Future evolution of the Sahel precipitation zonal contrast in CESM1

The main focus of this study is the zonal contrast of the Sahel precipitation shown in the CMIP5 climate projections: precipitation decreases over the western Sahel (i.e., Senegal and western Mali) and increases over the central Sahel (i.e., eastern Mali, Burkina Faso and Niger). This zonal contrast in future precipitation change is a robust model response to climate change but suffers from a lack of an explanation. To this aim, we study the impact of current and future climate change on Sahel precipitation by using the Large Ensemble of the Community Earth System Model version 1 (CESM1). In CESM1, global warming leads to a strengthening of the zonal contrast, as shown by the difference between the 2060–2099 period (under a high emission scenario) and the 1960–1999 period (under the historical forcing). The zonal contrast is associated with dynamic shifts in the atmospheric circulation. We show that, in absence of a forced response, that is, when only accounting for internal climate variability, the zonal contrast is associated with the Pacific and the tropical Atlantic oceans variability. However, future patterns in sea surface temperature (SST) anomalies are not necessary to explaining the projected strengthening of the zonal contrast. The mechanisms underlying the simulated changes are elucidated by analysing a set of CMIP5 idealised simulations. We show the increase in precipitation over the central Sahel to be mostly associated with the surface warming over northern Africa, which favour the displacement of the monsoon cell northwards. Over the western Sahel, the decrease in Sahel precipitation is associated with a southward shift of the monsoon circulation, and is mostly due to the warming of the SST. These two mechanisms allow explaining the zonal contrast in precipitation change

Microbially-mediated chromate reduction in highly alkaline groundwater systems

Chromium ore processing residue (COPR) has been deposited at a site in the North of England, probably at the end of the nineteenth century. The site covers an area of approximately 2.2 ha, and is situated between a canal and a river that are about 150m apart. It is in a glacial valley underlain by millstone grit and in-filled with alluvial deposits (silt, clay and sand). The original surface deposit is a thin layer of sandy clay that was probably deposited during over-bank flow of the river. COPR has been tipped onto the hillside between the river and canal (which is ~7m above the river), possibly to support the canal bank. At some time in the past top-soil has been placed over the COPR, and the site is now covered with grass. Ground level on the tip is about 1.5m higher than the canal towpath. Currently the site is a cause for environmental concern because groundwater emerging from the waste is alkaline, visibly yellow and has an elevated Cr(VI) concentration. This paper reports an investigation into the possible fate of any Cr(VI) that migrates downwards from the waste into the underlying soils. Sandy clay from immediately beneath the waste (assumed to be the topsoil layer prior to waste tipping) contains 30-70% acid extractable iron as reduced Fe(II), and between about 3,000 and 600 mg.kg 1 of Cr decreasing with depth. DNA fragments from soil bacteria were extracted from this soil, and microcosm experiments with this soil where the pH was reduced showed that it contains a viable bacterial population capable of iron-reduction. This sandy clay layer, despite a pH value of 10.5, appears to be acting as a natural reactive zone beneath the waste as it is accumulating chromium. It is thought that the mechanism of Cr(VI) reduction is most likely to be an abiotic reaction with the Fe(II) present in the soil, and that Fe(II) in the soil is being replenished by microbial iron reduction (albeit probably at a slow rate)

Effect of Microbially Induced Anoxia on Cr(VI) Mobility at a Site Contaminated with Hyperalkaline Residue from Chromite Ore Processing

This paper reports an investigation of microbially mediated Cr(VI) reduction in a hyper alkaline, chromium contaminated soil-water system representative of the conditions at a chromite ore processing residue (COPR) disposal site. Soil from the former surface layer that has been buried beneath a COPR tip for over 100 years was shown to have an active microbial population despite the pH value of 10.5. This microbial population was able to reduce nitrate using an electron donor(s) that was probably derived from the soil organic matter. With the addition of acetate, nitrate reduction was followed in turn by removal of aqueous Cr(VI) from solution, and then iron reduction. Removal of ~300uM aqueous Cr(VI) from solution was microbially mediated, probably by reductive precipitation, and occuredoccurs over a few months. Thus, in soil that has had time to acclimatize to the prevailing pH value and Cr(VI) concentration, microbially mediated Cr(VI) reduction can be stimulated at a pH value of 10.5 on a time scale compatible with engineering intervention at COPR contaminated sites