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Flow through soil into groundwater has been classically conceptualised as taking place through a set of aligned capillary tubes. In solute transport models these approximations are also present. Pore-Cor (a network model) has been used to model the void structure of soil by using water retention and mercury porosimetry curves. The model successfully predicts trends in saturated hydraulic conductivity. The effect of the assumptions used in the Pore-Cor geometry have been investigated by comparing of two dimensional slices of the simulated networks with two dimensional image analysis data. The geometric limitations of the model cause packing inefficiencies which prevent the model from representing the size distribution of voids found in real samples. The observation of environmental events is dependent upon the implementation of rapid and reliable analytical techniques. This work presents an adaptation of an FI method for the determination of dissolved reactive phosphorus (DRP) and a new method for the determination of total dissolved phosphorus (TDP). Both are ideally suited to the detection of phosphorus species in soil leachate and runoff waters over the concentration range 3 to 1000 Ilg ri. The effect of compaction on solute transport is described and the experimental data have been modelled using a modified form of the convection dispersion equation (CDE). The parameters of the CDE have been given structural interpretation by the network model. The model was used to interpret a change in dispersivity and the behaviour of reactive phosphorus species on compaction.

Cassiterite extraction in West Malaysia has left a legacy of some 80,000 hectares of nutrient-poor, drought-prone, quartz sandy tailings. Their proximity to urban centres has long made them a target for agricultural and forestry enterprises, and various techniques have been developed to overcome their intractability, including amendment with bulky organic matter and replacement with natural soils. Although reasonably successful, these approaches have often failed to acknowledge that a percentage of the original heavy mineral suite remains in sandy tailings, exerting possible phytotoxic effects and certainly raising concerns about the suitability of sandy tailings for agronomic crop production. To determine the nature and extent of trace element contamination of sandy tailings, ten different areas were identified and sampled in the states of Selangor and Perak. Preliminary analyses failed to corroborate earlier discoveries of elevated mercury and cadmium status, but lead, zinc, arsenic, uranium and rare earth elements were all common. However, greater concentrations of all these elements were found in clay (slime) tailings, which had been colonized by dense vegetation cover. This indicated that levels of trace elements in sandy tailings were not of themselves phytotoxic, but in the context of reduced nutrient availability, might exert a disproportionate influence on their colonization by plants. The colonization of sandy tailings by Melastoma malabathricum was therefore examined, revealing that in two of four sites, water contents were greater in colonized areas than the rest of each site. Macronutrient concentrations were also greater in colonized tailings, as were concentrations of zinc and manganese, but whether these were the cause or result of plant establishment could not be determined conclusively. Overall, trace element distribution in sandy tailings did not influence colonization by Melastoma, although in three of four populations, foliar concentrations of uranium were greater than those of a population growing on a natural soil. This has serious implications for the agronomic use of sandy tailings. The effects of bulky organic and clay amendment of sandy tailings on the availability and uptake of trace elements has never before been examined. A tropical greenhouse experiment was undertaken to examine the effects of peat and clay on the growth and trace element chemistry of Panicum miliaceum and Pueraria phaseoloides. The use of symbiotic mycorrhizal fungi was also included, since although these often improve the phosphorus nutrition of their hosts, they might also increase trace element uptake from pools sorbed by organic matter. Neither clay nor peat improved the growth of either plant species. However, addition of clay substantially increased the availability of aluminium and trace elements, and its use in rehabilitation attempts cannot be recommended. In contrast, peat reduced the availability of Co, Cu, As, La, Ce and U, whilst simultaneously improving the ECEC of sandy tailings. The effects of mycorrhizal inoculation were difficult to interpret, since the carrier medium used (predominantly zeolite) drastically altered the chemistry of soils to which it was added. Growth of Panicum and Pueraria was equally poor on two of three naturally sandy soils. Adding peat to sandy tailings produced plants with similar foliar chemistry to those grown on natural soils, suggesting that plant communities established on these natural siliceous media could act as models for the sustainable rehabilitation of anthropic mine tailings - something which has never before been suggested. A range of native species are recommended as having potential for further research into naturalistic rehabilitation.

In semi-arid environments, the combination of a non-uniform distribution of vegetation, an often highly irregular terrain and complex geological, pedological and management histories have frequently given rise to considerable spatial variability in the physical and hydrological properties of soils. Heterogeneity within the soil's physical and hydrological properties can result in pronounced differences in infiltration and soil moisture. The hydrological response of semi-arid landscapes to rainfall events may therefore be spatially non-uniform. Quantifying the spatial pattern of hydrological response is important for identifying those areas within the landscape which arc vulnerable to runoff and erosion. Since soil moisture is considered to be a key factor in determining hydrological response and its spatial distribution is a function of the soil's physical and hydrological properties, the spatial and temporal measurement of soil moisture may be used to identify contrasting areas of hydrological response. In a badlands environment located approximately 70 km north of Madrid, central Spain, an experiment was established to describe the temporal and spatial variability in soil moisture at three scales, with the primary aim of furthering the understanding of the hydrological and geomorphological processes operating in semi-arid landscapes. At each measurement scale, the macroscale (25m sampling interval), the mesoscale (gully catchments, 5m sampling interval) and the microscale (1 m sampling interval), two distinct groups of soil moisture conditions emerged related to dry and wet weather conditions. At each measurement scale the maximum variability in soil moisture is similar (>20% volumetric content difference between immediately adjacent sampling points). At the meso and microscale the spatial pattern of soil moisture could be described as a mosaic pattern which during the dry period was more fragmented and variable than during the wet period. The spatial pattern of soil moisture during wet conditions is more uniform due to the development of extensive wet areas within the catchments. During these conditions the range of spatial correlation in soil moisture may double (to greater than 30m) compared to dry conditions, indicating an increase in the spatial continuity of soil moisture. The spatial variability in soil moisture therefore displays a temporal dependency; the mosaic soil moisture pattern is more fragmented and spatially discontinuous during dry than wet conditions. A striking characteristic of the study area is the near horizontal interbedding of sediment horizons which may strongly contrast in their textural composition over relatively short distances. This variability in soil texture and the associated changes in pore size characteristics, were the principal controlling factors in determining the spatial patterns of soil moisture and overrides the known influence of vegetation and topography on soil moisture. During dry conditions the non-uniform uptake of soil moisture by vegetation may partly explain the greater variability in soil moisture observed during this period. The mosaic patterns of soil moisture represent areas of contrasting hydrological response. During dry periods when the mosaic pattern is more fragmented, source areas of overland flow are spatially isolated and surrounded by 'sink' areas capable of re-absorbing runoff and sediment deposition. Hydrological pathways are therefore discontinuous resulting in minimal runoff reaching the catchments channels. Since soil moisture values during this period are below saturation, any runoff which does occur is generated as infiltration excess overland flow. In semi-arid areas spatial variability in soil properties or vegetation patterns may therefore be beneficial for runoff and erosion control by creating a self-regulating system in which runoff producing areas are surrounded by buffer zones capable of re-absorbing the runoff. During wet periods extensive areas of the catchments may be saturated. source areas are no longer spatially isolated and continuous hydrological pathways may develop rapidly during this period. During the wet period when conditions arc above a critical saturation threshold value widespread runoff will occur regardless of the spatial variability in the soil's physical and hydrological properties. The creation of a mosaic pattern in which buffer zones are adjacent to potential runoff producing areas, as identified from spatial soil moisture patterns, may provide the most effective management strategy in runoff and erosion control for degraded semi-arid environments. The creation of a mosaic pattern is most applicable at the watershed scale allowing several land uses, including those which are potentially degrading, to co-exist. Increasing the critical threshold value above which widespread runoff occurs should also.be included as part of this management strategy.

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