Use and application of CFC-11, CFC-12, CFC-113 and SF6 as environmental tracers of groundwater residence time: a review

Groundwater residence time is a fundamental property of groundwater to understand important hydrogeological issues, such as deriving sustainable abstraction volumes, or, the evolution of groundwater quality. The anthropogenic trace gases chlorofluorocarbons (CFC-11, CFC-12 and CFC-113) and sulphur hexafluoride (SF6) are ideal in this regard because they have been released globally at known rates and become dissolved in groundwater following Henry's Law, integrating over large spatial (global) and temporal (decades) scales. The CFCs and SF6 are able to date groundwater up to ∼100 years old with the caveat of certain simplifying assumptions. However, the inversion of environmental tracer concentrations (CFCs and SF6) to derive groundwater age rests on the accurate determination of groundwater recharge parameters, namely temperature, elevation, salinity and excess air, in addition to resolving the potential for contamination, degradation and unsaturated zone effects. This review explores the fundamentals of CFC-11, CFC-12, CFC-113 and SF6 as environmental tracers of groundwater age and recommends complementary techniques throughout. Once this relatively simple and inexpensive technique has been used to determine initial concentrations at the recharge zone, setting the groundwater dating ‘clock’ to zero, this review then explores the meaning of groundwater ‘age’ in relation to measured environmental tracer concentrations. It is shown that the CFCs and SF6 may be applied to a wide-range of hydrogeological problems and suggests that environmental tracers are particularly powerful tools when integrated with numerical flow and transport models

Influence of tree species and forest land use on soil hydraulic conductivity and implications for surface runoff generation

Forest planting is increasingly being incorporated into land management policies to mitigate diffuse pollution and localised flooding because forest soils are associated with enhanced hydraulic properties and lower surface runoff compared to soils under other vegetation types. Despite this, our understanding of the effects of different tree species and forest land use on soil hydraulic properties is limited. In this study we tested for the effects of two tree species, sycamore (Acer pseudoplatanus) and Scots pine (Pinus sylvestris), subject to contrasting land use systems, namely ungrazed forest and livestock grazed forest, on soil surface saturated hydraulic conductivity (Kfs) at a long term (23 year) experimental site in Scotland. Additionally these forest land use systems were compared to grazed pasture. Kfs was found to be significantly higher under ungrazed Scots pine forest (1239 mm hr− 1) than under ungrazed sycamore forest (379 mm hr− 1) and under both of these forest types than under pasture (32 mm hr− 1). However, this measure did not differ significantly between the sycamore and Scots pine grazed forest and pasture. It was inferred, from comparison of measured Kfs values with estimated maximum rainfall intensities for various return periods at the site, that surface runoff, as infiltration excess overland flow, would be generated in pasture and grazed forest by storms with a return period of at least 1 in 2 years, but that surface runoff is extremely rare in the ungrazed forests, regardless of tree species. We concluded that, although tree species with differing characteristics can create large differences in soil hydraulic properties, the influence of land use can mask the influence of trees. The choice of tree species may therefore be less important than forest land use for mitigating the effects of surface runoff

Global patterns of nitrate storage in the vadose zone

Global-scale nitrogen budgets developed to quantify anthropogenic impacts on the nitrogen cycle do not explicitly consider nitrate stored in the vadose zone. Here we show that the vadose zone is an important store of nitrate that should be considered in future budgets for effective policymaking. Using estimates of groundwater depth and nitrate leaching for 1900–2000, we quantify the peak global storage of nitrate in the vadose zone as 605–1814 Teragrams (Tg). Estimates of nitrate storage are validated using basin-scale and national-scale estimates and observed groundwater nitrate data. Nitrate storage per unit area is greatest in North America, China and Europe where there are thick vadose zones and extensive historical agriculture. In these areas, long travel times in the vadose zone may delay the impact of changes in agricultural practices on groundwater quality. We argue that in these areas use of conventional nitrogen budget approaches is inappropriate

Radon in Chalk streams: Spatial and temporal variation of groundwater sources in the Pang and Lambourn catchments, UK.

Variations in dissolved 222Rn (radon) concentrations in rivers and groundwater are observed in the Cretaceous Chalk catchments of the Pang and Lambourn. Stream radon concentrations and flow data were used to model radon inputs to rivers from groundwater, with the modelled radon input concentrations (CI) varying between 0.2 Bq/l and 3.8 Bq/l, consistent with measured groundwater values. Groundwater in both catchments was found to have higher and more variable radon concentrations (2-12 Bq/l) in the near surface, weathered horizons, compared to a consistent 1 Bq/l from the solid Chalk. The variations in CI can be related to flow generation pathways and hydrological events. In the Lambourn, the radon budget is controlled by diffuse groundwater inputs, supporting the hypothesis that the alluvial aquifer plays a greater role during periods of high accretion. The Pang is more complex than the Lambourn having a combination of diffuse and point source inputs, with spring inputs dominating both flow and radon signatures in the lower part of the catchment. Significant temporal and spatial variations were determined for CI in both catchments reflecting their differing geologies and flow regimes. One use of radon in hydrology is the determination of groundwater discharges to rivers, but the observed variations in CI mean this approach may not be appropriate to all situations and that changes in source need further evaluation. Nonetheless, radon is shown to be a useful tracer of flow paths and processes within these catchments

A three dimensional numerical investigation of hillslope flow processes

SIGLEAvailable from British Library Document Supply Centre- DSC:DX76155 / BLDSC - British Library Document Supply CentreGBUnited Kingdo

Typology and metrics in hyporheic hydrology

This paper evaluates the perceptions and conceptual basis in hyporheic flow (HF) modeling. The combination of flow processes at several scales may currently not be well described by mechanistic models which are often limited by the calibration data and the modeler's understanding of the system. For example, transient storage models calibrated on tracer tests may underestimate long residence time (days) flow paths; the Darcy approach based on hydraulic heads tends to underestimate the shorter ones (hours). This has important implications for predicting contaminant fluxes through the hyporheic zone at the catchment scale. A set of standard measurements (metrics) in the hyporheic physical environment, combined with a typology of HF systems are discussed as promising lines to predict the relative magnitude of changes of residence time, flow budgets and spatial extent of flow paths. Since models do not need to be strictly realistic in terms of structure, the potential for hyporheic metrics to inspire simplified models is stressed. Such an approach may be appealing to ecologists who must integrate the hydrological component in more complex systems. These metrics are discussed on the basis of both experimental evidence and theoretical studies. Because some metrics may only apply to specific environments, a typology of hyporheic systems is presented which is a function of the physiographic setting, including scale and dynamics, and the biogeochemical problem addressed. There has been speculation that stream morpho-dynamic controls on HF deal more with shallow exchange, which is often easier to measure. Such a bias may be addressed by building on the hydrogeological experience and concepts in dealing with heterogeneous media, in particular in groundwater contaminant literature. More widely, environmental modelling principles such as equifinality and predictive uncertainty, which are often ignored in hyporheic studies, are discussed on the basis of generic HF models. The scope for hyporheic metrics and a physical typology of hyporheic systems is illustrated by the study case of a groundwater-fed river in the UK. A range of methodological tools, such as electrical resistivity profiles, environmental and artificial tracers, piezometric network development, has been applied from the bedform to the kilometre scale

Vadose zone model uncertainty as conditioned on geophysical data.

An approach to estimating the uncertainty in model descriptions based on a landscape space to model space mapping concept is described. The approach is illustrated by an application making use of plot scale geophysical estimates of changes in water content profiles to condition a model of recharge to the Sherwood Sandstone Aquifer in the United Kingdom. It is demonstrated that the mapping is highly uncertain and that many different parameter sets give acceptable simulations of the observations. Multiple profile measurements over time offer only limited additional constraints on the mapping. The resulting mapping weights may be used to evaluate uncertainty in the predictions of vadose zone flow dynamics for the site