Stable isotope patterns reveal widespread rainy-period-biased recharge in phreatic aquifers across Greece

The stable isotopic composition of water (δ18O, δ2H) is used to trace different components of the hydrological cycle, particularly interactions between precipitation, surface, and ground water. In Greece, insufficient spatial coverage for isotopes in precipitation hinders investigations of relationship between ground water and precipitation, information that is required to help quantify aquifer replenishment rates. We used precipitation, geospatial, ground water isotope data and General Linear Models (GLM) to predict the spatial distribution of isotopes in phreatic ground water in Greece. Prediction covariates of elevation, latitude, distance to nearest coastline, drainage basin (western vs. eastern Greece), and amount of precipitation predicted 62% of the isotopic variance in ground water. The GLM model yielded predictions of the isotopic composition of shallow aquifers, which allowed us to construct maps for use in hydrological and other forensic applications in Greece. A comparison of the stable isotope values predicted by our GLM to those of existing precipitation models revealed that phreatic aquifers in Greece are mainly recharged during the annual wet period, between November and March, as documented by the high overlap coefficients of 0.77 and 0.82, respectively. Rainy-period-biased aquifer recharge has implications for water quality and management issues, for example, nitrate pollution may be enhanced in the non-growing rainy period when crop uptake is the lowest, or for water resource management if climatic changes alter these temporal rainfall patterns. © 2018 Elsevier B.V

Stable nitrogen isotopes in waterfowl feathers reflect agricultural land use in western Canada

Stable nitrogen and carbon isotope analysis was performed on secondary feathers collected from flightless, mallard ducklings (Anas platyrhynchos) from 17 locations across western Canada. The δ15N values of mallard feathers ranged from +6.1 to +23.7‰(AIR). Mean δ15N feather values from the 17 locations were strongly correlated with the percentage of land under agricultural development. Higher δ15N values in waterfowl feathers collected from agricultural areas possibly reflected the entry of excess fertilizer nitrogen into local water bodies. However, other processes may have also been important. These results provide evidence that nitrogen isotope values in avian feathers may reflect long-term nitrogen additions to surface waters in agricultural areas and may also provide important clues in elucidating the origin of nonpoint source nitrogen inputs

Isotopic composition (¹³C, ¹⁴C, ²H) and geochemistry of aquatic humic substances from groundwater.

Aquatic humic substances from eight confined and unconfined aquifer/aquitard groundwater systems were analyzed for their stable carbon, radiocarbon, deuterium, and elemental contents. Fractionation of groundwater dissolved organic carbon (DOC) showed that humic substances are an important part of the organic solute load in all groundwaters. Groundwater humates are distinct from other terrestrial humates, and are characterized by low oxygen (36%) and high carbon (53%) contents. Elemental data from water table wells suggests this characteristic oxygen depletion is mainly a result of biochemical processes that occur in the vadose zone. The stable carbon isotopic composition of groundwater humates range between -31 and -24%. (PDB) with ann average of -26%., and reflect their terrestrial origins. The deuterium values of groundwater humate vary widely, and are mainly a reflection of 2H/H variability in the continental water cycle. Radiocarbon analyses suggest a predominant soil zone origin for most groundwater humates, although some groundwater systems are influenced to varying degrees by buried peat or coal. Soluble humate 14C mean residence times of up to several hundred years in the vadose zone before recharging to groundwater are due to the mixing of young and old organic carbon sources, rather than advective residence times

Organic carbon isotope geochemistry of clayey deposits and their associated porewaters, southern Alberta.

The organic carbon cycle of slowly permeable, clayey glacial till deposits in the Western Interior Great Plains, southern Alberta, was investigated by examining the relationship between solid organic matter (SOM) in the till sediments and dissolved organic carbon (DOC) in the till porewaters. Geochemically, the tills can be divided into two distinct zones: an upper oxidized (low SOM) till zone, and a lower unoxidized (high SOM) till zone. Till porewaters in both zones are characterized by high DOC contents. Radiocarbon dating and comparison of SOM and DOC fractions suggest DOC in the deep unoxidized zone originated during deglaciation, and is probably representative of groundwater ages in this till zone. In the oxidized zone, DOC originates from variable mixtures of soluble organic matter emplaced during deglaciation, and Cretaceous age coal fragments in this till zone. SOM in the upper till zone was mainly oxidized to CO2 gas during lowered water table conditions of the Altithermal climatic period. The subsurface production of fossil CO2 gas has serious implications for using the conventional dissolved inorganic carbon (DIC) 14C groundwater dating method in these clayey till porewaters

Correcting laser-based water stable isotope readings biased by carrier gas changes.

Recently, laser-based water stable isotope spectrometers have become popular as they enable previously impossible approaches of environmental observations. Consequently, they have been subjected to increasingly heterogeneous atmospheric conditions. However, there is still a severe lack of data on the impact of nonstandardized gas matrices on analyzer performances. Against this background, we investigated the influence of changing proportions of N2, O2, and CO2 in the carrier gas on the isotope measurements of a typical laser-based water stable isotope analyzer (Picarro L2120-i). We combined environmentally relevant mixtures of N2, O2, and CO2 with referenced, flash-evaporated water and found that isotope readings of the same water were altered by up to +14.57‰ for δ(18)O and -35.9‰ for δ(2)H. All tested relationships between carrier gas changes and respective isotope readings were strongly linearly correlated (R(2) > 0.99). Furthermore, an analyzer-measured variable allowed for reliable postcorrection of the biased isotope readings, which we additionally tested on field data. Our findings are of importance for environmental data obtained by analyzers based on the same technology. They are relevant for assays where inconsistent gas matrices or a mismatch in this regard between unknown and reference analyses cannot be excluded, which is in particular common when investigating the soil-vegetation-atmosphere continuum

Correcting for biogenic gas matrix effects on laser-based pore water-vapor stable isotope measurements.

The isotopic composition (δ2H, δ18O) of pore water is an invaluable tracer for the minimally invasive study of subsurface water flow and transport processes. Here, we evaluated a method for pore water isotope analysis that combines laser-based isotope analyzers and water-vapor isotope equilibration using evaporation- proof metalized sample bags. We tested inflation atmospheres (dry air vs. pure N2) and the impact of biogenic gas (CO2, CH4) accumulation for storage times of up to 4 wk. Samples were analyzed with a water isotope analyzer (Picarro L2120-i) and a gas chromatograph. Air-inflated water vapor samples showed a greater range of gas matrix effects (δ18O: 9.63‰; δ2H: 21.7‰) than N2–inflated samples (δ18O: 7.49‰; δ2H: 10.6‰) induced by nonuniform buildup of biogenic CO2, starting immediately after sample preparation. However, only air-inflated samples could be reliably corrected using instrument-specific sensitivity factors that were empirically determined by interpretation of periodically repeated isotope measurements. Corrected water isotope data were confirmed by similarity with local precipitation and suction cup isotope data. Residual uncertainties were well below the natural variations of soil water isotope values and independent of storage time, thus allowing for consistently reliable interpretations of soil water isotope profiles. We conclude that, especially for pore water sampling that requires small sample volumes and/or long storage times, metalized sample bags should be used to prevent evaporation notwithstanding the enhanced buildup of biogenic gases. Further, if gas matrix effects cannot be excluded, air inflation is preferred over pure N2, as only in that case can reliable postcorrections be performed by using internal data only