OR/15/042 Chalk karst and groundwater flow in the Pang and Lambourn Catchments
| MAURICE, L, FARRANT, A R, BUTCHER, A AND ATKINSON, T C. 2015. Groundwater in Cretaceous carbonates: KG@B field trip 21st June 2015. British Geological Survey Internal Report, OR/15/042. |
The Pang and the Lambourn catchments contain ample evidence of karst. The upper parts of both catchments are developed on the Chalk, but in the lower reaches, Palaeogene strata are present. Around the margins of these Palaeogene outcrops surface karst features are widespread (Figure 5). At least 18 stream sinks have been recorded on the west and south side of the River Pang between Hermitage and Bucklebury, with another 25 on the north side of the River Pang in the Yattendon area. Sediment filled dissolution pipes (buried sinkholes) are widespread and well developed, especially beneath thin superficial deposits. In this area they may be up to 20 m deep and 5 m across, and commonly merge to form a very irregular rock-head. This can cause problems during construction, and dissolution pipes presented a significant engineering hazard during the widening of the A34 north of Newbury.
There is also evidence of karstic groundwater flow. Tracer testing between stream sinks and springs has demonstrated very rapid groundwater flows of several kilometres per day over distances of several kilometres suggesting fully integrated karstic flow (Banks et al., 1994[1]; Maurice et al., 2006[2]; Maurice et al., 2010[3]).
Although surface karst features are less prevalent in the upper parts of the catchments, there are good examples of ephemeral winterbourne streams. At the head of the rivers there are seasonally active springs, sometimes known as bourne holes. These can be several kilometres upstream of the perennial river head. These seasonal springs become rapidly reactivated over a short period of time, suggesting that they are fed by karstic fissures and conduits.
Single borehole dilution testing in 24 boreholes in the Pang and Lambourn catchments showed that flow horizons are prevalent in areas with and without surface karst (Maurice et al., 2012[4]). Borehole imaging and geophysical logging data revealed that most flowing features identified from the borehole dilution tests were dissolutional in nature, and there is a strong lithological influence on flow (Figure 6).
References
- ↑ BANKS, D, DAVIES, C, and DAVIES, W. 1995. The Chalk as a karstic aquifer: evidence from a tracer test at Stanford Dingley, Berkshire, UK. Quarterly Journal of Engineering Geology 28, S31–S38.
- ↑ 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.
- ↑ MAURICE, L D, ATKINSON, T C, WILLIAMS, A T, BARKER and FARRANT, A R. 2010. Catchment Scale tracer testing from karstic features in a porous limestone. Journal of Hydrology 389 (1–2), 31–41.
- ↑ MAURICE, L D, ATKINSON, T C, BARKER, WILLIAMS, A T and GALLAGHER, A. 2012. The nature and distribution of flowing features in a porous limestone aquifer with small-scale karstification. Journal of Hydrology 438–439, 3–15.