Geochemical evolution of inactive pyritic tailings in the Elliot Lake uranium district

N. M. DUBROVSKY, J. A. CHERRY, AND E. J. REARDON Department of Earth Sciences, University of Waterloo, Waterloo, Ont., Canada N2L 3GI

AND

A. J . VIVYURKA Rio Algorn Limited, Elliot Lake, Ont., Canada P5A 2Kl

Received March 30, 1984 Accepted October 18, 1984

Geochemical data obtained between 1979 and 1983 from a network of piezometer nests and cores from three inactive uranium tailings impoundments in the Elliot Lake district indicate that oxidation of pyrite taking place in the shallow part of the zone above the water table is causing the chemistry of the pore water above and below the water table to change. A two-layer hydrochemical zonation has developed in which infiltration water from rain and snow has resulted in an upper zone of low-pH water with high concentrations of SO4, Fe, and heavy metals. This zone is gradually expanding downward at rates generally between 0.2 and 2 mlyear, causing displacement of the original mill process water, which has neutral pH and low concentra- tions of heavy metals. High concentrations of Fe(II1) at shallow depth in the zone above the water table indicate that ferric iron is an important oxidizer of pyrite in the presence of free oxygen.

The pe of the groundwaters is controlled by the ferric-ferrous redox couple, and trends in the data indicate iron solubility control by siderite at high pH, by ferric hydroxide at moderate to slightly acid pH values, and possibly by jarosite at low pH. Aluminum solubility controls are complex, and precipitation of amorphous aluminum hydroxide, allophane, and basic aluminum sulfates may occur over different pH ranges. Transport of low-pH conditions is retarded relative to the rate of groundwater flow in the tailings, because of the buffering effect of small amounts of carbonate minerals added during tailings neutralization; primary aluminosilicates such as sericite; and secondary aluminum hydroxides.

Field data show that the flux of dissolved iron from the vadose zone to the groundwater zone in the Nordic Main tailings has been decreasing in recent years. However, mass-balance calculations indicate a potential for the generation of high-Fe groundwater for several decades to several hundred years. A long-term potential for acid and iron production is also shown by data from two tailings impoundments that have been inactive 8- 10 years longer than the Nordic Main area. Presently only a small portion of the Nordic Main and West Arm tailings areas has become acidic through the entire tailings thickness; however, under existing infiltration conditions more extensive acidification will occur in future decades.

Les donnCes gCochimiques obtenues entre 1979 et 1983 2 I'aide d'un rCseau de pikzomktres et de carottes dans trois anciens bassins de stockage de rksidus d'uranium dans le district d'Elliot Lake montrent que l'oxydation de pyrite se produisant dans la zone peu profonde au dessus de la nappe phrCatique provoque une modification de la chimie de l'eau interstitielle au dessus et au dessous de la nappe phreatique. Un zonage hydro-chimique a deux couches s'est form6 dans lequel l'eau d'infiltration de pluie et de neige a result6 en une zone supCrieure B faible pH et fortes concentrations en SO,, Fe et mCtaux lourds. Cette zone se d6veloppe progressivement vers le bas une vitesse de l'ordre dc 0 ,2 B 2 m/an, provoquant un dkplacement de l'eau de traitement du minerai qui posskde un pH neutre et de faibles concentrations en mktaux lourds. Les fortes concentrations en Fe(II1) a faible profondeur dans la zone au dessus de la nappe phreatique indiquent que le fer ferrique est un oxydant important de la pyrite en presence d'oxygkne libre.

Le p e de l'eau est contrBlt par le couple redox ferrique-ferrcux et les tendances dans les donnees indiquent un contrBle de la solubilitC du fer par la siderite a fort pH, par I'hydroxyde ferrique a pH modCrCment a ICgkrement acide et peut &tre par la jarosite B faible pH. Les contrbles de solubilitC de I'aluminium sont complexes, et la precipitation d'hydroxyde aluminium amorphe, de sulfates d'aluminium allophane et basique peut se produire dans diffirents domaines de pH. Le transport des conditions de pH faible est ralenti par rapport a 1'Ccoulement de l'eau B travers le rCsidu par suite de I'effet tampon produit par la solution de faibles quantites de minCraux carbonatCs ajoutes lors de la neutralisation du rksidu, de silicates alumineux tels que la sericite et les hydroxydes d'aluminium.

Les donnCes du terrain montrent que le flux de Fe dissout vers la zone phriatique dans le bassin de rCsidus Nordic Main, a diminuC ces dernikres annkes. Cependant, des calculs dc balance de masse indiquent la possibilitC de gCnCration d'eau 2 fort Fe pour plusieurs dCcades a plusieurs sikcles. Un potentiel de production a long terme d'acide et de Fe est dgalement mis e n Cvidence par les donnCes recueillies sur deux bassins qui ont CtC inactifs depuis 8 a 10 ans de plus que la zone Nordic Main. A I'heure actuelle seule une petite quantitC des zones de rCsidus Nordic Main et West Arm est devenue acide sur toute 1'Cpaisseur du dCpot de rksidus, mais sous les conditions d'infiltration actuelles une acidification plus Ctendue se produira dans les prochaines decades.

[Traduit par la revue] Can. Geotech. J . 22, 110-128 (1985)

Introduction Mining of uranium in the Elliot Lake area of Ontario began in

1954. The ore body is an Archean quartz pebble conglomerate, which has uraninite, brannerite, and monazite as the major uranium minerals and which contains from 3 to 8% pyrite. More than 1 X lo8 t of tailings have been produced to date, and it is estimated that nearly 1 X lo9 t will be produced during the next few decades. Tailings produced by the Elliot Lake mines are stored in surface impoundments constructed in preexisting topographic depressions.

During milling, the ore is first ground until 50% is finer than 75 p.m; it is then leached in hot dilute sulfuric acid. After removal of the uranium by precipitation of ammonium diuranate ('yellow cake') the acidic waste liquid/solid mixture is neutral- ized with lime to a pH between 8 and 10 and discharged as a slurry to a tailings impoundment. Prior to 1969 the pH of the slurry was occasionally less than 8.

In the Elliot Lake mining district, there are 10 tailings impoundments. Three are currently in use; the other seven were filled to capacity prior to 1969. Three of these inactive tailings

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FIG. 1. Regional and local maps showing the location of the Elliot Lake mining district and the tailings areas studies.

areas have had their surfaces stabilized by the establishment of a grass cover to prevent wind erosion. With or without a grass cover, water from rain and snow infiltrates into the tailings. The water table lies within the tailings and is generally within a few metres of the surface. In the zone above the water table, referred to here as the vadose zone, the presence of water and oxygen causes pyrite oxidation, which results in a deterioration of the quality of groundwater in the tailings. The groundwater seepage from the tailings can influence the quality of surface runoff from the tailings and thus the quality of nearby ponds and streams.

The first study of groundwater chemistry in a tailings impoundment in the Elliot Lake area was done by Moffett and Tellier (1978) on the Nordic West Arm tailings, which had been inactive since 1960 (Fig. 1). By sampling water table wells they found that much of the shallow groundwater in the tailings had become acidic owing to the oxidation of pyrite in the vadose zone. The acidic water contained high concentrations of Fe and SO4. These 'high acid' (> 1000 ppm acidity as CaC03) ground- waters were also found to have greater concentrations of most radionuclides and heavy metals than groundwaters with less than 1000 ppm acidity.

The Nordic Main tailings impoundment, which is adjacent to the Nordic West Arm impoundment, received tailings during the period from 1957 to 1968. In 1970 the decant pond was drained from the tailings surface. Between 1973 and 1978 grass cover was established over most of the surface. In 1979 the first hydrological and geochemical investigations of the Nordic Main tailings were initiated by Blair et al. (1980) and Blair (1981), who found highly acidic, low pH groundwaters at shallow depth underlain by neutral or alkaline groundwaters. They considered the slightly acidic, and neutral- to high-pH groundwaters to be a residual of the mill process water originally discharged with the tailings prior to 1968. The shallower, highly acidic groundwater, usually with a pH of less

than 6 , was identified as postdepositional recharge water, which transports the products of pyrite oxidation deeper into the tailings. This water was found to contain high concentrations of heavy metals such as Cu, Zn, Pb, and Co.

Blair et al. (1980) also identified a small zone in the southern part of the tailings where low-pH tailings groundwater with high iron and sulfate concentrations enters the underlying sand. This plume has been studied in detail by Morin et al. (1982) and Morin (1983).

A potential environmental problem is posed by the exit of contaminated groundwater from the tailings areas through two pathways: by discharge through the base of the tailings into the groundwater flow system underlying the tailings (Morin 1983) or by groundwater discharge to surface-water courses on the tailings. ' At present, all surface drainage from the Nordic Main and West Arm tailings area is collected by a ditch around the periphery of the tailings and treated by BaC12 and lime addition to maintain an acceptable effluent water quality. This treatment will have to be maintained indefinitely or until the quality of the drainage from the tailings area improves.

To evaluate the potential for contaminant migration by groundwater seepage through the bottom of the tailings, it is necessary to understand the geochemical processes that have given rise to the present geochemical conditions observed in the tailings. The purpose of this study is to gather detailed information on the spatial variation of groundwater geochemis- try in the Nordic Main tailings and, to a lesser extent, the Nordic West Arm and Lacnor tailings, and to use these data to develop a hydrogeochemical explanation for the extent and degree of the observed acidification.

'D. W. Blowes and R. W. Gillham. Geochemical evolution of in- active pyritic tailings in the Elliot Lake uranium district: Runoff quality and mechanisms of streamflow generation. In preparation.

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CAN. GEOTECH. J. VOL. 22, 1985

NORTH SOUTH

- metres

TAILINGS MEDIUM SAND GLACIAL TILL

a BEDROCK a VERY FINE SAND PEAT (Discont inuous)

FIG. 2. Schematic cross section of the Nordic Main tailings (from Morin et 01. 1982)

Hydrogeologic and geochemical setting western end of the area (Fig. 3b). During the spring the water ~h~ ~ ~ ~ d i ~ ~~i~ and west areas are situated in table is at or very close to ground surface in much of the tailings

an east-west trending valley formed by the dip and scarp topography of the northward-dipping Lower Proterozoic bed- rock. The valley is partially filled with Pleistocene deposits, which consist of a basal till unit overlain by very permeable glaciofluvial sands and gravels (Fig. 2). The glaciofluvial sands form an extensive aquifer, which can be subdivided into an upper layer of 4-8 m of medium-grained sand and a lower layer of fine-grained sand (Morin et al. 1982).

Prior to tailings deposition, much of the valley was occupied by spruce bog and a discontinuous layer of peat 0.5-1 .O m thick formed on the glaciofluvial aquifer. The peat layer was removed along the east and southeast edges of the Nordic Main tailings area before the deposition of tailings, and may also be absent from areas that were formerly topographic highs.

Tailings were deposited as a slurry with a water content of approximately 66%. The thickness of the tailings in the West Arm impoundment varies from 4 to 7 m, and in the Nordic Main impoundment it varies from approximately 8 to 15 m.

Substantial grain-size segregation occurs during settling of the suspended solids from the slurry. In areas close to the discharge point coarser-grained components of the tailings are deposited. Finer-grained fractions are transported to areas of ponded water. In the Nordic Main tailings, discharge occurred from the south and southwest, and the decant pond was located in the northeast. In the Nordic West Arm tailings, deposition began in the middle of the area, and the discharge point was moved westward with time (Moffett and Tellier 1978). The process of grain-size segregation, coupled with the frequent relocation of the slurry discharge point, produced a deposit with complex layering.

The depth below ground surface of the water table in the Nordic Main tailings measured in the summer of 1980 increased from less than a metre near the northern limit of the area where the tailings are bounded by bedrock to a depth of 6- 10 m along the southern edge where the tailings are bounded by a permeable dam and bedrock knobs (Fig. 3a). Seasonal changes in the depth to the water table are substantial, and fall and spring recharge cause groundwater elevations to increase by as much as 1.5 m. The depth to the water table in the Nordic West Arm tailings is generally much less than that in the Nordic Main area, ranging from 0 .4m at the eastern end of the Nordic West Ann area where it abuts the Nordic Main area to 2.4 m near the dam at the

area. No records of the initial chemistry of the tailings water

discharged from the Nordic Mill at the time of tailings deposition are known to exist. However, the general chemical characteristics of the solutions are believed to be similar to the water chemistry of effluent from one of the two large acid-leach mills (the Quirke mill) currently in operation in the Elliot Lake area (Table 1). The major anions present in solution are sulfate and nitrate introduced during the ore processing. The dominant cation is calcium, originating from the addition of lime and limestone during tailings neutralization. Lesser amounts of Na and K are also present. The Quirke effluent contains low concentrations of Fe, Al, and the heavy metals Cu, Zn, Ni, Co, and Pb, and has a pH between 10 and 11. Information from personnel that worked at the Nordic Mill indicates that effluent from this mill had a pH of 7-9.

Methods A network of piezometer nests (Fig. 3) was installed in the

tailings to monitor the groundwater levels in the tailings and in the underlying aquifer. Most of the piezometers were a modified Casagrande drive-point type, consisting of a solid PVC plastic drive point connected to 3.18 cm (If in.) diameter PVC pipe by a 15 cm length of perforated PVC pipe (Fig. 4). The intake screen consists of a section of porous polyethylene (Vyon) fitted over the perforated PVC. The piezometers were installed by augering to within 1 m of the depth desired, and then hand- driving the piezometer the final distance in order to minimize the extent of the disturbed zone around the screen.

Piezometer bundles of the type described by Cherry et al. (1983) were installed in the aquifer underlying the Nordic Main tailings. The piezometer bundles were installed by insertion through the stem of a string of hollow-stem augers. A schematic representation of a piezometer nest and bundle piezometer is shown in Fig. 4. The sampling network also included three piezometer nests installed in the Nordic Main tailings area by Blair (1981).

Groundwater samples from the tailings piezometers were collected for chemical analyses using a syringe sampling technique (Gillham 1982). This collection technique was chosen because it allowed downhole filtering through 0.45 pm filters in a manner that avoided oxidation of the sample.

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PIEZOMETER NEST

4 I Nordic

FIG. 3. Maps of depth to water table, Nordic Main (a) and West Arm ( b ) tailings, and piezometer nest locations.

Samples for cation analysis were preserved by acidifying with concentrated HC1 and refrigerating; filtered samples for anion analysis were refrigerated without treatment. Ferrous iron was determined in the field by redox titration with potassium dichromate (Waser 1966), total iron was determined by atomic absorption spectrophotometry, and ferric iron was determined by difference.

Alkalinity titrations on filtered samples were performed in the field as soon as the samples were taken. The pH and redox potential of the unfiltered groundwaters were measured in a closed flow-through system maintained at groundwater tempera- ture, using a sealed gel-type combination pH electrode (Canlab No. H5503) and a combination platinum electrode (Orion 96-78), respectively. The pH metre was calibrated using buffers of pH 4 and 7, which were maintained at groundwater tempera- ture. Electrical conductance of unfiltered samples of ground- water was measured using a YSI 33 S-C-T conductivity bridge.

Samples of dissolved SO4 for sulfur isotope analysis were collected by precipitation as BaS04 through the addition of BaC12 to filtered water samples. Samples of SO4 from gypsum in the tailings were obtained by leaching the gypsum and then reprecipitating the SO4 as BaS04.

Samples of pore waters from the zone above the water table were obtained at piezometer nest T5 by coring with 7.62cm (3 in.) thin-walled aluminum tubing. The cores were taken in 1 m sections for ease of handling and to maximize recovery.

Recovery was complete except in the zone immediately above the water table, where the high water content of the tailings caused caving of the access hole and loss of core. After recovery of each section, the core was cut into 0.2 m lengths. Pore water samples were extracted from each length of core using the methodology described by Patterson et al. (1978).

Cores of tailings solids were taken at piezometer nests T1, T5, and T6 to examine grain-size variation and carbonate mineral content in the tailings. The cores were extracted by augering to the water table with 7.62 cm hollow-stem augers, and then percussion-driving a continuous section of 7.62 cm diameter thin-walled aluminum tube. A Dames-and-Moore-type sediment trap was used to prevent the cohesionless tailings from falling out of the sampling tube during recovery. Cores were immediately sealed with plastic end caps and silicon sealant.

Solid samples were analyzed for carbonate mineral content and grain-size distribution. The analysis of the carbonate mineral content of the tailings solids was complicated by the high concentrations of ferrous iron (Fe(I1)) in much of the tailings groundwaters. When these samples are exposed to atmospheric oxygen, the ferrous iron oxidizes rapidly to ferric iron (Fe(III)), which subsequently hydrolyzes and precipitates as ferric hydroxide. The overall reaction decreases the pH of the pore waters, which may result in the dissolution and loss of a large percentage of the carbonate mineral content of the tailings solids. In order to avoid this loss the following procedure was

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CAN. GEOTECH. J . VOL. 22, 1985

DETAIL OF D R I V E - POINT PIEZOMETER

TAILINGS

bentonite plugs

GLACIAL AQUIFER (SAND)

-0.95 cm I . D . polyethylene tubing with 10 cm length nylon mesh screen

FIG. 4. Schematic of a piezometer nest at a site with a downward hydraulic gradient, with a detail showing the design.

TABLE 1. Chemical analyses of water samples from the Nordic Main tailings (piezometer nest T6) and Quirke tailings area (neutralized tail-

ings) (all analyses mg/L except pH and pe, or as noted)

Quirke T6, T6, T6, Parameter Laboratory tailingsn 12 m 8.3 m 4.6 m

pH Pe Fe total Al Si02 Ca Mg Na K so4 PO4 C1 F Cu Zn Ni Co Pb Mn Alk (as HC03-) Ra total, p ~ i / ~ f NO3

Field Field 2O 2O 1 ' 2" 2O 2O 2O 1 1' 1' 1 ' 2O 2" 2O 2O 2O 2O Field

"Analysis done by Rio Algom Laboratory, March 1977, A. J. Vivyurka, written communication.

b ~ i o Algom Environmental Control laboratory. 'University of Waterloo laboratory. dSO, value adjusted to achieve charge balance. 'Result for '26Ra, from Monenco Analytical laboratory. f l pCi/L = 37 x Bq/L.

adopted. The sealed cores were opened in the laboratory with a pipe cutter and a 20-30g sample immediately transferred to a Buckner funnel. Samples were then washed under suction with three successive 40 mL aliquots of methanol. By limiting the exposure time of the core samples to oxygen and displacing the high-Fe water with methanol, the problem of Fe oxidation and precipitation was minimized. After air drying, the carbonate mineral content of the sample was determined by the method described by Barker and Chatten (1982).

Results Approximately one-third of the analyses of the water samples

collected in 1980 were found to have charge-balance errors above 15%. This error was attributed primarily to difficulties in the analysis of aqueous sulfate. The method was subsequently refined and all analyses of samples collected in 198 1 had less than 15% charge-balance error and 80% had less than 10% error.

Results of chemical analyses of Fe, SO4, and C1 and of pH measurements on groundwater samples from six piezometer nests in the Nordic Main tailings area are presented in Fig. 5. The pH of groundwater below the water table in the Nordic Main tailings ranges from a high of 8.00 at approximately 13 m below ground surface at piezometer nest T4 to a low of 2.73 measured at 5.3 m depth at T5. There is a general trend of increasing pH with greater depth in the tailings.

Iron and sulfate, both of which are products of pyrite oxidation, show similar trends with depth (Fig. 5). Their concentrations increase to peak values at 6-8 m depth, then decrease at greater depth. Essentially all of the iron below the water table was found to be Fe(I1). Concentrations of Fe range from a few mg/L to almost 18000 mg/L, whereas SO4 con- centrations were found to range from 1960 to 28 000 mg/L.

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DUBROVSKY ET AL. 115

FIG. 5. Profiles of groundwater geochemistry for piezometer nests T1, T2, T3, T4, T5, and T7 showing Fe, C1, and SO4 concentrations and pH.

The concentrations of C1 found in the tailings range from less than 0.5 mg/L to 196 mg/L. The trend in the vertical distribu- tion of C1 is inverse to that of Fe and SO4, with C1 increasing with increased depth. This pattern is most clearly seen in the data for piezometer nests T2, T4, T5, and T6 (Figs. 5 and 6a).

Additional aspects of the groundwater chemistry of the Nordic Main tailings area can be illustrated by examining more-detailed analyses from piezometer nest T6 (Fig. 6 and Table 1). The water sample from the piezometer at 12 m shows the effects of a minor amount of pyrite oxidation: the pH is slightly below that of effluent originally discharged to the tailings area and the Fe concentration is elevated. At shallower depths the concentrations of Fe and SO4 increase greatly to 6030 and 14 100 mg/L, respectively, and pH declines. The low-pH waters at shallower depths are characterized by higher pe's, indicating more oxidizing conditions in the shallow ground-

waters. The shallow, low-pH groundwaters also contain ele- vated concentrations of aluminum.

Cobalt concentrations in the shallow, low-pH groundwaters at T6 are much higher than in deeper groundwaters and show a pattern of distribution similar to that of Ni and Zn. The peak concentrations of Pb and Cu occur at shallower depths.

There are substantial variations in the depth to which waters of high iron and sulfate concentrations have penetrated the tailings in the Nordic Main and West Arm areas. In the Nordic Main tailings, recharge waters with high Fe concentrations occur throughout the entire tailings thickness at piezometer nests T3, T7, and T8 (Fig. 5) . The most extreme case of acidification of water in the tailings is found at piezometer nest UW14 in the West Arm tailings, where the pH was found to be below 2 at all sampling depths, with the lowest pH being 0.97. Iron and sulfate concentrations are high throughout this profile

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CAN. GEOTECH. J . VOL. 22, 1985

FIG. 6. Profile of groundwater geochemistry

(Fig. 7). The shallowest depth of penetration of high-Fe waters was found at piezometer nest UW6 in the West Arm tailings, where groundwaters with high concentrations of Fe and SO4 extend to only a depth of 4 m, and the lowest groundwater pH measured was 5.4.

Analyses from the one piezometer next in the Lacnor tailings area are presented in Fig. 7. The Lacnor area has been inactive for approximately 8 years longer than the Nordic Main area and results show that high-Fe waters occur through the entire thick- ness of these tailings. Low-pH waters extend to a depth of approximately 6 m. Cobalt concentrations are variable but are often in excess of 1 mg/L.

Carbonate mineral contents of tailings solids from piezometer nests T1, T5, and T6 in the Nordic Main tailings are shown in Fig. 8. The analytical technique used has a detection limit of about 0.003 wt% calcium carbonate. The data show a similar pattern of distribution of solid carbonate at each site, varying from below detection to 0.009 wt% at depths of 4-8m, followed by a sharp increase to values as high as 0.056 wt% at greater depth.

Identification and displacement of process water The first task in interpreting the profiles of aqueous chemistry

in the tailings is to distinguish between the original water from mill processing that was discharged to the impoundments when the tailings were deposited, referred to here as process water, and infiltration water that has infiltrated into the groundwater zone in the tailings since discharge of tailings to the impound- ments ceased. This water is referred to as recharge water.

Of all of the chemical constituents that have been measured, C1 in the tailings is the least reactive chemically. Nearly all of the C1 in the tailings groundwater is derived from milling reagents, although small amounts may have been released in the mill from dissolution of primary minerals associated with the ore. In addition, the occurrence of C1 at shallow depths may result from the application of fertilizer to the tailings surface during revegetation. Thus, tailings groundwater that originated as process water is identified by the markedly higher C1 concentrations found in it than in recharge water.

Based on high C1 concentrations, it is concluded that process water now exists in the deepest part of the tailings at sites T1,

at piezometer nest T6, Nordic Main tailings

T2, T4, T5, and T6 (Figs. 5, 6a , and 9c). Based on the low C1 values at shallow depths at these sites, the depths of penetration of recharge waters range from 6 to 12 m. At T3 all of the process water has been replaced by recharge water, as indicated by the high concentrations of Fe and low concentrations of C1 throughout the profile.

At sites T2, T4, T5, and T6, the position of the fronts of high Fe and SO4 coincides with the zone of maximum decrease in C1 concentrations. This indicates that the downward migration rates of Fe and SO4 have not been noticeably retarded by geochemical reactions such as the incongruent dissolution of calcite and precipitation of siderite and gypsum. Therefore, recharge waters can be identified on the basis of elevated concentrations of Fe and SO4 in addition to C1.

If it is assumed for the Nordic Main tailings that downward movement of recharge water from the water table began in 1969 and that the recharge front is represented now by the main inflection on the C1 versus depth profiles, then it is possible to determine the average annual rate of migration of the recharge front in these tailings. The calculated rates range from 0.6 to 1.2 m/year at sites T1, T2, T4, T5, and T6. The rate at which recharge has moved downward is important for prediction of the future chemical evolution of the tailings.

In addition to the use of C1, SO4, and Fe profiles, three other approaches are available for estimating the rate of downward displacement of the front of recharge water: by examining changes in the geochemical profiles in piezometer nests in the tailings over a period of several years, by estimating the annual infiltration flux into the tailings surface, and by calculating the downward groundwater velocity using the Darcy equation with hydrologic parameters measured in the field.

The current rates of downward migration of recharge water and the products of pyrite oxidation have been studied by annual sampling of piezo&ter nests in the Nordic Main tailings during the period 1979- 1983. Profiles of Fe, C1, and pH for piezometer nests T1, T2, T3, and T7 are presented in Figs. 9- 12.

Different geochemical criteria were used at each piezometer nest to estimate the downward rate of migration. At T1 the movement of the 3000 mg/L concentration at the leading edge of the high-Fe front indicates a downward velocity of approxi- mately 0.3 m/year for the period of record (Fig. 9). Concentra-

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UW 14 Co, m g / L Fe, m g / L x 1000

16 20 I I I I I I I ,

I l l 1 1 1 1

8 16 24 32 4 0

SO4, m g / L x 1000

LACNOR Co, mg /L

0 I 2 3 4

FIG. 7. Profiles of groundwater geochemistry at piezometer nests UW 14 and UW6 from the West Arm tailings, and the Lacnor piezometer nest showing pH and concentrations of Co, Fe, and SO4.

-

FIG. 8. Profiles of carbonate mineral content in tailings solids at T1, T5, and T6, Nordic Main tailings.

I I I I I I I ( I I I I I

2 4 6 8 Fe mg/L x 1000; pH

6

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0 2 4 6

1979 NOV. 0 1980 JULY A 1981 J U N E + 1982 . 1983

FIG. 9. Profiles of Fe, Cl, and pH for groundwater samples from piezometer nest TI during the period 1979-1983.

- E -6 I t- a 2 8

10 1979 NOV.

0 1980 JULY

12 A 1981 J U N E + 1982

FIG. 10. Profiles of Fe, C1, and pH for groundwater samples from piezometer nest T2 during the period 1979-1983.

tions of C1 show little change over the same period. At T2, CI concentrations were found to decrease at the 8.8 and 10 m depths between 1979 and 1983, and the downward movement of the 25 mg/L concentration indicates a rate of displacement of approximately 0.24 m/year (Fig. 10).

At piezometer nest T3, a major increase in Fe concentration in the groundwaters occurred from 1979 to 1980, indicating a downward migration rate of approximately 2 m/year (Fig. 1 1). Only small changes in C1 concentration occurred during this period because all of the process water was displaced from the profile prior to 1979. Although there are marked decreases in Fe concentration in the shallower points at T7 during the sampling period, the number of sampling points is insufficient to confidently quantify the rate (Fig. 12).

The above profiles show apparent rates of downward migra- tion of the high-Fe groundwaters in the range of 0.24 m/year at T2 and 2 m/year at T3. Part of the variation is due to the large vertical spacing of the piezometers, which makes it difficult to define a 'leading edge' of the high-Fe or high-C1 water and estimate the rate of downward migration.

Decreases in Fe concentration at the shallow sampling points were observed at sites T1, T2, T3, and T7 in recent years (Figs. 9-12). These reductions indicate that the rate of Fe input from the vadose zone to the groundwater zone has decreased over the sampling period.

An examination of the pH data for piezometer nests T1, T2, T3, and T7 for this same period shows that pH values have been less variable in time than the Fe, SO4, and C1 concentrations. The relative constancy of pH is attributed to pH buffering reactions in the tailings. As a result of this buffering, downward movement of low-pH conditions has lagged behind the down- ward movement of the Fe and SO4 fronts. This retardatiorl of movement of the low-pH front is particularly evident in the profiles at T 1, T2, and T3.

The mean annual precipitation in the Elliot Lake area is 0.81 m, of which 38%, or 0.31 m, is estimated to recharge the groundwater system. In the approximately 10 years between drainage of the decant pond on the Nordic Main tailings and the 1980 sampling, 3 m of precipitation would have infiltrated the tailings. Using a representative tailings porosity of 0.5 calcu-

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1979 NOV. 0 1980 JULY A 1981 J U N E

- + 1982

E 4 m 1983 - I

FIG. 1 1. Profiles of Fe, C1, and pH for groundwater samples from piezometer nest T3 during the period 1979- 1983.

FIG. 12. Profiles of Fe, C1, and pH for groundwater samples from piezometer nest T7 during the period 1979- 1983.

lated from bulk densities (Smyth 1981), this volume of water would occupy 6 m of tailings thickness. By accounting for additional water yielded by an estimated 50% drainage of pore space in the zone above the water table at each site, a depth of penetration ranging from 6.8 to 8.5 m is calculated for the recharge front. This represents a downward velocity of 0.65- 0.85 m/year, which is consistent with chemically based esti- mates presented above.

The rate of displacement of the process water can also be calculated using the Darcy equation:

where u = average linear velocity, K = hydraulic conductivity, i = hydraulic gradient, and n = porosity. This calculation requires measured values of the hydraulic conductivity and hydraulic gradient of the tailings.

In the Nordic Main tailings, hydraulic conductivities meas- ured by rising head tests on piezometers range from 1 x lop6 to 3.1 x lo-' m/s (Blair 198 1). Nelson et al. (1977) and Kealy and Busch (1971) have observed a high degree of anisotropy in a uranium tailings area in the United States, with horizontal to vertical hydraulic conductivity ratios ranging from 2.5 to 10. Thus, the higher values observed in the Nordic Main tailings are more representative of the range in horizontal hydraulic conductivity.

The vertical hydraulic gradients measured at piezometer nests

T1, T2, T3, and T7 are shown in Fig. 13. The vertical gradients at T1 and T2 are similar, averaging 0.066. By applying the Darcy equation and assuming a range of vertical hydraulic conductivity of 4 X lop7 to 3 X lop8 m/s and a porosity of 0.5, the vertical average linear groundwater velocity is found to range from 0.12 to 1.7 m/year. The vertical gradient at piezometer nest T3 is 0.21, significantly higher than the gradients at T1 and T2. Using the values for hydraulic conductivity and porosity presented above, a vertical average groundwater velocity of 0.4-5.3 m/year is calculated for T3. More-detailed calculations on the rate of downward migration based on hydrologic parameters are not warranted because of the heterogeneous and anisotropic nature of the tailings.

Over most of the tailings area the tailings are separated from the underlying aquifer by a low-permeability peat layer. The presence of the peat layer is indicated by the large head loss across this unit at piezometer nests T1 and T2 (Fig. 13). In areas where the peat is absent, the flux of tailings water downward into the aquifer is probably much greater than in areas where peat is present. For example, relatively high downward flux occurs at T3 and T7.

The results of the four methods for calculating the downward migration rate range from 0.12 to 5.3 m/year. Most of the methods produced a downward migration rate in the order of 1 m/year, and are considered to be in good agreement in view of the different parameters that had to be assessed for each method.

Geochemical processes Pyrite oxidation

The production of acid by pyrite oxidation at shallow depths in the unsaturated zone of the tailings is evidenced directly by the rapid depletion of O2 in the gas phase with increasing depth (Smyth 1981). The primary mechanism by which pyrite is oxidized under low pH (<4) conditions is oxidation by Fe(II1). Because the oxidation of Fe(I1) to Fe(II1) by oxygen is slow in this pH range (Singer and Stumm 1970), the amount of acid produced is limited by the availability of Fe(II1). However, the iron-oxidizing bacteria, Thiobacillus ferrooxidans, catalyze the oxidation of Fe(II), producing a reaction rate about lo6 times faster than abiotic oxidation of iron by oxygen. Thus the bacteria ensure an ample source of Fe(II1) so long as oxygen is available. The oxidation of pyrite by Fe(II1) and subsequent oxidation of the Fe(I1) produced can be represented by the following reaction:

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CAN. GEOTECH. J . VOL. 22, 1985

DEPTH TO W A T E R ( m e t r e s ) DEPTH TO WATER ( m e t r e s ) 0 2 4 6 8 10 0 2 4 6 8 10

DEPTH TO WATER ( m e t r e s )

FIG. 13. Depth to static water level in piezometer nests in the Nordic Main tailings.

T. ferrooxidans [21 J I

14 ~ e ~ + + FeS2(s) + 8H20+ 1 5 ~ e ' + + 2~0, ' - + 16H+

The presence of iron-oxidizing bacteria in low-pH ( 4 ) zones of the Elliot Lake tailings has been established by McCready (1976) and Silver and Taylor (1981).

In addition to oxygen depletion, the above model predicts that Fe(II1) may be abundant in solution where oxygen is present and bacterial action rapid; but once all available oxygen is utilized, Fe(II1) will be reduced by pyrite oxidation and the dominant aqueous form of iron species will be Fe(I1). The analytical data on iron speciation were obtained to test this hypothesis. Samples from the vadose zone at site T5 contain up to 62% Fe(III), while the iron in samples from directly below the water table was almost entirely Fe(I1) (Fig. 14). We conclude that the model of pyrite oxidation by Fe(II1) mediated by bacterial oxidation of Fe(I1) is supported by the available data. It is likely that some Fe(II1) is lost from solution above the water table due to the precipitation of amorphous Fe(OH)3 (designated (am)Fe(OH)3) or of basic iron sulfates such as jarosite, KFe3(S04)2(OH)G. Precipitation reactions would cause a decrease in the amount of Fe in solution relative to the SO4 concentration. However, molar ratios of Fe:S in solution in excess of the 1:2 ratio predicted by the stoichiometry of pyrite indicate that dispropor- tionate amounts of Fe have probably not been lost from solution.

The sulfur isotopic composition of aqueous SO4 and sulfur- containing solid phases was determined for samples from one site in the Nordic Main tailings. The 634S of SO4 in three samples of recharge water from 6 to 10 m depth at piezometer nest T6 varied from +0.2 to + 1.8%0 (Fig. 6). Samples of pyrite and gypsum were found to have a 634S of + 1% and +2.8%0

Fe, m g / L x 1000 0 4 8 12 16 20 0.2 0.4 0.6 0.8 1.0 Fe2+/FeT ratio

FIG. 14. Profile of pore water geochemistry in the vadose zone at piezometer nest T5 showing the depletion of oxygen in the gas phase of the pores with increased depth and the change in Fe speciation.

respectively. The similarity between the 634S of the aqueous SO4 and the pyrite indicates that no significant amount of isotopic fractionation of sulfur occurred during pyrite oxidation in the tailings. However, these results contrast with field evidence, which indicates that SO4 produced from pyrite through bacterial oxidation by Thiobncillrls is depleted in the order of - 1 0 % ~ 6 3 4 ~ in the heavy isotopes of sulfur relative to the pyrite (Nissenbaum and Rafter 1967), and a laboratory study showing that sulfide oxidation by T. concretivorus will produce a depletion of the same order (Kaplan and Rittenberg 1964). The most plausible explanation for the lack of fractionation in the

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{SKY ET AL. 121

tailings is that fractionation effects that occur during pyrite oxidation are subsequently obscured by rapid attainment of isotopic equilibrium between the aqueous SO4 and the gypsum present in the tailings.

Application of chemical equilibrium concepts In the development of predictions of the downward migration

of low-pH water with high metal concentrations in the tailings, an understanding of the geochemistry of Fe and A1 is important because hydrolysis and precipitation/dissolution reactions in- volving these metals act as strong pH buffers. In addition, as much as 2.96 X lo3 Bq/L (80000 pCi/L) of radium-226 has been found associated with the basic iron sulfate jarosite, which has been identified in both fresh and weathered tailings (Kaiman 1977, 1978). Although kinetic factors prevent the attainment of equilibrium in some cases, equilibrium concepts can be used to calculate the aqueous speciation of ions, and they can be used to identify relationships between solution chemistry and solid phases present in the tailings. Most usefully, such calculations can indicate particular solid phases that may limit the concentra- tion levels of important aqueous constituents. Equilibrium calculations in this study were performed using the WATEQ2 speciation model (Ball et al. 1979).

Aqueous speciation of Fe and A1 is a function of the pH and availability of complexing ligands, such as F, C1, and SO4. Iron speciation is also a function of the redox potential of the water, expressed as pe (-log electron activity). The ratio of Fez+ to ~ e ~ + can be calculated from the relationship

Because of the interrelations of Fe activity, pH, and pe, Fe speciation and solid-phase stabilities are often illustrated by means of pe-pH diagrams. The progressive change in the pH and pe of groundwater at site T6 in the Nordic Main tailings is shown in Fig. 15. The data show that with increasing depth in the tailings, the increase in pH is coupled with a decrease in pe, which results in a distribution of data points along the ~ e * + / ( a m ) ~ e ( O H ) ~ boundary line. A similar trend has been noted for groundwaters in sulfide tailings in New Brunswick (Boorman and Watson 1976).

A more rigorous interpretation of the apparent equilibrium with (am)Fe(OH)3 necessitates a discussion of the limitations of pe-pH diagrams. The diagram is constructed for one set of conditions, in this case total free Fe = 5 mg/L and pCOz = low2. A change in these concentrations will alter the position of the boundaries between the stability fields. Because each plotted point represents a water sample with a specific Fe concentration and pCOz, the proximity to equilibrium of the individual samples must be examined by other methods. This comparison is facilitated by the calculation of saturation indices (SI) from ion activity products (IAP) and solubility products (K,,) where

[4] SI = log (IAPIK,,)

A negative SI indicates undersaturation with respect to the particular mineral, while a positive SI indicates supersatura- tion. Most of the IAP values for (am)Fe(OH)3 calculated for the 26 samples from piezometer nests T4, T5, and T6 in the Nordic Main tailings fall between -36.3 and -40.4 and average -37.7, indicating approximate equilibrium with (am)Fe(OH), (Langmuir and Whittemore 1971; Whittemore and Langmuir 1975).

While near-equilibrium is indicated by the data, the calcula- tion does not indicate whether equilibrium is being maintained

FIG. 15. Plot of pe-pH for Fe speciation in groundwaters from piezometer nest T6, drawn for [Fe] = 10 -~ .~ ' , pCOz = pK of Fe(OH)3 = 38.5.

by dissolution or precipitation of the solid phase. Equilibrium in the tailings groundwaters could be maintained by either process: (am)Fe(OH)3 may be precipitating as the pH of high-Fe water increases owing to H+-consuming reactions during downward flow; or (am)Fe(OH), precipitated during tailings neutralization prior to deposition may be dissolving owing to removal of Fe(II1) from solution by pyrite oxidation. The removal of Fe(II1) from solution by pyrite oxidation is a rapid reaction and the possibility that (am)Fe(OH)3 may act as a source of Fe(II1) for pyrite oxidation has been suggested elsewhere (Stumm and Morgan 1970; van Breemen 1972). Langmuir and Whittemore (1971) propose that high values of log IAP (close to -37) sug- gest that water is actively precipitating (am)Fe(OH)3 whereas lower values (-39 to -44) indicate that the water may be dissolving or in equilibrium with a more stable iron hydroxide. In view of the high values of log IAP we conclude that it is likely that (am)Fe(OH)3 is being precipitated below the water table in the tailings because of pH increases caused by H+ neutralization.

Equilibrium calculations also show that tailings groundwaters in the neutral pH range are supersaturated with respect to siderite (FeC03) with SI's ranging from 0.34 to 1.7. In addition, a pe-pH diagram showing all measurements made in the Nordic Main, Nordic West Arm, and Lacnor tailings areas indicates that siderite may control Fe concentration and pe (Fig. 16). Siderite supersaturation has been noted by other researchers in tailings seepage (Morin et al. 1982) and in a landfill leachate plume (Buszka 198 1). The positive SI values could be produced if pH-increasing reactions occur more rapidly than siderite precipitation, or if the equilibrium constant for pure siderite is lower than the actual equilibrium constant for the impure siderite. It is believed, on the basis of the SI values and the grouping of points along the siderite field boundary in Fig. 16, that siderite precipitation is occurring in the neutral-pH tailings zone.

The stability fields of jarosite (KFe3(S04)2(OH)6) for two water chemistries (high Fe and SO4, and low Fe and SO4) are shown in Fig. 16. The jarosite field was obtained using the pK for jarosite dissolution of 94.6 calculated by van Breemen

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122 CAN. GEOTECH.

FIG. 16. Plot of pe-pH for Fe species and solid phases for groundwaters from the Nordic Main, Nordic West Arm, and Lacnor tailings. Heavy line defines fielddrawnfor [Fe] = [K] = [SO4] = lo-'; thin line is for [Fe] = 10-~.", [K] = [SO4] = 10-1.8

(1976) from the experimental data of Brown (1970) at 25OC. Ball et al. (1979) averaged the K values from work by Zotov et al . (1973), Vlek et al. (1974), and Kashkai et al. (1975), obtaining a pK value of 98.80 -+ 1.1. We chose to follow the rationale of Miller (1979), preferring the higher value, because in Brown's experimental conditions equilibrium was approached from supersaturation, a condition likely to occur in the tailings.

The calculations show that jarosite has a large stability field for typical K, SO4, and Fe activities found in the low-pH tailings waters, and all samples with a pH below approximately 5 are highly supersaturated. High degrees of jarosite supersaturation in acid mine drainage have been reported by other researchers (Nordstrom et al . 1979; Miller 1979; Peterson and Krupka 1981) and it is suggested that jarosite may precipitate only in favourable microenvironments (Nordstrom et a / . 1979). The five data points with the lowest pH in Fig. 15 are from site UW14 in the Nordic West Arm tailings. These points fall close to the line of jarosite equilibrium and it is possible that at this location jarosite solubility is effective in controlling solution composition. However, although jarosite has been identified by X-ray diffraction in old tailings (Kaiman 1977) and could be precipitating, equilibrium with jarosite is not realized in most groundwaters in the Nordic and West Arm tailings.

Boorman and Watson (1976) have proposed that the pe of low-pH (oxidation zone) tailings groundwater sampled by them in a base-metal tailings area in New Brunswick is controlled by the 02/H202 redox couple described by the reaction proposed by Sato (1960):

For waters with low oxygen partial pressures Sato (1960) assumes a lower limit of unity for the H202:02 ratio, producing the equation

which has been plotted in Fig. 16. Redox control by this mechanism has received support in the literature by Beck (1974)

and Langmuir (1970). Langmuir noted that the electrode- measured pe of oxygenated waters usually falls far below that predicted by the equation

and observed that Sato's proposed reaction is rapid enough to act as a pe control. Control by the 02/H202 couple has been indirectly challenged by Whitfield (1974), who expressed doubt that H202 exists in sufficient quantities to act as a p e control in the reaction

[8] H202 + e- S OH + OH-

Whitfield contends that many past researchers were measuring potentials produced by reaction of oxygen and platinum on the electrode surface to form a coating that responds to pH as described by the equation

The line for this equation plots well above the tailings pe-pH values (Fig. 16). We believe that the field measurements of oxidation potential are reflective of the redox conditions in the tailings groundwater. In view of the high Fe activities in the tailings groundwaters, we feel that the measured potentials reflect control by the Fe(II)/Fe(III) redox couple.

The aqueous chemistry of A1 is similar to that of Fe except that it occurs in only the 3+ valence state. The solubility of aluminum solid phases therefore is not pe dependent. The concentration of A1 in natural waters of near-neutral pH is often limited by the solubility of amorphous or crystalline aluminum hydroxide or clay minerals. However, it has been proposed that A1 concentrations in acidic, high-sulfate waters are controlled by the precipitation of basic aluminum sulfates (Nordstrom 1982; van Breemen 1973, 1976).

The SI values calculated for several aluminum and silica- containing solid phases are given in Table 2. The data show that most of the tailings groundwater with a pH > 5.5 is super- saturated with respect to the amorphous aluminosilicate allo- phane ([A1(OH)3]cl~,,[Si02]c,,). Allophane solubility control is also suggested by the fact that SiO, concentrations are depressed below the solubility product of amorphous silica in waters in which allophane is supersaturated, a phenomenon that has been observed in other waters actively precipitating allophane (Wells et al. 1977). In this pH range most samples are close to saturation with respect to amorphous aluminum hydroxide, (am)A1(OH)3. All samples with a pH > 5.5 are highly super- saturated with respect to basaluminite, A14S04(OH)lo.5H20, indicating that if basaluminite precipitation is occurring, it is not controlling the equilibrium of A1 concentrations in solution.

While saturation indices may be useful for suggesting solid phases that may limit A1 concentrations, caution must be exercised since large errors in an ion activity product can be generated by small analytical errors when the activity of a component is raised to a large exponent. Thus the SI for basaluminite, which has an IAP of [ A ~ ] ~ [ S O ~ ] [ O H ] ~ ~ , will be highly sensitive to errors in the measurement of pH and Al.

Based on a review of the literature on acid sulfate waters, Nordstrom (1982) has proposed that for acid mine waters with a pH between 3.3 and 5.7 and containing 10-'M sulfate, alunite (KA13(S04)2(OH)b) likely controls the solubility of Al. How- ever, the precipitation of alunite is kinetically inhibited and the first precipitate from such solutions is amorphous basaluminite, which may later alter to alunite (Nordstrom 1982). It is also

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DUBROVSKY ET AL. 123

TABLE 2. Saturation indices for selected Al- and Si-bearing minerals at piezometer nests T4, T5, and T6 (arranged by pH)

Jurbanite, Basaluminite, Amorphous Amorphous Allophane, Piezometer Depth AISO,OH, AI~(SO,)(OH)IO, Af(oH)3, SiO?, [Al(OH)31 I.,[S~O~I,,

nest (m) pH pK = 17.23" pK = 117.3' pK = 31.611' p~ = 2.71" p Kc

T4 2.0 4.06 1.40 0.25 -2.57 0.09 -0.44 3.2 4.40 1.32 1.70 -2.02 -.f -f

1.7 4.55 0.80 1.39 -2.20 0.02 -0.38 4.0 5.80 0.67 7.29 -0.40 -0.33 0.39 6.0 5.90 0.62 7.44 -0.44 -0.23 0.49 8.8 6.05 0.28 7.24 -0.29 -0.5 1 0.44 7.0 6.50 -0.78 6.36 -0.19 -0.70 0.52 9.4 6.85 - 1.24 5.73 -0.07 -0.52 0.60

10.0 7.09 -1.81 5.42 -0.34 -0.88 0.66 13.2 8.00 -3.49 4.36 -0.03 -0.57 1.17

T5 5.6 2.31 0.89 - 10.96 -6.55 0.51 -0.33 4.7 3.85 1.84 0.07 -3.02 0.10 -0.48 8.0 5.49 0.29 5.00 -0.86 -0.27 0.10

12.0 5.98 0.56 8.04 -0.11 -0.37 -0.58 T6 6.3 3.91 1.88 0.56 -2.87 0.15 -0.42

4.6 3.99 1.36 -0.32 -2.99 0.16 -0.46 7.1 4.30 2.11 3.74 -2.10 0.14 -0.19 8.3 5.66 2.05 11.50 0.58 -0.18 0.94 9.3 5.90 -0.39 3.15 -0.97 -0.29 -0.10

12.0 6.51 -1.80 2.26 -0.72 -0.86 -0.16

'Rawajfih (1975). bSingh (1969). 'Latimer (1952). "Morey et a/. (1964). 'Log K = -5.71 + 1.68pH, Paces (1973). /No SiOz analyzed.

suggested that amorphous jurbanite (A1S040H.6H20) is the Al-limiting phase in acid mine waters with pH < 3.3 (Nord- strom 1982; van Breemen 1976).

Tailings groundwaters with a pH between 3.3 and 5.5 are undersaturated with respect to amorphous aluminum hydroxide and allophane. In these waters basaluminite is close to satura- tion and may limit A1 concentration in solution. In the groundwater with a pH < 3.3 , both basaluminite and jurbanite are undersaturated in almost all of the samples.

In order to graphically illustrate the above relationship between pH and aluminum solubility, data for the tailings groundwater and the lines describing solution equilibrium with the solid phases amorphous aluminum hydroxide, basalu- minite, and jurbanite were plotted on a graph of log ~ l ~ + activity versus pH (Fig. 17). The graph was drawn for a constant S042- activity of lo-'.'. At high pH the data follows the trend of the (am)Al(OH)3 line. At a pH of 4 t 0.5 the data indicates that (am)A1(OH)3 is undersaturated and suggests equilibrium with respect to basaluminite. At pH < 3.5 the A13+ activity appears to be independent of pH.

While the above discussion considers equilibrium relations for the solid phases one at a time, it is likely that multiple equilibria occur between aqueous A1 and several solid phases. In addition, disequilibrium may occur locally due to the inability of precipitation/dissolution reactions to keep up with changes in pH.

The above evaluation of the field data suggests that in shallow groundwaters in the tailings, Fe and A1 may be precipitating as hydroxysulfate solid phases such as jarosite and basaluminite. The presence of these solid phases would have important consequences for the future evolution of tailings groundwater. As the concentrations of SO4 and Fe in the recharge waters

FIG. 17. Data for calculated aluminum activity plotted versus pH showing lines of equilibrium with respect to the aluminum solid phases jurbanite, basaluminite, and amorphous aluminum hydroxide. The lines are plotted for the average sulfate activity of the tailings groundwaters analyzed and the shaded fields indicate one standard deviation in sulfate activity.

decrease owing to exhaustion of the pyrite at shallow depths in the tailings, these minerals will dissolve, adding Fe, Al, and SO4 to the groundwater. The Fe and A1 will then reprecipitate as hydroxides, with the release of hydrogen ions, causing a decrease in pH. During this period radionuclides or other metals associated with the hydroxysulfate solid phases would go into solution. Thus acidic, low-pH conditions will continue to be

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124 CAN. GEOTECH. J. VOL. 22, 1985

produced in the tailings groundwater until all of the previously precipitated hydroxysulfate minerals are dissolved.

It should be noted that there is little direct evidence for the presence of specific Fe and A1 solid phases. This is due to the great difficulty of isolating and making positive identification of solids present in small quantities in the tailings, along with the amorphous nature of many of the precipitates. More work is needed in this area.

Acid neutralization processes Surface water in the Elliot Lake area has very low buffer

capacity, and therefore the migration of low-pH, high-Fe waters from the tailings into surface water can have a major effect on surface water quality. Pyrite in the tailings may be oxidized by oxygen or Fe(II1) as described by the two following reactions:

In both cases Fe(I1) and SO4 are released in a 1:2 ratio. Additional H+ will be generated if the Fe(I1) released is subsequently oxidized and precipitated as ferric oxyhydroxide or as basic iron sulfate, as illustrated by the reactions

Minimal Ht production occurs if the Fe(I1) is oxidized but does not hydrolyze:

The oxidation of pyrite as represented by [lo]-[14] produces 1- 16 mol H+ per mol pyrite oxidized. As essentially all of the Fe in solution below the water table is in the 2+ oxidation state, we can assume that a minimum ratio of 1:2 for Fe and H+ production results from pyrite oxidation ([lo]). It follows that the relative rate of downward movement of the low-pH (high H+ activity) water can be appraised by a consideration of the depth to which the other pyrite oxidation products, Fe and SO4, and the low-C1 water have penetrated the tailings.

Figure 5 shows that the peak concentrations of Fe and SO4 are located at greater depths in the tailings than the low-pH condition, indicating the presence of H+-consuming reactions. Several mechanisms are probably involved in the pH neutraliza- tion. These include dissolution of calcium carbonate, dissolu- tion of aluminosilicate minerals, dissolution of iron and aluminum hydroxides, and sorption of the H+ onto tailings solids. Profiles of pH at shallow depths in the tailings show that much of the H+ consumption occurs above the water table (Fig. 14). The high concentrations of A1 and Si02 in solution indicate that this neutralization in the vadose zone is likely the result of the dissolution of primary aluminosilicates. Although the pH of the solution is increased by this dissolution, substantial acidity persists in the form of various aluminum hydroxide species.

Neutralization in the groundwater zone in the tailings by

dissolution of calcium carbonate was first suggested by Blair (1981). The calcium carbonate probably originated as an impurity in the lime added to the tailings during tailings neutralization before discharge. The consumption of Ht by calcium carbonate dissolution is described by the reaction

The above reaction, which describes calcite dissolution below a pH of 6, shows that 1 mol CaC03 will neutralize 2 mol H+. We emphasize this mechanism because the reaction rate is fast and because it can cause the pH to rise to neutral or alkaline levels. This is particularly important because toxic metals and radio- nuclides mobilized at a lower pH will tend to be lost from solution due to enhancement of sorption and solubility controls at higher pH.

Sampling of solids in the Nordic Main tailings revealed that the highest weight percentages of calcite occur at T1, T5, and T6 at levels of approximately 0.06, 0.05, and 0.025, respec- tively (Fig. 8). These values are similar to the average of 0.06 wt% determined on samples from another inactive tailings, the Williams Lake tailings, in the Elliot Lake district (Feenstra et al. 1981), and 0.048 wt% for recent tailings from the Quirke mill (R. Nicholson, personal communication). Although there is a strong correlation between grain size and high carbonate content in the Quirke and Williams Lake samples, no such relationship was found for the samples from the Nordic Main tailings. This lack of correlation with grain size is probably due to the overwhelming control of depth on carbonate mineral content.

The sharply reduced carbonate mineral contents of the tailings at shallower depths in the profiles are attributed to the removal of calcium carbonate from the tailings solids by low-pH infiltration. The very low amount of solid-phase carbonate that remains at shallow depths may be due to protection from acid dissolution by precipitated coatings of gypsum, ferric hydrox- ides, or aluminum hydroxides, or to the presence of secondary siderite. Precipitated siderite may persist in the tailings at a lower pH than does calcite because the high concentrations of Fe(I1) in solution maintain siderite saturation.

One indirect check on the hypothesis of low-pH retardation by calcium carbonate dissolution is to compare the amount of pH retardation observed in the field with the amount of retardation predicted for the levels of calcium carbonate found in the deep tailings. For this comparison the observed retarda- tion factor (R) can be calculated as a function of the amount of calcium carbonate in the tailings solids and total hydrogen ion sources in the groundwater:

R represents the ratio of the downward component of the groundwater velocity to the rate of downward movement of the front of low-pH water. R is derived from mass-balance considerations without inclusion of the effects of mechanical dispersion or molecular diffusion.

As the downward velocities are approximated by the depths of penetration of the recharge water and the low pH condition, these depths must be specified. The leading edge of peak Fe and SO4 concentrations and increasing C1 concentrations will be used to delimit the depth to which recharge waters have penetrated the tailings. The position of the low-pH front is taken as that depth at which pH drops below 4.5, because at this pH

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DUBROVSK :Y ET AL. 125

essentially all of the available carbonate solids will have been dissolved. The retardation factors calculated from the relative positions of the low-pH and recharge water fronts vary from 1.3 to >3.

In order to calculate the retardation using [16] for known calcium carbonate levels, we have to define the total hydrogen ion content of the initial solution that contacts the solid-phase carbonate in the tailings. For the tailings water, if it is assumed that no Fe(I1) oxidation occurs and that Fe(I1) hydrolysis is negligible, the sum of the important acid-forming species can be written:

The coefficient of 3 on the A1 is derived from the assumption that all A1 will be precipitated as amorphous aluminum hydroxide or allophane. Estimation of the initial solution composition is difficult because the pyrite oxidation rate that controls solution composition has not been constant in time. It is useful, however, to estimate the initial solution as the lowest-pH water found below the water table at each site. The initial solution composition was estimated from vadose zone samples at sites where no groundwaters with pH 4.5 occur below the water table. The resulting total hydrogen ion contents of this initial-condition tailings groundwater calculated using [17] (Table 3) show the major source of acid to be precipitation of aluminum solid phases.

Retardation factors calculated using [16] and a calcium carbonate value representative of the Nordic Main alkaline tailings (0.05 wt%) are presented in Table 4. Comparison of the R values shows that the values calculated from [16] are close to the retardation factors calculated from the relative positions of the low-pH and high-C1 fronts.

I A better illustration of the relationship of R and carbonate / content of the tailings results from plotting [16] for the various I

i input solutions (mH;) considered (Fig. 18). This graph shows I that the relationship of R to carbonate content is strongly

influenced by assumptions about initial solution composition. The retardation factors calculated from the relative depth of penetration of the low-pH and high-Fe fronts for each nest are marked on the appropriate line. These values of R correspond to carbonate mineral contents varying from 0.017 to 0.153 wt%. This compares well to the range of 0.001-0.056%. The varia- tion in the predicted values again reflects uncertainty in the initial solution composition as well as possible omission of active H+-consuming reactions such as adsorption.

In spite of the wide range, the predicted calcium carbonate contents bracket values found in samples from the deep alkaline zone in the Nordic Main tailings. These results indicate that the

TABLE 3. Calculation of H+ total for hypothetical input solution (on a per litre basis)

Piezometer nest mH' 3rnA13+ mHS04- rnHf total

Tla 0.0003 0.020 0.001 0.0213 T3" 0.001 0.009 0.001 0.01 1 T4 0.000 1 0.0129 0.0001 0.0131 T5 0.002 0.107 0.005 0.114 T6 0.0001 0.066 0.0001 0.0662

"At these sltes there was no groundwater with a pH < 4.5 below the water table, so an Input solutlon chemistry was estimated from the analyses of unsaturated zone samples from Snlyth (1981).

TABLE 4. Comparison of observed and calculated retardation factors

R, Depth to Depth to R , calculated

Piezometer high-Cl low-pH (<4.5) ratio of for 0.05 wt% nest water (m) water (m) depths CaC03

"Minimum depth of penetration; no high-CI water encountered. bEstimated from unsaturated zone data of Smyth (1981).

weight % Ca C03

FIG. 18. Plot of retardation factor, R , versus carbonate mineral content of the tailings. Lines represent input solutions with total hydrogen ion concentrations calculated in Table 3. The maximum carbonate mineral content found at each piezometer nest is indicated under the piezometer nest number. The retardation factors calculated from the relative depths of penetration of the low-pH front and the corresponding carbonate mineral content are noted.

proposed mechanism of acid neutralization and retardation by dissolution of carbonate minerals is in agreement with the observed retardation of the downward movement of the zone of low-pH water in the tailings.

Summary of conclusions For more than 15 years the precipitation that has been

recharging the groundwater zone in the inactive Nordic Main, Nordic West Arm, and Lacnor tailings has been acidified as a result of the oxidation of pyrite at shallow depths in the tailings. Downward movement of acidic, high-Fe recharge water has displaced the residual mill process water to a depth of 4 m or more at all monitoring sites except one. The recharge water is distinguished from the process water on the basis of the higher Fe and SO4 and lower C1 concentrations in it than in the process water. The coincidence of decreasing C1 concentrations and increasing Fe and SO4 concentrations in the geochemical profiles indicates that the downward movement of the front of high-Fe and high-SO4 water is not significantly retarded by geochemical reactions.

On the basis of observed changes in Fe and C1 profiles in the Nordic Main tailings between 1979 and 1983, downward migration rates were estimated to be between 0.2 and 2 m/year, varying considerably from site to site. These migration rates are in the same range as velocities calculated by the Darcy equation and from estimates of annual recharge from precipitation on the

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126 CAN. GEOTECH. J . VOL. 22, 1985

tailings. Observed decreases in Fe concentration below the water table at most sites in the Nordic Main tailings indicate that the bulk rate of pyrite oxidation above the water table in the tailings has been decreasing.

Data on aqueous Fe in the zone above the water table at one site in the Nordic Main tailings showed that Fe speciation shifted from dominantly Fe(II1) (62%) near the tailings surface to almost entirely Fe(I1) at the water table. It is hypothesized that Fe(II1) in the pore water is reduced by the oxidation of pyrite in the absence of oxygen during downward migration in the deeper part of the vadose zone.

The tailings groundwaters are near equilibrium with respect to amorphous ferric hydroxide. Groundwaters with near-neutral pH are supersaturated with respect to siderite; siderite precipi- tation may result from the incongruent dissolution of calcite. Calculations also indicate that most groundwaters are highly supersaturated with respect to jarosite. The oxidation potential of the tailings groundwater is controlled by the iron system, and removal of Fe(II1) by precipitation of ferric hydroxide causes the pe of the groundwater to decrease as pH increases.

Equilibrium calculations indicate that in near-neutral ground- waters (pH > 5.5) in the tailings the precipitation of allophane or amorphous aluminum hydroxide may limit concentrations of aqueous aluminum. In the pH range of 3.5-5.5 the ground- waters are close to saturation with respect to basaluminite, and precipitation of this solid phase may limit A1 concentrations in solution. Data for groundwaters with a pH less than 3.5 indicate that the A1 concentration is independent of pH in these waters, and there is no evidence for equilibrium between aqueous A1 and an Al-containing solid phase.

Several H+-consuming geochemical processes in the tailings buffer the pH of the tailings groundwater. Much of the Hf consumption occurs above the water table and it is likely that in this zone dissolution of primary aluminosilicates or secondary hydroxide-containing solid phases is important. In the Nordic Main tailings most of the carbonate mineral content has been removed to a depth of approximately 8 m; below this depth maximum carbonate mineral content of the tailings solids varies from 0.025 to 0.06 wt% CaC03.

The downward migration of low-pH conditions (pH < 4.5) has been retarded relative to the downward groundwater velocity. The observed retardation factors, which are in the range of 1.3 to -3, compare well with retardation factors calculated assuming H+ consumption due to dissolution of a representative carbonate mineral content of 0.05 wt%.

Prognosis for the future Under the present conditions, water recharging the tailings

will continue to be acidified for a long period of time. Pyrite depletion due to oxidation at depths of less than 2 m in the tailings has been observed (Smyth 1981) and a decrease in iron concentrations between 1979 and 1982 in shallow groundwaters at most piezometer nests indicates that the rate of Fe flux from the vadose zone to the groundwater zone has been decreasing. However, even at these diminished oxidation rates, recharge waters continue to have high Fe concentrations and a low pH.

For example, using an average pyrite content of 5%, a tailings porosity of 0.5, and an annual recharge rate of 0.3 m, oxidation of the pyrite contained in just a 1 m thickness of tailings could produce an Fe concentration of 1000 mg/L in the groundwater for 54 years even if one-half of the Fe released by pyrite oxidation was precipitated as a solid phase. In areas of the tailings with a high water table, rapid pyrite oxidation may be limited to a zone as thin as 1 m because free gas-phase oxygen

will not diffuse below the water table at a significant rate. However, in areas where the water table is deep and the tailings texture relatively coarse, an unsaturated zone of several metres in thickness is available for oxygen entry and pyrite oxidation. The depth to water table is greater than 3 m over much of the Nordic Main tailings area (Fig. 3). Given the range in water table depth and pyrite contents, this analysis suggests that the period of time during which active production of high-Fe, low-pH water in the shallow tailings zone will continue is several decades to several hundred years.

Sampling of groundwater in tailings areas that have been abandoned for longer periods of time than the Nordic Main tailings supports the view of long-term acid production. In the West Arm tailings, which are approximately 10 years older than the Nordic Main tailings, the groundwater at piezometer nest UW14 (Fig. 7) was found to have a pH of approximately 1 and concentration of iron of - 10 000 mg/L at all depths sampled. Groundwaters sampled from depths less than 4.6m in the Lacnor tailings area, which has been abandoned approximately 8 years longer than the Nordic Main tailings, were found to have low pH values, ranging from 2 to 3.7.

Even after all of the pyrite is consumed in the vadose zone, it is likely that the pH will remain low for many more decades. Various causes of low-pH maintenance can be envisioned. For example, there is a potential for continued release of Hf due to the alteration of jarosite to amorphous ferric hydroxide:

The stoichiometry of the reaction dictates the release of 3 mol Hf per mol jarosite altered. Equilibrium of jarosite and (am)Fe(OH)3 in coal strip mine spoil was demonstrated by Miller (1980), who concluded that, even in the absence of pyrite oxidation, spoil containing these minerals would remain acidic with a pH of approximately 3.2 until the jarosite was entirely removed by leaching. Although the presence of jarosite in the Nordic Main tailings has been reported (Kaiman 1977), the quantity present has not been determined, and the period of time over which this pH buffer would operate cannot be calculated. Additional acidity would result from similar hydrolysis reac- tions involving basaluminite or alunite if these phases are present in the tailings. Even after all of the pyrite, jarosite, and other acid-producing minerals are removed by leaching, the pH of the vadose zone water will be kept below 4 or 5 by the acidity of rain and snow and by the production of C 0 2 by roots and decaying organic matter in the soil that is developing on the tailings.

It is difficult to predict the downward rate of migration of the low-pH conditions in the tailings owing to uncertainties of the buffer reactions and variations in solid-carbonate content. Retardation factors calculated considering only pH buffering by carbonate mineral dissolution indicate that low-pH conditions have a downward velocity component of approximately 30- 75% of the average linear groundwater velocity. Downward average linear groundwater velocities calculated previously are variable but on the order of 1 m/year. At this rate low-pH conditions will reach the bottom of the Nordic Main tailings within several decades.

At the present time, only a very small percentage, probably less than 5%, of the bottom area of the Nordic Main and Nordic West Arm tailings has acquired a high-Fe and low-pH condi- tion. Therefore, very little high-Fe, low-pH water, which has high trace metal contents, is migrating from the tailings into the

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'SKY ET AL. 127

underlying groundwater flow regi~nes in the sand aquifer and fractured bedrock. Although local variations in the magnitude of parameters such as pyrite content, permeability, thickness of the vadose zone, and hydraulic gradient influence the severity of pyrite oxidation effects and the rate of downward movement of the reaction products, there is little doubt that if the existing infiltration conditions continue, high-Fe, low-pH water will eventually occupy the entire tailings mass, probably within several decades o r a century. As this condition develops, the flux of contaminants to the sand aquifer and bedrock will increase proportionately, eventually resulting in an increase in contaminant flux from the groundwater zone to the surface- water environment.

Although the pyrite oxidation rate in the tailings is decreas- ing, the tailings contain sufficient pyrite to produce groundwater with a low pH and high F e concentration for several decades to several hundred years. Even if a cover is emplaced that prevents further pyrite oxidation, other pH-lowering processes will continue to cause downward movement of low-pH and high-Fe water for a long time. The acidified zone of the tailings already contains large volumes of contaminated water that will continue to migrate downward regardless of surface treatment. The long- term impact of this seepage on groundwater beneath the tailings and eventually on nearby streams or lake water warrants investigation.

Acknowledgements Thanks are extended to Dr. Nand Dave, T. P. Lim, and the

staff of the Elliot Lake laboratory of CANMET for assistance during the field work and to Drs. Kevin Morin and Carl Palmer of the University of Waterloo for their helpful discussions during preparation of the manuscript. Tim Cosgrave and Martin Cheung also provided field assistance, and Ray Blackport installed the network of piezometers in the West Arm tailings.

Direct financial support of this investigation was provided by the Canada Department of Energy, Mines and Resources and by Rio Algom Limited. Additional materials and services were provided by Rio Algom Limited and by the Elliot Lake laboratory of CANMET. Thanks are also extended to R . Keller- man of the Geochemistry Laboratory and R. Drimmie of the Isotope Laboratory, both at the University of Waterloo.

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