Geochemical characterization of acid mine drainage from a waste rock pile, Mine Doyon, Québec, Canada

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Journal of Contaminant Hydrology 69 (2004) 45–71

Geochemical characterization of acid mine drainage

from a waste rock pile, Mine Doyon,

Quebec, Canada

O. Sraceka,1, M. Choquettea, P. Gelinasa,R. Lefebvreb,*, R.V. Nicholsonc

aDepartment of Geology, Laval University, Quebec City, Quebec, Canada G1K 7P4b INRS-Eau, Terre et Environnement, Institut National de la Recherche Scientifique, 880 Chemin Sainte-Foy,

Bureau 840, C.P. 7500 Sainte-Foy, Quebec, Canada G1V 4C7cDepartment of Earth Sciences, University of Waterloo, Waterloo, Ontario, Canada N2L 4GI

Received 22 October 2002; accepted 2 July 2003

Abstract

Water quality in the unsaturated and saturated zones of a waste rock pile containing sulphides was

investigated. The main objectives of the project were (1) the evaluation of geochemical trends

including the acid mine drainage (AMD)-buffering mechanism and the role of secondary minerals,

and (2) the investigation of the use of stable isotopes for the interpretation of physical and

geochemical processes in waste rock. Pore water in unsaturated zone was sampled from suction

lysimeters and with piezometers in underlying saturated rocks. The investigation revealed strong

temporal (dry period vs. recharge period), and spatial (slope vs. central region of pile) variability in

the formation of acid mine drainage. The main secondary minerals observed were gypsum and

jarosite. There was a higher concentration of gypsum in solid phase at Site TBT than at Site 6,

suggesting that part of the gypsum formed at Site 6 in the early stage of AMD has been already

dissolved. Formation of secondary minerals contributed to the formation of AMD by opening of

foliation planes in waste rock, thus increasing the access of oxidants like O2 and Fe3 + to previously

encapsulated pyrite. The behavior of several dissolved species such as Mg, Al, and Fe2 + can be

considered as conservative in the leachate. Stable isotopes, deuterium and 18O, indicated internal

0169-7722/$ - see front matter D 2003 Elsevier B.V. All rights reserved.

doi:10.1016/S0169-7722(03)00150-5

* Corresponding author. Tel.: +1-418-654-2651; fax: +1-418-654-2615.

E-mail addresses: [email protected] (O. Sracek), [email protected] (R. Lefebvre).1 Present address: Institute of Geological Sciences, Faculty of Science, Masaryk University, Kotlarska 2, 611

37 Brno, Czech Republic.

O. Sracek et al. / Journal of Contaminant Hydrology 69 (2004) 45–7146

evaporation within the pile, and were used to trace recharge pulses from snowmelt. Isotope trends for34S and 18O(SO4) indicated a lack of sulfate reduction and zones of active oxidation of pyrite,

respectively. Results of numerical modeling of pyrite oxidation and gas and water transport were

consistent with geochemical and isotopic trends and confirmed zones of high evaporation rate within

the rock pile close to the slope. The results indicate that physical and chemical processes within the

pile are strongly coupled and cannot be considered separately when oxidation rates are high and

influence gas transport as a result of heat generation.

D 2003 Elsevier B.V. All rights reserved.

Keywords: Acid mine drainage; Waste rock pile; Geochemistry; Stable isotopes; Internal evaporation; Transport

modeling

1. Introduction

Acid mine drainage (AMD) represents significant environmental and financial

liability for the mining industry. It is caused by the oxidation of sulfides in mine

waste with resulting acidic conditions and mobilization of metals. There are two types

of mine waste. Mine tailings are the final product of ore treatment or processing and

are composed of sand and silt size particles. Waste rock is the non-economic material

removed from a mine to access the ore body. Particle size is generally large,

depending on the mechanical properties of the rock, and on the type of blasting

operation.

The AMD production in mine tailings has been studied extensively, for example by

Dubrovsky et al. (1985), Morin et al. (1988), and Blowes et al. (1991). In contrast,

much less is known about AMD generation from waste rock. Several waste rock

studies have evaluated temperature and gas concentration profiles (Harries and Ritchie,

1985; Lefebvre et al., 1993, 2001; Kuo and Ritchie, 1999). These studies revealed a

significant role of air convection in the unsaturated zone of very reactive waste rock

piles. However, the geochemical environment within waste rock piles is less well

known. Until recently, the focus on waste rock geochemical studies has been on the

quality of drainage water flowing out of the waste rock base (Morin et al., 1994). This

type of monitoring represents an integral over space and time of the acid drainage

produced in various parts of the dump. However, this information does not provide a

clear picture of geochemical processes within the pile (Ritchie, 1994). There have

been attempts to obtain data on water chemistry within the unsaturated zone of waste

rock piles (Shafer et al., 1994), but they were not very successful because of low

water content and correspondingly high suction values characteristic of semiarid

climate conditions in the Northwest USA. The acquisition of geochemical data from

the unsaturated zone of a waste rock pile remains a topic of on-going research (for

example, Smith et al., 1995; Stockwell et al., 2001). Our approach to the assessment

of water quality within waste rock deposits was based on sampling of leachate from

both unsaturated and saturated zones within the pile, combined with transport and

geochemical modeling, and solid phase analysis.

https://www.researchgate.net/publication/223826105_The_formation_and_potential_importance_of_cemented_layers_in_inactive_sulfide_mine_tailings_Geochim_Cosmochim_Acta_55_965-978?el=1_x_8&enrichId=rgreq-30e854d1-466d-41fd-b444-a085459ebfd3&enrichSource=Y292ZXJQYWdlOzczNTM5NTI7QVM6OTg1MzM1MjA3MDc1OTFAMTQwMDUwMzYzMTg4Mg==

https://www.researchgate.net/publication/237377109_Geochemical_evolution_of_inactive_pyritic_tailings_in_the_Elliot_Lake_uranium_district?el=1_x_8&enrichId=rgreq-30e854d1-466d-41fd-b444-a085459ebfd3&enrichSource=Y292ZXJQYWdlOzczNTM5NTI7QVM6OTg1MzM1MjA3MDc1OTFAMTQwMDUwMzYzMTg4Mg==

O. Sracek et al. / Journal of Contaminant Hydrology 69 (2004) 45–71 47

2. Site description

The study site is located between Rouyn-Noranda and Val d’Or in Abitibi, Quebec,

Canada (Fig. 1). The Mine Doyon is a gold mine that has been operating since 1978.

Initially, open pit mining was conducted with two pits excavated along the strike of the ore

body. Mining operations have been underground since 1989. Most of the excavated waste

rock exists in the South Dump covering an area of more than 50 ha and a volume of about

11.5 million m3 (Gelinas et al., 1994). More than half of this volume consists of sericite

schist, which is the host rock of the ore body. The height of local relief of the pile is 30–35

m between the highest points and the level of drainage ditches (Fig. 2). The AMD

production in the South Dump became apparent 2 years after the construction of the pile

and since 1988 the acid production appears to have attained steady state.

The sericite schist is the main contributor to acid generation. This rock type contains

more than 7% pyrite, generally distributed along schistosity planes. This rock occurs at

Site 6 and Site 7 in the southern part of the pile (Fig. 2). The northwest and central zones

of the pile contain low-grade ore associated with diorite at the top of the pile (a 5-m-thick

layer) at Site TBT (Fig. 2).

Acid drainage from the pile is discharged at the toe because the permeability of the

underlying rocks is very low. The effluent is pumped to a water treatment plant, where the

acidity is neutralized and heavy metals are precipitated. High-density sludge is stored in

special basins and the effluent is released to the Bousquet river in the proximity of the pile.

The south pile was studied by researchers from Laval University from 1990 to 1996

(Gelinas et al., 1992). There was drilling and instrumentation of seven boreholes through

Fig. 1. Site location.

Fig. 2. Site map.


the entire thickness of the pile and in the upper part of the underlying rocks. The boreholes

were instrumented for multilevel sampling of gas and measurement of temperature in the

unsaturated zone, and for sampling of water in the saturated zone. Gravity drainage

lysimeters were also installed in the unsaturated zone, allowing sampling down to a depth

of about 4.0 m. Periodic sampling of water in ditches around the pile was also conducted.

Maximum temperatures in the profiles within the pile ranged from 45 to 70 jC.Lefebvre et al. (1993), used temperature data to derive thermal properties of waste rock

and to determine the pyrite oxidation rate (POR) using 1-D analytical models. That


investigation revealed an important role for air convection within the pile. There is a high

convection zone with upward air flow component adjacent to the slope and in the upper

part of the pile and low convection or diffusion zones at the lower elevations within and far

from the edges of the pile. Lefebvre (1994) modeled the AMD generation in the pile using

the non-isothermal multiphase flow model TOUGH AMD expanded from the code

TOUGH2 (Pruess, 1991).

This study focused on geochemical aspects of the AMD behavior with the following

main objectives: (1) characterization of pore water chemistry in the unsaturated zone

including determination of the main geochemical processes, and identification of the

AMD-buffering mechanisms, mineralogical transformations, and role of secondary min-

erals, and (2) assessment of the potential of stable isotopes for the investigation of AMD.

3. Methods and materials

3.1. Sampling devices

Sampling of water in the saturated zone and gas in the unsaturated zone, respectively,

was conducted using piezometers and gas sampling ports installed during previous

investigations (Gelinas et al., 1994). All new boreholes were drilled by reverse-circulation

air-drilling with an eccentric bit. Each borehole contained several sampling devices,

including two ground water piezometers. A shallow piezometer was completed with 19-

mm PVC pipe in the saturated waste rock at the base of the pile (depth of about 32.0 m)

and a deeper one with 38-mm PVC pipe was completed in the underlying bedrock (depth

of about 39.0 m). Polyethylene gas sampling tubes were strapped to the shallower

piezometer with openings at different depths. The gas sampling tubes were located in a

sand pack in the annular space between the wall of boreholes and the piezometer pipes.

Thermistors were also installed in all boreholes to measure temperature with depth.

Water sampling of the unsaturated zone represented a challenge because of the coarse

grain composition of waste rock material. Nests of suction lysimeters (1920L24, Soil

Moisture, Santa Barbara, CA) were installed at three sites: Site 6, Site TBT, and Site 7

(Fig. 2). Suction lysimeters with a 5.0-cm diameter porous cup and with an air-entry

pressure of 1.9 bar were installed in boreholes of 0.15-m diameter. Silica flour with a grain

size of about 200 mesh was deposited at the base of each hole. The thickness of the silica

flour layer was about 0.5 m. A bentonite seal, 1.0 m thick, was placed above the silica

flour and the boreholes were filled with medium sand to the surface of the pile.

3.2. Solids analysis

All boreholes drilled for suction lysimeters were sampled at approximately 1.5-m

intervals. Samples of waste rock cuttings from rotary air-drilling were collected at the pile

surface. Mineralogical composition of the samples was investigated by X-ray diffraction

using a Siemens D-5000 diffractometer. Drilling cutting samples were split and sieved to

less than 2-mm fraction. Characteristic peaks of X-ray diffraction were used for semi-

quantitative determination of selected minerals.


Selected samples were observed under Scanning Electron Microscopy (SEM), with

emphasis placed on detection of secondary minerals. Drill cuttings were observed in

secondary electron mode (SE), using a JEOL 840-A microscope with EDX attachment

capable of light elements analysis. The carbonate content of samples was determined using

the Chittick device described by Dreimanis (1962). Total sulfur in drill samples was

determined using a LECO SC432 furnace based on release of SO2 during combustion of

samples. Total sulfate was extracted with 10% Na2CO3. Sulfate was split into a gypsum

fraction, determined by leaching of 50 g of drilling powder in 200 ml of distilled water,

and a jarosite fraction, determined as weight difference after conversion of jarosite to

goethite by the application of NaOH (Choquette in Gelinas et al., 1994).

3.3. Water analysis

Piezometers were sampled with a WATERRA (foot valve) pump. Suction lysimeters

were sampled by the combination of pressure and suction pumping. Parameters such as

pH, temperature, and Eh were measured immediately after recovery of samples in the

field. The pH electrode (Orion Ross combination pH electrode, model 815600) was

calibrated with standard buffers, and the Eh electrode (Orion platinum redox electrode,

model 96-7800) was checked with a ZoBelle’s solution. Samples were then filtered

through a 0.45-Am membrane and split into two subsamples. One, designated for cation

analysis, was acidified with HNO3 to a pH of about 1.0. The second, designated for anion

analysis, was unacidified. All samples were analyzed for selected index parameters (Ca,

Mg, K, Fe2 +, Fetotal and SO4). Samples from Spring 1996 were also analyzed for Na, Cu,

Zn, Pb and some samples were also analyzed for Cl and PO4. The majority of the analyses

were conducted by atomic adsorption spectrometry (AAS). Sulfate, chloride and phos-

phate were determined by ion chromatography and Fe2 + by the potassium dichromate

method. Concentrations of Fe3 + were determined from the difference between Fetotaland Fe2 +.

3.4. Isotopic analysis

Samples for D and 18O analysis were collected during both dry and wet periods. Several

samples had such high TDS content that they had to be diluted by toluene distillation prior

to the analysis. Results from the mass spectrometer were expressed as d18O and d D per

mil with respect to the Standard Mean Ocean Water (SMOW) standard, with analytical

precision F 0.15xand F 1x, respectively.

For 13C analysis, dissolved carbonate was precipitated as BaCO3 by the addition of

NaOH and BaCl2. Analysis of13C was then conducted on CO2 evolved via reaction with

H3PO4 and results were reported in d13C per mil notation with respect to the PDB standard

with a precision of F 0.5x.

Dissolved sulfate was co-precipitated with BaCO3 as BaSO4. Then BaCO3 was

removed by acidification with 10% HCl. The oxygen in the sulfate samples was converted

to CO2 for18O(SO4) analysis with the graphite reduction method. Analyses were reported

as d18O per mil with respect to the SMOW with a precision F 0.5x. Concentration of34S in the sulfate was analyzed using the SO2 gas produced by thermal decomposition of


BaSO4. Values were reported as 34S per mil with respect to the Canyon Diablo Troilite

(CDT) standard with a precision of F 0.3x.

3.5. Other measurements and modeling

Temperature and oxygen concentrations were also measured during dry and wet

periods. Concentrations of oxygen were determined with a temperature compensated

electrode of 0.1% resolution.

The water content in the unsaturated waste rock was determined by neutron activation

in boreholes about 12.0 m deep with PVC casings. The neutron probe was calibrated in a

barrel filled by waste rock and results were reported in g/cm3, corresponding to water

contents on a mass basis.

Flow and gas transport in the unsaturated zone were modeled using the code TOUGH

AMD adapted from the code TOUGH2 (Pruess, 1991), and described by Lefebvre (1994)

and Lefebvre et al. (2001). Hydrogeochemical modeling was performed using the code

SIMUL (Reardon, 1992). The code is based on Pitzer’s equations and can handle

extremely high ionic strength (at Mine Doyon in some cases >10 mol/l). The Pitzer’s

equation parameters were available for only 25 jC. Saturation indices were expressed in

logarithmic form (SI = log (IAP/Ksp)), e.g., values higher than zero indicate supersatura-

tion, and values lower than zero indicate undersaturation with respect to a given mineral.

4. Physical properties of waste rock and oxygen and heat transport

4.1. Physical properties

Physical properties of the waste rock were determined on samples from shallow

trenches. Selected parameters are presented in Table 1. Fragments larger than 70 mm

represented between 17% and 60% of waste rock material. The sericite schist from Site 6

was more brittle and produced a greater amount of fines than diorite in the upper zone of

the pile at Site TBT. Inter-block porosity was determined as 0.33 by gravimetric

measurements. Intra-block porosity determined for fresh samples was about 0.05, but

much greater values were evident for weathered material. The average volumetric water

Table 1

Selected physical parameters (after Gelinas et al., 1994)

Parameter Value/units

Bulk density 1836 kg/m3

Inter-block porosity/intra-block porositya 0.33/0.05

Grain size d10 0.5–1.0 mm

Grain size d60 15–100 mm

Van Genuchten’s a/n 0.23/0.504 Pa� 1

Saturated hydraulic conductivity 10� 3 m s� 1

Hydraulic conductivity at 0.40 saturation 10� 8 m s� 1

a Fresh material.


content at shallow depths was 0.078 for sericite schist and 0.038 for diorite during the dry

period. These values correspond to saturation values of 0.42 and 0.21, respectively.

Significant differences between the dry (August 1995) and wet period (March 1996)

existed only at shallow depth (less than 2.0 m) (Sracek, 1997).

4.2. Temperature and oxygen profiles

A temperature profile for Site 6 is shown in Fig. 3a. There was an increase in

temperature down to a depth of 6.0 m. At that depth, the temperature was as high as 60 jCand increased slightly with depth. However, data between 5 and 15 m depth were not

available because those thermistors were damaged. Measurement of the phase and

amplitude of temperature variations were used in analytical solutions by Lefebvre et al.

(1993) to determine the thermal properties of waste rock. The temperature profile at Site

TBT (Fig. 3b) indicated a much lower maximum temperature of about 35 jC at 12-m

depth. This observation is consistent with less reactive diorite material at the top of that

Fig. 3. Temperature and oxygen profiles: (a) Site 6, (b) Site TBT, (c) Site 7.


site. The temperature profile at Site 7 (Fig. 3c) indicated an increase of temperature down

to about 32.0 m.

The oxygen concentration profile at Site 6 is shown in Fig. 3a. The concentration of

oxygen was high down to a depth of 10.0 m (about 15 vol.%) and then decreased to 2–3

vol.%. This behavior was interpreted to result from lateral convective transport of oxygen

from the slope of the pile at intermediate depths and diffusive transport at depth. The

decrease of oxygen concentration at Site TBT started deeper, at about 15.0 m (Fig. 3b) and

dropped to 2–3% at a depth of 25.0 m. High concentrations of oxygen at the top of the

profile appeared to be related to the less reactive diorite layer at the top. The profile at Site

7 (Fig. 3c) showed almost exponential decrease with depth, indicating a diffusive supply

of oxygen in the central zone of the pile as expected (Lefebvre et al., 1993; Ritchie, 1994).

Thus, both convective and diffusive regimes of the oxygen supply appear to be present

within the Mine Doyon pile. The presence of convective cells close to the slope and in the

upper zone of the pile was also revealed by the airborne survey using an infrared

thermographic camera (Gelinas et al., 1994). High-temperature anomalies were typical

especially for the upper zone close to the edges of the pile indicating that hot air was

leaving the pile. The horizontal distance separating temperature maxima and minima in the

central zone of pile varied from 10 to 15 m.

5. Solid phase composition

The composition of fresh samples of waste rock was determined by Savoie et al.

(1991). The average concentrations of pyrite in the sericite schist and in the diorite were

7.0 and 5.5 wt.%, respectively. The calcite concentration was almost identical in both

types of fresh rock, 2.25 and 2.3 wt.% in the sericite schist and diorite, respectively. The

ratio Al/Mg was 4.26 in the sericite schist and 5.07 in the diorite. The main silicates

consisted of plagioclase feldspar, muscovite and chlorite. The representative formulas

were (Mg3.90Fe0.71Al1.39)(Al1.21Si2.79)O10(OH)8 for chlorite, and (Na0.20K0.80)Al2(Al0.90Si3.10)O10(OH)2 for muscovite The Al/Mg ratio in chlorite is about 0.75, and the observed

bulk Al/Mg ratio that was much larger than 1.0 resulted from the relatively high quantities

of muscovite and plagioclase present in waste rock.

The results of X-ray diffraction analysis (Sracek, 1997) revealed the presence of

gypsum and jarosite throughout the complete thickness of the pile. There were no

pronounced trends of secondary mineral formation with depth. Muscovite, plagioclases,

and chlorite were present in all samples. Pyrite was found in all samples and calcite was

absent in all samples, except for the sample at 2.0 m depth at Site TBT with diorite at the

surface. This is consistent with extremely acid conditions in the pile.

The scanning electron microscopy (SEM) analyses indicated a variable degree of pyrite

alteration, in some cases independent of depth in the pile. There was a high degree of

alteration of chlorite, which was preferentially depleted in Mg, especially at the margin of

the crystallographic plates. The alteration of muscovite during the smectization phase was

also observed. This was accompanied by a loss of K, Al, and by a gain in Mg. This

suggests that the behavior of Mg in water may not be conservative in some cases,

especially during the early stages of smectization at immature profiles. K-jarosite was

Fig. 4. Reduced and oxidized S profiles: (a) Site 6, (b) Site TBT, (c) Site 7.


observed in almost all samples. This mineral tends to crystallize close to the source of

potassium, epitaxially on muscovite (Sracek, 1997). Aggregates and clusters of gypsum

were observed on planes parallel to the schistosity of the sericite schist. The formation of

gypsum and jarosite observed under SEM seems to accelerate the oxidation of pyrite by

physically breaking-up the rock and thus increasing the accessibility of oxidants to fresh,

unoxidized pyrite grains within the waste rock fragments (Sracek, 1997).

Analysis of reduced sulfur concentrations in the solid phase, corresponding to the pyrite

content, showed an S-shaped curve close to the slope of the pile at Site 6 (Fig. 4a). The

deeper concentration minimum likely corresponded to the entry of oxygen supplied by

convection from the slope with a significant upward flow component. Trends at Site TBT

were less evident (Fig. 4b), but there was an initial increase of reduced sulfur concentration

followed by decrease with depth. A maximum of oxidized sulfur concentration was at a

depth of about 12.0 m (Sracek, 1997). The deeper oxidized sulfur maximum compared to

Site 6 was related to the less reactive diorite cover with lower concentrations of pyrite at

the top of the profile. Finally, there was an increase down to 5 m and then a gradual

decrease of reduced sulfur concentration at Site 7 (Fig. 4c). However, there was a decrease

of concentration below the depth at 15 m, which was interpreted as to be the result of a

paleo-surface (break of waste rock deposition) with high degree of pyrite oxidation at

shallow depths prior to ongoing deposition. The oxidized sulfur profile exhibited a

maximum at shallow depths (about 5.0 m) and then there was a general trend of decreasing

Table 2

Remaining pyrite and sulfate within the pile (1995)

Site Pyrite

(kg)

Pyrite

(kg/tonne)

Sulfate

(kg)

Sulfate

(kg/tonne)

Pyrite/sulfate

(kg/kg)

Site 6 3433 65.61 1124 21.48 3.054

Site TBT 3326 63.56 1519 29.03 2.190

Site 7 4059 84.38 560 11.64 7.248

Table 3

Average gypsum and jarosite sulfate concentrations in the solid phase (1995)

Site Gyp. SO4

(wt.%)

Gypsum

(wt.%)

Jar. SO4

(wt.%)

Jarosite

(wt.%)

Total SO4

(wt.%)

Calcite

(wt.%)

Site 6 0.63 1.13 2.20 5.70 2.84 0.59

Site TBT 2.29 4.10 2.35 6.03 4.64 1.04


concentrations with depth down to 15 m, followed by increasing concentrations and

second maximum at about 21 m depth.

Concentrations of both forms of sulfur at all sites were numerically integrated with

depth and results are presented in Table 2. The difference in remaining pyrite between Site

6 and Site TBT was relatively small. In contrast, there was a higher sulfate content and

thus, lower pyrite/sulfate ratio in the solids at Site TBT. This is related to higher

concentration of gypsum at Site TBT (see below). Site 7 at the central zone of the pile

had a higher pyrite content, and a lower sulfate content than other sites. This is consistent

with diffusive supply of oxygen and resulting lower pyrite oxidation rates at greater

distances from the edge of the pile.

Total sulfate in the solid phase at Site 6 and Site TBT was split into water-soluble

(gypsum) and water-insoluble (jarosite) contents. There were some discrepancies between

total oxidized sulfur and the sum of gypsum and jarosite sulfate, but we believe that

concentration trends are correct. Average gypsum and jarosite sulfate contents together

with remaining calcite contents are presented in Table 3. The total content of sulfate was

higher at Site TBT than at Site 6. The total sulfate content was highest (5.10 wt.%) at

depths of 7.3 m at Site 6 and 16.5 m (5.19 wt.%) at Site TBT (Sracek, 1997). However, the

main difference in sulfate content between Site 6 and Site TBT was related to the

difference in gypsum content. This was also consistent with a higher average remaining

calcite content at Site TBT (1.04 wt.%) than at Site 6 (0.59 wt.%). The initial calcite

content at both sites was almost identical (about 2.3 wt.%), suggesting that Site 6 may

represent an environment with higher rates of acid generation and consumption of calcite,

which was depleted earlier than at Site TBT. At Site 6, most of calcite in solid phase was

probably depleted earlier than at Site TBT and a part of gypsum formed during early stage

of AMD generation has already been dissolved.

6. Hydrogeochemical behavior

This section presents the results of sampling of suction lysimeters in the unsaturated

zone and piezometers in the saturated zone. Temporal and spatial trends of index

parameters (Ca, Mg, K, Fe3 +, Fetotal and SO4) with saturation indices for secondary

minerals are presented and discussed.

6.1. Temporal and spatial trends

There were two distinct periods of temporal trends based on the infiltration pattern: a

recharge period (late March–early June) and a dry period (mid-June–February). The


profiles of pH, Eh, Ca, Mg, Al, Fe3 +, Fetotal, and SO4 at Site 6 during both periods

(October 1995 and April 1996) are presented in Fig. 5a.

At Site 6, there was a slight increase of pH from values of about 2.0 at shallow depths

to values of about 3.0 in deeper zones in both periods. The concentrations of Mg and Al

reached their maximum values at a depth of 10.0 m in October 1995. The concentration of

Al was about 33,500 mg/l during the dry period, which appears to be the highest

concentration of Al reported for AMD. Concentration of Mg was also high, with a

maximum value of 24,000 mg/l during the dry period. The maximum concentrations

observed were much lower during the recharge period. There were only moderate

increases of concentrations with depth and maximum values were less than 10,000 mg/

l for both dissolved species discussed above.

The behavior of Fetotal was more complex. There was a minimum at shallow depths

(7.3 m), a local maximum at a depth of 10.0 m, and a steady increase from 22.0 m down

to water table. The peak disappeared during the recharge period, but the relative

minimum at 7.3 m was similar for both sampling periods. The behavior of sulfate was

similar, with a maximum value of 230,000 mg/l at 10.0 m depth during the dry period

and lower values (less than 100,000 mg/l) during the recharge period. Behavior of other

species is discussed later.

Concentrations for the same species at Site TBT are presented in Fig. 5b. Concen-

trations generally increased downward in both periods and reached maximum values at a

depth about 22.0 m. During the recharge period, concentrations were generally lower, but

the decreases were not as large as at Site 6. The profiles at Site 7 are not presented here

because only data from August 1995 were available. However, concentration maximums

at intermediate depths, followed by gradual decreases downward were observed at this site

(Sracek, 1997).

6.2. The Eh values and iron speciation

The Eh values probably reflect the variations in the dominant Fe2 +/Fe3 + redox couple

because concentrations of iron were generally higher than 20 g/l. Nordstrom et al. (1979)

have shown that Eh values are likely to be meaningful in high iron and low pH waters. The

values of Eh and Fe3 + concentrations at Site 6 are shown in Fig. 5a. Both parameters

decreased with some fluctuations below 10 m depth.

The principal reactions responsible for generation of dissolved iron are (Stumm and

Morgan, 1981):

FeS2ðsÞ þ 7=2O2 þ H2O ! Fe2þ þ 2SO2�4 þ 2Hþ ð1Þ

Fe2þ þ 1=4O2 þ Hþ ! Fe3þ þ 1=2H2O ð2Þ

FeS2ðsÞ þ 14Fe3þ þ 8H2O ! 15Fe2þ þ 2SO2�4 þ 16Hþ ð3Þ

Reaction (1) represents the oxidation of pyrite by atmospheric oxygen. Reaction (2)

represents the oxidation of Fe2 + to Fe3 + by oxygen and is considered to be the rate-

Fig. 5. Pore water concentrations: (a) Site 6, (b) Site TBT.



limiting step for the oxidation of pyrite (Singer and Stumm, 1970). Finally, reaction (3)

represents the oxidation of pyrite by ferric iron. The last reaction is dependent on reaction

(2) because it can take place only if the Fe2 + generated by the oxidation of pyrite is

recycled back to Fe3 +. Thus, when the concentration of O2 decreases significantly below

atmospheric values at depth greater than 10.0 m (Fig. 3), the Fe2 + is no longer oxidized

and remains in solution. This situation results in decreasing Eh values with depth.

Melanterite (FeSO4�7H2O) represents a potential solubility control on Fe2 +, but there

was undersaturation with respect to this phase for water samples below 10.0 m depth

(Sracek, 1997). Thus, ferrous iron likely behaved conservatively in the zones of the pile

with low oxygen concentrations.

The chemical behavior of iron is somewhat complicated by precipitation of Fe3 +minerals

including K-jarosite (KFe3(SO4)2(OH)6). Precipitation of K-jarosite is expressed as

Kþ þ 3Fe3þ þ 2SO2�4 þ 6H2O ¼ KFe3ðSO4Þ2ðOHÞ6ðsÞ þ 6Hþ ð4Þ

and this reaction requires a source of K+. Concentrations of K+ in pore water were

generally less than 1 mg/l. The K+ is probably consumed by the precipitation of K-jarosite

in micro-layers on the surface of muscovite, and the concentrations of K+ in bulk solution

may not be representative of what is available for K-jarosite formation in the muscovite

interlayer spaces. Saturation indices for K-jarosite at Site 6 from April 1996 are shown in

Fig. 6a. There are no results available for 1995 because potassium was not included in

earlier analyses. The samples at a depth of 4.0 m and at 10.0 m were supersaturated with

respect to K-jarosite. Calculations are based on data from the recharge period, with lower

dissolved Fe3 + concentrations. It can be assumed that more samples were supersaturated

with respect to K-jarosite in the dry period. The maximum accumulation of jarosite based

on solid phase analyses was at a depth of 7.3 m, corresponding to a sharp decline of

Fetotal concentrations. Precipitation of other Fe3 + mineral phases such as Fe(OH)3 was

prevented by low pH conditions in most samples.

Fig. 6. (a) SI values for K-jarosite (April 1996), (b) dissolved Si (October 1995).

6.3. Behavior of magnesium and aluminum

Concentrations of aluminum and magnesium were very high, especially at Site 6 (Fig.

5a). This was a consequence of an advanced stage of neutralization by silicate minerals.

The likely source of magnesium is the dissolution of chlorite that can be represented by the

reaction:

ðAl;Mg4; FeÞðSi3AlÞO10ðOHÞ8ðsÞ þ 16Hþ

¼ 2Al3þ þ Fe2þ þ 4Mg2þ þ 3H4SiO4 þ 6H2O ð5Þ

In this reaction, 16 mol of H+ is consumed by the dissolution of 1 mol of chlorite.

However, dissolution of silicates is constrained kinetically (Appelo and Postma, 1993). A

possible sink for magnesium is precipitation of epsomite (MgSO4�7H2O), but all samples

were undersaturated with respect to that mineral (Sracek, 1997). Temporary implementa-

tion of magnesium into clays during smectization was found under SEM in several

samples. Transformation of micas follows the sequence muscovite!vermiculite! smec-

smectite! amorphous silica (Gelinas et al., 1994). Thus, when smectite is later converted

to amorphous silica, magnesium is released back to pore water. This means that the

behavior of magnesium can be relatively conservative in the long term.

The amount of Mg stored in pore water between the pile surface and a depth of 16.5 m

was estimated through the waste rock for a column with a 1 m2 base (Sracek, 1997).

There was a decrease from 27 kg in August 1995 to 12 kg in April 1996. The quantity of

Mg decreased to 44.5% during the recharge period, suggesting a mass loss by a factor

of 2.25.

The behavior of Al was similar to the behavior of Mg, but concentrations were

generally higher for Al (Fig. 5a). The Al/Mg ratio in chlorite is 0.75. This means that in

water samples with Al/Mg ratios greater than 0.75, the additional Al may originate from

muscovite (KAl3(AlSi3O10)(OH)2), which has no Mg. The Al/Mg ratios were generally

>1.0, and in some cases as high as 1.65. This supports the conclusion that there is a

significant contribution from muscovite dissolution to AMD neutralization. The majority

of samples were undersaturated with respect to gibbsite, Al(OH)3, due to low pH values.

According to Nordstrom (1982) and Nordstrom and Ball (1986), the gibbsite solubility

controls dissolved Al starting at a pH of about 4.2. Some samples were supersaturated with

respect to alunite (KAl3(SO4)2(OH)6), but that mineral phase does not appear to control Al

concentrations in this system. Precipitation of alunite may be prevented kinetically, and

also by preferential incorporation of potassium into K-jarosite. Thus, it is possible to

conclude that both Mg and Al behave in a relatively conservative fashion under low pH

conditions, which correspond to most samples.

6.4. Behavior of sulfate

Sulfate was practically the only anion present in water samples at the Mine Doyon

site. Its principal source is the oxidation of pyrite. Regardless of the mechanism of

reaction (Eq. (1) or Eq. (3)), 1 mol of pyrite produces 2 mol of sulfate. Reduction of



sulfate and precipitation of secondary sulfides was not expected because there is no

significant reductant available and 34S(SO4) values (Table 5) were almost constant.

Thus, the potential sinks for sulfate include the precipitation of K-jarosite in the shallow

zone of the pile with ferric iron and precipitation of gypsum as shown in the following

equation:

Ca2þ þ SO2�4 þ 2H2O ¼ CaSO4 � 2H2OðsÞ ð6Þ

The precipitation of K-jarosite requires Fe3 + and, thus, maximum concentrations of K-

jarosite in the solid phase were found at relatively shallow depths (7.3 m at Site 6) where

Fe3 + is produced in the presence of oxygen. However, measurable quantities of K-jarosite

were also found at depth because of the extremely high sulfate concentrations that counter-

balanced the decreasing concentrations of ferric iron.

The precipitation of gypsum is not directly dependent on redox or pH conditions, but

requires the input of Ca2 +. Thus, more gypsum was found at Site TBT that contained

higher residual calcite concentrations than at Site 6. All samples from the Site TBTwere at

equilibrium with respect to gypsum. Samples from Site 6 were at equilibrium during the

dry period, but several samples were undersaturated with respect to gypsum during the

recharge period (Sracek, 1997). At Site 6, part of the gypsum formed in the earlier stage of

AMD generation has probably been dissolved as a consequence of low calcium

concentration in pore water and sulfate was flushed out of the pile.

6.5. Other dissolved species

The behavior of calcium was controlled by calcite dissolution and by equilibrium with

gypsum (Eq. (6)). Samples with extremely high sulfate concentrations in the summer of

1995 generally had lower concentrations of calcium (Fig. 5a) because extremely high

dissolved sulfate concentrations force Eq. (6) to the right.

The behavior of dissolved silica was strongly influenced by the ionic strength. There

was an inverse relation between these parameters in which maximum values of ionic

strength corresponded to the minimum concentrations of dissolved silica and vice versa

(Sracek, 1997). The concentration of Si vs. depth at Site 6 in October 1995 is shown in

Fig. 6b. Dissolved silica at the observed pH range is present as the uncharged species

H4SiO40 (Drever, 1997), and the activity coefficient for H4SiO4

0 increases with increasing

ionic strength. In this case, the reaction

SiO2ðamÞðsÞ þ 2H2O ¼ H4SiO04 ð7Þ

is pushed to the left and amorphous silica precipitates as the ionic strength increases.

There was also a correlation observed between the concentration of Fe3 + and that of

several dissolved species including Na, Cu, Zn, and Pb (Sracek, 1997). No other saturation

values for minerals containing these species were observed. This suggests that co-

precipitation of these metals with K- and Na-jarosite may have occurred. However, data

were limited (only from April 1996) and this remains a matter of conjecture.

https://www.researchgate.net/publication/247890890_The_Geochemistry_of_Natural_Waters_Surface_and_Groundwater_Environment?el=1_x_8&enrichId=rgreq-30e854d1-466d-41fd-b444-a085459ebfd3&enrichSource=Y292ZXJQYWdlOzczNTM5NTI7QVM6OTg1MzM1MjA3MDc1OTFAMTQwMDUwMzYzMTg4Mg==


7. Isotopic results

Results of stable isotope analyses presented in this section are discussed with the

purpose of relating it to the internal evaporation process within the pile, to elucidate the

behavior of sulfate, and to verify the possibility of neutralization of AMD by carbonates

present in the solid phase.

7.1. Deuterium and oxygen-18

The local meteoric water line (LMWL) was determined for the Mine Doyon region on

the basis of sampling of local precipitation. This equation of LMWL is

dD ¼ 7:02d18Oþ 7:67 ð8Þ

The principal characteristic of precipitation at the Mine Doyon site was the large

difference between winter snow precipitation, (dD less than � 120x, d18O less than

� 16x) and summer rain precipitation (dD greater than � 90x, d18O greater than

� 13x). The difference was used to trace recharge pulses from the melting of snow.

The interpretation of processes within the pile is based on the departure from the

LMWL. When evaporation from a free water surface takes place, there is an enrichment in

heavier isotopes with a shift of values from the LMWL and the expected slope of the

resulting evaporation line is about 5.0 (Clark and Fritz, 1997). When evaporation takes

place within a porous media, the slope of the evaporation line changes (Allison, 1982;

Allison et al., 1983). The slope decreases to about 2.0–3.0 due to more resistance for

diffusion of water vapor in partially saturated porous media. This behavior was described

in arid regions by Dincer et al. (1974). Internal evaporation was observed at the Mine

Doyon site during the drilling operation in June 1994, when a borehole 22.0 m deep at Site

6 acted as a chimney for upward vapor transport, and the temperature of the vapor was 54

jC. The isotopic data for the summer 1995 samples are presented in Fig. 7a. There was

clear deviation from the LMWL, which can be expressed as

dD ¼ 2:4d18O� 66:0 ð9Þ

The slope of 2.4 is consistent with findings of Allison (1982), and suggests internal

evaporation is occurring within the waste rock pile. The internal evaporation line intersects

the LMWL at a point with coordinates dD=� 96x, d18O =� 13.6x. This is mid-point

between typical composition of snow and early spring precipitation (May), suggesting an

almost equal contribution of melted snow and spring precipitation to the recharge in the

pile. There is a potential isotopic exchange with 18O in silicates, which would result in a

similar deviation from the LMWL. However, isotopic exchange of water with silicates is

generally limited at temperatures below 150 jC (Truesdell and Hulston, 1980; Clark and

Fritz, 1997).

The data from the recharge period (April 1996) are presented in Fig. 7b. There was

a shift towards snow isotopic values, indicating a significant recharge from melted

snow. At Site 6, only shallow samples (above 13.2 m depth) were influenced. On the

Fig. 7. Stable isotopes data: (a) Summer 1995, (b) Spring 1996.


other hand, a portion of snow recharge penetrated to the saturated zone at Site TBT

(Sracek, 1997). The contribution of snow recharge for different samples is shown in

Table 4. The mixing equation was applied, with an initial isotopic composition based

on the summer 1995 data and resulting isotopic compositions based on the early spring

1996 data.

The depth of snow recharge penetration seems to be dependent on the location in the

pile and on the type of rock, which also determines the oxidation rate, temperature, and,

thus, internal evaporation in the pile.

Table 4

Penetration of snow recharge based on stable isotope data (spring 1996)

Site Depth (m) Snow recharge (%) Comment

Site 6 4.0 51.1 unsaturated zone



Site 6 32.0 none saturated zone

Site TBT 7.3 50.5 unsaturated zone

Site TBT 16.5 16.6 unsaturated zone

Site TBT 32.0 16.2 saturated zone

7.2. Sulfur-34 and oxygen-18 in sulfate

Both 34S and 18O(SO4) were analyzed (Table 5). There was a slight depletion of

the AMD sulfate (average � 0.30x) in d34S(SO4) compared to reported values of

d34S for hydrothermal pyrite at the Mine Doyon site (average + 2.19, Savoie et al.,

1991).

A depletion of resulting sulfate compared to pyrite was also observed by Toran (1987),

and was attributed to the bacterial oxidation of pyrite. This is also a probable mechanism at

Mine Doyon because several bacterial strains were observed in the waste rock (Guay, in

Gelinas et al., 1994). No depletion of d34S(SO4) was observed (Sracek, 1997), suggesting

that sulfate reduction was not significant (Clark and Fritz, 1997; Strebel et al., 1990). The

fractions of O2 in sulfate derived from air and from water can be estimated using the

equation (Taylor and Wheeler, 1994; Toran, 1987):

d18Os ¼ ðfwÞðd18Ow þ ewÞ þ ðf aÞðd18Oa þ eaÞ ð10Þ

where d18Os, d18Ow, and d18Oa are18O values in sulfate, water and air, respectively, and

ew and ea are fractionation factors for water and air, respectively. Fractionation factors are

site-specific and they were not available for the Mine Doyon waste rock. Simple

combinations of O2 from water and from air were assumed, resulting in a possible error

F 10% (Toran and Harris, 1989). The estimated contribution of O2 from water was from

85% to 95% (Table 5), which was consistent with the expected important role of Fe3 + in

the oxidation of pyrite. Any combination of fractionation factors presented by Taylor et al.

(1984) gave the contribution of water O2 close to 100%. Plotting of both d18Ow and

d18O(SO4) with depth revealed a difference between Site 6 and Site TBT (Fig. 8a,b). At

Site 6, there was only slight increase in d18O(SO4) values between 16.5 and 32.0 m. In

contrast, there was an enrichment of 18O in water in the same depth interval, caused by

evaporation. If the oxidation of pyrite by Fe3 + had occurred below 16.5 m depth, then the

resulting sulfate would implement O2 from water enriched in 18O, and this reaction would

have resulted in a shift of d18O(SO4) values. This did not occur, and pyrite oxidation in the

deep zone at Site 6 must have been limited. At Site TBT, both water and sulfate became

more enriched in 18O with depth. It therefore appears that some oxidation of pyrite

occurred in the deep zone at Site TBT.


Table 5

Sulfate isotopic data

Site/depth Sulfate (mg/l) 34S (x) 18Owater (x) 18O(SO4) (x) O2 from water (%)

S.6/1.67 m 37,507 1.00 � 11.85 � 6.83 85

S.6/16.5 m 101,225 � 0.25 � 11.67 � 8.93 92

S.6/32.0 m 116,795 � 1.44 � 10.08 � 8.33 95

S.TBT/7.3 m 21,511 � 2.25 � 13.39 � 8.94 88

S.TBT/22 m 171,922 1.41 � 10.47 � 6.95 89.5

Average 89,792 � 0.30 � 11.49 � 7.99 89.9

Fig. 8. d18O water and sulfate data: (a) Site 6, (b) Site TBT.


7.3. Carbon-13

The average d13C value of calcite in waste rock was � 2.63x(Savoie et al., 1991).

Carbon generated by decomposition of organic matter has d13C values about � 25x, and

the fractionation factor for dissolution of CO2 in water is almost negligible at pH less than

4.5 (Clark and Fritz, 1997), that was observed in the pile. Thus, the d13C(DIC)approximates the carbon isotopic composition of a carbon source. The average d13C(DIC)C(DIC) values in the saturated zone water were � 21.42xand � 8.12xat Site 6 and

Site TBT, respectively. These values correspond to 16.0% and 75% contributions of

carbon from calcite at Site 6, and Site TBT, respectively. These results are consistent with

the low content of residual calcite at Site 6 and with organic matter (wood and grass)

observed in the saturated zone at that site. At Site TBT, a significant input of carbon from

calcite dissolution masked the fingerprint of carbon from decomposition of organic matter.

No data on d13C were available for water from the unsaturated zone because CO2 was

expelled from water by combined effects of extreme ionic strength and temperature, and

also by suction applied in the suction lysimeters.

8. Recharge and role of internal evaporation

Two main approaches can be used for the estimation of recharge into a waste rock pile:

recharge can be considered as the water entering the unsaturated zone on the top of the pile

or recharge can be considered as the water entering the saturated zone at the base of the

pile. Both terms can be different for calculation on an annual basis because a part of the

water can be retained in the unsaturated zone of the pile by: (1) retention of water by

capillary forces in fine grained material, (2) retention of water in hydrated minerals like

gypsum, and (3) evaporation and partial re-condensation of water in the pile.


The quantity of water entering and discharging from the saturated zone was estimated

by base flow hydrograph separation for the trench surrounding the pile (Bourque, 1994).

The average recharge value for the pile based on hydrograph analyses was determined to

be 205 mm/year, which comprises about 24% of the average precipitation. The mass

balance based on simulation of saturated flow at the base of pile (Sracek, 1997) indicated

that less than 1.0% of water in the saturated zone infiltrates into bedrock underlying the

pile. Thus, discharge into the collection trench approximates well the recharge into the

saturated zone of the pile.

The quantity of water retained in fine grained material does not seem to change

significantly during the year except for depths shallower than 2.0 m (Sracek, 1997). The

retention of water within the pile was important only at the initial stage of the pile

construction, when fresh and relatively dry waste rock was deposited and infiltration was

initiated. Some time is required to ‘‘wet up’’ the pile to ‘‘field capacity’’ before water in the

unsaturated zone migrates to the saturated zone.

Retention of water in gypsum was determined by data on the composition of solids. The

average gypsum content in the waste rock was 1.13 wt.% at Site 6 and 4.10 wt.% at Site

TBT (Table 3). Water retained in gypsum was calculated using the bulk density of waste

rock (1836 kg/m3), and the thickness of the unsaturated zone (32.0 m). Assuming that

infiltration occurred over 13.5 years at an average rate of 205 mm/year, the estimated

quantity of water retained in gypsum is about 5.0% at Site 6, and 18.2% at Site TBT. This

means that a non-negligible quantity of water remains in the unsaturated zone locked in

secondary gypsum.

Internal evaporation has been confirmed by visual observation (vapor leaving boreholes

during drilling) and by isotopic data presented above. The AMD water was also sampled

for chloride, assuming that the only factor contributing to the increase of chloride

concentration was evaporation (Table 6). The concentration factors were expressed with

respect to the shallowest samples because the chloride input from precipitation was not

known. The concentration factors approached values from 6 to 7 in the deep unsaturated

zone, which indicates that up to 83–86% of deeper water was evaporated compared to the

shallowest suction lysimeter at a depth of 4.0 m. The evaporated water was convected

towards the upper zone of the pile. Comparison of data at a depth of 16.5 m indicated that

evaporation was two times larger at Site 6 than at Site TBT.

At the Mine Doyon site, the internal source of heat results in evaporation within the

porous media. It is assumed that there is a threshold value of infiltration necessary for

penetration of recharge to the water table. The water balance is complicated by re-

Table 6

Dissolved chloride data (April 1996)

Sample Cl (mg/l) Concentration factor

Site 6: 4.0 m 3 1.0

Site 6: 10.0 m 8 2.7

Site 6: 16.5 m 21 7.0

Site TBT: 7.3 m 6 1.0

Site TBT: 16.5 m 20 3.3

Site TBT: 22.0 m 38 6.3


condensation of evaporated water close to the surface of the pile and by the existence of

preferential flow zones in large openings between blocks.

Hypotheses for the roles of infiltration, evaporation and re-condensation were tested

using the numerical multiphase flow code TOUGH AMD. The TOUGH AMD model was

adapted for acid mine drainage modeling and comprises the oxidation of pyrite, convective

and diffusive fluxes of oxygen, transport of heat, and advective transport of sulfate

(Lefebvre et al., 2001). However, geochemical reactions are not considered. Simulations

were run for a period of 13.5 year, corresponding to the lifetime of the pile in 1995. The

domain was wedge-shaped with the unsaturated zone 30 m thick and a base length of 85

m. The boundary conditions were: temperature 5 jC, water saturation 0.42, infiltration 350mm/year and atmospheric oxygen fraction 0.2315. The water retention curve used for

modeling was derived by Lefebvre (1994) based on the grain size distribution of the

material from the pile. The right boundary corresponds to a symmetry line at the center of

the pile.

The calculated distribution of temperature is presented in Fig. 9a. The calculated

temperature maximum was more than 65 jC at 15 m from the upper left margin of the pile.

This is a consequence of the oxygen supply in the region. The region corresponds to the

zone of maximum evaporation and is represented by Site 6. The calculated temperature

was much lower at the right boundary of the modeling domain, corresponding to the

central zone of the pile (less than 25 jC). These results are consistent with simulations by

Pantelis and Ritchie (1993), Lefebvre (1994), and Lefebvre et al. (2001). Vapor flux (Fig.

9a) was high at distance of about 15 m from the slope and was directed upward. The vapor

Fig. 9. Numerical modeling: (a) Temperature and water vapor flux, (b) water saturation and water flux.


flux was limited in the central zone of the pile. The presence of an eddy in the central zone

was not confirmed in the field and remains questionable. In contrast, water flux (Fig. 9b)

was always downward and the maximum recharge of re-condensated water was estimated

to occur closer to the central zone of the pile, where convective vapor transport occurred to

the pile surface. This has a direct impact on the hydrogeochemical behavior because water

evaporated close to the slope does not return to its original location, but contributes to the

recharge closer to the central zone of the pile. The zones close to the slope also had lower

water contents (Fig. 9b). Thus, evaporation contributes to extremely high concentrations in

pore water observed at Site 6 near the edge of the pile. Concentrations of dissolved species

are lower at the much colder Site TBT that is closer to the central zone of the pile. This

suggests that both physical and geochemical processes in the pile are interconnected and

contribute to water quality within the pile.

9. Character of transport in waste rock

The data originally presented an apparent paradox that involved significant concentra-

tion changes for conservative species like Mg and Al, accompanied by almost insignificant

changes of water content below a depth of 2.0 m. The water content in the deeper zone

varied between 0.20 and 0.25 (water saturation 0.60–0.75) (Sracek, 1997). At the

beginning of the recharge period at Site 6, the quantity of Mg stored between the surface

and a depth of 16.5 m decreased to 45% of the quantity in the dry period and the extreme

concentrations at a depth of 10 m completely disappeared. There was also a significant

decrease of other species concentrations. A similar trend was observed at Site TBT.

At Site 6, recharge effects were apparent at a depth of 13.2 m, but the recharge pulse did

not seem to penetrate deeper. In contrast, the recharge pulse penetrated to a depth of at

least 22.0 m at Site TBT (Sracek, 1997).

The apparent paradox can be explained when it is understood that the waste rock at

Mine Doyon can be considered as an example of double or even multiple porosity media

with an important role of matrix diffusion. This topic has been studied, for example, by

McKay et al. (1993), Parker et al. (1994) and others. Relatively small changes in water

content can cause significant changes in concentrations of dissolved species, when reverse

diffusion occurs from decomposed waste rock blocks into relatively fresh water migrating

in preferential flow zones. During recharge periods, mobile water in preferential flow

zones becomes gradually more concentrated with depth as a consequence of diffusion

from lower permeability zones and partially decomposed blocs containing very high

concentrations. It was impossible to precisely estimate the time of recharge available for

reverse diffusion. However, the maximum time limit is less than 6 months because the last

sampling before the recharge period sampling was in October 1995. The distance

influenced by 1-D transient diffusion was calculated (Sracek, 1997), and reduction of

concentration at a distance of 1 cm from the preferential flow zone was about 0.12 times

the initial concentration (C0) in the blocks. For similar conditions, a reduction factor of

0.55 was calculated for a travel distance in the block of 5 cm. Thus, diffusion may

equilibrate concentrations in the mobile water phase within the waste rock even during

relatively limited time periods.


10. Summary and conclusions

This hydrogeochemical and isotopic investigation of acid mine drainage at Mine Doyon

in the unsaturated zone of waste rock contributed to a better understanding of the processes

within a sulphide-bearing waste rock pile. There were strong spatial (slope vs. core) and

temporal (dry period vs. recharge period) variations in geochemical and physical behavior.

Site 6 represented an environment close to the slope of the pile containing friable sericite

schist material, with high temperatures and a convective supply of oxygen. Site TBT

represented a central zone of the pile containing diorite material and affected by the

proximity of a deep depression, with low temperature and a mixture of convective and

diffusive oxygen supply. Site 7, with limited data, represented the central zone of the pile

dominated by diffusive oxygen transport.

The main secondary minerals found during mineralogical investigations were K-

jarosite, and gypsum. Concentrations of gypsum in solid phase were higher at Site TBT

than at Site 6, suggesting that a part of gypsum formed at early stage of AMD at Site 6 has

already been dissolved. Crystallization of secondary minerals along foliation planes of

sericite schist contributed to the splitting of exfoliation in waste rock followed by

penetration of oxidants to previously unoxidized pyrite within the blocks of waste rock.

Thus, pyrite oxidation rates were higher in sericite schist than in diorite due to different

textures in each rock type.

Concentrations of several dissolved species in the unsaturated zone of the pile exhibited

extreme values, especially close to the slope of pile. A concentration maximum was

observed at a depth of 10.0 m at Site 6 during the dry period. Concentrations of iron in

pore water at shallow depths were controlled by the precipitation of K-jarosite and by the

oxidation of pyrite by dissolved ferric iron. Concentrations of ferrous iron were not

controlled by precipitation of a mineral phase. Concentrations of Al and Mg exhibited

maximum values of 33 and 24 g/l, respectively, at Site 6 during the dry period, and both

species seemed to behave conservatively in most of the samples. There was a decrease of

concentrations during the recharge period even though water content remained constant

and the concentration peak at 10 m depth completely disappeared. Concentrations at Site

TBT were lower than at Site 6, but there was also a relative decrease in observed pore

water concentrations during the recharge period.

Isotopic data based on dD and d18O revealed an important role of internal evaporation

within the pile. There was a shift from the internal evaporation line with a slope of 2.4

towards the dD and d18O values typical for snow during the recharge period. The colder

Site TBT exhibited a greater influence from snow recharge than warmer Site 6. The

d34S(SO4) values indicated bacterial oxidation of pyrite and an absence of sulfate

reduction within the pile. The d18O(SO4) suggested that 85–95% of oxygen was derived

from water and a comparison with d18O in water suggested that pyrite oxidation occurred

at depth at Site TBT, but not at Site 6.

Internal evaporation was also confirmed from increasing chloride concentrations with

depth. A significant portion of recharged water was retained in gypsum within the pile.

Numerical modeling of pyrite oxidation with the TOUGH AMD code coupled with heat

transport and multiphase flow gave results consistent with geochemical and isotopic data,

supported the observed high temperatures and upward air convection close to the slope of


the pile, and zones of lower temperature and diffusion-limited oxygen transport in the

central zone of the pile. Evaporated water does not return to the zone of evaporation close

to slope, but re-condensates and infiltrates closer to the central zone of the pile and,

therefore, contributes to the extreme concentrations of dissolved species observed in the

pore water in the unsaturated zone close to the slope during period without recharge.

These results indicate that both physical and geochemical processes within a waste rock

pile are interconnected for highly reactive sulphide waste rock, and they cannot be

considered separately. Furthermore, there is an important role of rock texture and strength

in the generation of acid mine drainage, where AMD generation rates are much faster in

more friable material even when the pyrite content of rocks is similar.

Acknowledgements

The authors acknowledge the assistance of Pierre Therrien from the Universite Laval

with application of the code TOUGH AMD. Thanks also go to Eric Reardon from the

University of Waterloo, who provided us with his code SIMUL, to Michel Giguere from

the Universite Laval, who performed the chemical analysis, and to Robert Drimmie and

his staff from the Isotope Laboratory of the University of Waterloo, who performed the

isotopic analysis. Assistance of Dominique Marceau during field sampling is appreciated.

Fieldwork was financed by the Mine Doyon, which belonged to the Barrick/Cambior

group during the project period. The first author gratefully appreciates financial support

awarded by NSERC Canada and FCAR Quebec. We also thank Uli Mayer from the

University of British Columbia in Vancouver and John Molson and Michel Aubertin from

Ecole Polytechnique in Montreal for their review of the paper, which helped to improve

this manuscript.

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Documents

Geochemical characterization of acid mine drainage from a waste rock pile, Mine Doyon, Québec, Canada