Preserving and adapting functions to limited fresh water supply

For agriculture/horticulture and nature, adaptation to decreasing fresh water availability is crucial in the growing seasons. Rainfall becomes concentrated in fewer, but heavier showers, the inlet of good quality water from main water courses will be under pressure, while evapotranspirative demand grows. Particularly for coastal provinces, this causes an increasing influence of brackish/saline ground water that upwells or directly enters the water courses. This influences which plants can be grown, at which infrastructural and other costs, whether agri/horticultural production remains sustainable, how nature develops at „abandoned‟ agricultural areas, and how nature areas and their protection, restoration, and management costs change. A central issue is how agro/ecosystems react to changing salinity

Stochastic soil water dynamics of phreatophyte vegetation with dimorphic root systems

As the direct uptake of deep groundwater by vegetation may be essential in semiarid regions, we incorporated this process in stochastic root zone water balance models. The direct water uptake by vegetation via deep tap roots is simulated using one additional empirical parameter. This is considered for the case of feedback with root zone saturation and without such feedback. The model that accounts for feedback between shallow root zone saturation and groundwater uptake by deep roots takes up less water if the shallow root zone is wet. The behavior of the models demonstrates that for certain combinations of climate and groundwater depths this feedback becomes important in determining differences in total evapotranspiration (ET). This feedback mechanism also captures hydraulic redistribution processes. The range of relative contributions of groundwater to ET predicted by the models was similar to values derived in isotope studie

Measurement network design including traveltime determinations to minimize model prediction uncertainty

Traveltime determinations have found increasing application in the characterization of groundwater systems. No algorithms are available, however, to optimally design sampling strategies including this information type. We propose a first-order methodology to include groundwater age or tracer arrival time determinations in measurement network design and apply the methodology in an illustrative example in which the network design is directed at contaminant breakthrough uncertainty minimization. We calculate linearized covariances between potential measurements and the goal variables of which we want to reduce the uncertainty: the groundwater age at the control plane and the breakthrough locations of the contaminant. We assume the traveltime to be lognormally distributed and therefore logtransform the age determinations in compliance with the adopted Bayesian framework. Accordingly, we derive expressions for the linearized covariances between the transformed age determinations and the parameters and states. In our synthetic numerical example, the derived expressions are shown to provide good first-order predictions of the variance of the natural logarithm of groundwater age if the variance of the natural logarithm of the conductivity is less than 3.0. The calculated covariances can be used to predict the posterior breakthrough variance belonging to a candidate network before samples are taken. A Genetic Algorithm is used to efficiently search, among all candidate networks, for a near-optimal one. We show that, in our numerical example, an age estimation network outperforms (in terms of breakthrough uncertainty reduction) equally sized head measurement networks and conductivity measurement networks even if the age estimations are highly uncertain

Spatial moment analysis of transport of nonlinearly absorbing pesticides using analytical approximations

Analytical approximations were derived for solute transport of pesticides subject to Freundlich sorption, and first-order degradation restricted to the liquid phase. Solute transport was based on the convection-dispersion equation (CDE) assuming steady flow. The center of mass (first spatial moment) was approximated both for a non-degraded solute pulse and for a pulse degraded in the liquid phase. The remaining mass (zeroth spatial moment) of a linearly sorbing solute degraded in the liquid phase was found to be a function of only the center of mass (first spatial moment) and the Damköhler number (i.e., the product of degradation rate coefficient and dispersivity divided by flow velocity). This relationship between the zeroth and first spatial moments was shown to apply to nonlinearly sorbing pulses as well. The mass fraction leached of a pesticide subject to Freundlich sorption and first-order degradation in the solution phase only was found to be a function of the Damköhler number and of the dispersivity, so independent of sorption. Hence perceptions of the effects of sorption on pesticide leaching should be reconsidered. These conclusions equally hold for other micropollutants that degrade in the solution phase onl

Solute Transport in Soil

Solute transport is of importance in view of the movement of nutrient elements, e.g. towards the plant root system, and because of a broad range of pollutants. Pollution is not necessarily man induced, but may be due to geological or geohydrological causes, e.g. in the cases of pollution with arsenic, and salt. For the polluting species, a distinction can be made between dissolved and immiscible, and between conservative and reactive. Dissolved pollutants (aqueous phase pollutants) will spread with the groundwater due to groundwater flow, diffusion and dispersion

Modelingof air sparging in a layered soil : numerical and analytical approximations

Air sparging in an aquifer below a less permeable horizontal layer is modeled using a two-phase flow approach. Supported by numerical simulations we show that a steady state situation is reached. For an analysis of the steady state we distinguish three different flow regimes, which occur between the well screen and the unsaturated zone. Just below the interface, that separates the high and the low permeable layers, a regime with almost hydrostatic capillary pressures develops. We use this observation to derive an ordinary differential equation for the pressure at the interface, which leads to an approximation of the air flow pattern just below and within the low permeable layer. The approximation provides an estimate for the radius of influence as a function of the physical parameters. The agreement between the analytical approximation and the numerical steady state results is almost perfect when heterogeneity is increased. With a few modifications the analysis applies also to a DNAPL spill above a less permeable layer. Comparison with an illustrative numerical simulation shows that the analytical approximation provides a good estimate of the radial spreading of the DNAPL flow on top of and within the low permeable layer

Analysis of oil lens removal by extraction through a seepage face

Removal of LNAPL (oil) from an aquifer is described using a multi-phase flow model. At the well boundary seepage face conditions are imposed. These conditions are implemented in a numerical model and withdrawal in a two-dimensional domain is simulated for two different geometries of the oil lens and for varied values of the physical parameters. Assuming vertical equilibrium, the oil flow equation is reduced by vertical integration. The well boundary condition is approximated by imposing zero oil lens thickness. Similarity solutions of the reduced equations for the two geometries show good agreement with the numerical results in most cases

Transport of reactive contaminants in heterogeneous soil systems

Transport of reactive contaminants was studied in soil systems that exhibit pronounced variability with respect to the flow and sorption parameters and the solute feed function at the inlet boundary. Emphasis was given to the sorption and transport of orthophosphate (P) in soil. An approximate P sorption kinetics model was derived, that is based on a mechanistic description of reaction processes at the microscopic scale. The approximate model, that involves a reversible adsorption according to Langmuir kinetics, and an irreversible diffusionprecipitation reaction, as a function of a concentration scaled time variable, described experimental sorption and desorption data well. Validation by predicting P- transport using independently assessed sorption parameter values, showed a reasonable agreement between experimental and numerical results. With the distributions of sorption model parameters and soil variables found for a field and a watershed, transport was simulated for homogeneous and heterogeneous soil systems at conditions resembling those of the field. It appeared that only a smaller part of sorbed P is subject to desorption, and that transport in many cases conforms to shock front displacement. Transport at field conditions, furthermore, appeared to be largely controlled by P-sorption and to a lesser extent by flow.Assuming soil in the field may be described as a number of parallel, noninteracting columns, and assuming piston type displacement, P-transport was described if the sorption capacity and P-input differ for each column, using stochastic theory. The main trends for P-displacement in such a heterogeneous field were in agreement with experimental data, and showed large differences with the solutions of the convection-dispersion equation for average parameter values. An analytical solution for a more specific case supported these findings, for heavy metal transport, and showed faster breakthrough in the small concentration range than expected using average parameter values. The effect of transversal Interaction between two layers with different properties showed that the loss of solute from the layer with the largest transport velocity may be significant, when at the sharp interface, in the direction of flow, soil properties vary much