John P. Bloomfield

Climate change and water in the UK - past changes and future prospects

Climate change is expected to modify rainfall, temperature and catchment hydrological responses across the world, and adapting to these water-related changes is a pressing challenge. This paper reviews the impact of anthropogenic climate change on water in the UK and looks at projections of future change. The natural variability of the UK climate makes change hard to detect; only historical increases in air temperature can be attributed to anthropogenic climate forcing, but over the last 50 years more winter rainfall has been falling in intense events. Future changes in rainfall and evapotranspiration could lead to changed flow regimes and impacts on water quality, aquatic ecosystems and water availability. Summer flows may decrease on average, but floods may become larger and more frequent. River and lake water quality may decline as a result of higher water temperatures, lower river flows and increased algal blooms in summer, and because of higher flows in the winter. In communicating this important work, researchers should pay particular attention to explaining confidence and uncertainty clearly. Much of the relevant research is either global or highly localized: decision-makers would benefit from more studies that address water and climate change at a spatial and temporal scale appropriate for the decisions they make.

Characterisation of hydrogeologically significant fracture distributions in the Chalk: an example from the Upper Chalk of southern England

Fractures pervade the Chalk, and are the primary pathways for rapid flow and contaminant migration. Improved quantitative descriptions of their geometry and scaling characteristics are essential for the development of predictive transport models. Fracture aperture and fracture connectivity are the most critical properties in controlling transport. To provide examples of fracture characteristics in the Upper Chalk, four fracture parameters—fracture orientation, trace length, spacing and aperture—have been measured using section and scan-line surveys at Play Hatch Quarry, near Reading, Berkshire. Three fracture types have been investigated: joints, bedding plane fractures and faults. Unrefined, or global, joint trace length and spacing measurements approximate to negative exponential distributions, with means of 0.09 m and 0.02 m. The trace length and spacing distributions of the two dominant joint sets, i.e. the bedding parallel joint set and the joints at a high angle to bedding, approximate to log-normal distributions, with geometric mean trace lengths of 0.15 m and 0.3 m, and spacings of 0.10 m and 0.12 m, respectively. Calculated fracture interconnectivity indices suggest that the bedding parallel joint set is likely to be of greater hydraulic importance than the high-angle joint set. Aperture measurements obtained for a single bedding plane fracture range from less than 0.5 mm to 23.5 mm. Apertures approximate to a negative exponential distribution below 7 mm, and to a log-normal distribution above 7 mm. It is inferred that the larger apertures have been affected by solution processes and that flow through bedding plane fractures is channelled across 10–20% of the fracture surface area. There are insufficient data for a rigorous analysis of the faulting at Play Hatch Quarry, but it is expected that faulting in the Chalk will show scale-invariant length and spacing characteristics. The results of the study are consistent with a visualisation of the Chalk consisting of scale-invariant fault-bounded segments, where the internal fracture architecture of each segment is dominated by continuous bedding plane fractures, and subordinate, scale-dependent, arrays of joints. The scale of jointing within a given fault-bounded segment is a function of bedding thickness.

A review of the impact of climate change on future nitrate concentrations in groundwater of the UK

This paper reviews the potential impacts of climate change on nitrate concentrations in groundwater of the UK using a Source-Pathway-Receptor framework. Changes in temperature, precipitation quantity and distribution, and atmospheric carbon dioxide concentrations will affect the agricultural nitrate source term through changes in both soil processes and agricultural productivity. Non-agricultural source terms, such as urban areas and atmospheric deposition, are also expected to be affected. The implications for the rate of nitrate leaching to groundwater as a result of these changes are not yet fully understood but predictions suggest that leaching rate may increase under future climate scenarios. Climate change will affect the hydrological cycle with changes to recharge, groundwater levels and resources and flow processes. These changes will impact on concentrations of nitrate in abstracted water and other receptors, such as surface water and groundwater-fed wetlands. The implications for nitrate leaching to groundwater as a result of climate changes are not yet well enough understood to be able to make useful predictions without more site-specific data. The few studies which address the whole cycle show likely changes in nitrate leaching ranging from limited increases to a possible doubling of aquifer concentrations by 2100. These changes may be masked by nitrate reductions from improved agricultural practices, but a range of adaption measures need to be identified. Future impact may also be driven by economic responses to climate change.

Correlating mechanical data with microstructural observations in deformation experiments on synthetic two-phase aggregates

Abstract A simple method of analysing the results of deformation experiments on synthetic two-phase aggregates is presented and applied to some tests on calcite-halite aggregates. The approach involves replacing the observed mechanical behaviour of real two-phase materials of coarse microstructure, with a phase volume fraction weighted mechanically equivalent representation in which each phase is assigned its own single-valued stress and its own single-valued strain. Such a representation can be solved for the stresses and strains in the two phases and the results compared with the stresses and strains as determined from the deformation microstructures. In principle, it is then possible to constrain which of the features of the microstructure are mechanically significant. The stresses supported by the synthetic calcite-halite aggregates suggest a strong partitioning of the deformation into the halite at all volume fractions of calcite, and yet analysis of the halite strains from the deformation microstructures indicates that there is little or no strain partitioning between the phases. However, these conflicting results may be reconciled by using the calcite contiguous volume, rather than the actual calcite volume fraction, to characterize the strength of the aggregates with respect to those of their component phases.

An empirical liquid permeability—gas permeability correlation for use in aquifer properties studies

Abstract Laboratory permeability studies can contribute significantly to the quantification of aquifer heterogeneity. However, intrinsic permeabilities obtained by standard core analysis techniques using gas are different from those obtained using water. This is because gas measurements may be affected by a molecular phenomenon known as gas slippage. An empirical correlation is presented for liquid and gas permeability measurements obtained for a suite of Permo-Triassic sandstones and shales from the Sherwood Sandstone Group of northern England. Liquid permeability tests were performed using synthetic formation brines and deionized water. Gas permeability tests used nitrogen as the permeant. Liquid permeabilites, kl, ranged from 9.0 x 10-19 m2to 2.4 x 10-12m2 and gas permeabilites, kg, ranged from 1.7 x 10-17m2 to 2.6 x 10-12m2.The liquid and gas permeability data exhibit log-normal frequency distributions; the log transformed liquid and gas permeability data have means of 5.1 x 10-16m2 and 4.3 x 10-15m2respectively. A linear least-squares fit to the data has the form log10kl = 1.17log10kg + 1.51. kl/kg ratios, in the range 0.03 to 0.9, indicate that Hagen-Poiseuille type models may not provide appropriate descriptions of gas flow in the Sherwood Sandstone.

Regional trends in matrix porosity and dry density of the Chalk of England

Abstract Laboratory measurements of porosity and dry density are presented for 2045 core samples from the Chalk of England. The data are subdivided on the basis of gross stratigraphy, i.e. Lower, Middle and Upper Chalk, and into four geographical areas: Northern England, East Anglia, Thames & Chilterns and Southern England. Statistical analysis of the data shows (i) that the porosity distributions for the Upper Chalk of the Southern and Thames & Chilterns regions are indistinguishable, (ii0 that the porosity distributions for the middle and Lower Chalk of the East Anglian region are indistinguishable, and (iii) that the porosity distributions for each of the gross stratigraphical units from all other regions are statistically discrete. Porosities range from 3.3% to 55.5%, with a mean porosity of 34.0%. Dry densities range from 1210 kg/m3 to 2510 kg/m3, with a mean dry density of 1790 kg/m3. In a given region there is a trend of increasing porosity from Lower to Middle to Upper Chalk. There are systematic variations in porosity between the regions. There is a trend of increasing porosity from the Northern England region to the Southern England region, to the Thames & Chilterns region, to East Anglia. No significant systematic variations in porosity-depth gradients were observed. Chalk porosity-depth gradients are typically high, of the order of -0.07 to -0.1 porosity per cent per metre.

Sediment filled fractures in the Permo-Triassic sandstones of the Cheshire basin: observations and implications for pollutant transport.

Fracture mapping in a tunnel system and at nearby outcrop on the Runcorn Penninsula, UK, suggests the need for a review of the potential pathways for pollutant transport in Permo-Triassic sandstone aquifers. Sediment infilling is pervasive in the largest sub-vertical multi-layer fractures in the study area, both at the surface and to a depth of about 40 m below ground level. Sediment infill is inferred to have formed in situ. The conventional models of pollutant transport in fracture networks assume that they comprise open fractures, with pollutant mobility depending on fracture connectivity (a function of density, length, orientation and intersection) and aperture. The presence of extensive sediment fills in fractures will materially change their permeability, thereby reducing pollutant flux, and be of significance in the assessment of risks arising from chemical spillages. There has been little or no substantive evidence for such fills in Permo-Triassic sandstones in the UK, apart from observations at outcrop and anecdotes of sand being pumped from boreholes. Here, we report surface and rare, but complementary, subsurface observations of extensive fills in the Cheshire basin, and argue that they will only act as preferential pathways where they crosscut low-permeability horizons such as mudstones.

Stygobitic Invertebrates in Groundwater — A Review from a Hydrogeological Perspective

Abstract Groundwater-adapted species (known as stygobites) provide animportant contribution to biodiversity. Groundwater ecosystems are some of the oldest on earth, and contain many endemic species adapted to live in an environment with no light and limited resources. The controls on stygobite distributions are not yet fully resolved because of the complex interaction between many processes operating at different scales. Many of these processes are geological or hydrogeological in nature and therefore more detailed geological and hydrogeological studies could provide improved understanding of stygobite distributions. Hydrogeologists can assist ecologists by providing expertise on both general geological characteristics of sampling sites, and how groundwater at sampling sites relates to the wider aquifer setting. Geological input would be especially useful in stygobite dispersal studies because dispersal depends upon habitat continuity associated with geological dispersal corridors, and is limited where rocks that do not provide a suitable habitat form geological barriers. Stygobite studies are of benefit to hydrogeology because stygobite distributions can provide information on ground-water-surface water interaction and aquifer connectivity over a range of spatio-temporal scales. Future studies using DNA analysis of stygobites may provide much more detailed information on hydraulic connectivity within and between aquifers. There is also potential for the development of stygobites as indicators of groundwater quality. The biogeochemical function of stygobites is of interest to both hydrogeologists and ecologists. Studies have demonstrated that stygobites graze biofilms and bacteria but their role in biogeochemical cycles is still not fully understood. Ecosystem services provided by groundwater fauna depend upon their abundance and biomass. Future studies using hydrogeological data (e.g. borehole packer techniques) may provide an improved understanding of where in aquifers stygobites live and how many there are, which would be an important step towards assessing the significance of their role in biogeochemical cycling of nutrients and carbon.

An early warning system for groundwater flooding in the Chalk

Abstract An early warning system has been developed for groundwater flooding and trialled in the Patcham area of Brighton. It provides a fit-for-purpose approach for forecasting groundwater flood events in the Chalk and is capable of operating across longer time scales than had previously been possible. The method involves a set of nested steps or tasks. Initially, the catchments response to recharge is determined and, using a representative hydrograph, a simple regression model that relates annual groundwater level minima and autumn and winter rainfall to subsequent annual maxima is developed. The regression model is then applied at the end of each summer recession using the observed annual minimum and estimates of winter rainfall to predict the following groundwater level maximum. Based on the results of this prediction a variety of steps may then be appropriate. Where the model predicts potentially high groundwater levels the frequency of groundwater level monitoring observations can be increased. A novel element of the method developed is the monitoring of changes in the matric potential of the unsaturated zone. Specific trigger levels to initiate either the next step of the method or promulgation of warnings of varying severity will be developed through experience of use of the system.

Evidence for changes in historic and future groundwater levels in the UK

We examine the evidence for climate-change impacts on groundwater levels provided by studies of the historical observational record, and future climate-change impact modelling. To date no evidence has been found for systematic changes in groundwater drought frequency or intensity in the UK, but some evidence of multi-annual to decadal coherence of groundwater levels and large-scale climate indices has been found, which should be considered when trying to identify any trends. We analyse trends in long groundwater level time-series monitored in seven observation boreholes in the Chalk aquifer, and identify statistically significant declines at four of these sites, but do not attempt to attribute these to a change in a stimulus. The evidence for the impacts of future climate change on UK groundwater recharge and levels is limited. The number of studies that have been undertaken is small and different approaches have been adopted to quantify impacts. Furthermore, these studies have generally focused on relatively small regions and reported local findings. Consequently, it has been difficult to compare them between locations. We undertake some additional analysis of the probabilistic outputs of the one recent impact study that has produced coherent multi-site projections of changes in groundwater levels. These results suggest reductions in annual and average summer levels, and increases in average winter levels, by the 2050s under a high greenhouse gas emissions scenario, at most of the sites modelled, when expressed by the median of the ensemble of simulations. It is concluded, however, that local hydrogeological conditions can be an important control on the simulated response to a future climate projection.