M. Jim Hendry

Distribution of arsenic(III), arsenic(V) and total inorganic arsenic in porewaters from a thick till and clay-rich aquitard sequence, Saskatchewan, Canada

Abstract Inorganic arsenic species were measured in the porewaters collected from eighteen piezometers installed between 3 and 91.4 m below ground in a thick till and clay-rich aquitard sequence located in southern Saskatchewan to investigate the distribution of, and controls on arsenic speciation in the sequence. Aqueous concentrations of As(V), As(III) and total As are in the range of 0.31-97, 0.71-21 and 3.2-98 ppb, respectively. Profiles of As(III) and As(V) concentration distribution with depth broadly track that of total As: erratic increases to 15.2 m, then more uniform concentrations to 88 m. Aqueous arsenic is accumulated at the upper redox transition zone (6–14 m). The alkaline porewater at 91.4 m contains the highest concentrations of As(V) and total As, which might result from the facilitated desorption of arsenate from the host solid due to decrease of positive surface charge of the oxides in alkaline solution. The ratio of As(V)/As(III) is greater than unity in the uppermost oxidized porewater (3 m), less than unity from 4.6 to 71.6 m, and greater than unity in the lowest four porewaters (76.2 to 91.4 m). In the 3 m porewater low As(III) but high As(V)/As(III) is due to the oxidized nature of the near surface weathered till. The high As(V)/As(III) in the deepest porewater at 91.4 m likely results from the enhanced and heterogeneous oxidation of As(III) to As(V) on clay mineral surfaces in the alkaline solution. Total As and arsenic speciation may not be controlled by As, Fe or Mn concentrations in the host till or clay. Dissolved As(V) and total As positively covary with aqueous chloride, whereas dissolved As(III) is independent of aqueous chloride. Aqueous As(III), and to a less extent As(V) and total As are positively correlated with dissolved Mn in the till. In the clay, aqueous As(V) and total As show strong negative covariation with Mn. However, aqueous As(III), As(V) and total As exhibit almost no correlation with total dissolved Fe in the till. The As(V)/As(III) ratio has strong negative correlation with dissolved Mn, but positive covaration with dissolved chloride. Generally good agreement between the redox potentials (Eh) calculated from aqueous As(V) and As(III) concentrations and those measured by a Pt electrode throughout most of the unoxidized till suggests the suitability of using As(V)-As(III) redox couple as a redox indicator for the studied aquitard system. However, large negative bias of the calculated Eh from the measured Eh in the oxidized till/upper unoxidized till and the clay is attributed to errors associated with the field measurements of Eh.

Transformation of Two-Line Ferrihydrite to Goethite and Hematite as a Function of pH and Temperature

Under oxic aqueous conditions, two-line ferrihydrite gradually transforms to more thermodynamically stable and more crystalline phases, such as goethite and hematite. This temperature- and pH-dependent transformation can play an important role in the sequestration of metals and metalloids adsorbed onto ferrihydrite. A comprehensive assessment of the crystallization of two-line ferrihydrite with respect to temperature (25, 50, 75, and 100 °C) and pH (2, 7, and 10) as a function of reaction time (minutes to months) was conducted via batch experiments. Pure and transformed phases were characterized by X-ray diffraction (XRD), X-ray absorption near-edge spectroscopy (XANES), atomic force microscopy (AFM), and scanning electron microscopy (SEM). The rate of transformation of two-line ferrihydrite to hematite increased with increasing temperature at all pHs studied and followed first-order reaction kinetics. XRD and XANES showed simultaneous formation of goethite and hematite at 50 and 75 °C at pH 10, with hematite being the dominant product at all pHs and temperatures. With extended reaction time, hematite increased while goethite decreased, and goethite reaches a minimum after 7 days. Observations suggest two-line ferrihydrite transforms to hematite via a two-stage crystallization process, with goethite being intermediary. The findings of this study can be used to estimate rates of crystallization of pure two-line ferrihydrite over the broad range of temperatures and pH found in nature.

Rare earth element geochemistry of groundwaters from a thick till and clay-rich aquitard sequence, Saskatchewan, Canada

Rare earth elements (REE) were determined in groundwater samples collected from a thick till and clay-rich aquitard sequence located in southern Saskatchewan, Canada. The groundwaters are Na-Mg-SO4-type waters that range from highly concentrated brines (e.g., I up to 2 moles/kg) near the ground surface to relatively dilute waters (I ≈ 0.04 moles/kg) at depth. The majority of these groundwaters have pH values between 7 and 8, although the deepest samples are more alkaline (9 ≤ pH ≤ 9.6). Groundwater REE concentrations are relatively constant in the overlying till but increase by up to a factor of 50 in the underlying clay bedrock. Shallow groundwaters have heavy REE (HREE)—enriched shale-normalized patterns, whereas the REE patterns of the deep groundwaters are relatively flat. Solution complexation modelling indicates that variations in REE patterns reflect differences in solution complexation across the REE suite. In the shallow groundwaters, strongly adsorbed, positively charged carbonate complexes (LnCO3+), sulfate complexes (LnSO4+), and free metal ion species (Ln3+) dominate the speciation of light REEs (LREE), whereas HREEs occur chiefly as more stable, negatively charged dicarbonato complexes [i.e., Ln(CO3)2−)]. For the deepest groundwaters, however, all of the REEs are predicted to occur in solution as dicarbonato complexes. The large HREE enrichments of the shallow groundwaters reflect the greater affinity of the positively charged LREE solution species to adsorb to clay minerals or coatings on clay minerals in the aquitard sequence compared to the more stable, negatively charged HREE dicarbonato complexes. On the other hand, the flat REE patterns of the deep groundwaters reflect the dominance of the negatively charged dicarbonato complex for all REEs. The solution complexation model along with the strong positive correlation between REEs and [CO32−]F(0.79 ≤ r ≤ 0.95), and to a lesser extent pH (0.57 ≤ r ≤ 0.72), indicates that carbonate ion concentrations, and thus pH, exert important controls on aqueous REE concentrations in these groundwaters.

Flow injection on-line group preconcentration and separation of (ultra)trace rare earth elements in environmental and geological samples by precipitation using a knotted reactor as a filterless collector for inductively coupled plasma mass spectrometric determination

A flow injection (FI) on-line filterless precipitation-dissolution system was developed for the ICP-MS determination of (ultra)trace REE in environmental and geological samples. A knotted reactor (KR), laboratory-made from PTFE tubing (200 cm×0.5 mm id), was used as a filterless collector. On-line precipitation of REE was achieved by mixing the sample with an ammonia buffer solution. The resulting precipitates were collected on the inner walls of the KR without filtration. A flow of 1 mol l –1 nitric acid was introduced to dissolve the collected precipitates and to transport the analyte to the ICP-MS system. Group REE preconcentration was achieved with separation from alkali and alkaline earth elements at pH 8.3-9.0. The method was demonstrated on-line with a Perkin-Elmer FIAS-400 FI system without modification. Using a preconcentration time of 120 s and a sample flow rate of 5 ml min –1 , a single preconcentration and separation with an enhancement factor of 55-75 were achieved within 5 min. The detection limits (3s) ranged from 0.06 to 0.27 ng l –1 . The precision for 16 replicate determinations of 0.1 µg l –1 of individual REE spiked in a porewater sample was in the range 1.8-4.2% (RSD). The accuracy of the method was demonstrated by analyzing a number of geological and environmental standard reference materials. The method was also applied to determination of ultratrace REE in porewater samples.

Effects of Adsorbed Arsenate on the Rate of Transformation of 2-Line Ferrihydrite at pH 10

2-Line ferrihydrite, a form of iron in uranium mine tailings, is a dominant adsorbent for elements of concern (EOC), such as arsenic. As ferrihydrite is unstable under oxic conditions and can undergo dissolution and subsequent transformation to hematite and goethite over time, the impact of transformation on the long-term stability of EOC within tailings is of importance from an environmental standpoint. Here, studies were undertaken to assess the rate of 2-line ferrihydrite transformation at varying As/Fe ratios (0.500-0.010) to simulate tailings conditions at the Deilmann Tailings Management Facility of Cameco Corporation, Canada. Kinetics were evaluated under relevant physical (~1 °C) and chemical conditions (pH ~10). As the As/Fe ratio increased from 0.010 to 0.018, the rate of ferrihydrite transformation decreased by 2 orders of magnitude. No transformation of ferrihydrite was observed at higher As/Fe ratios (0.050, 0.100, and 0.500). Arsenic was found to retard ferrihydrite dissolution and transformation as well as goethite formation.

Molybdenum Speciation in Uranium Mine Tailings Using X-Ray Absorption Spectroscopy

Uranium (U) mill tailings in northern Saskatchewan, Canada, contain elevated concentrations of molybdenum (Mo). The potential for long-term (>10,000 years) mobilization of Mo from the tailings management facilities to regional groundwater systems is an environmental concern. To assist in characterizing long-term stability, X-ray absorption spectroscopy was used to define the chemical (redox and molecular) speciation of Mo in tailings samples from the Deilmann Tailings Management Facility (DTMF) at the Key Lake operations of Cameco Corporation. Comparison of Mo K near-edge X-ray absorption spectra of tailings samples and reference compounds of known oxidation states indicates Mo exists mainly as molybdate (+6 oxidation state). Principal component analysis of tailings samples spectra followed by linear combination fitting using spectra of reference compounds indicates that various proportions of NiMoO(4) and CaMoO(4) complexes, as well as molybdate adsorbed onto ferrihydrite, are the Mo species present in the U mine tailings. Tailings samples with low Fe/Mo (<708) and high Ni/Mo (>113) molar ratios are dominated by NiMoO(4), whereas those with high Fe/Mo (>708) and low Ni/Mo (<113) molar ratios are dominated by molybdate adsorbed onto ferrihydrite. This suggests that the speciation of Mo in the tailings is dependent in part on the chemistry of the original ore.

Geochemical and transport properties of dissolved organic carbon in a clay‐rich aquitard

[1] The properties and controls on the diffusive transport of dissolved organic carbon (DOC) in a thick clay-rich till aquitard were investigated. DOC was measured in 14 piezometers ranging in depth from 1.2 to 43 m belowground (BG). The DOC data showed a decrease in concentration with depth from 168 mg/L in the surficial, fractured, and oxidized zone (1.2 m BG) through the thick underlying unoxidized and nonfractured aquitard, reaching minimum values of between 12 and 16 mg/L below 15 m. Flow field-field flow fractionation analyses showed that the DOC was uniform with respect to hydrodynamic size (2.5 nm), molecular weight (approximately 900 Da), and aqueous diffusion coefficient (1.9 x 10 - 1 0 m 2 /s). Specific UV aborbance (UV/TOC) varied with depth but was significantly lower than surface water DOC. Results of batch experiments showed that DOC exhibited negligible sorption (i.e., k d = 1.1 x 10 - 3 mL/g) to the aquitard matrix. Double-reservoir diffusion tests showed that DOC diffuses through this clay-rich media. Results of best fit numerical modeling of the diffusion cell data yielded an effective diffusion coefficient of 9 x 10 - 1 1 m 2 /s for the DOC and suggested that the effective porosity for the DOC may be approximated by the total porosity. Pore aperture measurements on core samples and the results of the diffusion cell experiments revealed that straining of DOC by the aquitard matrix does not occur. Overall, our results suggested that DOC can diffuse through clay-rich aquitards in a similar manner to conservative inorganic solutes.

Sequential leachates of multiple grain size fractions from a clay-rich till, Saskatchewan, Canada: implications for controls on the rare earth element geochemistry of porewaters in an aquitard

Porewater REE concentrations vary by an order of magnitude over 45 vertical m in a thick clay-rich till aquitard in southern Saskatchewan. To address controls on aqueous REE, the till was disaggregated into seven size fractions (>850, 425–850, 295–425, 180–295, 150–180, 75–150, and <75 μm). Each fraction was sequentially leached with water (L1); 1 M NaOAc (pH 5) (L2); 0.25 M NH2OH·HCl (L3); 1 M NH2OH·HCl (L4); 12 M HCl+KClO3+4 M HNO3 (L5); and the residue digested in Na2O2 (L6). Aqueous leachate of the <75 μm size fraction has a near flat REE pattern at ∼0.1 PAAS and carries 99% of REE, relative to the total from all L1 leached size fractions. Aqueous leachates of the coarser size fractions have flat patterns 1 to 2 orders of magnitude lower. Leachate L2 extracts metals held electrostatically on inorganic or organic material: the REE also plot at ∼0.1 PAAS with a convex up pattern and a maximum at Gd. Leachate L3 of amorphous Fe- and Mn-oxyhydroxides carries more REE than L1, L2 or L4, and like L2, has a mildly convex up pattern. There are lower REE abundances associated with crystalline Fe- and Mn-oxides (L4) than with amorphous counterparts, and HREE are fractionated and depleted. Leaching of organic matter (L5) produces REE patterns comparable to those of L4. The residue (L6) has convex down REE patterns at 0.1 to 0.4 PAAS with prominent positive Eu anomalies from plagioclase feldspar, excepting the <75 μm size fraction. Natural porewaters have mildly fractionated patterns with LREE and MREE depletion versus convex up patterns for leachates (L1, L2 and L3). LREE and MREE may readily sorb onto clays, or clay coatings, in the till, and be desorbed during leaching.

Hydrogeology and hydrochemistry of the Milk river aquifer system, Alberta, Canada : a review

Abstract The Milk River aquifer system consists of 30–60 m of Cretaceous sandstone located within the Milk River Formation, southern Alberta, Canada. The Milk River Formation is confined below by > 500m of shale of the Colorado Group and above by up to 120 m of shale the Pakowki Formation. The dominant recharge area for the aquifer is the Sweetgrass Hills, Montana, where the aquifer crops out. From the recharge area, the groundwater flows to the north, east and west. Calculated groundwater residence times at the north end of the aquifer (about 100 km north of the recharge area) range from 250 to 510 ka. Limited hydrological data from the confining shales suggest that cross-formational flow does not occur. Systematic patterns are observed in major ions (Na, Cl, HCO 3 + CO 3 , and SO 4 ), stable isotopes ( 18 O and deuterium), and field pH on a regional scale. Several mechanisms have been proposed to explain the geochemical evolution of the groundwaters.

Inferring Heterogeneity in Aquitards Using High-Resolution δD and δ18O Profiles

Vertical depth profiles of pore water isotopes (deltaD and delta18O) in clay-rich aquitards have been used to show that solute transport is dominated by molecular diffusion, to define the timing of geologic events, and to estimate vertical hydraulic conductivity. The interpretation of the isotopic profiles in these studies was based on pore water samples collected from piezometers installed in nests (typically 4 to 15 piezometers) over depths of 10 to 80 m. Data from piezometer nests generally have poor vertical resolution (meters), raising questions about their capacity to reveal the impact of finer scale heterogeneities such as permeable sand bodies or fractured till zones on solute transport. Here, we used high-resolution (30-cm) depth profiles of deltaD and delta18O from two continuously cored boreholes in a till aquitard to provide new insights into the effects of sand bodies on solute transport. High-resolution core-derived profiles indicate that such heterogeneities can cause major deviations from one-dimensional diffusion profiles. Further, comparison of piezometer-measured values with best-fit diffusion trends shows subtle deviations, suggesting the presence of heterogeneities that should not be ignored. High-resolution profiles also more clearly defined the contact between the highly fractured oxidized zone and the underlying unoxidized zone than the piezometers.