Kerr-McGee Nuclear Corporation and the Scientific Laboratory Division of the Health and Environment Department, State of New Mexico. Natalie Keller of the New Mexico Environmental Improvement Division provided assistance with the computer modeling.
REFERENCES
AMER. PUB. HEALTH. ASSOC., AMER. WATER WORKS ASSOC., WATER POLL. CONT. FED. (1980) Standard Methods for the Examination of Water and Wastewater, 15th ed., Washington, D.C. AMER. SOC. FOR TESTINGAND MATERIALS(1981) Standard method for particle-size analysis of soils, ASTM D 42263, pp. 113-123.
343
BARNESi. and CLARKEF. E. (1969) Chemical properties of ground waters and their corrosion and encrustation effects on wells. U.S. Geol. Surv. Prof. Pap. 498-D. BARTONA. F. M. and WILDEN. M. (1971) Dissolution rates of polycrystalline samples of gypsum and orthorhombic forms of CaSO4 by a rotating disc method. Trans. Faraday Soc. 67, 3590-3597. BLADHK. W. (1982) The formation of goethite, jarosite and alunite during the weathering of sulfide-bearing felsic rocks. Econ. Geol. 77,176-184. BROOKINS D. G. (1977) Uranium deposits of the Grants Mineral Belt: geochemical constraints on origin. In Rocky Mountain Association of Geologists -- I977 Symposium, pp. 337-352. BROOKINS D. G., THOMSON B. M. and LONGMIREP. A. (1982) Early diagenesis of uranium mine stope backfill. In Proc. Fifth Ann. Uranium Mill Tailings Management Conf., Fort Collins, CO, pp. 27-37. CHENOWETHW. L. (1979)Grants uranium region, prepared for field trip symposium on Grants uranium region, U.S. Dept. of Energy, Grand Junction office, CO. CHRISTOFFERSONJ. and CHRISTOFFERSONM. R. (1976) The kinetics of dissolution of calcium sulfate dihydrate. J. Crystal Growth 35, 29-88. CRAWFORD M. (1985) Mill railings: a $4-billion problem. Science 229,537-538. DEVOTO R. H. (1978) Uranium Geology and Exploration. Colorado School of Mines Press, Golden, CO. LANGMUIRD. (1978) Uranium solution-mineral equilibria at low temperatures with applications to sedimentary ore deposits. Geochim. Cosmochim. Acta 42,547-569. LONGMIREP. (1985) Geochemistry and alteration processes of uranium railings in groundwater, Grants Mineral Belt, New Mexico. In Practical Applications of Groundwater Geochemistry (eds B. Hitchon and E. I. Wallick), Proc. 1st Can./Amer. Conf. on Hydrogeology, pp. 190-199. Natn. Water Well Assoc., Dublin, OH. LONGMIREP. A. (1983) Geochemistry, diagenesis and contaminant transport of uranium tailings, Grants Mineral Belt, New Mexico. M.S. thesis, Dept. of Geology, University of New Mexico, Albuquerque, NM. PARKHURSTD. L., THORSTENSOND. C. and PLUMMERL. N. (1980) PHREEOE - - a computer program for geochemical calculations. U.S. Geol. Surv. Water-Resour. Invest. 80-96. RUNNELLS D. D. and LINDRERG R. D. (1981) Hydrogeochemical exploration for uranium ore deposits: use of the computer model WATEQFC. J. Geochem. Explor. 15, 37-50. THOMSON B. M., BROOKINSD. G. and LONGMIREP. A. (1984) Investigation of geochemical and hydrological transformation in backfilled uranium mill tailings. Final report submitted to U.S. Bureau of Mines, Spokane, WA. THOMSONB. M. and HEGGENR. J. (1983) Water quality and hydrologic impacts of disposal of uranium mill tailings by backfilling. In Management of Wastes From Uranium Mining and Milling, pp. 373-384. Inter. Atomic Energy Agency, IAE-SM-262/51, Vienna. THOMSON B. M.. LONGMIREP. A. and GALLAHERB. M. (1982) The hydrologic environment and uranium production in New Mexico. In Proc. of Conf. on Water and Energy, Technical and Policy Issues, pp. 525-531. Amer. Soc. of Civil Engrs., Fort Collins, CO. U.S. NUCLEAR REGULATORY COMMISSION (1980) Final Generic Environmental Impact Statement On Uranium Milling, NUREG-0706. Vol. 1, Ch. 8, pp. 18-28. Office of Nuclear Material.