Abstract. Characterization of the dynamics of moisture migration in the unsaturated zone of aquifers is essential if reliable estimates of the transport of pollutants threatening such aquifers are to be made. Electrical geophysical investigation techniques, such as ground-penetrating radar, offer suitable methods for monitoring moisture content changes in the vadose zone. Moreover, these tools permit relatively large measurement scales, appropriate for hydrological models of unsaturated processes, and thus they offer a distinct advantage over conventional measurement approaches. Ground-penetrating radar, when applied in transmission mode between boreholes, can provide high-resolution information on lithological and hydrological features. The technique may be applied in tomographic mode and in a much simpler vertical profile mode. Both modes of measurement have been utilized using two boreholes 5 m apart located at a field site in the UK Sherwood Sandstone aquifer. Radar transmission measurements have been used to characterize the change in moisture content in unsaturated sandstone due to controlled water tracer injection. Continual monitoring of cross-borehole radar measurements over an 18 month period has also permitted determination of travel times of natural loading to the system and has revealed the impact of subtle contrasts in lithology on changes in moisture content over time. The time series of inferred moisture contents show clearly wetting and drying fronts migrating at a rate of approximately 2 m month -• throughout the sandstone. IntroductionGeophysical methods have been widely employed for many years as an aid to hydrogeological characterization. The velocity of high-frequency (10 MHz to 1 GHz) electromagnetic waves is directly related to the bulk (composite) dielectric constant of the subsurface. In this frequency range the dielectric constant is primarily controlled by the polarization of individual water molecules and is therefore strongly dependent on volumetric water content [Arulanandan, 1991]. Thus the bulk dielectric constant changes with changes in the water content (0) Site DescriptionThe field site selected for detailed study of the vadose zone dynamics in the sandstone is located 10 km NE of Doncaster, South Yorkshire, at the Lings Farm smallholding near the small town of Hatfield (National Grid Reference SE 653 078). The site was chosen because of the following criteria: (1) close proximity to a sand/gravel quarry permitting detailed largerscale hydrogeological surveys, (2) minimal drift cover, and (3) flat topography and reasonably undisturbed grass cover.Following preliminary surface geophysical surveys and trial auguring, eight boreholes were drilled at the Hatfield site during June/July 1998. The boreholes were drilled using a 127 mm

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[1] The relationship between dielectric permittivity, water saturation, and clay content for the Sherwood Sandstone from NE England was characterized as part of a wider study of the vadose zone moisture dynamics and pollution vulnerability of this aquifer. Dielectric permittivity was measured over the full range of saturation levels, for various lithologies ranging from clean medium-grained sandstone to fine-grained sandstone containing up to 5% clay, using a specially constructed dielectric cell. Dielectric constant, K r , is largely independent of frequency between 350 MHz and 1000 MHz. Below 350 MHz, K r of fine-grained, clay-rich sandstone shows frequency dispersion. Tests on physical models of the sandstone consisting of a fine fraction of Ottawa Sand and montmorillonite clay indicate that the clay minerals within the sandstone are responsible for its frequency dispersive behavior. These tests also show that increasing pore fluid salinity increases dielectric dispersion at the lower end of the frequency range, which indicates that this arises from the interfacial Maxwell-Wagner mechanism associated with platy clay particles. Water saturated sand:clay mixtures show very low dielectric constants at high frequencies (over 650 MHz). This effect is independent of salinity and probably results from the layered geometric arrangement of solids, bound and free water within the swelling clay. The complex refractive index method (CRIM) with a mineral dielectric constant of about 5 provides a good match to the water saturation versus dielectric constant data for all Sherwood Sandstone lithologies at frequencies between 350 and 1000 MHz and for clay-poor sandstone at lower frequencies. Below 350 MHz the presence of a few percent of clay in some Sherwood Sandstone lithologies raises their CRIM best fit mineral dielectric constant substantially.

Environmental change caused by urban development, land drainage, agriculture or climate change may result in accelerated decay of in situ archaeological remains. This paper reviews research into impacts of environmental change on hydrological processes of relevance to preservation of archaeological remains in situ. It compares work at rural sites with more complex urban environments. The research demonstrates that both the quantity and quality of data on preservation status, and hydrological and chemical parameters collected during routine archaeological surveys needs to be improved. The work also demonstrates the necessity for any archaeological site to be placed within its topographic and geological context. In order to understand preservation potential fully, it is necessary to move away from studying the archaeological site as an isolated unit, since factors some distance away from the site of interest can be important for determining preservation. The paper reviews what is known about the hydrological factors of importance to archaeological preservation and recommends research that needs to be conducted so that archaeological risk can be more adequately predicted and mitigated. Any activity that changes either source pathways or the dominant water input may have an impact not just because of changes to the water balance or the water table, but because of changes to water chemistry. Therefore efforts to manage

Geological discontinuities have a controlling influence for many rock-engineering projects in terms of strength, deformability and permeability, but their characterisation is often very difficult. Whilst discontinuities are often modelled as lacking any strength, in many rock masses visible rock discontinuities are only incipient and have tensile strength that may approach and can even exceed that of the parent rock. This fact is of high importance for realistic rock mass characterisation but is generally ignored. It is argued that current ISRM and other standards for rock mass characterisation, as well as rock mass classification schemes such as RMR and Q, do not allow adequately for the incipient nature of many rock fractures or their geological variability and need to be revised, at least conceptually. This paper addresses the issue of the tensile strength of incipient discontinuities in rock and presents results from a laboratory test programme to quantify this parameter. Rock samples containing visible, natural incipient discontinuities including joints, bedding, and mineral veins have been tested in direct tension. It has been confirmed that such discontinuities can have high tensile strength, approaching that of the parent rock. Others are, of course, far weaker. The tested geological discontinuities all exhibited brittle failure at axial strain less than 0.5 % under direct tension conditions. Three factors contributing to the tensile strength of incipient rock discontinuities have been investigated and characterised. A distinction is made between sections of discontinuity that are only partially developed, sections of discontinuity that have been locally weathered leaving localised residual rock bridges and sections that have been 'healed' through secondary cementation. Tests on bedding surfaces within sandstone showed that tensile strength of adjacent incipient bedding can vary considerably. In this particular series of tests, values of tensile strength for bedding planes ranged from 32 to 88 % of the parent rock strength (intact without visible discontinuities), and this variability could be attributed to geological factors. Tests on incipient mineral veins also showed considerable scatter, the strength depending upon the geological nature of vein development as well as the presence of rock bridges. As might be anticipated, tensile strength of incipient rock joints decreases with degree of weathering as expressed in colour changes adjacent to rock bridges. Tensile strengths of rock bridges (lacking marked discolouration) were found to be similar to that of the parent rock. It is concluded that the degree of incipiency of rock discontinuities needs to be differentiated in the process of rock mass classification and engineering design and that this can best be done with reference to the tensile strength relative to that of the parent rock. It is argued that the science of rock mass characterisation may be advanced through better appreciation of geological history at a site thereby improving the ...