Chlorofluorocarbons (CFCs) and sulphur hexafluoride (SF 6 ) provide a technique for dating groundwater up to 50 years old. When used together, CFCs and SF 6 can help to resolve the extent to which groundwater mixing occurs, and therefore provide indicators of the likely groundwater flow mechanisms. Modelling shows that diffusive retardation of these tracers is likely to be low owing to the high moisture content of the chalk unsaturated zone. Data collected from groundwater and surface water from a lowland Chalk catchment in southern England suggest that groundwater movement can be divided into three regimes: on the interfluves of the catchment, 'piston' flow dominates with a bulk groundwater age of several decades; at the valley bottom, there is mixing between shallow groundwater and stream water; and in an intermediate zone between the top and the bottom of the valley there is approximately 3:1 mixing between new and pre-tracer groundwaters. A conceptual model of groundwater movement has been developed to describe the catchment processes. Surface water-groundwater interactions are found to take place down to depths in excess of 10 metres bgl.The nitrate found at the greatest depth are thought to originate from the mid 1950s.

Understanding the processes controlling groundwater-surface water interaction is essential for effective resource management and for protecting sensitive ecosystems. Through intensive monitoring of Chalk groundwater, surface water, and shallow gravel groundwater along a river bank and below the river, using a combination of hydrochemical and hydrophysical techniques a complex pattern of interactions has been elucidated. The river is broadly in hydraulic contact with the river bed and adjacent gravels and sands (although with local variability), but these sediments are mainly hydraulically separate from the underlying Chalk at the site. The relationship between the river and underlying alluvium is variable, involving components of groundwater flow both parallel and transverse to the river and with both effluent and influent behaviour seen. The degree of groundwater-surface water interaction within the hyporheic zone at this site seems to be controlled by a number of factors including lithology, topography, and the local groundwater flow regime. While the gravel aquifer is significant in controlling groundwater-surface water interaction, its importance as a route for flow down the catchment is likely to be modest compared with river discharge.

Groundwater is by far the largest unfrozen freshwater resource on the planet. It plays a critical role as the bottom of the hydrologic cycle, redistributing water in the subsurface and supporting plants and surface water bodies. However, groundwater has historically been excluded or greatly simplified in global models. In recent years, there has been an international push to develop global scale groundwater modeling and analysis. This progress has provided some critical first steps. Still, much additional work will be needed to achieve a consistent global groundwater framework that interacts seamlessly with observational datasets and other earth system and global circulation models. Here we outline a vision for a global groundwater platform for groundwater monitoring and prediction and identify the key technological and data challenges that are currently limiting progress. Any global platform of this type must be interdisciplinary and cannot be achieved by the groundwater modeling community in isolation. Therefore, we also provide a high‐level overview of the groundwater system, approaches to groundwater modeling and the current state of global groundwater representations, such that readers of all backgrounds can engage in this challenge.

It is widely acknowledged that waterbodies are becoming increasingly affected by a wide range of drivers of change arising from human activity. To illustrate how this can be quantified a linked modelling approach was applied in the Thames river basin in southern UK. Changes to river flows, water temperature, river and reservoir quality were predicted under three contrasting future "storylines"; one an extension of present day rates of economic development, the others representing more extreme and less sustainable visions. Modelling revealed that lower baseflow conditions will arise under all storylines. For the less extreme storyline river water quality is likely to deteriorate but reservoir quality will improve slightly. The two more extreme futures could not be supported by current management strategies to meet water demand. To satisfy these scenarios, transfer of river water from outside the Thames river basin would be necessary. Consequently, some improvement over present day water quality in the river may be seen, and for most indicators conditions would be better than in the less extreme storyline. However, because phosphorus concentrations will rise, the invoked changes in water demand management would not be of a form suitable to prevent a marked deterioration in reservoir water quality.

14Factor analysis and (two-component) end member mixing analysis (EMMA) were 15 applied to high resolution stream chemistry data from three catchments to infer 16 sources of iron and manganese-rich runoff in the catchment area and to evaluate their derived from surface waters (MacDonald, 1994). 45 While generally regarded as relatively undisturbed natural environments, increasing 46 environmental pressures on upland water quality arise from acidification through 47 sulphur and nitrogen deposition (Kirchner and Lydersen, 1995; Neal et al., 1998), 48 high sediment loads from forestry and overgrazing (Carling et al., 2001) and 49 mobilisation of colour and metals (Mitchell and McDonald, 1995). This has provided 50 a major impetus for water quality research in the UK during the last few decades. 51 Much research has been undertaken in upland catchments in Wales (Neal et al., 52 1997a) and North-east Scotland (Soulsby et al., 1998; and the results have 53 significantly improved the understanding of the water quality functioning in such 54 upland environments (Langan et al., 2001;Neal et al., 2004). 55 56 While iron (Fe) and manganese (Mn) have been included in many of these studies on 57 upland catchment hydrogeochemistry (Reid et al., 1981; Giusti and Neal, 1993; Heal, 58 and water bodies is of growing concern to many water authorities in the UK (Little 68 and McFadzean, 1991;Schofield et al., 1991; Graham et al., 2002) (Reid et al., 1981; Giusti and Neal, 1993; Gavin et al., 2001), Northern England 82 (Stunell and Younger, 1995) and Wales (Neal et al., 1997a matter of hours or minutes (Foster et al., 1997) and are often associated with high 96 loadings in colour (TOC), Fe and Mn (Reid et al., 1981; Giusti and Neal, 1993; Neal 97 et al., 1997a; Heal et al., 2002 analysis (Davis, 1986; Kovach, 1995). Starting from the same standardised data set of 257 n samples and m variables, an n x n similarity matrix is constructed, representing the 258 similarities between sample pairs in terms of the way they respond to all variables. 259Factors are then extracted from the data using the same procedure applied in R-mode 260 analysis and producing a factor matrix of all sample factor loadings. This time, the 261 size of the resulting factor loading is related to the amount of variance contributed by 262 a sample to a particular factor. Neal, 1990; Neal et al., 1997b). 281The main difficulty in EMMA arises from the identification of viable end-members. 282For the purpose of this modelling effort, a simple two-component flow model was 283 deemed appropriate in view of the hydrochemical response observed at the streams. In this study, no direct measurements of groundwater or soilwater chemistry were 306 available. Thus, the alkalinity of the groundwater end-member (Alk groundwater ) was 307 inferred from the three lowest f low samples, collected during summer low flow, at 308 each particular stream ( Figure 3, Table 2). For the definition of the soilwater end-309 member (Alk ...