Shaun K. Frape

Water-rock interaction and chemistry of groundwaters from the Canadian Shield

The chemical and isotopic compositions of groundwaters in the crystalline rocks of the Canadian Shield reflect different degrees of rock-water interactions. The chemistry of the shallow, geochemically immature groundwaters and especially of the major cations is controlled by local rock compositions, whereby dissolution reactions dominate. Conservative constituents, such as chloride and bromide, however, are not entirely a result of such reactions but appear to be readily added from leachable salts during the initial stages of the geochemical evolution of these waters. Their concentration changes little as major cations increase, until concentrations of Total Dissolved Solids (TDS) reach 3000 to 5000 mg 1−1. The isotopic composition of these shallow waters reflects local, present day precipitations. In contrast to the shallow groundwaters, the isotopic and chemical compositions of the deep, saline waters and brines are determined by extensive, low-temperature rock-water interactions. This is documented in major ion chemistries, 18O contents and strontium isotopic compositions. These data indicate that the deep brines have been contained in hydrologically isolated “pockets”. The almost total loss of primary compositions make discussions on the origin of these brines very speculative. However, all brines from across the Canadian Shield have a very similar chemical composition, which probably reflects a common geochemical history. The concentrations of some major and most minor elements in these fluids appear to be governed by reactions with secondary mineral assemblages.

Saline groundwaters in the Canadian Shield — A first overview

Abstract The chemical and isotopic compositions of saline groundwaters and brines in the Canadian Shield are described. The occurrence of such waters is very widespread, not related to any specific rock type or mineralization and, generally, can be expected at depths below 1 km. The most saline brines have salinities exceeding 200 g l−1. Dissolved solids and their chemistries tend to suggest that, in the past, saline waters penetrated the Canadian Shield. The original chemistry of these solutions was subsequently modified through strong rock-water interactions, and it appears that their final chemical compositions are independent of the original chemistries. Stable-isotope analyses indicate that these waters are not simple evaporated seawater, sedimentary-basin brines, hydrothermal solutions, or residual ore-forming fluids, although any of these could be a precursor of the brines. Secondary isotope-exchange reactions, and the hydration of primary silicates, accompanied by the formation of clay minerals, could have modified not only the primary isotopic composition but may have also profoundly influenced the chemistries of these brines. To be effective these processes would have had to occur in closed systems.

Stable chlorine and carbon isotope measurements of selected chlorinated organic solvents

Abstract Stable Cl and C isotope ratio results for 3 selected chlorinated solvents, perchloroethylene (PCE), trichloroethylene (TCE) and 1,1,1-trichloroethane (TCA) provided by 4 different manufacturers are presented. The isotope ratio for all compounds range between −3.5 and +6.0‰ for δ 37 Cl and from −37.2 to −23.3%. for δ 13 C . The greatest 37 Cl difference between manufacturers is observed in the TCE samples which show δ 37 CI values of −2.5%o for PPG, +2.43‰ for ICI and +4.4‰ for DOW. TCAs show a smaller range (−2.4 to +2.0‰), while the TCEs have slightly different 37 Cl contents. The 13 C data show the most distinct δ 13 C values for PCEs (−23.3 for DOW, −24.1 for Vulcan, −33.8 for PPG and −37.2‰ for ICI) while both TCEs and TCAs show a smaller δ 13 C range, but still distinct differences. These preliminary data suggest that each manufacturer and solvent type may have distinctive δ 637 Cl and δ 13 C values. These results show that by using a combination of 37 Cl and 13 C, there is a potential to indicate a specific source of chlorinated solvents, as well as an ability to delineate contamination episodes caused by these compounds in groundwaters.

The 87Sr86Sr values of Canadian Shield brines and fracture minerals with applications to groundwater mixing, fracture history, and geochronology

Analyses of saline waters, fracture minerals, and host rocks from seven localities on the Canadian Shield demonstrate the utility of the 87Sr86Sr ratio in the study of groundwater systems in crystalline rocks. The ratios range from 0.704 to 0.753 and have obtained their signatures by mineral/rock interactions, primarily involving the feldspars. We have identified brines from isolated pockets in the same mines where extensive flow regimes exist. There is mixing of different brines as well as mixing with meteoric waters. The isotopic results on calcites from fractures and shear zones show more than one generation of mineral growth in a given fracture. The 87Sr86Sr ratios of the calcites vary from values identical to the present-day brine in the fracture zone to ratios with Archean signatures. This implies that activity may occur in fault zones over a very long time. The brines are very rich in Sr (up to 2400 mg/1), very low in Rb, and have relatively radiogenic 87Sr86Sr ratios. They are ubiquitous in Shield rocks and, if they were present throughout geological time, they may be one reason why RbSr ages of felsic plutons are commonly younger than associated UPb ages.

A comparison of groundwater dating with 81Kr, 36Cl and 4He in four wells of the Great Artesian Basin, Australia

The isotopic ratios 81 Kr/Kr and 36 Cl/Cl and the 4 He concentrations measured in groundwater from four artesian wells in the western part of the Great Artesian Basin (GAB) in Australia are discussed. Based on radioactive decay along a water flow path the 81 Kr/Kr ratios are directly converted to groundwater residence times. Results are in a range of 225^400 kyr with error bars in the order of 15% primarily due to counting statistics in the cyclotron accelerator mass spectrometer measurement. Additional uncertainties from subsurface production and/or exchange with stagnant porewaters in the confining shales appear to be of the same order of magnitude. These 81 Kr ages are then used to calibrate the 36 Cl and the 4 He dating methods. Based on elemental analyses of rock samples from the sandstone aquifer as well as from the confining Bulldog shale the in situ flux of thermal neutrons and the corresponding 3 He/ 4 He and 36 Cl/Cl ratios are calculated. From a comparison of: (i) the 3 He/ 4 He ratios measured in the groundwater samples with the calculated in situ ratios in rocks and (ii) the measured N 37 Cl ratios with the 4 He concentrations measured in groundwater it is concluded that both helium and chloride are most likely added to the aquifer from sources in the stagnant porewaters of the confining shale by diffusion and/or mixing. Based on this ‘working hypothesis’ the 36 Cl transport equation in groundwater is solved taking into account: (i) radioactive decay, (ii) subsurface production in the sandstone aquifer (with an in situ 36 Cl/Cl ratio of 6U10 315 ) and (iii) addition of chloride from a source in the confining shale (with a 36 Cl/Cl ratio of 13U10 315 ). Lacking better information it is assumed that the chloride concentration increased linearly with time from an (unknown) initial value Ci to its

Stable hydrogen, carbon and chlorine isotope measurements of selected chlorinated organic solvents

Stable hydrogen isotopes of two chlorinated solvents, trichloroethylene (TCE) and 1,1,1-trichloroethane (TCA), provided by five different manufacturers, were determined and compared to their carbon and chlorine isotopic signatures. The isotope ratio for delta2H of different TCEs ranged between +466.9 per thousand and +681.9 per thousand, for delta13C between -31.57 per thousand and -27.37 per thousand, and for delta37Cl between -3.19 per thousand and +3.90 per thousand. In the case of the TCAs, the isotope ratio for delta2H ranged between -23.1 per thousand and +15.1 per thousand, for delta13C between -27.39 per thousand and -25.84 per thousand, and for delta37Cl between -3.54 per thousand and +1.39 per thousand. As well, a column experiment was carried out to dechlorinate tetrachloroethylene (PCE) to TCE using iron. The dechlorination products have completely different hydrogen isotope ratios than the manufactured TCEs. Compared to the positive values of delta2H in manufactured TCEs (between +466.9 per thousand and +681.9 per thousand), the dechlorinated products had a very depleted delta2H (less than -300 per thousand). This finding has strong implications for distinguishing dechlorination products (PCE to TCE) from manufactured TCE. In addition, the results of this study show the potential of combining 2H/1H analyses with 13C/12C and 37Cl/35Cl for isotopic fingerprinting applications in organic contaminant hydrogeology.

The isotope geochemistry of carbon in groundwater at Stripa

The carbon isotopic composition of the total dissolved inorganic carbon in groundwater associated with a granitic pluton at Stripa (Sweden) reflects both inorganic and organic carbon sources. Following the uptake of soil carbon-dioxide, calcite dissolution dominates the geochemical evolution of shallow groundwater. Calcite saturation is reached at a depth of about 100 m. In deeper waters geochemical release of Ca and increasing pH cause calcite precipitation. Radiocarbon contents suggest carbon (and water ?) ages in excess of 20 000 years for waters at 300–400 m depth. In deep groundwaters with enhanced salinities organic carbon is added to the dissolved inorganic carbon either through bacterial activity (e.g, sulphate reducing bacteria) or the oxidation of organic compounds such as methane. The lowest radiocarbon contents were measured at the 300–400 meter levels and not in the deepest fluids. The distribution of 13C in the deep groundwaters suggests the existence of well-defined flowsystems with limited active hydraulic interaction. Isotope analyses on fracture calcites substantiate the complex geochemical history of the pluton.

A strontium, oxygen and hydrogen isotopic composition of brines, Michigan and appalachian basins, Ontario and Michigan

Abstract 87 Sr/ 86 Sr ratios of brine from samples from the Michigan and Appalachian Basins, in Ontario and Michigan, covering the stratigraphic interval from the Cambrian to Mississippian, vary from 0.708 to 0.711. With the exception of the salt units of the Salina Formation (Silurian), most values are greater than seawater for the time in question, indicating water-rock interaction. The sources of the radiogenic Sr has not been identified. All samples plot below the GMWL in δ 18 O−δ 2 H space, with the Cambrian and Ordovician samples closest to the line. Mixing of brines meteoric and glacial (Pleistocene) water is indicated in some cases. The more concentrated brines from each stratigraphic unit show a very narrow spread in values. All the Ordovician brines show a narrow range over a 200 km area for Sr, O and H isotopes, indicating extensive lateral migration of the fluids. Strontium in the brine has not equilibrated isotopically with its host rock. In some cases the late-stage minerals saddle dolomite, calcite and anhydrite have the same 87 Sr/ 86 Sr ratios as the brine, indicating that they precipitated from the brine in isotopic equilibrium.

Strontium isotopic composition of some brines from the Precambrian Shield of Canada

Abstract Twenty-four groundwater samples from seven operating mines at Sudbury, Yellow-knife and Thompson (Ontario, North West Territories and Manitoba, resp.), all from depths greater than 1 km and ranging in total dissolved solids (TDS) from 1900 to 250,000 mg l−1, were measured for their 87 Sr 86 Ar values. Each geographic location gives a limited range in values and each location is distinct from the others. This is interpreted as the result of extensive water-rock interaction on a local scale. For most of the time, these brines were isolated and only recently have been exposed to surface water as a result of the mining operations. The extent of the isolation is shown by the contrasting isotopic values of two “pockets” of water (0.711 vs. 0.716) located on opposite sides of the same fault system on the North Range at Sudbury. The exchange at all sites probably has continued until the present, as indicated by the close agreement between water and present-day 87 Sr 86 Sr whole-rock values. If so, it suggests that there is no single age for such brines, but it may be possible to date stages in the waters evolution by determining the age of secondary minerals that equilibrated with the water.

Methane occurrences in the Canadian Shield

Methane is ubiquitous and discharging freely from numerous exploration boreholes in the Canadian Shield. Methane concentrations in the free gas phase vary from a few % to over 80%. The gas occurs in association with He (up to 20%), Ar (up to 4%) and H2 (up to 30%), as well as with N2 (up to 80%) and minor amounts of noble gases and higher hydrocarbons. The δ13C-values of the methane vary between ∼ −56 and ∼ −25‰ PDB, whereas deuterium contents range from δD = −130‰ to values as low as −450‰ No equilibration with associated fluids (usually CaNaCl brines) or known carbon compounds has occurred. Chemical and isotopic analyses tentatively indicate an abiogenic origin for these gases. In each mining district the observed isotopic compositions are unique, and tightly grouped, suggesting local gas production under specific environmental conditions. A positive correlation between δ13C and δD in each mining district is not readily explainable in terms of carbon sources and formation mechanisms, or in terms of migration and mixing of gases from more than one source. He isotope results typical of crustal He indicate that a mantle origin for methane is unlikely; but as yet there are no definitive indications of the role of secondary processes such as diffusion and bacterial oxidation in establishing the observed isotopic signatures. The occurrence of high levels of free hydrogen gas in certain mining districts indicates that hydrogen may be involved in kinetically controlled, inorganic processes of methane formation. Reactions involving light hydrogen typical of that generated in the Canadian Shield and in other ultramafic-mafic environments could explain the deuterium-depleted nature of the Canadian Shield methanes. However, no specific generating mechanism has yet been identified.