Sulfur isotope signatures in gypsiferous sediments of the Estancia and Tularosa Basins as indicators of sulfate sources, hydrological processes, and microbial activity

In order to reconstruct paleo-environmental conditions for the saline playa lakes of the Rio Grande Rift, we investigated sediment sulfate sources using sulfur isotope compositions of dissolved ions in modern surface water, groundwater, and precipitated in the form of gypsum sediments deposited during the Pleistocene and Holocene in the Tularosa and Estancia Basins. The major sulfate sources are Lower and Middle Permian marine evaporites (δ³⁴S of 10.9-14.4‰), but the diverse physiography of the Tularosa Basin led to a complex drainage system which contributed sulfates from various sources depending on the climate at the time of sedimentation. As inferred from sulfur isotope mass balance constraints, weathering of sulfides of magmatic/hydrothermal and sedimentary origin associated with climate oscillations during Last Glacial Maximum contributed about 35-50% of the sulfates and led to deposition of gypsum with δ³⁴S values of −1.2‰ to 2.2‰ which are substantially lower than Permian evaporates. In the Estancia Basin, microbial sulfate reduction appears to overprint sulfur isotopic signatures that might elucidate past groundwater flows. A Rayleigh distillation model indicates that about 3-18% of sulfates from an inorganic groundwater pool (δ³⁴S of 12.6-13.8‰) have been metabolized by bacteria and preserved as partially to fully reduced sulfur-bearing minerals species (elemental sulfur, monosulfides, disulfides) with distinctly negative δ³⁴S values (−42.3‰ to −20.3‰) compared to co-existing gypsum (−3.8‰ to 22.4‰). For the Tularosa Basin microbial sulfate reduction had negligible effect on δ³⁴S value of the gypsiferous sediments most likely because of higher annual temperatures (15-33 °C) and lower organic carbon content (median 0.09%) in those sediments leading to more efficient oxidation of H₂S and/or smaller rates of sulfate reduction compared to the saline playas of the Estancia Basin (5-28 °C; median 0.46% of organic carbon).The White Sands region of the Tularosa Basin is frequently posited as a hydrothermal analogue for Mars. High temperatures of groundwater (33.3 °C) and high δ¹⁸O(H₂O) values (1.1‰) in White Sands, however, are controlled predominantly by seasonal evaporation rather than the modern influx of hydrothermal fluids. Nevertheless, it is possible that some of the geochemical processes in White Sands, such as sulfide weathering during climate oscillations and upwelling of highly mineralized waters, might be considered as valid terrestrial analogues for the sulfate cycle in places such as Meridiani Planum on Mars.     

Open to closed system transition traced through the TDIC isotopic signature at the aquifer recharge stage, implications for groundwater C dating

¹⁴C dating models are limited when considering recent groundwater for which the carbon isotopic signature of the total dissolved inorganic carbon (TDIC) is mainly acquired in the unsaturated zone. Reducing the uncertainties of dating thus implies a better identification of the processes controlling the carbon isotopic composition of the TDIC during groundwater recharge. Geochemical interactions between gas, water and carbonates in the unsaturated zone were investigated for two aquifers (the carbonate-free Fontainebleau sands and carbonate-bearing Astian sands, France) in order to identify the respective roles of CO₂ and carbonates on the carbon isotopic signatures of the TDIC; this analysis is usually approached using open or closed system terms. Under fully open system conditions, the seasonality of the ¹³C values in the soil CO₂ can lead to important uncertainties regarding the so-called “initial ¹⁴C activity” used in ¹⁴C correction models. In a carbonate-bearing unsaturated zone such as in the Astian aquifer, we show that an approach based on fully open or closed system conditions is not appropriate. Although the chemical saturation between water and calcite occurs rapidly within the first metre of the unsaturated zone, the carbon isotopic contents (δ¹³C) of the CO₂ and the TDIC evolve downward, impacted by the dissolution-precipitation of the carbonates. In this study, we propose a numerical approach to describe this evolution. The δ¹³C and the A¹⁴C (radiocarbon activity) of the TDIC at the base of the carbonate-bearing unsaturated zone depends on (i) the δ¹³C and the A¹⁴C of the TDIC in the soil determined by the soil CO_2, (ii) the water’s residence time in the unsaturated zone and (iii) the carbonate precipitation-dissolution fluxes. In this type of situation, the carbonate δ¹³C-A¹⁴C evolutions indicate the presence of secondary calcite and permit the calculation of its accretion flux, equal to . More generally, for other sites under temperate climate and with similar properties to the Astian sands site, this approach allows for a reliable determination of the carbon isotopic composition at the base of the unsaturated zone as the indispensable “input function” data of the carbon cycle into the aquifer.     

The origin and mechanisms of salinization of the lower Jordan river

The chemical and isotopic (⁸⁷Sr/⁸⁶Sr, δ¹¹B, δ³⁴S_sulfate, δ¹⁸O_water, δ¹⁵N_nitrate) compositions of water from the Lower Jordan River and its major tributaries between the Sea of Galilee and the Dead Sea were determined in order to reveal the origin of the salinity of the Jordan River. We identified three separate hydrological zones along the flow of the river:

(1)

A northern section (20 km downstream of its source) where the base flow composed of diverted saline and wastewaters is modified due to discharge of shallow sulfate-rich groundwater, characterized by low ⁸⁷Sr/⁸⁶Sr (0.7072), δ³⁴S_sulfate (−2‰), high δ¹¹B (∼36‰), δ¹⁵N_nitrate (∼15‰) and high δ¹⁸O_water (−2 to-3‰) values. The shallow groundwater is derived from agricultural drainage water mixed with natural saline groundwater and discharges to both the Jordan and Yarmouk rivers. The contribution of the groundwater component in the Jordan River flow, deduced from mixing relationships of solutes and strontium isotopes, varies from 20 to 50% of the total flow.

(2)

A central zone (20-50 km downstream from its source) where salt variations are minimal and the rise of ⁸⁷Sr/⁸⁶Sr and SO₄/Cl ratios reflects predominance of eastern surface water flows.

(3)

A southern section (50-100 km downstream of its source) where the total dissolved solids of the Jordan River increase, particularly during the spring (70-80 km) and summer (80-100 km) to values as high as 11.1 g/L. Variations in the chemical and isotopic compositions of river water along the southern section suggest that the Zarqa River (⁸⁷Sr/⁸⁶Sr∼0.70865; δ¹¹B∼25‰) has a negligible affect on the Jordan River. Instead, the river quality is influenced primarily by groundwater discharge composed of sulfate-rich saline groundwater (Cl^-=31-180 mM; SO₄/Cl∼0.2-0.5; Br/Cl∼2-3×10^-3; ⁸⁷Sr/⁸⁶Sr∼0.70805; δ¹¹B∼30‰; δ¹⁵N_nitrate ∼17‰, δ³⁴S_sulfate=4-10‰), and Ca-chloride Rift valley brines (Cl^-=846-1500 mM; Br/Cl∼6-8×10^-3; ⁸⁷Sr/⁸⁶Sr∼0.7080; δ¹¹B>40‰; δ³⁴S_sulfate=4-10‰). Mixing calculations indicate that the groundwater discharged to the river is composed of varying proportions of brines and sulfate-rich saline groundwater. Solute mass balance calculations point to a ∼10% contribution of saline groundwater (Cl⁻=282 to 564 mM) to the river. A high nitrate level (up to 2.5 mM) in the groundwater suggests that drainage of wastewater derived irrigation water is an important source for the groundwater. This irrigation water appears to leach Pleistocene sediments of the Jordan Valley resulting in elevated sulfate contents and altered strontium and boron isotopic compositions of the groundwater that in turn impacts the water quality of the lower Jordan River.

     

Controls on dissolved inorganic carbon and δC in cave waters from DeSoto Caverns: Implications for speleothem δC assessments

Unraveling the factors controlling the carbon chemistry and transport of carbon within extant karst systems has important implications concerning the assessment of time-series δ¹³C records of speleothems. Here we report the results of a 3-year study of total dissolved inorganic carbon [DIC] and δ¹³C_DIC from cave waters at DeSoto Caverns (Southeastern USA) that offer valuable insight on carbon transport and the accompanied isotope fractionations from end-member sources to speleothems.[DIC] and δ¹³C_DIC values of cave waters range from 0.2 to 6.0 mM and 2.7 to −12.9 (‰ VPDB), respectively. [DIC] and δ¹³C_DIC of “seasonal drips” show seasonal, albeit noisy, variability and are inversely related (δ¹³C_DIC = −2.49[DIC] + 0.64, r² = 0.84). A shallow pool fed by multiple drips shows a bimodal δ¹³C_DIC distribution with an isotopically heavier mode during winter (−4‰ to −5‰ VPDB) relative to summer months (−9‰ to −10‰ VPDB). A multi-year trend of decreasing water availability during the study period is not reflected in a response of cave water carbon chemistry suggesting that rainfall amount may not be a significant controlling factor of the carbon chemistry. Coupled cave air winter ventilation/summer stagnation and varying CO₂ fluxes through the soil horizon and epikarst exert the strongest influence on seasonal [DIC] and δ¹³C_DIC variability. Measured values of high [DIC] and low δ¹³C_DIC from cave waters collected during the summer/early fall closely approximate isotopic equilibrium conditions. Conversely, low [DIC] and high δ¹³C_DIC values during winter/early months indicate kinetically enhanced isotopic fractionations within the cave waters. The kinetically enhanced isotopic fractionation of partitioned between degassed CO₂ and precipitated CaCO₃(1000lnα_{[(CO2-HCO3)+(CaCO3(AR)-HCO3)]/2}) is greater by about a factor of two (−6.7 ± 0.3‰) relative to the same isotopic fractionation under equilibrium conditions (−3.1‰).On the basis of ¹⁴C mass balance and paired ¹⁴C-U/Th measurements we estimate that on average about ∼23% of C delivered annually by the drips to the aragonite stalagmites is derived from ¹⁴C-dead dolomite cap while the remainder of ∼77% is derived from ¹⁴C-live biomass. δ¹³C measurements of aragonite (n = 12) sampled from the tips of active speleothems during the summer months are consistent with theoretical aragonite δ¹³C values calculated using the shallow pool summer/early fall data thus confirming the δ¹³C seasonality in both drips and coeval aragonite. δ¹³C values of an active stalagmite section spanning the last 200 years show a normal distribution with a mean of −7.1 ± 1.2‰ (n = 81) and a mode of −7‰ to −8‰ that are statistically indistinguishable from the annual mean and mode of all dripwaters. Thus secular time-series δ¹³C records of stalagmites at DeSoto Caverns with resolving power >10⁻¹ year will likely carry the imprints of drip annual means that record climate-driven δ¹³C seasonal biases.     

Solution behavior of reduced COH volatiles in silicate melts at high pressure and temperature

The solubility and solution mechanisms of reduced COH volatiles in Na₂OSiO₂ melts in equilibrium with a (H₂ + CH₄) fluid at the hydrogen fugacity defined by the iron-wüstite + H₂O buffer [f_H2(IW)] have been determined as a function of pressure (1-2.5 GPa) and silicate melt polymerization (NBO/Si: nonbridging oxygen per silicon) at 1400 °C. The solubility, calculated as CH₄, increases from ∼0.2 wt% to ∼0.5 wt% in the melt NBO/Si-range ∼0.4 to ∼1.0. The solubility is not significantly pressure-dependent, probably because f_H2(IW) in the 1-2.5 GPa range does not vary greatly with pressure. Carbon isotope fractionation between methane-saturated melts and (H₂ + CH₄) fluid varied by ∼14‰ in the NBO/Si-range of these melts.The (C..H) and (O..H) speciation in the quenched melts was determined with Raman and ¹H MAS NMR spectroscopy. The dominant (C..H)-bearing complexes are molecular methane, CH₄, and a complex or functional group that includes entities with CCH bonding. Minor abundance of complexes that include SiOCH₃ bonding is tentatively identified in some melts. There is no spectroscopic evidence for SiC or SiCH₃. Raman spectra indicate silicate melt depolymerization (increasing NBO/Si). The [CH₄/CCH]^melt abundance ratio is positively correlated with NBO/Si, which is interpreted to suggest that the (CCH)-containing structural entity is bonded to the silicate melt network structure via its nonbridging oxygen. The ∼14‰ carbon isotope fractionation change between fluid and melt is because of the speciation changes of carbon in the melt.     

Soils at the hyperarid margin: The isotopic composition of soil carbonate from the Atacama Desert, Northern Chile

We evaluate the impact of exceptionally sparse plant cover (0-20%) and rainfall (2-114 mm/yr) on the stable carbon and oxygen composition of soil carbonate along elevation transects in what is among the driest places on the planet, the Atacama Desert in northern Chile. δ¹³C and δ¹⁸O values of carbonates from the Atacama are the highest of any desert in the world. δ¹³C (VPDB) values from soil carbonate range from −8.2‰ at the wettest sites to +7.9‰ at the driest. We measured plant composition and modeled respiration rates required to form these carbonate isotopic values using a modified version of the soil diffusion model of [Cerling (1984) Earth Planet. Sci. Lett.71, 229-240], in which we assumed an exponential form of the soil CO₂ production function, and relatively shallow (20-30 cm) average production depths. Overall, we find that respiration rates are the main predictor of the δ¹³C value of soil carbonate in the Atacama, whereas the fraction C₃ to C₄ biomass at individual sites has a subordinate influence. The high average δ¹³C value (+4.1‰) of carbonate from the driest study sites indicates it formed—perhaps abiotically—in the presence of pure atmospheric CO₂.δ¹⁸O (VPDB) values from soil carbonate range from −5.9‰ at the wettest sites to +7.3‰ at the driest and show much less regular variation with elevation change than δ¹³C values. δ¹⁸O values for soil carbonate predicted from local temperature and δ¹⁸O values of rainfall values suggest that extreme (>80% in some cases) soil dewatering by evaporation occurs at most sites prior to carbonate formation. The effects of evaporation compromise the use of δ¹⁸O values from ancient soil carbonate to reconstruct paleoelevation in such arid settings.     

Multi-proxy approach (H/H, O/O, C/C and Sr/Sr) for the evolution of carbonate-rich groundwater in basalt dominated aquifer of Axum area, northern Ethiopia

Isotopic and chemical composition of groundwater from wells and springs, and surface water from the basalt-dominated Axum area (northern Ethiopia) provides evidence for the origin of water and dissolved species. Shallow (depth < 40 m) and deep groundwater are distinguished by both chemical and isotopic composition. Deep groundwater is significantly enriched in dissolved inorganic carbon up to 40 mmol l⁻¹ and in concentrations of Ca²⁺, Mg²⁺, Na⁺ and Si(OH)₄ compared to the shallow type.The δ²H and δ¹⁸O values of all solutions clearly indicate meteoric origin. Shifts from the local meteoric water line are attributed to evaporation of surface and spring water, and to strong water–rock interaction. The δ¹³C_DIC values of shallow groundwater between −12 and −7‰ (VPDB) display the uptake of CO₂ from local soil horizons, whereas δ¹³C_DIC of deep groundwater ranges from −5 to +1‰. Considering open system conditions with respect to gaseous CO₂, δ¹³C_DIC = +1‰ of the deep groundwater with highest PCO₂ = 10^−0.9 atm yields δ¹³C_CO2(gas) ≈ −5‰, which is close to the stable carbon isotopic composition of magmatic CO₂. Accordingly, stable carbon isotope ratios within the above range are referred to individual proportions of CO₂ from soil and magmatic origin. The uptake of magmatic CO₂ results in elevated cations and Si(OH)₄ concentrations. Weathering of local basalts is documented by ⁸⁷Sr/⁸⁶Sr ratios of the groundwater from 0.7038 to 0.7059. Highest values indicate Sr release from the basement rocks. Besides weathering of silicates, neoformation of solids has to be considered, which results in the formation of, e.g., kaolinite and montmorillonite. In several solutions supersaturation with respect to calcite is reached by outgassing of CO₂ from the solution leading to secondary calcite formation.     

Silicon isotopic composition of dissolved silicon and suspended particulate matter in the Yellow River, China, with implications for the global silicon cycle

The silicon isotopic composition of dissolved silicon and suspended particulate matter (SPM) were systematically investigated in water samples from the mainstem of the Yellow River and 4 major tributaries. The SPM content of the Yellow River varied from 1.4 to 38,560 mg/L, averaging 3568 mg/L, and the δ³⁰Si of suspended particulate matter (δ³⁰Si_SPM) varied from 0.3‰ to −0.4‰, averaging −0.02‰. The major factors affecting the SPM content and the δ³⁰Si_SPM values in the Yellow River were inferred to be the mineralogical, chemical and isotopic characteristics of the sediments from the Loess Plateau and a combination of the climate and the flow discharge of the river.The major ions in the Yellow River water were Na⁺, Ca²⁺, Mg²⁺, HCO₃⁻, SO₄²⁻ and Cl⁻. High salt concentration was observed in samples from the middle and lower reaches, likely reflecting the effects of evaporation and irrigation because the Na⁺, Mg²⁺, SO₄²⁻ and K⁺ concentrations were correlated with the Cl⁻ concentration. The dissolved Si concentration (D_Si) increased downstream, varying from 0.016 to 0.323 mM. The δ³⁰Si of dissolved Si (δ³⁰Si_Diss) varied from 0.4‰ to 2.5‰, averaging 1.28‰. The major processes controlling the D_Si and δ³⁰Si_Diss of the Yellow River are (a) the weathering of silicate rocks, (b) the formation of phytoliths in plants, (c) the evaporation of water from and the addition of meteoric water to the river system, which only affects concentrations, (d) the adsorption and desorption of aqueous monosilicic acid on iron oxide, and (e) the dissolution of phytoliths in soils.The D_Si and δ³⁰Si_Diss values of global rivers vary spatially and temporally in response to changes in climate, chemical weathering intensity and biological activity. The moderately positive δ³⁰Si_Diss values observed in the Yellow River may be attributed to the higher rates of chemical weathering and biological activities that have been observed in this catchment in comparison with those of other previously studied catchments, excluding the Yangtze River. Human activities may also potentially influence chemical weathering and biological activities and affect the D_Si and δ³⁰Si_Diss values of the major rivers of the world. Further river studies should be performed to gain a better understanding of the global Si isotope budget.     

Tracing sources of carbon in urban groundwater using δ<Superscript>13</Superscript>C<Subscript>TDIC</Subscript> ratios

Total dissolved inorganic carbon (TDIC) and its stable isotope ratio δ¹³C_TDIC are used to trace the evolution of the carbon system of groundwater in three UK Permo-Triassic sandstone aquifers. Samples were collected from multilevel piezometers, open boreholes and sewer sampling points in the British Midlands (Nottingham, Birmingham and Doncaster) to evaluate both local and regional variations in δ¹³C_TDIC. δ¹³C samples of matrix and pore water have also been analysed in each aquifer to further constrain the interpretations. Combining δ¹³C_TDIC ratios with measurements of TDIC and pH clearly distinguishes the principal processes underlying the geochemical evolution of groundwater in Triassic sandstone aquifers, where processes can be both natural (e.g. carbonate dissolution) and anthropogenic (sewer-derived recharge). The paper shows that δ¹³C_TDIC resolves ambiguities that arise from the interpretation of TDIC and pH measurements in isolation. Field measurements demonstrate that, under natural conditions, the carbonate system evolves similarly in each aquifer. An open-system evolution during recharge largely saturates the groundwater with carbonate depending upon its availability in the sandstone matrix. The contribution of sewer exfiltration to urban recharge is readily distinguished by lower pH and higher TDIC values without significant changes in δ¹³C_TDIC.     

Abundance of mass 47 CO2 in urban air, car exhaust, and human breath

Atmospheric carbon dioxide is widely studied using records of CO₂ mixing ratio, δ¹³C and δ¹⁸O. However, the number and variability of sources and sinks prevents these alone from uniquely defining the budget. Carbon dioxide having a mass of 47 u (principally ¹³C¹⁸O¹⁶O) provides an additional constraint. In particular, the mass 47 anomaly (Δ₄₇) can distinguish between CO₂ produced by high temperature combustion processes vs. low temperature respiratory processes. Δ₄₇ is defined as the abundance of mass 47 isotopologues in excess of that expected for a random distribution of isotopes, where random distribution means that the abundance of an isotopologue is the product of abundances of the isotopes it is composed of and is calculated based on the measured ¹³C and ¹⁸O values. In this study, we estimate the δ¹³C (vs. VPDB), δ¹⁸O (vs. VSMOW), δ47, and Δ₄₇ values of CO₂ from car exhaust and from human breath, by constructing ‘Keeling plots’ using samples that are mixtures of ambient air and CO₂ from these sources. δ47 is defined as , where is the R⁴⁷ value for a hypothetical CO₂ whose δ¹³C_VPDB = 0, δ¹⁸O_VSMOW = 0, and Δ₄₇ = 0. Ambient air in Pasadena, CA, where this study was conducted, varied in [CO₂] from 383 to 404 μmol mol⁻¹, in δ¹³C and δ¹⁸O from −9.2 to −10.2‰ and from 40.6 to 41.9‰, respectively, in δ47 from 32.5 to 33.9‰, and in Δ₄₇ from 0.73 to 0.96‰. Air sampled at varying distances from a car exhaust pipe was enriched in a combustion source having a composition, as determined by a ‘Keeling plot’ intercept, of −24.4 ± 0.2‰ for δ¹³C (similar to the δ¹³C of local gasoline), δ¹⁸O of 29.9 ± 0.4‰, δ47 of 6.6 ± 0.6‰, and Δ₄₇ of 0.41 ± 0.03‰. Both δ¹⁸O and Δ₄₇ values of the car exhaust end-member are consistent with that expected for thermodynamic equilibrium at∼200 °C between CO₂ and water generated by combustion of gasoline-air mixtures. Samples of CO₂ from human breath were found to have δ¹³C and δ¹⁸O values broadly similar to those of car exhaust-air mixtures, −22.3 ± 0.2 and 34.3 ± 0.3‰, respectively, and δ47 of 13.4 ± 0.4‰. Δ₄₇ in human breath was 0.76  ± 0.03‰, similar to that of ambient Pasadena air and higher than that of the car exhaust signature.     

Loading and fate of particulate organic carbon from the Himalaya to the Ganga-Brahmaputra delta

We use the evolution of river sediment characteristics and sedimentary C_org from the Himalayan range to the delta to study the transport of C_org in the Ganga-Brahmaputra system and especially its fate during floodplain transit.A detailed characterisation of both mineral and organic particles for a sampling set of river sediments allows taking into account the sediment heterogeneity characteristic of such large rivers. We study the relationships between sediment characteristics (mineralogy, grain size, specific area) and C_org content in order to evaluate the controls on C_org loading. Contributions of C3 and C4 plants are estimated from C_org stable isotopic composition (δ¹³C_org). We use the evolution of δ¹³C_org values from the Himalayan range to the delta in order to study the fate of C_org during floodplain transit.Ganga and Brahmaputra sediments define two distinct linear relations with specific area. In spite of 4-5 times higher specific area, Ganga sediments have similar C_org content, grain size and mineralogy as Brahmaputra sediments, indicating that specific area does not exert a primary control on C_org loading. The general correlation between the total C_org content and Al/Si ratio indicates that C_org loading is mainly related to: (1) segregation of organic particles under hydrodynamic forces in the river, and (2) the ability of mineral particles to form organo-mineral aggregates.Bed and suspended sediments have distinct δ¹³C_org values. In bed sediments, δ¹³C_org values are compatible with a dominant proportion of fossil C_org derived from Himalayan rocks erosion. Suspended sediments from Himalayan tributaries at the outflow of the range have low δ¹³C_org values (−24.8‰ average) indicating a dominant proportion of C3 plant inputs. In the Brahmaputra basin, δ¹³C_org values of suspended sediments are constant along the river course in the plain. On the contrary, suspended sediments of the Ganga in Bangladesh have higher δ¹³C_org values (−22.4‰ to −20.0‰), consistent with a significant contribution of C4 plant derived from the floodplain. Our data indicate that, during the plain transit, more than 50% of the recent biogenic C_org coming from the Himalaya is oxidised and replaced by floodplain C_org. This renewal process likely occurs during successive deposition-erosion cycles and river course avulsions in the plain.     

Application of carbon isotopes to detect seepage out of coalbed natural gas produced water impoundments

Coalbed natural gas (CBNG) production from coal bed aquifers requires large volumes of produced water to be pumped from the subsurface. The produced water ranges from high quality that meets state and federal drinking water standards to low quality due to increased salinity and/or sodicity. The Powder River Basin of northeastern Wyoming is a major coalbed natural gas producing region, where water quality generally decreases moving from the southeastern portion of the basin towards the center. Most produced water in Wyoming is disposed into impoundments and other surface drainages, where it may infiltrate into shallow groundwater. Groundwater degradation caused by infiltration of CBNG produced water holding impoundments into arid, soluble salt-rich soils is an issue of immense importance because groundwater is a major source for stock water, irrigation, and drinking water for many small communities in these areas. This study examines the potential of using stable C isotope signatures of dissolved inorganic C (δ¹³C_DIC) to track the fate of CBNG produced water after it is discharged into the impoundments. Other geochemical proxies like the major cations and major anions were used in conjunction with field water quality measurements to understand the geochemical differences between CBNG produced waters and ambient waters in the study area. Samples were collected from the CBNG discharge outfalls, produced water holding impoundments, and monitoring wells from different parts of the Powder River Basin and analyzed for δ¹³C_DIC. The CBNG produced waters from outfalls and impoundments have positive δ¹³C_DIC values that fall within the range of +12‰ to +22‰, distinct from the ambient regional surface and groundwaters with δ¹³C_DIC values ranging from −10‰ to −14‰. The results from the study demonstrate that these contrasting δ¹³C_DIC signatures can be used to trace seepage out of CBNG produced water impoundments into shallow groundwaters.     

Geochemical processes controlling the elevated fluoride concentrations in groundwaters of the Taiyuan Basin,Northern China

High fluoride groundwater with F⁻ concentration up to 6.20 mg/L occurs in Taiyuan basin, northern China. The high fluoride groundwater zones are mainly located in the discharge areas, especially in places where shallow groundwater occurs (the groundwater depth is less than 4 m). Regional hydrogeochemical investigation indicates that processes including hydrolysis of silicate minerals, cation exchange, and evaporation should be responsible for the increase in average contents of major ions in groundwater from the recharge areas to the discharge areas. The concentration of F⁻ in groundwater is positively correlated with that of HCO₃⁻ and Na⁺, indicating that groundwater with high HCO₃⁻ and Na⁺ contents help dissolve some fluoride-rich minerals. The water samples with high F⁻ concentration generally have relatively higher pH value, implying that alkaline environment favors the replacement of exchangeable F⁻ in fluoride-rich minerals by OH⁻ in groundwater. In addition, the mixing of karst water along the western mountain front and the evaporation may also be important factors for the occurrence of high fluoride groundwater. The inverse geochemical modeling using PHREEQC supports the results of hydrogeochemical analyses. The modeling results show that in the recharge and flow-through area of the northern Taiyuan basin, interactions between groundwater and fluoride-rich minerals are the major factor for the increase of F⁻ concentration, whereas in the discharge area of the northern basin, the evaporation as well as the mixing of karst water has greater contribution to the fluoride enrichment in groundwater.     

Stable isotope geochemistry of ground and surface waters associated with undisturbed massive sulfide deposits; constraints on origin of waters and water-rock reactions

Stable isotopes (H, O, C) were determined for ground and surface waters collected from two relatively undisturbed massive sulfide deposits (Halfmile Lake and Restigouche) in the Bathurst Mining Camp (BMC), New Brunswick, Canada. Additional waters from active and inactive mines in the BMC were also collected. Oxygen and hydrogen isotopes of surface and shallow groundwaters from both the Halfmile Lake and Restigouche deposits are remarkably uniform (− 13 to − 14‰ and − 85 to − 95‰ for δ¹⁸O_VSMOW and δ²H_VSMOW, respectively). These values are lighter than predicted for northern New Brunswick and, combined with elevated deuterium excess values, suggest that recharge waters are dominated by winter precipitation, recharged during spring melting. Deeper groundwaters from the Restigouche deposit, and from active and inactive mines have heavier δ¹⁸O_VSMOW ratios (up to − 10.8‰) than shallow groundwaters suggesting recharge under warmer climate or mixing with Shield-type brines. Some of the co-variation in Cl concentrations and δ¹⁸O_VSMOW ratios can be explained by mixing between saline and shallow recharge water end-members. Carbon isotopic compositions of dissolved inorganic carbon (DIC) are variable, ranging from − 15 to − 5‰ δ¹³C_VPDB for most ground and surface waters. Much of the variation in the carbon isotopes is consistent with closed system groundwater evolution involving soil zone CO₂ and fracture zone carbonate minerals (calcite, dolomite and siderite; average = − 6.5‰ δ¹³C_VPDB). The DIC of saline Restigouche deposit groundwater is isotopically heavy (∼+ 12‰ δ¹³C_VPDB), indicating carbon isotopic fractionation from methanogenesis via CO₂ reduction, consistent with the lack of dissolved sulfate in these waters and the observation of CH₄-degassing during sampling.     

Geochemical characterization of secondary microbial gas occurrence in the Songliao Basin, NE China

A suite of natural gases from the northern Songliao Basin in NE China were characterized for their molecular and carbon isotopic composition. Gases from shallow reservoirs display clear geochemical evidence of alteration by biodegradation, with very high dryness (C₁/C₂₊ > 100), high C₂/C₃ and i-C₄/n-C₄ ratios, high nitrogen content and variable carbon dioxide content. Isotopic values show wide range variations (δ¹³C_CH4 from −79.5‰ to −45.0‰, δ¹³C_C2H6 from −53.7‰ to −32.2‰, δ¹³C_C3H8 from −36.5‰ to −20.1‰, δ¹³C_nC4H10 from −32.7‰ to −24.5‰, and δ¹³C_CO2 from −21.6‰ to +10.5‰). A variety of genetic types can be recognized on the basis of chemical and isotopic composition together with their geological occurrence. Secondary microbial gas generation was masked by primary microbial gas and the mixing of newly generated methane with thermogenic methane already in place in the reservoir can cause very complicated isotopic signatures. System openness also was considered for shallow biodegraded gas accumulations. Gases from the Daqing Anticline are relatively wet with ¹³C enriched methane and ¹³C depleted CO₂, representing typically thermogenic origin. Gases within the Longhupao-Da’an Terrace have variable dryness, ¹³C enriched methane and variable δ¹³C of CO₂, suggesting dominant thermogenic origin and minor secondary microbial methane augment. The Puqian-Ao’nan Uplift contains relatively dry gas with ¹³C depleted methane and ¹³C enriched CO₂, typical for secondary microbial gas with a minor part of thermogenic methane. Gas accumulations in the Western Slope are very dry with low carbon dioxide concentrations. Some gases contain ¹³C depleted methane, ethane and propane, indicating low maturity/primary microbial origin. Recognition of varying genetic gas types in the Songliao Basin helps explain the observed dominance of gas in the shallow reservoir and could serve as an analogue for other similar shallow gas systems.     

Flow of river water into a Karstic limestone aquifer. 1. Tracing the young fraction in groundwater mixtures in the Upper Floridan Aquifer near Valdosta, Georgia

The quality of water in the Upper Floridan aquifer near Valdosta, Georgia is affected locally by discharge of Withlacoochee River water through sinkholes in the river bed. Data on transient tracers and other dissolved substances, including Cl⁻, ³H, tritiogenic helium-3 (³He), chlorofluorocarbons (CFC-11, CFC-12, CFC-113), organic C (DOC), O₂ (DO), H₂S, CH₄, δ¹⁸O, δD, and ¹⁴C were investigated as tracers of Withlacoochee River water in the Upper Floridan aquifer. The concentrations of all tracers were affected by dilution and mixing. Dissolved Cl⁻, δ¹⁸O, δD, CFC-12, and the quantity (³H+³He) are stable in water from the Upper Floridan aquifer, whereas DOC, DO, H₂S, CH₄, ¹⁴C, CFC-11, and CFC-113 are affected by microbial degradation and other geochemical processes occurring within the aquifer. Groundwater mixing fractions were determined by using dissolved Cl⁻ and δ¹⁸O data, recognizing 3 end-member water types in the groundwater mixtures: (1) Withlacoochee River water (δ¹⁸O=−2.5±0.3‰, Cl⁻=12.2±2 mg/l), (2) regional infiltration water (δ¹⁸O=−4.2±0.1‰, Cl⁻=2.3±0.1 mg/l), and (3) regional paleowater resident in the Upper Floridan aquifer (δ¹⁸O=−3.4±0.1‰, Cl⁻=2.6±0.1 mg/l) (uncertainties are ±1σ). Error simulation procedures were used to define uncertainties in mixing fractions. Fractions of river water in groundwater range from 0 to 72% and average 10%. The influence of river-water discharge on the quality of water in the Upper Floridan aquifer was traced from the sinkhole area on the Withlacoochee River 25 km SE in the direction of regional groundwater flow. Infiltration of water is most significant to the N and NW of Valdosta, but becomes negligible to the S and SE in the direction of general thickening of post-Eocene confining beds overlying the Upper Floridan aquifer.     

Experimental studies of oxygen isotope fractionation in the carbonic acid system at 15°, 25°, and 40°C

In light of recent studies that show oxygen isotope fractionation in carbonate minerals to be a function of HCO₃⁻ and CO₃²⁻ concentrations, the oxygen isotope fractionation and exchange between water and components of the carbonic acid system (HCO₃⁻, CO₃²⁻, and CO_2(aq)) were investigated at 15°, 25°, and 40°C. To investigate oxygen isotope exchange between HCO₃⁻, CO₃²⁻, and H₂O, NaHCO₃ solutions were prepared and the pH was adjusted over a range of 2 to 12 by the addition of small amounts of HCl or NaOH. After thermal, chemical, and isotopic equilibrium was attained, BaCl₂ was added to the NaHCO₃ solutions. This resulted in immediate BaCO₃ precipitation; thus, recording the isotopic composition of the dissolved inorganic carbon (DIC). Data from experiments at 15°, 25°, and 40°C (1 atm) show that the oxygen isotope fractionation between HCO₃⁻ and H₂O as a function of temperature is governed by the equation: