Environmental Geology & Geotechnical Consultants Ltd delivered oral and poster presentations at the recent Hydrogeology of the Chalk meeting held at the Geological Society in London on 25th & 26th April. Abstracts and links to the presentations are given below.

Spatial and temporal variation in the effective porosity of the Chalk and its significance for groundwater quality

Aidan Foleya, Stephen R.H. Worthingtonb

Email: aidan.foley@eggconsult.co.uk

a Environmental Geology & Geotechnical Consultants, 22A Beswick Street, Manchester, M4 7HR

b Worthington Groundwater, 55 Mayfair Avenue, Dundas, Ontario L9H 3K9, Canada

The Chalk is widely recognised as a dual or multi-porosity medium, with porosity types (matrix / fracture / fissure / conduit) controlling recharge, flow and transport processes on multiple spatial and temporal scales. Chalk studies attempting to predict groundwater quality variations and contaminant transport behaviour must frame the spatio/temporal scales relevant to the problem, which in turn dictate which porosity types are likely to exert dominant process controls. For example, there will be rapid transport in the Chalk of contaminants that enter the aquifer at sinking streams or dolines because the pathways are solutionally-enhanced fissures and conduits, with an effective porosity likely to be within the range of 0.00001 to 0.001. By contrast, agricultural nitrate pollution, occurring at whole catchment scales over decades, involves matrix diffusion by which up to ~0.4 (~40%) matrix porosity may become ‘effective’. A number of typical scenarios, based on differing recharge mechanisms & presence/type of overlying deposits, are presented within this framework. These scenarios are analysed in terms of how they define the differing scales at which effective porosity is recruited, and how broadly understanding scale effects may inform our understanding and management of groundwater quality.

The full presentation may be viewed HERE.

Chalk porosity continuum adapted from Eslinger & Pevear (1988)

Quantifying the geomorphic relationship between Chalk dolines and overlying Palaeogene and Quaternary cover and its significance for groundwater vulnerability

Aidan Foleya, Hanna Landquista

Email: aidan.foley@eggconsult.co.uk

a Environmental Geology & Geotechnical Consultants, 22A Beswick Street, Manchester, M4 7HR

Dolines represent surface expression of subsurface karst processes operative within limestone aquifers. Within the English Chalk, dolines have been frequently noted in relation to allochthonous point-source recharge from runoff generated on overlying Palaeogene clay or Quaternary Clay-with-Flints cover. As the Palaeogene cover has retreated over geological time, dolines formed by such concentrated recharge at the boundary between the Chalk and the clays have been exposed to subaerial weathering for longer periods of time with distance from that boundary. Hence, fewer of them are apparent at the surface with increasing distance (Maurice et al, 2006). This poster presents results from the quantification of this relationship, which has been made possible by the development of a new solution features database for the Chalk. Different types of doline and other geo- and anthropomorphic features of the Chalk and overlying deposits will be presented. As dolines represent karst, and thus enhanced groundwater vulnerability, the knowledge of their distribution may help inform groundwater vulnerability mapping and modeling.

You may see the poster HERE.

Reference: Maurice, L.D., Atkinson, T.C., Barker, J.A., Bloomfield, J.P., Farrant, A.R., and Williams, A.T. (2006). Karstic behaviour of groundwater in the English Chalk. Journal of Hydrology. 330(1-2) 63-70.

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