Abstract

The chalk bedrock of the Hampshire Basin, Southern England is an important aquifer and is highly susceptible to dissolution, making the development and presence of karstic features a widespread occurrence. These features are hazardous as they provide possible pathways to the underlying aquifer presenting potential site-specific contamination risks. Extensive extraction, both through mining and surface quarrying, of chalk, flint and clay over many centuries is also in evidence. Geophysical techniques consisting of electromagnetic surveys (EM31) and ground penetrating radar (GPR) were used to identify and characterise target features identified from desk study data. The GPR and EM31 interpretations enabled the classification of the non-anthropogenic target features as diffuse buried sinkholes with disturbed and subsiding clay-rich infill and varying symmetrical and asymmetrical morphologies. The investigations of such features identified at Holme Farm, Stansted House, Hampshire are described in this paper. The combination of EM31 data and GPR profiles facilitated the identification of a palaeovalley, cavities and irregular rockhead. This investigation enabled the identification of locations of aquifer contamination risk, as some sinkholes have been the sites of illegal dumping of waste or infiltration of fertilisers, leaking sewage pipes or animal waste. This potential contamination source utilises the sinkhole as a pathway into the highly transmissive White Chalk Subgroup of Hampshire and has caused contamination of the aquifer. This study concluded that an integrated approach of geophysical techniques linked to aerial photo and LiDAR image interpretation proved highly effective in the location and characterisation of dissolution, infilled former quarries and mining features at this site.

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