Assesing the hydrogeological functioning of an evaporite karst system coupling tritium and physicochemical data

Discharge rate, electrical conductivity and water temperature have been continuously recorded in an evaporite karst spring located in S Spain and 3H determinations of spring water were performed. Results evidence a complex hydrogeological functioning, including rapid conduit flows (unsaturated zone), and diffuse flow (saturated zone) with diverse residence time within the systemUniversidad de Málaga. Campus de Excelencia Internacional Andalucía Tech International Atomic Energy Agency (IAEA

Fluid and gas fluxes from the Logatchev hydrothermal vent area

The Logatchev hydrothermal field at 14°45′N on the MAR is characterized by gas plumes that are enriched in methane and helium compared to the oceanic background. We investigated CH4 concentration and δ13C together with δ3He in the water column of that region. These data and turbidity measurements indicate that apart from the known vent fields, another vent site exists northeast of the vent field Logatchev 1. The distribution of methane and 3He concentrations along two sections were used in combination with current measurements from lowered acoustic Doppler current profilers (LADCP) to calculate the horizontal plume fluxes of these gases. According to these examinations 0.02 μmol s−1 of 3He and 0.21 mol s−1 of methane are transported in a plume that flows into a southward direction in the central part of the valley. Based on 3He measurements of vent fluid (22 ± 6 pM), we estimate a total vent flux in this region of about 900 L s−1 and a total flux of CH4 of 3.2 mol s−1

On the Propagation of Reaction Fronts in a Sandy Aquifer Over 20+ Years: Lessons From a Test Site in Northwestern Germany

Despite reduction measures, nitrate and aluminum concentrations remain high in aquifers in northwestern Europe. To evaluate the effectiveness of groundwater protection policies, the long-term fate of these contaminants in groundwater needs to be understood. The groundwater catchment of the Haren water works, NW Germany, was characterized hydrogeochemically in the late 1990s, which provides an opportunity to study the solute fronts over a two-decade period and conduct a post-audit of the predicted front movement. Results indicate that, despite a significant reduction of the atmospheric acid loads, the acidification of soil and groundwater at the forest site persists. Removal of sorbed aluminum is required to induce a noticeable improvement, which will take at least several decades. The unexpected appearance of nitrate at the site, caused by a land use change in 1998, highlights the need for long-term monitoring. Core data at the agricultural site show that the denitrification front has moved very little between 1998 and 2017, in accordance with previous forecasts. Denitrification by-products, mainly sulfate and nitrogen, have migrated from the upper into the lower aquifer. A reactive transport model demonstrated how the link between the regional groundwater flow, pyrite oxidation, and the temporal variability of the nitrate concentration in recharge water, as reconstructed from age tracers, result in the observed vertical distribution of sulfate and nitrogen. This study demonstrates how long-term monitoring, aided by model-based data interpretation, can be used to successfully study and predict the fate of contaminants in groundwater. © 2021. The Authors

Tritium tracers of rapid surface water ingression into arsenic-bearing aquifers in the Lower Mekong Basin, Cambodia

Arsenic (As) contamination of groundwaters in South and Southeast Asia is a major threat to public health in these areas. Understanding the source and age of the groundwaters is critically important to understanding the controls on As mobilization in these aquifers. Using tritium (3H) and noble gas (He and Ne) signatures, model groundwater ages and dominant hydrological controls were identified in a transect oriented broadly parallel to inferred groundwater flowpaths in Kandal Province, Cambodia in the lower Mekong Basin. Apparent 3H-3He ages showed that most groundwaters are modern (< 55 years), indicating relatively fast recharge even in the absence of large-scale groundwater abstraction. The age-depth relationship indicates a strong vertical component of groundwater flow and allows for recharge rates to be estimated. Vertical and horizontal flow velocities are heterogeneous and site-specific. The conceptual framework will be used to better understand As mobilization and subsequent transport with these and similar aquifers

Flux and dispersion of gases from the "Drachenschlund" hydrothermal vent at 8°18' S, 13°30'W on the Mid-Atlantic Ridge

Hydrothermal emission of mantle helium appears to be directly related to magma production rate, but other processes can generate methane and hydrogen on mid-ocean ridges. In an on-going effort to characterize these processes in the South Atlantic, the flux and distribution of these gases were investigated in the vicinity of a powerful black smoker recently discovered at 8°17.9' S, 13°30.4' W. The vent lies on the shoulder of an oblique offset in the Mid-Atlantic Ridge and discharges high concentrations of methane and hydrogen. Measurements during expeditions in 2004 and 2006 show that the ratio of CH4 to 3He in the neutrally buoyant plume is quite high, 4 × 108. The CTD stations were accompanied by velocity measurements with lowered acoustic Doppler current profilers (LADCP), and from these data we estimate the methane transport to have been 0.5 mol s− 1 in a WSW-trending plume that seems to develop during the ebb tidal phase. This transport is an order of magnitude greater than the source of CH4 calculated from its concentration in the vent fluid and the rise height of the plume. From this range of methane fluxes, the source of 3He is estimated to be between 0.14 and 1.2 nmol s− 1. In either case, the 3He source is significantly lower than expected from the spreading rate of the Mid-Atlantic Ridge. From the inventory of methane in the rift valley adjacent to the vent, it appears that the average specific rate of oxidation is 2.6 to 23 yr− 1, corresponding to a turnover time between 140 and 16 days. Vertical profiles of methane in the surrounding region often exhibited Gaussian-like distributions, and the variances appear to increase with distance from the vent. Using a Gaussian plume model, we obtained a range of vertical eddy diffusivities between 0.009 and 0.08 m2m2 s− 1. These high values may be due to tidally driven internal waves across the promontory on which the vent is located

Groundwater ages, recharge conditions and hydrochemical evolution of a barrier island freshwater lens (Spiekeroog, Northern Germany)

Freshwater lenses below barrier islands are dynamic systems affected by changes in morphodynamic patterns, groundwater recharge and discharge. They are also vulnerable to pollution and overabstraction of groundwater. Basic knowledge on hydrogeological and hydrochemical processes of freshwater lenses is important to ensure a sustainable water management, especially when taking into account possible effects of climate change. This is the first study which gives a compact overview on the age distribution, recharge conditions and hydrochemical evolution of a barrier island freshwater lens in the southern North Sea (Spiekeroog Island, Eastfrisian Wadden Sea). Two ground- and surface water sampling campaigns were carried out in May and July 2011, supplemented by monthly precipitation sampling from July to October. 3H–3He ages, stable oxygen and hydrogen isotopes and major ion concentrations show that the freshwater lens reaches a depth of 44 mbsl, where an aquitard constrains further expansion in vertical direction. Groundwater ages are increasing from 4.4 years in 12 mbsl up to >70 years at the freshwater– saltwater interface. Stable isotope signatures reflect average local precipitation signatures. An annual recharge rate of 300–400 mm was calculated with 3H–3He data. Freshwater is primarily of Na–Ca–Mg–HCO3– and Ca–Na–HCO3–Cl type, while lowly mineralized precipitation and saltwater are of Na–Cl types. A trend towards heavier stable isotope signatures and higher electric conductivities in the shallower, younger groundwater within the freshwater lens may indicate increasing atmospheric temperatures in the last 30 years

Isotope fractionation and mixing in methane plumes from the Logatchev hydrothermal field

As methane is consumed in the deep sea, its 13C/12C ratio progressively increases because of kinetic isotope fractionation. Many submarine hydrothermal vents emit methane with carbon isotope ratios that are higher than those of background methane in the surrounding ocean. Since the latter exists at low concentrations, mixing of background methane with vent fluid tends to decrease the 13C/12C ratio as concentration decreases, opposite to the trend produced by consumption. We investigated CH4 concentration and δ13C together with δ3He in plumes from the Logatchev hydrothermal field (LHF) located at 14°45′N, 45°W, which generates relatively heavy methane (δ13C ≈ −13‰) by serpentinization of ultramafic rock. The measured methane and δ3He were well correlated at high concentrations, indicating a CH4/3He ratio of 1 × 108 in the vent fluids. These tracer distributions were also simulated with an advection-diffusion model in which methane consumption only occurs above a certain threshold concentration. We utilized δ3He to calculate the methane remaining in solution after oxidation, f, and the deviation of δ13C from the value expected from mixing alone, Δδ13C. Both in the model and in the data, the entire set of Δδ13C values are not correlated with log f, which is due to continuous oxidation within the plume while mixing with background seawater. A linear relationship, however, is found in the model for methane at concentrations sufficiently above background, and many of the samples with elevated CH4 north of LHF exhibit a linear trend of Δδ13C versus log f as well. From this trend, the kinetic isotope fractionation factor in the LHF plumes appears to be about 1.015. This value is somewhat higher than found in some other deep-sea studies, but it is lower than found in laboratory incubation experiments

Dual in-aquifer and near surface processes drive arsenic mobilization in Cambodian groundwaters

Millions of people globally, and particularly in South and Southeast Asia, face chronic exposure to arsenic from reducing groundwater in which arsenic release is widely attributed to the reductive dissolution of arsenic-bearing iron minerals, driven by metal reducing bacteria using bioavailable organic matter as an electron donor. However, the nature of the organic matter implicated in arsenic mobilization, and the location within the subsurface where these processes occur, remains debated. In a high resolution study of a largely pristine, shallow aquifer in Kandal Province, Cambodia, we have used a complementary suite of geochemical tracers (including 14C, 3H, 3He, 4He, Ne, δ18O, δD, CFCs and SF6) to study the evolution in arsenic-prone shallow reducing groundwaters along dominant flow paths. The observation of widespread apparent 3H-3He ages of 30 m, and the relationships between age-related tracers and arsenic suggest that this surface-derived organic matter is likely to contribute to in-aquifer arsenic mobilization. A strong relationship between 3H-3He age and depth suggests the dominance of a vertical hydrological control with an overall vertical flow velocity of ~0.4 ± 0.1 m·yr−1 across the field area. A calculated overall groundwater arsenic accumulation rate of ~0.08 ± 0.03 μM·yr−1 is broadly comparable to previous estimates from other researchers for similar reducing aquifers in Bangladesh. Although apparent arsenic groundwater accumulation rates varied significantly with site (e.g. between sand versus clay dominated sequences), rates are generally highest near the surface, perhaps reflecting the proximity to the redox cline and/or depth-dependent characteristics of the OM pool, and confounded by localized processes such as continued in-aquifer mobilization, sorption/desorption, and methanogenesis

Basal melt and freezing rates from first noble gas samples beneath an ice shelf

A climatically‐induced acceleration in ocean‐driven melting of Antarctic ice shelves would have consequences for both the discharge of continental ice into the ocean and thus global sea level, and for the formation of Antarctic Bottom Water and the oceanic meridional overturning circulation. Using a novel gas‐tight in‐situ water sampler, noble gas samples have been collected from six locations beneath the Filchner Ice Shelf, the first such samples from beneath an Antarctic Ice shelf. Helium and neon are uniquely suited as tracers of glacial meltwater in the ocean. Basal meltwater fractions range from 3.6% near the ice shelf base to 0.5% near the sea floor, with distinct regional differences. We estimate an average basal melt rate for the Filchner‐Ronne Ice Shelf of 177 ± 95 Gt/year, independently confirming previous results. We calculate that up to 2.7% of the meltwater has been refrozen, and we identify a local source of crustal helium

Auswirkungen von anthropogenen Einflüssen auf einen Küstengrundwasserleiter am Beispiel des Ems-Ästuars

Im ostfriesischen Moormerland sind Einflüsse auf einen Küstengrundwasserleiter zu finden. Zum einen wird der Tidefluss Ems durch Ausbaggerungen und den Betrieb des Emssperrwerks bei Gandersum stark bewirtschaftet, zum anderen bewirken das Wasserwerk Tergast in unmittelbarer Umgebung der Ems sowie der durch die Entwässerungsverbände künstlich erniedrigte Grundwasserspiegel ein hydraulisches Gefälle von der Ems hin zum Binnenland. In mehreren Projekten wurden von 1999 bis 2013 die hydrochemische Beschaffenheit und die hydraulischen Bedingungen untersucht, um die zukünftige Entwicklung des Küstengrundwasserleiters abzuschätzen. Mit den Umwelttracern Tritium, Helium-3, Helium-4, Argon-39 und Kohlenstoff-14 konnte nachgewiesen werden, dass die heutige Verteilung von höher und gering mineralisierten Grundwässern das Resultat einer Überprägung des älteren, anthropogen unbeeinflussten Grundwasserströmungsfeldes durch ein jüngeres, anthropogen verändertes Strömungsfeld ist. Modellrechnungen zeigen, dass im Laufe der nächsten Jahrhunderte die Relikte des älteren Strömungsfeldes vollständig durch höher mineralisierte Grundwässer, die der künstlich induzierten Abflussrichtung folgen, überprägt werden