Applied Geochemistry (v.18, #12)

Mineralogical and isotopic studies were carried out on the natural nuclear reaction zone 2 from the Oklo deposit to evaluate the mobility of several nuclear reaction products in response to the alteration of the reaction zone and to identify the mechanisms which could retard the transport of released radionuclides. To address these issues, in situ isotopic analyses by SHRIMP and a selective extraction procedure were performed to constrain the structural location of nuclear reaction products (exchangeable and non exchangeable) and their association with mineral phases. The distribution patterns of U, REE, Zr and Mo isotopes reveal that substantial amounts were released from the core and migrated through the hydrothermal alteration halo over metric distances, owing to uraninite dissolution and advective transport by hydrothermal solutions during and soon after criticality. The results emphasize the mobility of Zr at Oklo, this element being often considered as “immobile” during water–rock interactions. The main output is the demonstration of the net effects of sorption and coprecipitation processes. Chlorite and to a lesser extent illite were found to have adsorbed significant amounts of U, REE, Zr (and probably Th) and less sorbing elements such as Mo. Coprecipitation of secondary UO2 and P-rich coffinite within the alteration halo is also an important means of retardation. The concentration of radionuclides released from the reactor were probably high and they display solubility limited transport behaviour. No retention effect was found for Se in the immediate vicinity of the reactor and this element may have moved farther from its source of production. These results have interesting implications for the evaluation of long-term containment of radionuclides. They provide a simple illustration of the performance of a clay barrier in the uptake of radionuclides by sorption onto clays and reincorporation in secondary U-minerals. This study also demonstrates the robustness of these retention processes over extremely long periods of time.

One of the proposals for large-scale sequestration of fossil fuel-derived CO2 is deep geologic disposal in depleted oil/gas reservoirs or deep aquifers. Previously published scenarios for this inadequately proven technology have either ignored or dismissed the possibility of vertical migration of gases caused by overpressure. Overpressuring of a reservoir or aquifer will be necessary in order to have acceptable rates for dispersal of injected CO2. This research describes methodology and the results of measurement of microseepage of CO2 and CH4 at a large-scale CO2-enhanced oil recovery (EOR) operation at Rangely, Colorado, USA. Shallow and deep soil gas concentrations, and direct transport of CO2 and CH4 into the atmosphere were measured. The interpretation of the measurements was complemented by both stable and radiogenic isotopic measurements of C. The results have demonstrated an estimated microseepage to the atmosphere of approximately 400 metric tonnes of CH4/a from the 78 km2 area of the Rangely field. Preliminary estimates of deep-sourced CO2 losses are <3800 tonnes/a, based on stable isotope measurements of soil gases. Several holes up to 10 m deep were drilled on, and off the field for nested gas sampling of composition and stable C isotopic ratios for CO2 and CH4. Carbon-14 measurements on CO2 from these holes indicate that deep-sourced CO2 microseepage losses were approximately 170 tonnes/a.

Measurements of CH4 and CO2 concentrations in soil gas, and as exchange rates (fluxes) with the atmosphere were made over the Rangely oil field, Colorado, USA. The Rangely field has been operated as a large CO2 enhanced oil recovery (EOR) project since 1986, and may serve as a prototype for CO2 sequestration in depleted oil/gas fields. The first of this pair of papers reported on the details of the methodology and results of field measurements of these gases, including light alkanes. The interpretation was enhanced by the extensive use of 13C and some 14C measurements. The fluxes of both CH4 and CO2 to the atmosphere were estimated. This research has demonstrated and quantified the methanotrophic oxidation of microseeping CH4 in the unsaturated zone. Strong spatial variations were found and studied further by nested soil gas sampling in holes up to 9 m deep. Areas of microseepage were found, as well as areas where atmospheric CH4 was the only source for methanotrophic oxidation. Computer modeling was used to determine rate constants for the oxidation process. Strong seasonal differences exist in the rates of CH4 oxidation because of the semiarid and severe climate of Rangely. Rate constants of <1 to >1000 day−1 were determined. During the summer, the highest rates were near the surface, but shifted downward to about 2 m depth in the winter. Methanotrophic oxidation of CH4 in the unsaturated zone will be the most important component of amelioration of CH4 microseepage induced by overpressuring of the reservoir undergoing CO2 injection. These types of measurements and interpretation will be necessary in order to fully understand the environmental effects of CO2 sequestration in oil/gas fields.

In recent years environmental geochemical mapping has assumed an increasing relevance and separation of background values to evaluate pollution is, probably, even more critical than the separation between background and anomalies in mineral prospecting studies. The recognition of background values assumes particular relevance as a function of national environmental legislation, which fixes intervention limits for some elements, such as the harmful ones (e.g. As, Cd, Hg, Pb). In this paper a recently developed multifractal IDW interpolation method and a fractal filtering technique are applied to separate natural background and anthropogenic values for the compilation of environmental geochemical mapping from stream sediment samples of Campania region (Italy), where no mineralization occurs. To discuss the application of these recently developed techniques the elements Pb and U were selected because they show two completely different situations, the high Pb values being mostly of anthropogenic origin and high U values being mostly of geogenic origin. The new fractal filtering method works well in both extreme situations.

Iodine-129 and 238Pu, 239Pu and 240Pu are radionuclides posing a long-term safety concern due to their potential integration in bio and geo-chemical cycles and their significant half-lives. They are present throughout the environment at very low levels, and more particularly, nuclear fuel reprocessing plants (NRP) have been identified as local sources of these radionuclides. However, due to measurement difficulties, published data concerning their activity levels in terrestrial environments around NRP facilities remain scarce. The aim of the present paper is to communicate 129I, 238Pu and 239+240Pu measurements results from the area surrounding the Marcoule NRP, which is situated in SE France. Several vegetation samples were collected around the nuclear installation in 1999 and 2000, in order to examine the possible impacts of its atmospheric I and Pu discharges. Based on 238Pu/239+240Pu activity ratios and 129I/127I isotopic ratios, local increases in Pu and 129I were detected and related to industrial activity.