Christian Sonntag

Tritium/3He dating of shallow groundwater

Combined tritium/3He data from three multi-level sampling wells (DFG 1, DFG 4, DFG 7) located at Liedern/ Bocholt, West Germany, are presented and principles of the tritium/3He method in shallow groundwater studies are discussed. The3He excess produced by radioactive decay of bomb tritium (released mainly between 1952 and 1963) is clearly reflected in the data. The tritiogenic3He signal can be detected with a good resolution (signal/1σ error: ≈ 350). The confinement of the tritiogenic3He is estimated to approximately 77–85% at site DFG 4. For the bomb tritium peak the deviation of the tritium/3He age from the age determined by identifying the groundwater layer recharged between 1962 and 1965 is about 3 years (15%). The deviation can be explained by diffusive3He loss across the groundwater table and by flow dispersion.

Tritiogenic 3He in shallow groundwater

Tritium, helium isotope and neon data from a multi-level sampling well (DFG 7) at Liedern/Bocholt (West Germany) are presented and discussed. The presence of a radiogenic helium component leads to 3He/4He ratios below that of atmospheric helium (minimumδ3He values≈ −60%) below about 20 m depth. The3He profile can be corrected for the nucleogenic3He component using the neon measurements. Based on the “Vogel” model of a shallow aquifer the tritium/3He distributions are simulated for the years 1987, 2000 and 2025. The model results show that under favourable conditions the tritiogenic3He peak will be detectable in shallow aquifers for at least the next 4 decades. The influence of the vertical flow velocity and the transversal dispersion coefficient on simulated distributions are estimated.3He confinement is calculated as a function of the vertical flow velocity and the transversal dispersion coefficient. There is a critical value of the vertical flow velocity (about 0.25–0.5 m/year) below which the3He loss increases rapidly to high values.

Groundwater age dating with chlorofluorocarbons

CFCs are useful tracers for age dating young water. Two critical assumptions are typically invoked: (1) the initial concentration needs to be known, and (2) the tracer must be stable. A series of 8000 CFC air data from four sites on the west European continent (from high-alpine clean air down to urban sites) show site-specific CFC excesses relative to the global background ranging from 125% at an urban site (Heidelberg) to only 30% at a rural site (Wachenheim). In both cases we find regular diurnal, weekly, and annual variations but also episodic variations due to changing air mass and sporadic local emission as well as a decrease of the CFC excess due to decreasing emissions in Europe. However, in soil air above the groundwater table the CFC excess variation is low-pass filtered, allowing reconstruction of the site-specific CFC input anomaly in relation to the global background. Thus local CFC soil air measurements can provide a valuable local correction factor for CFC dating and extend the applicability of CFC studies in specific environments. To investigate the chemical stability assumption, five field sites and two laboratory experiments show CFC 11 and CFC 12 degradation under anoxic conditions with degradation rates ranging from 0.05 up to 3×105 pmol L−1 yr−1. However, the CFC 12/CFC 11 degradation ratio is found to be constant over six orders of magnitude, with CFC 12 always being less reactive by a factor of about 10.

Helium in deep circulating groundwater in the Great Hungarian Plain: Flow dynamics and crustal and mantle helium fluxes

Observed helium concentrations in deep circulating groundwater of the sedimentary basin of the Great Hungarian Plain (GHP), Hungary, cover a range of three orders of magnitude (≈4 ·10−8 to 4 · 10−5 ccSTP g−1). 3He4He ratios and noble gas concentrations are used to separate helium components originating from the atmosphere, tritium decay, crustal production, and mantle degassing. The characteristic distribution of measured helium concentrations and isotope ratios can be reproduced qualitatively by a simple two-dimensional advection/diffusion model. Other simple models isolating parts of the regional flow domain (recharge, discharge, and horizontal flow) are discussed and applied to derive quantitative information on helium fluxes due to degassing of the Earths crust /mantle and on the dynamics of groundwater flow. The estimated helium flux of 0.7–4.5 · 109 atoms 4He m−2 s−1 is lower than values derived from other deep groundwater circulation systems, probably because the relatively young upper few thousand meters of the sedimentary basin (Tertiary to Quaternary age) shield the flux from the deeper crust. The high mantle helium flux of up to 4.2 · 108 atoms 4He m−2 s−1 is probably related to the Miocene volcanism or to continuing intrusion accompanying extension. By fitting calculated helium depth profiles to measured data in the discharge area, vertical flow velocities of the order of 1.5 mm y−1 are estimated. Assuming that a flux of 0.7–4.5 · 109 atoms 4He m−2 s−1 is representative for the entire basin, the turnover time of the regional groundwater flow system is estimated to be about 106 y.

231Pa dating of deep-sea cores via 227Th counting

A simple procedure for simultaneous Io and 231 Pa dating of deep-sea sediments is described. The 231 Pa content is determined from the specific activity of its grand-daughter 227 Th. Fast thorium (and uranium) separation is carried out with a newly developed procedure using the ion-exchangers Aliquat-336 and TTA. The procedure was applied for Io and 231 Pa dating of deep-sea cores Vema 18–258, Valdivia 10127 and Valdivia 10141 from the North and South Pacific. In the case of core Vema 18–258 our results confirm previous Io and 231 Pa data of T.L. Ku. For the Valdivia core 10127 sedimentation rates of respectively 2.7 and 3.2 mm/1000 years were calculated from the decrease of the Io and 231 Pa excess activities. For the Valdivia core 10141 different sedimentation rates in the core section above and below 18 cm were found (0–18 cm: 3.9 mm/1000 years (Io) and 5 mm/1000 years ( 231 Pa); 18–37 cm: 20 mm/1000 years (Io and 231 Pa)). These findings give proof that 227 Th in the sediment is essentially in radioactive equilibrium with 231 Pa. 231 Pa dating via 227 Th counting simultaneously to Io dating is suggested.

Variations of Deuterium and Oxygen-18 in Continental Precipitation and Groundwater, and Their Causes

Spatial and temporal variations in the deuterium- and 18O-content of precipitation are caused by isotope fractionation resulting from evaporation and condensation processes during circulation of atmospheric water vapour. During these phase transitions, the isotope-labelled water molecules are preferentially transferred into the liquid phase. Water loss by moist adiabatic cooling of air masses therefore leads to progressive isotopic depletion, which is described by a Rayleigh condensation formula. Application of this formula to the moist adiabatic ascent of discrete air masses yields exponential vertical profiles of the moisture and its D- and 18O-content assuming a constant relative humidity and a mean temperature lapse rate of −6°C km−1. These isotope profiles agree fairly well with observed profiles in tropospheric water vapour. The spatial variation of D and 18O in precipitation and groundwater across Europe and North America can also be described by a simple Rayleigh condensation model, which links the local isotope data with the parameters of the vertically-integrated water vapour flux. This model uses as input data monthly means of local temperature, relative humidity, precipitation and evapotranspiration. The model also yields an estimate of the variation of the isotope content of local precipitation with temperature. A one-year record of D and 18O in daily mean values of water vapour at Heidelberg shows the seasonal variation already known from monthly mean precipitation data and successfully simulated by the model. Superimposed on this seasonal pattern are strong short-term variations with a dominant periodicity of 22 days. This periodicity seems to be related to the phase velocity of long waves in the free atmosphere, which steer the water vapour transport in the lower troposphere.

Zur Paläoklimatik der Sahara

Von einer gr613eren Anzahl ~4C-datierter Grundwfisser de r algerischen [1] und libyschen[2] Zentralsahara liegeu auch Deuteriumund Sauerstoff-18-Messungen vor. Die meisten tiefen Grundw/isser sind mit 14C-Altern zwischen 20000 und 40000 Jahren vor Heute dem letzten grogen Pluvial zuzuordnen, die jfingereu flachen Grundw~isser (einige tausend bis ca. 14000 a alt) geh6ren postpluvialen Feuchtphasen an. Eine neuerliche Sichtung der Isotopendaten der SaharawS.sser ergab ein signifikantes West-Ost-Geffilte im Dund l so_Gehalt (kurz: Isotopengehalt) yon ungeMhr -2,6%o/100 km bzw. 0,3%0/100 km