Isotopic tracing of perchlorate sources in groundwater from Pomona,California

The groundwater of Pomona, California, is contaminated with perchlorate (${\text{ClO}}_4^{-}$). This water is treated to reduce the ${\text{ClO}}_4^{-}$ concentration to less than 6 μg L⁻¹ for compliance with California Department of Public Health drinking water regulations. A study of the isotopic composition of oxygen and chlorine in ${\text{ClO}}_4^{-}$ has been conducted to determine the source of the contamination. Isotopic compositions were measured for ${\text{ClO}}_4^{-}$ samples extracted from 14 wells, yielding ranges of δ¹⁸O values from −10.8‰ to −8.0‰, Δ¹⁷O values from +4.6‰ to +7.5‰, and δ³⁷Cl values from −12.8‰ to −8.9‰. Evaluation of mixing proportions using published isotopic data for three ${\text{ClO}}_4^{-}$ end-members (synthetic, Atacama, and indigenous natural ${\text{ClO}}_4^{-}$) indicates that contamination is dominantly (85–89%) Atacama ${\text{ClO}}_4^{-}$ derived from past use of imported Chilean nitrate fertilizer in citrus cultivation. This interpretation is consistent with (1) aerial photography archives showing extensive citrus fields surrounding Pomona in the early- to mid-20th century, (2) mass-balance estimates for ${\text{ClO}}_4^{-}$, and (3) numerical hydrologic models yielding travel-times for ${\text{ClO}}_4^{-}$ from fields to wells that are in the range of 15 to >100 years. The hydrologic models predict that ${\text{ClO}}_4^{-}$ contamination of Pomona groundwater will persist for decades into the future.     

Geochemistry of surface and ground water in Guiyang,China: Water/rock interaction and pollution in a karst hydrological system

The chemical compositions of the surface/ground water of Guiyang, the capital city of Guizhou Province, China are dominated by Ca²⁺, Mg²⁺, ${\text{HCO}}_3^{-}\text{and}{\text{SO}}_4^{2-}$, which have been derived largely from chemical weathering of carbonate rocks (limestone and dolomite). The production of ${\text{SO}}_4^{2-}$ has multiple origins, mainly from dissolution of sulfate evaporites, oxidation of sulfide minerals and organic S in the strata, and anthropogenic sources. Most ground water is exposed to soil CO₂ and, therefore, the H₂CO₃ which attacks minerals contains much soil C. In addition, the H₂SO₄ produced as a result of the oxidation of sulfides in S-rich coal seams and/or organic S, is believed to be associated with the chemical weathering of rocks. The major anthropogenic components in the surface and ground water include K⁺, Na⁺, Cl⁻, ${\text{SO}}_4^{2-}\text{and}{\text{NO}}_3^{-}$, with Cl⁻ and ${\text{NO}}_3^{-}$ being the main contributors to ground water pollution in Guiyang and its adjacent areas. The seasonal variations in concentrations of anthropogenic components demonstrate that the karst ground water system is liable to pollution by human activities. The higher content of ${\text{NO}}_3^{-}$ in ground water compared to surface water during the summer and winter seasons, indicates that the karstic ground water system is not capable of denitrification and therefore does not easily recover once contaminated with nitrates.     

Bulk modulus of Fe-rich olivines corrected for non-hydrostaticity

In situ X-ray diffraction was used to measure the isothermal bulk modulus at room conditions (K_T0) of synthetic olivines with different iron contents. The chemical formulae of the olivine samples were ${({\mathrm{Fe}}_x,{\mathrm{Mg}}_{1?x})}_2\mathrm{Si}{\mathrm{O}}_4$ with x = 0.45; 0.64; 0.82; 1, with 1% standard deviation (referenced as Fa45, Fa64, Fa82 and Fa100, respectively). All experiments were performed in the multi-anvil apparatus installed at NSLS beamline X17B2, to pressures up to about 7 GPa. Unit-cell volumes under hydrostatic conditions and differential stresses present in the samples were calculated using the method developed by Singh et al. (1998), and pressures measured using NaCl as a standard were then corrected for these stresses. Using a second-order Birch–Murnaghan equation of state, we obtained the isothermal bulk modulus of each composition: $K_{T0}^{\mathrm{Fa45}}=131.4\pm 2.6$ GPa, $K_{T0}^{\mathrm{Fa64}}=132.1\pm 3.1$ GPa, $K_{T0}^{\mathrm{Fa82}}=136.3\pm 1.7$ GPa and $K_{T0}^{\mathrm{Fa100}}=134.8\pm 1.4$ GPa. These values combined with data available in the literature show that the K_T0 of Fe-rich olivines increases very slowly with the Fe content, but possibly not in a simple linear trend.     

Soil organic matter transport along an sub-Arctic river–sea transect

Bacteriohopanepolyols (BHPs) and glycerol dialkyl glycerol tetraethers (GDGTs) have potential as soil-tracing biomarkers for the extensive shelves of the Arctic Ocean. In this work these biomarkers were analysed in surface sediments along a well characterised sub-Arctic transect in the northernmost Baltic Sea from the Kalix River to the central Bothnian Bay to assess their environmental behaviour and potential for tracing the contribution of soil in this type of system. There was a high BHP diversity and enhanced total BHP concentration in the estuarine sediments, whereas a much less diverse pattern could be observed in the open bay with lower total BHP concentration. In addition, both soil marker BHPs (adenosylhopanes) and branched GDGTs were substantially more abundant in the estuarine than the open bay sediments.The $R_{\text{soil}}^{\prime }$ index, based on the R_soil index minus the contribution from the methylated soil marker BHPs, is suggested as a new approach for tracing soil derived organic matter (OM) in the (sub)-Arctic region. The index decreased along the transect in an off-river direction, correlating strongly with both the branched and isoprenoid tetraether (BIT) index and the stable carbon isotopic composition of the sedimentary organic carbon. These field results indicate that both the $R_{\text{soil}}^{\prime }$ and the BIT indices have potential for tracing soil derived OM in sub-Arctic to Arctic waters.     

Model reconstruction of nitrate pollution of riverbank filtration using 15N and 18O data,Karany, Czech Republic

Stable isotopes of O (δ¹⁸O) in water and N (δ¹⁵N) in ${\text{NO}}_3^{-}$ have been used as natural indigenous groundwater tracers for sources of water and of ${\text{NO}}_3^{-}$ at two riverbank filtration (RBF) water supply systems. Both RBF systems (Skorkov and Sojovice) have wells in unconsolidated Quaternary sediments close to the Jizera River (Czech Republic) that have been affected by increasing ${\text{NO}}_3^{-}$ concentrations. The area is underlain by Turonian sandstones and marls that form a deeper bedrock aquifer. Sources of ${\text{NO}}_3^{-}$ are local sewerage systems and landfills (point sources) and seasonal application of manure and inorganic fertilizers (diffuse sources).At RBF Skorkov recharge to wells can be modelled using a two-component model with 60% river water contribution and 40% of very shallow groundwater with an average residence time of one month. During periods of abundant precipitation, groundwater originates entirely from the unsaturated zone of the Quaternary aquifer; extensive pumping for over 40 a has created new, bypassing flow paths that preferentially drain the contaminated unsaturated zone. During dry periods, wells are recharged by longer residence time groundwater from the Quaternary aquifer.At RBF Sojovice there is an additional recharge component of groundwater from the Turonian aquifer, which is sandier at this locality; this contains denitrified ${\text{NO}}_3^{-}$ with highly positive δ¹⁵N values.