Applied Geochemistry (v.20, #4)

5th International Symposium on Applied Isotope Geochemistry by Anita S. Andrew; Suzanne D. Golding (671-672).

Sulfur mobility in peat by Martin Novák; Marie Adamová; R. Kelman Wieder; Simon H. Bottrell (673-681).
Lead-210 chronologies, vertical S concentration gradients and δ 34S values are presented for 5 Sphagnum-dominated peat bogs located in Central Europe (Rybarenska slat and Ocean Bog; Czech Republic) and the British Isles (Thorne Moors, England; Connemara, Ireland; and Mull, Scotland). Sulfur concentrations were measured in three 40-cm deep peat cores per site, sectioned into 2-cm segments. The coefficient of variation in S concentrations was low across all depths and sites (mean of 16%), indicating a high degree of within-site homogeneity in vertical S patterns. Similar S concentration trends and similar δ 34S trends were found at all study sites. With an increasing peat depth, S concentrations first increased and then decreased. S concentrations peaked in layers which were deposited in ca. 1959, 1907, 1945, 1899 and 1799 at Rybarenska slat, Ocean, Thorne Moors, Connemara and Mull, respectively. Atmospheric S deposition peaked in 1972 in the UK and in 1987 in the Czech Republic. Due to downward S mobility in peat, S concentration maxima were found in layers 59 (±19) a older that the year of the actual peak in S input. With an increasing depth, the maturating peat substrate at all sites exhibited first a negative δ 34S shift, resulting from dissimilatory bacterial SO4 reduction, and then a positive δ 34S shift, which may be related to advancing S mineralization. Minimum δ 34S values were detected in layers which were deposited in ca. 1988, 1982, 1945, 1940 and 1978. A comparison of historical δ 34S signatures of atmospheric S in England, measured on archived grain from the Broadbalk experiment (1845–1994), with δ 34S values of Thorne Moors peat (1830–1994) also indicated mobility of S in peat. Sulfur mobility in water-logged peat is of concern during the present period of easing industrial pollution because SO4 released from peatlands may increase the acidity of the output.

Methane oxidation: isotopic enrichment factors in freshwater boreal reservoirs by Jason J. Venkiteswaran; Sherry L. Schiff (683-690).
Methane oxidation plays a vital role in controlling the flux of CH4 from many ecosystems. Release of the green house gas CH4 to the atmosphere during creation and operation of hydroelectric reservoirs is of concern because of the dramatic changes in C and nutrient cycling that result from flooding. Experimentally flooded reservoirs in the boreal forest at the Experimental Lakes Area, northwestern Ontario, Canada, have been under study for a decade. In these large-scale ecosystem experiments, stable C isotopic ratios are used to determine the importance of CH4 oxidation but quantification requires knowledge of the C isotope enrichment factor associated with CH4 oxidation under the appropriate environmental conditions. Laboratory incubations were used to assess the CH4 oxidation enrichment factors in 3 experimental boreal reservoirs with different soil and vegetation, and flood histories. As a result of flooding, new flooded surfaces were created with different temperature and hydrologic regimes and the importance of CH4 oxidation in controlling the flux of CH4 to the atmosphere changed significantly. However, isotopic ratio data from different systems could not be compared directly because the enrichment factor changed between systems. The enrichment factor in a flooded boreal wetland ecosystem (ELARP) decreased with temperature and the rate of CH4 oxidation increased with temperature. This was in contrast with two flooded upland boreal forest reservoirs (Flooded Upland Dynamics Experiment) where the enrichment factor was smaller than in ELARP and there was little or no temperature effect on the enrichment factors or rates of CH4 oxidation.

Following summer drought periods, pulses of elevated SO 4 2 - concentrations are frequently observed in streams draining forested catchments that contain wetlands. Delays in the recovery of freshwater streams and lakes in eastern Canada from historically high levels of acidic precipitation have been partially ascribed to these periodic pulses of SO 4 2 - . Climate in eastern Canada has also changed within the last 25 a, with a documented increase in summer dryness and duration of droughts.In small forested catchments in the Turkey Lakes Watershed (TLW), SO 4 2 - concentrations in streams draining wetlands can be elevated by up to a factor of 7 during post-drought discharge events compared to the annual average. Two neighbouring catchments, one with a series of cascading wetlands and one without any wetlands, were selected for comparison. Stable S and O isotope ratios were analyzed in samples of bulk precipitation, streams, and groundwaters to examine sources of SO 4 2 - in post-drought pulses. δ 34 S – SO 4 2 - in the streams and groundwaters show that SO 4 2 - is retained in the wetland via SO 4 2 - reduction and stored in the upper peat profile. Nitrate is elevated in soil and groundwaters at TLW due to high rates of nitrification in forest soils and the presence of NO 3 - can be used to identify piezometers unaffected by SO 4 2 - reduction. δ 18 O – SO 4 2 - shows that higher concentrations of SO 4 2 - in deeper groundwater are likely due to oxidation of organic S and not a geologic source of reduced S. Following drought, the low δ 34 S – SO 4 2 - in streams is consistent with wetland retention by SO 4 2 - reduction and much lower than SO 4 2 - released by weathering in deep glacial till and bedrock. High SO 4 2 - groundwaters and geologic sources do not contribute to the SO 4 2 - pulses in streams. Isotopic patterns over 6 a were similar. Pulses of SO 4 2 - in the wetland catchments following drought are a result of the oxidation of S previously reduced and stored in the wetland.

Modern flood protection projects are often combined with measures for river restoration, which enlarge the river bed to improve the flow capacity during peak discharge. For the planning of such projects it is essential to quantify the river–groundwater exchange. To address this question in the highly regulated upper Rhone River basin, a combination of stable isotope techniques with geochemical and transient tracers has been used. The δ 18O signal in precipitation decreases towards more negative values with a slope of 0.34‰ per 100 m altitude, precipitation during winter was about 5.5‰ more negative than in summer. Since in winter about 55% of the water in the River Rhone comes from high alpine hydropower reservoirs with a known δ 18O value, this isotopic signature provides direct information of the source region and the seasonality in samples from groundwater wells. On a spatial scale SO 4 2 - measurements help to constrain groundwater components, because the tributaries and groundwater sources south of the Rhone are rich in SO 4 2 - with concentrations of more than 12 mM in spring water. In winter the Rhone water reaches concentrations of up to 1.5 mM, and during snowmelt in summer, this value drops below 0.5 mM. Finally the transient tracer 3H/3He is used to estimate groundwater inflow in deep gravel pits and to calculate an average travel velocity in the alluvial aquifer of about 1.7 km a−1.

Stable isotope characterization of porewater, and dissolved species, in mudrocks and argillaceous rocks is notoriously difficult. Techniques based on physical or chemical extraction of porewater can generate significant analytical artefacts. The authors report a novel, simple approach to determine the δ 18O of porewater and δ 13C of dissolved C in argillites. The method uses core samples placed in specifically-designed outgassing cells, sealed shortly after drilling and stored in well-controlled conditions. After 1–2 months, CO2 naturally outgassed by argillite porewater is collected, purified and analyzed for C and O isotopes. Porewater δ 18O and dissolved C δ 13C are calculated from CO2 isotope data using appropriate fractionation factors. This methodology was successfully applied to the Callovo-Oxfordian argillites from Bure (eastern Paris Basin, France) and the Opalinus Clay formation from Mont Terri (Switzerland). In both clay formations, results indicate that porewater is meteoric and dissolved C is of marine origin. The main advantage of the approach is that it does not induce any major physical or chemical disturbance to the clay–water system investigated. Further testing on argillaceous rocks of variable composition and organic content is needed to assess extent of applicability.

Origin of salinity in produced waters from the Palm Valley gas field, Northern Territory, Australia by Anita S. Andrew; David J. Whitford; Martin D. Berry; Stuart A. Barclay; Angela M. Giblin (727-747).
The chemical composition and evolution of produced waters associated with gas production in the Palm Valley gas field, Northern Territory, has important implications for issues such as gas reserve calculations, reservoir management and saline water disposal. The occurrence of saline formation water in the Palm Valley field has been the subject of considerable debate. There were no occurrences of mobile water early in the development of the field and only after gas production had reduced the reservoir pressure, was saline formation water produced. Initially this was in small quantities but has increased dramatically with time, particularly after the initiation of compression in November 1996.The produced waters range from highly saline (up to 300,000 mg/L TDS), with unusual enrichments in Ca, Ba and Sr, to low salinity fluids that may represent condensate waters. The Sr isotopic compositions of the waters (87Sr/86Sr = 0.7041–0.7172) are also variable but do not correlate closely with major and trace element abundances. Although the extreme salinity suggests possible involvement of evaporite deposits lower in the stratigraphic sequence, the Sr isotopic composition of the high salinity waters suggests a more complex evolutionary history.The formation waters are chemically and isotopically heterogeneous and are not well mixed. The high salinity brines have Sr isotopic compositions and other geochemical characteristics more consistent with long-term residence within the reservoir rocks than with present-day derivation from a more distal pool of brines associated with evaporites. If the high salinity brines entered the reservoir during the Devonian uplift and were displaced by the reservoir gas into a stagnant pool, which has remained near the reservoir for the last 300–400 Ma, then the size of the brine pool is limited. At a minimum, it might be equivalent to the volume displaced by the reservoired gas.

A synthesis of Sr isotope data from shallow and deep groundwaters, and brines from the Fennoscandian and Canadian Shields is presented. A salinity gradient is evident in the water with concentrations varying from approximately 1–75 g L−1 below 1500 m depth in the Fennoscandian Shield and from 10 up to 300 g L−1 below 650 m depth in the Canadian Shield. Strontium isotope ratios were measured to assess the origin of the salinity and evaluate the degree of water–rock interaction in the systems. In both shields, the Sr concentrations are enriched relative to Cl, defining a positive trend parallel to the seawater dilution line and indicative of Sr addition through weathering processes. The depth distribution for Sr concentration increases strongly with increasing depth in both shields although the variation in Sr-isotope composition does not mirror that of Sr concentrations. Strontium-isotope compositions are presented for surface waters, and groundwaters in several sites in the Fennoscandian and Canadian Shields. Numerous mixing lines can be drawn reflecting water–rock interaction. A series of calculated lines links the surface end-members (surface water and shallow groundwater) and the deep brines; these mixing lines define a range of 87Sr/86Sr ratios for the deep brines in different selected sites. All sites show a specific 87Sr/86Sr signature and the occurrence of large 87Sr/86Sr variations is site specific in both shields. In Canadian Shield brines, the Sr isotope ratios clearly highlight large water rock interaction that increases the 87Sr/86Sr ratio from water that could have been of marine origin. In contrast to the Canadian Shield, groundwater does not occur in closed pockets in the Fennoscandian, and the well-constrained 87Sr/86Sr signatures in deep brines should correspond to a large, well-mixed and homogeneous water reservoir, whose Sr isotope signature results from water–rock interaction.

The supergiant Pb–Zn–Ag Broken Hill orebody and numerous other minor mineral deposits occur within the limited outcrop of the Proterozoic Curnamona Province of Australia. The vast majority of this Province is concealed by up to 200 m of transported regolith, hampering conventional exploration strategies. Approximately 300 groundwater samples were collected over the southern Curnamona Province to test whether this medium could be helpful in the search for hidden mineral deposits. Sulphur, Sr and Pb isotope composition of the groundwaters were determined and S excess (SXS), i.e., the amount of S that can be ascribed neither to evaporation nor to mixing, was calculated. Many samples were recognised to have undergone an addition of 34S-depleted S, which can be attributed to oxidation of sulfides with a Broken Hill type δ 34S signature (average ∼0‰ V-CDT). Furthermore, Sr isotopes identify the broad types of bedrock that the groundwater has been interacting with, from the less radiogenic Adelaidean rocks (and minerals) in the west (groundwater 87Sr/86Sr ratio as low as 0.708) to the highly radiogenic Willyama Supergroup in the east (87Sr/86Sr ratio up to 0.737). The groundwaters have 207Pb/204Pb and 206Pb/204Pb ratios comparable to, or intermediate between, various mineralisation types recognised in the area (Broken Hill, Rupee, Thackaringa, etc., types). The few samples taken in the vicinity of known mineralisation yield positive indicators (positive SXS, low δ 34S, 87Sr/86Sr signature of bedrock type and Pb isotope fingerprinting of mineralisation type). This study also highlights several new locations under sedimentary cover where these indicators suggest interaction with mineralisation.

Mineralogical, hydrochemical and S isotope data were used to constrain hydrogeochemical processes that produce acid mine drainage from sulfidic waste at the historic Mount Morgan Au–Cu mine, and the factors controlling the concentration of SO4 and environmentally hazardous metals in the nearby Dee River in Queensland, Australia. Some highly contaminated acid waters, with metal contents up to hundreds of orders of magnitude greater than the Australia–New Zealand environmental standards, by-pass the water management system at the site and drain into the adjacent Dee River.Mine drainage precipitates at Mt. Morgan were classified into 4 major groups and were identified as hydrous sulfates and hydroxides of Fe and Al with various contents of other metals. These minerals contain adsorbed or mineralogically bound metals that are released into the water system after rainfall events. Sulfate in open pit water and collection sumps generally has a narrow range of S isotope compositions (δ 34S = 1.8–3.7‰) that is comparable to the orebody sulfides and makes S isotopes useful for tracing SO4 back to its source. The higher δ 34S values for No. 2 Mill Diesel sump may be attributed to a difference in the source. Dissolved SO4 in the river above the mine influence and 20 km downstream show distinctive heavier isotope compositions (δ 34S = 5.4–6.8‰). The Dee River downstream of the mine is enriched in 34S (δ 34S = 2.8–5.4‰) compared with mine drainage possibly as a result of bacterial SO4 reduction in the weir pools, and in the water bodies within the river channel. The SO4 and metals attenuate downstream by a combination of dilution with the receiving waters, SO4 reduction, and the precipitation of Fe and Al sulfates and hydroxides. It is suggested here that in subtropical Queensland, with distinct wet and dry seasons, temporary reducing environments in the river play an important role in S isotope systematics.

A rapid and precise procedure for Pb isotopes in whole blood by Fe co-precipitation and MC-ICPMS analysis by Steven N. Chillrud; N. Gary Hemming; James M. Ross; Sean Wallace; Nancy LoIacono (807-813).
Elevated Pb levels in humans through environmental exposure are a significant health concern requiring scientific study of the sources of, and physiological response to this toxin. This requires a simple and precise method for measuring radiogenic Pb isotopes and Pb levels in blood. Presented here is a combination of methods for separation and analysis of Pb previously used predominantly for geologic samples. This includes separation of Pb from the complex matrix of blood samples using an Fe co-precipitation method, followed by isotopic analysis by multi-collector inductively coupled plasma mass spectrometry. Evaluation of the efficacy of this procedure shows that the precision of sample preparations as measured by % difference between the 207Pb/206Pb of duplicate analyses averages 0.064% (n  = 48). Using the same preparation and analysis techniques to measure Pb concentrations by isotope dilution resulted in a reproducibility of better than 6%. The method was successfully used to measure uptake of ingested soil Pb in a study of the bioavailability of Pb in contaminated soils.