Prediction in hydrogeology: two case histories

An inherent aspect of hydrogeology is the dynamic character of groundwater flow within the more passive lithospheric medium. Prediction therefore requires insight into the spatial pattern of aquifer properties as well as the dynamic character of groundwater flow systems. Both aspects together determine hydrogeological system behaviour in space and time. The spatial characteristics of an aquifer are mainly based on geological and geomorphological structure, whereas the dynamics of the system result predominantly from climate and topography; knowledge of paleoconditions are a prerequisite for sound understanding of present systems. The integration of exploration and forecast is illustrated by two case studies. The first study focuses on an assessment of groundwater resource and their sustainable development in semi-arid Botswana. The second study concerns the impact of land and water management in the Netherlands coastal area on the groundwater regime and its future development, including the redistribution of salt and fresh groundwater on a long time scale.     

Sediment origin and budget in Sepetiba Bay (Brazil) - an approach based on multielemental analysis

 Sediment origin and transport were determined in a shallow 447 km² coastal lagoon in Brazil, using the distribution of major elements in bottom and suspended sediments. Applying multivariate analysis, the sum of normalized concentrations of Ti, Fe, and Al in sediments was selected to trace the terrigenous influence in bottom sediments, whereas Ca, Si, and K were used as indicators of marine influence. Sepetiba Bay bottom sediments are dominated by up to 80% terrigenous inputs. The open bay sediments are enriched in P, Mg, K, and Ca because of the intensive water column primary production. The inner bay forms an independent circulation cell with a predominance of terrigenous sediments. The results suggest a long residence time for particles in the bay, demonstrated by the magnitude of resuspension flux compared to the total sediment input to the bay. Received: 29 May 1996 · Accepted: 17 December 1996     

Analysis of DNAPL infiltration in a medium with a low-permeable lens

In this paper we study the infiltration of DNAPL in a porous medium containing a single low-permeable lens. Our aim is to determine whether or not DNAPL infiltrates into the lens. A key role is played by the capillary pressure: DNAPL cannot infiltrate into the lens unless the capillary pressure exceeds the entry pressure of the lens. In the model this is reflected by an interface condition, the extended capillary pressure condition. To derive analytical approximations we first consider a steady-state DNAPL plume in a homogeneous medium. This results in an estimate of the DNAPL plume width as a function of depth, and an asymptotic solution for small saturations. Assuming that the extent of the lens is much larger than the width of the unperturbed DNAPL plume in the homogeneous medium, we derive an explicit criterion for DNAPL infiltration into the lens in terms of a critical inflow rate. A numerical algorithm is presented in which the extended capillary pressure condition is incorporated. The numerical and analytical results show good qualitative agreement. This revised version was published online in August 2006 with corrections to the Cover Date.     

The Influence of Geomorphologic Heritage on Present Nival Erosion: Peñalara, Spain

This article examines how snow plays a role in current erosive processes in a high mountain area (1800—2400 m a.s.l.) known as Peñalara, located in Spain's Central Range (40°50' N; 3°58' W). The hypothesis maintains that snow becomes an important erosive factor when it accumulates over sedimentary or weathered materials, therefore geomorphological heritage is a key factor in nival erosion. To test this hypothesis, the authors identified the landforms in the study area and determined their relative ages by weathering and lichenometry ( Rizocarpon geographicum ag. ), differentiating between preglacial, glacial (Recent Pleistocene) and postglacial (Holocene) forms. The information was used to plot a reticulate pattern of observation sites for the study area. Snow depth and the movement of selected blocks at each site were recorded from October 1991 to June 1995. The relationship between late-lying snowpatches, geomorphological heritage and current erosive processes was determined. Between 1800 and 2000 m a.s.l., there is an indirect relationship between snowpatches and predominant processes (stream incision and gelifluction) on terminal moraines. Between 2000 and 2200 m, direct action is present where there are late-laying snowpatches on lateral moraines and some glacial steps. Between 2200 and 2400 m, gelifraction and gravity processes are also in direct relation to snowpatches.     

A general correspondence principle for time-domain electromagnetic wave and diffusion fields

A general correspondence principle is presented that relates any time-domain electromagnetic diffusion field to an electromagnetic wavefield in a 'corresponding' configuration. The principle applies to arbitrarily inhomogeneous and anisotropic media and arbitrary transmitters and receivers. For the correspondence between the two types of electromagnetic fields to hold, the electric conductivity in the diffusive case and the permittivity in the wavefield case should have the same spatial variation, while the permeability distributions in space in the two cases are to be identical. Essential steps in the derivation of the correspondence principle are the use of the time Laplace transformation of causal signals, taken at real, positive values of the transform parameter, the Schouten-Van der Pol theorem in the theory of the Laplace transformation, and the reliance upon Lerch's theorem of the uniqueness of the interrelation between causal field quantities and their time-Laplace-transform representations at real, positive values of the transform parameter. Correspondence is then established between the tensorial Green's functions in the two cases, where the Green's functions are the point-receiver responses (either electric or magnetic field) to point-transmitter excitations (either electric- or magnetic-current source).
Through the correspondence principle, all transient electromagnetic wavefields (where losses are neglected) have as a counterpart a transient diffusive electromagnetic field (where the electric displacement current is neglected). The interrelation yields the tool to compare quantitatively the potentialities of the two types of fields in transient electromagnetic geophysical prospecting.
Finally, a general medium-parameter scaling law for time-domain electromagnetic wavefields is presented.     

Calculating the two-dimensional magnetotelluric Jacobian in finite elements using reciprocity

To speed up the calculation of the field Jacobian for 2-D magnetoteliuric inversion using finite elements, the principle of electromagnetic reciprocity is applied. The governing relationship for the Jacobian of the field along strike is obtained by differentiating the Helmholtz equation with respect to the resistivity of each region in the finite-element mesh. The result is a similar Helmholtz equation for the Jacobian, with new sources distributed over all nodes within the parameter medium. However, according to the principle of electromagnetic reciprocity, the roles of sources and receivers are interchangeable. Utilizing reciprocity, the field values obtained from the original forward problem and for new unit sources imposed at the receivers are then utilized in the calculation of the Jacobian by simple multiplication and summation with finite-element terms at each rectangle in the mesh. For the auxiliary (across-strike) fields, the Jacobian terms are obtained by solving source vectors loaded with parabola coefficients used in the approximation to Maxwell's equations. Jacobian terms for the apparent resistivity ( p _a), the impedance phase (φ) and the vertical magnetic field ( K _zy) are then calculated utilizing the parallel- and auxiliary-field Jacobians. Comparison of Jacobian values obtained from reciprocity calculations and by differencing two forward solutions show that the reciprocity method is accurate and can be used to decrease the number of calculations required to obtain sensitivities by one to two orders of magnitude.     

Limiting the soil degradational impacts of wildfire in pine and eucalyptus forests in Portugal: A comparison of alternative post-fire management practices

In newly burnt and unburnt pine and eucalyptus forest in Portugal, overland flow and soil losses were monitored to assess the impacts of the following post-fire treatments: application of different quantities of logging litter; rip-ploughing compared with minimum tillage prior to planting eucalyptus seedlings; and clearance of pine needles and vegetation. Eucalyptus logging litter reduced soil losses by up to 95 per cent. The impact of pine logging litter was equivocal, but removal of pine needles increased soil losses elevenfold. Implications for soil longevity, soil quality and land management strategy are discussed.     

Transport in a 2-D saturated porous medium: A new method for particle tracking

A new method for solving the transport equation based on the management of a large numbe of particles in a discretized 2-D domain is presented. The method uses numerical variables to represent the number of particles in a given mesh and is more complex than the 1-D problem. The first part of the paper focuses on the specific management of particles in a 2-D problem. The method also would be valid for three dimensions as long as the medium can be modeled similar to a layered system. As the particles are no longer tracked individually, the algorithm is fast and does not depend on the number of particles present. The numerical tests show that the method is nearly numerical dispersion free and permits accurate calculations even for simulations of low-concentration transport. Because each mesh is considered as a closed system between two successive time steps, it is easy to add adsorption phenomenon without any problem of numerical stability. The model is tested under conditions that are extremely demanding for its operating mode and gives a good fit to analytical solutions. The conditions in which it can be used to best advantage are discussed.