Helmut Dörr

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.

Soil texture parameterization of the methane uptake in aerated soils

Long-term records of the methane uptake by five different aerated soils in South-West Germany and spot measurements in Western Europe, North America, and North-West Africa are presented. A good correlation between the methane uptake rate and soil permeability, obtained from parallel 222Radon flux and concentration measurements of methane and 222Radon in soil air indicates, that methane consumption in aerated soils is mainly controlled by the gas transport resistance within the soil. Soil temperature is found to be of minor influence on the methane flux from the atmosphere into the soil. The soils investigated were classified with respect to the three major soil texture classes - coarse, medium, and fine - representing soils with low, medium, and high gas transport resistance, respectively. Average methane uptake rates for each soil texture class were determined and extrapolated to fit into the scheme of the NASA-GISS global digital data set of soil types (Staub et al., 1987). Assuming that the observed relation between methane uptake rate and soil permeability (soil texture) is valid for all kinds of aerated soils, methane uptake rates are estimated on a regional and global scale. From this parameterization we calculate a global CH4 soil sink in the range of 9.0 to 55.9 Tg CH4/a with a best estimate of 28.7 Tg CH4/a. This corresponds to about 7% of the total global destruction rate by OH radicals.

Carbon-14 and carbon-13 in soil CO2.

Carbon isotope measurements in soil CO 2 are presented and discussed. Soil CO 2 concentration and 13 C profiles were measured using a new technique. A simple model describing the CO 2 transport from the soil to the atmosphere is derived. The finding that CO 2 in the soil is richer in 13 C than the CO 2 leaving the soil is attributed to isotopic fractionation in molecular diffusion.

Stable isotopic signature of methane from major sources in Germany

Abstract Measurements of the isotopic composition (δD, δ13C) of methane originating from major sources in Germany and northern Italy (i.e. dairies and dairy wastes, waste deposits, swamps, natural gas, rice paddies) are presented. All methane samples collected from recent biogenic sources are strongly depleted in δ13C and δD if compared to the isotopic composition of the substrates, organic material (δ13C, δD) and ground- and precipitation water (δD). δ13C of simultaneously produced carbon dioxide, on the other hand, shows a slight isotopic enrichment with respect to the original organic matter. The following systematical effects influencing the isotopic signature of the generated methane have been observed: 1. (1) δ13C of methane eructated from cows strongly depends on the diet (100% C3 diet: δ 13 C = −65.1±1.7‰ ; 60–80% C4 diet: δ 13 C = −55.6±1.4‰ ). This suggests fractionation factors during methane generation in the rumen being independent from the substrate. 2. (2) The δD values of methane generated in dairy wastes ( δ D = −298±6‰ ) and produced by cows ( δ D = −300±10‰ ) are very similar. This and the absolute δD depletion with respect to the substrates suggests that acetate fermentation is the major methanogenic pathway in all sources from dairy farming - although CH4 in the rumen of cows is known to be mainly generated via CO2 reduction. The unexpected δD signature in ruminant methane can perhaps be explained by an additional δD fractionation of the intracellular water of M. ruminantium if compared to the methanobacteria species in marine and freshwater environments. 3. (3) Large variations in the δ13C of methane generated from different waste deposits were observed. The fractionation factors α CO 2 CH 4 calculated from parallel δ13CO2 measurements are, however, very similar. The large spread of δ13CH4 (and δ13CO2) is explained by differences in the δ13C of the organic substrate in the source systems. 4. (4) Methane samples collected from swamps show significant isotopic enrichment relative to the original methane generated if partial oxidation had occurred. Isotope enrichment due to partial oxidation of methane was also observed in a waste deposit in the uppermost aerobic layers. From the concentration profile in the waste deposit investigated we estimated a reduction of the effective methane flux to the atmosphere due to oxidation by more than 60%.

PCB and PAH fluxes to a dated UK peat core

Concentrations of PCBs and PAHs have been determined from the individual sections of peat cores obtained from an ombrotrophic bog in rural north-west England. Chronological intervals throughout the core were determined from both radiometric (210Pb, 137Cs, 241Am) and independent, non-chemical characteristics (pollen, magnetics) information. Net fluxes of 25 individual PCB congeners and 14 PAH compounds to the bog were then derived. PCB inputs were apparent from the late-1930s/ early-1940s, with maximum sub-surface fluxes ( 1300 pg cm−2 year−1) observed at a depth corresponding to 1964. Loadings decreased by 65% over the following 15 year period before showing a surface enrichment. Initial increases in PAH loadings appear to coincide with the beginning of the Industrial Revolution, with fluxes peaking in the early-1930s (305 ng cm−2 year−1). Introduction of emission controls and the decline of heavy industry has led to an 80 % reduction in the net flux of PAHs to the bog over the last three to four decades. Potential effects of postdepositional diagenesis are considered, with particular reference to alteration of contaminant chronologies.

Lead and cesium transport in european forest soils

The downward velocity of Pb and Cs in undisturbed European forest soils is determined from the depth distribution of atmospheric 210Pb , and from the penetration depth of the bomb Cs peak in the soil. The downward velocity of Pb and Cs shows no correlation with soil type and pH. The downward migration of Pb and Cs is found to be due to an apparent downward movement of organic material, caused by the sedimentation of soil organic matter and subsequent turnover into CO2 . Cs migration, however, shows an additional velocity component which is found to depend on the turnover rate of soil organic matter.

Mobilization of cesium in organic rich soils: Correlation with production of dissolved organic carbon

A study of the downward movement of 137Cs in an undisturbed forest soil is presented. Seasonal variations and depth profiles of 137Cs activities were measured in seepage water, which is the transport medium for the downward movement of anthropogenic substances in soils. Furthermore the correlation of 137Cs mobilization and production of dissolved organic carbon (DOC) was investigated. Seasonal variations of both 137Cs and DOC fluxes in the seepage water in a depth of 5 cm depth were observed, where the maximum fluxes in the summer months were about one order of magnitude higher than the minimum fluxes in the winter months. 137Cs fluxes are found to be correlated with DOC fluxes with a correlation coefficient of r = 0.63, and both are highly correlated with soil temperature. This indicates that cesium is bound to soil organic material. The production of DOC is controlled by microbial decomposition of soil organic matter and we assume that this holds true for the 137Cs release as well. The actual transport velocity (0.2 ± 0.14 mm/a) of 137Cs (calculated by the weighed mean of 137Cs concentration in the seepage water and the total 137Cs content of the soil) is about one order of magnitude less than the mean transport velocity (1.2 ± 0.3 mm/a) over the past 25 years (calculated from the 137Cs depth profile). It is possible that the transport velocity of 137Cs in undisturbed soils decreases with time as it binds to aged organic material which is less easily decomposable than fresh organic material.

14C Measurements of Soil Organic Matter, Soil Co2 and Dissolved Organic Carbon (1987–1992)

For several undisturbed sites in Germany, 14 C data are reported for soil organic matter (SOM) (4 sites), soil CO 2 (10 sites) and dissolved organic carbon (DOC) (1 site). With the assumption of a fast degradable component (lifetime ca. 1 yr) and a slow degradable component (lifetime ca 100 yr), a range between 0.6 and 1.6 mm yr -1 has been determined for the d ,ward migration rates of soil organic carbon at the sampling sites from the soil 14 C data. The soil measurements show that in deciduous forests the fast degradable component is ca. 60% and the slow degradable component is ca. 40% of the SOM. In coniferous forests this ratio is reversed. The 14 C results for DOC could not be explained with the assumption of a first-order decay process. The removal of soil organic carbon by DOC is of minor importance for the estimation of carbon budgets for the investigated site.