The heterogeneous nature of upland hard-rock catchments in terms of geology, geomorphology, superficial deposits, soil type and land use gives rise to a range of hydrochemical characteristics in stream waters. This is further complicated by the large and often rapid changes in stream flow typical of storm events. The sources of solutes and flow pathways in hard-rock catchments are still poorly understood, in particular the role of bedrock groundwater. Spatial variations in water chemistry are presented for stream waters, soils and groundwaters in the forested Plynlimon catchment of Wales, UK. The results highlight a large degree of spatial heterogeneity in each of these systems. This has major implications for the application of end-member mixing analysis and presents serious problems for modelling in scaling up from study sites to catchment scale. However, such data provide important constraints on sources, flow pathways and residence times within individual catchment compartments, knowledge of which is essential for understanding how such catchments function. The characterisation of sub-surface waters in upland catchments requires a great deal of care during sampling as well as high spatial and temporal resolution of sampling, and further work is required to characterise the Plynlimon catchments fully. Nevertheless, the presence of an active and highly stratified groundwater system is considered important as a source of solutes and water to streams. It also provides a storage medium that is likely to make a major contribution to explaining the strongly damped rainfall Cl and d 2 H signals measured in the streams.

Abstract. A series of boreholes of up to 50 m depth, drilled into Lower Palaeozoic mudstone, shale and greywacke bedrock in the headwater catchment areas of the River Severn at Plynlimon in Central Wales, shows an extensive chemically- and hydrologically-active shallow groundwater fracture flow system. Groundwater chemistry varies in space and time with lowest water levels and highest alkalinities occurring during the drier summer months. The groundwaters are enriched in base cations, silica, sulphate and alkalinity relative to surface waters indicating significant silicate weathering sources and sulphide oxidation. These sources provide important contributions to both stream water quality and flow. At one site, the introduction of a borehole near to the main river opened bedrock fractures which increased the amount of groundwater entering the river. This had a profound effect on the river water quality by increasing the pH, alkalinity and calcium concentrations. As well as pointing to the possibility of the wider availability of groundwater resources in upland areas, the results highlight (a) the potential value of groundwater as a acid neutralizing resource, (b) the importance of weathering processes and flow routing within the groundwater environment for stream water chemistry, (c) the potential for altering stream water quality by manipulation of groundwater routing and (d) the need to include groundwater characteristics in hydrochemical management models of surface water acidification.

Geophysical logging is shown to be a useful technique to support investigations of coastal aquifer hydrogeology. Formation logging can identify the geological layering and fluid logging can characterize the salinity distribution. The measurements also reveal wellbore flow to be common in coastal boreholes, which can mask the salinity stratification in the aquifer matrix. Geophysical logging can be used to guide water sampling and to provide information on the palaeohydrogeology. In combination with water sampling and age determinations, it has shown modern groundwaters overlying Holocene age groundwaters, in turn overlying groundwaters of Pleistocene age, within 150 m of the surface in some of the aquifers studied. Sea-level change in response to Pleistocene glaciations and deglaciations is recognized as a major control on the salinity of groundwaters and on the development of permeable flow routes in coastal aquifers. The permeable routes that developed by groundwater circulation to older and deeper base levels are now partly or wholly occupied by groundwaters of modern flow systems, and can be the focus for saline intrusion. The effects of Pleistocene sea-level change on aquifer development appear to be worldwide and are being increasingly recognized. Examples are described to illustrate the development of flow horizons in relation to rock layering, structure and base levels.

Drilling and geophysical logging of Deccan basalt during groundwater exploration in Central India demonstrated that the Deccan lavas comprise a series of simple lava flows, 15–20 m thick, having a porous upper cooling surface with thin clay development and a layered structure. Geophysical logging has proved capable of resolving the layering and produced consistent log responses. Gamma-ray logs were especially useful to identify the weathered flow surfaces but further revealed certain lavas have distinctly higher radioactivity. The higher radioactivity flows were found to be of regional significance. The resolution of the layering and positive identification of flows by geophysical logging permitted individual lavas to be recognized and traced over distances of at least 100 km with no apparent displacement. The logs provided a framework for a detailed stratigraphic sub-division of the lava sequence, and also a key to investigate lateral and vertical variation in aquifer properties.

The marked climatic changes that occurred during the Late Quaternary and Holocene have had a significant impact on the evolution of the groundwater systems at and near the English coastline. Lowered sea levels and the emergence of a much larger landmass over most of the past 100 ka have also ensured deeper groundwater circulation in the vicinity of the modern coastline. The impacts on the Chalk and Lower Greensand (Albian) aquifers along the English Channel and North Sea coasts are examined, using mainly geochemical and isotopic evidence, especially from borehole depth profiles and interstitial waters.Along the south coast, fresh groundwaters are found to depths of 250-300 m below OD (ordnance datum) in the Brighton-Worthing area, as well as beneath Poole Harbour, which are related to deeper circulation during lowered sea levels, controlled by the central palaeovalley of the English Channel. In contrast, pockets of saline groundwater are found, protected in east-west structures, which are considered to be little-modified Chalk formation waters. In the Albian sands, near Worthing, freshwaters dating to 7 ka Be are found at a depth of -450 m OD, suggesting that movement of groundwater towards the shoreline and possibly beyond is still occurring. The east-west structures also influence groundwater migration in north Kent, where fresh palaeowaters can be identified beneath saline water which invaded during the Holocene sealevel rise. In the East Midlands Sherwood Sandstone aquifer, freshwater is found to a depth of -500 m OD, showing continuous geochemical evolution probably over a period of 100 ka, although an 'age gap' of between c. 20 and 10 ka Bp corresponds to permafrost cover. These palaeowaters in coastal and near-coastal areas remain effectively isolated from the active presentday meteoric flow system but represent high-value resources that may, in some cases, extend offshore.The British Isles lie in a strategic position in Europe for any consideration of the impacts of Late Quaternary climate change on the landscape in general and water resources in particular. The land surface and nearshore areas have been strongly affected by glacial advances and permanent ice cover, by permafrost (especially in southern Britain) and by sea-level changes. Glacio-isostatic changes have had a strong impact on the coastline, especially along the western seaboard, in addition to the global eustatic changes in sea levels. The