The vulnerability of groundwater from chemical leaching through soil is a concern at some locations. Because measurements are laborious, time-consuming, and expensive, simulation models are frequently used to assess leaching risks. But the significance of simulated solute movement through a layered soil is questionable if vertical homogeneity of physical soil properties has been assumed.In the present study, a semi-analytical model for solute leaching in soils is presented. The model is relatively simple, but it does account for soil layers having different physical properties. The model includes the mobile-immobile model (MIM) to describe one-dimensional (1-D) nonequilibrium, transient solute transport under steady-state flow conditions. The MIM is rewritten as a second-order differential equation and solved by a numerical scheme. Differing from fully analytical or fully numerical solutions, the new approach solves the differential equation numerically with respect to time and analytically with respect to distance.Numerical experiments for a single layered soil profile show that the semi-analytical solution (SA-MIM) is numerically stable for a wide range of parameter values. The accuracy of SA-MIM predictions is comparable to that of analytical solutions. Numerical experiments for a multilayered profile indicate that the model correctly predicts effluent curves from finite layered soil profiles under steady-state flow conditions. The SA-MIM simulations with typical parameter values suggest that neglecting vertical heterogeneity of flow paths in a layered soil can lead to inaccurate prediction of soil-solute leaching. The quality of predictions is generally improved if parameter estimates for the different soil layers are considered. However, the mobile-immobile-parameter estimates obtained in a number of previous studies may not be transferable to a field situation that is characterized by a slow and steady flow of water. Further field experiments to determine mobile-immobile parameters under such conditions are desirable.

A three-year long lysimeter experiment with a fine-grained aluminum (Al) recycling by-product and a mixture of this by-product and a coal combustion waste was conducted. The wastes were proposed as possible soil substitutes in an engineered surface barrier covering a potash mining residue mount. To evaluate the suitability of the wastes as surface barrier material, their hydrological behavior under field conditions must be known. Lysimeter experiments provide one means to study the hydrological behavior of soils or soil-like materials. However, it is difficult to estimate the long-term hydrological behavior from short-term lysimeter studies. The present study was conducted therefore to derive from short-term lysimeter observations the long-term hydrological behavior of the two waste materials. The lysimeter data were used to calibrate the one-dimensional soil water flow model HYDRUS-1D. With the calibrated model, hydrological simulations for the site of the residue mount were carried out for a period of 31 yr. Calculated long-term annual seepage from the lysimeters was 237 mm for the pure Al waste and 186 mm for the mixture, or 39% and 24% of the average annual precipitation (764 mm). The average discharge of the bare mount is 482 mm or 63%. We conclude that a soil cover could considerably reduce the discharge and that the mixture is better suited as surface barrier than the pure Al waste.

It seems that the frequency of floods along the main rivers in Germany increased during the second half of the past century. A number of causes for this phenomenon have been suggested. We hypothesise that postwar changes in agricultural land use also play a role. For example, the meadowland area in former West Germany decreased between 1951 and 1989 from 15.7 to 10.8%. Simultaneously, the small grain acreage grew from 18.5 to 22.3%. Additionally, nearly 20% of the agricultural land area was drained artificially during this period. We used the US Soil Conservation Service rainfall-runoff model for small drainage basins to estimate the possible increase in surface runoff during heavy rainstorms because of the observed changes in agricultural land use. Our model calculations suggest that increased surface runoff during large-scale heavy rainstorms may contribute substantially to the present flood problem in Germany.

Preferential flow in soil can enhance the leaching of agricultural chemicals. In a number of studies it has been shown that the mobile-immobile solute transport model (MIM) is a useful tool to characterize preferential flow. In the present study, a new laboratory method for determining the MIM parameters theta(m) and theta(im) (mobile and immobile water content), as well as alpha (mass transfer coefficient), is developed. The computations are uncomplicated and the method requires only simple equipment. It is applied to short, undisturbed soil columns. Measured values ranged from 0.11 to 0.27 for theta(im) theta(-1) and from 0.015 h(-1) to 0.034 h(-1) for alpha for an Iowan soil (Nicollet silt loam). For two sandy Eutric Gleysols from Germany, low values for theta(im) theta(-1) from 0.04 to 0.07 and from 0.001 h(-1) to 0.008 h(-1) for alpha were determined. Although the new method is a flow-interruption technique, values for the Nicollet silt loam compare well with those from conventional leaching experiments. Values for the Eutric Gleysols agree with the observation that these soils were poorly structured. Because the new method does not assume negligible dispersion, it is applicable to a wider range of soils and boundary conditions than comparable approaches. We conclude that the new method provides parameter values that are suited to describe non-equilibrium solute transport.

In Germany, field-average soil NO3- measurements are used to identify agricultural risks of groundwater pollution and to evaluate the compliance of land users with environmental regulations. In the present study, it was tested at three typical agricultural sites if common practices of soil NO3- sampling were appropriate to obtain reliable estimates of the mean field NO3- content in fall. Three sites of 1 ha area were extensively sampled for NO3-. Ordinary statisticial analyses were conducted on the NO3- data in order to calculate how many samples per ha were required to predict the mean field NO3- with a given accuracy. Variograms were derived to assess applicability and relevance of ordinary statistical methods for heterogeneous fields.Results from statistical analysis suggest that at two sites common practices of soil NO3- sampling would have been adequate to obtain estimates of the actual mean field NO3- content with a sampling error less than 10kg NO3--N ha(-1) at a 95% probability level. At the other site, common practices obviously would have failed because NO3- contents varied much more spatially. It remains a problem of soil sampling for NO3-x analysis that information on field heterogeneity is frequently not available a priori.