Evidence for periods of wetter and cooler climate in the Sahel between 6 and 40 kyr BP derived from groundwater

Concentrations of noble gases, stable isotopes and 14C in samples from the Continental Terminal groundwaters of Niger provide evidence for more humid and cooler climate phases in West Africa in the Holocene and the late Pleistocene. During humid phases, even within the Holocene, the soil temperature was up to 5.5°C cooler than today, which is partly attributed to atmospheric cooling, but also to a change in the relationship between air and soil temperature due to increased vegetation. Intense rainfall events and increased groundwater recharge are consistently indicated by stable isotope data and excess air concentrations, i.e., the component of dissolved atmospheric gases in excess of solubility equilibrium. This finding encourages the use of excess air as an additional, humidity-related climate indicator

Estimating groundwater mixing ratios and their uncertainties using a statistical multi parameter approach

The estimation of groundwater mixing ratios is a crucial task in groundwater research and management. The quantity as well as the quality of groundwater resources are sensible to, e.g. the admixture river water, the interaction between different groundwater layers and the spatial distribution of such mixing zones. In this paper, we present a procedure to calculate mixing ratios and their uncertainties using a set of chemical and isotopic parameters. The advantages compared to estimations based on only few parameters are better-constrained mixing ratios and the limitation of systematic errors due to the parameter choice. The statistical consistence of the resulting mixing ratios is checked with a χ2-test. In particular in larger aquifers, where sampling resolution is often coarse, the composition of end members may vary considerably from one borehole to another due to rock water interaction or varying recharge conditions. After selecting sampling sites where no mixing occurs, the spatial distribution of the end member composition was calculated using Kriging. This provides the best estimates of end member compositions and their uncertainties at any location of the area of investigation. The method is, in principle, applicable for any number of end members. In this paper, the spatial distribution of the interaction of two layers of the Continental Terminal aquifer (Niger, Africa) was investigated. Mixing between the deep CT2 and the shallow CT3 aquifer occurs where the separating layers are thinning out or due to improper sealing of some boreholes. Radiogenic 4He concentrations, which differ significantly in the two aquifers, were used as independent parameter to crosscheck the mixing calculations

A comparative study of the modelling of cement hydration and cement-rock laboratory experiments

The use of cement and concrete as fracture grouting or as tunnel seals in a geological disposal facility for radioactive wastes creates potential issues concerning chemical reactivity. From a long-term safety perspective, it is desirable to be able model these interactions and changes quantitatively. The 'Long-term Cement Studies' (LCS) project was formulated with an emphasis on in situ field experiments with more realistic boundary conditions and longer time scales compared with former experiments. As part of the project programme, a modelling inter-comparison has been conducted, involving the modelling of two experiments describing cement hydration on one hand and cement-rock reaction on the other, with teams representing the NDA (UK), Posiva (Finland), and JAEA (Japan).This modelling exercise showed that the dominant reaction pathways in the two experiments are fairly well understood and are consistent between the different modelling teams, although significant differences existed amongst the precise parameterisation (e.g. reactive surface areas, dependences of rate upon pH, types of secondary minerals), and in some instances, processes (e.g. partition of alkali elements between solids and liquid during cement hydration; kinetic models of cement hydration). It was not conclusive if certain processes such as surface complexation (preferred by some modellers, but not by others) played a role in the cement-rock experiment or not. These processes appear to be more relevant at early times in the experiment and the evolution at longer timescales was not affected. The observed permeability profile with time could not be matched. The fact that no secondary minerals could be observed and that the precipitated mass calculated during the simulations is minor might suggest that the permeability reduction does not have a chemical origin, although a small amount of precipitates at pore throats could have a large impact on permeability. The modelling exercises showed that there is an interest in keeping the numerical models as simple as possible and trying to obtain a reasonable fit with a minimum of processes, minerals and parameters. However, up-scaling processes and model parameterisation to the timescales appropriate to repository safety assessment are of considerable concern. Future modelling exercises of this type should focus on a suitable natural or industrial analogue that might aid assessing mineral-fluid reactions at these longer timescales. © 2011 Elsevier Ltd.This work is a component of the international Long-Term Cement Studies (LCS) Project managed by Nagra (http://www.grimsel.com/gts-phase-vi/lcs/lcs-introduction). Modelling teams were funded by JAEA, Posiva (IDAEA-CSIC), and the Radioactive Waste Management Directorate of the UK Nuclear Decommissioning Authority (Quintessa). The manuscript was substantially improved by perspicacious reviews from two anonymous referees.Peer Reviewe

Assessing the impact of modern recharge on a sandstone aquifer beneath a suburb of Doncaster, UK

A major water quality issue in urban areas underlain by a productive aquifer is the impact of modern recharge. Using a variety of sample sources including multi-level boreholes, detectable concentrations of CFCs and SF6 have been found throughout the upper 50 m of the saturated aquifer beneath a suburb of Doncaster, UK, indicating that modern (<50-year old) recharge has penetrated to at least this depth. Additional support for this deep penetration is provided by the detection of sulphite-reducing clostridia and faecal streptococci. Despite the upper aquifer being a poorly cemented sandstone, the residence time indicators suggest that some modern recharge is travelling via fracture systems in addition to that moving down by simple piston flow. However, the overall impact of 80 years of steady urbanisation on water quality in the aquifer beneath this suburb has in general been limited. This is attributed to a combination of factors including previous land use, dilution by direct recharge of rainfall through green-space areas including gardens, and locally high storage in the friable upper aquifer