Tracing the Source of Fluoride and Boron in Groundwaters Associated With the Lake Saint- Martin Impact Structure, Manitoba
Investigations by the Geological Survey of Canada have confirmed reports of naturally elevated F- and B concentrations in groundwaters of the Lake Saint-Martin region of Manitoba. Fluoride and B concentrations are highly correlated and reach 15.1 and 8.5 mg/L, respectively. Virtually all groundwaters with F- concentrations greater than the drinking water limit of 1.5 mg/L are from wells within the Lake Saint-Martin impact structure, a 208 Ma complex crater 23 km in diameter underlying a large part of the study area. These high-F- groundwaters can be classified into two groups according to their anionic and isotopic compositions. Group I samples consist of Na-mixed anion groundwaters, with Cl greater than 100 mg/L and highly depleted 18O compositions indicative of recharge under much cooler climatic conditions. Samples from this group exhibit a striking relationship to crater morphology, and are found in an arcuate belt within the southern rim of the impact structure. Group II high-F- samples consist of Na-HCO3-SO4 groundwaters, with little Cl, and less depleted 18O compositions. Samples from this group are associated with groundwaters recharged locally, on a ridge within the impact structure. The probable source of high-F- groundwaters is traced to phosphatic pellets in shales of the Winnipeg Formation, a regional basal clastic unit which sub-crops at shallow depth beneath the crater rim as a result of more than 200 m of structural uplift associated with the impact event. This extensive aquifer is known elsewhere in southern Manitoba for its naturally softened groundwaters and locally elevated F- concentrations. Group I groundwaters are interpreted as discharge from the Winnipeg Formation where it abuts against crater-fill deposits. Group II high-F- groundwaters are interpreted as modern recharge from within the impact structure, displacing Group I groundwaters. Elevated F- and B concentrations observed in groundwaters of the Lake Saint-Martin area represent the geochemical signature of upwelling from a deep regional aquifer. The previously unsuspected discharge zone occurs within an isolated sub-crop of the aquifer formed as a result of structural uplift caused by the impact event.
A Multi-Tracer Approach to Understanding the Groundwater-Hydrothermal-Surface Water System of Kawah Ijen Volcano (East Java, Indonesia)
The hyperacidic Banyu Pahit stream in East Java, Indonesia, contains elevated levels of anions and trace metals. This toxic water is transported through the downstream environment and is eventually used for irrigation, contaminating soils and crops over a large area. The source of fluid contamination is the Kawah Ijen volcanic system. Four sources and sinks of elements to the Banyu Pahit stream have been identified. These are seepage from the hyperacid crater lake, hydrothermal fluids, groundwater, and water-rock interaction. Whereas dilution by relatively neutral waters dominates the observed changes in water chemistry over the downstream Banyu Pahit (more than 4 km downstream from the headwaters), most of the contamination by fluids containing high concentrations of ions and trace metals is restricted to the upstream reaches (within 4 km of the headwaters). Detailed chemical analysis of this 4 kilometer section of the stream, the crater lake, and all springs (neutral and acid) that could have contributed waters to this section of the Banyu Pahit, has been undertaken as part of this study. Each sample was analyzed for 59 major, minor and trace elements, including the rare earth elements and anion species. The distinct chemical signature of each water source mentioned above, as well as that of water-rock interaction has been determined. Moreover, several elements have been identified that can be used as conservative tracers of these sources and sinks as they enter and move through the surface water system. These tracers, along with stream discharge measurements and mass-balance principles are integrated in an end-member mixing analysis of the Banyu Pahit stream to quantify the fluid and solute mass contribution of each source to the stream. The results of this analysis indicate that fluids from the hydrothermal system form a significant part of the contamination from Kawah Ijen, in addition to crater lake seepage. The volcanic fluids are responsible for the high concentration of anions to the Banyu Pahit stream, whereas water-rock interaction is responsible for high cation concentrations, except for Pb, which is removed from the fluid.
Reconstructing Temporal Changes in Solute Concentrations From Mineral Precipitates in the Hyperacidic Banyu Pahit River and Kawah Ijen Crater Lake, East Java, Indonesia
The composition of an aqueous fluid provides a chemical indicator of the processes that have acted upon it and the sources that have contributed to its solute load. When the characteristic signatures of individual sources and processes are known, the respective contribution of each to the overall system can be quantified. Furthermore, by monitoring water composition over time, the temporal variations in the relative contributions of these sources can be determined, providing important insights into the dynamics of the hydrologic system. However, direct measurements of water chemistry are generally only available for the recent past, whereas in many cases it is desirable to look further back in time. Mineral precipitates have the potential to provide this record, as they store a chemical signature of the host environment in their mineral composition. Specifically, given precise mineral-fluid element partition coefficients at relevant conditions, the chemical composition of a fluid can be reconstructed from the compositions of associated precipitates. Here, we present results of the application of predictive lattice-strain modelling of mineral-fluid element partition coefficients, combined with the composition of precipitates in a sequence of sedimentary deposits, to reconstruct temporal variations in the chemistry of hyperacidic waters of the Banyu Pahit river and Kawah Ijen crater lake in East-Java, Indonesia. The Banyu Pahit - Kawah Ijen system provides an excellent opportunity to demonstrate the power of this approach, as the chemical signatures of all sources and processes are known and variations in their relative contribution have major implications for the environmental impact of these fluids as well as for volcanic hazard monitoring. Preliminary results indicate that the system has been remarkably stable over time, but that distinct variations in the water-rock interaction contribution, especially, are present.
18O/16O Isotope Ratios of Nitrate Produced by Nitrification: New Insights From Soil Incubation Experiments and Implications for Source Partitioning
δ15N and δ18O values of nitrate (NO3-) derived from (i) the nitrification of soil organic matter (SOM) (microbial NO3-); (ii) synthetic fertilizer; (iii) sewage and manure; and, (iv) atmospheric deposition are isotopically distinct. Many researchers have utilized dual NO3- isotope analysis for NO3- partitioning studies. The expected range of δ18O values for microbial NO3- reported in the literature is based upon our knowledge of the systematics of NO3- formation by ammonia (NH3) and nitrite (NO2-) oxidizing bacteria. During chemolithoautotrophic nitrification, one-third of the oxygen atoms in NO3- originate from molecular oxygen (O2) (δ18O-O2 = +23.5‰ vs. SMOW). The remaining two-thirds of the oxygen atoms in NO3- are derived from water (H2O), which has a variable isotopic signature that depends upon geographical location (δ18O-precipitation = < -25‰ to 0‰ vs. SMOW). As such, the theoretical range of δ18O-NO3- produced from nitrification varies from -9‰ to +8‰ vs. SMOW. There are very few actual measurements of δ18O of microbial NO3- and the values vary between < 0‰ to +16‰. Controlled laboratory nitrification experiments were conducted with three soils collected from a temperate hardwood forest and a small agricultural catchment to determine the relative proportions of NO3--oxygen that originated from O2 and from H2O during nitrification in soils. Soils were incubated under an artificial headspace (20.9% O2; δ18O-O2 = +25.7‰) for several weeks at 50% water holding capacity using soil waters with variable δ18O-H2O (from -10‰ to +106‰). Replicate δ18O-NO3- values from all soils were very close, leading to a consistent interpretation of our results. None of the extractable NO3- had a δ18O signature that corroborates our current understanding of NO3- formation by nitrification in soils. The results of this study suggest that the expected range of δ18O values for microbial NO3- should be extended to include values that more closely resemble δ18O-H2O. Further studies are currently underway to determine if the results found for the lab incubation studies are also applicable to soils under in situ field conditions.
Stable Isotopes of N2O in a Large Canadian River Impacted by Agricultural and Urban Land Use
N2O is a potent greenhouse gas. Although denitrification is an important process in the global N cycle, N2O flux measurements from rivers worldwide are scarce. The two main processes producing N2O in rivers -- nitrification and denitrification -- result in N2O that is widely separated in isotopic signature. However, studies on the stable isotopes of N2O in rivers are almost non-existent. Here, we report the N2O fluxes and isotopic signatures in the Grand River, a large, heavily impacted river in southern Ontario. Land use in the basin is predominately agricultural and the river receives effluent from 26 wastewater treatment plants (WWTPs). River samples were collected over a 28 hour period to capture diel variation, along the entire length of the river to capture changing land use and throughout the year to capture the seasonal variability. A dynamic model was used to correct the measured N2O values for the effects of atmospheric exchange. Isotopic analysis of both the NH4+ and the NO3- end members in the WWTP effluent and in the river allowed the determination of N2O production pathways. N2O is produced along the entire length of the river but N2O from denitrification increases dramatically in the river below WWTPs at night when dissolved oxygen is low and nitrification of NH4+ decreases.
Nitrogen and Oxygen Isotopes of Low-Level Nitrate in Groundwater For Environmental Forensics
Sources of nitrate in water from human activities include fertilizers, animal feedlots, septic systems,
wastewater treatment lagoons, animal wastes, industrial wastes and food processing wastes. Nitrogen and
Oxygen isotopic analysis of nitrate in groundwater is essential to source identification and environmental
forensics as nitrate from different sources carry distinctly different N and O isotopic compositions. Nitrate is
extracted from groundwater samples and converted into AgNO3 using ion exchange techniques. The purified
AgNO3 is then broken down into N2 and CO for N and O isotopic measurement. Since nitrate concentrations in
natural ground waters are usually less than 2 mg/L, however, such method has been limited by minimum
sample size it requires, in liters, which is highly nitrate concentration dependent. Here we report a TurboVap-
Denitrifier method for N and O isotopic measurement of low-level dissolved nitrate, based on sample
evaporation and isotopic analysis of nitrous oxide generated from nitrate by denitrifying bacteria that lack N2O-
reductase activity. For most groundwater samples with mg/L-level of nitrate direct injection of water samples in
mLs is applied. The volume of sample is adjusted according to its nitrate concentration to achieve a final
sample size optimal for the system. For water samples with ug/L-level of nitrate, nitrate is highly concentrated
using a TurboVap evaporator, followed by isotopic measurement with Denitrifier method. Benefits of TurboVap-
Denitrifier method include high sensitivity and better precision in both isotopic data. This method applies to
both freshwater and seawater. The analyses of isotopic reference materials in nitrate-free de-ionized water and
seawater are included as method controls to correct for any blank effects. The isotopic data from groundwater
and ocean profiles demonstrate the consistency of the data produced by the TurboVap-Denitrifier method.
Use of Zn isotopes as a probe of anthropogenic contamination and biogeochemical processes in the Seine River, France
Metal contamination is a major issue of human impact on the aqueous environment. River water is particularly susceptible to contamination for both dissolved and particulate loads, displaying a major challenge in understanding the dominant sources and pathways of metals in polluted drainage basins. Recent improvements in mass spectrometry allow isotopic measurements of "non-traditional" metals (Zn, Cu, Fe, etc.), making their isotopes a new potential device to investigate contamination of metals under dissolved and particulate forms in rivers. We focus here on Zn isotope geochemistry in the largely anthropized Seine River (France). A new protocol of two-column separation of Zn from dilute aqueous solution has been developed and proven to be reproducible and satisfactory for accurate measurement of Zn isotopic ratios in water samples by MC-ICP-MS (2σ = 0.04‰). Preliminary results show a total variation of 0.65‰ for δ66Zn in dissolved phases of the Seine basin, and a light isotope enrichment in anthropogenic sources compared to other water samples. The determined conservative behavior of Zn in river water makes its isotopes an effective probe of anthropogenic contamination. The natural and anthropogenic inputs were clearly identified and calculated based on Zn isotope compositions for dissolved loads. Suspended particular matters (SPM) display different Zn isotope compositions compared to dissolved loads, with a total δ66Zn variation of 0.22‰. Zn concentrations and its isotope compositions in SPM reveal inverse relationships as function of the distance from the headwater and the SPM content for geographical and temporal samples, respectively. The δ66Zn data in SPM are interpreted as reflecting the mixture of natural and anthropogenic particles. The correlation between dissolved and particulate δ66Zn shows that adsorption processes are not the dominant process making Zn enrichment in SPM. We report here for the first time systematic δ66Zn data in waters of a whole river basin, showing Zn isotopes a powerful probe to trace contamination sources and biogeochemical processes in hydrologic systems.
The oxygen isotope composition of dissolved chromate: a new tool for determining sources of chromium contamination in groundwater
Hexavalent chromium (Cr(VI)) is a widespread carcinogen in groundwater, derived from both anthropogenic and natural sources. A large range of chromium isotope composition has been demonstrated for dissolved Cr(VI) in groundwater, resulting from the large isotope fractionation accompanying reduction of Cr(VI) to trivalent chromium (Cr(III)). As a result, the isotopic composition of chromium in dissolved chromate is beginning to prove useful for determining the sources of chromium in contaminated groundwater, but considered alone can likewise be non-diagnostic due to overlapping compositional ranges of potential anthropogenic and natural sources. Based on the strong Cr-O bond in the chromate molecule implied by the large chromium isotope fractionation accompanying Cr(VI) reduction, we have proposed that oxygen will remain closely linked to chromium in the chromate molecule and thus can be used to better constrain chromate sources through a Cr-O "multi-tracer" approach. In a series of laboratory experiments using isotopically "enriched" water and "normal" chromate, we have demonstrated that there is insignificant isotopic exchange between oxygen in chromate and water for residence times as long as one year, and thus chromate will retain the oxygen isotope composition of its source during extended transport in groundwater. We have likewise demonstrated that sufficient chromate for oxygen isotope analysis can be successfully isolated from a chemically complex groundwater sample through a series of precipitation, ion exchange and heating procedures. Although our current approach of measuring 100 micromolar samples of chromate using TCEA- gas mass spectrometry is straightforward and robust, we are also developing a negative-ion thermal ionization mass spectrometry technique in order to greatly reduce the sample size requirement. We are currently applying this novel technique at an electric power facility in California and a metal plating facility in France in order to better determine chromate sources at those sites.