This thesis presents a study into the flow processes in the Chalk unsaturated zone at a research site in east Yorkshire. The unsaturated zone plays an important role in reducing groundwater flooding by temporally spreading the impact of storm recharge events and provides a buffer in which surface contaminants may be delayed and attenuated before reaching the water table. Due to the complex dual porosity nature of the Chalk, flow and solutes can be rapidly moved through the unsaturated zone via preferential fracture pathways or more slowly through the porous matrix. Rising nitrate concentrations in excess of the legislated maximum permissible concentration level (11.3 mg/l NO,-N) have been observed in Chalk catchments across the UK in recent decades. However due to difficulties in obtaining in-situ measurements the characterization of flow and transport processes and identification of dominant flow pathways remains elusive. This research project focuses on the quantification of water and nitrate fluxes through the Chalk unsaturated zone through the fusion of intensive field monitoring and soil zone modelling. High temporal resolution matric potential, moisture content, rainfall and deep unsaturated zone discharge measurements are reported. Field data are used to develop a soil zone model from which water fluxes entering the Chalk unsaturated zone are estimated; results show that the soil profile stores water and suddenly releases it, creating several distinct drainage events through the recharge season. The flux of drainage water from the soil zone is compared to deep unsaturated zone discharge fluxes at three inflow sites within the tunnel, where the signals from the soil drainage events remain distinct. Water level rise events coincide with discharge events in the tunnel indicating that the events observed in the tunnel are transmitted to the depth of the water table. Nitrate concentrations from shallow cores down to 1 m depth are compared to concentrations obtained from deep unsaturated zone discharge samples. Short lag times exceeding the rate at which the Chalk matrix can transmit water are reported between the soil and the deep unsaturated zone indicating bypass flow via fractures. The magnitude of deep unsaturated zone discharge exceeds the magnitude of soil zone drainage by a factor of up to 28 revealing that flow pathways converge with depth. Observed nitrate concentrations exceed the natural baseline concentration of N03-N expected in the Chalk aquifer of northern England by nearly four times and are nearly double the legislated maximum permissible concentration. Comparison of current nitrate fluxes at the study site to previous show no signs that nitrate levels are decreasing.
| Identifer | oai:union.ndltd.org:bl.uk/oai:ethos.bl.uk:598030 |
| Date | January 2013 |
| Creators | Keim, Dawn |
| Publisher | University of Leeds |
| Source Sets | Ethos UK |
| Detected Language | English |
| Type | Electronic Thesis or Dissertation |
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