Hydrogeology PhD research proposals

THE ROLE OF GLACIAL DEPOSITS IN PROTECTING GROUNDWATER

GEOPHYSICAL STUDIES OF GLACIAL SEDIMENTS USING MULTI-CHANNEL REFLECTION SEISMICS AND TEM


THE ROLE OF GLACIAL DEPOSITS IN PROTECTING GROUNDWATER.
Dr K.M. Hiscock, Mr P. Dennis

Modern agricultural practises in Norfolk including application of fertilisers to increase productivity has led to an increase in nitrate levels in the unconfined chalk aquifers.

This is worrying since 70% of public drinking water in Norfolk comes from these aquifers. The European drinking water directive states that the maximum acceptable level of nitrate is 50mg per litre. High concentrations can lead to blue baby syndrome and it is thought to stomach cancer.

However, semi-confined aquifers which are overlain by glacial till have been found to contain little or no nitrate even though recharge is occurring through the till. his suggests that denitrification is occurring.

Denitrification is the reduction of nitrate or nitrite to molecular nitrogen or nitrogen oxides by microbial activity. Many of the products are gaseous and can therefore escape from the soil.

This attenuation of nitrate can take several forms depending on what compound acts as the reducing agent. Such compounds are organic carbon, sulphur compounds and ferrous iron.

This study will examine denitrification in the chalky boulder clay till in Norfolk. It will involve developing geochemical profiles of aquitards in various locations to investigate the flow pattern of water through the till and also the natural attenuation reactions taking place. This will provide additional information to previous isotope studies of the till.


GEOPHYSICAL STUDIES OF GLACIAL SEDIMENTS USING MULTI-CHANNEL REFLECTION SEISMICS AND TEM
Dr P N Chroston, Dr K M Hiscock and Dr M Meju (University Of Leicester)

One of the distinctive features of the Pleistocene glacial history in Eastern England was the development of over-deepened valleys, now buried or part buried, and often termed ‘tunnel’ valleys. Whereas the thickness of the Pleistocene deposits over much of northern East Anglia is generally less than 30 m, Chalk water supply boreholes show that in these over-deepened valleys it can reach 80 m or more.

The formation of the valleys is of generic interest in relation to Quaternary geology but are also important in that they may have an important structural and lithological role in influencing hydrogeological conditions. For example, depending on the nature of the fill, they may act as barriers or conduits for regional groundwater flow, and thus may have a role in the development of Chalk groundwater resources. The pattern and nature of the infilling sedimentary units can also provide clues to their (controversial) origin and the nature of groundwater and contaminent movement through glacial sediments.

Geophysical methods are potentially important in supplementing the sparse borehole data to provide information on their fill. Gravity and conventional d.c. resistivity soundings have been used in some locations but whilst they can contribute to the location of the valley they provide poor constraints for their detailed internal structure and composition.

In this project, a major tool to be used is Transient (or Time Domain) Electromagnetic (TEM or TDEM) sounding, which suffers less from lateral variation and ambiguity of interpretation than conventional d.c. resistivity sounding. One dimensional and three dimensional software packages are available for modelling. For constraining the models, the borehole data will be supplemented by micro-gravity profiles and multi-channel seismic reflection surveys. The use of TEM and the reflection seismic surveys are a new application to this problem, and evaluation of their success in this challenging geophysical environment is an important part of the study. Ultimately, the target is to produce an integrated model based on field observations to allow a better constraint on structure and lithologies than has been obtained hitherto. There is also potential for the project to be extended into looking at the electrical resistivity of the Chalk aquifer at deeper levels.

This project provides training in several geophysical methods, including field survey, data processing and modelling, but it will also require knowledge of the Quaternary geology and hydrogeological principles. Applicants for this project should have an Earth Science background, ideally with a significant geophysics component.

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Last updated Dec.'99