Ground- and surface water mass balances to ensure protection of St Lawrence River ecostystems

No abstract available

Carbon isotope fractionation during aerobic biodegradation of trichloroethene by Burkholderia cepacia G4: a tool to map degradation mechanisms

The strain Burkholderia cepacia G4 aerobically mineralized trichloroethene (TCE) to CO2 over a time period of similar to20 h. Three biodegradation experiments were conducted with different bacterial optical densities at 540 nm (OD(540)s) in order to test whether isotope fractionation was consistent. The resulting TCE degradation was 93, 83.8, and 57.2% (i.e., 7.0, 16.2, and 42.8% TCE remaining) at OD(540)s of 2.0, 1.1, and 0.6, respectively. ODs also correlated linearly with zero-order degradation rates (1.99, 1.11, and 0.64 mumol h(-1)). While initial nonequilibrium mass losses of TCE produced only minor carbon isotope shifts (expressed in per mille delta C- 13(VPDB)), they were 57.2, 39.6, and 17.0parts per thousand between the initial and final TCE levels for the three experiments, in decreasing order of their OD(540)s. Despite these strong isotope shifts, we found a largely uniform isotope fractionation. The latter is expressed with a Rayleigh enrichment factor, E, and was -18.2 when all experiments were grouped to a common point of 42.8% TCE remaining. Although, decreases of epsilon to -20.7 were observed near complete degradation, our enrichment factors were significantly more negative than those reported for anaerobic dehalogenation of TCE. This indicates typical isotope fractionation for specific enzymatic mechanisms that can help to differentiate between degradation pathways

Water mixing in a St. Lawrence river embayment to outline potential sources of pollution

Water mass balances on an isolated embayment (Hoople Bay) in the St. Lawrence River revealed a small stream (Hoople Creek), local groundwater and the St. Lawrence Main Channel as the 3 principal water sources. The latter had an average evaporative isotope signal that was inherited from the Great Lakes (delta(18)O(H2O) = -7.0parts per thousand) and an average Cl- content of 0.55 mmol/l. Hoople Creek and Hoople Bay waters were more variable in their isotopic composition and Cl- contents, while local groundwater was assumed to have a homogeneous composition year around. These parameters constituted an equation system that was solved with matrix operations to yield monthly contributions of the 3 endmembers. Influx of groundwater and Hoople Creek dominated the embayment only after higher snowmelt discharges, while the Main Channel contributed more than 50% during the remainder of the year. Preliminary results suggest that potential pollution in the Main Channel would strongly affect Hoople Bay and similar ecosystems along the river. Nevertheless, more detailed data are needed for a better water balance over longer time periods. The 3-component mixing technique serves as a good tool to evaluate seasonal water fluxes and may also become useful in other mass balances

Automated analyses of 18O/16O ratios in dissolved oxygen from 12-mL water samples

We introduce a new technique to routinely determine the 18O/16O ratio of O2(aq) from 12-mL Exetainer vials. Results were expressed in a ?-notation versus air and the Vienna Standard Mean Ocean Water (VSMOW). Samples were prepared by creating a He-headspace and stripping O2(aq) from solution by shaking for 30 min on a wrist shaker. Subsequent isotope analysis of the extracted O2(g) was achieved by converting the entire headspace into a large sampling loop using a double-hole needle. This enabled admission of sufficient O2(g) into a packed A5-Å-molecular sieve column, where it was separated from N2 before admission to the isotope ratio mass spectrometer. The latter was tuned to an m/z ratio of 32, thus enabling direct determination of molecular O2(g) without conversion to CO2. External standards consisted of dry air samples in helium-flushed vials and had between 1.5 and 16.8 parts per thousand O2(g) in a He matrix and a known isotopic composition of 0‰ air (+23.8‰ VSMOW). The method allows automated analyses of up to ~180 samples in one single batch and will provide new quantitative information about oxygen turnover in aqueous systems, including rates of gas transfer, redox processes, respiration, and photosynthesis. Repeat ?18OO2(aq) measurements on samples with concentrations between 15.6 µmol L–1 and saturation revealed standard deviations of 0.3‰. This is a typical precision encountered in continuous flow applications, and the method is available for studies using either 18O-labeled water to evaluate O2 gross production by incubation experiments or for natural abundance studies when isotope shifts are larger than 0.8‰. It may also become useful in microbiological and medical applications and can serve to quantify plant-gas exchange and soil gas processes

Automated analyses of ¹⁸O/¹⁶O ratios in dissolved oxygen from 12-ml water samples

We introduce a new technique to routinely determine the 18O/16O ratio of O2(aq) from 12-mL Exetainer vials. Results were expressed in a d-notation versus air and the Vienna Standard Mean Ocean Water (VSMOW). Samples were prepared by creating a He-headspace and stripping O2(aq) from solution by shaking for 30 min on a wrist shaker. Subsequent isotope analysis of the extracted O2(g) was achieved by converting the entire headspace into a large sampling loop using a double-hole needle. This enabled admission of sufficient O2(g) into a packed A5-Å-molecular sieve column, where it was separated from N2 before admission to the isotope ratio mass spectrometer. The latter was tuned to an m/z ratio of 32, thus enabling direct determination of molecular O2(g) without conversion to CO2. External standards consisted of dry air samples in helium-flushed vials and had between 1.5 and 16.8 parts per thousand O2(g) in a He matrix and a known isotopic composition of 0‰ air (+23.8‰ VSMOW). The method allows automated analyses of up to ~180 samples in one single batch and will provide new quantitative information about oxygen turnover in aqueous systems, including rates of gas transfer, redox processes, respiration, and photosynthesis. Repeat d18OO2(aq) measurements on samples with concentrations between 15.6 µmol L–1 and saturation revealed standard deviations of 0.3‰. This is a typical precision encountered in continuous flow applications, and the method is available for studies using either 18O-labeled water to evaluate O2 gross production by incubation experiments or for natural abundance studies when isotope shifts are larger than 0.8‰. It may also become useful in microbiological and medical applications and can serve to quantify plant-gas exchange and soil gas processes

Can conductivity and stable isotope tracers determine water sources during flooding? An example from the Elbe River in 2002

Despite drastic runoff variations, stable isotope data of water in the upstream part of the Elbe River showed remarkable similarity during, and two months after the flood of 2002. This homogeneity indicates that water sources remained the same and most likely represents dominant groundwater input regardless of the flood. While the latter remains the most plausible explanation, it was difficult to prove with water stable isotopes for the upstream part of the river. Overlapping isotope compositions of long-term average precipitation data (as a proxy for the groundwater end member) and of the weighted average from precipitation events in August 2002 did not allow quantification of the groundwater component during flooding. This shows that better spatial and temporal sample resolution is necessary for enhanced understanding of water sources and mass balances during floods. Such mass balances were only possible in the estuary where conductivity and stable isotope tracers both revealed much stronger freshwater fluxes to the estuary during the flooding event of 2002 when compared to those of two months after the flood. Conductivity was also a good indicator for additions of more saline waters from the Saale River when the system was not affected by flooding. However, during the flood in August 2002, large volumes of low-saline waters, most likely from shallow groundwater, masked these usually high conductivity values. This indicates that conductivity better reveals input of dissolved constituents, while the stable isotopes can better indicate sources of water input. Both tracers become most powerful when applied in combination for better understanding of water sources and river basin management

Carbon isotope fractionation during abiotic reductive dehalogenation of trichloroethene(TCE)

Dehalogenation of trichloroethene (TCE) in the aqueous phase, either on palladium catalysts with hydrogen as the reductant or on metallic iron, was associated with strong changes in delta C-13. In general, the delta C-13 of product phases were more negative than those of the parent compound and were enriched with time and fraction of TCE remaining. For dehalogenation with iron, the delta (13) C of TCE and products varied from -42 parts per thousand. to + 5 parts per thousand. For the palladium experiments, the final product, ethane, reached the initial delta C-13 of TCE at completion of the dehalogenation reaction. During dehalogenation, the carbon isotope fractionation between TCE and product phases was not constant. The variation in delta C-13 of TCE and products offers a new monitoring tool that operates independently of the initial concentration of pollutants for abiotic degradation processes of TCE in the subsurface, and may be useful for evaluation of remediation efficiency

Recharge velocity and geochemical evolution for the Permo-Triassic Sherwood Sandstone, Northern Ireland

The Triassic Sherwood Sandstone Group is a major European aquifer system. It is also the principal groundwater source in Northern Ireland. However, key aspects of its regional hydrogeology, such as age distribution and geochemical evolution, remain largely unknown. Here the groundwater geochemistry and isotopic composition were investigated in order to evaluate groundwater recharge and flow processes in a complex regional hydrogeological setting. The dominant geochemical processes, such as dissolution of carbonate cements were determined from the major and trace element chemistry. Stable and radio-isotope measurements were taken as residence time and flow path indicators and, together with physical and geochemical groundwater modelling, revealed groundwater ages of up to 9000 years. The importance of infiltration from overland flow from springs deriving water from the adjacent Cretaceous chalk aquifer and subsequently re-infiltrating into the Sherwood Sandstone was confirmed. In addition, evidence was found of a slow recharge component through low conductivity mudstones that yielded significant groundwater resident times throughout the Lagan Valley. These findings provide improved understanding of groundwater flow processes in Northern Ireland and serve as an example of methods that can be applied to water management elsewhere. (c) 2005 Elsevier B.V. All rights reserved