Analysis of Sulfur Isotopes Along a Wide Sulfide Concentration Gradient
In this study we investigate the influence of increasing sulfide concentrations on the fractionation of stable sulfur isotopes during microbial sulfate reduction. Sulfide concentrations range from 0.2 to 40~mM in batch culture experiments with the marine sulfate reducer Desulfobacter latus. We discuss δ34S ratios of dissolved sulfate from two independent batch culture experiments where the 2nd experimental set reaches maximum sulfide concentrations of 40~mM, twice the amount of the 1st set. Both data sets are interpreted using a Rayleigh type fractionation model. The model results show that an individual Rayleigh type interpretation of the two S-isotope data sets reveals two different enrichment factors. More importantly, only the Rayleigh model curve for Exp.~1 is able to match its corresponding data. In contrast, the Rayleigh model curve for Exp.~2, where higher sulfide concentrations were present, results in a distinct S-isotope offset to its experimental data. Therefore, we introduce a single step sulfate reduction model that allows for a sulfide-S reflux (reoxidation) into the sulfate pool, to interpret our experimental data. Our proposed model is able to explain both experimental data sets including the aforementioned offset. As a consequence of the new reflux model, the apparent enrichment factor is not constant, unlike a Rayleigh model derived enrichment factor.
Estimation of Nuclear Volume Dependent Fractionation of Mercury Isotopes Using Octanol- Water Partitioning of Inorganic Mercury
Mercury is a globally distributed pollutant; the toxicity and biomagnifications in aquatic food chains, even in remote areas, makes it a serious worldwide problem. Similar to other stable isotope systems, the isotopic composition of environmental Hg is potentially a new tool to better understand the biogeochemical cycling, fluxes and anthropogenic impacts of Hg. The promise of Hg isotopes is even more exciting with the recent discovery of large mass independent fractionation (MIF) displayed by the odd Hg isotopes (199Hg and 201Hg). Based on current theory MIF of Hg isotopes can arise either from the non-linear scaling of nuclear volume with mass for heavy isotopes (Nuclear Volume Effect, NVE) or from the magnetic isotope effect (MIE), which is due to the non-zero nuclear spin and nuclear magnetic moments of odd-mass isotopes. In order to interpret and use Hg MIF signatures in nature, both experimental and theoretical work is needed to better understand the controls and expression of MIF along with the underlying mechanisms of MIF. The goal of the current study was to design an experiment that would express the NVE in order to confirm theoretical predictions of the isotopic signature of the NVE for Hg. Unfortunately, both NVE and MIE predict MIF for only the odd isotopes. However since MIE is a kinetic phenomenon only, MIF observed in equilibrium reactions should be attributable to the NVE only. Thus it should be possible to isolate NVE driven MIF from MIE driven MIF. A laboratory experiment was designed on equilibrium octanol-water partitioning of different Hg chloride species. Octanol-water partitioning of Hg depends on the hydrophobicity of the Hg species, so non polar lipophilic species partition into the octanol phase while polar species remain in water phase. At 25 degree Celsius, a Cl- concentration of 1 molar and pH <2, the dominant aqueous phase is HgCl42- while non polar HgCl2 will partition into the octanol phase. Since HgCl42- has a stronger ionic bonding than HgCl2, this contrast in bond character between the two species is predicted to cause significant nuclear volume MIF when the phases equilibrate. This experiment also has relevance to Hg bioaccumulation in nature where bioaccumulation in aquatic organisms is often attributed to the hydrophobic character of both inorganic and organic Hg species. The results of this model could be an analogy to a natural aquatic system where nonpolar HgCl2 readily crosses the cell membrane of an organism, whereas species like HgCl42-would dominate the surrounding aqueous reservoir. Consequently, the organism would have a distinctive NV-MIF signature in Hg isotopes relative to the aqueous reservoir.
Tracing Sources Of Nitrate And Sulfate In The Bow River, Alberta Canada, Using Isotope Techniques
The Bow River in Alberta is a major tributary to the South Saskatchewan River in western Canada. Urban development and agricultural activities including feedlot operations within the Bow River Basin can potentially impact the river water quality by elevating nitrate and sulfate concentrations. In this project, we applied hydrological, chemical and isotopic techniques to identify sources of nitrate and sulfate in the Bow River. The study area stretches approximately 570km along the Bow River from Lake Louise in the Rocky Mountain headwaters to near its confluence with the Oldman River in the prairies. Between June 2007 and July 2008, monthly samples were taken from the Bow River for major ion chemistry and stable isotope ratio measurements of H, O, C, N and S. Flow data from Alberta Environment were used in combination with chemical data to estimate fluxes of nitrate, sulfate and other ionic solutes along the river. Isotope results show that Bow River water near Lake Louise was characterized by δ15N-NO3 values between 0 and +4‰ and δ18O-NO3 values between +7 and +11‰ falling within the range typical for nitrate produced by nitrification in forest ecosystems. Between Canmore and Calgary, δ15N- NO3 increased to values between +3 and +8‰, and δ18O-NO3 ranged between -5 and +5‰. Nitrate discharged from the Bonnybrook wastewater treatment plant in Calgary has elevated δ15N-NO3 values of +8‰ and low δ18O-NO3 values of -10‰. Nitrate flux increased over an order of magnitude in the river as a result of wastewater effluent discharge at Calgary. In the agricultural irrigation districts downstream of Calgary, δ15N-NO3 values varied between +6 and +11‰, whereas δ18O-NO3 values ranged between -11 and +1‰. The elevated δ15N-NO3 and low δ18O-NO3 values indicate that sewage derived nitrate from the wastewater treatment plant is the major cause for increased nitrate fluxes in the Bow River downstream of Calgary. At Lake Louise, δ34S-SO4 values varied between +17 and +20‰ indicating that sulfate is mainly derived from dissolution of evaporite minerals in the headwaters. Downstream of Lake Louise, sulfate concentrations increased with decreasing δ34S- SO4 values. The Bow River downstream of Calgary showed δ34S-SO4 values between +7 and -1‰ whereas tributaries in the irrigation districts had δ34S-SO4 values between +1 and -13‰. These are isotopic values typically found in sulfate derived from oxidation of reduced sulfur species. The sulfur isotope values of the wastewater effluent discharged at Calgary were between -2 and +6‰. The trend of increasing sulfate concentrations with flow distance accompanied by decreasing δ34S-SO4 values suggests there is a mixture of sulfate sources from dissolution of sulfate minerals in the headwaters, municipal effluent in Calgary, and oxidation of pyrite or other reduced sulfur species in the irrigation districts. The result suggests that Bow River in the irrigation districts is affected by sulfate sources from the oxidation of reduced sulfur and the wastewater effluent in Calgary. In Calgary above the wastewater treatment plant, sulfate mainly comes from dissolution of evaporite in the Rock Mountains. We conclude that isotopic techniques can enhance the understanding of the sources and the transport of nitrate and sulfate in the Bow River.
Contaminant Source Misidentification due to "Chromatographic" Separation in Fractured Bedrock
Identification of the composition of an upgradient contaminant source using downgradient monitoring well data is a common practice in applied hydrogeology. A common assumption underlying this sort of analysis is that weathering is innocuous enough that chromatographic data can be used to characterize a multicomponent source without any sort of inverse modeling. We have analyzed the feasibility of identification of PCB sources in terms of former commercial mixtures (Aroclors), based on simulated measurements from a plume that has migrated through fractured sandstone. Our conclusion is that visual identification from a chromatogram, and also algebraic fitting methods that ignore domain properties, are unreliable. This conclusion holds even over very short plume travel distances (well under 5 meters).
Unsafe Practice of Extracting Potable Water From Aquifers in the Southwestern Coastal Region of Bangladesh
The groundwater resource is of paramount importance to the lives and livelihoods of the millions of people in Bangladesh. Unfortunately, high levels of arsenic have been found in groundwater in many parts of Bangladesh. Besides, the salinity in water systems in the coastal areas has increased as a consequence of the flow diversion from the upper reaches of Ganges River by the neighboring country India. Since hand- pumped groundwater (tube) wells are the only viable sources of drinking water, maintaining drinking water security for over 6 million people in the south-west (SW) region has been a major challenge for the Bangladesh Government. Due to rapid exploitation of groundwater resources in excess of recharge capacity, non-saline water sources in the SW region have already been depleted and the hand tube wells have mostly been abandoned. Meanwhile, shrimp farming has resulted in saline water infiltration into the perched aquifer system in many areas. A recent survey covering123 wells out of 184, extending to a depth of 330 m, showed high salinity in water. Combined factors of rapid exploitation of shallow groundwater, depletion of the deep aquifers and the subsequent saline water intrusion into these aquifers have put long-term sustainability of the remaining fresh groundwater resource into jeopardy. Very high concentrations of nitrite are found in this study in many tube wells in the area where samples have been drawn from aquifer systems up to 244 m deep. Nitrite concentrations in 35 wells randomly sampled in this study range from 16.98 to 43.11 mg/L, averaging 27.55 mg/L. This is much higher than the Maximum Contaminant Level (MCL) of 1 mg/L set by the U.S. EPA for human consumption. Simultaneously, dissolved oxygen (DO) is found to be very low (0.1 to 2 mg/L). There are numerous reports and anecdotal evidences of "Blue Baby Syndrome" (methemoglobinemia) in the region, which is generally due to gradual suffocation caused by poor transport of oxygen from the lungs to the body parts. This situation is known to occur as a result of methemoglobin compounds deposited in the blood stream in association with excess nitrite consumption. Nitrogen isotope analysis from eight random samples recorded an average δ 15 N value of 6.7‰ thereby indicating that the nitrite is probably derived from the nitrogen based fertilizers that are commonly applied to the paddy fields and the shrimp farms. The observed δ 15 N values, ranging from 0.35‰ to 11.82‰ are higher than the typical numbers expected for nitrogen based fertilizers. In this context, the high content of ammonium, ranging from 0.59 to 28.01 mg/L, clearly indicates that the nitrogen in the system has undergone denitrification in a bio-chemically reduced condition, which resulted in the increased delta values. The above results clearly show that in addition to the already known risks of arsenocosis from groundwater, there are serious threats of blood disorders among the population that routinely consumes water with high levels of nitrite. Nevertheless, the existing problems of arsenic and other metals in water are expected to become worse over the time because of highly anoxic condition. All the above results from this study warrant immediate actions by the appropriate government agencies to arrange for alternative potable water for more than 6 million people living in the region.