Interaction of basin-scale topography- and salinity-driven groundwater flow in synthetic and real hydrogeological systems

Salinization of groundwater has endangered e.g. drinking water supply, agricultural cultivation, groundwater-dependent ecosystems, geothermal energy supply, thermal and hydrocarbon well production to a rising degree. In order to investigate the problem of coupled topography- and salinity-driven groundwater flow on a basin-scale, a systematic simulation set has been carried out in a synthetic numerical model. Detailed sensitivity analysis was completed to reveal the effect of the salinity, permeability, permeability heterogeneity and anisotropy, mechanical dispersivity and water table head on the salt concentration field and the flow pattern. It was established that a saline dome with slow inner convection formed beneath the discharge zone in the base model due to the topography-driven regional fresh groundwater flow. An increase in the salinity or the anisotropy or decrease in the water table variation weakens the role of the forced convection driven by the topography, thus facilitating the formation of a saline, dense, sluggish layer in the deepest zone of the basin. In the studied parameter range, the variation in permeability and dispersivity affects the shape of the saltwater dome to less degree. However, the decrease in permeability and/or the increase in dispersivity advantage the homogenization of the salt concentration within the saline zone and strengthen the coupling between the saltwater and freshwater zone by growing the relative role of diffusion and transverse dispersion, respectively. The interaction of the topography-driven forced and salinity driven free convection was investigated along a real hydrological section in Hungary. Simulation elucidated the fresh, brackish and saline character of the water sampled the different hydrostratigraphic units by revealing the connection between the topography-driven upper siliciclastic aquifer and the lower confined karstic aquifer through faults in high-salinity clayey aquitard. The current study improves the understanding of the interaction between the topography-driven forced and the salinity-driven free convection, i.e. topohaline convection, especially in basin-scale groundwater flow systems

The seismic signature of Upper‐Mantle Plumes: application to the Northern East African Rift

Several seismic and numerical studies proposed that below, some hotspots upper‐mantle plumelets rise from a thermal boundary layer below 660 km depth, fed by a deeper plume source. We recently found tomographic evidence of multiple upper‐mantle upwellings, spaced by several 100 km, rising through the transition zone below the northern East African Rift. To better test this interpretation, we run 3‐D numerical simulations of mantle convection for Newtonian and non‐Newtonian rheologies, for both thermal instabilities rising from a lower boundary layer, and the destabilization of a thermal anomaly placed at the base of the box (700–800 km depth). The thermal structures are converted to seismic velocities using a thermodynamic approach. Resolution tests are then conducted for the same P and S data distribution and inversion parameters as our traveltime tomography. The Rayleigh Taylor models predict simultaneous plumelets in different stages of evolution rising from a hot layer located below the transition zone, resulting in seismic structure that looks more complex than the simple vertical cylinders that are often anticipated. From the wide selection of models tested, we find that the destabilization of a 200 °C, 100 km thick thermal anomaly with a non‐Newtonian rheology, most closely matches the magnitude and the spatial and temporal distribution of the anomalies below the rift. Finally, we find that for reasonable upper‐mantle viscosities, the synthetic plume structures are similar in scale and shape to the actual low‐velocity anomalies, providing further support for the existence of upper‐mantle plumelets below the northern East African Rift

The number of hotspots in three-dimensional numerical models of mantle convection

Thermal convection has been modelled in a 3D model box, in order to estimate the areal density of upwellings and compare it to the density of hotspots, assumed as surface imprints of the cylindrical upwellings of the mantle. The number of the hotspots of the Earth is 40 to 100. If this is translated to a nondimensional areal plume density, using the depth of the convecting layer as length unit, a value of 2-6 is obtained for whole-mantle convection, while this value is 0.04-0.10 for a separately convecting upper mantle. The nondimensional theoretical areal plume density has been found about 0.2-1.0 for reasonable numerical models of the mantle. The fact, that the theoretical value lies between the densities estimated for one- and two-layer mantle systems, supports the possibility of a mixed regime, where some of the plumes come from the base of the mantle, some others from the 660 km boundary

Effective buoyancy ratio: a new parameter for characterizing thermo-chemical mixing in the Earth's mantle

Numerical modeling has been carried out in a 2-D cylindrical shell domain to quantify the evolution of a primordial dense layer around the core–mantle boundary. Effective buoyancy ratio, <i>B</i>_eff was introduced to characterize the evolution of the two-layer thermo-chemical convection in the Earth's mantle. <i>B</i>_eff decreases with time due to (1) warming of the compositionally dense layer, (2) cooling of the overlying mantle, (3) eroding of the dense layer through thermal convection in the overlying mantle and (4) diluting of the dense layer through inner convection. When <i>B</i>_eff reaches the instability point, <i>B</i>_eff = 1, effective thermo-chemical convection starts, and the mantle will be mixed (<i>B</i>_eff = 0) over a short time period. A parabolic relationship was revealed between the initial density difference of the layers and the mixing time. Morphology of large low-shear-velocity provinces and results from seismic tomography and normal mode data suggest a value of <i>B</i>_eff &ge; 1 for the mantle