Sustainable groundwater resources exploration and management in a complex geological setting as part of a humanitarian project (Mahafaly Plateau,Madagascar)

Southwestern Madagascar is a semi-arid region and a hot-spot of global change. On the Mahafaly plateau, people live with quasi-permanent water stress and groundwater, the only available resource, is difficult to exploit due to a complex hydrogeological environment. A methodology (suitable for humanitarian projects; <?40 k€) was developed in four phases to assess the sustainable exploitation of the water resource: (A) regional scale exploration, (B) village scale exploration, (C) drilling campaign, and (D) hydro-climatic monitoring. This integrated hydrogeophysical approach involves geophysical measurements (262 TEM-fast soundings, 2588 Slingram measurements, 35 electrical soundings), hydrochemical analyses (112 samples), and a piezometric survey (127 measurements). Two groundwater resources were identified, one deep (below 150 m) and one shallow (<?20 m). Hydrochemical results highlighted the vulnerability of both resources: anthropic contamination for the shallower and seawater intrusion for the deeper. Therefore, subsequent geophysical surveys supported the siting of six boreholes and three wells in the shallow aquifer. This methodological approach was successful in this complex geological setting and requires testing at other sites in and outside Madagascar. The study demonstrates that geophysical results should be used in addition to drilling campaigns and to help monitor the water resource. In fact, to prevent over-exploitation, piezometric and meteorological sensors were installed to monitor the water resource. This unique hydro-climatic observatory may help (1) non-governmental organization and local institutions prevent future water shortages and (2) scientists to understand better how global change will affect this region of the world.     

Holocene Refreshening and Reoxygenation of a Bothnian Sea Estuary Led to Enhanced Phosphorus Burial

Salinity variations in restricted basins like the Baltic Sea can alter their vulnerability to hypoxia (i.e., bottom water oxygen concentrations <2 mg/l) and can affect the burial of phosphorus (P), a key nutrient for marine organisms. We combine porewater and solid-phase geochemistry, micro-analysis of sieved sediments (including XRD and synchrotron-based XAS), and foraminiferal δ¹⁸O and δ¹³C analyses to reconstruct the bottom water salinity, redox conditions, and P burial in the Ångermanälven estuary, Bothnian Sea. Our sediment records were retrieved during the Integrated Ocean Drilling Program (IODP) Baltic Sea Paleoenvironment Expedition 347 in 2013. We demonstrate that bottom waters in the Ångermanälven estuary became anoxic upon the intrusion of seawater in the early Holocene, like in the central Bothnian Sea. The subsequent refreshening and reoxygenation, which was caused by gradual isostatic uplift, promoted P burial in the sediment in the form of Mn-rich vivianite. Vivianite authigenesis in the surface sediments of the more isolated part of the estuary ultimately ceased, likely due to continued refreshening and an associated decline in productivity and P supply to the sediment. The observed shifts in environmental conditions also created conditions for post-depositional formation of authigenic vivianite, and possibly apatite formation, at ～8 m composite depth. These salinity-related changes in redox conditions and P burial are highly relevant in light of current climate change. The results specifically highlight that increased freshwater input linked to global warming may enhance coastal P retention, thereby contributing to oligotrophication in both coastal and adjacent open waters.     

Phosphorus Cycling and Burial in Sediments of a Seasonally Hypoxic Marine Basin

Recycling of phosphorus (P) from sediments contributes to the development of bottom-water hypoxia in many coastal systems. Here, we present results of a year-long assessment of P dynamics in sediments of a seasonally hypoxic coastal marine basin (Lake Grevelingen, the Netherlands) in 2012. Sequential phosphorus extractions (SEDEX) and X-ray absorption spectroscopy (XAS) indicate that P was adsorbed to Fe-(III)-(oxyhydr)oxides when cable bacteria were active in the surface sediments in spring. With the onset of summer hypoxia, sulphide-induced dissolution of the Fe-(III)-(oxyhydr)oxides led to P release to the pore water and overlying water. The similarity in authigenic Ca-P concentrations in the sediment and suspended matter suggest that Ca-P is not formed in situ. The P burial efficiency was ≤ 32%. Hypoxia-driven sedimentary P recycling had a major impact on the water-column chemistry in the basin in 2012. Water-column monitoring data indicate up to ninefold higher surface water concentrations of phosphate in the basin in the late 1970s and a stronger hypoxia-driven seasonal P release from the sediment. The amplified release of P from the sediment in the past is attributed to the presence of a larger pool of Fe-bound P in the basin prior to the first onset of hypoxia. Given that P is not limiting, primary production in the basin has not been affected by the decadal changes in P availability and recycling over the past 40 years. The changes in P dynamics on decadal time scales were not recorded in sediment profiles of total P or organic C/total P.     

The TST3D Method for Automated Structural Interpretation in Horizontal Wellbores

Horizontal wells dominate the development of unconventional shale reservoirs. Using real time drilling data to steer in a target zone is the key to economic success. Today structural interpretation in unconventional horizontal wells is a manual process that is time-consuming, tedious, and error-prone, especially because gamma-ray (GR) logs are commonly the only available logging-while-drilling data. For the first time, a method named TST3D is developed to automate interpretation of subsurface structure. TST3D (true stratigraphic thickness in three-dimensional space) automates structural interpretation using pattern recognition. Given an initial structural model, TST3D automatically computes true stratigraphic thickness (TST) as the shortest distance from each wellbore survey location to the initial surface, then matches GR patterns in the horizontal well to those seen in a vertical pilot well in TST domain. TST3D inserts fold hinges, bends the structure, then recomputes the modeled GR response, progressively matching the pilot well log signature, from heel to toe in the horizontal well. There are three assumptions in the current version of TST3D: constant layer thickness across the drilled interval, GR variation follows stratigraphic layering, and no faults are present in the drilled section. Those assumptions are reasonable in most shale plays. The TST3D method can be applied in either a post-drill mode for structural interpretation or real-time mode for aiding geosteering. Field tests in different shale plays and complex well trajectories demonstrate that TST3D runs quickly: a structural model of a 10,000-ft horizontal section can be computed in minutes, and a real-time update of 100 ft of new data takes less than a minute. Automating the geosteering correlation process would allow well placement engineers to cover multiple wells simultaneously, increasing the efficiency of the team while potentially improving service quality.

     

A thermo‐hydro‐mechanical model for unsaturated soils based on the effective stress concept

A simple thermo‐hydro‐mechanical (THM) constitutive model for unsaturated soils is described. The effective stress concept is extended to unsaturated soils with the introduction of a capillary stress. This capillary stress is based on a microstructural model and calculated from attraction forces due to water menisci. The effect of desaturation and the thermal softening phenomenon are modelled with a minimal number of material parameters and based on existing models. THM process is qualitatively and quantitatively modelled by using experimental data and previous work to show the application of the model, including a drying path under mechanical stress with transition between saturated and unsaturated states, a heating path under constant suction and a deviatoric path with imposed suction and temperature. The results show that the present model can simulate the THM behaviour in unsaturated soils in a satisfactory way. Copyright © 2010 John Wiley & Sons, Ltd.     

Comparison of numerical methods for simulating strongly nonlinear and heterogeneous reactive transport problems—the MoMaS benchmark case

Although multicomponent reactive transport modeling is gaining wider application in various geoscience fields, it continues to present significant mathematical and computational challenges. There is a need to solve and compare the solutions to complex benchmark problems, using a variety of codes, because such intercomparisons can reveal promising numerical solution approaches and increase confidence in the application of reactive transport codes. In this contribution, the results and performance of five current reactive transport codes are compared for the 1D and 2D subproblems of the so-called easy test case of the MoMaS benchmark (Carrayrou et al., Comput Geosci, 2009, this issue). This benchmark presents a simple fictitious reactive transport problem that highlights the main numerical difficulties encountered in real reactive transport problems. As a group, the codes include iterative and noniterative operator splitting and global implicit solution approaches. The 1D easy advective and 1D easy diffusive scenarios were solved using all codes, and, in general, there was a good agreement, with solution discrepancies limited to regions with rapid concentration changes. Computational demands were typically consistent with what was expected for the various solution approaches. The differences between solutions given by the three codes solving the 2D problem are more important. The very high computing effort required by the 2D problem illustrates the importance of parallel computations. The most important outcome of the benchmark exercise is that all codes are able to generate comparable results for problems of significant complexity and computational difficulty.