Ruud J. Schotting

Characterization of geochemical constituents and bacterial populations associated with As mobilization in deep and shallow tube wells in Bangladesh.

While millions of people drink arsenic-contaminated tube well water across Bangladesh, there is no recent scientific explanation which is able to either comprehensively explain arsenic mobilization or to predict the spatial distribution of affected wells. Rather, mitigation strategies have focused on the sinking of deep tube wells into the currently arsenic-free Pleistocene aquifer. In this study, Bangladesh shallow tube wells identified as contaminated and uncontaminated, as well as deep tube wells, were analyzed for geochemical and in situ microbiological composition. Whereas arsenic was detected in all Holocene aquifer wells, no arsenic was found in wells accessing the Pleistocene aquifer. Bacterial genera, including Comamonadaceae, Acidovorax, Acinetobacter, and Hydrogenophaga, associated with tolerance of high arsenic concentrations, rather than dissimilatory Fe(III) or As(V) reduction, were identified in shallow tube wells, indicating that mobilization may not occur at depth, but is rather due to drawdown of surface contaminated water. Deep tube wells contained microbes indicative of aerobic conditions, including the genera Aquabacterium, Limnobacter, and Roseomonas. It is concluded that through drawdown of arsenic or organic matter, further utilization of the Pleistocene aquifer could result in contamination similar to that observed in the Holocene aquifer.

Analytical modelling of fringe and core biodegradation in groundwater plumes.

Biodegradation can be divided into two categories depending on the location at which it occurs within the plume: degradation at the plume fringes, and degradation in the interior (core). Available analytical solutions are limited to the consideration of either fringe or core degradation, which in turn limits the applicability of these solutions. Here, a new analytical approach to modelling plumes with both fringe and core degradation is presented. The approach relies on the use of readily available analytical solutions for solute transport. Using a well-known solution for three-dimensional solute transport from a planar source, an approximate solution is derived for the maximum plume length at steady-state conditions. This is verified through the use of a numerical solution. The solution suggests that the parameters controlling the plume length are: (i) the size of the contaminant source, (ii) electron acceptor to electron donor ratio, (iii) transverse dispersivities and (iv) the ratio between degradation rate constant and velocity (lambda/v). The latter term provides a simple check on the relative weights of transport to core degradation and can be used to estimate the importance of core degradation in the overall plume attenuation. The well-documented Bemidji field site has both fringe and core degradation. The new combined degradation model estimates the length of the plume with 10 m of the observed length; core only and fringe only solutions overestimate the length by more than a factor of 2.

The Interface Between Fresh and Salt Groundwater in Horizontal Aquifers: The Dupuit–Forchheimer Approximation Revisited

We analyze the motion of a sharp interface between fresh and salt groundwater in horizontal, confined aquifers of infinite extend. The analysis is based on earlier results of De Josselin de Jong (Proc Euromech 143:75–82, 1981). Parameterizing the height of the interface along the horizontal base of the aquifer and assuming the validity of the Dupuit–Forchheimer approximation in both the fresh and saltwater, he derived an approximate interface motion equation. This equation is a nonlinear doubly degenerate diffusion equation in terms of the height of the interface. In that paper, he also developed a stream function-based formulation for the dynamics of a two-fluid interface. By replacing the two fluids by one hypothetical fluid, with a distribution of vortices along the interface, the exact discharge field throughout the flow domain can be determined. Starting point for our analysis is the stream function formulation. We derive an exact integro-differential equation for the movement of the interface. We show that the pointwise differential terms are identical to the approximate Dupuit–Forchheimer interface motion equation as derived by De Josselin de Jong. We analyze (mathematical) properties of the additional integral term in the exact interface motion formulation to validate the approximate Dupuit–Forchheimer interface motion equation. We also consider the case of flat interfaces, and we study the behavior of the toe of the interface. In particular, we give a criterion for finite or infinite speed of propagation.

The Effect of Grain Size Distribution on Nonlinear Flow Behavior in Sandy Porous Media

The current study provides new experimental data on nonlinear flow behavior in various uniformly graded granular materials (20 samples) ranging from medium sands (

Editorial to the first issue of modeling earth systems and environment

d_{50 }>0.39

Heat and brine transport in porous media: the Oberbeck-Boussinesq approximation revisited

d50>0.39 mm) to gravel (

Modeling Transverse Dispersion and Variable Density Flow in Porous Media

d_{50}=6.3