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.

In Germany, mounds of solid wastes (mainly rock salt, NaCl) from potash mining produce large amounts of briny runoff which are frequently conveyed into surface waters. This study was conducted to evaluate a saline, fine-grained aluminum recycling by-product as soil substitute in a surface barrier over potash mining waste mounds. Four free-drainage lysimeters were monitored for three years under field conditions. Two lysimeters were filled with a by-product from aluminum recycling, and the other two with a mixture of this by-product and a coal combustion waste. Precipitation, evaporation, discharge, as well as pressure head in three depths were measured continuously. Electrical conductivity in the discharge and in suction cup solutions from four depths was used to monitor desalinization. It was found that mean annual discharge from the pure metallurgical waste and from the mixture was 39% and 24% of rainfall. Materials were sufficiently leached to support growth of ryegrass (Lolium perenne, L.) after 444 (pure metallurgical waste) and 281 mm (mix) of seepage, or 28 months of lysimeter operation. We conclude that the mix seems the better material for an engineered mine-waste surface barrier.

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.

Disposal of highly saline industrial by-products in landfills is not permitted in member states of the European Union, such as Germany. Large amounts of such by-products thus have to be disposed of in alternative ways. In many countries bare potash mining residue mounds, consisting almost entirely of rock salt (NaCl), pose environmental problems. Covering such mounds with soil or soil-like material could help to reduce the yearly amount of briny runoff. A fine-granular saline aluminum recycling by-product (ALRP) has been proposed as a soil substitute to cover rock salt residue mounds. Use of this by-product as a combined soil substitute and surface barrier is not considered to be a landfill disposal, but as a beneficial by-product reuse. To judge the feasibility of ALRP for this purpose, its properties must be known. In this study physical characteristics of an industrially produced ALRP, mixed with the flue gas desulfurization by-product (FGDP) of a coal combustion power plant, were determined. It was found that the texture of both ALRP and ALRP-FGDP mix was silt loam. Bulk densities of ALRP and ALRP-FGDP were 0.93 and 0.88 Mg x m(-3) and the corresponding salt contents were 50.0 and 35.5%, respectively. The erodibility factor K of pure ALRP was estimated as 0.65 Mg h x ha (-1)N(-1). Because of the stabilizing effect of FGDP, this factor was reduced considerably in ALRP-FGDP. The water-holding capacity of unwashed ALRP was 44.5% and of washed ALRP-FGDP 61.8%. In view of its physical properties, ALRP-FGDP seems to be suitable as an evaporation enhancing, runoff reducing cover material for potash mine residue mounds, even on steep slopes. Use of ALRP, mixed with FGDP, as a soil substitute in a surface barrier, thus seems to be environmentally meaningful. However, the high salt content initially prevents plant growth. With time, after the salt has been leached, the material seems able to support plant growth, which would further reduce runoff. The physical and hydraulic parameters determined in this study may serve future users of similar by-products.

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.

Despite a common interest of soil scientists, groundwater hydrologists, agronomists, and climatologists in quantifying soil nitrogen losses through denitrification, these are often neglected when the I nitrogen turnover of agricultural soils is modelled. This is the first of two papers describing a semi-quantitative denitrification model that has been derived from data collected from soils of a catchment area near Viersen in the Lower Rhine Valley in West Germany. A field study was conducted to study the soil environment of denitrifying microorganisms. The results of this study were used to evaluate the effect of soil water, soil air and soil temperature, bf the amount and quality of the organic matter, and of the pH-value on denitrification. To this end an existing model to quantify denitrification was extended. Provided the seasonal nitrogen excess of a field is known, the model in its new form enables an estimation of the extent of field-specific denitrification rates. This paper describes the effects of the parameters under consideration on denitrification, the model components used to describe these parameters, and the development of the denitrification model.

This is the second of two papers describing a denitrification model. Whereas in the first paper the model approach was described, the second paper deals with results obtained with the model. To evaluate the model's performance, 14 profiles in the catchment area of a water-work were studied in detail. For each site the potential and the actual annual denitrification rater were estimated. It was found that the least favorable conditions for denitrification occurred in the sandy soils of the study area. Consequently, estimated denitrification rates were lowest (< 10 kg N ha(-1) a(-1)) in these soils. Estimated denitrification rates were highest (50 kg N ha(-1) a(-1)) in peat and river plain soils, with either a high amount of organic matter and/or a high groundwater table. In silty loess soils, denitrification losses were estimated to be 20 to 35 kg N ha(-1) a(-1). With use of the anion concentrations in the surface layer of the groundwater, a plausibility study was conducted to evaluate the estimated denitrification losses. Its results show that the values obtained are realistic. However, a true calibration of the model is still necessary.