Modelling flow and recharge in the Chalk unsaturated zone and influence of subsurface geology, Brighton block, South East England

  • Peshawa Al-Jaf

Student thesis: Doctoral Thesis

Abstract

The Chalk aquifer is one of the main sources of water in South East England. In the city of Brighton and Hove, it provides about 98% of total water consumption. It is also a significant aquifer in North East England, Northern France and Denmark. The Chalk unsaturated zone plays an important role in the water cycle, controlling the timing and magnitude of recharge and the main pathway of contaminant transport. A range of previous work has addressed flow processes in the Chalk unsaturated zone, but our physical understanding is still incomplete. This thesis attempts to understand the flow mechanism in the Chalk unsaturated zone using statistical analysis and novel laboratory methods. New laboratory scale instruments have been used that are suitable for this condition and can measure matric potential and volumetric water content simultaneously. In addition, a statistical analysis has been applied to field recorded rainfall, potential evapotranspiration, groundwater level and groundwater electrical conductivity at three sites in the study area, North Heath Barn, Pyecombe East and Preston Park. With help of a physically based model, flow and recharge in the study area were simulated using parameters obtained from laboratory and field observation analysis.

Results show that flow and recharge occur predominantly through the Chalk matrix, which requires months till the pulse of water reaches the water table. The slow matrix flow occurs when the Chalk unsaturated zone is not completely saturated, and it will continue through summer even when the rainfall has ceased. Faster flow occurs during recharge seasons when the Chalk unsaturated zone is saturated or close to saturation. At that point, flow occurs through the matrix by the action of piston flow and the time lag can be counted in days. Relatively modest increases in effective rainfall during winter trigger fracture flow which minimizes the time lag to hours. Delay in response and slower drainage results from the influence of the subsurface geology, mainly the presence of marl seam and fracture discontinuities. Simulating results from the laboratory and statistical analysis using a physically based model (MODFLOW) shows that the model poorly matches the observed data, but it performs well at the scale of seasonal variation.
Date of Award2018
LanguageEnglish
Awarding Institution
  • University of Brighton
SupervisorMartin Smith (Supervisor) & Friederike Gunzel (Supervisor)

Cite this

Modelling flow and recharge in the Chalk unsaturated zone and influence of subsurface geology, Brighton block, South East England
Al-Jaf, P. (Author). 2018

Student thesis: Doctoral Thesis