Groundwater chemistry in the Schefferville, Quebec iron deposits

CATENA

Vol. 10, 149-158

Braunschweig 1983

GROUNDWATER CHEMISTRY IN THE SCHEFFERVILLE, QUEBEC IRON DEPOSITS J.J. Drake, Hamilton SUMMARY Input precipitation quality, soil clay mineral reactions and aquifer mineral dissolution control the quality of groundwater in an area of discontinuous permafrost in the Labrador Trough iron deposits near Schefferville, Quebec. Na and CI are derived from precipitation and K from clay minerals. Ca, Mg and HCO3 concentrations and pH are governed by the extent of mixing of saturated water from a dolomite unit with water from other, non-carbonate units. Local conditions govern the equilibrium concentrations in the dolomite-saturated water. On ridge tops carbonate materials are absent and the closed-system evolution path is followed but in one lower area, where a carbonate-containing till is present, evolution is open-system. Iron species concentrations and pE values suggest that groundwater in equilibrium with the iron oxyhydroxide minerals is anoxic. Groundwater temperatures are similar to the annual recharge temperature (4.7°C) and pumping associated with mining may cause an alteration in the permafrost distribution by inducing groundwater flow.

1. INTRODUCTION The Schefferville, Quebec area is the centre of the Knob Lake iron ore deposits in the discontinuous permafrost region of the Labrador Trough. Ore has been mined since 1954, and is currently extracted from a series of open-pit mines located in a 50 km long belt close to the SW margin of the Trough. This paper examines the hydrochemical processes and environmental controls that are operative in groundwater in the area based on samples from mine de-watering wells, a spring and a water well. The main features of the geology of the area, which are described in detail by GROSS (1968) and HARRISON et al. (1972), are a series of NW-SE trending ridges and vales which are the surface expression of a series of folded and overthrust repeats of the Proterozoic Knob Lake Group of sedimentary rocks of the Labrador Trough (Figure 1). A generalized crosssection of a ridge is shown in Figure 2a. In sequence from the base the Knob Lake Group includes the Attikamagen Formaton (slates), the Denault Formation (dolomite), the Fleming Formation (chert breccia; some dolomite), the Wishart Formation (primarily quartzite and arkose), the Ruth Formation (ferruginous, carbonaceous slates), the Sokoman Iron Formation and the Menihek Formation (slates). The Sokoman Formation generally consists of iron-silicates which are resistant to erosion, and it therefore tends to form the ridges. The large thrust faults that lie along the ridge sides are zones of locally higher permeability, and GROSS (1968, 59-67) has explained the local alteration of the hematite- and magnetite-rich protore to hematite-goethite earthy ore on the basis of differential silicate leaching and secondary goethite emplacement in warm, humid Mezozoic climates. The mines are consequently located on the ridge flanks close to the crest, and may penetrate the permafrost zone if it is present.

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The ridges in the area are generally, but not completely, underlain by permafrost which may be up to 100 m thick. Permafrost distribution is largely controlled by surface conditions, chiefly exposure and winter snow cover (NICHOLSON 1978), but has been mapped directly only in several small experimental areas. Geophysical methods (e.g. SEGUIN 1974) have been used to map permafrost distribution over larger areas, but these show only approximate agreement with direct borehole temperature measurements. Permanently unfrozen zones (taliks) exist beneath some lakes and drainage lines within the permafrost area, and permafrost is entirely absent beneath the main valleys in the area. There is little information on the hydrology of the Schefferville area, especially so in areas undisturbed by mining. Suprapermafrost groundwater flow has been shown to concentrate along drainage lines (wet-lines) beneath small dry valleys on the ridges thereby main-

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mining taliks (LEWIS 1977). Taliks beneath lakes on the ridges (NICHOLSON 1978) serve as recharge points to the subpermafrost groundwater body. Hydraulic properties of the rocks, even in the absence of permafrost, are very variable (STUBBINS & MUNRO 1965). The piezometric surface beneath the ridges, as shown in wells or mines that penetrate the permafrost, or where permafrost is absent, generally lies within 30 m of the surface and apparently follows the topography. The ridges are generally associated with recharge zones although, on the side of some ridges, small springs may be found at the base of the Sokoman Formation. The vales are occupied by lakes and swamps and the water table is at the surface in the lower-lying areas. Water budgets of several lakes in the vales (F.H. RIGLER, Dept. of Biology, McGill Univ., pers. comm.) have shown that a considerable quantity of groundwater discharges into them, and the vales are generally associated with discharge zones. Figure 2b shows a generalized hydrogeologic cross-section of a ridge and vale. Control of water in the mines is achieved by two methods: inflowing surface water and suprapermafrost groundwater is directed to a mine-floor sump from which it is pumped, and perimeter inteceptor wells are used to lower the local piezometric surface (associated with subpermafrost groundwater where permafrost is present) below the mine floor (GARG & STACEY 1973). The sump pumps and perimeter wells are linked into common piping systems which discharge into surface streams close to the mines. Mines 2 and 6 have two such outfalls, mine 8 has four and the rest only one.

152

2.

DRAKE

SAMPLING SITES AND METHODS

Water samples were taken from the outfalls of eight mines (Figure 1): seven (1-7) in the main mine zone north-west of SchefferviUe and one (8) 20 km south-east of the town. Mines 1-7 extract earthy ore formed in situ and mine 8 extracts ore that was emplaced as rubble by earth movements in the Cretaceous. Samples were also taken from a spring at the base of the Sokoman Iron Formation (9) and from a single water-supply well driven into the Denault dolomite (10). The stratigraphic and hydrogeologic positions of the sampling sites are shown schematically in Figures 2a and 2b. Samples were taken as pumping allowed, approximately semi-monthly in the summers of 1975 and 1976 (except for mine 8), and for mines 2, 4 and 5 once or twice during the intervening winter of 1975-76. Mines 4-7 are adjacent to one another and form part of a single dewatering zone. Only 4 and 5 were being worked during the study period. Mines 6 and 7 were pumped to aid in the de-watering of 4 and 5, but were not themselves kept empty of standing water. Aconsiderable portion of their total pumpage thus came from mine ponds. Number 5 lies on a major fault zone which passes to the south under Ruth Lake. This lake has suffered considerable drawdown since the mine has been in operation, and it appears that the fault zone is of relatively high permeability, allowing flow from the lake to the mine. All other sampling sites are independent of each other, lying in separate surface and (presumably) underground drainage basins. Water samples were analysed in the field for temperature (with a thermistor probe and digital multi-meter), pH and Eh (with a platinum electrode and calomel reference). Samples were briefly stored under refrigeration, and within 24 hours of collection all were analysed at "he McGill Subarctic Research Station for Ca and Mg by standard Swartzenbach titration ,sing BDH kits, and in summer the alkalinity of samples was determined by potentiometric .tration with dilute H2SO 4. Total Fe was determined by the bipyridine spectrophotometric method and for SiO2 by spectrophotometry using a B & L Spec 20. Subsequently some samples were analysed for Na and K by AA spectrophotometry and for C1 by titration. Methods for Fe, SiO:, and C1 were those of BROWN et al. (1970). Occasional checks with acidified replicate samples showed that no alteration of ionic concentrations of an unacidified sample occured within 24 hours of collection. Occasional samples were also analysed for Fe(III) by the bipyridine method (BROWN et al. 1970) but in no case was the concentration greater than the limit of detection of 0.05 mg 1-1. Comparisons of total Fe concentrations in filtered (0.45 #) and unfiltered replicate samples showed no difference. Chargebalance errors for most samples were less than 5%.

3.

RESULTS AND INTERPRETATION

Information on these routinely determined chemical constituents is tabulated in Table 1, which provides data on the mean, the standard deviation and the number of samples analysed from each mine. Table 1 also contains data for the analyses of the spring and the well water. No temporal trends could be distinguished in the data from any outfall, and analysis of variance showed that multiple outfalls from the same mine were not different at the 0.05 significance level. Few wells sample water from a single formation, and even in those that appear to do so, the proximity of fault zones probably causes a considerable mixing of waters from various formations (see Table 1). It is therefore difficult to separate the samples into groups based on their differing geological provenance, and the following inter-

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pretation emphasizes the probable sources and controlling processes that determine groundwater quality in this subarctic area.

3.1.

TEMPERATURE

The temperatures of the groundwaters (Table 1) are generally between 3 and 50C, with the exception of sites 6 and 7. Samples from these sites in summer include a considerable amount of water pumped from the partly water filled mines that has been warmed by exposure to the atmosphere. All other sites yield water that has little admixture of surface water, and the limited winter temperature data available suggest that 3-5°C is a representative figure for the whole year. NICHOLSON & GRANBERG (1973) found the aquifer rock temperature in non-permafrost zones to be between 0 and 0.5°C in a relatively high area near site 1, and suggested that that was the characteristic groundwater temperature. In this study the characteristic groundwater temperature is much higher. The mean annual air temperature at Schefferville is -4.5°C, but the approximate annual average recharge temperature, given by 12 =

P i " MAX(T i, 0)

Pi where T i and Pi are the average monthly temperature and precipitation, is 4.7°C, a value similar to the groundwater temperature. In the area studied by NICHOLSON & GRANBERG (1973), groundwater circulation was limited, and aquifer temperatures were largely controlled by heat fluxes between the atmosphere and the ground, and the geothermal flux. In the area in this study, groundwater flow is considerable and to some extent induced by pumping, and becomes the major source of heat for the aquifer. It is therefore possible that pumping may affect the distribution of permafrost in the area, but no direct evidence is available to support this.

3.2.

MINOR IONS

Concentrations of Na and C1 were less than 2 mg/ -1 in all samples, and were in approximately sea-water proportion. Precipitation in the areas has Na and CI concentrations similar to those in the groundwater, and precipitation is therefore considered to be the source of these ions. Concentrations of Kare of the order of 1 mg/-1, but only of 0.1 mg/-1 in precipitation inputs. MOORE (1978) has considered clay-mineral transformations in soils inthe Schefferville area, and has suggested that concentrations of K in equilibrium soil solutions are probably governed by kaolinite solubility, and are of the same magnitude as those in groundwaters in this study. The same source and control are therefore inferred for K concentrations in groundwater. Concentrations of SiO2 are in the range 3 - 7 mgl -l which is generally higher than the equilibrium value for quartz solution( - 3 mg! -x at 5oc), but considerably less than that for amorphous silica ( - 30 m g / - l . It appears, therefore, that quartz solubility is the basic control of SiO2 concentrations in the groundwaters.

GROUNDWATERCHEMISTRY:QUEBECIRONDEPOSITS 3.3.

155

MAJORIONS

Ca and Mg are the dominant cations, and HCO% the dominant anion in all samples. They are in approximate charge-balance. The molar Ca/Mg ratios shown in Table 1 are not significantly different from the molar value of 0.96 reported for whole-rock analysis of unaltered Denault dolomite by GROSS (1968, 20). It can therefore be assumed that this widespread geological unit is the source of these ions, and that their concentration and the pH in sampled waters is controlled by the strongly buffering carbonate reaction system. The mean and standard deviation of the calculated equilibrium partial prssure of carbon dioxide (PCO2, atm) are shown for each site in Figure 3, which also shows the saturation index with respect to dolomite. This index is defined as SId=

0.5 log([Ca 2+} [Mg 2+} {CO32-]2/Kd)

where Kd is the dissociation constant for dolomite and [x] represents the activity of species x. The values of PCO2 and SId were calculated with the program WATSPEC (WlGLEY 1977).

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Fig. 3: Means (symbols) and standard deviations (bars) of Sic and PCO2 values from each site. Vertical solid line shows the open-system evolution path and the curved line the closed-system evolution path for an initial PCO2 of 10-Ls4 atm at 4"C. The evolution paths are identical with the mixing curves for waters at various stages of evolution.

The equilibrium chemistry of groundwater in carbonate terrains in various climate regions of the world can be modelled as dependent on mean annual recharge temperature and on the carbonate content of the surficial material (DRAKE 1980). The mean annual recharge temperature is a measure of the annual recharge-weighted soil temperature which

156

DRAKE

determines the partial pressure of CO2 in the soil air through the biologic response of soil respiration and organic matter decomposition processes. For the mean annual recharge temperature at Schefferville, the predicted soil air and hence groundwater initial PCO2 is 10-l" 84 atm. Equilibrium groundwater conditions depend on the evolution path that is followed during the solution process. If carbonate materials are present in the surficial materials, solution will proceed under open system conditions in which dissolved CO2 is replenished from the soil air reservoir as it is combined into HCO-3:PCO2 remains constant throughout the process. ff carbonate materials are not present in the surficial materials, but are present in an aquifer at depth, then dissolved CO2 cannot be replenished as it is combined: PCO2 therefore decreases during evolution. The solid lines shown on Figure 3 represent the open system (vertical) and closed system (curve) evolution paths for an initial PCO2 of 10-]" 84 atm from a value of SId of- oo to saturation (SId = 0). The evolution curves are identical to the curves representing the mixing of various fractions of waters at their endpoints under the stated conditions: thus the closed-system evolution curve also represents the mixing curve for a water at SId = - oo, PCO2 = 10-1'84 atm and one at the closed system equilibrium point for those initial conditions. In this case, however, position along the curve represents relative mixing volumes rather than the extent of progression towards equilibrium. Values of SId and PCO2 for sites 1-9 generally conform to the closed-system evolution or mixing curve shown in Figure 3. The fact that they all lie to its left is due to the degassing of CO2 to the atmosphere during turbulent flow in the partly-full, sub-horizontal outflow pipes. The observed patterns of Ca 2+, Mg 2÷ and HCO-3 concentrations and ofpH can therefore be explained by the mixing of water with a PCO2 of 10-1"84atm that has not been in contact with dolomite with water that has equilibrated with dolomite under closed system conditions. The two waters are mixed in different proportions at each site. The ridge tops were sites of glacial erosion, and the Mini Humo-Ferric Podzols and Degraded Dystric Brunisols subsequently developed are carbonate-free (NICHOLSON & MOORE 1977). Closed system evolution of waters sampled at sites on the ridges is therefore to be expected. The values of SId and PCO2 for site 10 are close to the open system curve in Figure 3. This site is from low on the flank of a ridge in an area where dolomite outcrops. The valleys in the area are sites of glacial deposition, and locally the till and soils contain considerable quantities of carbonate material. Local recharge therefore equilibrates with dolomite under open system conditions. Degassing does not occur at this site because samples were taken immediately from a tap fed from the well with no free air space in the piping. The single well and lithology at this site are reflected in the small standard deviation of the data.

3.4.

REDOX POTENTIAL AND Fe CONCENTRATION

The Eh values obtained from measurements have been conveted to the electron activity, pE, by the relationship pE -

F Eh 2.3RT

where F is the faraday, Rthe gas constant and T the absolute temperature of the sample. This quantity is analogous to pH and provides a simple, dimensionless description of redox state.

GROUNDWATER CHEMISTRY: QUEBEC IRON DEPOSITS

157

Values ofpE show considerable variability, presumably because of the aeration in the outfall pipes. Values shown in Table 1 for total Fe essentially represent Fe 2÷because filtration did not reduce the concentration and Fe 3÷ was below the lower limit of the method used (0.05 mg/-1) which is less than 10% of the values reported for total Fe. No definitive modelling similar to that used above for the carbonate system is possible because of the uncertainty of the pE values and the unknown Fe speciation. Assuming that the Fe is derived from the iron minerals of the Sokoman and Ruth Formation, however, values of pQ, defined as pQ = . log([Fe3+] [OH-]3) and discussed in detail by WHITIEMORE & LANGMUIR (1975), range from 30 to 35, suggesting that the measured pE is considerably higher than that to be expected if it were controlled by any Fe(II) - Fe(III) couple associated with the minerals present. The aeration of the samples in the outfaU pipes leads to a rapid re-oxygenation of the groundwater, but the absence of any particulate Fe(III) oxyhydroxides indicates that the precipitation reaction is slow. After standing open to the air for some hours, a colloidal precipitate of aorphous Fe(OH)3 formed from most samples, a phenomenon which is also found in samples drawn from anoxic hypolimnion lake waters in the area. The total Fe concentrations observed in the groundwater samples would be in equilibrium with amorphous Fe(OH)3 at the observed pH values at a p E of approximately 3.75. The partial pressure of oxygen in equilibrium with water with pH - 6 and pE - 3.75 is of the order of 10-44 atm. The discharge of this essentially anoxic groundwater to lakes in the vale may contribute to their winter de-oxygenation (D .RAKE & FREUND 1980).

4. CONCLUSIONS

The data considered above represent summer conditions, but in no case was any constituent of a winter sample where these were taken significantly different from the mean shown in Table 1. Winter conditions cannot be confidently determined because of the paucity of the winter data, but it is likely that the values shown approximate annual average aquifer conditions in the general mine area. The quality of groundwater pumped from sedimentary rocks in the Schefferville area of the Labrador Trough is controlled by several factors. Some dissolved ions, notably Na ÷ and CI-, are derived from precipitation, some (K÷) from the clay minerals in the area and some (Ca2÷, Mg 2÷, HCO-3, Fe, SiO2) from the major dolomite, silicate or iron-silicate rock formations. Ions in water derived from the dolomite occur in concentrations predicted by a general model based on soil-air CO2 behaviour. In each outfall the Ca, Mg and HCO-3 concentrations are governed by the mixing of dolomite and non-dolomite waters. Definitive statements on the control of iron species concentrations cannot be made with any certainty because of doubt about the meaning ofpE values as measured in the field. Total iron concentrations are generally similar to those in surface waters in the area reported by GROSS (1968, Table II), but represent dissolved species rather than particulates as is the case for the surface waters. Re-aeration of the almost anoxic ground waters causes ferric oxy-hydroxides to precipitate, but in sufficiently small concentrations and sufficiently small particles that they do not settle out of the streamwaters. The discharge of groundwater into lakes may contribute to their winter de-oxygenation. Groundwater temperatures in the mine areas are higher than

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DRAKE

rock temperatures in other areas where pumping is absent, and the induced flow in areas of heavy pumping may induce thawing and influence permafrost distribution.

ACKNOWLEDGEMENTS Funding provided by the Natural Science and Engineering Research Council Canada and Ministre de I'Education du Qu6bec (FCAC program). Field and laboratory assistance by P. Hess and G. Hess. Site access courtesy of Iron Ore Company of Canada. Facilities: McGill Subarctic Research Station, Schefferville (then Director, F.H. Nicholson). REFERENCES BROWN, E., SKOUGSTAD, M.W. & FISHMAN, M.J. (1970): Methods for collection and analysis of water samples for dissolved minerals and gasses. Chapter A1, Book 5 in Techniques of WaterResources Investigations, 169 p. United States Geological Survey, Washington, D.C. DRAKE, J.J. (1980): The effect of soil activity on the chemistry of carbonate groundwaters. Water Resources Research 16(2), 381-386. DRAKE, J.J. & FREUND, I.J. (1980): The fate of sewage phosphorus input to a subarctic lake chain. Water, Air and Soil Pollution 14, 331-337. GARG, O. & STACEY, P. (1973): Techniques used in the delineation of permafrost in the Schefferville, P.Q. area. Proceedings, Seminar on the Thermal Regime and Measurements in Permafrost. National Research Council Canada, Technical Memorandum 108, 76-83. GROSS, G.A. (1968): Geology of Iron Deposits in Canada, Vol. III - Iron ranges of the Labrador Geosyncline. Economic Geology Report 22, 179 p. Geological Survey of Canada, Ottawa. HARRISON, J.M., HOWELL, J.E. & FAHRIG, W.F. (1972): Ageological cross-section ofthe Labrador Miogeosyncline near Schefferville, Quebec. Paper 70-37, 34 p. Geological Survey of Canada, Ottawa. LEWIS, J.S. (1977): Active layer depths and supra-permafrost groundwater in a small subarctic catchment, Schefferville, Quebec. Unpubl. M.Sc. thesis, McGill University, Montreal. MOORE, T.R. (1978): Soil development in Arctic and Subarctic areas of Quebec and Baffin Island. Quaternary Soils, Proceedings, 3rd York Quaternary Symposium, 379-411. Geo Abstracts, Norwich, England. NICHOLSON, F.H. (1978): Permafrost distribution and characteristics near Schefferville, Quebec: recent studies. Proceedings, 3rd International Permafrost Conference, 427-433. National Research Council of Canada, Ottawa. NICHOLSON, F.H. & GRANBERG, H.B. (1973): Permafrost and snowcover relationships near Scheffeville. Permafrost: the North American contribution to the 2nd International Conference, 151158. National Academy of Sciences, Washington, D.C. NICHOLSON, H.M. & MOORE, T.R. (1977): Pedogenesis in a subarctic ironrich environment: Schefferville, Quebec. Canadian Journal of Soil Science 57, 35-45. SEGUIN, M.K. (1974): Etat de recherches sur le perg61isoldans la partie centrale de la Fosse du Labrador, Qu6bec subarctique. Revue de g6ographie de Montr6.a128, 343-356. STUBBINS, J.B. & MUNRO, P. (1965): Open pit mine de-watering - Knob Lake. Canadian Institute of Mining, Bulletin 640, 229-237. WHITIEMORE, D.O. & LANGMUIR, D. (1975): The solubility of ferric oxyhydroxides in natural waters. Ground Water 13(14), 360-365. WlGLEY, T.M.L. (1977): WATSPEC: a computer program for determining the equilibrium speciation of aqueous solutions. British Geomorphological Research Group, Technical Bulletin 20, 48 p. Geo Abstracts, Norwich, England.

Address of author: John J. Drake, Department of Geography, McMaster University Hamilton, Ontario, L8S 4KI, Canada