We were commissioned by the Environment Agency to develop groundwater models of the London Basin and North Downs aquifer. A conceptual groundwater model was developed over the period November 2008 to November 2009. The development of a numerical groundwater model commenced in March 2011 and was completed in March 2012. The model has since been used for a variety of purposes by the Environment Agency and other stakeholders.
A summary poster of our work on the conceptual model, presented to the Geological Society Thames Region Group in October 2010, can be viewed here.
The Chalk and Thanet Sands aquifer (Figure 1) provides the main groundwater resource in the London area, supporting significant abstraction for a variety of uses including public water supply. Beneath London the aquifer is confined by clayey strata of the Lambeth Group and London Clay.
As a consequence of historical abstraction, a regional cone of depression developed beneath Central London, in the centre of which the Chalk aquifer developed an unsaturated zone (see Figure 3 below). But since 1960, the reduction in pumping from industrial decline resulted in the groundwater levels rising over most of the area. This recovery would potentially cause issues with the structural integrity of infrastructure, which has led to a strategy of abstraction management to control groundwater levels in the area, a strategy known as GARDIT (General Aquifer Research Development and Investigation Team) – see www.groundwateruk.org/Rising_Groundwater_in_Central_London.aspx for details.
Aquifer recharge schemes have been developed in the areas where historical abstraction caused the development of an unsaturated zone in the Thanet Sands, such as along the River Lee (NLARS – North London Aquifer Recharge Scheme). The Environment Agency monitors the situation and produces an annual review of groundwater levels.
The London Basin aquifer is one of the most densely investigated and data-rich groundwater bodies in the UK. Following consolidation and analysis of the available data and a comprehensive literature review, we developed a detailed conceptual understanding of the key hydrogeological processes which occur within the London Basin aquifer. Conceptualisation focused on understanding and quantifying the following key aspects of the hydrogeology of the aquifer:
Recommendations were then made for the development of a numerical groundwater model to simulate the hydrogeology of the London Basin.
We developed a numerical model of the London Basin and North Downs aquifer in MODFLOW. From the original conceptual study, the model area was extended eastwards to the estuary of the River Medway. The key technical challenges were:
We created the most up-to-date and accurate model of the London Basin and North Downs aquifer. The conceptual and numerical models aid the Environment Agency and other stakeholders in managing water resources of the basin, comprising tools which enable sound science-based regulatory decision making to quantify the sustainable balance of inflows and abstraction in the aquifer. It has been used, for instance, to quantify impacts of groundwater abstraction on flows in watercourses, to estimate the potential impacts of climate change on aquifer recharge and deployable output and to optimise the use of artificial recharge schemes.
For further information on this project, please contact Mike Streetly, Water Group Director.
As described by John Doherty in the article ‘Improved Calculations for Dewatered Cells in MODFLOW’ published in the academic journal Groundwater.
Citation: Doherty, J. (2001), Improved Calculations for Dewatered Cells in MODFLOW. Groundwater, 39: 863– 869. doi: 10.1111/j.1745-6584.2001.tb02474.x