High precision
stable isotope imaging of groundwater flow dynamics in the chalk aquifer
systems of Cambridgeshire and north Norfolk.Completed PhD research 1994-1999
Chlorofluorocarbons in Groundwater.
Nitrous Oxide
in British Limestone Aquifers.
Radon in the
groundwater in the Chalk of East Anglia.
Application of
Nitrogen and Sulphur Isotope Hydrochemistry in Groundwater
Studies.
Controls on
Saline Intrusion in the Crag Aquifer of North-East Norfolk.
High precision stable isotope imaging
of groundwater flow dynamics in the chalk aquifer systems of Cambridgeshire and
north Norfolk.
Mike George - September 1998
High precision ð18O imaging and major and minor ion hydrochemistry have been studied to identify and contrast permeability differences and temporal changes in groundwater flow dynamics in the Chalk aquifer in Cambridge and north Norfolk. In north Norfolk, the Chalk is covered by extensive Pleistocene glacial deposits.
A survey of spatial and temporal groundwater hydrochemistry in the largely unconfined Chalk aquifer of Cambridgeshire identified regions of preferential winter recharge at the glacial till boundary. These occur primarily where the River Cam flows off the Chalky Boulder Clay and within sub-valleys at the till margin. The interaction between recharge waters (ð18O » ð18Oprecipitation) and sub-glacial groundwater (ð18O £ -7.5 per mille) determines the ð18O composition of bulk groundwater sampled at the glacial till margin.
The stable isotopic evidence also highlighted a north-south trending zone of enhanced permeability, extending northwards until the characteristic ð18O signature of the sub-glacial till groundwater is masked by the increasingly dominant isotopic signature of direct groundwater recharge.
A specially designed and constructed high temperature vacuum distillation line was used to abstract porewaters from the Chalky Boulder Clay of north Norfolk for ð18O analysis and ð2H analysis. The results demonstrated how the highly heterogeneous nature of the till deposits controls its hydrogeological behaviour. The sand-rich layers act as pathways along which relatively modern meteoric water ((ð18O £ -6.5 per mille) can travel. The relatively impermeable clay-rich layers retain Late Pleistocene groundwaters (ð18O £ -8.1 per mille) which are slowly undergoing alteration due to diffusive mixing with the more mobile and modern porewaters in the sand-rich layers.
The spatial and temporal hydrochemistry and water balance data for the chalk aquifer of north-Norfolk indicated that winter recharge to the unconfined Chalk occurred via direct recharge in the upper Bure valley and as runoff at the interfluve valley margins. Winter recharge to the confined Chalk aquifer occurred via infiltration of relatively modern meteoric water through fractures and the more permeable horizons in the glacial till deposits, and along the subsurface boundary between the glacial sands and gravels and Chalky Boulder Clay.
Chlorofluorocarbons in
Groundwater.
Alison Bateman - September 1998
This thesis examines the application of chlorofluorocarbons (CFCs) and carbon tetrachloride (CCl4) as tracers of recent groundwater recharge in groundwaters from three study areas in the UK. Concentrations of CFC-11, CFC-12, CFC-113 and CCl4 were measured in groundwaters in the Norfolk Chalk, the Lincolnshire Limestone and the unconfined Chalk of East Anglia.
The spatial distributions of CFCs and CCl4 in the Norfolk Chalk aquifer indicate that recharge is controlled by the confining Pleistocene drift. High concentrations of CFCs and CCl4 were measured in groundwaters close to the principal river valleys suggesting active circulation of recently recharged groundwaters in these areas where the pleistocene deposits are largely absent. CFCs and CCl4 were undetectable, or present only in low concentrations, in groundwaters sampled from interfluvial plateau locations indicating that little modern recharge to the Chalk occurs through the Pleistocene drift in these locations. Most of the groundwaters from the Norfolk Chalk aquifer contain concentrations of CFCs and CCl4 in proportions which cannot be explained by dissolution of these compounds from the atmosphere into recharging groundwater and conservative behaviour within the aquifer. There is some evidence that microbial degradation of CFCs and CCl4 may be occurring within anoxic microzones in the predominantly oxic groundwaters.
Spatial distributions of the CFCs and CCl4 in the Lincolnshire Limestone suggest that "islands" of aquifer may be isolated from the dominant west-east regional gropundwater flow. Differences in the estimates of the amount of recent recharge in the confined zone pumped groundwaters from the CFC and CCl4 data compared with estimates from other data may be due to CFC and CCl4 removal. Compounds recognised by previous workers as products of the microbial degradation of CFC-11 and CCl4 were detected in groundwaters from the Lincolnshire Limestone and the Norfolk Chalk aquifers.
An investigation of CFC concentrations in groundwater and river water adjacent to a landfill site in Norfolk indicates that CFCs are very sensitive tracers of landfill contamination of groundwater and river water.
The widespread contamination of groundwaters by CFCs and CCl4 from non-atmospheric sources found in many of the sampled groundwaters, and the suggestion that a removal process, probably microbial degradation, may modify dissolved CFC and CCl4 concentrations, even in ostensibly oxic groundwaters complicates the use of CFCs and CCl4 as indicators of groundwater recharge age and mixing history.
Nitrous Oxide in British Limestone
Aquifers.
Ingo Hans Mühlherr - April 1997
Nitrous oxide and other nitrogen species were measured in groundwater from the most important limestone aquifers in the UK. Nitrous oxide levels were generally very high, with concentrations exceeding the concentration of air equilibrated water by up to 320 times. The correlations between nitrous oxide and nitrate and dissolved oxygen were used to identify nitrification as the main nitrous oxide production mechanism in the investigated aquifers. Most of the nitrous oxide in groundwater seems to be produced in the unsaturated zone and recharge conditions strongly affect the nitrous oxide concentrations in the aquifer. Nitrous oxide production in the saturated zone is less substantial and can also be denitrification mediated; denitrification under very reducing aquifer conditions can result in nitrous oxide consumption.
A maximum nitrous oxide emission rate from groundwater of 0.14kg N2O ha-1 a-1 was calculated for the main Chalk study area in Cambridgeshire, which might be representative for sedimentary aquifers underlying arable land. In a further step, the contribution of groundwater nitrous oxide emissions to the total emissions in the UK and on a global scale was assessed. It was estimated that groundwater is responsible for slightly less than 10% of the total global nitrous oxide emissions, while its contribution on a national scale is probably above 10%.
An additional background sampling programme revealed that natural nitrous oxide background concentrations are low and only slightly in excess of the air equilibrium concentartion. It was also observed that nitrous oxide concentartions increase drastically with the degree of human influence on groundwater. This implies that agricultural activitires, and land applications in particular, are mainly responsible for the high nitrous oxide concentrations in groundwater found during this study, as well as for most of the global nitrous oxide emissions from groundwater, which contribute to climatic forcing induced by higher atmospheric nitrous oxide concentrations.
Radon in the groundwater in the Chalk
of East Anglia.
Rob Low - September 1996
The Cretaceous Chalk, underlying large areas of South and East England is the most important formation for groundwater abstraction in the UK. It is classically described as a dual-porosity aquifer incorporating a low permeability matrix with a high permeability fissure system. Continuing concern over security of water supply and the fate of contaminants in the Chalk has highlighted weaknesses in the understanding of fluid dynamics in the aquifer, particularly relating to the fissure network and the degree of water exchange with the matrix. It was decided to study the behaviour of Rn-222 in order to evaluate its usefulness as a groundwater tracer in the Chalk.
Over 300 one litre groundwater samples were obtained between July 1993 and March 1995 from boreholes and springs in the outcropping Chalk between and the north Norfolk coast. Rn-222 concentrations ranged 0.31-10.33 Bq./kg, verifying the minimal radiological health risk associated with Chalk groundwater.
Significant spatial variation in groundwater Rn-222 concentration was discovered. Using surrogate variables attempts were made to confirm Chalk radionuclide content and fissure aperture as the main determinants of Chalk groundwater Rn-222 concentration. These attempts were unsuccessful, probably due to the poor quality of the surrogate variables.
Regular sampling of a subset of sites every four weeks over 18 months revealed seasonal variations in groundwater Rn-222 concentration of up to 20-30% of the mean at each site, with additional larger more abrupt episodes of reduced Rn-222 in some cases. A model of groundwater/Rn-222 dynamics at Fleam Dyke, Cambridgeshire, using aquifer and climatic variables measured at the site, showed that the influence of unsaturated zone matrix drainage and fissure-directed recharge could be detected in the Rn-222 content of the fissure water. It is argued in conclusion that Rn-222 has the potential to become a uniquely useful groundwater tracer for the Chalk aquifer.
Application of Nitrogen and Sulphur
Isotope Hydrochemistry in Groundwater Studies.
Nicholas A Feast - October 1995
The hydrochemistry of nitrogen and sulphur isotopes in selected UK groundwaters has benn studied in order to reveal information concerning the sources od nitrate and sulphate and the occurrence of hydrochemical processes in aquifers, such as natural denitrification. The Lincolnshire Limestone and the Norfolk Chalk aquifers have been studied in detail.
In the south Lincolnshire Limestone aquifer, the lowering of nitrate concentrations in groundwater on passing beneath confining strata is associated with an enrichment in N-15 in the remaining nitrate, which is characteristic of denitrification. However, the isotopic enrichment factor, E=-1 permil, associated with this process is lower than previously reported for other aquifers and it is shown that this is due to dispersion and dilution processes that occur within the limestone aquifer. Sulphur isotope ratios become progressively lighter down the dip of the aquifer as the proportion of isotopically heavy sulphur from modern recharge, relative to isotopically lighter sulphur from the oxidation of pyrite, decreases. The combination of nitrogen, sulphur and oxygen isotope techniques has revealed a variety of processes controlling nitrate concentrations, of which dilution of modern recharge in the aquifer is the most important.
Nitrogen isotope measurements have been combined with dissolved Nitrogen:Argon ratios to demonstrate the controls on nitrate in the Chalk aquifer of the Bure Valley region of north Norfolk. In the valley zone, the unconfined Chalk aquifer is contaminated with nitrate as a result of agricultural activities. Beneath valley margins, low concentrations of nitrate have isotopically light signatures which cannot have been produced by in-situ reduction of nitrate from modern recharge. However, high Nitrogen:Argon values show that large quantities of excess nitrogen have been produced by denitrification. This evidence, and the increase in delta15N values of nitrate contaminated Chalk groundwater away from the valley, show that denitrification is most likely occurring in the overlying glacial deposits. Sulphur isotope measurements show that dissolved sulphate is largely derived from modern recharge, with minor amounts from oxidation of sulphur within the glacial deposits. Bacterial sulphate reduction has been shown to occur within the aquifer and mixing with saline water may also occur locally.
Nitrogen isotope ratios have also been used to demonstrate the absence of significant denitrification in the unsaturated zone of the Chalk at a site in Hampshire. The occurrence of isotopically heavy N-15 values in urban areas and at a landfill site has been used to show the potential application of nitrogen isotopes in tracing non-agricultural sources of nitrate.
Controls on Saline Intrusion in the
Crag Aquifer of North-East Norfolk.
Ian P Holman - 1994
The River Thurne catchment on the north-east coast of Norfolk contains large areas of marshland, including several Ramsar-designated sites. Land drainage of much of the marshland over the previous centuries has lowered groundwater levels to below sea-level. Consequently, saline groundwater has intruded into a large proportion of the underlying Norwich Crag aquifer, thereby raising the salinity of the surface water networks. Changes in the economics of arable farming and the perception that changes in the land drainage regimes will produce beneficial effects on the water quality of the River Thurne and its associated broads have necessitated the need for a better understanding of the hydrogeology of this complex, highly managed aquifer.
To provide the necessary understanding, this study has investigated the hydrogeology, hydrology and land management of the catchment using a variety of geophysical techniques, including electrical resistivity soundings, EM surveys and reflection seismology to supplement information collected using standard hydrogeological measurements. The distribution od dyke water levels as maintained by the drainage pumps are shown to exert important controls on the extent and depth of saline intrusion. In the north of the catchment a further control is the internal structure of the Crag aquifer. A clay layer of probable Baventian age divides the Crag aquifer into two units and appears to prevent the salinization of the aquifer above this layer. A catchment water balance has shown that land drainage pumps discharge about 95% of the catchment recharge, so that raising dyke water levels will also result in raised groundwater levels.
Future land use change to grazing marsh, while improving the water quality of drianage water entering the River Thurne may lead to increased salinization of the aquifer, as a result of changes in the distribution of dyke water levels.
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