Applied Geochemistry (v.22, #3)

The geochemistry of groundwater resources in the Jordan Valley: The impact of the Rift Valley brines by Efrat Farber; Avner Vengosh; Ittai Gavrieli; Amer Marie; Thomas D. Bullen; Bernhard Mayer; Amir Polak; Uri Shavit (494-514).
The chemical composition of groundwater in the Jordan Valley, along the section between the Sea of Galilee and the Dead Sea, is investigated in order to evaluate the origin of the groundwater resources and, in particular, to elucidate the role of deep brines on the chemical composition of the regional groundwater resources in the Jordan Valley. Samples were collected from shallow groundwater in research boreholes on two sites in the northern and southern parts of the Jordan Valley, adjacent to the Jordan River. Data is also compiled from previous published studies. Geochemical data (e.g., Br/Cl, Na/Cl and SO4/Cl ratios) and B, O, Sr and S isotopic compositions are used to define groundwater groups, to map their distribution in the Jordan valley, and to evaluate their origin. The combined geochemical tools enabled the delineation of three major sources of solutes that differentially affect the quality of groundwater in the Jordan Valley: (1) flow and mixing with hypersaline brines with high Br/Cl (>2 × 10−3) and low Na/Cl (<0.8) ratios; (2) dissolution of highly soluble salts (e.g., halite, gypsum) in the host sediments resulting in typically lower Br/Cl signal (<2 × 10−3); and (3) recharge of anthropogenic effluents, primarily derived from evaporated agricultural return flow that has interacted (e.g., base-exchange reactions) with the overlying soil. It is shown that shallow saline groundwaters influenced by brine mixing exhibit a north–south variation in their Br/Cl and Na/Cl ratios. This chemical trend was observed also in hypersaline brines in the Jordan valley, which suggests a local mixing process between the water bodies.

The distribution of dissolved halides (Cl, Br, I) in interstitial waters of deep drill holes in the ocean is discussed for a number of environments: hemi-pelagic sediments; hydrate containing sediments; sediments affected by halide brines; and also in hydrothermal environments. The importance of the diagenesis of organic C in the production of I and Br is discussed, with reference to the observations of dissolved SO 4 2 - and NH 4 + in the pore fluids. More importantly, in the zone of methano-genesis, below the SO 4 2 - reduction zone, and usually associated with high organic C accumulation rates, increases in I and Br are of importance and usually are much higher than the increases observed in the SO 4 2 - reduction zone. In pore fluids affected by dissolution of evaporites the Br/Cl is often a strong indicator of the degree of evaporation of seawater demonstrated from results obtained in the Mediterranean Sea. Dissolution of halites on the other hand leads to Br/Cl much lower than those of sea water. In hydrothermal sediments, there is no doubt that I and Br contents of the fluids are influenced by hydrothermal alteration of the sediments and their organic C, but signals are not clear. The present study provides an overview of previous DSDP and ODP studies of halide distributions in sedimentary pore waters, in addition to other contributions on halide distributions in sedimentary pore waters in this symposium volume.

Halogen concentrations in pore waters and sediments of the Nankai Trough, Japan: Implications for the origin of gas hydrates by Y. Muramatsu; T. Doi; H. Tomaru; U. Fehn; R. Takeuchi; R. Matsumoto (534-556).
Presented here are halogen concentrations (Cl, Br and I) in pore waters and sediments from three deep cores in gas hydrate fields of the Nankai Trough area. The three cores were drilled between 1999 and 2004 in different geologic regions of the northeastern Nankai Trough hydrate zone. Iodine concentrations in all three cores increase rapidly with depth from seawater concentrations (0.00043 mmol/L) to values of up to 0.45 mmol/L. The chemical form of I was identified as I, in accordance with the anaerobic conditions in marine sediments below the SO4 reduction depth. The increase in I is accompanied by a parallel, although lesser increase in Br concentrations, while Cl concentrations are close to seawater values throughout most of the profiles. Large concentration fluctuations of the three halogens in pore waters were found close to the lower boundary of the hydrate stability zone, related to processes of formation and dissociation of hydrates in this zone. Generally low concentrations of I and Br in sediments and the lack of correlation between sediment and pore water profiles speak against derivation of I and Br from local sediments and suggest transport of halogen rich fluids into the gas hydrate fields. Differences in the concentration profiles between the three cores indicate that modes of transportation shifted from an essentially vertical pattern in a sedimentary basin location to more horizontal patterns in accretionary ridge settings. Because of the close association between organic material and I and the similarity of transport behavior for I and CH4, the results suggest that the CH4 in the gas hydrates also was transported by aqueous fluids from older sediments into the present layers.

Chlorine stable isotope studies of old groundwater, southwestern Great Artesian Basin, Australia by Min Zhang; Shaun K. Frape; Andrew J. Love; Andrew L. Herczeg; B.E. Lehmann; U. Beyerle; R. Purtschert (557-574).
Stable Cl isotope ratios (37Cl/35Cl) were measured in groundwater samples from the southwestern flow system of the Great Artesian Basin, Australia to gain a better understanding of the Cl sources and transport mechanisms. δ 37Cl values range from 0‰ to −2.5‰ (SMOC), and are inversely correlated with Cl concentration along the inferred flow direction. The Cl isotopic compositions, in conjunction with other geochemical parameters, suggest that Cl in groundwaters is not derived from salt dissolution. Mixing of the recharge water with saline groundwater cannot explain the relationship between δ 37Cl and Cl concentration measured. Marine aerosols deposited via rainfall and subsequent evapotranspiration appear to be responsible for the Cl concentrations observed in wells that are close to the recharge area, and in groundwaters sampled along the southern transect. δ 37Cl values measured in the leachate of the Bulldog shale suggest that the aquitard is the subsurface source of Cl for the majority of groundwater samples studied. Diffusion is likely the mechanism through which Cl is transported from the pore water of the Bulldog shale to the aquifer. However, a more detailed study of the aquitard rocks is required to verify this hypothesis.

Stable chlorine isotopes in Phanerozoic evaporites by C.J. Eastoe; T.M. Peryt; Oleh Y. Petrychenko; D. Geisler-Cussey (575-588).
Modern seawater has a uniform δ 37Cl value (0.0‰), with an exception in the upper current of the Bosphorus (0.4‰). Marine halite ranging in age from Cambrian to Miocene has δ 37Cl values of 0.0 ± 0.9‰, with most of the data in the range 0.0 ± 0.5‰. Mean δ 37Cl values differ measurably between basins, with no evident relationship to basin size or to age. Smaller evaporite bodies have the largest δ 37Cl ranges. Potash facies halite has mean δ 37Cl values lower than those of halite facies salt in the East Siberia and Zechstein basins. The bulk δ 37Cl of bedded halite preserving sedimentary textures cannot be shifted measurably after deposition under plausible natural conditions. During the Phanerozoic, a detectable change in the δ 37Cl values of the oceans is unlikely as a result of Cl fluxes to and from the mantle and evaporites. In halite, the values of δ 37Cl that cannot be explained by fractionation occurring on crystallization are best explained by the addition of non-marine Cl with δ 37Cl ≠ 0.0‰ to evaporite brine.

The chemical composition and the isotopic characteristics of formation waters from the Siberian Platform are presented. The study involved samples of formation brines from depths ranging from 100 to ∼4000 m at five different sites covering a large area of the Siberian Platform. Four water types were identified. The two main water types that were found are: (1) Ca–Cl brines that are believed to be the residual of an evaporated paleoseawater; and (2) Na–Cl brines that are derived mainly from halite dissolution. The origin of a third group of highly saline samples was not determined. However, the chemical and isotopic characteristics of this group of samples suggest that they were produced by various complex scenarios such as metamorphism, water–rock interaction, permafrost freezing and mixing. The last group of samples represents fresh and brackish waters across the area.A wide range for the natural variation of Br stable isotopes (between −0.80‰ and +3.35‰) was found. The δ81Br results obtained during this study indicate that Br stable isotopes can have large natural variations and that different evolutionary processes affecting water chemistry may cause significant fractionation.

129I anthropogenic budget: Major sources and sinks by A. Aldahan; V. Alfimov; G. Possnert (606-618).
Data are presented here on the anthropogenic 129I inventory in regions that have been strongly affected by releases from European reprocessing facilities which, to the authors’ knowledge, presently account for >90% of the global isotope source in the Earth’s surface environment. The results show that >90% of the isotope inventory occurs in marine waters with the Nordic Seas and Eurasian basin of the Arctic Ocean containing most of the 129I. Within the terrestrial environment of Europe, soils contain the largest part of the isotope inventory. However, the inventory of the terrestrial system did not provide clues on the most plausible atmospheric source of 129I to Europe, thus supply from both gaseous and marine releases is proposed. The sum of the total inventory in both the marine and terrestrial environments did not match the estimated releases. This imbalance is likely to relate to unconstrained inventory estimates for marine basins (Irish Sea, English Channel and North Sea) close to the facilities, but also to the occurrence of 129I in the biosphere, and possible overestimated releases from the nuclear reprocessing facilities. There is no doubt that the available data on 129I distribution in the environment are far from representative and further research is urgently needed to construct a comprehensive picture.

129I/127I ratios in Scottish coastal surface sea water: Geographical and temporal responses to changing emissions by Christoph Schnabel; Valérie Olive; Mariko Atarashi-Andoh; Andrew Dougans; Robert M. Ellam; Stewart Freeman; Colin Maden; Martin Stocker; Hans-Arno Synal; Lukas Wacker; Sheng Xu (619-627).
This work constitutes the first survey of I isotope ratios for Scottish sea water including the first data for the west of Scotland. These data are of importance because of the proximity to the world’s second largest emission source of 129I to the sea, the Sellafield nuclear reprocessing plant, because of the increasing importance of the sea to land transfer of 129I and also as input data for dose estimates based on this pathway of 129I. 129I/127I ratios in SW Scotland reached 3 × 10−6 in 2004. No strong variation of I isotope ratios was found from 2003 to 2005 in Scottish sea waters. Iodine isotope ratios increased by about a factor of 6 from 1992 to 2003 in NE Scotland, in agreement with the increase of liquid 129I emissions from Sellafield over that time period. It is demonstrated that 129I/127I ratios agree better than 129I concentrations for samples from similar locations taken in very close temporal proximity, indicating that this ratio is more appropriate to interpret than the radionuclide concentration.

129I/127I ratios in surface waters of the English Lake District by M. Atarashi-Andoh; C. Schnabel; G. Cook; A.B. MacKenzie; A. Dougans; R.M. Ellam; S. Freeman; C. Maden; V. Olive; H.-A. Synal; S. Xu (628-636).
Accelerator Mass Spectrometry (AMS) was used to measure 129I/127I ratios in surface sea, lake, and river water samples collected in 2004 and 2005 from the English Lake District and from SW Scotland, areas which are in relatively close proximity to the Sellafield nuclear fuel reprocessing plant in NW England. The 129I/127I ratios in surface water collected from the shore of the Irish Sea were in the range 2.8 × 10−6 to 8.2 × 10−6. These ratios are one order of magnitude higher than that of seawater collected from the Irish Sea in 1992, correlating with the increase in 129I content of the Sellafield liquid effluent discharge over the last decade. The 129I/127I ratios in lakes in the Lake District were in the range 0.7 × 10−6 to 6.4 × 10−6 and decreased exponentially as a function of distance from Sellafield. Consideration of the relative variation of stable I concentrations and 129I/127I ratios suggests that Sellafield gaseous discharges may be the dominant source of 129I to the lakes.

Iodine-129 enrichment in sediment of the Baltic Sea by A. Aldahan; E. Englund; G. Possnert; I. Cato; X.L. Hou (637-647).
Sediments are an excellent archive for evaluation of time-series environmental contamination of water systems. Measurements of ultra-trace radioactive species, such as 129I, provide information for both chronologic calibration and anthropogenic emissions during the nuclear era. Here data are presented on 129I and other chemical parameters from two sediment cores collected in the Baltic Sea during 1997. The sediment sections have a relatively uniform grain size (clay–mud) and cover a period of about 50 a. Distribution of 129I in the sediment strongly relates to the liquid release records from the nuclear reprocessing facilities at Sellafield and La Hague. However, syn- and post-depositional alteration of organic matter at the sediment–water interface and within the sediment column may have contributed to slightly obliterating the anthropogenic 129I signals. Indication of Chernobyl-derived 129I occurs in the sediment profile, but is apparently overridden by the overwhelming flux from the nuclear reprocessing facilities. Although the record did not cover the pre-nuclear era (before 1945) sections, the ultra sensitive 129I profile provides a potential tool for relative dating and monitoring sources of water and sediment to the region.

Analysis of 129I in groundwater samples: Direct and quantitative results below the drinking water standard by Christopher F. Brown; Keith N. Geiszler; Michael J. Lindberg (648-655).
This paper highlights an analytical method based on mass measurement that can be used to directly quantify 129I in groundwater samples at concentrations below the maximum contaminant level (MCL) without the need for sample pre-concentration or extraction. Samples were analyzed on a Perkin Elmer ELAN DRC II ICP-MS after minimal dilution using O2 as the reaction gas. Analysis of continuing calibration verification standards indicated that the dynamic reaction cell (DRC) mode could be used for quantitative analysis of 129I in samples below the MCL (0.0057 ng/mL or 1 pCi/L). The low analytical detection limit of 129I analysis in the DRC mode coupled with minimal sample dilution (1.02x) resulted in a final estimated quantification limit of 0.0051 ng/mL. Subsequent analysis of three groundwater samples containing 129I resulted in fully quantitative results in the DRC mode, and spike recovery analyses performed on all three samples confirmed that the groundwater matrix did not adversely impact the analysis of 129I in the DRC mode. This analytical approach has been proven to be a cost-effective, high-throughput technique for the direct, quantitative analysis of 129I in groundwater samples at environmentally relevant concentrations that reach below the current MCL.

The authors report here halogen concentrations in pore waters and sediments collected from the Mallik 5L-38 gas hydrate production research well, a permafrost location in the Mackenzie Delta, Northwest Territories, Canada. Iodine and Br are commonly enriched in waters associated with CH4, reflecting the close association between these halogens and source organic materials. Pore waters collected from the Mallik well show I enrichment, by one order of magnitude above that of seawater, particularly in sandy layers below the gas hydrate stability zone (GHSZ). Although Cl and Br concentrations increase with depth similar to the I profile, they remain below seawater values. The increase in I concentrations observed below the GHSZ suggests that I-rich fluids responsible for the accumulation of CH4 in gas hydrates are preferentially transported through the sandy permeable layers below the GHSZ. The Br and I concentrations and I/Br ratios in Mallik are considerably lower than those in marine gas hydrate locations, demonstrating a terrestrial nature for the organic materials responsible for the CH4 at the Mallik site. Halogen systematics in Mallik suggest that they are the result of mixing between seawater, freshwater and an I-rich source fluid. The comparison between I/Br ratios in pore waters and sediments speaks against the origin of the source fluids within the host formations of gas hydrates, a finding compatible with the results from a limited set of 129I/I ratios determined in pore waters, which gives a minimum age of 29 Ma for the source material, i.e. at the lower end of the age range of the host formations. The likely scenario for the gas hydrate formation in Mallik is the derivation of CH4 together with I from the terrestrial source materials in formations other than the host layers through sandy permeable layers into the present gas hydrate zones.

Fluid migration in subduction zones is one of the key phenomena to understand the global mass transfer system. While active volcanoes provide the most recognizable conduits for fluid flow in active margins, the existence of a large number of active fluid seepages demonstrates that other forms of fluid release are also important in subduction zone settings. The authors collected fluid samples from springs and wells across the forearc area in Kyushu, a southwestern island of Japan, covering hot spring activities associated with active volcanism and the Median Tectonic Line (MTL), a major fault system present in the southwestern part of Japan. In order to determine sources of these fluids, halogen concentrations as well as 129I/I and 36Cl/Cl ratios were measured in samples from several locations. While Cl concentrations of the forearc fluids in Kyushu range between seawater and meteoric water value, I concentrations are considerably higher than seawater value. Fluids in the Miyazaki area are much higher in I, and somewhat higher in Br, than waters in the Oita area, which is closely associated with the MTL. The differences between those two areas are also pronounced in 129I/I ratios, which range between 800 and 900 × 10−15 in the Oita area and between 100 and 360 × 10−15 in the Miyazaki area. The 129I/I ratios obtained from the Oita area are compatible with an I derivation from subducting marine sediments, similar to findings from an earlier investigation of fluids collected from Satsuma-Iwojima, an active volcano south of Kyushu Island. In the Miyazaki area, on the other hand, I ages are too old to be derived from currently subducting marine sediments and point to a derivation from old organic-rich materials in the upper plate of the forearc region. The results demonstrate the presence of very different fluid systems in the forearc area of Kyushu: old CH4-rich fluids dominate in the seaward side of the forearc, while fluids close to the MTL and the Quaternary Volcanic Front demonstrate derivations from subducting marine sediments. The latter fluids in the MTL area probably are transported through the fractures associated with the fault activities, suggesting that this fault system reaches the transition zone between upper and lower plates in this region.

The long-lived halogen radioisotopes 129I and 36Cl provide valuable information regarding the source of fluids in hydrocarbon systems and in localized areas where infiltration of younger meteoric water has occurred. Despite the utility of these two isotopes in providing time-signatures for fluid end-members, considerable uncertainty remains regarding the interpretation of “intermediate-age” waters in hydrologic systems. These waters are likely the result of the combination of two or more halogen sources at some time in the past, each with its own characteristic concentration and isotopic composition. In order to unravel the evolution of these “intermediate-age” waters, the effect that infiltration of meteoric water has on the isotopic composition of older formation waters is modeled. Also evaluated is the effect that the timing of dilution has on 129I and 36Cl signatures observed in the present, specifically, the hypothesis that halogen isotopic signatures imparted by the mixing of brine and meteoric waters early in the development of a sedimentary basin are quantitatively different from those imparted by the mixing of old brines with recent meteoric waters.The modeled results are compared to previously published isotopic data from production wells in the Fruitland Formation coalbed methane system. Portions of the basin preserve enough of the original brine source to retain 129I/I ratios roughly recording the age of deposition of organic matter (73–76 Ma) even where some degree of dilution by meteoric water has occurred. However, the 129I signatures of waters that presently contain less than 10% of the original formation water component indicate that mixing has occurred within the past 10 Ma, consistent with recently published 4He data. Both 36Cl concentrations and 36Cl/Cl ratios show that waters containing more than 10% formation water are all in secular equilibrium with the host formation coals. In contrast, waters with less than 1% of the original formation water’s Cl content all have 36Cl/Cl ratios which are greater than secular equilibrium values, indicating the migration of meteoric water into the coal formation more recently than 2 Ma. Although 36Cl signatures in the Fruitland Formation are not significantly affected by anthropogenic input, 129I/I ratios in a subset of samples that retain less than 1% of the initial formation water component suggest input of anthropogenic 129I during the past 50 a.