Search

The Upper Jurassic aquifer (UJA) within the South German Molasse Basin (SGMB) is the most important exploration horizon for geothermal energy supply in Bavaria. The UJA shows a heterogeneous geology with karstic features and deep fault-zones. The result is a complex hydrogeology consisting of different groundwater types which differ significantly in their hydrochemical and isotopic composition.Despite the great interest in the Upper Jurassic aquifer for geothermal energy supply, leading to numerous scientific studies, the exact apparent groundwater age, the infiltration area and the regional flow system remain yet unknown.In this study we are using a multi parameter approach for the determination of apparent groundwater age distributions with the innovative 14CDOC and 81Kr methods and combine them with hydrochemistry data and stable water isotopes (δ18O/δ2H).Our results indicate that the UJA system consists of at least two groundwater components: an up to now unknown young meteoric water component from the Pleistocene/Holocene transition and an older Pleistocene component. The apparent 14CDOC ages increase from south to north and show some evidence that the infiltration area of the UJA is located in the southern part of the SGMB and a groundwater flow directed to the north.

Ebullition is a major transport pathway of methane from aquatic ecosystems to the atmosphere. Several studies have highlighted that methane ebullition plays an important role in reservoirs, but not much data is available from non-impounded stream sections. Quantifying river methane emissions on a global scale is a challenge due to the high spatiotemporal heterogeneity in these dynamic and variable systems. A better conceptual understanding of riverine methane ebullition is needed for sound carbon budgets, both in terms of flux volumes and predictive factors for hot-spot emission zones.This study targeted the knowledge gap in riverine methane ebullition with a time-resolved observation-based analysis of the ebullitive transportation pathway in a river cross section. We installed four bubble traps in a small stream in southern Germany and monitored volumes and concentrations of the two greenhouse gases CO2 and CH4 along with carbon stable isotopes (δ13C) of CH4 over the course of a year. The bubble traps were evenly distributed over one cross-section in a curve to represent different sediment-compositions and flow regimes between the undercut slope and slip-off slope. Sediment characterization at each site included grain-size distribution curves, porosity measurements and determination of organic carbon content.Ebullitive gas fluxes were extremely high at two of the locations centrally in the river: up to 1000 ml m-2 d-1 during summer and autumn, and 100-400 ml m-2 d-1 in December. CH4 concentrations of up to 65% were measured in the gas samples and CH4 exceeded CO2 concentrations (

The hyporheic zone (HZ) is of major importance for carbon and nutrient cycling as well as for the ecological health of stream ecosystems. However, biogeochemical observations in this ecotone are complicated by a very high spatial heterogeneity and temporal dynamics. Especially the latter are difficult to observe without disturbing the system. In this field study, we tested and combined three less common methods for time-resolved measurements with high vertical resolution. We installed Rhizon samplers for repeated pore-water extraction, an optical sensor unit for in-situ measurements of dissolved oxygen, and a depth-resolved temperature monitoring system in the HZ of a small stream. While Rhizon samplers were found to be highly suitable for pore-water sampling of dissolved solutes, measured gas concentrations, here CH4, showed a strong dependency of the pump rate during sample extraction, and an isotopic shift in gas samples became evident. This was presumably caused by a different behaviour of water and gas phase in the pore-space. The manufactured oxygen-sensor could locate the oxic-anoxic interface with very high precision. This is ecologically important and allows to distinguish aerobic and anaerobic processes. Temperature data could not only be used to estimate vertical hyporheic exchange, but also depicted sedimentation and erosion processes. Overall, the combined approach was found to be a promising tool to acquire data for the quantification of biogeochemical processes in the HZ with high spatial and temporal resolution.

Facing the challenges of climate change, policy making relies on sound greenhouse gas (GHG) budgets. Rivers and streams emit large quantities of the potent GHG methane (CH4), but their global impact on atmospheric CH4 concentrations is highly uncertain. In situ data from the hyporheic zone (HZ), where most CH4 is produced and some of it can be oxidized to CO2, are lacking for an accurate description of CH4 production and consumption in streams. To address this, we recorded high-resolution depth-resolved geochemical profiles at five different locations in the stream bed of the river Moosach, southern Germany. Specifically, we measured pore-water concentrations and stable carbon isotopes (δ13C) of dissolved CH4 as well as relevant electron acceptors for oxidation with a 1 cm vertical depth resolution. Findings were interpreted with the help of a numerical model, and 16S rRNA gene analyses added information on the microbial community at one of the locations. Our data confirm with pore-water CH4 concentrations of up to 1000 µmol L−1 that large quantities of CH4 are produced in the HZ. Stable isotope measurements of CH4 suggest that hydrogenotrophic methanogenesis represents a dominant pathway for CH4 production in the HZ of the river Moosach, while a relatively high abundance of a novel group of methanogenic archaea, the Candidatus “Methanomethyliales” (phylum Candidatus “Verstraetearchaeota”), indicate that CH4 production through H2-dependent methylotrophic methanogenesis might also be an important CH4 source. Combined isotopic and modeling results clearly implied CH4 oxidation processes at one of the sampled locations, but due to the steep chemical gradients and the close proximity of the oxygen and nitrate reduction zones, no single electron acceptor for this process could be identified. Nevertheless, the numerical modeling results showed potential not only for aerobic CH4 oxidation but also for anaerobic oxidation of CH4 coupled to denitrification. In addition, the nitrate–methane transition zone was characterized by an increased relative abundance of microbial groups (Crenothrix, NC10) known to mediate nitrate and nitrite-dependent methane oxidation in the hyporheic zone. This study demonstrates substantial CH4 production in hyporheic sediments, a potential for aerobic and anaerobic CH4 oxidation, and underlines the high spatiotemporal variability in this habitat.

Characterizing unsaturated water flow in the subsurface is a requirement for understanding effects of droughts on agricultural production or impacts of climate change on groundwater recharge. By employing an improved lumped-parameter model (LPM) approach that mimics variable flow we have interpreted stable water isotope data (δ

Rivers and streams are often supersaturated in methane (CH4) and emit significant amounts of this potent greenhouse gas to the atmosphere. Methane is produced by methanogenic archaea in anaerobic sediments where energetically more favorable redox processes are substrate-limited. Diffusing up towards the sediment surface, methane can be oxidized in the hyporheic zone (HZ) aerobically with oxygen or anaerobically with nitrate, nitrite, sulfate or iron and manganese oxides as electron acceptors. Yet, knowledge about net carbon emissions from streams is restricted, because high spatial heterogeneities in production and consumption zones make bottom-up global estimations particularly challenging.In this study we want to increase process understanding of riverine methane cycling, in particular production and oxidation in hyporheic stream sediments. Several studies have investigated predictors for potential methane production and oxidation in river sediments using incubation experiments. We chose a different approach by measuring high-resolution depth-dependent geochemical profiles at five different locations across a stream bed. An in-situ equilibrium dialysis sampler (peeper) was used to obtain pore-water samples with a 1 cm depth-resolution for the measurement of dissolved oxygen, relevant anion and cation concentrations as well as methane concentrations and stable carbon isotopes (δ13C) of methane. In the methanogenic zone stable carbon isotopes may provide information about the most relevant methane production pathway, while an isotopic enrichment in δ13C-CH4 towards the sediment surface linked with decreasing methane concentrations may indicate microbial degradation. Production and oxidation rates were estimated using inverse numerical modeling of measured concentration gradients. Additionally, the concentration of 16S rRNA genes (a measure of bacteria biomass) was quantified from one of the locations using quantitative PCR, which revealed an increase in microbial biomass at the nitrate-methane transition zone. Sequencing of the 16S rRNA genes shows clear shifts in microorganisms driving the streambed methane cycle at and below the nitrate-methane transition zone.The measured δ13C-CH4 values between -75 ‰ and -70 ‰ indicate that hydrogenotrophic methanogenesis is the dominant production pathway. However, we found that methane fluxes into the surface water were low. Mainly responsible for a strong decrease in methane concentrations towards the sediment surface were most likely diffusive processes. Decreasing methane concentrations linked with a significant enrichment in δ13C towards heavier isotopes was only observed at one of the sampling locations. The isotopic shift together with modeling results and microbiological analyses may reveal microbially driven aerobic and anaerobic methane oxidation in the HZ, but in most locations methane is only oxidized at low rates where the environment is already methane depleted by diffusion. Limiting for both aerobic and anaerobic methane oxidation was supposedly the low methane concentration in zones with available electron acceptors.

Globally occurring nitrate pollution in groundwater is harming the environment and human health. In situ hydrogen addition to stimulate denitrification has been proposed as a remediation strategy. However, observed nitrite accumulation and incomplete denitrification are severe drawbacks that possibly stem from the specific microbial community composition. We set up a microcosm experiment comprising sediment and groundwater from a nitrate polluted oxic oligotrophic aquifer. After the microcosms were sparged with hydrogen gas, samples were taken regularly within 122 h for nitrate and nitrite measurements, community composition analysis via 16S rRNA gene amplicon sequencing and gene and transcript quantification via qPCR of reductase genes essential for complete denitrification. The highest nitrate reduction rates and greatest increase in bacterial abundance coincided with a 15.3-fold increase in relative abundance of Rhodocyclaceae, specifically six ASVs that are closely related to the genus Dechloromonas. The denitrification reductase genes napA, nirS and clade I nosZ also increased significantly over the observation period. We conclude that taxa of the genus Dechloromonas are the prevailing hydrogenotrophic denitrifiers in this nitrate polluted aquifer and the ability of hydrogenotrophic denitrification under the given conditions is species-specific.

Nitrate pollution of freshwaters and methane emissions into the atmosphere are crucial factors in deteriorating the quality of drinking water and in contributing to global climate change. The n-damo (nitrite-dependent anaerobic methane oxidation), nitrate-dependent anaerobic methane oxidation and the anaerobic oxidation of ammonium (anammox) represent two microbially mediated processes that can reduce nitrogen loading of aquatic ecosystems and associated methane emissions to the atmosphere. Here, we report vertical concentration and stable-isotope profiles of CH4, NO3-, NO2-, and NH4+ in the water column of Fohnsee (lake in southern Bavaria, Germany) that may indicate linkages between denitrification, anaerobic oxidation of methane (AOM), and anammox. At a water depth from 12 to 20 m, a methane–nitrate transition zone (NMTZ) was observed, where δ13C values of methane and δ15N and δ18O of dissolved nitrate markedly increased in concert with decreasing concentrations of methane and nitrate. These data patterns, together with the results of a simple 1-D diffusion model linked with a degradation term, show that the nonlinear methane concentration profile cannot be explained by diffusion and that microbial oxidation of methane coupled with denitrification under anaerobic conditions is the most parsimonious explanation for these data trends. In the methane zone at the bottom of the NMTZ (20 to 22 m) δ15N of ammonium increased by 4 ‰, while ammonium concentrations decreased. In addition, a strong 15N enrichment of dissolved nitrate was observed at a water depth of 20 m, suggesting that anammox is occurring together with denitrification. The conversion of nitrite to N2 and nitrate during anammox is associated with an inverse N isotope fractionation and may explain the observed increasing offset (Δδ15N) of 26 ‰ between δ15N values of dissolved nitrate and nitrite at a water depth of 20 m compared to the Δδ15Nnitrate-nitrite of 11 ‰ obtained in the NMTZ at a water depth between 16 and 18 m. The associated methane concentration and stable-isotope profiles indicate that some of the denitrification may be coupled to AOM, an observation supported by an increased concentration of bacteria known to be involved in n-damo/denitrification with AOM (NC10 and Crenothrix) and anammox (“Candidatus Anammoximicrobium”) whose concentrations were highest in the methane and ammonium oxidation zones, respectively. This study shows the potential for a coupling of microbially mediated nitrate-dependent methane oxidation with anammox in stratified freshwater ecosystems, which may be important for affecting both methane emissions and nitrogen concentrations in lakes.

Fault zones (FZ) are major components of geothermal exploration concepts for the Southern German Upper Jurassic aquifer (UJA). Because these sections of possibly favorable hydraulic properties can be hidden in pumping test data, their explorational importance with respect to well productivity is still debated. In this work, the effect of hydraulically active FZ on the well-productivity-index (PI) is quantified in dependency of the UJA parameter space using numerical modeling and the Reduced Basis method. Results suggest that hidden FZ can increase PI significantly, while a hydraulically active FZ thickness of ≥ 100 m is unlikely for the Upper Jurassic aquifer.

Heat transport in natural porous media, such as aquifers or streambeds, is generally modeled assuming local thermal equilibrium (LTE) between the fluid and solid phases. Yet, the mathematical and hydrogeological conditions and implications of this simplification have not been fully established for natural porous media. To quantify the occurrence and effects of local thermal disequilibrium during heat transport, we systematically compared thermal breakthrough curves from a LTE with those calculated using a local thermal nonequilibrium (LTNE) model, explicitly allowing for different temperatures in the fluid and solid phases. For the LTNE model, we developed a new correlation for the heat transfer coefficient representative of the conditions in natural porous aquifers using six published experimental results. By conducting an extensive parameter study (>50,000 simulations), we show that LTNE effects do not occur for grain sizes smaller than 7 mm or for groundwater flow velocities that are slower than 1.6 m day(-1). The limits of LTE are likely exceeded in gravel aquifers or in the vicinity of pumped bores. For such aquifers, the use of a LTE model can lead to an underestimation of the effective thermal dispersion by a factor of up to 30 or higher, while the advective thermal velocity remains unaffected for most conditions. Based on a regression analysis of the simulation results, we provide a criterion which can be used to determine if LTNE effects are expected for particular conditions.

Intense farming is often associated with the excessive use of manure or fertilizers and the subsequent deterioration of the groundwater quality in many aquifers worldwide. Stable isotopes of dissolved nitrate (δ

A complete hydrogeological understanding of the deep Upper Jurassic carbonate aquifer in the South German Molasse Basin is essential for the future development of this important drinking water resource and geothermally used system. Water chemistry data, δ13CDIC, 14C of the dissolved inorganic carbon (14CDIC) and stable water isotope (δ18O and δD) measurements have been used to evaluate a promising groundwater dating approach with 14C of dissolved organic carbon (14CDOC). The pre-concentration of dissolved organic matter (DOM) was performed by the easy applicable solid phase extraction (SPE) with a styrene-divinylbenzene copolymer sorbent (PPL). Based on the sampling campaign of seven groundwater wells conducted between 2017 and 2019, it was shown that the groundwater is mainly of Ca–HCO3 type with some evidence of ion exchange between Ca2+ and Na+ at two of the investigated wells. The δD values ranged from −89.4‰ to −70.9‰ while δ18O values varied between −12.5‰ and −9.8‰. The obtained stable water isotope signatures indicated that the groundwater is of meteoric origin and was recharged during warm climate (Holocene), intermediate climate and cold climate (Pleistocene) infiltration conditions. The measured 14CDOC activities varied from 5.7 pmC to 51.1 pmC and the calculated piston-flow water ages (ORAs) ranged from 4200 years to 25,248 years using an initial 14C0DOC of 85 pmC. The calculated ORAs showed a very good correlation to the infiltration temperature-sensitive δ18O values which were affirmed with noble gas infiltration temperatures for two wells after Weise et al. (1991) and were also in good accordance with the atmospheric temperature record of the northern hemisphere from Dokken et al. (2015). The results reflect a consistent hydrogeological picture of the carbonate aquifer, which also supports the applicability of the SPE-PPL method for 14CDOC dating in groundwater with a low DOC content ( 30,000 years and generally to an overestimation of the apparent water ages. This study gives insights into the promising approach of 14CDOC groundwater dating in carbonate aquifers with low DOC contents and allows future sustainable groundwater resource management of the investigated aquifer system.

Intense farming is often associated with the excessive use of manure or fertilizers and the subsequent deterioration of the groundwater quality in many aquifers worldwide. Stable isotopes of dissolved nitrate (δ15 N and δ18 O) are widely used to determine sources of nitrate contamination and denitrification processes in groundwater but are often difficult to interpret. Thus, Monte Carlo simulations were carried out for a site in lower Bavaria, Germany, in order to explain δ15 N observations in a porous groundwater system with two aquifers, the main aquifer (MA) and several smaller perched aquifers (PA). For evaluating potential contributions, frequency distributions of δ15 N were simulated deriving from (I) the mixing of different nitrate sources, related to land use, as input to groundwater, combined with (II) transport of nitrate in groundwater and (III) microbial denitrification. Simulation results indicate a source-driven isotopic shift to heavier δ15 N values of nitrate in groundwater, which may be explained by land use changes toward a more intensified agriculture releasing high amounts of manure. Microbial denitrification may play a role in the PA, with simulated δ15 N distributions close to the observations. Denitrification processes are however unlikely for the MA, as reasonable simulation curve fits for such a scenario were obtained predominantly for unrealistic portions of nitrate sources and related land use. The applied approach can be used to qualitatively and quantitatively evaluate the influence of different potential contributions, which might mask each other due to overlapping δ15 N ranges, and it can support the estimation of nitrate input related to land use.

We have used stable water isotopes (δ18 O, δ2 H) in combination with lumped-parameter modeling for characterizing unsaturated flow in two lysimeters vegetated with maize. The lysimeters contained undisturbed soil cores dominated by sandy gravel (Ly1) and clayey sandy silt (Ly2). Stable water isotopes were analyzed in precipitation and lysimeter outflow water over about 3 years. The mean transit time of water T and dispersion parameter PD , obtained from modeling, were higher for the silt soil in Ly2 than for the gravel soil in Ly1 (T of 362 vs. 129 d, PD of 0.7 vs. 0.12). The consideration of preferential flow (PF) paths could substantially improve the model curve fits, with 13 and 11% contribution of PF for Ly1 and Ly2 as best estimates. Different assumptions were compared to estimate the input function, that is, stable water isotope content in the recharging water. Using the isotopic composition of precipitation as input (no modification) resulted in reasonable model estimations. Best model fits for the entire observation were obtained by weighting the recharging isotopes according to average precipitation within periods of 3 and 6 months, in correspondence to changing vegetation phases and seasonal influences. Input functions that consider actual evapotranspiration could significantly improve modeling at some periods, however, this led to deviations between modeled and observed δ18 O at other periods. This may indicate the influence of variable flow, so that dividing the whole observation period into hydraulically characteristic sub-periods for lumped-parameter modeling (which implements steady-state flow) is recommended for possible further improvement.

Microcosm experiments with the well-studied denitrifier Thaurera aromatica show a link between a higher maximum membrane concentration (MMC) of the toxic organic solvents 1-octanol and 4-chlorophenol and a higher degree of saturation (DoS) of the fatty acids in the cell membrane. This coincides with less pronounced stable isotope fractionation during denitrification. We suggest that the change in cell membrane fluidity and the cell's stress response leads to a decrease in nitrate transport across the cell membrane and/or an increase in the relative ratio of respiratory nitrate reduction rate versus efflux of unreacted nitrate. Both models show that the apparent kinetic isotope effect (AKIE) approach unity and thus reduce the extent of the resulting stable isotope enrichment factor epsilon N-15-NO3 in dissolved nitrate during denitrification, as experimentally and mathematically shown in this study. This may lead to an underestimation of nitrate reduction determined by nitrate stable isotope analysis in aquatic habitats where various types of stresses may affect the physiology of the driving microorganisms. (C) 2018 Elsevier Ltd. All rights reserved.

Dissimilatory sulphate reduction (DSR) has been proven to be one of the most relevant redox reactions in the biodegradation of contaminants in groundwater. However, the possible role of sulphur species of intermediate oxidation state, as well as the role of potential re-oxidative sulphur cycling in biodegradation particularly at the groundwater table are still poorly understood. Here we used a combination of stable isotope measurements of SO42−, H2S, and S0 as well as geochemical profiling of sulphur intermediates with special emphasis on SO32−, S2O32−, and S0 to unravel possible sulphur cycling in the biodegradation of aromatics in a hydrocarbon-contaminated porous aquifer. By linking these results to the quantification of total bacterial rRNA genes and respiratory genes of sulphate reducers, as well as pyrotag sequencing of bacterial communities over depth, light is shed on possible key-organisms involved. Our results substantiate the role of DSR in biodegradation of hydrocarbons (mainly toluene) in the highly active plume fringes above and beneath the plume core. In both zones the concentration of sulphur intermediates (S0, SO32− and S2O32−) was almost twice that of other sampling-depths, indicating intense sulphur redox cycling. The dual isotopic fingerprint of oxygen and sulphur in dissolved sulphate suggested a re-oxidation of reduced sulphur compounds to sulphate especially at the upper fringe zone. An isotopic shift in δ34S of S0 of nearly +4‰ compared to the δ34S values of H2S from the same depth linked to a high abundance (∼10%) of sequence reads related to Sulphuricurvum spp. (Epsilonproteobacteria) in the same depth were indicative of intensive oxidation of S0 to sulphate in this zone. At the lower plume fringe S0 constituted the main inorganic sulphur species, possibly formed by abiotic re-oxidation of H2S with Fe(III)oxides subsequent to sulphate reduction. These results provide first insights into intense sulphur redox cycling in a hydrocarbon contaminant plume, which widens the perspective of redox processes and microbial interactions ongoing in contaminated aquifers.

Pure cultures of Thauera aromatica were stressed by the addition of the toxic solvents 1-octanol and 4-chlorophenol causing adaptive modifications in their cell-membrane fatty acid composition. At the same time, we investigated the isotopic fractionation in δ15N-NO3- during denitrification. We observed a change in the degree of saturation (DoS) of the bacteria as a reaction to solvent stress and a higher degree of saturation was directly correlated to the concentration of solvents in the cell membrane. The enrichment factor ε15N-NO3- during denitrification showed a linear dependency to the DoS as well as to the membrane solvent concentration. Denitrifying bacteria thus may express less negative ε15N-NO3- when they are exposed to environmental stress that changes their cell membrane composition.

The stable isotopes 15N/14N and 18O/16O of nitrate are frequently used to determine sources of nitrate and to assess denitrification processes in the environment. Nitrate isotope ratios are thought to be conservative unless involved in (bio-) chemical conversion processes. Thus, stable isotopes are considered to be a reliable tool to determine sources of nitrate in aquatic habitats even after transport and dilution has occurred. Denitrification is known to shift both isotope ratios towards higher δ-values. A fixed ratio of 0.5 for Δδ18O/Δδ15N has been proposed and has been widely used to detect denitrification in terrestrial environments, predominantly in aquifers. However, it is observed in environmental and laboratory studies that this ratio actually varies between less than 0.5 and 1 for uncertain reasons with laboratory studies usually describing a ratio close to 1. Here we report results of anoxic incubation experiments with natural populations of nitrate-reducing microorganisms using sediments from three different environments. In our experiments we used water with a δ18O in excess of 500‰ and found a microbially mediated influence of the oxygen isotopic composition of ambient water on the isotopic composition of the residual dissolved nitrate. We found up to 5.7 ± 2.3% of the oxygen-atoms in the residual dissolved nitrate was exchanged by oxygen-atoms from ambient water within the limited timeframe of the experiments. The fastest incorporation of oxygen-atoms from water into dissolved nitrate correlated with the highest intermittent nitrite concentrations observed in our experiments. In a second series of batch experiments we also found that pure cultures of the nitrite-oxidizing bacterium Nitrobacter vulgaris promoted the incorporation of oxygen atoms from ambient water into dissolved nitrate under anoxic conditions. Presumably this happens via a reoxidation of intermediary formed nitrite by the enzyme “nitrite oxidoreductase” (NXR) in concurrence with respiratory nitrate reduction. In this context, our hypothesis is a reversibility of the reactions at the NXR enzyme even in the absence of external electron acceptors for nitrite oxidation. We suggest that the presence of nitrite-oxidizing microorganisms in aquatic environments may catalyse such an incorporation of oxygen-atoms stemming from ambient water into nitrate. This process may thus mask the original δ18O value of nitrate sources during denitrification and also distort the observed enrichment of 18O that is ascribed to denitrification. Our results are highly likely an explanation of the deviation of the described variable Δδ18O/Δδ15N ratios for denitrification in terrestrial field studies from the values observed in the laboratory on pure cultures.

In batch experiments, we studied the isotope fractionation in N and O of dissolved nitrate during dentrification. Denitrifying strains Thauera aromatica and "Aromatoleum aromaticum strain EbN1" were grown under strictly anaerobic conditions with acetate, benzoate, and toluene as carbon sources. (18)O-labeled water and (18)O-labeled nitrite were added to the microcosm experiments to study the effect of putative backward reactions of nitrite to nitrate on the stable isotope fractionation. We found no evidence for a reverse reaction. Significant variations of the stable isotope enrichment factor ε were observed depending on the type of carbon source used. For toluene (ε(15)N, -18.1 ± 0.6‰ to -7.3 ± 1.4‰; ε(18)O, -16.5 ± 0.6‰ to -16.1 ± 1.5‰) and benzoate (ε(15)N, -18.9 ± 1.3‰; ε(18)O, -15.9 ± 1.1‰) less negative isotope enrichment factors were calculated compared to those derived from acetate (ε(15)N, -23.5 ± 1.9‰ to -22.1 ± 0.8‰; ε(18)O, -23.7 ± 1.8‰ to -19.9 ± 0.8‰). The observed isotope effects did not depend on the growth kinetics which were similar for the three types of electron donors. We suggest that different carbon sources change the observed isotope enrichment factors by changing the relative kinetics of nitrate transport across the cell wall compared to the kinetics of the intracellular nitrate reduction step of microbial denitrification.

The geochemistry and isotopic composition of a karstic coastal aquifer in western Ireland has shed light on the effect of sea-water/groundwater interactions on the water quality of Ireland’s Atlantic coastal zone. The use of stable isotope data from the IAEA precipitation station in Valentia, located in SW Ireland has facilitated the characterization of groundwater recharge conditions in the western part of Ireland and suggests that groundwater is mostly replenished by the isotopically light winter precipitation. The dissolved SO 4 2 - in the karstic groundwater that was collected during baseflow conditions with δ34S values between 4.6‰ and 18‰ may be composed of S stemming from three principal sources: SO 4 2 - derived from precipitation which is composed of both sea-spray S (δ34S: 20‰) and an isotopically light anthropogenic source (δ34S: 1–5‰), SO 4 2 - stemming from animal slurries (δ34S: ∼5‰), and intruding sea-water SO 4 2 - (δ34S: 20.2‰). The isotopic composition of δ18O in dissolved groundwater SO 4 2 - collected during baseflow conditions is interpreted as reflecting sea-water intrusion to the karstic coastal groundwater system. The highest δ18O values in dissolved groundwater SO 4 2 - were in samples collected near the coast (4.8 ± 0.4‰) and the lowest (2 ± 0.5‰) were collected further inland. The δ15N and δ18O values of groundwater NO 3 - were between 3.4‰ and 11.4‰ and approximately 7.7‰, respectively, and reflect geochemical conditions in the aquifer that do not promote attenuation of NO 3 - through denitrification. As a result N loading to Kinvara Bay that is controlled by submarine groundwater discharge (SGD) was calculated as 5 tons/day on average compared to an estimated N-input that derives from precipitation of approximately 2.5 tons/a. SGD into the bay may result in near coastal sea-water quality changes. These results represent one of the first studies addressing the effect of groundwater quality in Ireland on the European Atlantic coastal zone.

The extent of seawater intrusion in a coastal karstic aquifer system along the southern shore of Galway Bay, western Ireland, was investigated using spatial and temporal variations in major ion chemistry and nutrient levels. The background water was Caand bicarbonate-rich with variable concentrations of Mg. In general, higher Mg concentrations were detected in the Gort lowlands region compared with sites in the Burren, likely due to contact with dolomite layers. Nitrate concentrations ranged from 1.3mg/L to 78.5mg/L as N03, and dissolved organic carbon concentrations ranged from 0.6mg/L to 4.9mg/L, suggesting that anthropogenic contamination due to surface activities occurs at some locations in the study area. Based on salinity values, on Na and CI concentrations and on CI/Br mass ratios, five of the 24 operational wells sampled were in a saltwater-influenced zone. Three of these wells contained

Phosphorus cycling in the ocean is influenced by biological and geochemical processes that are reflected in the oxygen isotope signature of dissolved inorganic phosphate (Pi). Extending the Pi oxygen isotope record from the water column into the seabed is difficult due to low Pi concentrations and small amounts of marine porewaters available for analysis. We obtained porewater profiles of Pi oxygen isotopes using a refined protocol based on the original micro-extraction designed by Colman (2002). This refined and customized method allows the conversion of ultra-low quantities (0.5 – 1 µmol) of porewater Pi to silver phosphate (Ag3PO4) for routine analysis by mass spectrometry. A combination of magnesium hydroxide co-precipitation with ion exchange resin treatment steps is used to remove dissolved organic matter, anions, and cations from the sample before precipitating Ag3PO4. Samples as low as 200 µg were analyzed in a continuous flow isotope ratio mass spectrometer setup. Tests with external and laboratory internal standards validated the preservation of the original phosphate oxygen isotope signature (δ18OP) during micro extraction. Porewater data on δ18OP has been obtained from two sediment cores of the Moroccan margin. The δ18OP values are in a range of +19.49 to +27.30‰. We apply a simple isotope mass balance model to disentangle processes contributing to benthic P cycling and find evidence for Pi regeneration outbalancing microbial demand in the upper sediment layers. This highlights the great potential of using δ18OP to study microbial processes in the subseafloor and at the sediment water interface.

The occurrence of two pharmaceuticals, ibuprofen and diclofenac, in a vulnerable karst groundwater system was investigated. The hydrogeology of the karst system was identified by collecting (3)H samples in groundwater over 27years and by performing tracer tests. The isotopes and tracer data were interpreted by mathematical modeling to estimate the mean transit time of water and to characterize the hydrogeological flow paths in the groundwater system. By this approach, a mean (3)H transit time of 4.6 years for the fissured-porous karst aquifer was determined, whereas the fast flowing water in the conduit system showed a mean transit time of days. Both pharmaceuticals which infiltrated along sinkholes and small streams into the karst system were detected in concentrations of up to approximately 1 microg/L in effluent water of the wastewater treatment plants. Diclofenac was present in most samples collected from four springs discharging the karst groundwater to the rivers Altmühl and Anlauter in concentrations between 3.6 and 15.4 ng/L. In contrast, ibuprofen was rarely detected in groundwater. The results of this study suggest that both pharmaceuticals move into the fractured system of the karst system and go into storage. Thus dilution processes are the dominant control on the concentrations of both pharmaceuticals in the fractured system, whereas biodegradation is likely less important.

High-resolution depth-resolved monitoring was applied to groundwater and sediments samples in a tar oil contaminated aquifer. Today, it is not fully clear, whether groundwater-based lines of evidence are always sufficient to adequately assess natural attenuation (NA) potentials and processes going on in situ. Our data unveiled small-scale heterogeneities, steep physical-chemical and microbial gradients, as well as hot spots of contaminants and biodegradation in the supposedly homogeneous sandy aquifer. The comparison of basic geochemical data revealed a fairly good agreement between sediment and groundwater samples. Nevertheless, a comprehensive understanding of both BTEX and PAH distribution, as well as redox processes involving insoluble electron acceptors, i.e., iron reduction, clearly asks for consideration of both, sediment and groundwater analysis. A thin BTEX plume right below the groundwater table was visible only in groundwater, while significant amounts of PAHs were present in sediments from deeper zones of the aquifer. Indications for sulfate reduction as a dominant process involved in BTEX degradation were largely obtained from groundwater, while the role of iron reduction in degradation and possible sulfide oxidation at the capillary fringe and the upper BTEX plume fringe, as well as in deeper PAH-contaminated zones was evident from sediments. Moreover, sediment analyses were essential to meaningfully recover cell abundances, distribution, activity and composition of the bacterial community. Sediments harbored 97.7% of bacterial cells and displayed enzyme activities 5 to 6 orders of magnitude higher than groundwater samples. Bacterial community T-RFLP fingerprints revealed important distinctions, but also similarities in depth-resolved microbial community distribution in sediments and water. An apparently highly specialized degrader population was found to dominate the lower BTEX plume fringe. However, even though sediment data seemed to comprise most community information found also in groundwater, this relation did not apply vice versa. In summary, our results show that groundwater sampled at an appropriate scale may contain sufficient information to identify and localize dominant redox reactions, but clearly fails to unravel natural attenuation potentials. This clearly emphasizes the importance of both groundwater and sediment samples for truly assessing natural attenuation potentials and activities at organically contaminated aquifers.

Summary The mean transit time of groundwater is commonly expected to increase gradually with increasing depth below water table. The present study provides evidence that the theoretical distribution of transit times may be significantly altered depending on the thickness of the unsaturated zone. An unconfined porous groundwater system formed by Tertiary sediments (Test Field Scheyern close to Munich in southern Germany) is overlain by an unsaturated zone with variable thickness between 4 and 60 m. Between 1992 and 2007 the groundwater system has been repeatedly sampled for tritium contents at different depths using two high-resolved wells. Modelled tritium concentrations by using a lumped parameter approach yielded depth profiles of mean transit times of tracer. In one well the profile was characterized by two local transit time maxima, each of approximately 100 years. A moving particle approach (MPA) developed in this study was used on the streamlines between the recharge zones linked to different sampling depths in the well. This suggested that the observed transit time in the profile was mainly governed by variable travel distances of the tracer through the unsaturated zone at the points of recharge. This finding was confirmed at a second multi-level well of the test site. The lumped parameter modelling of chlorofluorocarbon data yielded lower transit times as compared to those obtained from tritium data. This effect was explained by the different behaviour of tritium and chlorofluorocarbons in the unsaturated zone. The study clearly shows that the impact of a variable thickness of the unsaturated zone may overweigh the effect of local heterogeneities. Such transit time distributions of water in porous aquifers as observed in the present study can only be achieved with the help of environmental tracer data.

Summary 3 H, δ 18 O and δ 2 H measurements on groundwater and chemical and isotope analyses on groundwater sulfate were used to determine the origin and mean transit times of groundwater and the sources affecting groundwater sulfate in a high-alpine karstic catchment area in southern Germany. Modelling results using a lumped parameter approach yielded mean transit times of water between 4 and 12 years for different karst springs. However, results obtained from δ 18 O and δ 2 H measurements on groundwater showed that a calculated mean transit time of 12 years for groundwater flow in one karst system can only be explained by mixing of young and old tritium-free ice and snowmelt water. Groundwater sulfate characterized by δ 34 S values of around 20‰ and δ 18 O values of ca. 12‰ in concert with sulfate concentrations of approximately 11 mg/L are probably affected by evaporites. In contrast, δ 34 S values of around 6‰ and δ 18 O values up to 9.4‰ in concert with sulfate concentrations between 1 and 7 mg/L mainly derive from atmospheric deposition. However, a δ 34 S value of −7.3‰ identified for one of the karst springs demonstrates that the oxidation of sulfide-containing minerals must be considered as an additional source, affecting sulfate concentrations in groundwater. The estimated transit time distribution of groundwater shows relatively high mean transit times between 2 and 5 years with a low contribution of very short transit times of less than 1 year. The high contribution of long mean transit times of groundwater in the alpine karst system, which represents an important drinking water resource, is an important function for drinking water supplies.

Three rivers in North-Rhine Westphalia, Germany, were investigated for their hydrochemical properties including their stable isotopic composition of water ( δ 2 H, δ 18 O) and dissolved river compounds ( δ 13 C DIC , δ 34 S SO4 and δ 18 O SO4 , and δ 15 N NO3 and δ 18 O NO3 ). The study focused on two objectives: an assessment of potential sources for river solutes (anthropogenic vs. natural sources), and the quantification of changes in river chemistry over the past 15 a (for the rivers Lippe and Ruhr). Decreasing concentrations were found for most of those river constituents that are commonly linked to anthropogenic activities, such as [ NO 3 - ] , [Cl − ], [K + ], and [Na + ]. An observed increase in [ SO 4 2 - ] for the river Lippe reflects most likely varying discharges from mining activities. Variations in the isotopic composition of water display the influence of ocean water (river Ems) or of evaporation that occurred either in channels (river Ems), in reservoirs (river Ruhr) or due to the use of river water for cooling purposes (river Lippe). δ 13 C DIC values around −11‰ point to carbonate dissolution by carbonic acid as the major source for dissolved inorganic C. Modifications of this average δ 13 C DIC resulted from enhanced agricultural use, sewage inputs, and gas exchange with the atmosphere in reservoirs and channels. The isotopic composition of dissolved SO 4 2 - reveals atmospheric deposition and sulphide oxidation as its major sources. Sulphate from sulphide oxidation in parts reflects the local geology (river Ruhr); in the Kreidebecken leaching of sulphide seems to be linked to agriculture and drainage (rivers Lippe and Ems). However, SO 4 2 - introduced from mining activities into the Lippe and the Ems does not alter the isotopic composition of riverine SO 4 2 - , despite rather high discharges. Nitrogen and O isotopes reveal that manure and sewage are major sources of NO 3 in most parts of the river Ruhr. Only a single value from the headwaters displays the signature of soil NO 3 . Downstream increasing δ 15 N NO3 and δ 18 O NO3 values (both by 2‰ on average) point to denitrification and to additional inputs from atmospheric deposition.

Bacterial sulfate reduction (BSR) is a key process in anaerobic respiration in wetlands and may have considerable impacts on methane emissions. A method to determine sulfate production and consumption in situ is lacking to date. We applied a single-well, injection-withdrawal tracer test for the in situ determination of potential sulfate turnover in a northern temperate peatland. Piezometers were installed in three peat depth levels (20, 30, and 50cm) during summer 2004, and three series of injection-withdrawal cycles were carried out over a period of several days. Turnover rates of sulfate, calculated from first-order-reaction constant k (-0.097 to 0.053 h -1 ) and pore-water sulfate concentrations (approx. 10 μmol L -1 ), ranged from -1.3 to -9.0 nmol cm -3 d -1 for reduction and from +0.7 to +25.4 nmol cm -2 d -1 for production, which occurred after infiltration of water following a heavy rainstorm. Analysis of stable isotopes in peat-water sulfate revealed slightly increasing δ 34 S values and decreasing sulfate concentrations indicating the presence of BSR. The calculated low sulfur-fractionation factors of

Denitrification is an important natural attenuation process that has been observed in many fissured and porous aquifers. However, an important factor limiting denitrification in aquatic systems is the microbial availability of electron donors. Pyrite as the most abundant sulfide mineral in nature represents one of the potential electron sources for denitrifiers to reduce nitrate, but the reaction mechanisms coupling denitrification processes to pyrite oxidation are still questionable. We utilized hydrochemical data and stable isotopes of nitrate and sulfate in groundwater, isotope ratios of sulfur compounds in aquifer sediments and tritium based groundwater dating for assessing denitrification processes in a pyrite-bearing porous groundwater system. The oxic part of the aquifer with mean water transit times of approximately 60 years was characterized by nitrate concentrations of around 15 mg/l and δ 15 N values were similar to those typical for nitrification. In contrast, in the anoxic part with mean water transit times of up to 100 years, low nitrate concentrations accompanied by elevated δ 15 N values were observed. Furthermore, isotope data of groundwater sulfate and sulfur compounds in the aquifer sediment suggest that pyrite oxidation is the dominant source of sulfate in the aquifer. The trend of increasing δ 15 N values and decreasing nitrate concentrations in concert with depleted δ 34 S values of groundwater sulfate similar to δ 34 S values of pyrite, FeS 2 , suggests that denitrification is coupled to pyrite oxidation, particularly when water mean transit time is elevated.

The study of molecular transformation processes of dissolved organic carbon (DOC) in the environment significantly contributes to a better understanding of the global biogeochemical organic matter cycle. In an oxic karst groundwater system, in which the most powerful abiotic DOC degradative reactions, photodegradation and metal-mediated redox chemistry, are at best marginal contributors, a near complete turnover of fulvic acids (FAs) has been observed within decades (∼60 years). Depletion of oxygen for a very extensive range of aliphatic and aromatic carbon chemical environments has been confirmed as well as the formation of novel classes of compounds, suggesting a major contribution from biotic processes. From these results we infer that FAs must be perceived as a rather active participant in the global carbon cycle. Molecular-level alterations of such magnitude and rapidity on such short-time scales ought to be considered as widespread in the processing of “refractory” DOC in the environment.

Chemical and isotope analyses on groundwater sulfate, atmospheric deposition sulfate and fulvic acids (FAs) associated sulfur were used to determine the S cycling in a karstic catchment area of the Franconian Alb, Southern Germany. Sulfur K-edge X-ray absorption near edge structure (XANES) spectroscopy provided information on the oxidation state and the mechanism of the incorporation of sulfur in FAs. During base flow δ34S values of groundwater sulfate were slightly depleted to those of recent atmospheric sulfate deposition with mean amount-weighted δ34S values of around + 3‰. The δ18O values of groundwater sulfate shifted to lower values compared to those of atmospheric deposition and indicated steadiness from base flow to peak flow. The reduced sulfur species (S− 1/thiol; S0/thiophene, disulfide, S+ 2/sulfoxide) of soil FAs averaged around 49% of the total sulfur and δ34S value in FAs was found to be 0.5‰. The formation of polysulfides and thiols in FAs in concert with a decreasing isotope value of δ34S in FAs with respect to those of atmospheric deposition sulfate suggests oxidation of H2S, enriched in the 32S isotope, with organic material. The depletion of δ18O–SO42− by several per mil in groundwater sulfate with respect to those of atmospheric deposition is, therefore, consistent with the hypothesis that SO42− has been cycled through the organic S pool as well as that groundwater sulfate is formed by oxidation of H2S with organic matter in the mineral soil of the catchment area.

Freshwater discharge into the coastal sea is of general interest for two reasons: (i) It acts as vehicle for the transport of contaminants or nutrients into the ocean, and (ii) it indicates the loss of significant volumes of freshwater that might be needed for irrigation or drinking water supply. Due to the large-scale and long-term nature of the related hydrological processes, locating and quantitatively assessing freshwater discharge into the sea require naturally occurring tracers that allow fast, inexpensive and straightforward detection. In several studies, the standard water parameters electrical conductivity (EC) and pH have proven their suitability in this regard. However, while distribution patterns of EC and pH in the coastal sea indicate freshwater discharge in general, a separation between discharging surface water and submarine groundwater discharge (SGD) is not possible with these alone. The naturally occurring radionuclide radon-222 has been shown to be useful in the quantification of SGD and its distinction from surface runoff. This study aimed to evaluate and compare the informative value of the three parameters-EC, pH and radon concentration-in detecting and quantifying SGD by carrying out a case study in a bay located in western Ireland. The results reveal that radon activity is the most sensitive parameter for detecting SGD. However, only the combined evaluation of radon, EC and pH allows a quantitative allocation of groundwater and surface water contributions to the overall freshwater discharge into the sea. This conclusion is independently supported by stable isotope data measured on selected samples.