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Applied Geochemistry (v.27, #9)
The practicalities of using CFCs and SF6 for groundwater dating and tracing by W.G. Darling; D.C. Gooddy; A.M. MacDonald; B.L. Morris (pp. 1688-1697).
Knowledge of groundwater residence time is important in understanding key issues in the evolution of water quality, whether this occurs due to water–rock interaction or simply by mixing or contamination. The build-up in the atmosphere of the trace gases chlorofluorocarbons (CFCs) and sulphur hexafluoride (SF6) from the middle of the last century offers a convenient way of dating waters up to ∼60a old. The gases are well-mixed in the atmosphere so their input functions are not area-specific as is the case with3H. While any one of these trace gases can in principle provide a groundwater age, when two or more are measured on water samples the potential exists to distinguish between different modes of flow including piston flow, exponential flow and simple end-member mixing. As with all groundwater dating methods, caveats apply. Factors such as recharge temperature and elevation must be reasonably well-constrained. Primarily for SF6, the phenomenon of ‘excess air’ also requires consideration. Primarily for the CFCs, local sources of contamination need to be considered, as do redox conditions. For both SF6 and the CFCs, the nature and thickness of the unsaturated zone need to be factored into residence time calculations. However, as an inexpensive dating method, the trace gases can be applied to a wide range of groundwater problems where traditional age indicators might once have been used more sparingly. Examples include tracing flowlines, detecting small modern inputs in ‘old’ waters, and pollution risk assessment. In the future, with the main CFCs already declining in the atmosphere, new anthropogenic trace gases are likely to take their place.
Constraining groundwater flow, residence times, inter-aquifer mixing, and aquifer properties using environmental isotopes in the southeast Murray Basin, Australia by Ian Cartwright; Tamie R. Weaver; Dioni I. Cendón; L. Keith Fifield; Sarah O. Tweed; Ben Petrides; Ian Swane (pp. 1698-1709).
► Environmental isotopes constrain groundwater flow, mixing and recharge in Riverine Province of Murray Basin. ► Groundwater residence times mainly <30ka. ► Groundwater flow paths complex outside palaeovalleys. ► Groundwater does not preserve record of climate variation.Environmental isotopes (particularly δ18O, δ2H, and δ13C values,87Sr/86Sr ratios, and a14C) constrain geochemical processes, recharge distribution and rates, and inter-aquifer mixing in the Riverine Province of the southern Murray Basin. Due to methanogenesis and the variable δ13C values of matrix calcite, δ13C values are highly variable and it is difficult to correct14C ages using δ13C values alone. In catchments where δ13C values,87Sr/86Sr ratios, and major ion geochemistry yield similar a14C corrections, ∼15% of the C is derived from the aquifer matrix in the silicate-dominated aquifers, and this value may be used to correct ages in other catchments. Most groundwater has a14C above background (∼2pMC) implying that residence times are <30ka. Catchments containing saline groundwater generally record older14C ages compared to catchments that contain lower salinity groundwater, which is consistent with evapotranspiration being the major hydrogeochemical process. However, some low salinity groundwater in the west of the Riverine Province has residence times of >30ka probably resulting from episodic recharge during infrequent high rainfall episodes. Mixing between shallower and deeper groundwater results in14C ages being poorly correlated with distance from the basin margins in many catchments; however, groundwater flow in palaeovalleys where the deeper Calivil–Renmark Formation is coarser grained and has high hydraulic conductivities is considerably more simple with little inter-aquifer mixing. Despite the range of ages, δ18O and δ2H values of groundwater in the Riverine Province do not preserve a record of changing climate; this is probably due to the absence of extreme climatic variations, such as glaciations, and the fact that the area is not significantly impacted by monsoonal systems.
Stable carbon isotope behaviour of natural seepage of deep underground13C-rich methane detected along a fault zone and adsorbed in mudstone: Tokyo Bay area, Japan by E. Nakata; S. Tanaka; H. Suenaga; Y. Mahara; T. Nakamura; K. Nakagawa; T. Ohta (pp. 1710-1723).
Gas was sampled regionally, including by drilling into faults, in the South Kanto gas-field around Tokyo Bay, Japan. Gas samples were collected from cores in a gas sampling container immediately after drilling. A value of δ13C1=−44.3‰ was obtained for gas in the container and δ13C1=−36.3‰ for seeping gas in a fault zone. However, typical CH4 in this dissolved-in-water gas-field is mainly depleted in13C, and δ13C1 values range from −66‰ to −68‰ owing to microbial degradation of organic material.13C-rich CH4 is so far uncommon in the South Kanto gas-field. Seepages were observed from the surface along the north–south fault zone. The natural gas is stored below the sandstone layer by impermeable mudstone underlying the boundary at a depth of 30m. Gas seepages were not observed below a depth of 40m. Gas rises along the fault zone dissolved-in-groundwater up to the shallow region and then separates from the groundwater.13C-rich CH4 (adsorbed CH4) was found to have desorbed from drilled mudstone core samples taken at depths of 1400–1900m in the main gas-production strata. Similarly,13C-rich CH4 was found in black shale overlying the oceanic crust forming part of a sedimentary accretionary prism underling the Tokyo region. It also appears in the spring-water of spa wells, originating at a depth of 1200–1500m along a tectonic line. Methane generated by microbial degradation of organic material through CO2 reduction in the South Kanto gas-field mainly originates as biogenic gas mixed with a small amount of13C-rich CH4, derived from thermogenic gas without oil components in strata. It is assumed that12C-rich CH4 is easily detached from core or pore water through gas production, whereas13C-rich CH4 is strongly adsorbed on the surfaces of particles. The13C-rich CH4 rises along the major tectonic line or up the 50m wide normal fault zone from relatively deep sources in the Kanto region.
A new approach to multiphase geochemical speciation modeling by R.J. Perez; R.A. Heidemann; E.C. Perez (pp. 1724-1737).
Geochemical modeling based on equilibrium thermodynamics has become a recognized tool for understanding water–rock interaction processes and mechanisms. However, for the last 40years, little attention has been given to the interaction of water–rock–gas systems or water–rock–gas–petroleum systems, possibly due to the complexity of the equilibrium calculations. The program described here is particularly useful for modeling geochemical interactions in oil and gas reservoirs, where brines and minerals are just some of the phases present, together with gases, and petroleum liquids, or other liquid phases. It all seems that, through a global minimization procedure, the equilibrium state of all the components (or pseudo-components) present in a hydrocarbon charged reservoir can be easily calculated at any pressure and temperature, by combining classical fluid phase equilibrium concepts, electrolyte theory, and solid–liquid equilibrium theory.
Influence of pH and temperature on the early stage of mica alteration by K. Pachana; P. Zuddas; P. Censi (pp. 1738-1744).
Mineral dissolution and precipitation reactions actively participate in controlling fluid chemistry during water–rock interaction. In this study, the changes in the biotite and muscovite basal surface nano-morphology were evaluated during interaction with fluids of different pH (pH=1.1, 3.3 and 5.7) at different temperatures ( T=25°, 120°, and 200°C). Results show that at the nanometre scale resolution of the atomic force microscope (AFM), dissolution generates etch pits with a stair-shaped pattern over the (001) surface. The flux of dissolved elements decreases when pH increases. However, at pH 5.7, a change was found in the flux after 42h of reaction when abundant gibbsite and kaolinite coat the dissolving mineral surface. This phenomenon was widely observed at edges of the etch pits by AFM. It was also found that an increase in temperature produces an enhancement in the elemental flux in both micas. Dissolution regime changes after less than one day of interaction at high temperature because of abundant coating formation over the etch pits and edges. The results demonstrate the key role of nanometre size neogenic phases in the control of elemental flux from mica surfaces to solution. The formation of nanometre size coatings, blocking the sites active for dissolution, appears to control the alteration of phyllosilicates even at the early stage of the interaction.
Lake sediment cores as indicators of historical metal(loid) accumulation – A case study in Mexico by Anne M. Hansen (pp. 1745-1752).
To examine and compare historical accumulation of metal(loid)s in Mexican lakes and reservoirs,210Pb and137Cs dated sediment cores were evaluated: two from the remote Zempoala and Miramar Lagoons and three from Lake Pátzcuaro, and the Intermedia and Silva dams that are affected by human activities. Sediment ecotoxicology was assessed using consensus-based sediment quality guidelines for freshwater ecosystems. The+100a sediment core from the remote Miramar Lagoon had the highest concentrations of Cr and Ni these being higher than the Probable Effect Levels (PELs). Zinc concentrations were also higher in the Miramar Lagoon compared to the other lakes and reservoirs, with concentrations higher than the Threshold Effect Level (TEL). Mercury concentrations from this lagoon were comparable to those for the Intermedia dam that receives water from urban, industrial and agricultural areas. The higher metal concentrations in the core from the Miramar Lagoon suggest that metal concentrations in the rocks of the watershed are high. Another explanation for the higher metal concentrations is the slow sediment accumulation that causes metals to be accumulated over longer time-periods at the sediment–water interface. A decrease in the concentration of As in the Intermedia dam was observed in sediments corresponding to the last decades. This may be due to an increase in sediment accumulation rate or to the reduction in sources of this metalloid in the watershed. In the Miramar Lagoon, an increase was observed in concentrations of As and Cr in more recent sediments, probably related to increased deforestation in the area or the eruption of El Chichonal volcano in 1982. Concentrations of Pb showed a decreasing tendency over the past decades in the Lake Pátzcuaro, Miramar and Zempoala Lagoons sediment cores while such behavior was not be observed for the Intermedia dam. This reduction in concentrations of Pb was attributed to the decrease in use of leaded gasoline.
Formation of the hydrothermal system in Geysers Valley (Kronotsky Nature Reserve, Kamchatka) and triggers of the Giant Landslide by A.V. Kiryukhin; T.V. Rychkova; I.K. Dubrovskaya (pp. 1753-1766).
The Geysers Valley hydrothermal system is hosted within a system of two permeable faults (revealed by mapping thermal features), located above a suggested partially melted magmatic body and recharged by meteoric water along the outcrops of rhyolite–dacite extrusions. Fast erosion is stimulating the significant discharge rate, the geyser’s cycling mode and landslide events. Natural state thermal hydrodynamic modeling shows that 20–30ka of high temperature upflow of 250kg/s and an enthalpy of 900kJ/kg could build up the hydrothermal system in the Geysers Valley basin with output discharge parameters comparable to those at the current level. Modeling also shows that steam accumulation below an inclined caprock may have hydrothermal eruption potential. The Giant Landslide took place on June 3, 2007, when 20×106m3 of rocks were shifted 2km downstream, more than 23 geysers were buried or submerged, and Podprudnoe Lake was dammed, injecting cold water into submerged geysers. Possible triggers of the Giant Landslide include the inclination of the sliding plane towards the Geysernaya river basin, a pressure increase in the fluid-magma system, hanging block saturation by water during spring flooding, hydrothermal alteration weakening of the sliding plane, and steam explosions. Recent geysers cycling activity monitoring data (2007–2010), hydrogeochemical sampling, and thermal area infra red (IR) survey data are also discussed.
Hazardous gas emissions from the flanks of the quiescent Colli Albani volcano (Rome, Italy) by M.L. Carapezza; F. Barberi; M. Ranaldi; T. Ricci; L. Tarchini; J. Barrancos; C. Fischer; D. Granieri; C. Lucchetti; G. Melian; N. Perez; P. Tuccimei; A. Vogel; K. Weber (pp. 1767-1782).
Gas hazard was evaluated in the three most important cold gas emission zones on the flanks of the quiescent Colli Albani volcano. These zones are located above structural highs of the buried carbonate basement which represents the main regional aquifer and the main reservoir for gas rising from depth. All extensional faults affecting the limestone reservoir represent leaking pathways along which gas rises to the surface and locally accumulates in shallow permeable horizons forming pressurized pockets that may produce gas blowout when reached by wells. The gas, mainly composed of CO2 (>90vol.%), contains appreciable quantities of H2S (0.35–6vol.%), and both represent a potentially high local hazard. Both gases are denser than air and accumulate near ground where they may reach hazardous concentrations, and lethal accidents frequently occur to animals watering at local ponds. In order to evaluate the rate of degassing and the related hazard, CO2 and H2S diffuse soil flux surveys have been repeatedly carried out using an accumulation chamber. The viscous gas flux of some important discrete emissions has been evaluated and the CO2 and H2S air concentration measured by portable devices and by Tunable Diode Laser profiles. The minimum potential lethal concentration of the two gases (250ppm for H2S and 8vol.% for CO2) is 320times higher for CO2, whereas the CO2/H2S concentration ratio in the emitted natural gas is significantly lower (15–159). This explains why H2S reaches hazardous, even lethal, concentrations more frequently than CO2. A relevant hazard exists for both gases in the depressed zones (channels, excavations) particularly in the non-windy early hours of the day.
Geochemistry and Cr stable isotopes of Cr-contaminated groundwater in León valley, Guanajuato, México by Alejandro Villalobos-Aragón; Andre S. Ellis; María A. Armienta; Ofelia Morton-Bermea; Thomas M. Johnson (pp. 1783-1794).
León valley, located in the Mexican state of Guanajuato, has a long history (35+ a) of Cr contamination of groundwater and surface water. Here data are presented for Cr, major ion and trace element concentrations and Cr stable isotope measurements of groundwater in a heavily contaminated aquifer in Buenavista, where Cr ore processing residue piles (COPRPs) located in a chromate production factory are the main source of Cr. The aquifer directly beneath the factory still retains very high Cr(VI) concentrations (∼121mg/L). Ongoing pump and treat remediation is keeping the high concentration plume confined to the factory area and immediate vicinity, though Cr is also detected at some distance away. Chromium isotope data of the aquifer directly under the factory show only a small increase in δ53Cr (+0.33‰ to +0.81‰) and indicates minimal reduction of Cr(VI) to Cr(III). Very high Cr(VI) concentrations have possibly overwhelmed natural reductants and furthermore fresh Cr(VI) was being leached into the groundwater from the COPRP. From just one year of Cr isotope data it is clear that more aggressive remediation techniques will be necessary to reduce or eliminate the contamination. The fringes of the Cr plume have substantially lower concentrations and can be partially explained by transport of the main plume or mixing between waters from the nearby landfill and highly contaminated waters from QC. While the source of Cr at the fringes of the plume could be attributed to a source from the nearby landfill instead of the main plume from the factory, the Cr stable isotope data show enrichment in the heavier isotopes and point to varying amounts of reduction. Isotopic enrichment seen between 2007 and 2008 along the fringe may indicate either reduction or less unreacted Cr(VI) is being transported to the fringes. In either case some potential for natural attenuation of Cr(VI) exists at the western margin of the plume with the landfill playing a role.
Assessment of environmental hazards at abandoned mining sites: A case study in Sardinia, Italy by Rosa Cidu; Claudia Dadea; Paolo Desogus; Luca Fanfani; Pier Paolo Manca; Giampaolo Orrù (pp. 1795-1806).
In the last two centuries, several Pb–Zn mines were active in the Rio Mannu basin near Narcao (SW Sardinia, Italy), but are now abandoned. These abandoned mines pose a serious risk to the population and required an assessment of the hazard sources and the contaminant pathways in the area. The characterization plan of the Rio Mannu basin was carried out according to Italian protocols. Specifically, samples of soil, stream sediment, fine-grained ore-processing waste (from washery and flotation processes), surface water and groundwater have been analyzed in order to assess the levels of contamination in each mine area and the risks in the plains surrounding the site. Several potential chemical contaminants were considered both in solid materials and water samples. The Rosas mine was recognized as the most contaminated area in the basin, due to the presence of a large tailing pond, some fine-grained waste piles and two mine adits with concentrations of the toxic elements As, Cd, Cu, Pb, Sb and Zn exceeding Italian regulatory standards. The dispersion of contaminants occurs downstream from the small stream draining the area. In case of heavy rain the runoff into the streamlet transports the contaminated material far into the plain. The results of this study show that the characterization plan is a relatively cheap tool for establishing mitigation actions, prior to the realization of a complete, and usually expensive, remediation project at abandoned mine sites. Urgent recommended actions in the Rio Mannu basin include the treatment of the adit water prior to its discharge into the Rio Barisonis; the construction of drainage barriers on waste piles to reduce runoff and solid transport into the Rio Barisonis; the consolidation of the earthen dam containing the tailing pond of Rosas to avoid the deepening of erosion furrows that may threaten its stability.
Dynamics of xenobiotic transformation processes in soil systems: A zero-dimensional system model applied to ethylenethiourea by Astrid Möller; Anne M. Hansen (pp. 1807-1813).
A zero-dimensional model for the environmental fate of ethylenethiourea (ETU) in soils was formulated. The relevant physicochemical transformation processes were derived from published data, and their dynamics analyzed. Corresponding kinetic submodels for sorption, photodegradation, abiotic oxidation and biodegradation were then developed based on assumptions on the dominant process mechanisms. These submodels, and the overall model of biotic degradation in soil, were applied to literature data with determination coefficients of R2>0.9, validating the assumed process dynamics.
Hydrozincite seasonal precipitation at Naracauli (Sardinia – Italy): Hydrochemical factors and morphological features of the biomineralization process by D. Medas; R. Cidu; P. Lattanzi; F. Podda; R.B. Wanty; G. De Giudici (pp. 1814-1820).
Hydrozincite [Zn5(CO3)2(OH)6] precipitation from Naracauli waters (SW Sardinia) is, among other things, promoted by a microbial community made up of a filamentous cyanobacterium ( Scytonema sp.) and a microalgae ( Chlorella sp.). Hydrozincite bioprecipitation is responsible for the natural removal of harmful metals, especially Zn, from the stream waters. Thus, hydrozincite could be used to attenuate metal pollution in mining waters. Information on environmental conditions which promote the biomineralization process is fundamental for the development of remediation strategies. This paper aims to investigate the variables controlling the biomineralization process, and seasonal variation of the hydrochemical factors that affect hydrozincite precipitation. Hydrozincite morphology varies depending on the environmental conditions. Specifically, changes were observed between samples collected in late spring and samples collected in summer, and among samples precipitated under different water flow conditions. According to field observations, correlated with speciation and equilibrium calculation, the maximum intensity in hydrozincite precipitation occurs in the late spring during rainy years. In these conditions, the hydraulic regime in the stream reaches stationary conditions, and saturation index values with respect to hydrozincite reach their highest values. Concomitantly, the Zn2+/CO32- molar ratio reaches values close to 1, indicating that kinetic processes have a role on the hydrozincite biomineralization process.
Geochemistry of a continental saline aquifer for CO2 sequestration: The Guantao formation in the Bohai Bay Basin, North China by Zhonghe Pang; Yiman Li; Fengtian Yang; Zhongfeng Duan (pp. 1821-1828).
The Neogene Guantao formation in the Beitang sag in the Bohai Bay Basin (BBB) of North China, a Mesozoic–Cenozoic sedimentary basin of continental origin, has been chosen as a candidate for a pilot field test of CO2 sequestration. Hydrogeological and geochemical investigations have been carried out to assess its suitability, taking advantage of many existing geothermal wells drilled to 2000m or greater depths. Water samples from 25 wells and drill cores of three sections of the Guantao formation were collected for measurements of mineralogy, water chemistry and isotopes (δ18O, δD, δ13C,14C). Formation temperature estimated by chemical geothermometry is in the range of 60–80°C. Geochemical modeling of water–rock–CO2 interaction predicts a strong geochemical response to CO2 injection. Besides the elevated porosity (33.6–38.7%) and high permeability (1150–1980mD) of the Ng-III formation and a favorable reservoir–caprock combination, it is also found that the formation contains carbonates that will react with CO2 after injection. The low salinity (TDS<1.6g/L) offers high CO2 solubility. The14C age of the formation water indicates a quasi-closed saline aquifer system over large time scales, the lateral sealing mechanism for CO2 sequestration requires further investigation. The CO2 storage capacity of the Guantao formation within the Beitang sag is estimated to be 17.03Mt, assuming pure solubility trapping.
Geochemical and geo-electrical study of mud pools at the Mutnovsky volcano (South Kamchatka, Russia): Behavior of elements, structures of feeding channels and a model of origin by E.P. Bessonova; S.B. Bortnikova; M.P. Gora; Yu.A. Manstein; A.Ya. Shevko; G.L. Panin; A.K. Manstein (pp. 1829-1843).
This study presents data on the geochemical composition of boiling mud pools at the Mutnovsky volcano. The physicochemical characteristics of the pools and the concentrations of major, minor and trace elements in pool solutions vary widely. A comparison of the geochemical compositions of host rocks and solutions indicates that leaching from rocks is not the only source of chemicals in thermal solutions. Geophysical studies reveal the inner structure of thermal fields, which reflect the shapes of the underground reservoirs and feed channels. Using geophysical methods (electrical resistivity tomography and frequency domain investigations), it was shown that the vertical structure and complex geochemical zonation of the feed channels leads to a high contrast in the compositions of the mud solutions. These findings answer questions about the origin and composition of surface manifestations. To elucidate the mechanisms of solution formation, an attempt was made to describe the magmatic fluid evolution and the resulting mixing of waters by physical and mathematical models. The model illustrates fluid migration from a magma chamber to the surface. It is shown that the formation of brines corresponding to the mud pool composition is possible during secondary boiling.
Short-term changes in water chemistry in the Baccu Locci stream (Sardinia, Italy) affected by past mining by Franco Frau; Rosa Cidu; Carla Ardau (pp. 1844-1853).
Short-term changes in water chemistry, and especially in dissolved trace element concentrations, associated with diel cycles during base-flow conditions at a specific sampling station in the Baccu Locci stream draining the homonymous old mine area in Sardinia (Italy) were investigated. Diel fluctuations in pH and alkalinity were correlated with the temperature-dependent CO2 solubility and the biologically-induced CO2 production, both of which were higher during the night. Adsorption/desorption to/from streambed material, in particular ferrihydrite, is believed to be the main in-stream mechanism causing the observed diel variations in dissolved concentrations of As and Zn. Arsenic was mainly affected by the dual action of temperature and competing carbonate ions, whereas pH seemed less important. Temperature acted in accordance with the exothermic feature of anion sorption onto hydrous metal oxide surfaces; aqueous carbonate species exerted their competitive effect in relation to alkalinity variation. Zinc was primarily affected by temperature, in accordance with the endothermic feature of metal cation sorption onto hydrous metal oxide surfaces, and secondly by pH. Co-precipitation of As and Zn with calcite is another possible mechanism, which requires further investigation involving examination of inorganic and biological materials coating the streambed. All these processes potentially controlling the diel cycles of trace elements should be carefully considered to assess the effectiveness of remediation actions currently in progress at Baccu Locci. A normalization method for data from asynchronous sampling has been developed and proposed in order to eliminate or at least attenuate the effect of sampling time and provide an additional tool to identify the processes/mechanisms involved in trace element concentration fluctuations observed along a contaminated stream during base-flow conditions.
Streamwater chemistry in three contrasting monolithologic Czech catchments by Pavel Krám; Jakub Hruška; James B. Shanley (pp. 1854-1863).
Hydrochemical patterns resulting from differing bedrock geochemistry were ascertained by concurrent streamwater sampling in three small catchments, each underlain by geochemically contrasting silicate rock types in the western Czech Republic, Central Europe in 2001–2010. The catchments are situated 5–7km apart in the Slavkov Forest and are occupied by Norway spruce ( Picea abies) plantations. They have similar altitude, area, topography, mean annual air temperature, and atmospheric deposition fluxes. The amount of base cations oxides (Ca+Mg+Na+K) is markedly different among the three studied rocks (leucogranite 8%, amphibolite 22%, serpentinite 36%). The leucogranite contains a very small amount of MgO, while the serpentinite contains extremely large amounts of MgO. The amphibolite contains an intermediate amount of MgO and elevated CaO. The Lysina, on leucogranite, exhibited very small concentrations of Mg (median 0.4mgL−1) in streamwater; Pluhův Bor, on serpentinite, contained extremely high concentrations of streamwater Mg (18mgL−1). Streamwater in the Na Zeleném catchment, on amphibolite, contained an intermediate amount of Mg and an elevated Ca. Very low pH (4.2), negative alkalinity (−60μeqL−1) and high inorganic monomeric Al concentrations (0.3mgL−1) were found in the stream draining leucogranite. Serpentinite streamwater exhibited the highest pH (7.6), alkalinity (+940μeqL−1) and Ni concentrations (100μgL−1). Aquatic chemistry reflected the composition of the underlying rocks within the studied catchments. Contrasting streamwater compositions of the studied catchments were generated according to the MAGIC model simulations mainly by differences in chemical weathering rates of base cations (65meqm−2a−1 at Lysina, 198meqm−2a−1 at Na Zeleném, and 241meqm−2a−1 at Pluhův Bor).